Study: Polyethylene releases harmful gases when exposed to sunlight

15 Aug 2018 --- A team of researchers have found that polyethylene releases methane and ethylene into the environment when exposed to sunlight in what was an unplanned discovery. Dr. Sarah-Jeanne Royer of the University of Hawaii intended to measure methane gas coming from biological activity in seawater but instead found that plastic bottles were a greater source of the warming molecule than the bacteria in the water.

Published in the journal PLOS One, the experiment set out to demonstrate that polyethylene – the most common plastic and the material used to create shopping bags – produces two greenhouse gases, methane and ethylene, when exposed to ambient solar radiation.

The research found that polyethylene, which is the most produced and discarded synthetic polymer globally, is the most prolific emitter of both gases. Royer and her team of volunteers also found that that the production of the harmful gases from virgin low-density polyethylene increases in correlation to the duration of sun exposure.

“It was a totally unexpected discovery,” Dr. Royer tells BBC News. “Some members of the lab were experimenting with high-density polyethylene bottles looking at methane biological production, but the concentrations were much higher than expected. So we realized that the emissions were not just coming from the biology but from the bottle that we were using for the experiment.”

“At this stage we do not know the global implication of the GHGs produced from plastic under natural solar radiations. To get a better idea of the contribution of each gas we would need to get better technology to estimate: the total amount of plastic exposed to sunlight, the type of plastics and also the surface area of this exposed plastic with its level of biofouling and effect on the emissions,” Royers tells PackagingInsights.

Environmentally aged plastics incubated in water for at least 152 days were also found to produce hydrocarbon gases. In addition, low-density polyethylene emits these gases when incubated in air at rates ~2 times and ~76 times higher than when incubated in water for methane and ethylene, respectively.

“Our results show that plastics represent a heretofore unrecognized source of climate-relevant trace gases that are expected to increase as more plastic is produced and accumulated in the environment,” Royer claims.

The dangerous gases are emitted from the plastic through exposure to solar radiation. As the plastic breaks down and the surface area of the plastic increases, the sunlight exposure accelerates the gas emissions.

“I'm in the field every week,” Dr. Royer tells BBC News. “When I touch a piece of plastic, if there's a little impact on that plastic it's degrading into hundred of pieces pretty much as we look at it.”

“Low-density polyethylene (LDPE) does emit ethylene, methane and propane, even at low temperatures that contribute to greenhouse gas emissions,” Professor Ashwani Gupta from the University of Maryland, who was not involved in the study, tells BBC News.

“It is nice to see some quantified emissions on greenhouse gases for the selected polyethylene. The results clearly show variation in gas emission levels among the different polyethylene sources,” Gupta says.

Royer says that she approached suppliers for information on the chemistry of their plastics, the differing densities and processes involved, but no responses were forthcoming. “I think the plastic industry absolutely knows, and they don't want this to be shared with the world,” she says.

“Research on plastic waste is revealing it to be a disturbing Pandora’s box,” says Dr. Montserrat Filella, a chemist at the University of Geneva. “As research expands our knowledge, we are realizing that plastics can be insidious in many other ways. For instance, as vectors of 'hidden pollutants', such as heavy metals present in them or, now, as a source of greenhouse gases. And, in all cases, throughout the entire lifetime of the plastic.”

“No one knows how much methane and ethylene are being released from these sources,” adds Dr. Jennifer Lynch, a marine environment expert from the US National Institute of Standards and Technology (Nist). “We don't know if it is adding significant amounts of greenhouse gases to our atmosphere. It's another consequence of the use of plastics and it needs further examination.”

On the topic of how the world should proceed in the fight agaisnst plastic waste, Royers tells PackagingInsights, “My view is that single-use plastics should overall be banned. Only law and policy makers will allow this to happen. Changing people’s habits is possible but never as efficient as banning the products. There is a big movement around the plastic straws at the moment, it is a good start but might not make a significant difference on the total amount of plastic discarded in the environment globally.”

By Joshua Poole