Polyhydroxyalkanoates (PHAs): How to select the right grade?

Polyhydroxyalkonates (PHAs): Chemical Composition and Types

Chemical Structure of PHA

• n is the number of repeating units
• R is a functional group that varies depending on the specific type of PHA

1. Poly-3-hydroxypropionate (PHP) – Known for its high production yield over other bioplastics. It has excellent mechanical properties such as rigidity, flexibility, and tensile strength. Found to be more stable that polylactic acid, another biodegradable plastic. Researchers may need genetically modified microorganisms to synthesize PHP.


2. Poly-3-hydroxybutyrate (PHB) – It is a homopolymer composed of 3-hydroxybutyrate monomers. It is popular for its simple structure and versatile applications. Synthesized by bacteria like Cupriavidus necator, Ralstonia eutropha, and Bacillus species. Can be processed using thermoplastic processing techniques.


3. Poly-3-hydrovalerate (PHV) – Blends with PHB to improve the flexibility and toughness of a material. Used in trash bags and mulch films. Microorganisms used to synthesize PHV include Pseudomonas oleovorans.


4. Poly-3-hydroxyhexanoate (PHHX) – Contains a longer alkyl side chain which results in more flexible and elastic material. It is more biodegradable than PHB. Produced using Pseudomonas putida.


5. Poly-3-hydroxyoctanoate (PHO) – It has a longer side chain and exhibits improved flexibility. Used in applications where elastomeric properties. Some examples include medical devices, surgical sutures, and drug delivery systems. Pseudomonas species are used to produce PHO.


6. Poly-3-hydroxydodecanoate (PHOD) – A medium chain-length thermoplastic PHA with excellent mechanical properties composed of 3-hydroxydodecanoate monomer units. Synthesized using Pseudomonas citronellolis.


7. Poly-4-hydroxybutyrate (PAHB) – Composed of 4-hydroxybutyrate monomer units. It can be processed into films, fibers and 3D structures for medical applications. It uses Cupriavidus necator to synthesize PAHB.


8. Poly-5-hydrovalerate (PSHV) – Produced through microbial fermentation using renewable feedstocks. Contains 5-hydroxyvalerate monomer units. Synthesized using Cupriavidus necator.

Types of PHA based on chain length

PHA Biosynthesis and End-of-life Practices

Steps in the production of PHAs

1. Selection of microorganisms – Bacteria is the most commonly used microorganism in the production of PHA. In some cases, archaea and fungi are also used. Based on the PHA type, the formulator needs to select the microorganism of choice.


2. Growth and fermentation – The selected microorganisms are cultivated in a bioreactor under a controlled environment. The nutrient medium includes carbon, nitrogen, phosphorous, and other essential ingredients. These nutrients allow the microorganisms to grow rapidly. The fermentation process is split into two stages – biomass production and PHA buildup. The strategies for the recovery process include solvent and non-solvent based.


3. Monitoring and optimization – Parameters such as temperature, pH, and oxygen levels are monitored to optimize the growth of microorganisms.


4. Harvesting – As the microorganisms accumulate PHA polymers, the culture is harvested. After the biomass and fermentation broth are separated the PHA-rich cells are left behind.


5. Cell lysis and PHA extraction – The PHA-rich cells are disrupted into PHA granules. The disruption involves mechanical, chemical, and enzyme digestion methods. These granules are then purified.


6. Purification and drying – The extracted PHA is further purified and dried to remove residual moisture.


7. Processing – The purified PHA can be molded into various shapes depending on the intended application. The processing methods include injection molding, extrusion, thermoforming, or cast films to create final products.


8. Application – The PHA products are used in a wide range of applications ranging from packaging to medical devices. They offer eco-friendly and biodegradable alternatives over conventional plastics.


General Overview of PHA Production Process (Source: ResearchGate)

End-of-life management of PHAs

Closing the Loop with PHAs (Source: Go!PHA)¹

Applications of PHAs

• Biofuels, fine chemicals, bioplastics, industrial fermentation, upholstery, and carpet
• Food packaging (compostable bags, lids, thermoforming tubs)
• Disposable items (diapers, cosmetic containers, razors, cups, feminine hygiene products, utensils)
• Packaging films (containers, paper coatings, shopping bags)
• Medical surgical garments, drugs, and bio-implants

PHAs Key Material Properties vs. Other Polymers

Physical properties that make PHA unique

• Origin – PHA is produced through microbial processes, making it environmentally friendly. It can be either derived from biobased or renewable content.


• Transparency and colorability – PHA can be transparent or translucent. They exhibit good optical clarity which makes them ideal for clear packaiging applications. But they can also be pigmented to achieve good colorability PHA grades.


• Biodegradability – PHA can naturally degrade, reducing environmental impact and waste accumulation. These biodegradable plastics are utilized in the manufacturing of bioplastics.


• Density – The density of PHA can vary on the chemical composition. Typical range varies from 1 – 1.3 g/cm³.


• Melting point – They can be thermoplastic or elastomeric, with melting points ranging from 40 to 180 °C. For example, PHB has a Tm in the range of 173 – 180 °C.


• Solubility – PHAs are insoluble in water and organic solvents. They are soluble in halogenated solvents such as chloroform, dichloromethane, and dichloroethane.


• Barrier properties – PHA have good barrier properties to gas and oxygen making them suitable for packaging materials.


• Flexibility – PHAs range from rigid to flexible. Longer side chains tend to be more flexible than smaller side chains. View good flexibility PHA grades.


• Processability – PHAs tend to have good processing. This is because they can be converted to the final products using conventional processing methods.

Mechanical properties: Deeper evaluation of the values

Evaluating PHAs: The Positives and Negatives

Key benefits of PHAs

• Biodegradable – PHAs are completely broken down using mircoorganisms via biodegardation. As a result, they reduce plastic waste and pollution.


• Sustainable – PHAs are produced from renewable resources like plant sugars and vegetable oils. This in turn reduces their dependence on non-renewable fossil fuels.


• Carbon footprint – PHAs reduce the carbon footprint thus making it an environment friendly option for plastic manufacturers.


• Carbon sequestration – Some PHA production methods involving Cyanobacteria absorb and bind CO₂ in the polymer. It mitigates the release of CO₂ and reduces green house gases.


• Versatility – PHA can be produced in diverse settings like building rooftops, industrial sites, and oceans. It can be tailored for many applications based on chemical composition and properties.


• Biocompatibility – PHA exhibits compatibility with biological systems. This makes it suitable for medical applications.


• Transformation of toxic materials – Microbes involved in PHA fermentation can convert harmful substances like halogenated derivatives and CO₂.


• Non-toxic – PHAs are non-toxic and safe to use in contact with food and human body.

Limitations of PHAs

• Cost – The cost of PHA production is influenced by fermentation and downstreaming process. It can be more expensive to produce than traditional plastics.


• Customizability – Achieving a PHA type for your need may require a lot of research. It may be time consuming and costly process for formulators to create a PHA type with the desired properties.

How to combat the high cost of PHA?

• First is the development of a fermentation process allowing higher production yields than 10 to 20 g PHA per liter of media.
• Secondly, the origin and the price of the ingredients, (e.g., carbon sources) should be substituted with the waste materials from non-food chains (e.g., lignocellulosic biomass).
• Thirdly, the thermal processing stability of PHA polymers has to be improved.

Polyhydroxybutyrates (PHBs): The Most Popular PHA Type

• Cariamids necator,
• Ralstonia eutropha, and
• Bacillus species

Synthesis of PHB

• First approach — This involves ring-opening polymerization (ROP) of β-butyrolactone (BL).


• Second approach — It utilizes natural or transgenic plants, such as Linum usitatissimum L. (flax). They provide acetyl-CoA as an available substrate for PHA biosynthesis. Genetic modification of flax plants results in increased biomass growth and higher yields of PHA materials compared to control cultures. The cellulose in transgenic flax plant cell walls exhibit structural differences and lower crystallinity compared to the control callus. This approach shows significant potential for efficient bioprocesses in the future. This aligns with the increasing demand for biodegradable resources.


• Third approach — It involves bacterial fermentation. It is the most used method for PHB synthesis. Under optimal fermentation conditions, more than 90% of the cell's dry weight can consist of PHA materials. This approach relies on the conversion of acetyl-CoA. This is a central carbon metabolite, through a sequence of three enzymatic reactions as shown below.

Synthesis and Degradation Process of PHB
(Source: MDPI)

Material properties: Unveiling the key aspects of PHB

Role of functional groups

• excellent mechanical properties,
• high elasticity,
• modulus of roughly 2.5-3 GPa, and
• tensile strength at break of 35-40 MPa.

Analyzing thermal properties

• Glass transition temperature (Tg) for the amorphous phase and
• Melting temperature (Tm) for the crystalline phase.

• measure Tm, Tg, Td, and
• determine the degree of crystallinity (Xc)

• tacticity, hardness, modulus, density, transparency, and
• behavior during cold drawing or ductile flows

Evaluating mechanical properties in a gist

Limitations of PHB

1. Inherent physical aging effect — PHB experiences secondary crystallization, leading to embrittlement.


2. Slow crystallization rate and low nucleation density — This promotes the formation of large spherulites. This makes PHB more prone to cracking and fracturing. It is reflected in its low elongation at a break of 5-7%.


3. Thermal instability — PHB has a narrow thermal processing window. It degrades through random chain scission on the ester bond within the temperature range of 170-200 °C.


4. High production cost — The production cost of PHB limits its competitiveness in industrial and commercial applications.

Overcoming the cons of PHBs

• Modification through drawing and thermal treatment
• Blending with materials from natural sources and synthetic polymers with suitable molecular structures
• Incorporation of natural fibers or rigid fillers to form reinforced composites
• Chemical functionalization

PHB Toughening²

Explore All Polyhydroxyalkonate Grades

1. PHA, Global Organization for PHA (GO!PHA)
2. Recent advances in the development of biodegradable PHB-based toughening materials: Approaches, advantages and applications. Jayven Chee Chuan Yeo, Joseph K. Muiruri, Warintorn Thitsartarn, Zibiao Li , Chaobin He. Materials Science and Engineering: C, Volume 92, 1 November 2018, Pages 1092-1116.
3. Poly(lactic acid) (PLA) and polyhydroxyalkanoates (PHAs), green alternatives to petroleum-based plastics: a review. Ahmed Z. Naser ORCID, I. Deiab, and Basil M. Darras. Royal Society of Chemistry