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- Shining light on why plastics turn yellow
FOR IMMEDIATE RELEASE|September 06, 2022
“Could Superficial Chiral Nanostructures Be the Reason Polyethylene Yellows as It Ages?”
ACS Applied Polymer Materials
If you own a retro gaming console or have an old roll of packing tape, you’ve seen how plastics turn yellow as they age. Though the cause of this color change has long been attributed to the formation of molecules that act as dyes — the actual chemical changes that take place remained unexplained. Now, researchers reporting in ACS Applied Polymer Materials have identified surface-based chiral nanostructures as the potential culprit.
Understanding how and why polymers degrade with age is key to designing alternatives that can avoid these pathways, allowing plastic products to have a longer lifespan. For one of the most commonly used plastics, polyethylene, it’s long been suggested that ultraviolet (UV) light — the same light that gives us sunburns — initiates reactions in the backbone of the polymer’s structure that cause the yellow color change. However, though chemical changes to polyethylene’s polymeric backbone have been observed after exposure to UV light, those new structures cannot account for polyethylene’s yellowing. One emerging way to intentionally modify the color and the ways that plastics interact with light is to create nano-sized “supramolecular” structures on their surfaces that impact plastics’ properties in a controllable way. Inspired by these surface-based technologies, Margaret M. Elmer-Dixon, Melissa A. Maurer-Jones and colleagues wanted to see if such nanostructures formed unintentionally by UV light could be the cause of polyethylene yellowing.
The researchers first investigated if potential structures formed on yellowed polyethylene films’ surfaces interacted with circularly polarized light, a type of light whose waves travel with a right- or left-handed rotation. The amount of circularly polarized light absorbed by the film in these experiments changed depending on the film’s orientation, suggesting that the yellowed plastic contains new chemical structures that are chiral, that is, they are directional and aren’t identical to their mirror images. Additional experiments showed that most of the degradation during film yellowing occurred on the surface of the films. The team concluded that chiral chemical structures on the surfaces of the polyethylene films are formed during exposure to UV light and are a potential cause for the yellow color of old plastics. They say that these insights could help researchers design plastic products that last longer before becoming unsightly or unusable.
The authors acknowledge funding from the University of Minnesota, Duluth, the University of Minnesota McKnight Foundation, and the U.S. Department of Energy.
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Plastics, like what was used for this clock’s face, yellow over time; researchers now might know why this happens for polyethelene.
Credit: Dmitriev Mikhail/Shutterstock.com
As an expert in materials science and polymer chemistry, I've delved into the intricate world of polymer aging and color change, particularly focusing on the recent research published in ACS Applied Polymer Materials. The article, dated September 6, 2022, sheds light on a longstanding mystery: why plastics, especially polyethylene, turn yellow as they age.
The study proposes a novel explanation for the phenomenon, attributing it to surface-based chiral nanostructures. The authors, Margaret M. Elmer-Dixon, Melissa A. Maurer-Jones, and their colleagues, present compelling evidence supporting their hypothesis. They conducted thorough experiments to explore the relationship between yellowing in polyethylene films and the formation of chiral structures induced by UV light exposure.
One of the key aspects of their investigation involves the interaction of yellowed polyethylene films with circularly polarized light. Circularly polarized light, with its right- or left-handed rotation, revealed changes in absorption depending on the film's orientation. This suggests the presence of chiral chemical structures on the surface of the yellowed plastic—structures that are directional and distinct from their mirror images.
Moreover, the researchers observed that most of the degradation during film yellowing occurred on the surface. This observation further strengthens the conclusion that chiral chemical structures on the surfaces of polyethylene films are formed as a result of exposure to UV light, potentially causing the yellow coloration of aged plastics.
The significance of this research lies in its implications for designing more durable plastic products. By understanding the mechanisms behind polymer aging, scientists can work towards developing alternatives that resist these pathways, ultimately prolonging the lifespan of plastic products. The intentional modification of plastic properties through nano-sized "supramolecular" structures on their surfaces is highlighted as a promising avenue for future research and development in the field.
The funding acknowledgment to the University of Minnesota, Duluth, the University of Minnesota McKnight Foundation, and the U.S. Department of Energy further reinforces the credibility of the study. This interdisciplinary collaboration and financial support underscore the scientific community's recognition of the importance of unraveling the mysteries of polymer behavior and degradation.
In conclusion, the research not only deepens our understanding of why plastics turn yellow as they age but also opens new avenues for designing more resilient plastic materials. It exemplifies the crucial role of surface-based nanostructures in influencing the properties of polymers, showcasing the potential for controlled modifications that could revolutionize the durability and aesthetic longevity of plastic products.
