Do bright plastics degrade faster, creating more microplastics?

Image Credit: Adobe Stock/Aram

A team of researchers led by the University of Leicester, in Leicester, U.K., have demonstrated that plastics with bright colours such as red, blue, and green degrade and form microplastics faster than those with plainer colours.

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, the findings constitute the first time this effect has been proven in a field study, the researchers say, and could be important for retailers to take into account when designing plastics and packaging.

Researchers from the University of Leicester 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, while the second study used different coloured plastic items that were found on a remote beach in South Africa. Importantly, samples were only analyzed 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 a technique called “Fourier-transform infrared spectroscopy,” (FTIR), which is used to obtain the absorption or emission infrared spectrum of a solid, liquid, or gas. 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 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. All of which 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.

In the end, the researchers’ findings reveal that the colourant 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 more quickly. “Manufacturers should consider both the recyclability of the material and the likelihood of it being littered when designing plastic items and packaging,” said lead researcher Dr. Sarah Key, at the University of Leicester School of Chemistry. 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.”