Swiss researchers discover “unprecedented” design method for biodegradable plastics

17 Oct 2022 --- Researchers at ETH Zurich, Switzerland, say they have discovered a method of accurately measuring how plastic degrades in the environment. A team from the University’s Earth Sciences department claims its findings could, in the future, give industry the design tools to ensure and certify product biodegradability.

Lead study author Dr. Michael Sander tells PackagingInsights that a lot of plastic packaging is currently inaccurately labeled as “biodegradable” because tracking the degradation of polymers in the soil is a complex task. 

“It requires that we follow the polymer carbon throughout this degradation process. This is hard because there is a lot of other, naturally occurring organic carbon (in the form of things like leaves and roots) in the environments to which we add the polymer,” he explains. 

Sander and his team devised a method of distinguishing polymers from other naturally occurring carbons by replacing some of the carbons in the polymer with a stable carbon isotope that is naturally very rare.

Image credit: Michael Zumstein.“We now have the tools in hand to assess how polymers need to be designed and how systems need to be operated (like industrial composts) to ensure complete biodegradation of polymers,” he claims. “This finding makes a big difference.” 

Polymer tracing
The research, published in Nature Communications, details how biodegradable polymers are “eaten” by other microorganisms, which burn most of the polymer’s carbon. The team used a stable carbon isotope, poly(butylene succinate) (PBS), a synthetic polyester the authors say is easily identifiable. 

“Now, the polymer carbon can be tracked by that stable isotope. It’s a bit like painting some of the carbon atoms in the polymer in a slightly different color and, by doing so, making them analytically distinguishable from the other carbons,” says Sander. 

“It's the first time that the idea of using stable carbon isotopes to selectively follow polymer carbon during biodegradation has been experimented.” 

“This work provides an unprecedented view into the biodegradation process,” he asserts.  

The good from the bad  
Sander says the fact that the experiment only used PBS and only one type of soil is not a limitation because the team only needed to demonstrate that the method works. 

“We realize that there are many polymers to test and many other soils, environments like compost and many conditions like temperature or humidity. So the study opens the door to answer exciting research questions on biodegradable polymers that can now be answered,” he says. Lead researcher Dr. Michael Sander.

Sander adds that the packaging industry is currently approached by polymer producers with suggested techniques that are often inaccurate. 

“As usual, there are polymers that work and some technologies that don’t. For instance, (oxo) additive technologies that claim to render polyolefins (such as PE) compostable or biodegradable in the open environment – without adequate scientific proof.” 

“Our approach can clearly delineate the good from the bad,” he says.

Scaling up
Sander concedes the research will need continuing, and industry must make the technique economically viable. “Of course, stable carbon isotope labeled polymers need to be synthesized. We needed very small amounts for the analyses – such polymers are not cheap because of the label,” he says. 

However, he asserts that the packaging industry is well aware of the ongoing discussion of using polymers more sustainably and that biodegradability is a must in many applications. 

“The approach we present in this paper is equally applicable to polymers designed to biodegrade in engineered systems, such as composts. We are currently extending in this direction from soils to include industrial composts in our testing capabilities,” he continues. 

“The solvent extraction technique we used in the paper to quantify residual polymer in the soil is readily applicable to other matrices, such as compost, and would allow quantifying potentially present residual biodegradable (compostable) polymer without the need to go into expensive, time-consuming and non-quantitative microplastic particle analysis.”

“We now have the analytical methodologies in hand to determine how much polymer potentially stays behind and how much of the polymer carbon is incorporated into microbial biomass.” 

Pushing policy changes 
The ETH’s team may have found a viable solution to the problem of misinformation in the bioplastics industry. Last year, the policy compliance scheme Ecosurety called for clarification on terms such as “biodegradable” and “compostable,” noting broad consumer confusion.  

The scheme’s head of policy and innovation, Robbie Staniforth, argued there is a potential “to go down the wrong path that needs to be addressed. One example is that some novel [biodegradable or compostable] plastics simply break up into microplastics, making the pollution situation worse, not better.”

While biodegradable plastics still make up a tiny proportion of market materials, accurate design and testing methods will become increasingly important as environmental legislation tightens worldwide. 

By Louis Gore-Langton