A new study has shown that sunlight radiation alters the chemical properties of polyethylene, a component of many plastics
Hikes are fantastic opportunities to explore new places. However, during our journeys we often come across less captivating sights, such as discarded plastic items along the roadside, some of which have already been lying around for a long time, exposed to the elements. Over time, the plastic begins to wear away: some items will crack, others will crumble, and some undergo changes in color, often turning yellow. Certain plastic products, such as windows used in vending machines, parts of advertising signs and components of large packages, are not useful to the same extent once they turn yellow. Others, such as bottles and toys - simply look less appealing in their yellowed state, thus reducing the likelihood that we’ll continue using them. Therefore, understanding why plastic turns yellow, may be a step towards prolonging its durability and usability.
The term “plastic” encompasses a broad, diverse family of substances all made of organic polymers. A polymer is an especially large molecule composed of a repeated sequence of identical subunits. In fact, a significant portion of organic molecules familiar to us, such as proteins, DNA and polysaccharides, are polymers. One of the most common synthetic polymers is polyethylene, widely employed in the manufacture of plastic products such as bags, plastic containers, toys, and more. American physicist, Margaret Elmer-Dixon, and her colleagues, demonstrated that the reason for the appearance of the yellow coloration in polyethylene plastic results is a change in the material that results in a higher concentration of chiral molecules.
Chiral molecules possess unique traits: they exist in either right-handed or left-handed forms, much like your right and left hands. They share the same components but cannot be superimposed as they are non-superimposable mirror images due to their opposing orientations. They are not identical, rather, the order of the atoms on one of them is the mirror image of the other. In contrast, non-chiral molecules are identical to their own mirror images or can be superimposed one on the other, much like a ball, which does not have a right or left side, and its mirror image is identical to the ball itself.
Chiral molecules are similar to our hands - two molecules can have an identical chemical composition, but exhibit reversed configurations - one is left-handed and one is right-handed. Illustration of hands and chiral molecules | Wikipedia, Perhelion
More Chiral, More Yellowing
In their study, Elmer-Dixon and her colleagues exposed a layer of polyethylene plastic to ultraviolet radiation for varying periods of time, and examined the accumulation of chiral molecules on its surface. They chose to use ultraviolet radiation because it has sufficient energy to induce changes in the polymer and is a component of the solar radiation to which plastic is naturally exposed. After just one day of exposure, the material exhibited a significant increase in chiral molecules; and the longer the plastic was exposed to ultraviolet radiation, the more yellow it became.
The researchers concluded that exposing polyethylene plastic to ultraviolet radiation induces structural changes in the molecules, resulting in a higher concentration of chiral molecules on the surface, which increases with longer exposure. The scattering of light by these molecules is what causes the plastic to turn yellow. The results of this study shed new light on the processes of degradation of plastic products, and may help us in the future in devising strategies to prevent or delay such deterioration.