Colourful plastics may lead to more microplastics: new study

Plastics with bright colours such as red, blue and green degrade and form microplastics quicker than those with plainer colours, researchers led by the University of Leicester have demonstrated.

Their findings reveal that the colourant used in the formulation of a plastic product can significantly affect the rate at which it degrades and break down, potentially introducing harmful plastics into the environment more quickly.

Published in the journal Environmental Pollution, it is the first time this effect has been proven in a field study and could be important for retailers to take into account when designing plastics and packaging.

Researchers from the University of Leicester, UK and the University of Cape Town in South Africa used two complementary studies to show that plastics of the same composition degrade at different rates depending on what is added to colour them. 

One study used bottle lids of various colours and placed them on top of the roof of a university building to be exposed to the sun and the elements for three years. The second study used different coloured plastic items that were found on a remote beach in South Africa. Importantly, samples were only analysed when the date of the manufacture of the plastic was known by a date stamp embossed into the plastic items. 

The scientists measured how chemically degraded the samples were by looking at how much they had reacted with oxygen in the air using Fourier-transform infrared spectroscopy (FTIR). They also measured the structural integrity before and after, using a breaking strength test to measure how brittle and easy to break apart they were. 

The findings across both studies showed that black, white and silver plastics were largely unaffected whereas blue, green and red samples became very brittle and fragmented over the same time period. In fact, older samples in South Africa were all plain colours and no brightly coloured plastic items were found. But, the sand itself was full of many coloured microplastics. 

This demonstrates that the black, white and silver colourants protect the plastic from damaging ultraviolet (UV) radiation whereas other pigments do not. UV damage changes the plastic’s polymer structure, making it brittle and susceptible to fragmentation.

The research was led by Dr Sarah Key, who conducted the studies while a PhD student at the University of Leicester School of Chemistry and funded by CENTA – The Central England NERC Training Alliance, and is now a senior research analyst with climate action NGO WRAP (Waste & Resources Action Programme).

Dr Key said: “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.

“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.

“Next time you clean up some plastic litter, take note of the colour and think about how soon it would have otherwise broken down. Whatever the colour, always check the packaging for details of how to recycle plastic packaging.”

Microplastics display different properties from their original bulk materials and little is understood about their impact on the environment. We know that they can release toxic plastic additives into the environment and they can potentially be transferred to humans, as well as toxic chemicals on their surfaces, through the food chain and water supplies.

The study has significant implications for material design, and suggests that manufacturers should give more consideration to the colour of short-lived plastics.

Dr Key added: “Manufacturers should consider both the recyclability of the material and the likelihood of it being littered when designing plastic items and packaging. 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. Where the plastic is designed to break down, such as by using pro-oxidant additives, consider the role that colour could play in this.”

Co-author Professor Sarah Gabbott, from the University of Leicester School of Geography Geology and the Environment, said: “I’ve often wondered why microplastics in beach sand often 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. Turns out there really 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.”

• Influence of colourants on environmental degradation of plastic litter is published in Environmental Pollution, DOI: 10.1016/j.envpol.2024.123701