CM – A new process makes biodegradable plastics really compostable


By Robert Sanders, media relations | April 21, 2021 April 21, 2021

A modified plastic (left) disintegrates after only three days in standard compost (right) and completely after two weeks. (UC Berkeley photo by Ting Xu)

Biodegradable plastics have been promoted as a solution to the plastic pollution problem that threatens the world. However, today’s “compostable” plastic bags, utensils and cup lids do not break down during typical composting and contaminate other recyclable plastics, which is a headache for recyclers. Most compostable plastics, made primarily from the polyester known as polylactic acid or PLA, land in landfills and last as long as plastics forever. Scientists at the University of California, Berkeley, have now found a way like this Compostable plastics can more easily degrade within weeks with just heat and water to solve a problem that has messed up the plastics industry and environmentalists.

« People are now ready to switch to biodegradable polymers for single-use plastics. If however, if this turns out to be causing more problems than it’s worth, politics could return, ”said Ting Xu, a professor of materials science and engineering at UC Berkeley and chemistry. “We are basically saying that we are on the right track. We can solve this persistent problem that single-use plastics are not biodegradable. “

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

The new technology should theoretically be applicable to other types of polyester plastics and possibly make them compostable Allow plastic containers that are currently made from polyethylene, a type of polyolefin that does not degrade. Xu believes that polyolefin plastics are best made into higher quality products rather than compost, and is working on ways to convert recycled polyolefin plastics for reuse.

A melt-extruded PCL (polycaprolactone) plastic filament (left) with embedded nanoclusters of the RHP-coated enzyme lipase was almost completely degraded to small molecules within 36 hours in warm (104 F) water. (Photos by Christopher DelRe)

The new process involves embedding polyester-eating enzymes in the plastic as it is made. These enzymes are protected by a simple polymer coating that prevents the enzyme from untangling and becoming unusable. When the enzyme is exposed to heat and water, it shakes off its polymer casing and breaks the plastic polymer down into its building blocks – in the case of PLA, it reduces it to lactic acid, which can compost the soil microbes. The polymer coating also deteriorates.

The process eliminates microplastics, a by-product of many chemical breakdown processes and its own pollutant. Up to 98% of the plastic made with the Xu technique is broken down into small molecules.

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

Plastics are designed in such a way that they do not disintegrate during normal use. However, this also means that they will not disintegrate after being disposed of. The most durable plastics have an almost crystal-like molecular structure, with the polymer fibers aligned so tightly that no water can penetrate them, let alone microbes that could chew up the polymers, which are organic molecules.

Enzymes like lipase (green Spheres) can break down plastic polymers from the surface (top left), but they accidentally cut open the polymer and leave behind microplastics (top right). A UC Berkeley group embedded enzyme nanoclusters throughout the plastic (bottom left), protected by random heteropolymers (chains of colored spheres). The embedded enzymes are immobilized near the end of the polymer chains and, under the right conditions of heat and moisture, break down polymer molecules mainly from the chain end. This technique maintains the integrity of the plastic during use. However, when the user triggers a depolymerization, the plastic goes back down to recyclable low molecular weight by-products. (Graphic by Christopher DelRe)

Xu’s idea was to embed nanoscale polymer-eating enzymes directly into a plastic or other material so that they are bound and protected until the right conditions release them. In 2018 she showed how it works in practice. She and her UC Berkeley team have embedded an enzyme in a fiber mat that breaks down toxic organophosphate chemicals such as insecticides and chemical warfare agents. When the mat was immersed in the chemical, the embedded enzyme decomposed the organophosphate.

Their key innovation was a way to protect the enzyme from breaking apart, which proteins normally do outside of their normal environment, such as B. a living cell. She designed molecules, which she called random heteropolymers, or RHPs, that wrap around the enzyme and gently hold it together without compromising its natural flexibility. The RHPs are made up 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 less than 1% of the weight of the plastic – low enough not to be a problem.

For the research reported in the Nature article, Xu and her team used a similar one Technique in which the enzyme was trapped in RHPs and billions of these nanoparticles were embedded in plastic resin beads that are the starting point for all plastic production. She likens this process to embedding pigments in plastic to color them. The researchers showed that the RHP-coated enzymes did not change the character of the plastic, which could be melted at temperatures around 170 degrees Celsius or 338 degrees Fahrenheit and extruded into fibers like regular polyester plastic.

A film made from PLA plastic ( Polylactic acid) immediately after placing in the compost (left) and after a week in the compost (right). Embedded in an enzyme, the PLA plastic can biodegrade into simple molecules, making it promising as a future alternative to a non-degradable plastic. (UC Berkeley photo by Adam Lau / Berkeley Engineering)

All that was needed to do was add water and some heat to trigger degradation. At room temperature, 80% of the modified PLA fibers completely decomposed within about a week. The degradation was faster at higher temperatures. Under industrial composting conditions, the modified PLA degraded within six days at 50 degrees Celsius. Another polyester plastic, PCL (polycaprolactone), degraded within two days under industrial composting conditions at 40 degrees Celsius. For PLA, she embedded an enzyme called Proteinase K, which chews PLA into lactic acid molecules. For PCL she used lipase. Both are inexpensive and readily available enzymes.

« If you only had the enzyme on the surface of the plastic, it would be very slow to etch, » Xu said. « They want it to be nanoscopically dispersed everywhere, so essentially each of them just eats away their polymer neighbors and then all of the material breaks down. »

The rapid breakdown works well with municipal composting, which typically takes 60 to 90 days takes to turn food and plant waste into usable compost. Industrial composting at high temperatures takes less time, but the modified polyesters also decompose faster at these temperatures.

Doctoral student Ivan Jayapurna with a sample film made of PCL (polycaprolactone), a new, biodegradable polyester plastic. Enzyme-embedded PCL has mechanical properties very similar to low-density polyethylene, making it a promising future alternative to non-biodegradable plastics. (UC Berkeley photo by Adam Lau / Berkeley Engineering)

Xu suspects that the coated enzyme moves more at higher temperatures, so that it finds the end of a polymer chain faster, chews it up and then moves on to the next chain. The RHP-coated enzymes also tend to bind near the ends of the polymer chains, keeping the enzymes close to their targets.

The modified polyesters do not degrade at lower temperatures or during short periods of humidity, she said. For example, a polyester shirt made by this process would withstand sweat and moderate washing. Soaking the plastic for three months in water at room temperature didn’t break the plastic.

« It turns out that composting isn’t enough – people want to compost in their house without getting their hands dirty, they want in Compost water, « she said. “We tried to see that. We used warm tap water. Just heat it to the right temperature then put it in and we’ll see it go away in a couple of days. “

Xu is developing RHP encased enzymes that can degrade other types of polyester plastic, but she also modifies the RHPs so that the degradation can be programmed to stop at a certain point and not completely destroy the material. This can be useful when the plastic is to be remelted and converted into new plastic.

The project is supported in part by the Department of Defense’s Army Research Office, an element of the Army Research Laboratory of the US Army Combat Capabilities Development Command.

« These results provide a foundation for the rational design of polymeric materials that can degrade in relatively short periods of time and provide significant benefits for army logistics in the waste management context, » said Stephanie McElhinny, Ph.D., program manager with the Army Research Office . « In a broader sense, these results provide insights into strategies for incorporating active biomolecules into solid-state materials that could impact a variety of future Army capabilities, including sensing, decontamination, and self-healing materials. »

A film made from PLA plastic (Polylactic acid) that is embedded with an enzyme so that it biodegrades quickly in normal compost. (UC Berkeley photo by Adam Lau / Berkeley Engineering)

Xu said programmed degradation could be key to recycling many objects. Imagine using biodegradable glue to assemble computer circuits, or even entire phones or electronics, and then, when you’re done, dissolve the glue so the devices can fall apart and all of the parts can be reused.

« It’s good for millennials to think about it and start a conversation that will change the way we connect with the earth, » said Xu. “Look at all the wasted stuff we throw away: clothes, shoes, electronics like cell phones and computers. We take things off the earth faster than we can give them back. Don’t go back to earth to mine these materials, just mine whatever you have and then convert it to something else. « 

The paper is co-authored by Christopher DelRe, Yufeng Jiang, Philjun Kang, Junpyo Kwon, Aaron Hall, Ivan Jayapurna, Zhiyuan Ruan, Le Ma, Kyle Zolkin, Tim Li and Robert Ritchie from UC Berkeley; Corinne Scown from Berkeley Lab; and Thomas Russell of the University of Massachusetts at Amherst. Work was funded primarily by the US Department of Energy (DE-AC02-05-CH11231) with support from the Army Research Office and UC Berkeley’s Bakar Fellowship program.

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