Outdoor long-term sunlight insolation, high temperature and humidity is caused by coating cracks, loss of light, fading, yellowing, powdering of the main reasons, the weatherability of the coating is related to the spectral sensitivity of the coating components. Coating resin is mainly composed of epoxy resin, amino resin, polyester resin, including carbon and oxygen double bond, carbon carbon single bond and other functional groups. Due to different bond energies, different functional groups have different sensitivity to light and different weather resistance. To outdoor coating, ultraviolet ray is the main reason that causes coating ageing. Ultraviolet accelerated aging test can simulate the damage of ultraviolet part in sunlight to the coating, and reproduce the damage of outdoor for months or even years in a few days or weeks.

Analysis of aging factors of paint coating

Light, high temperature and humidity, any of these three factors will cause aging damage to the car paint coating, but they often work together, the harm caused by each factor will be greater than the single effect of any one of the factors.

Analysis of aging factors of paint coating

1. Sun exposure

The degree and type of degradation of the coating depends on the spectral sensitivity of the resin. Different wavelengths of light have different destructive effects. Each resin has a different spectral sensitivity, which determines whether a resin is sensitive to short wavelengths or long wavelengths or both. Each bond has a threshold wavelength, and that wavelength has enough energy to make the bond react. Wavelengths shorter than the threshold wavelength can break bonds, while longer wavelengths, no matter how strong the light is, cannot break bonds.

Automotive coating resin is mainly composed of polyester resin, amino resin, alkyd resin and acrylic resin one or more of the composition (including c-c, c-h, o-h, c-o and other functional groups), containing functional groups corresponding to the threshold wavelength range of 231-376nm (common coating resin sensitive wavelength is shown in table 1). This wavelength corresponds to the wavelength of short-wave ultraviolet light in sunlight.

2 high temperature

High temperature is also an important factor causing damage to automobile paint coating. As the temperature rises, the destruction of light increases. Although temperature does not affect the primary photochemical reactions, it does affect secondary chemical reactions. Laboratory aging tests must provide accurate temperature control and usually accelerate the aging process by heating.

3 damp

Dew, rain and high humidity are the main causes of moisture damage of automobile paint coating. Statistics show that cars left outside will stay wet for a long time every day. The dew water that forms by moisture is the main factor of outdoor moisture, the harm that dew causes to car paint coating is bigger than rain water, because it adheres to the time on paint coating is longer, cause more serious moisture to absorb.

Ultraviolet accelerated aging test

1. Test conditions

QUV aging machine was used in this test. QUV aging testing machine can achieve the following simulation functions.

2. Sunshine simulation

QUV can use fluorescent ultraviolet lamps to simulate the sun's rays to cause damage to the paint coating. The uva-340 lamp tube has a good simulation effect on the ultraviolet short-wave segment of sunlight, and its spectral energy distribution within the cut-off point of sunlight to about 360nm is in good agreement with the solar spectrum (as shown in figure 1).

3. Irradiance control

QUV aging machine is equipped with strong daylight vision controller, which can continuously and automatically control and accurately maintain the irradiance by using the feedback loop system of the solar eye. The solar eye can automatically compensate the light intensity change caused by the aging of the lamp tube and other factors by adjusting the power of the lamp. Damage done outdoors for months or even years can be simulated in just a few days or weeks.

4. Moisture simulation

The water at the bottom of the humidity simulation test room is used to heat to generate steam, which keeps the test room at 100% relative humidity. In QUV, the test sample actually forms the side wall of the test room, and the other side of the sample is exposed to the air around the room. The relatively cold air in the room makes the surface temperature of the test sample several degrees lower than that of the hot steam in the room. The temperature difference produces condensation cycle phenomenon, and the water in liquid form on the surface of the sample slowly condenses (as shown in figure 2). In addition, the condensation process is carried out at a higher temperature (usually 50), which greatly accelerates moisture erosion. Using QUV's long thermal condensation cycle to simulate outdoor moisture erosion is more effective than other methods, such as splashing, soaking or high humidity.

Thermal condensation cycles that simulate moisture erosion in the open air are more effective than other methods, such as splashing, soaking, or high humidity.

conclusion

After comprehensive comparison of the changes in gloss, color and adhesion of paint coatings of company A and company B before and after the test, it can be seen that the weathering resistance of paint coatings of company A is better than that of company B, the paint system of company B is not very stable, and the indicators fluctuate greatly among different colors.

Uv accelerated aging test, with controllable conditions, can shorten the development and selection cycle of new color coatings, predict problematic coatings, and reduce the losses of automobile manufacturers. However, the experimental data obtained by uv accelerated aging test are relative, which can only determine the weather resistance of similar materials.

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