Maya Ellison
It is a well-known phenomenon that plastics turn yellow over time. This colour change is usually attributed to increased exposure to ultraviolet (UV) light, which initiates a chemical reaction in the backbone of the polymer's structure, eventually resulting in a yellow tint. However, other factors such as visible light exposure, extreme temperatures, humidity, and solvents can also contribute to the degradation and discolouration of plastics. Understanding the underlying chemical processes involved in plastic degradation is crucial for the development of improved polymers that can resist colour changes and prolong the lifespan of plastic products.
| Characteristics | Values |
|---|---|
| Cause of yellowing | Exposure to ultraviolet (UV) light, visible light, extreme temperatures, humidity, solvents, and other environmental factors |
| Chemical changes | Oxidation of polymers, formation of free radicals, and disruption of polymeric backbone |
| Impact on plastic products | Discoloration, reduced lifespan, and potential failure or deterioration |
| Prevention and mitigation | Use of light stabilizers, improved polymer design, and controlled surface nanostructures |
Explore related products
UV light exposure
The colour change in plastics is often attributed to exposure to UV light. Ultraviolet (UV) light initiates reactions in the backbone of the polymer's structure, which eventually turn the plastic a yellow colour. The more exposed plastic is to UV light, the quicker it changes colour. However, this process is not instantaneous and requires prolonged and excessive exposure to UV light.
The chemical changes that occur in plastics due to UV light exposure have been observed in polyethylene, a commonly used plastic. Researchers have found that UV light can cause the formation of surface-based chiral nanostructures on polyethylene, leading to its yellowing over time. These chiral structures are directional and are not identical to their mirror images. Additional experiments revealed that most of the degradation during the yellowing process occurred on the surface of the polyethylene films.
The oxidation of polymers due to UV light exposure is another factor contributing to the colour change in plastics. This process, known as photodegradation, results in the formation of free radicals, which are atoms or groups of atoms with an odd number of electrons. While photodegradation is not reversible, the oxidation process can be addressed by adding hydrogen peroxide, which allows hydrogen to bond with the radicals, thus de-yellowing the plastic.
It is important to note that while UV light exposure is a significant contributor to the yellowing of plastics, other factors can also play a role. These factors include exposure to visible light, extreme temperatures, humidity, and solvents, which can also initiate the colour change in plastics over time.
Plastic Garden Sheds: Worth the Hype?
You may want to see also
Explore related products
Oxidation of polymers
The process of oxidation in polymers, also known as oxidative degradation, involves the disintegration of macromolecules due to oxygen exposure. This process is initiated by either physiological or external environmental factors, such as UV light, heat, or mechanical stress, and can lead to surface attacks and chemical changes in the polymer.
One of the critical aspects of oxidative degradation is the formation of free radicals. These highly reactive species can be generated through various mechanisms, such as the photolysis of hydroperoxides, which then react with oxygen to produce oxy- and peroxy-radicals. These radicals can further participate in multiple reactions, including reacting with each other or removing hydrogen from polymer chains. This leads to changes in the molecular weight and distribution of the polymer, resulting in increased brittleness and mechanical failure.
The rate of oxidation in polymers is influenced by several factors, including the presence of double bonds in the macromolecule. An increase in the number of double bonds enhances the stability of certain polymers, such as polyolefins. Additionally, the chemical structure of the polymer plays a significant role in determining its susceptibility to oxidation. Some polymers, like fluoropolymers, polyimides, silicones, and specific acrylate polymers, exhibit excellent stability against oxidative degradation.
One specific type of oxidative degradation is photo-oxidation, which is the degradation of a polymer surface due to the combined action of light and oxygen. Photo-oxidation is considered the most significant factor in the weathering of plastics. It causes the breakage of polymer chains, leading to increased material brittleness and, eventually, the formation of microplastics. Technologies have been developed to both accelerate and inhibit photo-oxidation, depending on the desired outcome. For example, in the case of single-use plastics, biodegradable additives are used to accelerate fragmentation.
Another form of oxidative degradation is thermal oxidation, which occurs when oxygen interacts with the polymer at high temperatures. This type of oxidation can extend throughout the material, causing the formation of hydroxyl, carbonyl, aldehyde groups, or peroxides along the polymer chain. Thermal oxidation results in significant reductions in the mechanical properties and utility of the polymer due to secondary cross-linking and molecular chain disruption.
Transforming Plastics: Hydrocarbon Conversion for a Greener Future
You may want to see also
Explore related products
Extreme temperatures
The colour change in plastics is a result of the degradation of polymer chains. When plastics are exposed to extreme heat, the process of thermal oxidation is disrupted, leading to the production of “yellow bodies” that impart a yellow coloration. Additionally, during manufacturing, extreme temperatures can initiate the degradation process by breaking and rearranging chemical bonds along the polymer chain.
To counteract the effects of extreme temperatures, process stabilizers and long-term stabilizers, such as sterically hindered phenols, are added to the polymer. However, these anti-oxidants can exhibit an undesirable side effect when exposed to ultraviolet radiation, resulting in the irreversible yellowing of the plastic.
Therefore, extreme temperatures play a significant role in the yellowing of plastics, and understanding this process is crucial for developing alternatives that can avoid discoloration and extend the lifespan of plastic products.
Plastic's Ancient Origins: 1800s and Beyond
You may want to see also
Explore related products
Humidity
Plastics are made of polymers, and polymers are chemically distinct from one another, which affects how they behave in use. Some polymers are more prone to UV damage than others. Aromatic polyurethanes (PU), for example, turn yellow by free radical pathways occurring in the backbone of the polymer. Over time, light or heat-induced oxidation forms yellow-coloured quinones.
The susceptibility of plastic to yellow or degrade is often related to its structure. The more exposed plastic is to UV light, the quicker it changes colour. However, the change in colour does not happen immediately. The increase in UV light exposure initiates and accelerates a chemical reaction in the backbone of the polymer's structure, which will eventually turn the plastic yellow.
Similarly, humidity, along with heat and oxygen, can also initiate this chemical reaction. Polyesters, like polyethylene terephthalate (PET) and polycarbonates (PC), turn yellow when used outdoors due to their sensitivity to UV radiation, and this sensitivity is doubled in the presence of heat, moisture, and oxygen.
Therefore, humidity plays a role in causing plastic to turn yellow by interacting with the polymer's structure and accelerating its degradation.
Hand Sanitizer: Killing Coronavirus on Plastic?
You may want to see also
Explore related products
Nanostructures
The phenomenon of plastics turning yellow has been attributed to the formation of molecules that act as dyes. However, the actual chemical changes that take place have remained unexplained for a long time. Recently, researchers have identified surface-based chiral nanostructures as the potential cause of this colour change.
For one of the most commonly used plastics, polyethylene, it has long been suggested that ultraviolet (UV) light initiates reactions in the backbone of the polymer's structure, leading to the yellowing of plastics. However, the chemical changes observed in polyethylene's polymeric backbone after UV exposure could not fully explain the yellowing. To address this discrepancy, researchers explored the possibility that nanostructures formed unintentionally by UV light might be responsible for the colour change.
Margaret M. Elmer-Dixon, Melissa A. Maurer-Jones, and their colleagues investigated whether nanostructures formed by UV light could be the cause of polyethylene's yellowing. They examined the interaction of potential structures on yellowed polyethylene films with circularly polarized light, which has right- or left-handed wave rotation. The amount of circularly polarized light absorbed by the film varied depending on its orientation, indicating that the yellowed plastic contained new chiral chemical structures.
Additional experiments revealed that most of the degradation during film yellowing occurred on the surface of the films. The researchers concluded that chiral chemical structures formed on the surfaces of polyethylene films during UV exposure, contributing to the yellowing of old plastics. These findings could aid in the development of more durable plastic products that resist unsightly discolouration.
The Color of Raw Polyethylene Terephthalate Plastic
You may want to see also
Frequently asked questions
Plastics turn yellow due to excess exposure to UV light, which initiates a chemical reaction in the polymer's backbone structure, causing it to change colour.
Increased exposure to visible light, extreme temperatures, humidity, and solvents can also cause the polymer to eventually fail and change colour.
Polyethylene, a commonly used plastic, is known to turn yellow over time. Polyurethane, used in medical tubing, has also been observed to turn yellow prematurely.
Yes, the oxidation process can be reversed by adding hydrogen peroxide, which allows hydrogen to bond with the free radicals formed during oxidation, thus de-yellowing the plastic.
