Polyhydroxyalkanoates utilizing nature’s own mechanisms
Possibly the most effective way to produce biopolymers is to utilize nature’s own mechanisms. Many bacterial species synthesize polyhydroxyalkanoates (PHAs), a type of polyester, to store their energy for future need, especially when limiting essential nutrients such as phosphor and nitrogen. The most commonly produced and studied PHA is poly (hydroxybutyrate) (PHB). Both its biobased origin as well as a very good biodegradability make it an interesting substitute for current petrochemical plastics. Currently, PHA finds application in many fields, amongst which are food apparel such as straws and cutlery, flexible packaging, agricultural foils and in a variety of medical applications.
PHB is a polymer with similar properties as other non-biobased polyesters and for example olefins such as polypropylene (PP). While this material suffers from brittleness and breaks easily, this can be modified by copolymerization with for example poly-hydro valerate. Various copolymers can be synthesized by utilizing different feedstocks as well as changing cultivation conditions. The use of waste vegetable oil for example can lead to high amounts of 3-hydroxyhexanoate and/or 3-hydroxyoctanoate depending on strains. Also, overexpression of enzymes in the metabolic pathway can be utilized to get different PHA copolymers and thereby specific properties can be installed in the ultimate polymer product.
Isolation of the bio-synthesized PHA – A real challenge
The biosynthesized polymer is stored in granules inside the cell and under optimized conditions +/- 90% of the cell can be PHA (based on dry weight). As with many chemical processes the real challenge is to obtain the desired product in sufficient yield and purity. While the classic recovery methods involve chlorinated solvents such as chloroform, their toxicity, high costs and environmental impact make them unsuitable for large-scale production.
Various other solvents have been tried, with green options being the most sustainable. Examples are dimethyl carbonate and ethanolic waste streams amongst others. After dissolving, the polymer can be precipitated by using various agents and reduced temperature. Amongst other methods tested are chemical and enzymatical digestions of the cells, the use of supercritical fluids (amongst which CO2 would be the most interesting) and mechanical disruption such as bead mills. Other advanced purification techniques such as aqueous two-phase extractions (ATPE, figure 1), which utilizes the transfer of the biopolymer to a second aqueous medium and dissolved air flotation are reported to be used in the downstream processing (DSP) of PHAs.
Figure 1. Schematic depiction of two-phase aqueous separation. The image was taken from the work of Pérez-Rivero et al. Biochem. Eng. J., 150, 107283, doi:10.1016/j.bej.2019.107283. EOPO = ethyleneoxide propylene oxide.
After the recovery of the polymer, further purification steps are required. Especially for medical applications, strict control of contaminants is essential as the use of microorganisms explicitly results in the presence of bioactive compounds during production and purification. Choice of recovery method and purification technique directly influences the mechanical and physical properties of the resulting PHA. Furthermore, the selection of the type of microorganism will have a massive impact on the overall DSP with recombinant organisms generally show a more favourable profile. Overall the use of solvents has been explored most widely and is known to produce polymer in a reliable way. However, with the inherently sustainable image of bio-polymers, it is impossible to ignore other environmental impacts such as energy demand and the creation of further waste streams. The fact that microorganisms are responsible for the chemistry and the use of waste streams as feedstock is a great asset of this polymer, however, the real challenge remaining is to improve DSP, both in costs as well as in greenness.
Commercialization and future of PHA
As such, the cost of PHA currently remains a major hurdle. In the 1990s several large companies such as Zeneca, Monsanto and Procter & Gamble submitted a series of patents on the production and processing of PHAs. Due to high costs and the resulting economic unviability these projects were not continued at that time. P&G’s intellectual property is however still used by Japanese manufacturer Kaneka for example. With the improvement of DSP techniques, increased knowledge of fermentation and most of all increasing societal attention towards plastics with a bio-based origin and biodegradability, this higher price is increasingly paid. Although bioplastics still are a fraction of the worldwide plastic production, rapid progress is being made over the last few years and there is an increasing demand from the public to offer options with biobased plastic. With the current demand for plastic in general it is unlikely that bio-plastics are able to replace this full demand. Instead, introducing bio-plastics in a stepwise manner will allow the industry volumes to increase, prices to decrease and techniques to be improved even further. For now, it is clear that petrochemical plastics and bio-plastics have to co-exist while shifting to increasing fractions of bio-plastic.
Bio-plastics at Will & Co
Will & Co is able to deliver several types of bio-plastics and is continuously expanding our portfolio of biobased materials.