Paints, plastics, and other organic materials can be exposed to natural weather conditions and light radiation for a period of time, such as loss of light, fading, yellowing, flaking, cracking, loss of tensile strength, and loss of the entire layer. Even indoor light or sunlight through the window glass can cause damage to substances such as pigments or dyes. Therefore, for outdoor coatings, such as architectural exterior coatings and automotive coatings, weather resistance and light resistance are important inspection items.
Although everyone agrees that the weatherability and lightfastness of paints are important, they have their own opinions on what they are good test methods for. There are also many methods for evaluating the weather resistance and light resistance of paints at home and abroad. The methods commonly used for natural accelerated weathering tests such as natural weathering test, xenon arc lamp irradiation, carbon arc lamp irradiation, and ultraviolet lamp irradiation are widely used. This article will explore how to choose the right test method.
l Natural climatic aging test The natural climatic aging test method is widely used at home and abroad. The main reason is that the natural climatic aging experiment results are more realistic, the cost is lower and the operation is simple and convenient. Although we can conduct natural weathering tests anywhere, the internationally recognized test site is Florida in the United States because of its sunny sunshine.
However, the shortcoming of the natural climatic aging test is that the test takes a long time, and the tester may not have such a long time waiting for the test result of a product. In addition, even in Florida, the climate cannot be exactly the same year after year, so the reproducibility of the test results is not satisfactory.
2 Xenon arc radiation test The xenon arc radiation test is considered to be an experiment in which zui can simulate the full solar spectrum because it produces ultraviolet, visible and infrared light. Because of this, it is considered to be a widely adopted method at Zui and abroad. This method is described in detail in GB/T1865-1997 (equivalent to IS0113411:1994).
However, this method also has its limitations, namely the stability of the xenon arc lamp source and the complexity of the test system. Xenon arc lamps must be filtered to reduce unwanted radiation. A variety of filter glass types are available to achieve different irradiance distributions. Which glass to use depends on the type of material being tested and its end use. Changing the filter glass can change the type of short-wavelength ultraviolet light that is transmitted, thereby changing the speed and type of material damage. There are three types of filtering commonly used: daylight, glazing, and extended ultraviolet light types (Methods 1 and 2 mentioned in GB/T1865-1997 correspond to the first two types).
Typical xenon arc radiation is equipped with an irradiance control system. The irradiance control system is important in the xenon arc radiation test because the inherent stability of the spectrum of the xenon arc lamp source is worse than that of the fluorescent ultraviolet lamp. Some people in foreign countries have examined the difference between a new xenon arc lamp and a spectrum of old xenon arc lamps that have been used for 1000 hours. It was found that the spectral energy distribution not only changed significantly with the long wavelength range of the light source, but also changed significantly in the short wavelength range. The reason for this change is the aging of the xenon arc lamp, which is its intrinsic property.
A variety of remedies are also available for this change. For example, increase the frequency of lamp replacement to reduce the effects of light aging. Or the sensor can be used to control the irradiance. Despite the variation in spectral energy distribution due to lamp aging, Xenon arc lamps are still a reliable and practical source of light for weathering and sunlight resistance testing.
Most xenon arc radiation tests use a water spray and/or temperature automatic control system (the surface water spray) proposed by the national standard GB/T1865-1997 when simulating the wetting conditions. A limitation of the water spray method is that when a relatively low temperature water is sprayed onto a relatively high temperature test panel, the test panel will cool down, which will slow down the process of material damage.
In the xenon arc radiation test, high purity water is required to prevent deposits from forming on the surface of the test plate. Therefore, the operating cost is high.
3 UV lamp irradiation test UV lamp irradiation aging test The fluorescent ultraviolet lamp is used to simulate the destructive effect of sunlight on durable materials. This is different from the above-mentioned xenon arc lamp. The fluorescent ultraviolet lamp is similar in electrical principle to the ordinary illumination cold fluorescent lamp, but can generate more ultraviolet light than visible light or infrared light.
For different exposure applications, there are different types of lamps with different spectra to choose from. The UVA-340 lamp is a good model for simulating sunlight in the main short-wavelength UV spectrum. The spectral energy distribution (SPD) of the UVA-340 lamp is similar to the spectrum extracted from 360 nm in the solar spectrum. The UV-B type lamp is also a commonly used accelerated artificial weathering test lamp. It destroys materials faster than UV-A lamps, but its shorter wavelength energy output than 360 nm can deviate from actual test results for many materials.
Irradiance (light intensity) control is necessary to obtain accurate and reproducible results. Most UV aging test devices are equipped with an irradiance control system. These precise irradiance control systems allow the user to select an irradiance metric when testing. Through the feedback control system, the irradiance can be continuously and automatically monitored and accurately controlled. The control system automatically compensates for insufficient illumination due to aging of the lamp or other causes by adjusting the power of the lamp.
Fluorescent UV lamps simplify irradiance control due to their inherent spectral stability. All sources of light will age over time. But fluorescent lamps, unlike other types of lamps, do not change their spectral energy distribution over time. This feature enhances the reproducibility of the test results and is therefore a major advantage.
Tests have shown that there is no significant difference in the output power of an aging test system equipped with an irradiance control when a lamp of 2 h is used and a lamp of 5 600 h is used. The irradiance control device can maintain a constant light intensity. . In addition, their spectral energy distribution does not change, which is very different from xenon arc lamps.
One of the main advantages of using the violet lamp aging test is that it can simulate the damage to the material in a more realistic outdoor humid environment. When the material is placed outdoors, it is estimated that it is frequently subjected to moisture for at least 12 hours per day. Because this moisturization is mostly manifested in the form of condensation, a special condensation principle is used in the accelerated artificial weathering test to mimic outdoor humidity.
During such a condensation cycle, the tank at the bottom of the chamber is heated to produce steam. The hot steam maintains the environment of the test chamber at 100% relative humidity at high temperatures. The test chamber is designed so that the test panel actually constitutes the side wall of the test chamber. The back side of the test panel is exposed to room temperature at room temperature. The cooling effect of the indoor air reduces the temperature of the surface of the test panel to a few degrees below the steam temperature. These degrees of temperature difference allow the water to continuously drop to the surface being tested during the condensation cycle.
The condensed water thus produced is a stable, pure distilled water. This water can improve the reproducibility of experimental results, eliminate water deposit contamination problems and simplify the installation and operation of test equipment.
Because the material is generally damp for a long time outside the room, the typical circulating condensing system has a test time of 4 h. The condensation process is carried out under heating (50 ° C), which greatly accelerates the rate of moisture damage to the material. Condensation cycles under prolonged, heated conditions are more effective in reproducing moisture-damaging materials than other methods such as water spray, dipping, and other high-humidity environments.
4 Conclusion Although the national standard regulations and the current aging test method in China are Xenon arc radiation, the Xenon arc radiation and UV aging tests abroad are widely used test methods. These two methods are based on completely different principles. The Xenon Illumination Test Chamber mimics all of the solar spectrum, including ultraviolet, visible, and infrared light, with the purpose of simulating sunlight. The UV aging test does not attempt to imitate the sun's rays, but only mimics the damage of sunlight. It is based on the principle that long-term outdoor exposed durable materials are subject to aging damage caused by short-wave ultraviolet light.
In addition, even in the aging test in the natural climate, there is an accelerated method, which is to install the sample to be rotated in the sun as the sun rises and falls, so that the sample remains in direct sunlight for most of the time. Get accelerated test results. Before the 1980s, carbon arc lamps or direct UV lamps were used for parallel testing, which also shortened the inspection cycle. Which test method is good? There is no simple answer. Which method to choose depends on the material to be tested, the final application of the material, the mode of damage to the material of interest, and financial factors.
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