Omar Khalid
Ultraviolet (UV) radiation, consisting of photons with high energy relative to visible light, can cause degradation in susceptible materials. UV light can cause physical and chemical changes in plastics, resulting in colour changes and reduced strength. This degradation is a concern for manufacturers and users of products that are intended for outdoor use and storage, as they will be exposed to sunlight. While some plastics have good UV resistance, others are highly susceptible to degradation by UV light. UV-C light, in particular, is extremely aggressive and can break down plastic over time, turning it yellow or causing it to crack and disintegrate.
| Characteristics | Values |
|---|---|
| UV light effect on plastics | UV light can cause degradation in plastics, resulting in physical and chemical changes. |
| UV-resistant plastics | Polyester, PTFE, PVDF, FEP, PEEKTM, Polyimide (PI), and Polyetherimide (PEI) are plastics with good to excellent UV resistance. |
| UV degradation prevention | Using stabilizers, absorbers, blockers, or antioxidants can help prevent UV degradation in plastics. Carbon black, titanium dioxide, benzophenones, and benzotriazoles are effective additives. |
| UV-C light effect on plastics | UVC light is highly damaging to plastics and can cause disintegration. It breaks bonds in plastics and generates ozone, which attacks organic materials. |
| UV degradation consequences | Degradation can result in changes in colour, texture, strength, and ductility. It can also release by-products into the environment. |
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What You'll Learn
UV-resistant plastics
Plastics are prone to UV damage, which causes photodegradation, or the breakdown of chemical bonds in polymers, leading to disintegration. This results in discoloration, loss of strength, and increased brittleness.
When choosing a plastic for a project, it is essential to consider the level of UV resistance required. Plastics used for outdoor applications and industrial applications generally require higher UV resistance to maintain optimal quality and performance.
While almost no material is completely UV-resistant, some plastics offer better resistance than others. Here are some examples of UV-resistant plastics:
- Acrylic is inherently UV-resistant, with excellent scratch resistance and transparency. However, it is not very strong and is brittle, making it unsuitable for load-bearing parts.
- Polyester is a good candidate for UV exposure, offering greater UV resistance than polypropylene.
- HDPE (high-density polyethylene) is a thermoplastic used in playground equipment and open-air infrastructure. It is durable yet soft, easy to mold, and resistant to acids, alcohols, and bases. However, untreated HDPE becomes very weak after sun exposure.
- Polycarbonate is impact-resistant, strong, and offers good transparency and temperature tolerance. It is ideal for heavy-duty products like helmets and safety goggles but is more expensive.
- PAI (polyamide-imide) is a high-performance thermoplastic with exceptional UV resistance and good malleability.
- Fluoropolymers exhibit outstanding dimensional stability, mechanical properties, and temperature range. They perform well in extreme temperatures and harsh chemical environments.
- EPDM is not a plastic but is often used in roofing as it withstands UV radiation well. It forms a layer of ash on its surface that prevents further deterioration.
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Degradation by UV light
Ultraviolet (UV) radiation consists of photons with high energy relative to visible light. UV radiation can cause degradation in susceptible materials, resulting in physical and chemical changes. This degradation is of particular concern for materials intended for outdoor use and storage, as they will be exposed to sunlight.
UV-C, a subclass of UV radiation with wavelengths between 200 and 280 nm, is not present in terrestrial sunlight as it is absorbed by the ozone layer. Therefore, data on the degradation caused by UV-C is limited. However, UV-C lamps are used for disinfection and photochemical equipment, so understanding their effects on materials is important.
UV-C light is highly damaging to plastics. Exposure to UV-C can cause the plastic to turn yellow, crack, or even disintegrate. This is because UV-C breaks the bonds in the plastic, and the resulting ozone also degrades the material. The damage is cumulative, so repeated exposure to UV-C light will lead to increased degradation over time.
Some plastics are more susceptible to degradation by UV light than others. Polypropylene, for example, has a high degradation rate when exposed to UV light, while polyester exhibits greater UV resistance. The presence of impurities can also act as receptors for UV radiation, causing degradation.
There are methods to avoid UV degradation in plastics, such as using UV-resistant coatings or adding stabilizers, absorbers, or blockers to the plastic. For instance, carbon black can be added to provide protection through the blocking process, and organic compounds like benzophenones and benzotriazoles can selectively absorb UV radiation and re-emit it as heat.
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Photons and photo-oxidation
Photodegradation is the degradation of a photodegradable molecule caused by the absorption of photons, particularly those wavelengths found in sunlight, such as infrared radiation, visible light, and ultraviolet light. Photodegradation includes photodissociation, the breakup of molecules into smaller pieces by photons, and the change of a molecule's shape to make it irreversibly altered. A common photodegradation reaction is oxidation.
In polymer chemistry, photo-oxidation is the degradation of a polymer surface due to the combined action of light and oxygen. It is the most significant factor in the weathering of plastics. Photo-oxidation causes the polymer chains to break, resulting in the material becoming increasingly brittle. This leads to mechanical failure and, at an advanced stage, the formation of microplastics.
UV radiation causes photooxidative degradation, which results in the breaking of the polymer chains, producing radicals, and reducing the molecular weight, causing deterioration of mechanical properties and leading to useless materials. Polyolefins such as polyethylene and polypropylene are susceptible to photo-oxidation. Around 70% of light stabilizers produced worldwide are used in their protection, despite representing only around 50% of global plastic production.
The photo-oxidation of plastics can be investigated through natural or accelerated weather testing. Natural weather testing involves exposing polymer samples to open weather for a continuous period, while accelerated weather testing uses a specialized test chamber that simulates weathering by sending a controlled amount of UV light and water to a sample. Through weather testing, the impact of photooxidative processes on the mechanical properties and lifetimes of polymer samples can be determined.
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UV-C light
Some plastics, such as PET, PP, HDPE, and PA6, are considered to have fair resistance to UV rays, while others like POM, PC, ABS, and PA6/6 are regarded as having unacceptable resistance. Polyimide (PI), used in the Hubble Space Telescope, and Polyetherimide (PEI) are the only plastics found to have excellent resistance to UV rays. PTFE, a fluoropolymer, also exhibits good UV resistance due to its strong carbon-fluorine (C-F) bond.
To prevent UV degradation in plastics, additives such as stabilizers, absorbers, or blockers can be used. For outdoor applications, carbon black or pigments like titanium dioxide can be added to block UV light. Organic compounds like benzophenones and benzotriazoles can also be used as absorbers, selectively absorbing UV light and re-emitting it as heat. Additionally, a UV-resistant coating, such as varnish, can be applied to the surface of plastics to provide protection from UV-C light.
It is important to note that UV-C light is dangerous and can cause severe eye irritation and respiratory issues if not used properly. Therefore, it should be handled with caution and appropriate protective measures.
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UV-resistant coatings
Plastics are sensitive to UV light, which can cause degradation and discolouration. This is particularly true in the case of automotive parts, which are often exposed to sunlight. To combat this, UV-resistant coatings can be applied to plastics to prevent UV damage.
NEI Corporation's NANOMYTE® UVP coatings, for example, block UV radiation and protect surfaces by preventing the penetration of UV light. This preserves the material's strength, coating adhesion, and appearance. NEI's coatings are available in liter and gallon quantities and can be applied by dipping, brushing, or spraying. They also offer in-house coating services and custom formulations to meet specific customer requirements.
AI Technology, Inc.'s SOLARBLOC™ SC7050-UVB and SC7130-UVB coatings provide protection against UV, salt-fog, and water exposure. These coatings are designed for large industrial applications but are also suitable for DIY projects. They can be applied to existing coatings to provide additional protection and reduce maintenance.
When choosing a UV-resistant coating, it is important to consider the specific plastic material and its intended application. Some plastics, such as polypropylene, have a high degradation rate when exposed to UV light, so a more resistant material may be necessary. Additionally, pretreatment of the plastic surface may be required to promote adhesion between the coating and the substrate.
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Frequently asked questions
UV light, or ultraviolet light, is a type of radiation with a shorter wavelength than visible light. It can be further divided into UVA, UVB, and UVC.
UV light can cause degradation in plastics through physical and chemical changes. The high-energy photons in UV light can break the chemical bonds in polymers, reducing their molecular weight and leading to a loss of strength and changes in colour and texture. This process is known as chain scission by photolysis. Additionally, UV light can create free radicals, which can further degrade the plastic through photo-oxidation.
Yes, some plastics have good resistance to UV light, such as PET, PP, HDPE, PA12, PA11, PA6, PES, PPO, PBT, and PPO. Polymers extruded by Zeus, such as PTFE, PVDF, FEP, and PEEKTM, also show good resistance. The best resistance is found in imides, such as Polyimide (PI) and Polyetherimide (PEI).
There are several ways to avoid UV degradation in plastics, including the use of stabilizers, absorbers, or blockers. Carbon black, titanium dioxide, benzophenones, and benzotriazoles can be added to plastics to provide protection by blocking or absorbing UV light and re-emitting it at a less harmful wavelength. Additionally, UV-resistant coatings, such as varnish, spray-on films, or paint-on films, can be applied to the surface of plastics to protect them from UV light damage.
