PHA Biodegradability Explained: In natural conditions and in composting conditions.

PHA is often described as biodegradable. That is true, but the word biodegradable on its own, does not tell the full story.

How quickly a PHA product biodegrades depends on much more than the polymer name. The exact PHA type, formulation, molecular weight, product thickness, shape, surface area and the environment in which the product ends up can all influence the degradation process.

A thin film in biologically active soil can behave very differently from a thick injection-moulded product made from the same base polymer. The surrounding substrate also matters: soil, home compost, industrial compost and aquatic environments all provide different combinations of temperature, moisture, oxygen and microbial activity.

In this article, we look at how PHA biodegradation works, what influences the degradation rate and why material certification should not automatically be interpreted as product certification.

What does biodegradable actually mean?

A material is biodegradable when microorganisms can break it down and use it as a source of energy by digestion.

During this process, microorganisms and enzymes convert the polymer into naturally occurring end products such as carbon dioxide, water and biomass, depending on the environmental conditions.

This is different from simple fragmentation. A conventional plastic will become brittle overtime and break into smaller pieces without actually being biologically converted. Those fragments can remain in the environment as persistent microplastics.

PHA behaves different because many naturally occurring microorganisms produce enzymes capable of breaking down PHA polymers biologically by digestion.

However, biodegradability should never be interpreted as a fact that every PHA product disappears immediately in every environment. The rate of biodegradation depends on the combination of material, product design and environment.

Why can PHA biodegrade?

PHA is naturally produced by microorganisms as an intracellular carbon and energy reserve. Because PHA already exists in natural biological systems, many microorganisms have also evolved mechanisms to break it down.

Under suitable conditions, microorganisms can release enzymes that split the polymer chain. Smaller molecules are then taken up and metabolised by the microorganisms.

A simplified pathway looks like this:

PHA product → enzymatic breakdown → smaller molecules → microbial metabolism → CO₂ + water + biomass

The exact process and rate depend on the PHA structure and surrounding conditions.

Source: PHAntastic

Why product thickness matters

One of the most important factors in biodegradation is product thickness. Biodegradation generally starts at the surface of a product, where microorganisms and enzymes can access the polymer. This means that the ratio between surface area and total material volume matters.

A thin film has a large surface area relative to its mass. A thick injection-moulded component has much more material beneath the surface that microorganisms must gradually reach. As a result, two products made from the same PHA material can have very different biodegradation times simply because their geometry and thickness are different.

This is why it is not enough to know that the raw polymer is biodegradable. Product design plays an important role in determining how that biodegradability translates into a real application.

Credits: Prof. S. Bruzaud

Why the environment and substrate matter

Biodegradation does not take place in isolation. The surrounding substrate or environment determines which microorganisms are present and which conditions they experience. Temperature, moisture, oxygen availability, nutrient availability and microbial population can all affect degradation.

This is why soil, home compost, industrial compost and aquatic environments cannot simply be treated as interchangeable. A PHA product that biodegrades relatively quickly in warm, biologically active compost may behave differently in cooler soil or water.

For product developers, the important question is therefore not only “Is this material biodegradable?”, but also “In which environment should this product biodegrade?”

PHA biodegradation in soil

Soil is one of the most relevant environments for PHA applications, particularly in agriculture.

Soil contains a diverse microbial community capable of interacting with biodegradable polymers.
However, soil conditions vary enormously depending on location, season, moisture content, temperature and biological activity.

Laboratory screening of soil microorganisms interacting with PHA, used to study which microorganisms are capable of degrading the polymer before validating their behaviour under real soil conditions. Source: PHAntastic

For that reason, there is no single universal soil degradation rate for PHA. The polymer type and product design matter, but so does the soil itself.

This is especially relevant for applications such as mulch films, agricultural clips, plant-support materials and other products that may intentionally remain in or on the soil after use.

Helian Polymers is involved in the EU-Horizon PHAntastic project, which is developing PHA-based biodegradable mulch films and growth foams for agriculture. These systems are intended to be demonstrated under real horticultural and nursery conditions in Europe, rather than only under laboratory conditions.

Source: PHAntastic

PHAntastic: testing PHA in real agricultural conditions

PHAntastic is a four-year Horizon Europe project focused on PHA-based agricultural solutions.

The project is developing biodegradable mulch films and growth foams containing bio-based fertilisers and natural plant protection agents. The aim is not only to replace conventional agricultural plastics, but also to create materials that interact positively with the soil and plant environment.

Helian Polymers supports the project with material formulation and compounding. By developing and compounding suitable PHA formulations, Helian prepares the materials that are used in the next stages of research and application testing. This helps translate material requirements into formulations that can be processed and evaluated under relevant agricultural conditions.

The project is particularly relevant to biodegradation because it moves beyond a simple material claim. The materials are being designed and tested for real agricultural applications, where soil conditions, product geometry, biological activity and practical use all influence performance.

For example, the project describes PHBV-based growth foams intended to biodegrade after transplantation, while releasing nutrients and beneficial microorganisms into the soil.

This kind of application testing is important because biodegradability in theory and biodegradation in a real product are not exactly the same question.

Source: PHAntastic

PHA in home compost

Home composting takes place at relatively moderate and variable temperatures. Unlike industrial composting, a home compost heap is not continuously controlled. Temperature, moisture and microbial activity depend on the size of the compost pile, season, feedstock and how often it is turned.

PHA can biodegrade under home-composting conditions, but again the rate depends strongly on the specific PHA and the product being tested. Thin products generally offer more surface area to microorganisms than thick components, which can influence degradation speed.

When evaluating a PHA for home compost, it is therefore important to distinguish between the behaviour of the raw material and the behaviour of the finished product.

A good example of an application where home compostability can be particularly relevant is
coffee capsules. After use, the capsule remains in contact with moist coffee grounds and other organic residues. When the complete system is suitable for home composting, the capsule and its organic contents can potentially be disposed of together, rather than requiring the user to separate the packaging from the coffee grounds. The moisture, organic matter and microbial activity present in a composting environment can support the biodegradation process, although the actual rate will still depend on the material, capsule thickness and composting conditions.

PHA in industrial compost

Industrial composting provides more controlled conditions than home composting. Commercial composting facilities typically maintain higher temperatures, controlled moisture and active aeration. These conditions can accelerate microbial activity and therefore biodegradation.

For many biodegradable polymers, industrial composting represents a favourable environment. However, even under controlled conditions, product thickness and formulation remain relevant. A thin article can still behave differently from a thick product made from the same base polymer. The acceptance of biodegradable products is also subject to local and national legislation.

This distinction is especially relevant when comparing PHA with materials such as PLA. PLA is generally associated with industrial composting conditions, where the higher and more controlled temperatures support its breakdown. Under home-composting or cooler natural conditions, PLA typically does not biodegrade. You can read more about the differences between these two materials in our PHA vs PLA comparison.

Industrial compostability should therefore be considered a combination of material behaviour and product design rather than only a raw-material property.

PHA in marine and aquatic environments

Marine biodegradation receives a lot of attention because conventional plastics can persist in aquatic environments for very long periods.

Certain PHA materials can be biodegraded by microorganisms present in aquatic environments. However, marine conditions are very different from compost or soil. Water temperature, nutrient levels, salinity, microbial population and water movement can all influence biodegradation.

Our sample took a dive - because marine biodegradation should be tested, not assumed.

This means that marine biodegradation should not be treated as a simple yes-or-no claim. The specific polymer and product should be tested under relevant conditions.

For applications where accidental environmental leakage is a realistic possibility, this can be an important consideration during material selection.

Raw material certification vs finished product certification

This is one of the most important distinctions when discussing biodegradable materials. A material can be tested and certified for biodegradation or compostability under a specific standard, but a finished product made from that material does not automatically inherit the same certification.

Once a raw polymer is turned into a product, factors such as thickness, geometry, surface area, additives, pigments, fillers, processing history, crystallinity and final formulation can all influence biodegradation. A certified raw material therefore provides valuable information about the polymer, but it does not automatically prove how every finished product will behave.

Where product-level certification is required, the finished article should be evaluated under the relevant standard and intended conditions. Certification and test documentation can differ between PHA grades. For the available certifications of a specific material, please check the corresponding material page in our PHA portfolio.

What affects the biodegradation rate of PHA?

The biodegradation rate of a PHA product is determined by multiple factors working together. Important variables include PHA type and composition, crystallinity, molecular weight, formulation, additives, product thickness, surface area, temperature, moisture, microbial activity and the surrounding substrate.

This is why biodegradation data should always be interpreted in context. A laboratory test using a thin specimen under controlled conditions can provide valuable material information, but it does not automatically predict the exact lifetime of a finished product in the real world.

For product developers, realistic application testing can therefore be just as important as raw-material data.

How do you measure PHA biodegradation?

Biodegradation is not only evaluated by looking at whether a product appears to disappear. It can also be measured quantitatively in laboratory tests.

One commonly used approach is respirometry. A PHA sample is placed in a controlled environment together with microorganisms, for example in soil or compost. As the microorganisms metabolise the polymer, they consume oxygen and produce carbon dioxide.

By measuring the amount of CO₂ produced over time, researchers can determine how much of the carbon contained in the material has been biologically converted. The measured CO₂ can then be compared with the theoretical amount of CO₂ that would be produced if the material were completely biodegraded.

This makes it possible to follow biodegradation as a curve rather than simply checking whether a sample has visually disappeared.

Depending on the environment being simulated, different standardised methods can be used. Soil biodegradation can, for example, be measured by oxygen consumption or CO₂ evolution according to ISO 17556, while controlled composting methods such as ISO 14855 also use evolved CO₂ to determine biodegradation. Marine test methods can use the same principle under seawater or seawater/sediment conditions.

PHA sample + soil / compost → microorganisms → CO₂ measurement → biodegradation curve

Important: degradation is more than weight loss

Losing mass or breaking into smaller pieces is not necessarily the same as biodegradation. A sample could fragment while the polymer itself is still present. Measuring microbial conversion through CO₂ evolution provides evidence that the carbon in the polymer is actually being biologically metabolised.

Biodegradable vs compostable

These terms are related, but they do not mean exactly the same thing.

Biodegradable describes the biological breakdown of a material by microorganisms.
Compostable generally means that a material meets a defined set of requirements under specific composting conditions and within a defined timeframe.

A material can therefore be biodegradable without necessarily meeting the requirements of a particular compostability certification. This distinction matters when communicating environmental claims and selecting end-of-life options for a product.

What should product developers consider?

If biodegradation is important to an application, material selection should start with the actual product and intended environment.

A few questions are particularly useful:

What PHA type is being used?
How thick is the product?
What additives or other polymers are present?
Where should the product biodegrade?
What temperatures and moisture conditions will it experience?
How biologically active is that environment?
Does the raw material have relevant test data or certification?
Does the final product itself require certification?

These questions make the discussion much more meaningful than simply asking whether PHA is biodegradable.

How Helian Polymers can help

At Helian Polymers, we work with different PHA types, formulations and applications. Our role goes beyond supplying polymer. We support customers with material selection, formulation, compounding, processing trials and application development.

For applications where biodegradation is an important requirement, this means looking at both material properties and product design.

We can help identify suitable PHA building blocks, understand available biodegradation data and support the development of formulations that match the intended application and end-of-life pathway.

Our involvement in projects such as PHAntastic also gives us practical experience with PHA materials designed for real agricultural environments.

Biodegradability is a system, not a single material property

PHA has an important advantage over many conventional plastics: microorganisms can biologically break down the polymer. However, how this works in practice depends on much more than the letters “PHA”. The polymer type, formulation, product thickness, geometry and surrounding environment all influence what happens after use.

For developers, this means the most useful question is not simply “Is PHA biodegradable?”, but rather “How will this specific PHA product biodegrade in the environment it is designed for?”

That is where material knowledge, product development and realistic testing come together.