Ethan Clarke
Have you ever wondered why some of your old white plastic items have turned yellow? Well, you're not alone. Many people have noticed this strange phenomenon, and scientists have been working to understand the root causes of this colour change. The yellowing of plastics is usually due to increased exposure to ultraviolet (UV) light, but other factors such as extreme temperatures, humidity, and even solvents can also contribute to this change. This is because UV light initiates a chemical reaction in the polymer backbone, causing the plastic to eventually turn yellow. The good news is that by understanding these root causes, we can develop new polymers or alternative materials that are more resistant to yellowing and create plastic products that last longer.
| Characteristics | Values |
|---|---|
| Cause of yellowing | Excess UV light exposure |
| Exposure to visible light, extreme temperatures, humidity, and solvents | |
| Formation of chiral chemical structures on the surface of polyethylene films | |
| Impact | Change in colour |
| Plastic becomes brittle, cracks, breaks, shatters, or melts | |
| Prevention | Use of UV-resistant topcoats |
| Use of light stabilizers | |
| Reversal | Sanding down the plastic and sealing with a topcoat |
| Use of peroxide |
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UV light exposure
The yellowing of white plastic is often attributed to excess exposure to UV light. This phenomenon is observed in various plastic items, from storage boxes to old video game consoles and white appliances. The process of yellowing is not instantaneous and requires prolonged exposure to UV light, which initiates and accelerates a chemical reaction in the polymer backbone, eventually turning it yellow.
The susceptibility of plastic to yellowing or degradation is influenced by its structure. Polymers and epoxies containing aromatic groups, such as aromatic polyurethanes (PU), are particularly vulnerable to UV damage. Over time, exposure to light or heat induces oxidation, forming yellow-colored quinones. This process is amplified in the presence of heat, moisture, and oxygen.
To mitigate the effects of UV light exposure, manufacturers can incorporate UV absorbers and hindered amine stabilizers into the plastic composition. These additives enhance weatherability and provide protection against UV light-induced oxidation. Additionally, light stabilizers and whitening chemicals can be introduced to address the yellowing issue. However, it is important to carefully select the appropriate additives to avoid unintended coloration or damage to the plastic.
While UV light exposure is the primary culprit, other factors can contribute to the yellowing of white plastic. These factors include increased exposure to visible light, extreme temperatures, humidity, and solvents. Therefore, it is advisable to be cautious about where plastic goods are stored and to consider using UV-resistant coatings or applying protective treatments to prevent and reduce yellowing.
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Visible light exposure
Prolonged exposure to light, particularly the violet and blue portions of the visible spectrum, can cause white plastics to yellow. This process is known as photodegradation or photo-oxidation and primarily affects plastics that are not protected with UV inhibitors or stabilizers.
When plastic is exposed to visible light, particularly the higher-energy violet and blue wavelengths, photons can interact with the chemical bonds in the polymer chains that make up the plastic. This interaction can energize and excite the electrons in the polymer, causing them to
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Extreme temperatures
The effect of temperature on the discolouration of plastics is also influenced by the quality of the plastic. Poor quality plastics are more prone to yellowing due to their inability to withstand environmental factors. Higher quality plastics, such as those made from pure acrylic, are more resistant to yellowing but can still turn yellow over time when exposed to extreme temperatures.
Additionally, the manufacturing process of plastics can also contribute to their susceptibility to yellowing. High-temperature exposure during manufacturing can increase the likelihood of yellowing in plastics. This is particularly relevant for plastics with aromatic groups in their structure, such as aromatic polyurethanes, which are commonly used in textiles.
To prevent yellowing due to extreme temperatures, it is essential to control the temperature around plastic products. This includes avoiding exposure to high temperatures, such as direct sunlight, or proximity to heat sources like ovens or radiators. Proper storage and temperature control can help slow down the degradation process and maintain the original colour of the plastic.
Furthermore, understanding the chemical composition of plastics is crucial in addressing the issue of yellowing. The presence of impurities or inadequate amounts of additives can accelerate discolouration. By improving the formulation of plastics and selecting appropriate additives, manufacturers can develop plastics that are more resistant to extreme temperatures and less prone to yellowing over time.
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Humidity
In addition to humidity, the quality of the plastic mixture also plays a role in its susceptibility to yellowing. Lower-quality plastics are more prone to discolouration due to their chemical composition and manufacturing processes.
To prevent or slow down the yellowing of plastic caused by humidity, it is essential to store plastic items in dry, well-ventilated areas. Avoiding exposure to direct sunlight or extreme temperatures can also help maintain the original colour of the plastic.
If the plastic has already turned yellow due to humidity, there are a few methods to restore its original colour. One approach is to use hair products containing peroxide, a chemical known for its bleaching properties. Placing the yellowed plastic in a plastic bag with peroxide-based hair products and leaving it in a warm place can help activate the peroxide and gradually eliminate the yellowing. Another method involves sanding down the plastic to remove the discoloured surface layer and then sealing the lower, whiter layers with a topcoat to prevent further discolouration.
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Solvents
The effect of solvents on plastics is similar to the impact of increased exposure to visible light, extreme temperatures, and humidity. These factors can cause the polymer structure of plastics to break down, resulting in discolouration and a change in the physical properties of the plastic. For example, exposure to solvents can make plastic more brittle or prone to cracking.
In some cases, the use of solvents may be intentional to create specific plastic products. For instance, researchers have experimented with creating nano-sized "supramolecular" structures on the surfaces of plastics to modify their colour and interaction with light. These structures are designed to impact the properties of plastics in a controlled manner.
However, in most cases, exposure to solvents is unintentional and can occur during the use or storage of plastic items. For example, plastic items stored in areas with high solvent concentrations, such as paint thinners or cleaning products, may be more susceptible to discolouration and degradation.
To prevent solvent-induced yellowing, it is essential to understand the specific solvents that can cause this effect and to store plastic items away from these solvents. Additionally, the development of new polymers or alternative materials that are more resistant to solvents and discolouration is an ongoing area of research.
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Frequently asked questions
Prolonged exposure to ultraviolet (UV) light causes plastics to yellow over time.
Polyesters, like polyethylene terephthalate (PET) and polycarbonates (PC), are sensitive to UV radiation from the sun and artificial sources. Aromatic polyurethanes (PU) are also prone to UV damage.
Increased exposure to visible light, extreme temperatures, humidity, and solvents can also cause plastic to yellow.
To prevent yellowing, plastic can be treated with UV absorbers, hindered amine stabilizers, and light stabilizers. Additionally, storing plastic items out of direct sunlight can slow down the yellowing process.
