Plastic films exposed to natural weather undergoes photo-oxidative reactions. Sunlight, together with oxygen, initiates the chemical reactions in the polymer matrix. The important aspects of sunlight include sunshine duration, total solar radiation, and UV radiation that may be used in modeling of the degradative processes [6-8].
The solar radiation reaching Earth's surface is only about one-half to two-third of its value at the outer limits of the atmosphere. The most damaging portion of the total solar radiation is the UV section of the radiation, which causes the weather-induced degradation of plastics [8, 9]. The sun sends a continuous spectrum of energy radiation on Earth. The radiation varies in terms of wavelength, and therefore, photon energy. The wavelength has an inverse relationship with quantum energy .
The extraterrestrial radiation of the sun is largely constant. The intensity of radiation depends on the angle of incidence. The changes in the intensity as a result of the tilt of Earth's axis and the rotation of Earth in a daily as well as annual cycle are linked to the geographic latitudes where radiations are monitored .
The radiation wavelength ranges reaching Earth's surface are from about 290 to 1400 nm [13-15]. The portion of the sun's spectrum between 290 and 400 nm is important to polymers because it includes the highest energy region and is the only portion that can cause direct harm to unmodified polymers . The visible region of the solar spectrum, 400-800 nm, does not usually cause direct harm to polymers but can do so by interaction with sensitizing substances in the polymers. The IR portion, less than 800 nm, is generally considered harmless in a photochemical sense, but it may have a role in the thermal oxidative degradation of some polymers.
The UV portion of radiation largely causes the degradation of polymers; the energy at visible and high wavelengths is too low to damage the chemical structure of a polymer. Because of its chemical structure every polymer is susceptible to photochemical degradation at a particular wavelength. The absorption of UV radiation and its concomitant degradative effects vary for each individual polymer. Stability is strongly dependent on the specific chemical and molecular structure of the polymer; variations in structure lead to differences in the absorption ranges of individual polymers. Theoretically, polyethylene material should not contain functional groups that would be capable of absorbing UV radiation. However, polyethylene absorbs, and gets degraded by, UV radiation. Sources of UV-absorbing chromophores are [39-41]:
1. Polymer structure
2. Catalyst residues
3. Thermal-processing degradation products
4. Antioxidants and transformation products
5. Colorants, fillers, flame retardants, etc.
Most polymers should not absorb at wavelength greater than 300 nm. The above-mentioned sources of UV-absorbing chromophores present in polymers may lead to the initiation of weathering processes . It is a UVB radiation with a shorter wavelength (280-315 nm) that is mainly responsible for photo-damaging, discoloration, and loss of mechanical properties of polyethylene. The degree of deterioration of UVB levels on the lifetime of polymer depends on the geographic location of exposure and the tendency of the material to absorb UVB radiation . In Saudi Arabia UV dosage of 170-220 kLy/year is received as compared to 130-140 kLy/year in Florida.
The weathering process depends significantly on temperature along with other factors. Though the maximum temperature attained by the polymers is not sufficient to promote bond cleavage of any structures likely to be found in polyethylenes and other plastics. The role of heat in the weather-induced degradation of plastics is in accelerating processes otherwise induced, such as hydrolysis, secondary photochemical reactions, or the oxidation of trace contaminants . It has been determined that the degradation rates of polymers in the tropical zones far exceed those in temperature zone. Also the simulated weathering experiments have shown that the oxidation rates of polyethylene exposed to 300-nm radiation increase fourfold from 10 to 50°C .
The annual average incident energy in different parts of the world is reported in the literature [20-22]. Table 5.4 presents the annual average incident energy measured in different parts of the world.
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