Ethan Clarke
Plastic products are known to turn yellow over time, and this phenomenon is often observed in items stored in cellars. The primary cause of plastic discoloration is exposure to ultraviolet (UV) light, which initiates a chemical reaction in the polymer structure, leading to the formation of yellow-colored molecules. Additionally, other factors such as visible light exposure, extreme temperatures, humidity, and solvents can also contribute to plastic degradation and discoloration. Understanding the root causes of plastic yellowing is essential for developing alternatives and improving the longevity of plastic products.
| Characteristics | Values |
|---|---|
| Reason | Excess exposure to UV light, extreme temperatures, humidity, and solvents |
| Process | Oxidation, specifically photo-oxidation and thermal oxidation |
| Prevention | Store away from sunlight and extreme temperatures |
| Reversal | Exposure to sunlight, hydrogen peroxide, or bleaching |
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What You'll Learn
UV light exposure
Plastic turning yellow is a common phenomenon, and it is mostly due to excess UV light exposure. The more exposed plastic is to UV light, the quicker it changes colour. However, this discolouration does not happen immediately. The increase in light exposure initiates and accelerates a chemical reaction in the polymer, which eventually turns the plastic yellow.
The UV component in sunlight is known to cause yellowing of plastic through photo-oxidation. In the presence of UV light, oxygen in the atmosphere produces carbonyl groups, which change the colour of the plastic to yellow-brown. The plastic then begins to flake away until it becomes useless. This process is known as photodegradation, and it causes the yellow pigment to return to a more neutral look.
Polymers with aromatic groups are more prone to UV damage. Aromatic polyurethanes (PU), for example, turn yellow due to free radical pathways occurring in the backbone of the polymer. Over time, light or heat-induced oxidation forms yellow-coloured quinones.
To prevent plastic from turning yellow, it must be protected from excess UV light exposure. This can be achieved by storing plastic goods away from direct sunlight and in covered spaces.
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Oxidation
The yellowing of plastic is a result of oxidation. This can occur in multiple ways, including thermal oxidation and photo-oxidation. Photo-oxidation is caused by exposure to UV light, which initiates a chemical reaction that changes the colour of the plastic. The UV component in sunlight is known to cause yellowing of plastic through photo-oxidation, and this process can be accelerated by heat and moisture, which cause thermo-oxidation.
During photo-oxidation, the oxygen in the atmosphere produces carbonyl groups in the presence of UV light, which changes the colour of the plastic to yellow-brown. This process can also be influenced by impurities in the plastic, which can absorb light and give a yellowish tint. Additionally, inadequate amounts of additives or fillers can discolour plastic during processing or storage.
Thermo-oxidation, on the other hand, occurs when plastic is exposed to sufficient heat. This can be caused by high-temperature exposure during manufacturing or end-use. Over time, light or heat-induced oxidation forms yellow-coloured quinones. Aromatic polyurethanes (PU) are particularly prone to thermo-oxidation, as free radical pathways occur in the backbone of the polymer, causing the plastic to turn yellow.
To prevent oxidation and the yellowing of plastic, antioxidants can be added during the moulding process. Plastics for outdoor use should also be thoroughly processed through the end-capping of polymer chains. By understanding the root causes of oxidation and yellowing, manufacturers can develop plastic products that last longer and maintain their original colour.
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Extreme temperatures
High temperatures can also cause plastic to yellow. This can occur during the manufacturing process, where extreme heat may be used to form the polymer. Additionally, exposure to heat sources such as ovens or radiators can contribute to the yellowing of plastic.
Low temperatures can also be a factor in plastic yellowing. For example, the cold in a cellar could potentially contribute to the discoloration of plastic over time, especially when combined with other factors such as humidity or exposure to visible light.
The type and quality of the plastic also play a role in its susceptibility to yellowing. For instance, low-quality polypropylene can begin to show signs of yellowing after just a few months of exposure to UV light and heat. On the other hand, higher-quality acrylic materials, such as those made from pure acrylic, tend to be more resistant to yellowing.
To prevent plastic yellowing, it is important to control the temperature around plastic products and avoid exposing them to extreme temperatures. Additionally, using UV-resistant coatings or films can help protect plastics from UV light exposure, which is often the primary cause of plastic yellowing.
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Atmospheric pollutants
Plastic turning yellow is usually due to increased exposure to light, particularly ultraviolet (UV) light. This triggers a reaction in the polymer, which eventually causes it to discolour. The presence of heat, moisture, and oxygen can also contribute to this process.
Commodity plastics like polyethylene (PE), polypropylene (PP), and polyvinyl chloride (PVC) are at increased risk of yellowing due to their inherent instability. Without additives, these plastics can degrade during processing. In the presence of UV light, oxygen in the atmosphere produces carbonyl groups, leading to a yellow-brown discolouration.
To prevent plastic from turning yellow due to atmospheric pollutants, it is essential to limit the interaction between the plastic and these pollutants. This can be achieved through proper processing, including end-capping of polymer chains and the use of UV absorbers and stabilizers. Additionally, adding light stabilizers and whitening chemicals can help resolve the yellowing issue.
Overall, understanding the factors contributing to plastic degradation and implementing appropriate measures to mitigate their impact are crucial for preventing the yellowing of plastics due to atmospheric pollutants.
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Impurities in plastics
The presence of impurities in plastics is a significant concern, especially when it comes to their recycling and reuse. Impurities can arise from various sources, and their impact on the properties and performance of plastics is an active area of investigation for scientists.
One source of impurities in plastics is the additives that are incorporated during their manufacturing. These additives are essential for enhancing the functionality of polymer products. However, they can also act as impurities during the recycling process, potentially migrating and contaminating the soil, air, water, and even food items that come into contact with recycled plastics. This issue underscores the importance of proper waste management and recycling practices to minimise the release of these additives into the environment.
Another source of impurities in plastics is the degradation and oxidation processes that occur over time. For example, exposure to ultraviolet (UV) light can initiate chemical reactions in the polymer structure, leading to discolouration and the formation of new chemical compounds on the surface. This phenomenon, known as photo-oxidation, results in the characteristic yellowing of plastics. Additionally, extreme temperatures, humidity, and exposure to solvents can also contribute to the degradation of plastics, further complicating the identification and management of impurities.
To address the challenge of impurities in recycled plastics, scientists employ advanced techniques such as chemometric analysis of attenuated total reflection (ATR) spectroscopy. This method, coupled with tools like Partial Least Squares (PLS) analysis, enables the identification and quantification of impurities in recycled plastic streams. By comparing the spectra of different polymers and their blends, scientists can optimise recycling processes to minimise impurities and enhance the reusability of the final product.
While the presence of impurities in plastics is inevitable, ongoing research and the development of advanced characterisation techniques provide hope for improved waste management and the creation of more sustainable plastic alternatives.
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Frequently asked questions
Plastic turns yellow due to oxidation, which can occur through photo-oxidation or thermal oxidation. Photo-oxidation is caused by exposure to UV light, while thermal oxidation is caused by sufficient heat.
Aside from UV light and heat, other factors that can cause plastic to turn yellow include exposure to visible light, extreme temperatures, humidity, and solvents.
To prevent plastic from turning yellow, you can reduce its exposure to the factors mentioned above. Additionally, during the manufacturing process, additives such as antioxidants and UV-absorbing stabilizers can be added to the plastic to prevent degradation and discoloration.
