In the United States (US), researchers at Colorado State University (CSU) have redesigned polyhydroxyalkanoates (PHAs), a class of “dream” biodegradable polymers created by living microorganisms, to improve the material’s durability, recyclability, and thermal stability.
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They’ve been called “dream” plastics – polyhydroxyalkanoates (PHAs) – a class of polymers naturally created by living microorganisms or synthetically produced from biorenewable feedstocks.
PHAs are also biodegradable in the ambient environment, including oceans and soil. However, crystalline PHAs are brittle, so are not as durable and convenient as conventional plastics.
Furthermore, they cannot easily be melt-processed and recycled, making them expensive to produce which is why PHAs have yet to take off as a sustainable, environmentally benign alternative to traditional fossil-derived plastics.
That could all change thanks to new research emerging from the US Department of Energy’s (DOE) Bioenergy Technologies Office (BETO) and Advanced Manufacturing Office Bio-Optimized Technologies to keep Thermoplastics out of Landfills and the Environment (BOTTLE) consortium.
Addressing inherent disadvantages
Colorado State University (CSU) polymer chemists led by Eugene Chen, a Distinguished University Professor in the Department of Chemistry, have created a synthetic PHA platform that addresses each of these problems, paving the way for a future in which PHAs can take off in the marketplace as truly sustainable plastics.
Professor Chen and colleagues report a new class of redesigned PHAs, readily accessible via chemical catalysis, in a paper called “Chemically circular, mechanically tough, and melt-processable polyhydroxyalkanoates” published in the journal Science.
Thermal stability
The researchers had been searching for a strategy to address the intrinsic thermal instability of conventional PHAs; their lack of heat resistance also makes it difficult to melt-process them into end products.
The CSU chemists made fundamental changes to the structures of these plastics, substituting reactive hydrogen atoms responsible for thermal degradation with more robust methyl groups.
This structural modification drastically enhances the PHAs’ thermal stability, resulting in plastics that can be melt-processed without decomposition.
Better performance and fully recyclable
Furthermore, these newly designed PHAs are mechanically tough, even outperforming the two most common commodity plastics – high-density polyethylene (HDPE), and isotactic propylene.
The former is used in products like milk and shampoo bottles. The latter is a thermoplastic polymer that features high tensile strength, low density, and good thermal and abrasion resistance and is used to make automotive parts and synthetic fibers.
The best part is that the new PHA can be chemically recycled back to its building-block molecule (monomer), with a simple catalyst and heat, and the recovered clean monomer can be reused to reproduce the same PHA again – in principle, infinitely.
We are adding three key desired features to the biological PHAs, including closed-loop chemical recycling, which is essential for achieving a circular PHA economy, Professor Eugene Chen said.
