The test method and influence factor of exterior coating aging

Exterior paint has a bright and diverse appearance and decorative effects, convenient coating construction and maintenance, thus becoming a decorative, economical as one of the new decorative materials, in recent years has been widely used in building exterior decoration. However, as a polymer material, there is an inevitable problem of aging, the aging of coating is the main reason for the loss of protective function and decorative effect of exterior coating, so how to predict and evaluate the aging performance of exterior coating is an important research content in the application of outer wall coating.1 External wall coating aging performance test type
(1) coating aging factors: the main factors determining the aging resistance of materials are radiation, temperature and water (humidity), these three factors on the effects of material aging there are complex synergies
: The solar spectrum consists mainly of ultraviolet, visible and infrared spectra. Ultraviolet radiation is the most damaging to the material, and the shorter the wavelength, the greater the absorption rate of the material, and the shorter the wavelength, the greater the amount of energy contained by its photons, and the greater the ability to destroy chemical bonds.
temperature: During the degradation of polymeric materials, the temperature affects the reaction rate by 1 times for every 10 degrees C increase in temperature. High temperature will have a greater impact on the second step reaction of free-based photochemical reaction, and high temperature will increase the movement of molecules, improve the diffusion efficiency of oxygen and the diffusion of free-forming reactions.
(humidity): The effect of water on the material is in the form of spray, condensation, immersion and humidity. In general, water is a carrier, rich in dissolved oxygen water penetrates into the inside of the material, a large amount of oxygen into the material, prompting chemical changes inside the material, while the material will be molded before the addition of additives pre-predate. The infiltration of water will cause the volume of the surface layer to expand, and the inner material to create stress, when evaporation and contraction, causing the opposite stress, this alternating transformation will produce fatigue on the material, resulting in the material surface easy to crack.
(2) Coating aging test method: the way to determine durability can be divided into two categories: artificial climate aging test and natural atmospheric exposure test. Artificial climate aging test according to the light source can be divided into carbon arc lamp, xenon lamp and ultraviolet fluorescent lamp, because the radiation spectrum of carbon arc lamp and the solar spectrum is too different, has been less used in China. The aging acceleration effect of the three light sources is the carbon arc lamp> xenon lamp > 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 model. Indirect exposure includes exposure under the glass plate and spggle exposure of specular reflections (e.g. EMMQUA), and the acceleration effect is emMEqua> direct exposure> exposure under the glass plate.
the test and determination method of the aging properties of external wall coatings in all countries of the world is different. For example, the United States uses natural atmospheric exposure tests as the primary method of determining aging performance, Germany is a common method of artificial climate aging and natural atmospheric exposure, focusing on natural exposure, Japan was artificial aging before 1995, and since 1995 the Japanese standard in addition to retaining artificial climate aging, the introduction of natural atmospheric exposure test projects. China still has artificial climate aging as the only evaluation criteria.2 Natural climate exposure test methods and influences
Foreign countries began to establish exposure farms in the 1920s, estimated to have more than 400 exposures worldwide, most of them in North and European countries, with very few in Africa and South America, and recently in Asia and Oceania. Famous for the exposure of Florida and Arizona in the United States. Moreover, international exposure farms have established test networks with different climatic characteristics, such as the AWSG Group of the United States, and some large industrial enterprise groups have established internal exposure zones. Domestic natural atmospheric exposure testing began in the 1950s, mainly in Guangzhou and Hainan. Since the 1980s, a national network of material environmental corrosion tests and some large-scale exposure fields, such as the Guangzhou Tiansinggang exposure field, but the main area of exposure test research is mechanical and electrical products. Due to the late start of the development of external wall coatings in China, there are fewer exposure test studies for exterior coatings.
(1) Selection of climate zones for exposure tests: The World Meteorological Organization (WMO) divides the global climate into seven categories, with dry desert and subtropical climates as optimal natural climate exposure test conditions. Dry desert climate: With an extremely high temperature, dry, high ultraviolet light and less polluted environment, large temperature difference fluctuations, can obtain accelerated testing in the natural environment, the physical properties of the coating has a significant impact. Subtropical climate: is the main living area of mankind, the world's developed industrial countries are mainly concentrated in this region, at the same time is the first natural atmospheric exposure test area, accumulated a large amount of historical data. The subtropical climate combines temperature, humidity and abundant precipitation with high ultraviolet rays, especially for humidity-sensitive materials such as coatings. Both climates have high commercial value. Exposure farms in Florida and Arizona in the United States are the most typical of these two climatic conditions, with Florida's exposure site being the baseline for polymer material exposure tests. The domestic subtropical climate region and the United States Florida exposure site is closer to the climate environment of Guangzhou, so the natural atmospheric exposure test of exterior coatings and resins its baseline exposure site can be located in Guangzhou.
in addition to typical climate exposure, it is also necessary to carry out product exposure tests in the area of use. The following examples are very illustrated, according to the 21st Century Economic Daily reported that last June, Shanghai magnetic levitation project technical problems, cable contacts burned, the reason for analysis is: Shanghai
natural environmental conditions such as: humidity, temperature and air purity and Germany, resulting in German technology in China "water and soil", but if the use of domestic cables may not have this problem, which indicates that small climate differences may have a greater impact on products. Because of the characteristics of the exterior coating, especially latex paint, it is decided that it is a more sensitive to the environment temperature coating, especially the low temperature has a greater impact on latex paint, different Tg emulsions are suitable for different climate areas, and the establishment of different formulation systems. Because the domestic paint use area is very wide, it covers different climate zones, the temperature gap is large, there are very different use environment, so according to the situation of the product use area to establish exposure.
We can find from the comparison of solar radiation in several areas of Table 1 that Guangzhou's total solar radiation relative to other regions may not have a particular advantage, it is only higher than other regions in terms of average annual temperature, humidity and rainfall, but the minimum temperature is too high, making the coating
temperature resistance and low temperature performance of the test effect is not good. The climate of each region in China has its own characteristics, and 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 beneficial to the accelerated effect of coating aging, and it is easy to obtain the final life of the coating.
Where solar radiation comparison
through analysis and research, we believe: (1) the city has a high population density, a large number of automobile exhaust emissions, fuel-fired coal soot emissions, air conditioning applications and high energy consumption environment, resulting in the "hot island" effect, high content of air suspended matter and rain acidity and so on, so the impact of external wall coating is greater. (2) The proportion of external wall coatings used in urban areas is higher, the use of external wall coatings more high-rise buildings, due to the requirements of urban landscape, the durability of coatings is also high. Therefore, we should take the urban climate environment to build exposure field as a good.
(2) Selection of exposure methods and angles: Depending on the characteristics of the exterior coating, the most appropriate exposure method is direct exposure and back plate exposure, the exposure angle can be selected as needed 90o angle or 45o angle. Although the 90o angle is more simulated, the test cycle is long because the 90o angle receives less solar radiation. In the subtropical region, 50% of the ultraviolet rays are diffuse, and cape 90o blocks half of the diffuse light in the sky. In addition, the 90o angle of the model temperature is lower, the wet time is short, the surface dust accumulation is less. Dust on the short-wave ultraviolet block is obvious, less dust is conducive to the sample surface solar radiation reception, but the model temperature is low, wet time is not conducive to accelerated testing. 45o angle is the oldest, the accumulation of data is also rich, the outer wall paint selected this angle is mainly to achieve accelerated effect, but for the less resistant coatings, surface contaminants of the accumulation of the results. Although 5o angle can get the maximum solar radiation including diffuse light, but the dust accumulation is serious, blocking solar radiation, rain water on the dirt flushing is not good, so 5o angle exposure is not desirable. Natural atmospheric exposure tests are now developing rapidly, with the emergence of devices such as E M Q U A, which are inefficient to use in the subtropical regions due to the fact that E M M Q U A is to be turned off in the rain. The more effective exposure field used abroad is the Arizona exposure field in the United States, i.e. areas with annual solar radiation of more than 8000MJ/m2. The only region in China that can meet this requirement is Lhasa in Tibet, so it is premature to use EMMQUA for external wall paint exposure testing. The accelerated exposure of the black box and the exposure under the glass plate do not match the characteristics of the use of the exterior coating, so these two methods are not used in the external wall coating exposure test.
(3) Uncertainties in natural aging: (1) Typhoons: The number, intensity and route of typhoons have an impact on the exposed climate. (2) Clouds: Cloud volume is affecting the time of daylight and ultraviolet light, due to the short wavelength of ultraviolet light encountered by water vapor particles will be refracted, reflected and absorbed. (3) Air pollution: dust weather, industrial and living activities to produce suspended particles will block the flow of ultraviolet light, air pollution can make the composition of rainwater complex. (4) El Ni?o and La Ni?a phenomenon: the phenomenon alternates, resulting in abnormal rainfall, low temperature and high temperature in some areas of the earth's surface. (5) Volcanoes: Volcanic eruptions send large amounts of ash into the atmosphere and stay in the atmosphere for a longer period of time, affecting the light on the Earth's surface due to the effects of volcanic ash. The effects of volcanic ash in the atmosphere on short wavelengths of ultraviolet light are particularly pronounced, with volcanic eruptions in South America reducing UVB radiation from exposed U.S. exposed areas by more than 50 percent for more than two years. (6) Changes in the ozone layer: Human activities have led to a tendency to thin the ozone layer and the expansion of the ozone hole, resulting in an increase in ultraviolet light. The use of natural atmospheric exposure test for exterior coatings is a need for development, but as the main indicator of aging performance, there are still many problems to be solved, and natural climate conditions are complex and difficult to control.3 Artificial climate aging methods and problems
(1) The relationship between artificial climate aging light sources: the international selection of artificial light sources exists two theoretical systems, namely, the xenon light source simulated by Atlas's solar full spectrum and the ultraviolet fluorescent light source with Q-Panel's simulation of the main destructive spectrum of polymer materials. The results of the two methods of measuring the macro-aesthetic properties of the material are similar, but the determination of the color change problem of the exterior coating is better with the xenon light source. The cost of testing and running the two light sources is larger, considering the economic problems, we use ultraviolet fluorescent light source for testing in the laboratory screening formula, and for the external measurement of xenon light source. Below is a comparative experiment we did, see Table 2.
solar radiation in various parts
the coating is water-based polyurethane exterior paint. Ultraviolet aging: the instrument is the U.S. Q-Panel company QUV/spray type, UVA light source 340nm, irradiance 0.65W/m2/nm, cycle 65 degrees C20h light / 45 degrees C4h condensation. Xenon lamp aging: Atlas Ci65 type, average irradiance 550W/m2, temperature 65 degrees C, 18min/102min spray drying cycle. Through the test we get, both methods do not make the coating powder damage, xenon lamp easy to make the paint appear color change, the change trend of the two is the same, but not equivalent in aging time.
(2) Differences between artificial climate aging and natural atmospheric exposure: (1) Differences in light sources: Even the most advanced artificial light sources and real daylight spectra still have significant differences, artificial aging lamps produce some short-wave light radiation outside the solar spectrum, although there has been great development in the spectral filtration system, but there is no denying that there is still short-wave light generation during the test. The spectral curve changes when the energy compensation of the lamp due to aging decay is made. There is a difference between continuous exposure and exposure with gap cycles during the test, and continuous exposure aging is now used, but the chemical reaction process of some materials requires a gap period, so continuous exposure cannot meet its requirements. The use of excessive artificial light sources in artificial aging can cause changes in certain photochemical reaction processes in high-radiation environments, such as the catalytic structure of titanium dioxide in coatings when artificial radiation is different from natural exposure. (2) Differences in humidity and rain: during the aging of radon light sources in the laboratory, a water injection cycle of 18min/102min is carried out, and the sample rack speed is approximately one turn per minute, so the sample surface is wet during the rotation. Also only about 1/3, in the next 2/3 of the rotation process sample surface will be 65 degrees C temperature and light environment baking dry, such a water spray process is essentially low temperature water on the high temperature of the sample's cross-change temperature cold impact process. At present, the water resistance of the coating is better, the surface energy of the coating is also low, the vertical model surface for a very short period of time sprayed water, almost can not penetrate into the inside of the model. This is different from the process of wetting the sample by rain, when it rains the sample is a long process of absorbing saturation and releasing drying, a process that creates internal stress on the sample. Because the stress of hot and cold shock is different from the type of stress of wetting and drying, the spray simulation in the laboratory is different from the rain conditions in nature. (3) Biology and pollution: air pollutants and biological factors in nature also have a great impact on the sample, acid rain caused by pollutants such as sulfur dioxide in the atmosphere, oily particles and microorganisms produced by automobiles and other fuels, so that the sample is affected by the laboratory can not be simulated. 4 Correlation between natural atmospheric exposure tests and laboratory
has limitations in laboratory performance testing. Samples under natural climate conditions are subject to a combination of factors, samples subject to solar radiation at the same time to withstand the four seasons and day and night temperature changes, abnormal rainfall, strong air flow caused by sudden changes in humidity, night condensation and other conditions. The properties of the coating test in the laboratory are relatively independent, the ambient temperature, humidity and air flow of the artificial aging instrument are constant, the water-resistant, stain-resistant, temperature-resistant samples are newly prepared and cannot be measured after the paint film aging test. If it's resistant to dirt, there are