Biodegradable plastics are used as a solution to the problem of plastic pollution, but there is a catch: today’s compostable plastic bags, utensils, and cup lids – made primarily from the polyester known as polylactic acid or PLA – don’t degrade during typical composting. potentially as durable as conventional plastics.
Now scientists at the University of California at Berkeley have invented a method that makes compostable plastics more easily broken down within a few weeks using just heat and water.
Under the direction of Ting Xu, professor of materials science and technology at UC Berkeley and chemistry, the scientists developed a process in which polyester-eating enzymes are embedded in the plastic. 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.
A key innovation in this new technique was to find a way to protect the enzyme from breaking apart, which proteins normally do outside of their normal environment, for example in a living cell. Xu designed molecules called random heteropolymers, or RHPs, that wrap around the enzyme, gently holding 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 one percent the weight of the plastic – low enough not to be a problem.
Xu and her team used a similar technique to coat the enzyme in RHPs and embed billions of these nanoparticles in plastic resin beads, which are the starting point for all plastic manufacturing. 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 ° C or 338 ° F and extruded into fibers like regular polyester plastic.
With this new process, 80 percent of the modified PLA fibers were completely degraded within about a week at room temperature. The degradation was faster at higher temperatures. Under industrial composting conditions, the modified PLA degraded within six days at 50 ° C or 122 ° F. Another polyester plastic, PCL (polycaprolactone), degraded within two days under industrial composting conditions at 40 ° C or 104 ° F. For PLA, Xu has embedded an enzyme called Proteinase K, which PLA chews up in lactic acid molecules. She used lipase for PCL. Both are inexpensive and readily available enzymes.
The process also eliminates microplastics, a by-product of many chemical breakdown processes and a self-contained pollutant. The scientists said that up to 98 percent of the plastic made using Xu’s technique breaks down into small molecules.
And the new technology should, in theory, be applicable to other types of polyester plastics and potentially allow the production of compostable plastic containers 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.
But there is still a lot to be done. Xu is currently developing RHP-encased enzymes that can break down other types of polyester plastic, but is also modifying the RHPs so that the breakdown 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.
Xu said programmed deterioration could be key to recycling many objects. “Imagine using biodegradable glue to assemble computer circuits or even entire phones or electronic devices. When you’re done with that, you dissolve the glue so the devices fall apart and all of the parts can be reused,” she said.
Source: University of California