The enormous variety and abundance of plastic is a major headache for the recycling industry. More than 420 million tons of plastics are produced worldwide every year. As a result, the reuse, recovery and recycling of plastics are essential.

Recycling plastic helps conserve resources and divert plastics away from landfills or unintended destinations like oceans.

Scientists at the University of California have found a way how these compostable plastics can be more easily broken down within weeks with just heat and water.

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

Compostable plastic containers currently in use are made of polyethylene, a type of polyolefin that does not degrade. This new technology also applies to other polyester plastics and enables the production of compostable plastic containers.

According to Xu, polyolefin plastics are best processed into higher quality products and not into compost.

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

In this new technology, scientists have embedded polyester-eating enzymes in the plastic as it is being manufactured. A simple polymer coating protects the enzymes from being disentangled and useless.

When these enzymes are exposed to heat and water, the polymer cover is shaken off and the plastic polymer is broken down into its building blocks. In the case of PLA, the reduction to lactic acid can feed the soil microbes in the compost. The polymer coating also deteriorates.

The process wipes out microplastics, a by-product of numerous chemical breakdown processes and a pollutant in its ritual. Up to 98% of the plastic made using the Xu process is broken down into small molecules.

In a previous study, Xu designed molecules called random heteropolymers, or RHPs, that wrap around the enzyme, gently holding it together without compromising its natural flexibility. These RHPs degrade under ultraviolet light and are less than 1% of the weight of the plastic – low enough not to be a problem.

The same approach was used in this study. Scientists encased the enzyme in RHPs and embedded billions of these nanoparticles in plastic resin beads, which are the starting point for all plastic production.

When scientists compared this process with embedded pimento in plastic, they found that the RHP-coated enzymes did not change the character of the plastic.

A film from PLAA film made of PLA plastic (polylactic acid) immediately after placing it 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)

Additional water and little heat were required for dismantling. At room temperature, 80% of the modified PLA fibers completely decomposed within about a week.

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

“At higher temperatures, the coated enzyme moves more so that it finds the end of a polymer chain more quickly, chews it up and then moves on to the next chain. The RHP-coated enzymes also 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. For example, a polyester shirt made by this process would withstand sweat and moderate washing. Soaking in water for three months at room temperature did not decompose the plastic.

A film from PLAA film made of PLA (polylactic acid) plastic that is embedded with an enzyme so that it is quickly biodegraded in normal compost. (UC Berkeley Photo by Adam Lau / Berkeley Engineering)

“It turns out that composting is not enough – people want to compost in their house without getting their hands dirty, they want to compost in water. 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. “

Stephanie McElhinny, Ph.D. The Army Research Office program manager said, “These results provide a foundation for the rational design of polymeric materials that can degrade in relatively short periods of time and that could provide significant benefits to the Army’s waste management logistics. 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 military capabilities, including sensor, decontamination, and self-healing materials. “

Journal reference:
  1. C. DelRe, Y. Jiang, P. Kang, et al. Almost complete depolymerization of polyesters with nanodisperse enzymes. Nature 592: 558-563 (2021). DOI: 10.1038 / s41586-021-03408-3