In the previous parts of this series, we introduced the current market of bioplastics, the generally available methods of processing and a deep dive into the processing characteristics of one of the most widely available polymers: PLA. We will continue this trend by providing in depth information about PHA, which is a biodegradable polymer with a very high potential for replacing current packaging practices.
Polyhydroxyalkanoates (PHAs) are polyesters that can be synthesized with the use of bacterial1 or plant-based2 methods. These biobased plastics show biodegradability in all relevant instances. This means that PHA is biodegradable in both marine and sweet water environments, and also when deposited in soil. It is also suitable for both home and industrial composting and is digestible using anaerobic digestion. All these instances follow EU certifications and are thus marketable properties for the bioplastic3. The properties of PHA also show biocompatibility with the human body and are thus suitable for medical purposes. In contrast to the other bioplastics mentioned in this series, PHA encompasses an entire family of polyesters. There are several types of PHA that are currently commercially available, among them PHB, PHBV and PHB-co-3HHx (Figure 1, Table 1) with similar market contributions. The PHA market is currently estimated to encompass a worth of USD 62 million in 2020 and shows an increasing market growth with a CAGR of 14,2%. This translates to a market value of USD 121 million in 20254.
The first discovered and commercially sold PHAs were considered quaint, but too brittle for common applications. The general linearity of PHAs is an important factor in the properties exhibited by these polyesters. This linearity also allows for an additional dimension in modifying properties. As such, there two types of PHAs that are generally identified. The most common one is the short-chain length PHA (scl-PHA), such as PHB or PHV. The similarity between these scl-PHA is the limited amount of carbon atoms in the monomer unit, i.e. up to 3 – 5 carbon atoms. When a melt of a scl-PHA is cooled a high crystallinity is observed with possible values of up to 60%. With such a high crystallinity a high brittleness, high tensile strength and low elongation/flexibility is commonplace. The second category is the medium-chain length PHA (mcl-PHA) such as PHH. The monomers for mcl-PHA contain between 6 – 18 carbon atoms and thus inhabit a larger molecular volume, which is extended even furhter due to side groups (Figure 2: R functional group). These side groups prevent regular ordering during cooling and thus mcl-PHA exhibits a lower crystallinity compared to scl-PHA. This lower crystallinity provides a polyester with elastomeric properties, but at the cost of a lower tensile strength. Hybridizing between scl-PHA and mcl-PHA by copolymerization, such as with PHBH, produces a polyester with intermediate properties that is ideal for utilization as packaging material5,6,7.
PHAs are thermosensitive, which results in significant disadvantages during processing. PHB, for example, exhibits molecular changes at temperatures ranging from 170 to 200 ˚C. When PHB is subjected to such a temperature a change in molecular weight is observed, primarily due to chain scissioning8. This temperature range is remarkably close to the melt temperature of 180 ˚C (Table 2). A similar conclusion can be drawn for the other PHA’s, however, improvements can be observed. PHBV (5%) has a melt temperature of 155 ˚C and a degradation temperature of 260 ˚C. It should be noted though, that changes of molecular weight are observed below these degradation temperatures. To improve processing the melt temperature of PHB can be reduced by copolymerizing it with other PHA monomers, such as 4-hyroxybutanoic acid to PHB4HB. As such, the melting temperature can de decreased from 180 ˚C to 54 ˚C when de 4HB content is increased to 38 mol% (plateauing after this value). This can be attributed to disruption of the crystal structure of the PHB due to the formation of irregularities. Similar trends can be observed with other copolymers (Table 2). The effectiveness of the application of this theory can be illustrated with the commercialization of such products. In Table 1 it can be observed that copolymers are commercially the most available product compared with the regular homopolymers such as PHB.
Additional ways to stabilise the thermosensitive nature of PHAs can be achieved with certain additives. Phosphinic acids, nucleants, and inorganic and organic metal salts seem to be applicable as such, with the added benefit of reducing viscosity of the melt (preventing shear degradation) and increasing plasticity of the product. Such salts can be composed out of group I to V metals with fatty acids, preferably with metals such as magnesium, or their respective metal oxides15. These nucleating agents also increase the crystallization rate, which is an important objective when it concerns PHA. PHA suffers from slow crystallization rates and thus is difficult to process16. Plant derived additives can also provide thermal and ageing stability for PHA in the form of flavonoids by inhibition of oxidation17. Additional stabilization of melt processing of PHA can be achieved by producing polymer blends, for example with PBAT, or performing prewashing of the feedstock with HCl solution, which increases the degradation temperature by up to 50 ˚C 18.
The density of PHAs is similar or higher compared to regular packaging materials. This means that, if used to replace a plastic that is of lower density (i.e. HDPE), the transport costs and emissions increase overall.
It is perfectly possible to injection mould PHA to produce mundane objects such as the plastic components present in pens. This is proven with the injection moulding of Mirel, Telles/Metabolix (currently discontinued) to produce these articles. It is, however, necessary to design a specifically built moulding system to do so. During the moulding it is recommended to provide vents with a depth of 0.00127 cm to prevent excess addition of material to the mould (flashing) because of the viscosity profile. Smaller gates cause too much shear, which lowers the viscosity and thus increases flow-speed. It is additionally advised to use geometrically balanced, multicavity moulds to reduce residence time and prevent degradation and prevent flashing20.
A PHA composite suitable for compression moulding can be made from PHB with, for example, pita fibres. Processing at 160 ˚C at 100 N/cm2 for 100 g of material. Rapid cooling after processing is performed, which has the added benefit of reducing the time the material is degrading thermally21.
Commercial grade PHA suitable for thermoforming has been available to the market in the form of Telles/Metabolix’ Mirel F300222, however this is currently discontinued. Other options for thermoforming of PHA are found in blending with PLA23.
Mirel P1004 (injection moulding grade) and Mirel P3001 (thermoforming grade) are both also suitable for blow moulding24.
Additive manufacturing for PHA is challenging due to the warping of the articles after printing and low mechanical properties. Such problems can be helped by adding additives, blending with bioplastics such as PLA, or the addition of natural fibres. Especially natural fibres improve mechanical strength in Fused Deposition Modeling (FDM)25.
Article by Wybren Kalsbeek
- G.-Q. Chen, Q. Wu, Y.K. Jung, S. Y. L. PHA/PHB. in Comprehensive Biotechnology (ed. Moo-Young, M.) 217–227 (Academic Press, 2011). doi:https://doi.org/10.1016/B978-0-08-088504-9.00179-3.
- Dobrogojski, J., Spychalski, M., Luciński, R. & Borek, S. Transgenic plants as a source of polyhydroxyalkanoates. Acta Physiologiae Plantarum 40, 1–17 (2018).
- Bauchmüller Verena et al. BioSinn Products for which biodegradation makes sense. (2021).
- BusinessWire. Global Polyhydroxyalkanoate (PHA) Market (2020 to 2025) – Increasing Scope in End-use Segments Presents Opportunities. https://www.businesswire.com/news/home/20210211005487/en/Global-Polyhydroxyalkanoate-PHA-Market-2020-to-2025—Increasing-Scope-in-End-use-Segments-Presents-Opportunities—ResearchAndMarkets.com (2021).
- Shuo Wang, Zhaohui An, Z.-W. W. Bioconversion of methane to chemicals and fuels by methane-oxidizing bacteria. in Advances in Bioenergy (ed. Yebo Li, S. K. K.) 169–247 (Elsevier, 2020). doi:https://doi.org/10.1016/bs.aibe.2020.04.005.
- Rigouin, C. et al. Production and characterization of two medium-chain-length polydroxyalkanoates by engineered strains of Yarrowia lipolytica. Microbial Cell Factories 18, 1–9 (2019).
- Hazer, B. & Steinbüchel, A. Increased diversification of polyhydroxyalkanoates by modification reactions for industrial and medical applications. Applied Microbiology and Biotechnology 74, 1–12 (2007).
- N. Grassie, E.J. Murray, P. A. H. The thermal degradation of poly(-(d)-β-hydroxybutyric acid): Part 2—Changes in molecular weight. in Polymer Degradation and Stability 95–103 (1984). doi:https://doi.org/10.1016/0141-3910(84)90075-2.
- McAdam, B., Fournet, M. B., McDonald, P. & Mojicevic, M. Production of polyhydroxybutyrate (PHB) and factors impacting its chemical and mechanical characteristics. Polymers 12, 1–20 (2020).
- Anbukarasu, P., Sauvageau, D. & Elias, A. Tuning the properties of polyhydroxybutyrate films using acetic acid via solvent casting. Scientific Reports 5, 1–14 (2015).
- Cinelli, P., Seggiani, M., Mallegni, N., Gigante, V. & Lazzeri, A. Processability and degradability of PHA-based composites in terrestrial environments. International Journal of Molecular Sciences 20, (2019).
- Kovalcik, A. Recent Advances in 3D Printing of Polyhydroxyalkanoates: A Review. The EuroBiotech Journal 5, 48–55 (2021).
- Rebia, R. A., Sadon, N. S. B. & Tanaka, T. Natural antibacterial reagents (Centella, propolis, and hinokitiol) loaded into poly[(R)-3-hydroxybutyrate-co-(R)-3-hydroxyhexanoate] composite nanofibers for biomedical applications. Nanomaterials 9, (2019).
- Ivorra-Martinez, J. et al. Manufacturing and properties of binary blend from bacterial polyester poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) and poly(caprolactone) with improved toughness. Polymers 12, (2020).
- Monsanto Company. WO1999005207A1.pdf. (1999).
- Goebel, L. & Bonten, C. Crystallization behavior of polyhydroxybutyrate (PHB). AIP Conference Proceedings 1779, 1–6 (2016).
- Masek, A. & Latos-Brozio, M. The effect of substances of plant origin on the thermal and thermo-oxidative ageing of aliphatic polyesters (PLA, PHA). Polymers 10, (2018).
- Larsson, M., Markbo, O. & Jannasch, P. Melt processability and thermomechanical properties of blends based on polyhydroxyalkanoates and poly(butylene adipate-: Co -terephthalate). RSC Advances 6, 44354–44363 (2016).
- Niaounakis, M. Properties. in Biopolymers: Processing and Products (ed. Niaounakis, M.) 79–116 (William Andrew Publishing, 2015). doi:https://doi.org/10.1016/B978-0-323-26698-7.00002-7.
- Pitzi, T. J. Injection molding PHA bioplastics: Validating moldability for paper mate pens. (2010).
- Torres-Giner, S., Montanes, N., Fombuena, V., Boronat, T. & Sanchez-Nacher, L. Preparation and characterization of compression-molded green composite sheets made of poly(3-hydroxybutyrate) reinforced with long pita fibers. Advances in Polymer Technology 37, 1305–1315 (2018).
- MatWeb Material Property Data. Telles MirelTM F3002 Thermoforming Grade PHA Bioplastic. http://www.matweb.com/search/datasheet.aspx?MatGUID=19fa89949f5041d9b8e1d153ec66dbc2&ckck=1.
- Renewable Carbon. PHA-Modified PLA Thermoform Materials for Sustainable Packaging Applications. https://renewable-carbon.eu/news/pha-modified-pla-thermoform-materials-for-sustainable-packaging-applications/ (2016).
- The Free Library. PHA biodegradable blow-molded bottles: compounding and performance. Society of Plastics Engineers https://www.thefreelibrary.com/PHA+biodegradable+blow-molded+bottles%3A+compounding+and+performance.-a0317588208 (2013).
- Mehrpouya, M., Vahabi, H., Barletta, M., Laheurte, P. & Langlois, V. Additive manufacturing of polyhydroxyalkanoates (PHAs) biopolymers: Materials, printing techniques, and applications. Materials Science and Engineering C 127, (2021).