Aging test method and influencing factors of the h

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Aging test methods and influencing factors of exterior wall coatings

aging test methods and influencing factors of exterior wall coatings

December 1, 2018

exterior wall coatings have bright and diverse appearance decoration effects, convenient coating construction and maintenance, so they have become a new decorative material integrating decoration and economy, and have been widely used in building exterior wall decoration in recent years. However, as a high polymer material, there is an inevitable aging problem. The aging of coating is the main reason for the loss of protective function and decorative effect of exterior wall coating. Therefore, how to predict and evaluate the aging performance of exterior wall coating is an important research content in the application of exterior wall coating

1 test type of aging performance of exterior wall coatings (1) influencing factors of coating aging: the main factors that determine the aging resistance of materials are radiation, temperature and water (humidity). These three factors also have complex synergistic effects on the aging of materials

light radiation: the solar spectrum is mainly composed of ultraviolet spectrum, visible spectrum and infrared spectrum. Ultraviolet radiation has the greatest damage to the material, and the shorter the wavelength, the greater the absorptivity of the material; At the same time, the shorter the wavelength, the greater the energy contained in the photon; The greater the ability to destroy chemical bonds

temperature: during the degradation of polymer materials, the effect of temperature on the reaction rate is that the reaction rate will increase by one time when the temperature rises by 10 ℃. High temperature will have a great impact on the second step of free radical photochemical reaction. High temperature will increase the movement of molecules, improve the diffusion efficiency of oxygen and the diffusion of free radicals formed by the reaction

water (humidity): the influence of water on materials is in the form of spray, condensation, soaking and humidity. Generally, water is a carrier. When water rich in dissolved oxygen penetrates into the interior of the material, it will bring in a large amount of oxygen, causing chemical changes in the interior of the material, and will precipitate the additives added before the material is formed. The infiltration of water will lead to the volume expansion of the surface layer, resulting in stress between the surface layer and the inner layer material. When it evaporates, it will shrink, resulting in opposite stress. This alternating transformation will produce fatigue effect on the material, resulting in easy cracking of the material surface

(2) coating aging test method: the methods for judging durability can be divided into two categories: artificial climate aging test and natural atmosphere exposure test. The artificial climate aging test can be divided into carbon arc lamp, xenon lamp and ultraviolet fluorescent lamp according to the light source. Because the radiation spectrum of carbon arc lamp is too different from the solar spectrum, it has been less used in China. The aging acceleration effect of the three light sources is carbon arc lamp, xenon lamp and ultraviolet fluorescent lamp. The natural atmospheric exposure test can be divided into direct exposure and indirect exposure according to the process of sunlight exposure to the sample. Indirect exposure includes exposure under glass plate and specular reflection convergence exposure (such as emmqua), and the acceleration effect is emmqua direct exposure under glass plate

at present, there are different test methods for determining the aging performance of exterior wall coatings in the world. For example, in the United States, the natural air exposure test is the main judgment method for aging performance; In Germany, artificial climate aging and natural atmospheric exposure are used together, with emphasis on natural exposure; Japan was artificially aged before 1995. Since 1995, the Japanese standard has not only retained the artificial climate aging, but also introduced the natural atmospheric exposure test project. So far, artificial climate aging is still the only evaluation standard in China

2 natural climate exposure test methods and influencing factors foreign countries began to establish exposure sites in the 1920s. It is estimated that there are more than 400 exposure sites in the world. Most of them are set up in North America and European countries. Africa and South America have a very small number. Asia and Oceania have developed rapidly recently. Famous are the exposure sites in Florida and Arizona. Moreover, international exposure sites have established tests with different climatic characteristics, such as the awsg group in the United States, and some large industrial enterprise groups have also established internal exposure sites. In China, natural atmospheric exposure test and research began in the 1950s, mainly in Guangzhou and Hainan. Since the 1980s, the national environmental corrosion test for materials and some large-scale exposure sites, such as Guangzhou tianxingang exposure site, have been gradually established, but the main field of exposure test research is electromechanical products. Due to the late development of exterior wall coatings in China, there are few exposure tests on exterior wall coatings

(1) selection of climatic zone for exposure test: the World Meteorological Organization (WMO) divides the global climate into 7 types, of which dry desert climate and subtropical climate are the best natural climate exposure test conditions. Dry desert climate: it has an extremely high temperature, dry, high ultraviolet and less pollution environment. With large temperature difference fluctuations, accelerated tests in the natural environment can be obtained, which has a significant impact on the physical properties of the coating. Subtropical climate: it is the main living area of human beings. The developed industrial countries in the world are also mainly concentrated in this area. At the same time, it is the area where the natural atmospheric exposure test was first carried out and has accumulated a large amount of historical data. Subtropical climate has both temperature, humidity and abundant precipitation, as well as high ultraviolet light. It is especially useful for humidity sensitive materials, such as coatings. The above two climates have high commercial value. The exposure sites in Florida and Arizona of the United States are the most typical exposure sites under the above two climatic conditions. Among them, the exposure site in Florida is the benchmark exposure site for the exposure test of polymer materials. The climate environment of the subtropical climate area in China is close to that of the exposure site in Florida in the United States. Therefore, the reference exposure site for the natural atmospheric exposure test of exterior wall coatings and resins can be located in Guangzhou

in addition to the exposure to typical climate, it is also necessary to carry out the product exposure test in the use area. The following example illustrates the problem. According to the 21st Century Business Herald, a technical problem occurred in the Shanghai Maglev Project last June, and the cable contacts were burned. After analysis, the reason is that the natural environmental conditions in Shanghai, such as humidity, temperature and air purity, are different from those in Germany, resulting in the "acclimatization" of German technology in China. However, if domestic cables are used, this problem may not exist, This shows that small climate differences may have a greater impact on products. Due to the characteristics of exterior wall paint, especially emulsion paint, it is a kind of paint that is sensitive to ambient temperature, especially low temperature has a great impact on emulsion paint. Different TG lotion are suitable for different climatic regions, and different formulation systems are established. As the domestic coatings are used in a wide range of areas, covering different climatic areas, with a large temperature difference and different use environments, exposure sites can be established according to the conditions of the product use areas

by comparing the solar radiation of several regions in Table 1, we can find that Guangzhou does not have special advantages over other regions in terms of total solar radiation. It is only higher than other regions in terms of annual average temperature, humidity and rainfall, but the minimum temperature is too high, which makes the test effect of temperature change resistance and low temperature performance of the coating poor. The climate of each region in China has its own characteristics. To obtain more accurate exposure test results, the test can be carried out according to the characteristics of each region. However, the climate in Guangzhou is very favorable for the accelerated aging effect of the coating, and it is easy to obtain the final service life of the coating

comparison of solar radiation in different places

through analysis and research, we believe that: ① the city has high population density, a large amount of automobile exhaust emissions, smoke and dust emissions from fuel and coal combustion, a large number of applications of air conditioning and a high energy consuming environment, resulting in the "heat Island" effect, high content of air suspended solids and large acidity of rainwater, so it has a great impact on the exterior wall coating. ② The proportion of exterior wall coatings used in urban areas is high, and there are many high-rise buildings using exterior wall coatings. Due to the requirements of urban landscape, the durability of coatings is also high. Therefore, it is better to build an exposure site in the urban climate environment

(2) selection of exposure method and angle: according to the characteristics of exterior wall coating, the most appropriate exposure methods are direct exposure and backplane exposure. The exposure angle can be 90o angle or 45o angle as required. Although the simulation of 90o angle is good, the test cycle is long because 90o angle receives less solar radiation. In the subtropical zone, 50% of the ultraviolet light is diffused, and the 90o angle will block half of the diffuse light in the sky. In addition, the sample at 90o angle has low temperature, short wetting time and less dust accumulation on the surface. The dust has obvious barrier to the short wave ultraviolet ray. Less dust is conducive to the reception of solar radiation on the surface of the sample plate, but the low temperature and short humid time of the sample plate are not conducive to the accelerated test. The 45o angle has the longest history and abundant accumulated data. This angle is mainly used for exterior wall coatings to achieve accelerated effect. However, for coatings with poor stain resistance, the accumulation of surface pollutants will affect the results. Although the maximum solar radiation, including diffuse light, can be obtained at 5O angle, the dust accumulation is serious, blocking the solar radiation, and the rainwater does not wash the dirt well, so it is not advisable to expose at 5O angle. At present, the natural atmospheric exposure test has developed rapidly, with the emergence of equipment such as e m q u A. because e m q u a needs to be closed in overcast and rainy days, the use efficiency of equipment in subtropical areas is not high. The exposure site with good effect abroad is Arizona exposure site in the United States, that is, the area with annual solar radiation of more than 8000mj/m2. Only Lhasa, Tibet, can meet this requirement in China. Therefore, emmqua is not mature for exposure test of exterior wall coating in China. If the black box accelerated exposure and glass plate are produced, used and consumed in strict accordance with relevant national rules, regulations and standards, the exposure is inconsistent with the use characteristics of exterior wall coating, so these two methods are not used in the exposure test of exterior wall coating

(3) uncertain factors in natural aging: ① typhoon: the frequency, intensity and route of typhoon have an impact on the exposed climatic environment. ② Cloud cover: the cloud amount affects the sunshine time and ultraviolet ray. The ultraviolet ray with short wavelength will refract, reflect and absorb water vapor particles. ③ Air pollution: suspended particles produced in dusty weather, industrial and living activities will block the transmission of ultraviolet rays. Air pollution can complicate the composition of rainwater. ④ El Nino and La Nina phenomenon: the alternation of this phenomenon has caused abnormal rainfall, low temperature and high temperature in some areas of the earth's surface. ⑤ Volcanoes: volcanic eruptions will send a large amount of volcanic ash into the atmosphere and stay in the atmosphere for a long time. Due to the role of volcanic ash, the light on the earth's surface will be affected. The impact of volcanic ash in the atmosphere on short wavelength ultraviolet radiation is particularly obvious. Volcanic eruptions in South America once reduced the ultraviolet UVB radiation of U.S. exposure sites by 50% for more than two years. ⑥ Changes in the ozone layer: human activities tend to thin the ozone layer and expand the ozone hole, resulting in an increase in ultraviolet radiation. Natural atmospheric exposure test for exterior wall coating is the need of development, but there are still many problems to be solved as the main judgment index of aging performance. The natural climate conditions are complex and difficult to control

3 artificial climate aging methods and existing problems (1) relationship between artificial climate aging light sources: there are two theoretical systems for the selection of artificial light sources in the world by adding 10-20 parts of talc powder

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