Axel Barrett Biodegradation & Composting Definitions

The Degradation of Plastics and Polymers (FREE)

A complete overview of the degradation processes of plastic. This is a FREE article.

Stability and Resistance

Most plastics are not meant to degrade in the environment. Stability and resistance are defining attributes of plastics. The best example to understand the resistance of plastics is to take a full plastic bottle and throw it on the floor to see if it breaks. Do the same with an empty bottle. In both cases, the plastic bottle will survive the impact.

Do you think a glass bottle, aluminium can or milk carton would survive this experiment?

Biodegradable and Compostable Plastics

Bioplastics can be divided into biobased and/or biodegradable plastics. Not all biobased plastics are biodegradable; not all biodegradable plastics are biobased.

The difference between regular (non-biodegradable) plastics and biodegradable plastics is that biodegradables have been engineered to intentionally degrade.

All compostable plastics are biodegradable; not all biodegradable plastics are compostable. Biodegradable and compostable plastics have been specifically engineered to degrade through a biological process, mostly microbial digestion.

The difference between biodegradable and compostable is that in the case of compostable plastics, (1) the “degradation” process is activated through human intervention by placing the plastic in a composting setting or infrastructure and (2) the end residue will be classified as compost.

This is the only correct etymological definition of compostable plastics. It’s a social convention resulting from lobbying activities by commercial interest if the legislation defines it differently.

Microplastics vs Nanoplastics

Microplastics and nanoplastics are not specific kinds of plastics but are plastic fragments. The difference is the size.

Micro- and nano- relate to a unit of length of the metric system:

  • a nanometer (nm) is equal to one billionth of a metre
  • a micrometer (μm) is equal to one millionth of a metre
  • a millimeter (mm) is equal to one thousandth of a metre

In theory, there’s a clear difference. Microplastics are bigger than 100 nm (nanometer) and smaller than 5 mm (millimeter). Nanoplastics  are smaller than 100 nm (nanometer).

Most people will only use the term “microplastics” because “nanoplastics” is unknown outside the scientific world. We can assume that “nanoplastics” are included in the term “microplastics” when referred to in mainstream media or by a wider audience.

Primary and Secondary

Primary microplastics are fragments that are already 5.0 mm in size or less before entering the environment. Most important examples and sources are:

  • Micro beads in cosmetic products such as creams, toothpaste, etc. They’re used for scrubbing, exfoliating, whitening, etc. They have or will be banned.
  • Medical vectors for drugs. Controlled drug delivery occurs when a polymer, whether natural or synthetic, is judiciously combined with a drug or other active agent in such a way that the active agent is released from the material in a predesigned manner. 
  • Air blasting technology to remove rust and paint from surfaces

Secondary microplastics are created from the degradation of plastics after they’ve entered the environment. The most important examples of secondary microplastics are

  • Microfibers from synthetic textiles and clothing (polyester) caused by washing machines
  • Erosion of tires
  • Fragmentation and degradation of plastic waste
  • Fishing nets
  • Cigarette filters

Impact of Microplastics

Micro and nano plastics have been found everywhere; from the bottoms of our oceans to the mountain tops. Microplastics is found in almost all living organisms including human placenta.

Micro and nano plastics can be ingested by organisms as small as zooplankton. These can block the gastrointestinal tracts of organisms, or trick them into thinking they don’t need to eat, leading to starvation.

Microplastics may contain toxic additives and chemicals may adhere to the surface of the micro plastic increasing the danger of ingestion.

Natural Degradation of Plastics and Polymers

Here are the natural mechanisms by which plastics can degrade:

Physical degradation

Plastics may degrade or erode due to naturally caused movements or friction. For instance, waves or wind may rub plastic bottles against rocks.

Photodegradation

This is a degradation process caused by photons. Photons are the basic unit of light. They have no mass but travel at the speed of light.

Plastics / Polymers are made from molecules and atoms. Atoms are made from a nucleus (proton and neutron) and electrons circulating around the nucleus.

Electrons can absorb photons. This absorbed energy can excite the electrons and alter the molecular configuration.

There are three theoretical types of photodegradation:

  • Photolysis is a process that uses radiation with ultraviolet (UV) light to generate such reactive species as radicals, ions, and excited molecules.
  • Photooxidation processes combine oxidants such as O3, H2O2, or Fenton reagent with UV light so that the degradation is more effective than with UV alone.
  • Photocatalysis consists of a photoinduced reaction which is accelerated by the presence of a catalyst. The process starts when a photon with equal or higher energy than the band gap energy of the catalyst is absorbed.

Thermooxidative degradation

Thermooxidative is a combination of the words “thermal” and “oxidation”

Thermal refers to heat or temperature. Thermal energy is produced by heating up molecules and atoms until they move fast enough to collide into each other.

Oxidation is the loss of electrons. In terms of oxygen transfer, oxidation may be defined as the chemical process in which a substance gains oxygen or loses electrons and hydrogen.

The majority of polymers are susceptible to thermooxidation aging in the open environment and it’s considered the most serious mechanism to degrade plastics that has not been engineered to intentionally degrade. The influence of thermooxidative degradation on polymers is depending on their chemical structure.

Hydrolytic degradation 

This process occurs in polymers that are watersensitive, especially those that absorb a lot of moisture. Plastics absorbs water to varying degrees, depending on their molecular structure, fillers and additives. Water breaks the polymeric chain.

Plastic will usually need to be exposed to moisture and elevated temperature to start the hydrolytic degradation.

Hydrolytic degradation is also useful in the case of polymeric drug delivery system.

Passive hydrolysis is the most important kind of degradation for many biodegradable / compostable plastics.

Biodegradation or biological degradation

Microbiologically speaking the word “biodegradation” refers to the biological processing or “digestion” by microorganisms or microbes. There are 6 major types of microorganisms / microbes: bacteria, archaea, fungi, protozoa, algae, and viruses.

The ability of bacteria to degrade plastics is directly related to the ability of bacteria to attach itself to the surface of the polymer.

Environmental and nutritional conditions may act as a stimuli to create favourable conditions for bacteria to degrade plastics.

There are two ways microorganisms degrade plastics: a metabolic and enzymatic process.

Bacteria produce enzymes. Enzymes will break down polymers into smaller molecules and enable the transport of molecules through the cell membrane.

Once the molecules are in the cell, the metabolic process will convert the carbon of the polymers into CO2 (mineralisation) or incorporate them into biomolecules.

Enzymatic degradation

The human body also produces enzymes.

Polymeric biomaterials used in the biomedical applications may be degraded in contact with body fluids and tissues by several enzymes either by oxidation or hydrolysis.

For instance, biodegradable polymers used for medical applications may undergo degradation involving two steps: the adsorption of enzyme on the surface enabling the hydrolysis of the polymer.

Non-Natural Degradation

These are man made processes. They do not occur naturally in the environment.

Human degradation or intervention

Humans may intentionally or unintentionally degrade or start the degradation process of plastics. We can drive with a car over a plastic bottle causing it to degrade. We can create an environment or the right settings for the plastic to degrade through a natural process.

Composting

What is not natural is cultural.

Composting is based upon a natural metabolic process, microbial digestion. Microbial digestion is a biological process that happens in nature …. it’s natural

Home and industrial composting like we know it needs a setting, technique and infrastructure that cannot be described as “natural”. Industrial composting is in fact the industrialisation of microbial digestion. Composting is not the same as biodegradation although both are based on biological degradation. Composting should not be described as as a natural process. It’s a man-made process; it’s a cultural activity. Theoretically, it should be classified as a non-natural human made process alongside mechanical and chemical degradation.

Incineration

Incineration of plastic waste materials converts the waste into ash and heat. The heat generated by incineration may be used to generate electric power. This process is called energy recovery. Unfortunately, many toxic gases such as dioxins, furans, mercury and polychlorinated biphenyls may be released during the incineration of plastics.

Mechanical Degradation

This is an intentional physical degradation caused by “mechanical” means; mechanical machines who have been developed to shred plastic waste. This is usually a fundamental step in what is commonly referred to as “mechanical “recycling. The molecular structure of plastic will be maintained.

Chemical Degradation

This is about degrading plastics into building blocks. This is usually a fundamental step in what is commonly known as “chemical “recycling. The molecular structure of plastics will not be maintained.

  1. Depolymerisation turns mono plastic (like PET bottles) back into monomers, which can be re-polymerised into new PET-based products.
  2. Solvolysis (dissolution) is used to break down certain plastics like expanded polystyrene (EPS) into monomers with the aid of solvents.
  3. Pyrolysis converts mixed plastics into tar oil which can be cracked down and further refined for new plastics production.
  4. Gasification is able to process unsorted, uncleaned plastic waste and turn it into syngas, which can be used to build bigger building blocks for new polymers.

Types of Erosion

  • Bulk erosion

The polymer degrades in a fairly uniform manner throughout the matrix

  • Surface erosion

Degradation occurs only at the surface of the polymer.

Land vs Marine Degradation

We differentiate between sea water (marine) and fresh water. Both may be referred to as “aqueous environments”.

Non-natural degradation will only take place on land. Natural degradation can take place on land and in aqueous environment. The presence of water will play a determinant role. It can slow down or accelerate the degradation process.

Seawater may slow down the photodegradative effect due to the lower temperature and oxygen availability while water will accelerate the degradation process in the case of water / marine soluble plastics.

Polemics and Politics

Here are a two political discussions in the world of bioplastics.

  • One vs Two Steps Degradation

Most biodegradable and compostable plastics have been engineered to intentionally degrade through a natural process; namely a biological process commonly referred to as microbial / bacterial digestion (metabolic and enzymatic degradation). This is called the “biotic” phase.

Some people claim that compostable plastics only have a single degradation phase; the biotic phase… and that’s it!

In the case of a unique and single biotic phase, the plastic packaging would start to degrade immediately after it is produced. It would start to degrade before it has left the factory and before the food has been packed in it.

I’m sure this is technically possible, but is it truly …

(1) functional and safe to have a packaging that starts to degrade before it’s been used.

(2) efficient, to rely solely on a biotic phase to degrade the plastic.

Using other natural abiotic (non-biotic) processes on top of the biotic phase may have several advantages:

(1) you accelerate the biotic phase by degrading the molecular configuration of plastic through abiotic processes.

(2) you control the moment when the biotic phase starts; you insert some kind of abiotic “trigger” for the biotic phase.

Let’s take the most famous compostable plastic … Polylactic Acid … PLA

One may argue that PLA has a single biotic phase. However, PLA will start to degrade outside in the sun on a hot summer day. This is abiotic degradation. Did it degrade because of the heat, photons, moisture or a bit of everything? Two observations: (1) technically speaking, it’s not biodegradation; (2) It wouldn’t degrade completely, you still need bacteria to finish the degradation completely.

However, one may argue that PLA would not biodegrade outside a composting facility. Compostable plastics need a composting setting to biodegrade. PLA packaging would biodegrade in the open environment if it had a single “biotic” degradation phase. In that case, it would be more correct to refer to PLA as biodegradable instead of compostable.

  • Is there a difference in the microbial degradation process of biodegradable and compostable plastics ?

The word “biodegradation” refers to the biological processing by micro-organisms; the microbial digestion.

There’s no metabolic difference in the way microorganisms consume biodegradable and compostable plastics. The metabolic mechanism remain the same.

Bacteria uses enzyme to weaken the polymeric chain and to transport the carbon inside the cell. Once inside the cell, the carbon will be converted into CO2 or it will be absorbed into biomolecules.

Microorganisms don’t change their metabolisms according to the brand or type of bioplastics.

Refs

%d bloggers like this: