Those of us that take our own bags to the supermarket or checkout sometimes opt for an extra plastic bag or two if we run out of room - as long as the bag is labelled biodegradable. But research shows that in some cases, so-called biodegradable bags will still hold your shopping after three years underground. By Jeremy Torr.
Plymouth, UK, 5 May 2019. A recent study by researchers at the Litter Research Unit of the University of Plymouth (UOP) discovered that plastic bags labelled as “biodegradable” were still capable of carrying full loads of shopping after being exposed in the natural environment for up to three years. Sample bags were left exposed to air, buried in soil, and dropped into the 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 and complex changes in tensile strength, surface texture and chemical structure.
After nine months in the open air, all the materials had completely disintegrated into fragments. The compostable bag completely disappeared from the experimental test holder in the marine environment within three months but, while showing some signs of deterioration, was still present and completely intact in soil after 27 months.
And over three years later, the so-called biodegradable and oxo-biodegradable plastic bags remained functional as shopping bags after being in the soil or in a marine environment for three years.
Writing in Environmental Science and Technology, researchers from the University’s International Marine Litter Research Unit say the study poses a number of questions.
“After three years, I was really amazed that any of the bags could still hold a load of shopping. For a biodegradable bag to be able to do that was the most surprising (thing),” said PhD researcher Imogen Napper, who led the study. “When you see something labelled in that way, I think you automatically assume it will degrade more quickly than conventional bags. But, after three years at least, our research shows that might not be the case.”
Indeed, the UOP research highlights whether biodegradable formulations can be relied upon to offer a sufficiently advanced rate of degradation that offer any realistic solution to the problem of discarded plastic litter – or even composted bags.
“This research raises a number of questions about what the public might expect when they see something labelled as biodegradable,” said Prof. Richard Thompson, Head of the International Marine Litter Research Unit at UOP. “We demonstrated that the materials tested did not present any consistent, reliable and relevant advantage in the context of … litter,” he added.
Previous UOP research showed that discarded plastic debris that incorporated special polymers that degrade more quickly than conventional plastics (so called biodegradable plastics) offered a possible solution to persistent litter in the environment. It found that both oxo-biodegradable plastics and compostable plastic will degrade over time; compostable plastic in particular disappeared from a test rig between 16 and 24 weeks whereas approximately 98% of the other plastics remained after 40 weeks.
But, says the study, some plastics require UV light to degrade. And in the real world, the likelihood of exposure to UV decreases as a consequence of fouling (in the sea or rivers) or being buried in soil (covered by dirt and gutter litter).
Accordingly, the research found that while compostable plastics may degrade relatively quickly compared to oxo-biodegradable and conventional plastics, “… there are limitations to their effectiveness in reducing hazards associated with plastic debris” due to many plastics not getting enough UV exposure to break them down.
“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, said Prof. Thompson.
“We need clearer policy and international standards to define what we mean when we say something is biodegradable. The key thing is, in what environmental setting,” he asserts. He pointed out the rate of biodegradation depends on whether the plastic is an industrial bio-reduction plant, or in the natural environment in the sea, a river, or half buried in soil.
As Napper put it, “We need to ask ‘is this doing what it says it will do?’. Is it actually biodegradable, is it compostable? This research opened our eyes that things can be labelled in a way that means we need to take great care understanding the way the (labels are used).”
“Our research shows that plastic bags (we looked at) weren’t reliably degrading in the natural environment,” said Prof. Thompson. So be careful when you take a “compostable” bag home. It may still be in your compost heap in several years time, ready for a return trip to the supermarket.