What plastic is that?

October 23, 2021

Degradable vs biodegradable vs compostable plastic. What does it all mean? And is there such a thing as a good plastic choice?

We’ve all been hearing a lot over recent years about the impact of plastics on the natural world, and many of us are actively embracing alternatives in a bid to protect our planet and wildlife. But what if plastics labelled ‘green and degradable’, ‘biodegradable’ or ‘planet friendly’ were actually worse than normal plastic? Unfortunately, due to being unregulated terms in Australia, this is often the case. Let’s sort the truth from the trash.

Degradable means microplastics

Some plastic bags that are labelled ‘green and degradable’ or ‘naturally degradable’ contain an additive to make them break down faster into smaller fragments. The correct term for these degradable bags is oxo-degradable. They are made from fossil fuel-derived polymers such as polyethylene, polypropylene, and polystyrene, with a metal salt added to speed up the oxidation process. In many cases, when this type of plastic degrades it fragments into tiny pieces, leaving residual microplastic pollution behind.

An independent study of oxo-biodegradable plastic bags in the marine environment found that after 40 weeks, more than 90 percent of the bag was still present. A global initiative by the Ellen MacArthur Foundation’s New Plastics Economy has united 150 organisations in a call to ban oxo-degradable plastic packaging from the market, due to the risks that microplastics pose to the ocean and other ecosystems, potentially for decades into the future.

Biodegradable labelling lowdown

Without laws to limit its use, even the term biodegradable doesn’t mean anything, because everything is biodegradable eventually. Something that is biodegradable doesn’t necessarily break down in a way that will save the planet.

Legislation in the US forbids the term biodegradable to appear on any product unless that product is shown to break down into elements in nature within five years. Unfortunately, in Australia we don’t have any legislation that limits the use of the term biodegradable.

There are plastic bags labelled biodegradable that are misleading us to think they’re made from natural materials, but this isn’t always the case. To be truly biodegradable, a bag must tick all three of these boxes:

  1. It must leave no toxic residues or traces of plastic behind.
  2. It must be capable of being decomposed by bacteria and other living organisms to avoid pollution.
  3. It must break down into organic matter anywhere microorganisms are present, including on land and in water.

The compostable solution

Compostable plastic is a type of bioplastic made from substances obtained from plants, such as non-genetically modified corn starch and biodegradable, compostable polymers. Compostable bags break down like plants anywhere there is oxygen and microorganisms – without leaving any microplastics behind. These bags break down at normal compost temperatures and don’t need to be heated to high temperatures. Composting is nature’s way of sequestering carbon back into the earth as organic matter with no toxic residues.

The Australasian Bioplastics Association has launched the ‘seedling logo’ certification in Australia and New Zealand to help us clearly identify certified compostable packaging materials. To be certified compostable and carry the seedling logo, suitable biopolymer materials must undergo a stringent test regime outlined by Australian Standard AS4736 and carried out by a recognised independent accredited laboratory.

What about plastic in landfill?

An independent study in the US compared how polyethylene bags, oxo-degradable bags and compostable bags break down. To simulate a composting environment, a set of bags were buried six inches deep in clay flower pots and left untouched in an open-air box. Another set of bags was submerged in a crate in the ocean. Within six months on land and two years in water, the compostable bags had completely disappeared, while the oxo-degradable bags had broken down into small fragments of microplastics, and the polyethylene bags remained the same.

So, if a compostable plastic bag goes into landfill, it is much better for the environment than polyethylene and oxo-degradable plastic because it has a lower carbon footprint and will not release microplastics.


Our increasing use of plastic and our inability to manage plastic waste has resulted in one-third of plastic waste becoming land or marine pollution. With awareness we can all make small changes and be part of the solution. In an ideal world, no plastic would go to landfill, but choosing compostable means it will properly break down without leaving harmful microplastics.

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