In the process of use, processing and storage of plastics, due to various external factors, such as light, oxygen, heat, radiation, odor, rain, etc., as well as molds, bacteria, etc., the chemical structure is destroyed and the original excellent performance This phenomenon is known as aging. The main manifestations of plastic aging are discoloration and cracking in appearance, and changes in physical and chemical properties such as mechanical properties and electrical properties. The aging
test can be roughly divided into two categories: one is natural exposure aging, and the other is artificial aging test. They are introduced separately here. 1. Natural exposure aging Natural exposure aging test is an aging test under natural environmental conditions. Commonly used methods for plastics are: 1. Atmospheric exposure test GB/T3681-2000 (neq ISO877:1994) Plastic atmospheric exposure test method GB/T17603-1998 (equivalent to ASTM D5272-1992) Photodegradable plastic outdoor exposure test method 2 . Indirect exposure test to sunlight after the glass plate is filtered GB/T14519-1993 (refer to ISO877:1976) The test method for indirect exposure of plastics to sunlight after the glass plate is filtered The natural exposure aging test is closer to the actual use environment of the material, according to its The results are more reliable for the weather resistance evaluation of the materials, and the results of artificial weather tests must also be verified by natural aging tests, so natural aging is a basic method. But the natural aging test is slow, has a long period, and is greatly affected by environmental factors, so its application range is limited. Now we use more laboratory artificial simulation methods to accelerate aging. Second, artificial aging test (1) Plastic laboratory light source exposure test Laboratory light source (artificial climate) exposure test is an artificial accelerated aging experiment method that simulates and strengthens the main factors in the atmospheric environment, which can be approximated in a short time The result of regular atmospheric exposure. This method simulates the five main factors in the atmospheric environment: sunlight, air (), temperature, and rainfall. Among them, sunlight is the most important. A, light source The principle of light source selection is: the spectral characteristics of artificial light sources should be close to the most sensitive wavelength that causes material aging damage, and consider the purpose of the test and the use environment of the material. Research has shown that radiation in the near ultraviolet region (300~400nm) wavelength range of sunlight is the main factor that causes degradation of plastics. Usually, the radiation intensity increases and the material aging rate accelerates. Therefore, the more similar the energy spectrum distribution of artificial light sources in the ultraviolet region is to the spectral lines of the sun in this region, the better the simulation of the light source. Three light sources are now recommended internationally: xenon arc lamps, carbon arc lamps, and fluorescent ultraviolet lamps. 1, Xenon arc lamp: The spectral range of the xenon arc lamp is ultraviolet, visible and infrared radiation with a wavelength greater than 270nm. After proper filtering, its spectral energy distribution is most similar to the ultraviolet-visible part of sunlight. Therefore, the xenon arc lamp split irradiance distribution is the best simulation among the currently used artificial light sources, but the xenon lamp ultraviolet distribution increases less, so the aging doubling rate is lower. 2, carbon arc lamp: The spectral energy distribution of carbon arc lamp is relatively close to sunlight, but the ultraviolet rays are more concentrated in the wavelength of 370~390nm. Although the simulation of carbon arc lamp is not as good as xenon lamp, its doubling rate is higher than that of xenon lamp. Japanese standards use more. 3, fluorescent ultraviolet lamp fluorescent ultraviolet lamp emits a considerable part of the light energy with a wavelength below 300nm, which changes the mechanism of material aging degradation reaction, so the simulation is poor, but the doubling rate is high. B, test temperature The selection of air temperature is based on the highest temperature of the environment in which the material is used, which is slightly higher than that, usually around 50℃. The black board temperature of the xenon lamp is based on the highest temperature of the material surface in the environment in which it is used, which is slightly higher than it. Choose 63±3 ℃. Of course, other temperatures can also be selected. But the sample irradiation temperature should not be too high, especially for temperature-sensitive materials. Because the temperature is too high, the test result may not be mainly due to photo-oxidation aging, but the result of thermal effects, such as PVC. Therefore, when the xenon lamp is aging, attention should be paid to prevent the sample from overheating. For the open carbon arc lamp, the air flow should be strengthened to avoid excessive temperature rise. C, relative humidity The influence of relative humidity on material aging varies with different material types. Based on the average annual relative humidity of the material in the environment where it is used, it is usually selected in the range of 50~70%. D. The selection of the rainfall cycle is based on the rainfall of the natural climate. Internationally, choose 18min/102min or 12min/48min, 3min/17min, 5min/25min for rain time/no rain time. However, moisture in the form of condensation may have a greater impact on accelerated exposure tests. Therefore, exposure in light and dark cycles often requires condensation time. Artificial aging rain uses distilled water or deionized water. The minerals in the water may cause stains on the surface of the sample.
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