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This presentation is mainly about the prevention of damage caused by uv radiation. But uv radiation is part of a general phenomenon, which is the called the weathering of materials that are exposed to an atmosphere. The factors that influence the weathering are of course uv radiation as one of the most important factors to cause degradation, but also heat, moisture from dew or rain, salt water, and pollutants have an influence on the behaviour of materials that are used outside. Of course these factors vary according to the exposure site. Mostly the weathering occurs because of a combination of these influences. But some materials are more sensitive to some factors than others. Polymers for example are known to be largely influenced by uv radiation. Some factors can also have a synergistic effect on materials, such as radiation and water. Looking at these weathering influences, you can see that exposure to oxygen or heat is difficult to avoid, but exposure to uv radiation can be diminished. And that is what this presentation is about.
Ultraviolet radiation, sometimes called ultraviolet light, is invisible electromagnetic radiation of the same nature as visible light, but having shorter wavelengths and higher energies. UV radiation is conventionally classified into 3 bands in order of increasing energy: UV-A (320–400 nm), UV-B (280–320 nm) and UV-C (100–280 nm). This division corresponds broadly to the effects of UV radiation on biological tissue. The main source of natural ultraviolet radiation is the sun. About 1000Wm2 of the incident solar irradiation reaches the Earth’s surface without being significantly scattered. Most of this radiation is infrared light (55%) and visible light (40%), and approximately 5% of the ground-level solar radiation is ultraviolet radiation, mostly in the UV-A range. UV-B radiation is strongly absorbed by stratospheric ozone. Solar UV-C radiation does not reach the Earth’s surface because it is absorbed by ozone and other gases in the atmosphere.
The loss of strength, impact resistance, and mechanical integrity of polymers exposed to UV light is well known. At the same time, polymers progressively lose their coloration and acquire a yellowish tint after prolonged exposure to UV light. The automotive industry employs a large number of plastic parts in a car, which are exposed for very long periods of time to direct solar radiation. There is a great interest in protecting these materials. Ultraviolet light is the main factor responsible for the degradation of wood. Exposure to UV has disintegrated or damaged many cultural monuments.The obelisk of Thutmosis I was covered by sand until the 19th century below the white point marked on the right side of the column. It is visible that stone has different colors above and below the point, even though stone is extremely resistant to UV radiation (Egypt does not have snow and has almost no rain). Sand has protected stone against degradation for many centuries. Artwork in museums suffers an important photodegradation, as art pieces are exposed for large periods of time to natural and artificial illumination. UV radiation is especially harmful to libraries and archival materials as it leads to the weakening and embrittlement of cellulose fibres and causes paper to bleach, yellow or darken, depending upon its constituents
Common organic materials, such as most plastics, polymers, wood, etc., absorb ultraviolet radiation and undergo a rapid photolytic and photo-oxidative reaction that results in their photodegradation. The energy of the photons in the ultraviolet region (290–400 nm) is sufficient to break chemical bonds in polymers, wood, paper and other organic based materials, resulting in the formation of free radicals. A photochemical reaction occurs as a result of activation of a molecule or polymer by light to its excited singlet (S*) and/or triplet (T*) states. Pure degradation, consisting of chain scission and crosslinking, occurs only in inert atmosphere, however, in the presence of air, polymers and other organic materials undergo a photo-oxidative degradation. The degradation of these materials can be described as follows: The free radicals produced by the effect of UV light react with other molecules of the polymer to form oxy and peroxy radicals, resulting in chain scission. The reaction continues until two free radical react with each other, forming stable non-radical compounds. The main reactions occurring during the photodegradation of polymers are given in Table 1.
The basic possibilities for stabilizing polymers against uv-radiation is based on: Uv absorbers or screeners: they absorb uv light in competition with the chromophores which are part of the polymer backbone or impurities. The aim is to prevent chromophores from being transferred in their excited states from which radicals can be formed. Quenchers destroy the excited state of chromophores resulting in the chromophore going back to its grand state. A quencher accepts the energy of the chromophore and releases the energy as harmless heat or radiation. Radcial scavengers destroy already formed radicals and interrupt the chain reaction of polymer degradation Peroxide decomposers specifically target the destruction of peroxide radicals These stabilizers can be used by mixing them in the material itslef or in a coating to be applied on the material. To be included in the coating, they should dissolve well in solvents used for coatings or have a melting point close to the extrusion temperature for powder coatings I will discuss these stabilisers now in more detail.
Harmful uv light can be absorbed by different organic molecules, called uv absorbers. The uv absorbers dissipate the absorbed energy in a way which is harmless for the polymer. The technically most important uv absorber classes are benzotriazoles, triazines, benzophenones and oxalic anilides. Each of the mentioned uv absorbers has a characteristic absorption spectrum. At the wavelength where the polymer has a maximum sensitivity, the uv absorber needs to be able to absorb well. It is important that the uvabsorber stops absorbing before 400 nm otherwise color formation will occur. Sometimes also a reaction can occur with other components of the coating, resulting in yellowing. This should be avoided. From the absorbers indicated in the graph, the benzotriazoles give the broadest protection, followed by the triazines. Therefore these classes of uv absorbers are of the greatest technical importance. Tailor made protection is also provided by combining different uv absorber classes. For the protection of a substrate, the Lambert Beer law is important to consider. It says that the extinction E depends on the concentration, thickness of the coating and extinction coefficient of the uv absorber. Usually low concentrations are used (a few %), the coating thickness is also not so thick, therefore the extinction coefficiatient of the absorber should be quite high compared to that of the chromophore, in order to provide enough protection. The absorber should thus absorb uv light better and faster than the substrate it is supposed to protect. It must dissipate the absorbed energy quickly and it must be able to run this cycle repeatedly.
The uv absorbers thus absorb the uv-light instead of the substrate and thus provide protection to the substrate. But why don’t they degradate themselves? Their fotostability is related to an efficient mechanism of rapid dissipation of the absorbed radiation via an appropriate intramolecular rearrangement, called ESIPT (excited state intramolecular proton transfer). After UV absorption, the molecule is excited to the S1 energy level, from where there is a proton transfer to a lower excited energy level S1’ occurring in the femtosecond timescale. From this state, the energy is lost as thermal energy, and a back proton transfer causes a return to the ground state phenol. This mechanism occurs for uv absorbers that have a phenolic group that forms O-H-O bridges or O-H-N bridges
Also inorganic oxide particles are widely used for protection against uv radiation. They are more and more used because they are more stable against thermal degradation than the organic uv absorbers. Also in sunscreens for humans they cause less allergic reactions. The attenuation of the uv radiation is in these materials accomplished by both bandgap absorption and scattering of light. The uv light is strongly absorbed by excitation of electrons from the valence band to the conduction band. TiO2 and ZnO have bandgap energies that correspond to wavelengths of 365 nm and 380 nm. Light below these wavelengths has sufficient energy to excite electrons and hence is absorbed by tiO2 and zno. Light having a wavelength longer than the bandgap is not absorbed. The intensity of scattered light is a function of particle size as well as the refractive index of the particles and the media. Through careful control of particle size a maximum scattering of uv light can be obtained, while the scattering of visible light must be eliminated to obtain high transparency. Nanoparticles of tio2, Sio2 coated TiO2 (to suppress the photocatalytic activity) and ZnO are used as uv absorbers in sunscreen cosmetics.
Also inorganic oxide particles are widely used for protection against uv radiation. They are more and more used because they are more stable against thermal degradation than the organic uv absorbers. Also in sunscreens for humans they cause less allergic reactions. The attenuation of the uv radiation is in these materials acclomplished by both bandgap absorpation and scattering of light. The uv light is strongly absorbed by excitation of electrons from the valence band to the conduction band. TiO2 and ZnO have bandgap energies that correspond to wavelengths of 365 nm and 380 nm. Light below these wavelengths has sufficient energy to excite electrons and hence is absorbed by tiO2 and zno. Light having a wavelength longer than the bandgap is not absorbed. The intensity of scattered light is a function of particle size as well as the refractive index of the particles and the media. Through careful control of particle size a maximum scattering of uv light can be obtained, while the scattering of visible light must be eliminated to obtain high transparency. Nanoparticles of tio2, Sio2 coated TiO2 (to suppress the photocatalytic activity) and ZnO are used as uv absorbers in sunscreen cosmetics.
Radical scavengers destroy the already formed radicals before damaging reactions can occur. Two main classes of radical scavengers are based on a mode of action that is cyclic or non-cyclic. Cyclic radical scavengers are nearly exclusively hindered amine light stabilizers or HALS. The basic structure of all the technically relevant HALS compounds is the 2266 tetrametylpiperidine group. During the reaction, the piperidine group is converted into a stable nitroxyl radical under the influnce of uv light and oxygen. The next step is the formation of an aminoether by recombination of radicals. The aminoether reacts with peroxy radicals which again leads to the formation of nitroxide radicals and the deactivitation of the harmfull peroxy radicals. It has been shown that the formation of the nitroxide radicals reduces the concentration of peroxy radicals in the coating significantly
The most important non-cyclic radical scavengers are the phenolic antioxidants also named primary antioxidants. Their mode of action is based on the formation of a phenoxy radical, that deactivates the harmfull peroxy radical. The important drawback of this antioxidant is its non cyclic mode of action. This means that after a certain period no more antioxidant is available to prevent undesirable free radical reactions. A cyclic mode of action is better. Therefore HALS are much more used that phenolic antioxidants.
The main application of light stabilizers is in the automotive coatings. Most of the results with light stabilizers were generated in this sector. In the clearcoat a combination of uv absorbers and HALS is widely used. The benefit of the use of UV absorbers is mainly color retention. The use of HALS guarantees long lasting gloss retention and prevents the coating from cracking. Typical use levels in clearcoats of 40 µm film thickness are 1,5-2% uv absorber and 1 % HALS. This figure shows the effectiveness of uv absorbers and hals in a clearcoat used for automotive after outdoor exposure in Florida. The evaluation critaria are the 20° gloss and the time for cracking. A signifcant loss of gloss after short exposure times can be found when no light stabilizers are added. Cracking occurs after 2,5 years. The single use of a uv absorber can not prevent the coating from loss of gloss and cracking after 4,5 years. The extra addition of 1% HALS results in slightly improved gloss retention and time before cracking. The best result could be achieved with a combination of 2% BTZ-1 and 1 %HALS1. The loss of gloss can be slowed down from the beginning.
The filter effect of different uv absorbers can also be monitored with the color change of a topcoat applied on basecoats with different colors, after 54 months of Florida exposure. The protection is worst with the anilide; The improvements achievable in this coating with benzophenone are minor. The color change is much lower with BTZ-3. The triazoles give the best protection due to their absorption edge that is shifted to longer wavelengths compared with that of anilides and benzophenones.
Artificial ageing tests play an important part in developing improved coating systems and make it possible to estimate the durability of coatings in a shorter period of time than when they are tested in an outdoor installation A machine that can be used for this purpose is the QUV tester from Q-Lab. This lets you set alternating cycles of illumination and condensation to simulate sunlight and humidity (dew). It is also possible to spray a mist of water over the test samples to simulate the sort of temperature shock you get from a sudden rainstorm. A specific QUV test procedure is selected, depending on the application, load severity and the climate the components are exposed to. The lamp type, the UV intensity and the temperatures and duration of testing, together with the settings for the UV and condensation cycles (optionally in combination with a spray step) determine the severity of the test. By evaluating the samples at various intervals during the test, degradation phenomena such as the loss of colour and gloss, crack formation and calcification can be revealed. > Correlation to Outdoor Exposure A direct correlation of Q.U.V. to outdoor exposure does not exist, and any empirical correlation needs to be used with caution. In a paper published by George Grossman titled, "Correlation of Laboratory to Natural Weathering," Journal of Coating, March 1977, empirical data was generated over a wide range of materials comparing Q.U.V. to natural weathering. Based on this paper, he concluded that on average, one hour of Q.U.V. equals 17 hours of natural radiation. This factor varied from 8:1 to 25:1, depending on the material being tested.