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The secret of color: Why do organic pigments disappear quietly in sunlight?
In our daily lives, colors seem to be so stable and eternal, but in fact, organic pigments are in danger of disappearing under the irradiation of sunlight. This phenomenon of color loss is mainly due to the chemical effect of light, that is, photodegradation. Different materials react differently to sunlight, especially in areas such as food, paint, pesticides and pharmaceuticals.
Photodegradation refers to the changes in materials caused by exposure to light. This process not only affects pigments, but also affects a variety of chemicals.
According to research, the photodegradation of organic pigments mainly results from their interaction with ultraviolet light. When these pigment molecules absorb light energy, they rise to an excited state, a process that makes them more susceptible to decomposition, causing the color to disappear. Compared to inorganic pigments, organic pigments are more susceptible to the effects of light, which explains why the colors of some artworks and objects gradually fade when exposed to daily light.
The impact of photodegradation in different fields
Food
Photodegradation is particularly important in food care because many nutrients are damaged by the action of light. For example, hop bitter compounds in beer degrade under the influence of ultraviolet light, which not only changes the taste of the beer but also affects its quality. In order to reduce the impact of light, beer bottles usually use amber glass to absorb ultraviolet rays and protect the quality of the product.
Paints, inks and dyes
The use of organic pigments in the production of paints and inks increases their risk of photodegradation. Many ceramic products choose to use inorganic pigments to maintain color stability during long-term exposure to sunlight.
Pesticides and Herbicides
In agricultural production, it is of great significance to understand the photodegradation of pesticides. Some pesticides are designed to resist photodegradation, ensuring that they can effectively perform their biocidal functions after application. Therefore, researchers are working hard to develop technologies that can increase the photodegradation of pesticides, using photosensitizers and photocatalysts to solve this problem.
Drugs
The photodegradation of drugs has an important impact on water sources and the environment. When these drugs enter the water body, they will cause toxic damage to aquatic organisms. For example, pharmaceuticals need to avoid photodegradation during handling and packaging, which has led some pharmaceutical packaging to use amber glass or special materials to prevent the intrusion of ultraviolet rays.
Polymer
Photodegradation of polymers in sunlight often leads to a decrease in their performance. For example, the structure of synthetic polymers such as polypropylene and low-density polyethylene are easily affected by ultraviolet rays, causing problems such as discoloration and cracking. Therefore, measures to prevent photodegradation are also crucial when designing polymer materials.
Many organic chemicals are unstable in the presence of oxygen, and photodegradation is a process that works against this chemical stability, causing many substances to begin to change after exposure to light.
Strategies to prevent photodegradation
Currently, photodegradation of many materials can be prevented by adding stabilizers. Antioxidants and UV absorbers are common additives that can effectively inhibit the deterioration process of materials. Typical examples of these stabilizers include benzene-substituted compounds that capture photons and convert them into heat energy, reducing the photokill effect of the material.
Maintaining color stability, both in works of art and in products used every day, is a technical challenge. Many products also use innovative methods to repair materials that have become discolored due to photodegradation, which is particularly prominent in the care of old toys. This process is nicknamed "vintage brightening."
Faced with these photodegradation phenomena and technical challenges, we can’t help but think about how to keep organic pigments under sunlight longer so that they can coexist better with our lives?
