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Plastics, including biodegradable ones, often have other components mixed in as additives for various reasons. Some may be natural and added for fragrance, some may be added to petroleum-based bags for quicker breakdown to meet certification, ultimately these bags disintegrate becoming thin slices of micro plastic subsequently absorbing toxins present in the environment and transporting them up the food chain.  

Plastics often incorporate additives that change their specific properties, such as their strength and durability. These include UV stabilisers, anti-oxidants, dyes and flame retardants.

Bio-based Plastics 
Plastics that can be derived from other sources, including microbes and plants.

A substance is considered biodegradable when living things, like fungi or bacteria and microbes, can break it down. In the case of biodegradable plastics, this process is enabled by enzymes that are produced by microbes that use the plastic as a source of energy – the microbes are essentially eating the plastic for food.

It is important that biodegradation is not confused with other natural processes. For instance, most plastics will eventually break down into fragments through the weathering effects of sunlight, heat and friction if given enough time. This is degradation, not bio-degradation. To biodegrade, the polymers need to be broken down through the action of living cells into simple chemical elements.

Biodegradable bags
Currently are made from a mix of petroleum as a base and from plant-based materials like corn and wheat starch. However, when it comes to this kind of plastic, there are certain conditions required for the bag to begin to biodegrade. For example if they are to be composted then firstly temperatures need to reach 50°C . Secondly, the bag needs to be exposed to UV light. These bags break down into micro plastic with toxins from the petrochemical component.

Many bags on the market are currently a 50% petroleum base mixed with a plant additive (PLA) the PLA is added so that the bag can breakdown faster to meet composting standards (European EN 13432 and Australian AS4736) however, this results in micro plastic and toxins from the petroleum and additives releasing faster into the environment. 

Organic composting business will not take mixed composition bags or mixed compostable products like coffee cups for example because they produce polyethylene mulch.

Kyrikou and Briassouliss, 2007 state that “organic mulch is a much better alternative to polyethylene mulch. The efficiency of the plastic degradation process varies by environment and may also be affected by the concentration of chemicals present that may react with the plastic”.

Also, if biodegradable bags are sent to landfill, they break down without oxygen to produce methane, a greenhouse gas with a warming capacity 21 times more powerful than carbon dioxide. However, this can be off set if the waste management site flares off the methane or produces electricity from it.

In an oceanic environment, the speed of biodegradation varies greatly and is highly dependent on the environment in which the plastics end up. Cold Sea or Rivers. This will affect the value of the biodegradable product – few would consider a plastic bag to be usefully biodegradable if the process takes hundreds of years, like petroleum, or years for mixed composition bags. Likewise, biodegradability will be of questionable value if the breakdown process gives rise to toxic products. This is where standards are helpful.

Bioplastic - A polymer made wholly or in part from a renewable, plant-based feedstock. Will biodegrade when placed in a commercial composting system but may be made up only partly of plant and largely of synthetic less environmentally friendly substances. Some producers claim their packaging to be biodegradable, when in reality it just contains additives that make the plastic break down into smaller pellets. Plant bottle Coca Cola is marketed as bio-based however it is only PARTLY plant-based and has a chemical structure of plastic the same as plastic made from oil, therefore should be recycled with petro-derived plastic.
Non-biodegradable bio-based plastics - This group of bio-based plastics are indistinguishable from conventional fossil-based plastics in terms of their chemical composition, properties and decomposition characteristics. As the name suggests, they are manufactured from at least some biologically produced compounds, typically of plant or microbial origin. However, the terms ‘bio-based’ or ‘plant-based’ can be ambiguous as they do not convey what proportion of the plastic is bio-based. Polyethylene terephthalate (PET) illustrates this problem.


Cassava - Manihot esculenta 
Cassava, also known as manioc, yuca, mandioca or Brazilian arrowroot, is a woody shrub native to South America of the spurge family, Euphorbiaceae.

ComPlast compostable bags
ComPlast’s products are made from a fully biodegradable starch-based resin (Cassava)  made entirely from natural ingredients – a revolutionary non-petroleum material that uses no petroleum, heavy metals or chemicals in its manufacture and breaks down to harmless biomass, CO₂, H₂O when placed in water or composted.

Compost is organic matter that has been decomposed in a process called composting. This process recycles various organic materials - otherwise regarded as waste products - and produces a soil conditioner. Compost is rich in nutrients.

Composting involves the breakdown of biological material such as green waste from garden clippings or food scraps into humus. Industrial compost facilities are designed to accelerate microbial growth by controlling moisture, airflow, microbial activity and the proportions of different organic wastes. As the microbes consume the waste, they generate heat, which accelerates the breakdown. The resulting product, compost, is then used to return organic matter to the soil.

Home composting involves a similar process but on a much smaller scale, so it may not generate as much heat and will likely result in a slower breakdown process. Home composting is also much more varied depending on which method is used.

Compostable bags biodegrade, some only under certain conditions. ie: Industrialised composting at temperatures between 55 to 60 degrees c.

It may take longer for compostable bags to biodegrade successfully in the lower temperatures of home composting. 

Degradable items don't have living organisms as a crucial part of the breakdown process. Degradable bags cannot be classed as biodegradable or compostable. Instead, non-natural chemical additives used in the plastic allow the bag to break down quicker than a standard plastic bag usually would. Degradable bags that disintegrate just become tinier and tinier pieces of micro plastic quicker, and still pose serious threats to marine life. 

The break down into fragments through the weathering effects of sunlight, heat and friction over time.

Early Life
The implications of plastic production impacting on an environment, ie: feedstocks derived from corn for bio-based products, is there a heavy reliance on petro-chemicals fertilisers? Was a finite or virgin resource exploited for extraction? for example, oil, wood or water. 

End of Life
Where the product ultimately ends up; landfill, industrialised composting, recycled, home composting etc.

Fossil fuel is a general term for buried combustible geologic deposits of organic materials, formed from decayed plants and animals that have been converted to crude oil, coal, natural gas, or heavy oils by exposure to heat and pressure in the earth's crust over hundreds of millions of years.

The process or state of breaking or being broken into fragments.

Green Washing - Green washing (a compound word modelled on "whitewash"), also referred to as "green sheen", is a form of spin in which green PR or green marketing is deceptively used to promote the perception that an organisation's products, aims or policies are environmentally friendly.

HDPE - High Density polyethylene – or polyethylene high-density (PEHD)
A polyethylene thermoplastic made from petroleum. It is sometimes called "alkathene" or "polythene" when used for pipes.

A chemical compound with the chemical formula CH(one atom of carbon and four atoms of hydrogen). It is a group-14 hydride and the simplest alkane. A very powerful greenhouse gas. One pound of methane traps 25 times more heat in the atmosphere than a pound of carbon dioxide. Methane is the primary component of natural gas – a common fuel source. If methane can leak into the air before being used — from a leaky pipe, for instance — it absorbs the sun's heat, warming the atmosphere. For this reason, it's considered a greenhouse gas, like carbon dioxide. Landfill where there is no oxygen produces methane gas.

Methane capture
Because methane can be captured from landfills, it can be burned to produce electricity, heat buildings, or power garbage trucks.

Micro-beads are manufactured solid plastic particles of less than one millimetre in their largest dimension. They are most frequently made of polyethylene but can be of other petrochemical plastics such as polypropylene and polystyrene. They are often used in cosmetics. New Zealand banned the manufacture and sale of products with micro-beads on 7th June 2018.

Micro plastic
Micro plastics are small barely visible pieces of plastic that enter and pollute the environment. To clarify, micro plastics are not a specific kind of plastic, but rather is any type of plastic fragment that is less than five millimetres in length according to the U.S. National Oceanic and Atmospheric Administration. In essence all plastics go through a phase of micro plastic, but some are toxic others are not.

A molecule that can be bonded to other identical molecules to form a polymer.

Oxo-degradable Plastics
Oxo-degradable plastics are essentially conventional plastics such as polyethylene and polypropylene that contain additives known as ‘pro-oxidants’. These additives are typically metal salts, which are intended to speed up degradation of the plastics into smaller fragments without any help from microbes.

Fragmentation is facilitated by exposure to sunlight and air, which causes oxidation – a chemical reaction that leads to the loss of electrons and often involves oxygen – hence the name oxo-degradable. Oxo-degradable plastics typically result in a large number of micro-fragments or micro-plastic pieces. A key concern is that oxo-degradable plastics may be contributing to micro-plastic pollution in the marine environment.

If fragmentation occurs, some biodegradation may occur in the right environment.

There are questions around the recyclability of these plastics as they include additives to make them fragment into microplastics, which may weaken the recycled plastic.

PBAT - Polybutylene adipate terephthalate
PBAT belongs to the polyester family of polymers and is produced from petroleum-based substances. It is known for its flexibility and strength and is used to make biodegradable plastics, including plastic bags and wraps.

PET - polyethylene terephthalate 
PET is one of the dominant plastics within the market. It is commonly used for bottles because it makes a rigid container that is very lightweight. However, because of the stability of PET, it is also highly resistant to biodegradation, and creates micro plastic posing a significant environmental problem because of the amount of PET produced, sold, used and thrown away daily.

Relating to or denoting substances obtained by the refining and processing of petroleum or natural gas.

PLA - Polylactic acid 
Polylactic acid is made from lactic acid, which is usually produced by fermenting sugar or starch from vegetable sources such as corn and is then processed to form a polymer. Other chemicals may also be used in the PLA production process or introduced as additives. Products made from PLA include food packaging and coffee cups (which may use PLA for the lid or cup lining). PLA also has medical uses, such as dissolvable stitches, as it breaks down into lactic acid, which is easily processed by the human body. PLA is not suitable for recycling in New Zealand. It can also pose challenges for PET recycling because it is difficult to sort without special technology (as it often resembles PET) and can reduce the quality of the recycled material. Consequently PLA products do not get composted in New Zealand rather they go to landfill. 

Derived from the ancient Greek word ‘plastikós’, meaning ‘shapable’ or ‘mouldable’. Plastics are made up of large chemical units known as polymers, which are in turn made up of repeated smaller units called monomers. Many polymers are capable of being moulded into different shapes when heated. A characteristic of many polymers is that they can be moulded when heated – hence the name.

We generally think of plastics as being manufactured from petrochemical (or fossil-fuel-based) this is because the gas ethene (ethylene) is produced by reacting fossil hydrocarbons with steam at very high temperatures and is used to create polythene (polyethylene), the polymer that we commonly see in the form of plastic bags at the supermarket.

Polyethylene (LDPE) 
LDPE isn't regarded as a 'bad' plastic. However potentially toxic industrial chemicals are involved in its manufacture, including butane, benzene and vinyl acetate. This plastic is considered safe but not very environmentally eco-friendly.

Poly(hydroxyalkanoates) (PHA's)
Polyhydroxyalkanoate (PHA): Biodegradable Plastics from Bacteria • PHA's are natural polyester polymers synthesised by bacteria • 100% biodegradable, their Physical properties are similar to conventional petro-plastics. Polymer composition can be tailored into different industrial applications. They can be produced from agro-industrial waste streams.  Bacteria can be genetically modified to enhance the polymer production. They represent a potential platform for bioplastic, bio-resins fine chemicals, and bio-composite materials. PHA's are polyesters that can be synthesised by microbes that are fed starches and cellulose. They have a wide range of potential uses – for example, disposable cutlery, bags, bottles and cups.

Substances which have a molecular structure built up chiefly or completely from a large number of similar units bonded together, e.g. many synthetic organic materials used as plastics and resins.

New Zealand does not have a standard for biodegradable or compostable plastics. However, some plastic manufacturers have voluntarily sought certification using some of the existing international standards (European EN 13432 and Australian AS4736) so that they can label their products accordingly. An industry working group has been set up to consider whether adoption of a standard in New Zealand would be useful.


The ability of plastics to degrade
Plastics which are marketed as biodegradable must meet standards that include a range of tests for biodegradability that consider different receiving environments, such as wastewater treatment plants, freshwater or marine environments.

These tests typically include the following:

Chemical analysis of the starting material: checking to ensure that the components do not contain unacceptable levels of harmful chemicals, such as heavy metals.

Biodegradation: measuring how much of the carbon that was present in the starting material has been consumed and respired by microbes. In some environments, microbes also produce methane, which needs to be measured.

Disintegration: checking that no large fragments remain.

Toxicity of the degraded material: testing whether the degraded material will have an impact on the growth of organisms that are commonly present in the environment where the material is intended to degrade.

Manufacturers must meet the set targets for these tests before passing the standards.

Sustainable living 
A lifestyle that attempts to reduce an individual's or society's use of the Earth's natural resources, and one's personal resources. Its practitioners often attempt to reduce their carbon footprint by altering their methods of transportation, energy consumption, and/or diet.

References and further information:

The Dirt on Biodegradable Plastics: Are they compatible with organic practices?

Kyrikou, I. D. Briassoulis 2007. Biodradation of agricultural plastic films; A critical view. Journal of Polymers and the Environment 15, 125-150.

Parliamentary Commissioner for the Environment - Biodegradable and Compostable Plastics in the Environment