Biodegradable plastic bags are still capable of carrying full loads of shopping after being exposed in the natural environment for three years, a new study by researchers at University of Plymouth shows.  

The researchers say that biodegradable plastics failed to demonstrate any clear or reliable advantage over conventional plastic in the context of marine litter – but that they also create problems for recycling.

The Plymouth team examined the degradation of five plastic bag materials widely available from high street retailers in the UK.

They were then left exposed to air, soil and sea, environments which they could potentially encounter if discarded as litter.

The bags were monitored at regular intervals, and deterioration was considered in terms of visible loss in surface area and disintegration as well as assessments of more subtle changes in tensile strength, surface texture and chemical structure.

After nine months in the open air, all the materials had completely disintegrated into fragments.

However, the biodegradable, oxo-biodegradable and conventional plastic formulations remained functional as carrier bags after being in the soil or the marine environment for over three years.

The compostable bag completely disappeared from the experimental test rig in the marine environment within three months but, while showing some signs of deterioration, was still present in soil after 27 months.

Writing in Environmental Science and Technology, researchers from the University’s International Marine Litter Research Unit say the study poses a number of questions.

A realistic solution?
The most pertinent is whether biodegradable formulations can be relied upon to offer a sufficiently advanced rate of degradation to offer any realistic solution to the problem of plastic litter.

In the research, scientists quote a European Commission report in 2013 which suggested about 100 billion plastic bags were being issued every year, although various Governments (including the UK) have since introduced levies designed to address this.

Many of these items are known to have entered the marine environment, with previous studies by the University having explored their impact on coastal sediments and shown they can be broken down into microplastics by marine creatures.

Professor Richard Thompson OBE, Head of the International Marine Litter Research Unit, was involved in those studies and gave evidence to the UK Government inquiry which led to the introduction of the 5p levy. He added:

Standards needed
“This research raises a number of questions about what the public might expect when they see something labelled as biodegradable. We demonstrate here that the materials tested did not present any consistent, reliable and relevant advantage in the context of marine litter. It concerns me that these novel materials also present challenges in recycling. Our study emphasises the need for standards relating to degradable materials, clearly outlining the appropriate disposal pathway and rates of degradation that can be expected.”

A spokesperson for Vegware, which produced the compostable bag tested by the Plymouth team told The Guardian that the study  timely reminder that “no material was magic, and could only be recycled in its correct facility. It’s important to understand the differences between terms like compostable, biodegradable and (oxo)-degradable,” a spokesperson said. “Discarding a product in the environment is still littering, compostable or otherwise. Burying isn’t composting. Compostable materials can compost with five key conditions – microbes, oxygen, moisture, warmth and time.”

‘Meaningless’ term
David Newman, managing director of the Bio-based and Biodegradable Industries Association (BBIA) told the UK trade magazine Packaging News: “The study shows that compostables will break down in marine environments but that ‘biodegradable’ and ‘oxo degradables’ will not.  This confirms the reason why the EU has announced its ban on oxos, they have no known usefulness – and why using the word ‘biodegradable’ is meaningless and indeed should be made illegal as regards compostable packaging. Either a packaging material biodegrades when composting under the 13432 standard or there is no legal definition. The study proves this.

“Compostables are made to compost. Whilst it may be comforting that they also break down in marine environments, we should not be advocating that compostables are suitable for littering. Compostables should be used to reduce waste, to promote biowaste collections to composting and AD, and to ensure high quality soil improvers are returned to soil without plastics contamination.

• Photo, University of Plymouth, via YouTube.