The removal of the unnecessary use of plastic is noble and necessary, but new research casts doubt on how environmentally friendly some forms of biodegradable plastic are.
The researchers found that the bags labeled as biodegradable and compostable were still capable of carrying a large shopping load after being left to degrade the elements for three years – they do not decay as fast as expected.
The point raised by the University of Plymouth study in the UK is whether the biodegradable chemical blends in these bags go far enough. If the plastic is still in the environment three years after being discarded, it is still a major problem of garbage and pollution that needs to be addressed.
"After three years, I was really impressed that any of the bags could still carry a lot of shopping," says researcher Imogen Napper. "For a biodegradable bag to be able to do that was the most amazing."
"When you see something labeled in this way, I think you automatically assume it will degrade faster than conventional bags. But after three years at least, our research shows that this may not be the case."
The team tested five different types of plastic bag material in total, all widely available from UK retailers: two types of oxy-biodegradable bags, one biodegradable bag, one compostable bag and one conventional high density polyethylene plastic bag.
Oxy-biodegradable plastics are designed to break fragments relatively quickly, but they can still leave behind small microplastics, unlike true biodegradable ones. Compostable plastics, in turn, are a subset of biodegradable plastics that are designed to degrade faster.
All five types of plastic were tested outdoors, buried in the ground and submerged in the "ocean". The researchers measured the loss and disintegration of the surface over time, as well as tensile strength, surface texture and chemical structure.
The open air did a short job with all the bags, which split into fragments in just nine months.
In terms of soil and water, however, it was a different story: biodegradable, oxy-biodegradable and conventional plastic bags could still be purchased three years after being buried or submerged.
The type of compostable bag did its best, disappearing from the aquatic environment in three months and also fragmented into the soil environment – although fragments were still detected after 27 months of testing.
None of the bags deteriorated completely in all three environments.
Through their analysis, researchers hope to raise awareness of what "biodegradable" really means when it comes to plastic bags – something that can be recycled in the heat and pressure of an industrial facility may not disappear so quickly in the natural environment.
In other words, biodegradable as a label does not mean much without the conditions and the time scale of the degradation attached.
The team also wants to see stricter international standards against what definitions of biodegradable really mean. In the UK, disposable plastic bags have been charged with a 5p tax since 2015 and are now being withdrawn from large stores.
A spokesman for Vegware, who provided one of the compostable bags, said The Guardian that "it is important to understand the differences between terms like compostable, biodegradable and oxodegradable … compostable materials can compose with five key conditions – microbes, oxygen, moisture, heat and time."
"This research raises a number of questions about what the public can expect when they see something labeled as biodegradable," says one of the researchers, Richard Thompson.
"We have demonstrated here that the materials tested did not present any consistent, reliable and relevant advantage in the context of marine litter."
The research was published in Environmental Science and Technology.