The principle of ultraviolet degradation of three major categories of materials

　　UV sterilization is arguably the hottest topic in the LED industry this year, but some argue that while UV is effective, it can damage various materials. Addressing the issue of "UV-accelerated material aging" is a topic worth considering.
　　
　　In this article, Dr. Chen Haiying of Jingke Electronics, along with her R&D team, has conducted extensive research and reviewed a wide range of materials to explain how to prevent or mitigate the degradation of materials caused by ultraviolet radiation, providing insights from the principles of materials and degradation mechanisms. This information is offered for your reference.
　　
　　Background Knowledge of Ultraviolet Radiation
　　
　　To discuss ultraviolet radiation, we must first understand its background.
　　
　　Ultraviolet (UV) radiation, compared to visible light, has higher photon energy. High-energy photons can cause degradation in some materials, resulting in physical or chemical changes (this is why items left in the sun for extended periods quickly age and decompose).
　　
　　Ultraviolet C (UVC) radiation, with wavelengths between 200 and 280 nm, is not present in sunlight at ground level because wavelengths below 300 nm are absorbed by the ozone layer in the atmosphere. Therefore, publicly available research data on UVC-induced material degradation is scarce.
　　
　　With the widespread recognition of UVC's effectiveness in sterilization and disinfection, more and more products are designed to use UVC as a disinfection method. This necessitates understanding the principles of material degradation, including by high-energy UVC photons, and incorporating the effects of UV degradation into product material design to extend product lifespan.
　　
　　Principles of Ultraviolet Degradation of Three Material Categories
　　
　　1. Metals
　　
　　Metals are characterized by metallic bonding, which consists of tightly packed atoms arranged in a periodic crystal lattice structure, with all atoms sharing a delocalized electron "cloud." Due to their highly mobile electrons, metals are good conductors of electricity and heat and readily interfere with electromagnetic radiation, such as light and radio waves. This explains why metals are opaque and reflect a certain amount of light, as free electrons can absorb photon energy without undergoing energy transitions or bond dissociation. Therefore, metals are virtually unaffected by ultraviolet radiation.
　　
　　2. Ceramics
　　
　　Ceramic materials are formed through ionic bonding, with a periodically structured lattice containing positively and negatively charged ions. Most ceramics are metal oxides; a smaller number are nitrides, borides, and carbides with strong covalent bonds. Unlike metals, ceramic ions have tightly bound electrons, resulting in high bond strength, ability to withstand extreme temperatures, generally high chemical inertness, and good electrical insulation. This high bond strength and chemical inertness make ceramics completely unaffected by UV radiation.
　　
　　3. Quartz
　　
　　Amorphous silicon dioxide (SiO2) in materials exhibits both ionic and covalent bonding and is capable of transmitting UVC, making it very important to the UV industry. The main mechanism of UV absorption in quartz is related to impurities and defects. Impurities such as iron and other metals have electrons that can be promoted to higher energy levels or released from the atom, thus interfering with electromagnetic radiation, forming so-called "color centers," and reducing the UV transparency of the glass over time. Inherent atomic defects in quartz, such as unbonded silicon and oxygen atoms, also absorb some vacuum ultraviolet (VUV) and UV-C radiation.
　　
　　4. Polymers
　　
　　Polymers encompass a wide range of materials characterized by long molecular chains that are entangled and interconnected. They exhibit covalent bonding and typically contain carbon. Covalent bonds are the sharing of electrons between two or more atoms to satisfy the outermost electron orbitals of the constituent atoms. Compared to metallic bonds, the covalent sharing of electrons is localized (i.e., electron migration is limited to the nearest bonded atoms), so polymers are almost always electrical insulators and poor thermal conductors. Compared to metallic and ionic bonds, covalent bonds between organic components are also relatively weak. Therefore, most polymers are easily degraded by exposure to UV-C. High-energy photons have enough energy to promote electrons to higher energy levels, thus breaking covalent bonds and degrading the material. Polymers with carbon-carbon double bonds are generally more susceptible to UV degradation and chemical changes.
　　
　　In summary, polymeric materials are most affected by ultraviolet radiation. Let's discuss how UV radiation damages polymers and the mechanisms involved.
　　
　　How Ultraviolet Radiation Damages Polymers and the Mechanisms Involved
　　
　　1. How does UV radiation damage polymeric materials?
　　
　　The most basic and common mechanism of UV damage in polymers is called photo-scission, where high-energy photons directly break long chains into shorter ones, thus destroying the molecular "backbone." This degradation leads to deterioration of physical properties such as polymer strength and ductility, as well as changes in color and deterioration of texture and appearance. Polymer degradation can also release byproducts such as gases into the surrounding environment, causing pollution.
　　
　　2. What are the mechanisms of UV damage to polymers?
　　
　　The mechanisms by which polymers are damaged by UV radiation include free radical degradation and surface oxidation/hydrolysis. Free radicals are formed when chemical bonds break; these radicals react with other available bonds nearby, causing the polymer molecule to break or degrade. UV-dissociated bonds also readily react with oxygen or water, usually causing oxidative and hydrolytic degradation reactions on the surface. These two mechanisms combine and act synergistically, ultimately causing chemical and microstructural changes in the material.
　　
　　Principles of Ultraviolet Degradation of Three Material Categories
　　
　　3. Common manifestations of UV degradation in polymers:
　　
　　1. Yellowing and "chalking" of PVC pipes installed outdoors;
　　
　　2. Fading of colors in billboards and posters exposed to sunlight;
　　
　　3. Chalking and embrittlement of wire insulation exposed to sunlight;
　　
　　4. Sunburn is also a type of polymer degradation; skin is composed of polymers, particularly the protein collagen;
　　
　　5. UV radiation can also damage long polymer molecules of DNA/RNA; this UV-induced DNA/RNA damage is the basis of UV sterilization.
　　
　　How to prevent or mitigate UV degradation?
　　
　　1. Shielding and Coating
　　
　　Shielding ultraviolet (UV) radiation is an excellent protective method. This can be achieved using thin aluminum foil or other UV-impermeable materials. If simple shielding is not possible, coatings that absorb or reflect UV radiation can be used. Additives with this absorption or reflection function are commonly used in coatings. For example, some coatings containing metal particles are very effective UV barriers. High-performance coatings used outdoors often contain polyvinylidene fluoride (PVDF), which is beneficial for maintaining gloss and color. Applying a UV-stabilizing coating to the polymer surface effectively prevents UV damage to the material.
　　
　　2. Selecting UV-Resistant Polymer Materials
　　
　　Certain polymer materials have stronger UV resistance. Since C=C double bonds are particularly susceptible to UV photolysis, polymers with fewer C=C double bonds can be chosen. For example, polyolefins (polyethylene), and fluoropolymers such as polytetrafluoroethylene (PTFE), fluorinated ethylene propylene (FEP), and polyvinylidene fluoride (PVDF) are all good choices. These polymers containing carbon-fluorine bonds possess excellent properties, such as high-temperature stability, high dielectric strength, extremely high chemical inertness, and very good UV degradation resistance. Therefore, PTFE or FEP can be used for wire insulation in UV lamps or UV equipment.
　　
　　3. Utilizing UV-Absorbing Additives
　　
　　The first type is inorganic compounds, which are almost unaffected by UV irradiation. By adding inorganic fillers to the polymer material to absorb UV photons, UV stability is improved, thus reducing damage to the polymer bonds. The most common inorganic materials used for UV stabilization are carbon black and oxide ceramics, including aluminum oxide or titanium dioxide. These fillers also have advantages such as wear resistance, but the disadvantages are also quite obvious: they change the physical properties and color of the polymer, so they need to be used judiciously.
　　
　　The second type is organic additives, including antioxidants, UV absorbers, quenchers, and free radical scavengers. The mechanisms of these UV additives are as follows:
　　
　　a. UV absorbers—These molecules absorb UV light, converting it into heat or emitting it at longer wavelengths (fluorescence) to dissipate photon energy.
　　
　　b. Free radical scavengers—These molecules preferentially react with free radicals produced by photochemical or oxidative changes, reducing the likelihood of free radicals damaging the polymer chains.
　　
　　c. Organic and inorganic additives—Organic additives require much lower concentrations than inorganic fillers to achieve the same UV stability. In fact, such additives also contribute to high-temperature processability and antioxidant capacity, so they are usually added regardless of the expected UV exposure. However, some additives are expensive and can alter the properties and processability of certain polymers, and there is also a risk of environmental pollution.
　　
　　In summary, how can the degradation of materials by UV radiation be prevented or mitigated?
　　
　　First, good product design is crucial. By using simple shielding principles, UV exposure of sensitive and critical components can be minimized;
　　
　　Second, select good materials, preferably those with inherent UV resistance or those with suitable UV-resistant additives added to slow down material degradation.