PHAs, or polyhydroxyalkanoates are a family of biopolyesters produced in nature by numerous microorganisms. They are synthesised directly via fermentation of a carbon substrate, varying from sugar and lipids to sewage, coffee grounds or methanol, inside microorganisms, and stored in the form of water insoluble granules as energy and carbon reserves inside their cells. PHAs are synthesised by microbes in nutrient-deficient conditions. They are separated from the microbes with the help of a solvent; the solvent is then removed, the harvested PHA is washed, dried and pelletised through an extruder.
Not only are PHAs derived from renewable sources, but they are also fully biodegradable – in soil and water - compostable and biocompatible. The properties of these materials – they can be thermoplastic or elastomeric - are highly ‘tuneable’: PHAs can be engineered to meet the performance requirements of a wide range of applications currently using fossil fuel-based PE, PP, PS, PET, PVC or TPU.
However, until very recently research was inconclusive on whether making textiles from PHA was possible. Early production methods worked by extracting PHA from a bacterial synthesis, a method which made it difficult for scientists to control the resulting backbone and properties of the PHA.
Now, a team of polymer scientists from the US National Research Lab National Renewable Energy Laboratory (NREL) have developed a portfolio of PHAs with different properties, including some that behave like conventional polyester but are biobased, biodegradable, and easier to recycle.
As part of their mission to create more sustainable plastics, scientists at the US Department of Energy’s Bio-Optimized Technologies to keep Thermoplastics out of Landfills and the Environment (BOTTLE) consortium have developed alternative techniques for turning succinic acid (made from glucose) and other biobased building blocks into PHA via synthetic routes. Chemo-catalytic approaches allow the team to tailor PHA’s chemical backbone to achieve key attributes, like melt-processability, crystallinity, and ductility. The BOTTLE team has reported PHA prototypes with a broad range of properties — from rigid PHA polymers for clear food packaging to ductile varieties for spinning into fibre.
“We’re actually showing we can control the microstructure to get PHA to behave like polyester fibres and textiles,” said NREL polymer scientist Katrina Knauer.
Together with outdoor apparel brand The North Face, the team will compare the textile sustainability credentials of PHA to conventional polyester. They will study the carbon intensity of making and recycling PHA fibres, simulate microfibre shedding, and measure PHA’s rate of biodegradation in a variety of environmental scenarios.
After reviewing the scientists’ initial results, The North Face aims to test prototype fibres with its suppliers to evaluate for a line of more-sustainable products.
A key feature of the team’s PHAs portfolio is its design for recycability. At the end of a product’s useful life, BOTTLE’s depolymerisation technology can deconstruct PHAs and other polymers back into building blocks pure enough to reuse for making plastics.
According to Ravikumar Gowda, a BOTTLE researcher based at Colorado State University (CSU), the material’s recyclability hinges on specific changes to PHA’s chemical backbone. By replacing reactive hydrogen atoms with robust alkyl groups, Gowda and other CSU researchers significantly enhanced the thermal stability of PHA to make it melt processable — a key advantage over PHAs derived from microbes. The change also enables the team to depolymerise the polymers.
“Our redesigned PHA structure substantially increases mechanical toughness and renders the new PHA chemically recyclable to its building-block monomer with a simple catalyst and heat,” he said. “The recovered monomer can be reused to reproduce the same PHA again, in principle, infinitely.”
One of the points the project remains to address is the cost of its synthetic production of PHAs versus the traditional methods, whose high costs have long hindered attempts to scale up PHA production.