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.
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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 , 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. She compares this process to embedding pigments in plastic to color them. The researchers showed that the RHP-shrouded enzymes did not change the character of the plastic, which could be melted and extruded into fibers like normal polyester plastic at temperatures around 170 degrees Celsius, or 338 degrees Fahrenheit.
To trigger degradation, it was necessary only to add water and a little heat. At room temperature, 80% of the modified PLA fibers degraded entirely within about one week. Degradation was faster at higher temperatures. Under industrial composting conditions, the modified PLA degraded within six days at 50 degrees Celsius (122 F). Another polyester plastic, PCL (polycaprolactone), degraded in two days under industrial composting conditions at 40 degrees Celsius (104 F). For PLA, she embedded an enzyme called proteinase K that chews PLA up into molecules of lactic acid; for PCL, she used lipase. Both are inexpensive and readily available enzymes.
&#;If you have the enzyme only on the surface of the plastic, it would just etch down very slowly,&#; Xu said. &#;You want it distributed nanoscopically everywhere so that, essentially, each of them just needs to eat away their polymer neighbors, and then the whole material disintegrates.&#;
The quick degradation works well with municipal composting, which typically takes 60 to 90 days to turn food and plant waste into usable compost. Industrial composting at high temperatures takes less time, but the modified polyesters also break down faster at these temperatures.
Xu suspects that higher temperatures make the enshrouded enzyme move around more, allowing it to more quickly find the end of a polymer chain and chew it up and then move on to the next chain. The RHP-wrapped enzymes also tend to bind near the ends of polymer chains, keeping the enzymes near their targets.
The modified polyesters do not degrade at lower temperatures or during brief periods of dampness, she said. A polyester shirt made with this process would withstand sweat and washing at moderate temperatures, for example. Soaking in water for three months at room temperature did not cause the plastic to degrade.
Soaking in lukewarm water does lead to degradation, as she and her team demonstrated.
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&#;It turns out that composting is not enough &#; people want to compost in their home without getting their hands dirty, they want to compost in water,&#; she said. &#;So, that is what we tried to see. We used warm tap water. Just warm it up to the right temperature, then put it in, and we see in a few days it disappears.&#;
Xu is developing RHP-wrapped enzymes that can degrade other types of polyester plastic, but she also is modifying the RHPs so that the degradation can be programmed to stop at a specified point and not completely destroy the material. This might be useful if the plastic were to be remelted and turned into new plastic.
The project is in part supported by the Department of Defense&#;s Army Research Office, an element of the U.S. Army Combat Capabilities Development Command&#;s Army Research Laboratory.
&#;These results provide a foundation for the rational design of polymeric materials that could degrade over relatively short timescales, which could provide significant advantages for Army logistics related to waste management,&#; said Stephanie McElhinny, Ph.D., program manager with the Army Research Office. &#;More broadly, these results provide insight into strategies for the incorporation of active biomolecules into solid-state materials, which could have implications for a variety of future Army capabilities, including sensing, decontamination and self-healing materials.&#;
Xu said that programmed degradation could be the key to recycling many objects. Imagine, she said, using biodegradable glue to assemble computer circuits or even entire phones or electronics, then, when you&#;re done with them, dissolving the glue so that the devices fall apart and all the pieces can be reused.
&#;It is good for millennials to think about this and start a conversation that will change the way we interface with Earth,&#; Xu said. &#;Look at all the wasted stuff we throw away: clothing, shoes, electronics like cellphones and computers. We are taking things from the earth at a faster rate than we can return them. Don&#;t go back to Earth to mine for these materials, but mine whatever you have, and then convert it to something else.&#;
Co-authors of the paper include Christopher DelRe, Yufeng Jiang, Philjun Kang, Junpyo Kwon, Aaron Hall, Ivan Jayapurna, Zhiyuan Ruan, Le Ma, Kyle Zolkin, Tim Li and Robert Ritchie of UC Berkeley; Corinne Scown of Berkeley Lab; and Thomas Russell of the University of Massachusetts in Amherst. The work was funded primarily by the U.S. Department of Energy (DE-AC02-05-CH), with assistance from the Army Research Office and UC Berkeley&#;s Bakar Fellowship program.
The researchers suggest that standards need to be developed for compostable, biodegradable and oxo-biodegradable materials, such as clearly outlining appropriate disposal and the rates of degradation that can be expected.
The lack of consumer education about the differences between compostable, biodegradable and oxo-biodegradable materials and where to properly dispose of them, coupled with a lack of facilities to deal with their decomposition, further challenges the notion that these products are better for the environment.
But that should not be the end of the conversation. We need to challenge the single-use product model itself.
As resistance to the plastic epidemic swells and cities and countries carry out bans against single-use plastic products worldwide, we need to carefully consider the implications of this research study. At the heart of the issue remains a key question: is the single-use, throwaway model sustainable in the first place?
&#;The problem is not just plastic: it is mass disposability. Or, to put it another way, the problem is pursuing, on the one planet known to harbour life, a four-planet lifestyle. Regardless of what we consume, the sheer volume of consumption is overwhelming the Earth&#;s living systems.&#; - George Monbiot, We Won&#;t Save the Earth With a Better Kind of Disposable Coffee Cup
Last year, someone tweeted at Starbucks to request they replace their plastic coffee cups with cups made from corn starch. This tweet was retweeted over 60,000 times before being deleted when someone raised a big red flag: those who were supporting this call failed to consider the environmental impacts of producing corn starch. As it turns out, an enormous amount of land needs to be cleared to grow it, displacing food production. Growing corn is also notorious for causing soil erosion and requires heavy use of pesticides and fertilizers.
At this stage in the game, it might be beneficial for us to carefully consider what comes after plastic. The impacts of growing and harvesting raw materials, producing the single-use item in a factory and shipping it all over the world just so someone can use it once and throw it away needs to be picked apart and examined.
Humans living in capitalistic societies have a tendency to unleash products on the world without giving much thought as to, you know, whether or not there are facilities available to properly deal with that compostable bag. Or effective ways to collect these single-use items in the first place - technically we have recycling facilities for plastic bags, and yet plastic bags continue to find their way into the ocean. So clearly it&#;s not just a matter of infrastructure.
Is the problem with the material, or with a model of consumption that relies on disposability and the infinite production of stuff?
&#;People have to buy less. Our economy is based on endless growth, endless production of what our landfills tell us is basically junk. The stuff wouldn't be in them if it wasn't junk&#;our economy is already failing us in the way it messes up the planet in the service of all this crap. The cycle just keeps going: manufacture, consume, discard.&#; Daniel Hoornweg, Canada&#;s Dirty Secret
Perhaps models that privilege reuse over single-use are a bigger part of the solution than simply pumping out a &#;better&#; kind of disposable product.
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