Alexei Volkov
The phenomenon of plastic turning yellow with age is a well-known issue, and researchers have identified several factors contributing to this colour change. One of the primary causes is exposure to UV light, which initiates a chemical reaction in the polymer backbone, leading to oxidation and the eventual yellowing of the plastic. Additionally, other factors such as visible light exposure, extreme temperatures, humidity, and solvents can also play a role in plastic degradation and discolouration. The structure of the plastic itself also influences its susceptibility to yellowing, with some plastics like polyethylene, polypropylene, and polyvinyl chloride being more prone to this issue. Understanding the root causes of plastic yellowing is essential for developing more durable and colour-stable plastic products, as well as exploring alternative materials that can withstand the test of time without discolouration.
| Characteristics | Values |
|---|---|
| Cause of yellowing | Exposure to UV light, extreme temperatures, humidity, and solvents |
| Chemical changes | Oxidation of polymers, formation of free radicals, and changes to polyethylene’s polymeric backbone |
| Susceptible plastics | Polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), polyesters, polyurethanes (PU), and polycarbonates (PC) |
| Prevention methods | Adding UV absorbers, hindered amine stabilizers, light stabilizers, and whitening chemicals |
| Reversal method | Using hydrogen peroxide to bond with the radicals |
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UV light exposure
Exposure to UV light is a major contributor to the yellowing of plastics over time. The more exposed plastic is to UV light, the quicker it changes colour. However, the 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.
In the presence of UV light, oxygen in the atmosphere produces carbonyl groups, which change the colour of plastics to a yellow-brown hue. This process causes the plastic to begin flaking away until it becomes useless. Polyesters, like polyethylene terephthalate (PET) and polycarbonates (PC), are especially vulnerable to yellowing when used outdoors.
To prevent and slow down plastic degradation, UV absorbers and hindered amine stabilizers can be added to the plastic during production. These additives improve weatherability and provide protection from UV light-induced oxidation. Additionally, light stabilizers and yellowing whitening chemicals can be incorporated to address the yellowing issue.
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Oxidation
The oxidation process is initiated and accelerated by increased exposure to UV light or sunlight. This exposure triggers a chemical reaction in the polymer, causing it to eventually turn yellow. The intensity and duration of light exposure play a crucial role, as the slight exposure may not immediately ruin the plastic, but it will eventually lead to discolouration over time.
Commodity plastics such as polyethylene (PE), polypropylene (PP), and polyvinyl chloride (PVC) are particularly susceptible to oxidation and subsequent yellowing. The presence of oxygen in the atmosphere, in combination with UV light, produces carbonyl groups, which contribute to the colour change. This oxidation process can also be influenced by other factors, including extreme temperatures, humidity, and solvents, which can further damage the polymer.
To mitigate oxidation and the resulting yellowing, plastics intended for outdoor use should undergo specific processing, such as end-capping of polymer chains. Additionally, the incorporation of UV absorbers, hindered amine stabilizers, and light stabilizers can enhance protection against UV-induced oxidation. These measures aim to improve the weatherability and longevity of the plastic, reducing the chances of discolouration.
While oxidation is a primary contributor to plastic yellowing, it is not the sole cause. Other processes, such as biodegradation, mechanical degradation, and the presence of brominated fire-retardant compounds, can also lead to discolouration. Furthermore, atmospheric pollutants and impurities can react with antioxidants in the plastic, resulting in a yellowish or pinkish tint.
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Atmospheric pollutants
Plastic turning yellow is a common phenomenon, and it is often the result of exposure to ultraviolet (UV) light. UV light initiates a chemical reaction in the backbone of the polymer's structure, leading to the yellow colour change. However, atmospheric pollutants also play a significant role in plastic discolouration.
Pollutants exposed to plastic surfaces are a significant cause of plastic discolouration. This can occur when plastics are stored or used in polluted environments, where pollutants can adhere to the surface and cause discolouration over time. Additionally, during the manufacturing process, pollutants in the air can interact with the plastic, initiating degradation and discolouration.
To prevent atmospheric pollutants from causing plastic to turn yellow, several measures can be taken. Limiting the interaction between NOx pollutants and plastic is crucial. This can be achieved through the use of processing additives, especially in warehousing applications. Furthermore, understanding the plastic's properties, processing conditions, and application environment is essential for selecting the appropriate additives to prevent discolouration.
While atmospheric pollutants are a contributing factor to plastic yellowing, UV light remains the primary culprit. The more plastic is exposed to UV light, the quicker it changes colour. However, the colour change does not happen immediately and takes time to occur. Newer polymers are designed to last longer, be more durable, and take longer to degrade and turn yellow.
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Brominated compounds
The yellowing of plastic products is a result of exposure to sunlight, heat, and various environmental conditions, which cause the plastic to degrade over time. One of the primary causes of yellowing in plastics is the presence of bromine. Bromine is a chemical element commonly used as a flame retardant in plastic products, particularly in electrical and electronic goods. The bromine molecules are incorporated into the plastic during the manufacturing process, acting as a barrier to prevent the spread of fire.
However, bromine-containing plastics have a tendency to turn yellow over time due to a process known as photodegradation. Photodegradation is the breakdown of a material into smaller molecules due to exposure to light. When plastic products containing bromine are exposed to sunlight, the ultraviolet (UV) light in the sun’s rays breaks down the bromine molecules, causing the plastic to turn yellow. The yellowing of the plastic is a result of the bromine molecules breaking down into smaller molecules that are absorbed by the plastic, leading to a change in colour.
Heat is another factor that contributes to the yellowing of bromine-containing plastics. The heat generated by electronic devices or vehicles can cause the plastic to become brittle and yellow over time. The combination of heat and UV light accelerates the photodegradation process and increases the yellowing of the plastic. The degraded plastic becomes weaker and more brittle, reducing its strength and making it more prone to cracking or breaking.
While bromine is an effective flame retardant, its use in plastic products can have negative consequences over time, affecting both the appearance and performance of the product. Manufacturers and consumers should be aware of the potential issues associated with bromine-containing plastics and consider alternative materials that offer similar flame-retardant properties without the risk of yellowing. The use of bromine in plastic production has come under scrutiny due to its potential environmental and health impacts, leading to restrictions or bans in some countries.
It is important to note that the studies on the effects of bromine in plastic production are still ongoing, and more research is needed to fully understand the long-term implications.
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Degradation
Plastic turning yellow is a sign of degradation. Degradation is the process of something becoming worse in quality or condition over time. In the case of plastics, degradation is often due to exposure to UV light, which initiates a chemical reaction that causes the plastic to change colour. This process is known as photodegradation or UV degradation.
The susceptibility of plastic to degradation and yellowing is often related to its structure and composition. Polymers with aromatic groups, such as aromatic polyurethanes (PU), are more prone to UV damage and yellowing. This is because the UV light causes the formation of free radicals in the polymer backbone, which leads to the oxidation of the polymers. The oxidation process results in the plastic turning yellow and, eventually, flaking away until it becomes useless.
Other factors that can contribute to plastic degradation and yellowing include exposure to visible light, extreme temperatures, humidity, solvents, and tobacco smoke. Additionally, the presence of atmospheric pollutants such as oxides of nitrogen and NOx can react with antioxidants in the plastic, leading to discolouration.
To prevent plastic degradation and yellowing, manufacturers can add UV absorbers, hindered amine stabilizers, and light stabilizers to the plastic during processing. End-capping of polymer chains is also recommended for plastics used outdoors. For clear plastics like PET and PC, special care is required, and the addition of yellowing whitening chemicals can help solve the yellowing issue.
While degradation is a natural process that occurs over time, it can be slowed down or prevented by taking certain precautions. Storing plastic goods away from direct light exposure and controlling the temperature and humidity can help reduce the risk of degradation. Additionally, using newer polymers that are more durable and resistant to degradation can also prolong the lifespan of plastic products.
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Frequently asked questions
Plastic turns yellow due to exposure to UV light, which causes oxidation of the polymers that make up the plastic.
Other factors that can contribute to plastic turning yellow include exposure to visible light, extreme temperatures, humidity, tobacco smoke, and solvents.
Yes, by adding UV absorbers, hindered amine stabilizers, and light stabilizers to the plastic, the yellowing effect can be reduced.
Yes, a process called "Retr0brite" can be used to restore the plastic by adding hydrogen peroxide, which allows hydrogen to bond with the discolored molecules.
