What are biodegradable, bioplastics?

There is a lot of confusion when it comes to distinguishing between different types of bioplastics, and biodegradable plastics in the market. This article will cover the main branches of biodegradable plastics and talk about the specifications used to test those materials. We will also look into the most widely used biodegradable plastic out in the market, PLA, and talk about the limitations of the product.

Biodegradable?

A material's ability to be completely converted into carbon dioxide, water, and biomass through the action of microorganisms, such as fungi or bacteria, found in the environment.

The term biodegradable refers to a material's ability to be completely converted into carbon dioxide, water, and biomass through the action of microorganisms, such as fungi or bacteria, found in the environment. The term biodegradable plastics used in the industry refers to plastic products or materials that can disintegrate under a designated timeframe set by international specifications.  

The scientific community has been working hard to develop better plastic alternative options, the latest being biodegradable plastics. These are the second generation that followed ‘indestructible plastics’ that were made with degrading additives, such as starch which would help to break down the structure but did not break down the plastic particles themselves. 

There are mainly two types of biodegradable plastics. In both cases, the degradation process starts with a chemical process, whether it be oxidation or hydrolysis, which is then followed by a biological process. Both of the processes emit carbon dioxide as they degrade and in some cases for hydro-biodegradable plastics, methane may also be emitted. 

Oxo Biodegradable Plastics vs. Hydro Biodegradable Plastics

Oxo-biodegradable vs. Hydro-biodegradable

1. Oxo-biodegradable (OBP)

OBPs are made by adding fatty acid compounds to traditional plastics, speeding the degrading process of the plastic. These materials claim biodegradability and can meet the requirements of the following tests: 

  • ASTM D5988
  • ASTM D6954-04

In oxygen-containing environments, OBPs will indeed degrade and fragment but the plastic itself still remains, leaving tiny fragments behind known as microplastics which can easily enter the environment, oceans and waterways.

OBPs, although technically can be used for recycling, negatively affect the quality and the economic value of the plastic recyclates or pellets produced. Many recyclers and converters have reported that OBPs cannot be detected by current sorting technology from conventional plastics. For recyclers, it is extremely difficult to estimate the proportion of stabilisers added and the extent of the degradation induced in the material, so the product is not preferred as it becomes more challenging to maintain the quality of the recycled product with every repeating recycling loop.

OBPs also do not fulfill the relevant international standards to be considered compostable as their biodegradation takes too long, and plastic fragments can remain in the compost. If added to a composting stream, they affect the quality and market value of the compost, and can lead to the release of plastics into the natural environment.Therefore, oxo-degradable plastic packaging is unfit to be included in the material stream intended for composting. This incompatibility is also clearly stated by many manufacturers of oxo-degradable additives and by the Oxo-biodegradable Plastics Association.

In the case of OBPs that have a degrading enzyme additive to the product, with the main example being TDPAs.

the biodegradation category was only tested under lab conditions, not the ASTM D6400 which is tested under real life composting conditions. Therefore, it is difficult to actually know for sure how the product will biodegrade in a real life example. If the bag ends up in a location where there is not enough oxygen, or sunlight, buried under soil or other objects, the degradation process might not be complete, and the plastic will just fragment, turning one big problem into many, many smaller problems.

OBPs have been banned by the European Commission. Therefore countries in the EU, through the European Single Use Plastics Directive have voted against the use of this type of plastic. The UK which through Brexit did not transpose this environmental framework, also outlined a potential ban on OBPs.

2. Hydro-biodegradable (HBP)

HBPs are usually made from bio-based sources, such as starch, corn, wheat or sugar cane. They can also be made from petroleum-based sources or be a blend of the two. Products that use HBPs usually meet the following standards:

  • ASTM D6400
  • EN13432

The above tests refer to the compostability of a product. HBPs will also degrade and biodegrade in a shorter time frame compared to OBPs.

Clearing up some common misunderstandings

Biodegradable is not always compostable infographic

“Compostable is always biodegradable” however “Biodegradable is not always compostable” 

Biodegradable and compostable are terms frequently used to describe environmentally friendly products. Whilst they are both commonly used alongside the description that organic material is broken down in a specific environment in a specific timeframe, it is important to understand that compostability is the term used to describe products that biodegrade under specific conditions. It can get a little confusing, but the primary difference is that compostable plastics are only biodegradable in specific composting conditions, while the other is a general term used to describe plastics that can degrade in the soil (landfills or anaerobic digesters). “Compostable is always biodegradable” however “Biodegradable is not always compostable” 

Standard tests that are used to measure the biodegradability of plastics

In an effort to inform and clarify the various testing methods in use today, and how #INVISIBLEBAG complies with those specifications, let's explore the following standards in more detail. 

ASTM D6400

ASTM D6400 covers plastics and products made from plastics that have been designed to be composted in municipal and industrial aerobic composting facilities only. The scope of the specification exists to determine that the products tested will compost at a comparable rate to known compostable materials.

This Testing is equivalent to ISO17088.

More information can be found on the ASTM Standard website.

EN 13432

The EN 13432 specifies the minimum requirements that a packaging product must meet in order to be successfully processed by Industrial composting methods, which may be the reason why there is an abundance of this testing method in the environmental product community.

The test states a material or product must meet the following requirements;

  • Disintegration, or fragmentation refers to the visible structure of the product that is left in the final compost. After a 3-month period in a lab prepared setting, the remaining residue for the test material needs to be less than 10% of the original mass.
  • Biodegradability, the capability of the material to be converted into CO2 under the action of microorganisms whereby over 90% biodegradation must be reached in less than 6 months.
  • Non-toxic, all components of a material must be disclosed and the amount of heavy metals cannot exceed a stated amount. Furthermore, the remaining compost must not contain toxins or negatively affect plant growth. 

A more detailed explanation can be found on the European bioplastics website.

ASTM 5511

ASTM 5511 test covers the biodegradability of plastic products when placed in a high solids anaerobic digester for the production of digestate from MSW (municipal solid waste). This condition may also resemble some conditions in biologically active landfills. 

OECD 208

OECD 208 is otherwise known as the seedling growth test which is a continuation from ASTM 5511, whereby the remaining soil that a product has degraded in after 90 days is then used for the testing of seedling growth. For a test to be considered valid, the seedling emergence needs to be over 70% with the seedlings not showing phytotoxic effects. 

There are other biodegradability tests that have stricter requirements,  like the OECD 301 series (Ultimate Biodegradability) which requires 60% of a product (70% for some tests) degrades within a 10-day window and within 28 days. Although products that pass this test can be useful in some applications, they tend to be fuels or surfactants in liquid form. A solid product like a packaging bag with such material would prove to be difficult to use as the fast deterioration of quality would lead to an incredibly short shelf life.

The Limitation of PLA

The most widely accepted and used bioplastic, but does it meet all the requirements?

Apart from those of us that are meticulous with our recycling processes, or if your local facility is lacking the necessary division on PLA and the conventional PET plastics, PLA can cause problems in the recycling process. Typically, PLA plastics will usually be sent to the same plastic recycling station where they enter Material Recovery facilities. The facilities then sell the material to processors, which break down the plastic into pellets or flakes, which are in turn, made into new products such as carpeting or containers for fuel oil and cleaning products. As PLA and PET do not mix together, recyclers consider PLA a contaminant. They have to pay significant costs to sort it out and pay again to dispose of it. The limited availability of composting facilities that can biodegrade these PLA products, the real-life impact of these plastics and how they usually end up, mixed in with conventional plastics make it a difficult material for the traditional recycling industry to handle.

The ongoing issue of plastics affecting the natural environment is highlighted every day via news and social media and although it is all areas of the environment that are suffering, the plight of our oceans and seas is especially visible and images of sea turtles ingesting plastic, mistaking it for jellyfish are sadly commonplace now.

In this heated public issue to ban plastic and limit plastic products, biodegradable plastics are believed by some to be the best possible alternative solution. Although there are some existing measures in place to prevent a product from reaching the ocean, we still need to take into account that in the long term, there will eventually be plastic products that stray off from this pathway and enter the marine environment. There they will continue to exist, slowly degrading into microplastics and nano plastics and eventually entering the food chain that we are part of. 

Although PLA can be decomposed in special conditions found in composting facilities, it performs very poorly when degrading in the marine (seawater) environment.

A graphical study on PVA degradation rate

PVA, PLA composite degradation graph in marine environment. (2020). [Graph]. “Seawater Degradation of PLA Accelerated by Water-Soluble PVA.”

The study above found that the molecular weight of PLA remained unchanged in seawater after 6 months. PLA films with an average thickness of around 320 micrometers in a simulated seawater environment at 25 degrees and simulated fluorescence light (16 h light and 8h dark) for a period of 1 year, failed to show significant weight loss. 

PLA needs a temperature gradient of about 60 degrees to begin it’s degradation process and allow for the entry of water molecules. However, it is unlikely that the ocean will be above 60 degrees.  

As of now, PVA is the only commercial water soluble and biodegradable polymer material.

The main concern that we heard and faced with plastic pollution is the fact that it remains unchanged when discharged to the environment. The newest concern that we connect with plastics now is the problems associated with microplastics and nanoplastics. The usage of PVA to replace the conventional plastics can be a fix to both the old and new problems connected with our plastic usage. 

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