Bioplastic with Potential—But Weaknesses So Far
PHB is one of the most intensively researched bioplastics worldwide. Microorganisms can produce it; it is biodegradable and can be derived from renewable raw materials. For many years, PHB has therefore been considered a promising alternative to petroleum-based plastics. Nevertheless, a commercial breakthrough has so far eluded it.
The reason lies in the material’s molecular structure. Naturally occurring PHB forms highly ordered crystal structures. This makes the plastic brittle and gives it a melting point that is close to its thermal decomposition temperature. Anyone seeking to process PHB must therefore operate within a narrow temperature window. This complicates production—and, later on, recycling as well.
Chemists were able to demonstrate years ago that these drawbacks could be overcome by specifically modifying the polymer’s microstructure. Until now, however, this had only been achieved through chemical means. This was hardly practical for economical and sustainable production.
A Textbook Assumption Is Being Challenged
For a long time, researchers assumed that the enzymes involved would accept only a specific spatial configuration of the polymer building blocks. According to this assumption, microorganisms would inevitably always produce the same highly ordered plastic.
“We wondered whether this assumption was actually correct,” says Prof. Dr. Volker Sieber of the Chair of Chemistry of Biogenic Resources at TUM. “Perhaps the necessary building blocks were simply not available in the cell until now, which is why it had never been observed that the enzymes could also incorporate other structures.”
To test this hypothesis, the researchers developed a new metabolic pathway in Escherichia coli and specifically produced the previously missing mirror-image polymer building blocks. They were indeed able to demonstrate that the microorganisms incorporate these building blocks into PHB. In doing so, they showed for the first time that the enzymes are not as strictly defined as had long been assumed.
Less Order, Better Properties
The new structure noticeably alters the material. The melting point dropped from about 179 degrees Celsius to 154 degrees Celsius. This significantly increases the margin between the processing temperature and thermal decomposition. At the same time, the polymer chains suffer less damage during processing.
“At first glance, this sounds like a minor change at the molecular level,” says Marcel Mayer, a doctoral student at the department. “However, this difference makes a huge difference when it comes to processing the material.”
New Possibilities for Sustainable Plastics
The researchers view their work as the beginning of a new direction in the development of biologically produced plastics. Until now, efforts to optimize PHB have focused primarily on combining different monomers. The new study now shows that the spatial arrangement of the monomers can also be specifically influenced.
“Plastics chemists have been using microstructure control for decades to specifically tailor material properties,” says Sieber. “With our work, this concept is now accessible for microbially produced PHB for the first time,” adds doctoral student Mayer.
In the long term, this could lead to the development of biodegradable plastics whose properties can be specifically tailored to different applications—from packaging to engineering materials. Accordingly, the research was also conducted at the TUM Center for Microplastic Prevention.
A First Step
This is still a proof of concept. The researchers have already achieved a concentration of just under seven percent of the new building blocks in the polymer and were able to experimentally demonstrate the improved material properties. Now they want to investigate how this concentration can be further increased and how the material’s properties can be controlled even more precisely.
“We are demonstrating for the first time that the microstructure of biologically produced PHB is not fixed,” says Sieber. “This opens up new possibilities for the development of high-performance and sustainable bioplastics.”