Canisters and bottles collected for the study at a remote stretch of beach in De Hoop Nature Reserve. (Peter Ryan)
Palaeontology professor Sarah Gabbott has often wondered why microplastics in beach sand frequently appear to be all the colours of the rainbow.
“Until our study, I assumed that my eyes were being deceived and that I was just seeing the more colourful microplastics because they were easier to spot,” said Gabbott, of the University of Leicester’s School of Geography Geology and the Environment.
“Turns out there are likely to be more brightly coloured microplastics in the environment because those plastic items pigmented red, green and blue are more susceptible to being fragmented into millions of tiny, yet colourful, microplastic particles.”
She is a co-author of a new study conducted with the University of Cape Town (UCT), which found that plastics of bright colours such as red, blue and green degrade to form microplastics quicker than those with dull colours. The study was published in the journal Environmental Pollution.
The team behind the research found that the colourants used in the formulation of a plastic product can significantly affect the rate at which it degrades and breaks down, potentially introducing harmful plastics into the environment faster.
This is the first time this effect has been proved in a field study and could be important for retailers to take into account when designing plastics and packaging.
Plastics mostly break down when ultraviolet radiation (UV) changes the polymer structure, making it brittle and susceptible to fragmentation. The researchers used two complementary studies to show that items made from high-density polyethylene degrade at different rates, depending on what is added to colour them.
In the first part of the study, differently coloured polypropylene bottle tops from the same manufacturer were exposed to the elements on the roof of a building at the University of Leicester for three years.
For the second part of the study plastic samples were collected from canisters and bottles that had been washed ashore on a remote beach in De Hoop Nature Reserve, in the Western Cape, and their condition was analysed. The dates of manufacture were known from stamps embossed into the plastic items.
The scientists measured how chemically degraded the samples were as well as their structural integrity. Both studies showed that black, white and silver colourants protect plastic from damaging UV radiation, whereas other pigments do not.
Blue, green and red lids became brittle and broke into microplastics within three years, even when exposed to the moderate British climate. Samples from South Africa were up to 45 years old but all the older items were plain colours.
“It’s amazing — some of these big canisters last forever,” said Peter Ryan, an emeritus professor at the FitzPatrick Institute of African Ornithology at UCT and co-author of the study. He contributed to the paper by providing known-age plastic samples from the De Hoop beach.
“We sampled in a very remote part of De Hoop … Generally speaking, our more accessible beaches don’t have very old things because they get cleaned, at least sporadically.
“The fact that we were working in De Hoop because it was a particularly remote beach and it was difficult for people to go and get in there and clean stuff, so we didn’t actually remove the items in their entirety; we just took samples from them.”
Ryan noted the combination of short-term degradation experiments and the longer-term beach litter samples clearly indicate that bottles with different colourants break down at different rates.
“From an environmental perspective, we’d prefer littered bottles to remain intact as long as possible so they can be removed from the environment and recycled or disposed of responsibly — by being incinerated for energy production, turned into fuels by pyrolysis or, in the worst case, buried in landfills.”
It is much harder to deal with environmental plastics once they have broken down into microplastics, he said.
The research was led by Sarah Key, who conducted the studies while a PhD student at the University of Leicester’s School of Chemistry.
“It’s amazing that samples left to weather on a rooftop in Leicester in the UK, and those collected on a windswept beach at the southern tip of the African continent, show similar results,” Key, a senior research analyst with the non-profit Waste & Resources Action Programme, said.
“What the experiments showed is that, even in a relatively cool and cloudy environment, for only three years, huge differences can be seen in the formation of microplastics. Colourful plastics, such as red and green, degrade and form microplastics pretty quickly. When you look at more plain colours, such as black and white, they’re actually quite stable and remain intact.”
Microplastics display different properties from their original bulk materials and little is understood about how they affect the environment.
It is known that they can release toxic additives into the environment, as well as the toxic chemicals on their surfaces, and they can potentially be transferred to humans through the food chain and water supplies.
The scientists said that the study has significant implications for material design, with the findings suggesting that manufacturers should give more consideration to the colour of short-lived plastics.
Manufacturers should consider both the recyclability of the material and the likelihood of it being littered when designing plastic items and packaging, Key said.
“For items that are used outdoors or extensively exposed to sunlight, such as plastic outdoor furniture, consider avoiding colours like red, green and blue to make them last as long as possible,” she said. “Where the plastic is designed to break down, such as by using pro-oxidant additives, consider the role that colour could play in this.”
Ryan added: “The suggestion from the authors was that maybe producers who are using high-density polyethylene in their packaging should be looking at the colours that they choose. I think going to Coke and telling them that they shouldn’t use red in their lids … They’ve got such a brand identity that they’re not going to be very keen to change.”
All things being equal, he said, there is increasing pressure on manufacturers to be more mindful of how they use plastics in packaging applications.
“This is just one way that they can be a little bit more sensitive to the environment by changing the colour to something that makes it last longer … and can be removed from the environment before it breaks up.”