The principle of ultraviolet degradation of three kinds of materials

The most popular word in the LED industry this year is estimated to be UV ultraviolet sterilization, but there are also voices that UV ultraviolet is very good, but it has certain damage to a variety of materials. How to deal with "ultraviolet accelerated material aging" is also a topic worth thinking about.
　　
In this article, Dr. Chen Haiying of Jingke Electronics led the research and development product team through a series of research and access to a variety of data, will explain how to prevent or reduce the degradation of materials by ultraviolet rays from the aspects of the principle and degradation mechanism of materials, for your reference.
　　
Background knowledge of UV
　　
When it comes to UV, you need to talk about the background knowledge of UV.
　　
Ultraviolet (UV) relative to visible light, photon energy is higher, high-energy photons may cause degradation of some materials, resulting in physical or chemical changes (this is the article long-term exposure to the sun will soon be aging, decomposition of the truth).
　　
Among them, the subclass UVC of ultraviolet radiation with wavelengths of 200 to 280 nm does not exist in the sunlight on the ground, because the wavelength light below 300 nm will be absorbed by the ozone layer in the atmosphere, so there has been little published research data on UV-C degradation materials.
　　
As the sterilization and disinfection effect of UVC is widely recognized, more and more products will be designed to use UVC as a disinfection scheme. At this time, we need to understand the principle of material degradation including high-energy ultraviolet UV-C photons, and take the influence of ultraviolet degradation into consideration when designing product materials to prolong the service life of products.
　　
The principle of ultraviolet degradation of three kinds of materials
　　
1. Metal
　　
Metals are characterized by metal bonding, which consists of closely packed atoms arranged in a periodic lattice structure, all sharing a "cloud" of delocalize electrons ". Because of their highly mobile electrons, metals are good conductors of electricity and heat, and are prone to interference with electromagnetic radiation, such as light and radio waves. This explains why metals are opaque and reflect some degree of light, because free electrons can be used to absorb photon energy without undergoing energy transitions or bond dissociation, so metals are almost completely immune to ultraviolet light.
　　
2. Ceramics
　　
Ceramic materials are formed by ionic bonding, with a periodic structural arrangement of lattices containing positively and negatively charged ions. Most ceramics are metal oxides, and a small number of ceramics are nitrides, borides and carbides with strong covalent bonds. In contrast to metals, ceramic ions have tightly bound electrons, so they have high bonding strength, can withstand extreme temperatures, are usually extremely chemically inert and are good electrical insulators. This high bonding strength and chemical inertness make the ceramic completely unaffected by UV irradiation.
　　
3. Quartz
　　
Amorphous silicon dioxide (SiO2) is a material that exhibits ionic and covalent bonding and can penetrate UVC, which is very important for the UV industry. The main mechanism of UV absorption in quartz is related to impurities and defects in it. Impurities in metals such as iron, whose atoms can be lifted to higher energy levels or released from the atoms, can be used to interfere with electromagnetic radiation, forming so-called "color cores" and reducing the glass's UV transparency over time. There are also inherent atomic defects in quartz, such as unbonded silicon and oxygen atoms, which absorb some vacuum ultraviolet (VUV) and UV-C.
　　
4. Polymers
　　
Polymers comprise a variety of materials characterized by long molecular chains, molecular chains entangled and interconnected, which themselves exhibit covalent bonds, usually containing carbon components. A covalent bond is the sharing of electrons between two or more atoms to satisfy the constituent atoms to fill their outermost electron orbitals. In contrast to metallic bonds, the covalent sharing of electrons is localized (I. e., electron migration is restricted to the nearest bonded atom), so polymers are almost always electrical insulators and poor thermal conductors. Covalent bonds between organic components are also relatively weak compared to metallic and ionic bonds. Therefore, most polymers are easily degraded by exposure to UV-C. High-energy photons have enough energy to promote electrons to higher energy levels, thereby breaking covalent bonds and degrading materials. In general, polymers with carbon-carbon double bonds are more susceptible to UV degradation and chemical changes.
　　
To sum up, the most affected by ultraviolet rays is the polymer material, the following to talk about the performance and mechanism of ultraviolet damage polymer.
　　
Manifestations and Mechanisms of UV Destruction of Polymers
　　
1. How does UV damage polymer materials?
　　
The most basic and prevalent UV damage mechanism in polymers is called photolytic chain scission, the breaking of long chains into shorter chains by the direct action of high-energy photons, thus breaking the "backbone" of the molecule ". This degradation almost results in deterioration of the physical properties of polymer strength, ductility, and deterioration of texture appearance such as color change. Degradation of the polymer may also release byproducts such as gases into the surrounding environment causing contamination.
　　
2. What is the mechanism of UV damage to polymers?
　　
The mechanisms by which polymers are damaged by UV light include free radical degradation and surface water and oxygen degradation. Free radicals formed when chemical bonds are broken. These free radicals will react with other available bonds nearby and cause polymer molecules to break or degrade. The UV-dissociated bonds are also susceptible to reaction with oxygen or water, typically causing oxidative and hydrolytic degradation reactions at the surface. These two mechanisms combine and act synergistically, ultimately causing chemical and microstructural changes in the material.
　　
The principle of ultraviolet degradation of three kinds of materials
　　
3. Common manifestations of polymer degradation by ultraviolet light:
　　
1. Yellowing and "chalking" of outdoor installed PVC pipes ";
　　
2. The fading of the color of billboards and posters exposed to the sun;
　　
3. Pulverization and embrittlement of wire insulation exposed to sunlight;
　　
4. Skin sunburn is also a kind of polymer degradation, the skin is composed of polymers, especially collagen protein;
　　
5. UV rays can also cause damage to the long polymer molecules of DNA/RNA, and these UV-induced DNA/RNA damage is the basis for UV disinfection.
　　
How to prevent or mitigate UV degradation?
　　
1. Shielding and coating
　　
Shielding ultraviolet rays is a good protection method, such as shielding with thin aluminum foil or other materials that do not transmit ultraviolet rays. If simple shielding is not possible, you can choose to use a coating that absorbs or reflects ultraviolet rays. For example, some coatings containing metal particles are very effective UV barriers, and high-performance coatings for outdoor use usually contain polyvinylidene fluoride (PVDF), which has a good effect on light retention and color retention. By coating a UV-stabilized coating on the polymer surface, it is possible to prevent UV damage to the material.
　　
2. Choose UV resistant polymer materials
　　
Certain polymer materials have a stronger resistance to ultraviolet light. Since C = C double bonds are particularly susceptible to UV photolysis, we can choose polymer materials with less C = C double bonds, such as polyolefins (polyethylene), fluoropolymers such as polytetrafluoroethylene (PTFE), fluorinated ethylene-propylene (FEP) and polyvinylidene fluoride (PVDF) are good choices. These polymers containing carbon-fluorine bonds possess excellent properties such as high temperature stability, high dielectric strength and very high chemical inertness and very good resistance to UV degradation. Thus, PTFE or FEP can be used for wire insulation in UV lamps or UV devices.
　　
3. Use some additives that can absorb ultraviolet rays
　　
The first is an inorganic compound, which is hardly affected by ultraviolet radiation. Inorganic fillers are added to the polymer material to absorb ultraviolet photons and improve ultraviolet stability, thereby reducing damage to polymer bonds. The most common inorganic materials used for UV stabilization are carbon black and oxide ceramics, including alumina or titanium dioxide. Such fillers also have the advantage of wear resistance, but the disadvantage is quite obvious, that is, they will change the physical properties of the polymer and its color, so it needs to be used in a trade-off.
　　
The second is organic additives, including antioxidants, UV absorbers, quenchers and free radical scavengers. The mechanism of action of these UV additives is as follows:
　　
a. Ultraviolet Absorbers-These molecules absorb ultraviolet light, convert it to heat or emit at longer wavelengths (fluorescence) to dissipate photon energy.
　　
B. Free Radical Scavengers-These molecules will preferentially react with free radicals generated by photochemical or oxidative changes to reduce the chance of free radicals damaging the polymer chain.
　　
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 oxidation resistance at the same time, so they are usually added regardless of the expected UV exposure. However, some additives are expensive and change the properties and processability of certain polymers, and there is also a risk of environmental pollution.
　　
In summary, how to prevent or reduce UV degradation of materials?
　　
One is to have a good design of the product, through a simple shielding principle to minimize the UV exposure of sensitive and critical components;
　　
The second is to choose a good material, preferably an inherent anti-ultraviolet material or add a suitable anti-ultraviolet additive to slow down the rate of material degradation.