1. Artificial accelerated aging test
The artificial accelerated aging test is based on the results of a large number of natural exposure tests to find out the rules and find out the relationship between climatic factors and paint film damage, so as to artificially create conditions in the laboratory to simulate these climatic factors and give certain conditions. Accelerated, to overcome the lack of natural exposure test takes too long.
(1) Relevant factors in the test
1. Light source
Judging from the existing test situation at home and abroad, the light source is a very important factor in the aging of the paint film. The solar spectrum can generally be divided into three regions: ultraviolet region, visible region and infrared region. It has been determined that when the sun is at the head on a sunny day (refers to noon at the equator), the distribution of solar energy at sea level is as follows:
Ultraviolet rays (290~400nm) account for 4%
Infrared (700~2500nm) accounted for 53%
Visible light (400~700nm) accounts for 43%
Although ultraviolet energy accounts for only 4% of the sun's total energy, many materials suffer damage in the ultraviolet region. For example, nitrocellulose decomposes under the radiation of light with a wavelength of 310nm; the copolymer of unsaturated polyester and styrene turns yellow quickly when the wavelength is 325nm; polypropylene, polyurethane, etc. Aging occurs at 370nm.
2. Temperature
Since the effect of light is always accompanied by the rise of temperature, the rise of temperature is also an important factor to promote the aging of paint film. The heat aging of the paint film is mainly due to the cross-linking process and the destruction of the polymer molecular chain; as a result of cross-linking, a three-dimensional structure is produced, which makes the paint film hard, brittle, and loses elasticity; The molecular chain is broken, reducing the molecular length and molecular weight, forming a high group, which is soft and sticky.
In the artificial accelerated aging test, a certain temperature is combined with periodic rainfall to cause frequent alternating temperatures, and its influence is more important than simple thermal aging, because it causes the coating and the bottom plate to continuously expand and shrink, which can be Lead to the formation of a large internal stress in the coating. In most cases, the paint film composition system contains substances with different components and structures: primer, putty, paint, enamel and varnish, so when subjected to alternating temperature, the stress in each layer is different. , coupled with strong light with a certain wavelength, it is easy to cause cracking of the paint film.
3. Humidity
When exposed to the atmosphere, the paint film is actually kept in a humid state for a long time, especially in hot and humid areas. Although low-humidity ultraviolet radiation can also cause loss of light and pulverization of the paint film, the effect is slow and color cracking occurs; and under high humidity, ultraviolet radiation becomes a more effective destructive factor, greatly reducing the above-mentioned destructive effects. enhanced. The main reason is that the absorption of water causes the swelling and volume change of the paint film, or dissolves the water-soluble substances in the paint film. When exposed to light, it is easy to destroy the structure of the paint film or accelerate the photochemical change. Of course, the influence of humidity should take into account the overall influence of temperature, moisture and light and other factors that promote each other.
4. Oxygen
It is very rare that the polymers in the paint film degrade due to sunlight alone, and the accelerated aging due to the interaction of sunlight and oxygen, the so-called solar oxidation, is noteworthy. Oxygen activated by solar energy causes oxidation of the paint film surface, resulting in increased film porosity and loss of gloss. It has been confirmed that in the artificial accelerated aging test cycle, increasing the oxygen treatment is of great significance, especially in the higher pressure oxygen treatment, which significantly increases the damage caused by cracks and color cracks.
(2) Artificial accelerated aging equipment
In order to imitate various climatic factors in nature, in order to create the requested artificial climate in the laboratory, and to achieve the purpose of accelerating the test, an artificial aging machine can be used. According to the light source used, the artificial aging machine can be divided into ultraviolet carbon arc type, sunlight carbon arc type, xenon lamp type, high pressure mercury lamp and combination light source and other types. The ultraviolet carbon arc type uses two ultraviolet carbon arc lamps, which use the arc of the upper and lower carbon rods to generate greater ultraviolet energy, which passes through the quartz lamp sheet. After filtering, it emits a continuous spectrum, and its 300~380nm part has a large spectral energy, while the energy of visible light and infrared light is very small.
High-pressure mercury lamps are arc discharge lamps of mercury vapor, available in straight and U-shaped shapes. Like UV carbon arc lamps, they also primarily produce UV energy. However, the spectrum emitted by the high-pressure mercury lamp is a linear spectrum, which is discontinuous, and the energy is concentrated near several characteristic spectral lines. In addition to the 300-380nm band, there are many short-wave components less than 290nm, so it is sensitive to paint. Membrane destruction has different effects than carbon arc lamps.
The sunshine carbon arc type utilizes the different metal elements contained in the carbon rod, so that in addition to the ultraviolet light range of 278~400nm, the visible light part of the visible light part is also very large, and the infrared light part is very small, so that the spectrum of the light source The spectral energy distribution is closer to the spectral energy distribution of the sun, which is a step higher than the carbon arc lamp and high-pressure mercury lamp that simply emit ultraviolet bands.
The xenon lamp that has been popularized recently is an arc discharge lamp filled with high-purity xenon gas. It is made of a transparent quartz glass tube with a metal electrode sealed at both ends. Compared with the above-mentioned light sources, the xenon lamp is professional in that its spectral energy distribution is closer to sunlight, so that the simulation of the test can be greatly improved; in addition, the radiation intensity of the xenon lamp is relatively uniform. When the lamp current changes within a considerable range , the characteristics of its spectral distribution remain unchanged. Of course, the xenon lamp also contains a small amount of short-wave components below 300nm and infrared parts with high heat generation that are not found in natural light. Therefore, in order to avoid affecting the test results, it is also necessary to filter the light and cool the lamp tube with cold water.
Through the accelerated aging test of these varieties, we can get the basic concept of the test effect of these equipment: using the ultraviolet carbon arc aging machine, the result is basically the same as the natural exposure test, the paint film is losing light; discoloration and cracks In terms of destruction phenomena, it is more consistent with natural exposure to Japan and the West. The single defect is that there is no chalking or little chalking. This is especially obvious in the two types of alkyd and nitro paints; It is characterized by normal pulverization, good reproducibility, and easy operation. However, due to its more short-wave components less than 290nm, some varieties have a certain degree of distortion in the test compared with natural ones.
From the actual test situation, the sunshine type aging machine is not much faster than the ultraviolet carbon arc type in terms of acceleration rate, and the damage phenomenon is basically the same, but in terms of some damage characteristics, the color of the paint film in the aging process The change is quite consistent with the hue change of natural exposure sun and west, which shows that it has superior performance than the ultraviolet carbon arc type and high pressure mercury lamp type aging machine.
Like the high-pressure mercury lamp, the xenon lamp is also a tube type. It is easy to use and operate, and the xenon lamp is very good at simulating nature. It is especially suitable for the fading test. The disadvantage is that the acceleration rate is not satisfactory. In order to solve the problem of simulation and improve the acceleration, there are suggestions at home and abroad to use combined light sources, that is, to install two of the above-mentioned various light sources in the aging machine at the same time with appropriate coordination to solve the problem of simulation and acceleration. However, until now, it has not been widely promoted and applied.
