Embedded enzymes key to creating fully biodegradable plasticSiobhan Treacy | April 22, 2021
Researchers from the U.S. Department of Energy’s Lawrence Berkeley National Lab and University of California Berkeley have created truly compostable plastic.
Only 9%of the plastic in the U.S. is actually recycled. Biodegradable plastic could be one option to start solving this problem, but the materials must be properly sorted to prevent biodegradable plastic from contaminating recyclable #1 and #2 plastics. Most biodegradable plastics take months to break down and form harmful microplastics.
Today’s biodegradable plastic is typically made of polylactic acid (PLA), a vegetable-based plastic material blended with cornstarch, or polycaprolactone (PCL), which is essentially a biodegradable polyester that is usually used for biomedical applications.
In recycling, biodegradable plastic is indistinguishable from single-use plastic, which is why it often ends up in a landfill. Even if it does end up in an organic waste facility, biodegradable plastic does not break down as fast as other organic materials and ends up contaminating other organic waste. Biodegradable plastic is not as strong as regular plastic, which causes it to break down into microplastics.
The team's new enzyme-activated compostable plastic could help lower microplastic pollution. The new plastic can be broken down into its basic building blocks and reformed into a new compostable product.
To create the plastic, the team embedded nanoconfining enzymes into plastics by carving out a safe place inside the material for enzymes to lie dormant until it is needed. Trace amounts of the commercial enzyme Burkholderia cepacium lipase (BC-lipase) and protein K were then embedded into PLA and PCL plastics. An enzyme protectant called four-monomer random heteropolymer (RHP) was also added to help disperse the enzyme a few nanometers apart in the plastic.
With testing, the team found that household tap water or standard soil composts converted the enzyme-embedded plastic material into monomers, which are small molecule building blocks. This method eliminated microplastics in a few days or weeks.
They found that BC-lipase is a picky eater. This means that before it can convert polymer chains into monnomers, it needs to catch the end of a polymer train. By controlling when the lipase finds the chain end, it is possible to guarantee that the materials don’t degrade until triggered by water or compost soil. Industrial enzymes can cost up to $10/kg, but with the new approach, only a few cents would be added to the production cost per kilogram of resin. The new material has a shelf life of over seven months.
A variety of methods was used to test the new plastic. X-ray scattering studies characterized the nanodispersion of enzymes in PCL and PLA plastic. Interfacial tension experiments showed in real-time how the size and shape of droplets changed as the plastic material decomposed. The lab differentiated between enzyme and RHP molecules, which gave the team information on how degradation is going.
Creating affordable and easily compostable plastic film could incentivize manufacturers to package fresh fruit and vegetables in compostable plastic. This would also save organic waste facilities the extra step of buying expensive plastic depackaging machines when accepting food waste at their facilities. The new approach could work well with hard and rigid plastics and soft, flexible plastics. The team wants to broaden their study to polyolefins, which are used to produce toys and electronics parts.
The study was published in Nature.