The aging of the coating is caused by the influence of light, heat, oxygen, wind, snow, rain, dew, temperature, humidity and various chemical media in the atmosphere, and the chain structure of the polymer in the coating is gradually broken and the strength of the coating decreases. The main manifestations of coating aging are loss of light, discoloration, chalking, cracks, blistering, gold panning, spots, mildew, falling off and other phenomena.
The aging test of the coating is to evaluate the durability of the coating itself to the atmosphere, that is, the actual service life. The advantages and disadvantages of this durability represent the real practical value of the coating, and it is the comprehensive performance of various technical performance indicators of the coating.
The most direct and reliable way to test the aging resistance of the coating is the atmospheric aging test, that is, the coating sample is exposed to certain atmospheric conditions, and its durability is identified through the appearance inspection of the sample. In order to fully assess the aging resistance of a product, exposure tests need to be carried out simultaneously under various climatic conditions.
Although the atmospheric aging test method is simple and the data is reliable, it takes too long, and some varieties with good weather resistance may take several years, more than ten years or even longer to see the results, so that it is not conducive to the development of new coating products, and is not conducive to the upgrading of products. Therefore, the artificial accelerated aging test has also been widely used in assessing the weather resistance of coatings. The artificial accelerated aging test is based on the results of a large number of natural exposure tests, from which to find out the law, find out the relationship between climatic factors and paint film damage, imitate various climatic factors in nature in the laboratory to create the so-called artificial climate, and achieve a certain acceleration.
The artificial accelerated aging test can quickly compare the advantages and disadvantages of the weather resistance of different coating products in parallel in the laboratory, but it is difficult to determine their actual service life.
That is, there is no accurate conversion coefficient between the results of the artificial accelerated aging test and the atmospheric exposure test, although a lot of work has been done at home and abroad, and the comparative relationship between the artificial acceleration and natural aging of some paint varieties in some regions has also been obtained, but because the natural exposure is not constant, the climate change is capricious, and it is not easy to repeat, and it is difficult to simulate in the laboratory. In addition, even if it is the same paint variety, due to the different colors, the experimental cycle of artificial accelerated aging is not the same, so the weather resistance of coating products ultimately depends on the atmospheric exposure test, although so, the artificial accelerated aging test is still a quick and practical method to evaluate the weather resistance of coating products.
Atmospheric aging
The resistance of a coating to atmospheric aging can be defined as the degree to which a coating or coating system resists the damaging effects of the exposure conditions to which they are subjected, which is affected by the exposure method, location, and season. Since the products are generally used in various types of natural climates, and the results of the atmospheric aging test are mainly related to the tested environment, the key to the atmospheric aging test is the selection of the exposure site. Exposure sites should be selected in areas that are most representative of a certain type of climate or under environmental conditions that approximate actual use. (Related Instruments: Aging Test Chamber)
(1) Classification of test environment
Due to the complexity of climatic conditions, the general classification cannot summarize all climates, but the following classifications can be used for reference: (1) the average temperature of the coldest month is greater than 18°C and the relative humidity is greater than 75% for most of the year, A. there is no wet tropical climate in the dry season; B. Humid tropical climate with dry season.
(2) The average temperature of the coldest month of the tropical climate is greater than 18°C and the relative humidity is less than 75% for most of the year.
(3) Environments in which trees cannot grow in extremely dry climates, A. Grassland climate; B. Desert climate.
(4) The average temperature of the coldest month in temperate climate is 3~18°C.
A. Temperate climate without dry season;
B. Temperate climate with dry summers; C. Temperate climate with dry winters.
(5) The average temperature of the coldest month in the cold zone is lower than 3 °C and the average temperature of the most ambiguous month is higher than 10 °C.
A. Cold-zone climate without dry season; B. Cold-zone climate with dry winters.
(6) The polar climate is the most ambiguous and the average temperature of the month is less than 10 °C.
A. Permafrost climate, the average temperature of the warmest month is greater than 0°C; B. Permafrost climate, the average temperature of the most ambiguous month is less than 0 °C.
(7) If the alpine climate has high ultraviolet radiation, and the temperature varies greatly on a daily and seasonal basis, the surrounding environment is also an important factor to consider when choosing an exposure site for atmospheric aging test. Because the composition of the corrosive medium in the air is different in different environments, it will also affect the results of atmospheric aging test. The surrounding environment can generally be divided into the following types.
A. Urban; B. Industrial type; C. Marine type; D. Rural; E. Mixed type (industrial - marine type, urban - rural type, etc.).
(2) Selection of test environment
Different climatic conditions and the surrounding environment have an impact on the results of the atmospheric aging test, so the weather resistance index of a certain coating variety generally refers to the results of the atmospheric exposure test of the coating under certain climatic conditions and surrounding environment, and the test results are very different when tested under different climatic conditions and environments. Therefore, when investigating the atmospheric aging resistance of coating products used in some large-scale long-term facilities, an exposure field can be set up at the location of the long-term facility or in the same place as its climate and environmental conditions, and if its weather resistance is comprehensively assessed, an exposure field should be set up in the place with the worst climate and environmental conditions in each climate type area, and exposure tests should be carried out at the same time to obtain statistical data, so as to determine the weather resistance of the coating under different climatic conditions.
The surrounding area should be empty, the site should be flat, and maintain the local natural vegetation state, the grass height should not exceed 30cm, if there is snow, do not destroy the natural state of the snow; The distance from the surrounding obstacle to the exposure site is required to be at least three times the height of the obstacle, so that the test plate can be fully affected by the atmospheric preparation factors. There should be no factory chimneys, vents and devices that can emit large amounts of corrosive chemical gases in the vicinity of the exposure site. The industrial climate exposure site should be located in the factory area, the coastal area or island exposure site should be located by the sea and in a representative place, and the exposure site far away from the meteorological station (station) should set up a meteorological observation station. The exposure site should also have a dedicated studio and storage room.
(3) Test equipment
The exposure frame can be made of steel and wood, and painted with light gray paint with good weather resistance, its structure strives to be simple and firm, and should be able to withstand the blowing of the maximum local wind, the structure of the exposure frame is shown in Figure 21-41 and Figure 21-42. The skeleton is made of 30mmx4mm angle steel.
(4) Sample preparation and evaluation of test results
The exposure test plate can be made of 150mmX250mmX(0.8~1.5)mm ordinary low-carbon thin steel plate or 150mmX250mmX(1~2)mm aluminum and magnesium alloy plate. The painting of the test board should be constructed by spraying method, and the requirements for the number of spraying channels and the thickness of the paint film are shown in Table 21-22.
The actual thickness of the product can also be applied when the product is painted as required.
After the last paint is completely dry, the test board is edged with good weather resistance of different color self-drying paint, and each coating variety should be prepared with the same construction method at the same time to meet the requirements of two exposure test plates and a standard plate (for comparison). The exposure test board should be placed indoors for 7 angel paint film to fully solidify before being put into the test.
During the first three months of the test plate exposure, the paint film changes are inspected every half month, and once every month from three months to one year, and every three months after one year. Due to the different types of coatings and the speed of destruction in the exposed areas, the inspection cycle can be appropriately changed according to the specific situation, but in the rainy season or when the weather changes suddenly, it should be inspected at any time, and if there is any abnormal phenomenon, it should be recorded or photographed. Located in an exposure area with too much wind and sand, the test plate should be cleaned frequently with a soft broom, so that the test plate is fully affected by other atmospheric factors.
Because it is easy to have human error in the evaluation of the aging degree of the test board, it is better to designate a special person to be responsible for the end from beginning to end when carrying out the aging test of the same batch of samples. A batch of test plates is better to be jointly responsible for two people to minimize human error.
After a period of aging test, the coating will inevitably appear light, discoloration, chalking and other phenomena, to be checked and recorded regularly, according to different situations, can be evaluated from 11 aspects of gloss, discoloration, chalking, cracks, blistering, rust, mildew, spots, gold, shedding, staining, according to the degree of change, 5 grades, grade 0 is better, grade 1 is second, grade 4 is the worst. The specific evaluation method refers to the national standard GB/T1766-1995 "Paint Film Weather Resistance Rating Method", and then carries out a comprehensive evaluation of the advantages and disadvantages of paint film aging resistance according to the changes of each individual item, and its grading standards are shown in Table 21-23 and Table 21-24.
Artificially accelerated aging
The artificial accelerated aging test is an experiment created in the laboratory under artificial climatic conditions that simulate various climatic factors in nature. Control a certain temperature, humidity, rainfall cycle and time to conduct the test, and evaluate the grade according to the appearance damage degree of the sample, which can evaluate the weather resistance of the coating product in a short time.
(1) Test equipment and operation
The artificial accelerated aging test is carried out in the artificial aging machine. The structure of the artificial aging machine can be roughly divided into three parts: upper, middle and lower. The upper part is mainly the control panel of the aging machine, which is equipped with various control instruments and switches, which can change the temperature, humidity, two reduction amount, two reduction cycle and other parameters of the accelerated aging test according to different conditions. The inner walls of the laboratory, which are aged in the middle of the sample, are made of stainless steel, and the inner walls are filled with insulation material to increase the thermal insulation performance of the room. There is a light source hanging in the middle of the laboratory, because the light source (such as carbon arc lamp and xenon lamp) has different spectral wavelength range and different distribution, so the acceleration of different light sources for aging test is also different, such as the spectrum of ultraviolet carbon arc lamp has great spectroscopic ability in the part with a wavelength of 300~380nm, and the energy of visible light and infrared part is very small, so the acceleration of the aging test is faster. The spectral energy distribution of xenon lamps is closer to sunlight than that of ultraviolet carbon arc lamps, so the simulation of aging test is better, but its acceleration is slower than that of the former. There is also a drum of a certain diameter that rotates around the light source at a constant speed to ensure that the specimens on the drum can receive uniform light. At the back or side of the laboratory, there are also several nozzles that automatically and regularly spray water on the specimen on the bowl to simulate natural descent. The temperature in the laboratory can be adjusted by the cooling nozzles and temperature converters in the room; The humidity can be generated by the Humidity Chamber, and the humidified air is sent to the laboratory by the blower, and the Humidity Chamber, blower and transmission mechanism are installed in the lower part of the artificial aging machine.
(2) Evaluation of test results
In addition to the light source, temperature, relative humidity, rainfall cycle and water quality for rainfall are all important factors in the process of artificial accelerated aging test. However, the standards of control vary from country to country. For example, temperature control requires that the air temperature in the laboratory be controlled at (45±2) °C, while Japan and the United States require the surface temperature of the sample in the laboratory to be controlled by observing the blackboard thermometer. The temperature of the blackboard for aging experiments specified in the Japanese standard is (63 soil 3) °C, and the relevant American standard stipulates that the temperature of the blackboard is (63±5) °C. CHINA requires the relative humidity in the laboratory to be 65% ~ 75%, the relevant standards of the United States stipulate that the light source in the test cycle does not spray water cycle, the relative humidity should be 50% ±5%, the rainfall cycle is 12min per hour, and the relevant standards of the United States stipulate that 18min of rain every 120min. The water used in the artificial accelerated aging test requires the use of distilled water and ion exchange resin to purify the water, the distilled water is used for cooling, and the ion exchange resin purified water is used for rainfall, so as to avoid the scale caused by the high hardness of the tap water affecting the test results of the paint film, especially the white paint film, and also avoid the test error caused by the different tap water quality in various places.
Coating products in the process of artificial accelerated aging test its change law is similar to the natural exposure test, that is, in the initial change is more obvious, the later change is gradually stable, so the inspection cycle in the initial stage is shorter, it is required to stop the inspection every 48 hours to check the sample, to be tested to 192 hours after the shutdown inspection of the sample every 96 hours, after each inspection, the sample in the testing machine up and down positions should be interchanged, the termination index should be determined according to the degree of aging damage of various paint films and specific requirements. Generally, when the damage of the paint film reaches any of the "poor grades" of the comprehensive rating of the "Paint Film Aging Test Rating Method" (GB/T1766-1995), the test can be stopped, and the rating is shown in Table 21-23 and Table 21-24 in the atmospheric aging test.
