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

Polyhydroxyalkanoates (PHAs) are a family of microbial polyesters. They form a large group of thermoplastic polymers produced by various prokaryotic organisms. They are formed as carbon and energy storage materials under unbalanced nutrition conditions.

PHAs encompass a wide range of materials with significant variations in their chemical structure. The general structure of PHA consists of repeating units in the polymer chain as shown in the below figure.

 

Chemical structure of PHA

Where:
 

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

What changes in n and R mean for different PHA types? 

The table below shows the different types of PHA based on the changing 'n' and 'R' groups.

 

Each type of PHA comes with distinct properties and applications. Let's brief you about them.

 

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.

All these PHA subtypes are biodegradable, biocompatible, and offer excellent mechanical properties. Amongst them, polyhydroxybutyrate (PHB) is the most popular PHA. Its simple chemical structure makes it easy for the researchers to study and implement their R&D. As PHB was the first discovered PHA, a lot of knowledge about its properties and production is known today. Despite its popularity, researchers are working on developing and optimizing various PHA variants to meet industry needs.


 

Types of PHA based on chain length

PHAs can be classified into two groups based on the length of the monomer units. They are as follows:

 

With PHA types and chain lengths in mind, let’s explore their production and end-of-life practices to understand their full potential.

All PHA production procedures begin with fermentation and end with polymer processing for specialized purposes.

 

Steps in the production of PHAs

PHA production involves a biotechnological process using microorganisms to synthesize the polymer. The steps in the production process include:
 

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

PolyHydroxyalkanoate offers many pathways for sustainability and waste management.

Recycling

The reusability makes PHA recyclable. This happens either by returning it back into the polymer for new uses or by recycling it into raw materials for renewable feedstock. Furthermore, PHA can be effectively recycled through industrial or residential composting. This contributes to the reduction of waste in the environment.

 

Incineration 

It is another disposable method, where the PHA can be utilized to generate renewable energy. But it's not the most environmentally preferred method as it contributes to greenhouse gas emissions.

 

Biodegradation 

PHA is biodegradable as it allows complete decomposition. This leads to the conversion of PHA into nutrients for living organisms.

 

Home composting 

PHA is compostable at home offering a convenient solution for waste management. However, the rate of degradation may vary depending on the PHA type. It exhibits biodegradability in soil, freshwater, and marine settings.

By leveraging these qualities, PHA holds the potential to combat plastic pollution at both the macro and micro levels. They minimize the impact of primary and secondary microplastics on the environment.

 

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

Choose PHA grades that fit your end-of-life strategies. Explore our platform to request samples and access technical datasheets:
 

Applications of PHAs

In addition to its eco-friendly characteristics, PHA is a versatile natural polymer that can be tailored to meet specific applications. Some examples include:
 

• 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

Select the PHA grades used in various applications from our master catalog:
 

Now that we’ve seen how PHAs are made and used, let’s uncover what makes them truly special. Their unique properties give them versatility and an edge over conventional polymers.

Physical properties that make PHA unique

PHAs offer a wide range of physical properties that vary depending on their type and chemical composition. Here are some of the common features below.

 

• 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. Select good flexibility PHA grades available on our platform.

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

Some grades of PHA are UV stable and have a low water permeability rate. A larger percentage of valerate in the material improves processability, impact strength, and flexibility.


 

Mechanical properties: Deeper evaluation of the values

The mechanical properties of PHA can vary depending on the specific type of PHA, molecular weight, or any other modifications. They can be altered by mixing, modifying the surface, or combining it with other polymers, enzymes, and inorganic components.

 

The below table depicts the comparison of the mechanical properties of PHAs with other commercial polymers.

 

Understanding the physical and mechanical traits of PHAs gives us a clear picture of what makes them stand out. Next, let’s weigh their strengths and limitations to see where they shine and where challenges remain.