assessing biodegradability
Biodegradability testing is a complicated topic, and even the terminology can be a little nuanced. Although third-party evaluation is common, it is important to know what "biodegradable" actually means and how it was assessed. The following (hopefully) provides a little background context to some of the issues.
material listings
A number of organisations provide approval schemes for biodegradability, and getting a product or material listed with them requires independent testing to international standards. There are a few subtleties to this process (see below), and controlled laboratory tests can never mimic the full range of conditions encountered in the natural world. However, they are a good indicator of what to expect from a material or product.
Although not the final word on the matter, the use of a listed material of known composition is a good start for producing biodegradable products. Accordingly, our ties are made in the UK from a well-documented pure homopolymer (poly 2-oxepanone). This material has been extensively tested, and is listed by BPI and TUV Austria as being biodegradable under industrial & home composting conditions, and also in soil, freshwater and marine environments.
The relevant listings published by TUV Austria for this fatty-acid polymer are as follows:
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Industrially Compostable: S0420 TA8012105536 1.0mm maximum thickness |
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Home Compostable: S0420 TA8022105542 0.49mm maximum thickness |
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Soil Biodegradable: S0420 TA8032307452 |
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Freshwater Biodegradable: S0420 TA8042307464 |
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Marine Biodegradable: S0420 TA80622066040.13mm maximum thickness |
The applicable test protocols require over 90% biodegradation (absolute or relative to cellulose) within the specified time frame, and require that thin films of the stated maximum thickness can disintegrate within half that period. Because thicker items take longer to biodegrade, finished products can exceed these allowed times and would therefore not themselves qualify for certification:
This means that material certification does not automatically transfer to products.
Material certification merely helps establish the intrinsic biodegradability of an item in a particular environment, it is not sufficient to determine how long the process will take. As far as is practical, products themselves must be tested to confirm they meet the requirements of certification.
Visit the TUV Austria document centre for more information on their specific testing requirements.
things to consider...
tests vs certificates
Many international test standards are available that define laboratory procedures for assessing the biodegradability of plastics under various conditions. These tests are generally quite straightforward (though lengthy), and in the main, they can be conducted in any suitably equipped lab. Some tests are pass/fail procedures, others report a level of biodegradability, usually against cellulose as a reference.
Overseeing this industry are approvals bodies, such as TUV Austria and BPI. These organisations have their own interpretation of what "biodegradable" (or "compostable") actually means in different environments, and they maintain online databases of materials & products that meet their criteria. To comply, testing must be conducted only at an approved test house, and usually a variety of tests all have to be met before they give their seal of approval.
As an example, the TUV Austria programmes for industrial and home composting require products to meet certain levels of biodegradation, disintegration and ecotoxicity. (Where materials are tested, thin films are used as a proxy for a physical product.)
To make the duration more manageable, biodegradation testing is normally conducted using samples of thin film.
Biodegradation: Conducted according to ISO 13432 (packaging items) or ISO 14995 (non-packaging items). Lasts for 6 months at 58°C (industrial) or for 12 months at 20-30°C (home). The level of biodegradation is measured by CO2 release according to ISO 14855, and must achieve 90% by the end of the test. |
Disintegration: Evaluated halfway through the test according to ISO 16929 or ISO 20200. At least 90% of the sample must pass through a 2mm sieve, and there must be no visual contamination of the test medium by the end of the test. The maximium sample thickness that passes this test is stated on the certificate. |
Eco-toxicity Tested according to OECD 208, which requires >90% germination of at least 100 seeds in the test medium at the end of the test. On top of this, there are also restrictions on the level of heavy metals and other regulated substances (SVHC). |
Similar requirements are needed to obtain certification for soil, freshwater and marine biodegradability, though different temperatures and durations apply.
In light of the above, testing materials or products for biodegradation can be seen as a lower bar than certifying them, and is therefore an easier way to assess performance. However, it does not guarantee a product achieves the generally accepted minimum degree of biodegradability seen as necessary to help safeguard the environment from plastic pollution.
On the other hand, the stricter requirements dictated by certification can preclude some inherently biodegradable items, simply because they do not biodegrade quickly enough. For example, when rubber balloons were tested for soil biodegradability, they achieved 87% relative to cellulose in 2 years. Despite being demonstrably biodegradable, they are not technically certified since 90% is required.
soil testing is a bit different...
Soil testing does not always use actual soil. Often, a synthetic "standard soil" is prepared, which comprises (very) fine sand, bentonite/kaolinite clay, chemical fertiliser, and about 20% of real soil and/or compost as an inolculant (i.e., a source of microbes).
Soil biodegradability is commonly tested according to ISO 17556, but this standard does not actually test finished products. Instead, it is intended to establish the intrinsic biodegradability of a material in soil at ambient temperatures (20-25°C) over the course of 24 months. (There are some caveats here, see below...)
To do this, ISO 17556 recommends (section 8.1) comparing powdered sample material against microcrystalline cellulose, stating that "experiments have shown that the ultimate degree of biodegradation is almost independent of the form and shape of the test material. The speed of biodegradation, however, does depend on the form and shape of the material."
Consequently, any object made from a soil biodegradable material will itself also (eventually) biodegrade in soil, though the time taken will depend on its size and shape.
For the purposes of soil certification, a material must generally achieve 90% biodegradation relative to cellulose within the allowed 24 months. (No disintegration test is needed, because the powdered samples used in ISO 17556 are already disintegrated.)
Apart from PCL (the common acronym for poly-oxepanone) and the PHAs (basically bacteria fat), very few materials currently meet these requirements.
Achieving soil certification is important because compost generally ends up being dug back into the ground. Products that fulfill the requirements of soil biodegradability can therefore be considered safe to compost. (See TUV Austria's OK2 Home Composting procedure.)
caveat emptor
Most "biodegradable" plastics are actually compounds, and exact recipes are rarely disclosed by manufacturers. Because some of the ingredients may degrade more rapidly than others, it is difficult to assess their overall biodegradability.
When presented with compounds, test standards can only reveal the maximum average level of biodegradability (usually relative to cellulose) over some period of time. This is not 100% conclusive, and they are careful to stress that the results cannot be extrapolated. A level of at least 90% is therefore generally used as a certifiable benchmark, but even this could allow up to 10% of non-degradable constituents.
Of particular concern are poly lactic acid (PLA) and starch based materials: Although PLA degrades rapidly under hot composting conditions, it is substantially inert at ambient temperatures; And because starches can degrade rapidly in a range of environments, they can easily give a false sense of overall biodegradability.
Because of these issues, biodegradable products should ideally only be made from well-documented compositions or from pure polymers.
PLA-based materials can be deceptive. They may be disintegrating ("perishing") rather than being bioassimilated. Measurements of CO2 release are needed to know the truth.
materials vs products
Products made from certified materials may not live up to expectations. Most materials will only biodegrade rapidly as thin films, and although larger items may biodegrade eventually, additional testing is needed to determine exactly how long it will take.
Certified materials do not make certified products!
There are several issues here. Firstly, the certification process is generally designed for finished products, and with the exception of soil biodegradability, materials themselves cannot be usefully certified as-is. To overcome this problem, materials are normally submitted for testing in the form of thin films or sheets.
This important point is frequently overlooked:
It is not the material that is certified, but a thin film made out of it.
Thin films also help with any disintegration testing requirements, since chunky items will usually not disintegrate fast enough to pass through a 2mm sieve after just 12 weeks. Films can achieve this feat because they have a very large surface area which promotes biodegradation. However, even this only works up to some maximum thickness, and this will stated on the issued certificate. (It is frequently less than 100µm, and rarely above 500µm.)
Because of this limit, any products with dimensions above the stated maximum thickness cannot be assumed to be certified.
(Products thinner than the stated maximum are accepted because they are in "equivalent form", as per EN13432 section 4.3.)
Secondly, there may be concerns over the composition of finished products. Conformity marks for a material should therefore not be used on any finished product unless the product itself has been certified. (Even sticking a label a certified item can invalidate its certification; The paper, ink and glue would all have to be assessed.)
Thirdly, conformity logos are licenced to specific companies or individuals, and are not transferable. In the case of TUV Austria, a licensee is assigned a unique "S-code", and logos displaying this code are usuable only by that organisation. Manufacturers of finished products cannot therefore just use a supplier's S-code, they must independently apply for their own product certification.
biodegradable vs compostable
(it's a labelling thing...)
"Plastics labelled as "biodegradable" are often only degradable under very specific conditions which are not met in the natural environment; especially not in the marine environment. Furthermore, plastic products labelled as "compostable" are not necessarily fit for home composting." (EU Plastics Strategy, January 2018.)
The term "compostable" generally applies to finished products, and is intended to confirm that the entire product will return to nature in a controlled environment within a definite time frame. In laboratory testing, this is taken to be 6 months at 58°C for industrial composting, and 12 months at 20-30°C for home composting. (These are considered representative of actual composting conditions.)
The composting standard EN 13432 requires that the product rapidly disintegrates, whereas the US FTC Green Guidelines state that compostable plastics should "break down into, or otherwise become part of, usable compost (e.g., soil-conditioning material)" in a time comparable to the other materials being composted, i.e., general green waste.
Although many materials are themselves described as being compostable, this is misleading since they are commonly tested only as thin films, sometimes less than 50 microns. (As discussed above.) Bags made from such films may well be compostable, but larger items will often not degrade rapidly enough to qualify, and should not be so labelled. (This is why finished products themselves need to be assessed; It is not sufficient to rely on material claims.)
Even when products are properly certified and labelled as compostable, they are still only intended for disposal where managed facilities exist. Unless specifically stated, "compostable" does not mean home compostable, nor can it be assumed that such products will biodegrade in the ambient environment.
This material contains poly lactic acid and therefore requires disposal by industrial hot composting. Although fungal hyphae are growing on it at ambient temperatures, this does not mean it will completely biodegrade in home compost or natural environments. (Note how the edges are still sharp; They are usually the first areas to be attacked.)
The term "biodegradable" is more ambiguous, because a compostable product can also be decribed as biodegradable, albeit only under specific conditions.
Furthermore, as per ISO 17556, a material can be legitimately described as biodegradable in a particular environment (soil) if it is capable of degrading sufficiently rapidly. And because any product made from that material would (eventually) return to nature, such a product could also be termed biodegradable.
However, natural conditions are highly variable, and the applicable environments & anticipated time frames need to be clearly stated. If this is not done, then the US FTC Green Guidelines assume natural biodegradation will occur in less than 12 months. (Specifically they say that biodegradable plastics should "decompose into elements found in nature within a reasonably short period of time", which they deem to be one year unless otherwise stated.)
With few exceptions, only certain thin films & bags are likely to meet the FTC guidelines and decompose in nature in under a year. Thicker items (such as our ties) are more likely to take at least 2-3 years, depending on conditions.
In summary of the above: Products (correctly) labelled as Industrially or Home compostable are suitable for disposal only in these very specific ways, and end users need to make sure they have access to the required facilities.
Products specifically described as biodegradable in the environment are more versatile, but they are also less common. They are restricted to using just a few particular materials, and they are not designed for rapid disposal. (They can take several years to fully return to nature). However, they may be the only option for outdoor applications where plastic items may get lost or become unrecoverable.
about microplastics
These are microplastic fragments of HDPE. Despite essentially being made from very heavy duty wax, they will not easily biodegrade if they enter the environment.
These fragments are non-persistent because they can biodegrade in soil. When chosen correctly, biodegradable polymers do not produce microplastics.
The European Chemicals Agency (ECHA) defines microplastic as "insoluble and non-biodegradable solid particles measuring less than 5mm".
Because poly-oxepanone (aka poly-caprolactone, PCL) is fully biodegradable in most natural environments, it does not (by definition) generate microplastics. However, the situation is not so clear-cut with other bioplastics, since some will only rapidly decompose under certain conditions.
Of particular concern are materials that are only suitable for disposal by industrial composting. If these are instead discarded into a natural environment, their very slow rate of decay can result in much longer-lived plastic fragments.
To prevent this, biodegradable products should use materials that match their expected disposal conditions: If they can end up in nature, then only materials certified as soil & marine biodegradable should be used.
breakdown products
Like other biodegradable polyesters, poly-oxepanone is depolymerised by microorganisms in the environment into short fatty acids which are then consumed as food.
Some polymers are steadily disassembled from their ends, whereas others can undergo random chain scission at any point; Evidence suggests the random process is predominant in poly-oxepanone.
Once these chopped-up fragment get to being less than 20-30 atoms long, they can more easily pass through a cell's membrane wall. They are then subjected to the standard β-oxidation process, with the resulting carboxylic acids entering the Krebs (TCA) cycle. These fatty acids are then converted into new biomass, plus some CO2 and water.
Because this metabolic process requires oxygen, beta-oxidation cannot function in anaerobic conditions. Different metaboic pathways can instead be utilised, but these are less efficient and generate methane. Many organic materials (including poly-oxepanone) will still biodegrade under such circumstances, but it proceeds much more slowly.
(Although we haven't seen any specific research on the topic, some microbial species can swap oxygen for sulfur, with the production of hydrogen sulfide instead of water. It's probable that certain species can also consume at least some biodegradable plastics via this process.)
The extensive amount of furry fungus covering the surface of this fragment illustrates how certain species see poly-oxepanone as a free lunch.
environmental impact
Low-temperature biodegradation of poly-oxepanone (PCL) does not impede plant growth, even at 5% concentrations.
Part of the certification procedure for biodegradable plastics is to test for eco-toxicity, and any formally certified material has probably been subjected to seed germination tests. We have replicated this experiment ourselves after biodegrading some of our cable ties, and there was no observable difference between the samples containing poly-oxepanone residues and the control specimens.
Along with some other polyesters, poly-oxepanone has also been used for in vivo medical applications for many decades, and is regarded as being substantially non-toxic in the body. If ingested, the MSDS states that the LD50 exceeds 2g/kg, which compares to around only 1.5g/kg for table salt.
In concensus, poly-oxepanone is therefore generally seen as a fairly safe material.
However, like most things, there are limits, and research by Asma Al Hosni in 2019 (University of Manchester, UK) suggested that compost containing (very) large amounts of poly-oxepanone (PCL) can have a negative impact on plant growth. The effect was not fully explained, and was only seen above 50°C. (It may have been a build-up of fatty acids.)
Based on this observation, we recommend that hot (industrial) compost heaps do not contain more than 5% PCL (poly-oxepanone) waste. (This equates to about 20 of our PCL-5-265 cable ties per kg of (dry) compost.)