The main reason is the change in the colorant to solar ultraviolet light and visible light in sunlight color products caused by the destruction.

Weathering is defined as a colorant and geometry without body after sunlight, under natural temperature and rain and dew wetting generated by color change.
The main reason for the changes in the atmosphere in addition to sun colorant and humidity and atmospheric constituents. Cold temperature and rapid changes in different temperature, can accelerate the change and destruction of colorant. Normally stable humidity and temperature can slow down the speed of destruction. Some of the atmosphere in the atmosphere will be associated with the light to make the pigment changes, such as oxygen and ozone. Oxygen is the root cause of product oxidation. The chemical structure of damaged colorants ozone, cause fading. Automobile exhaust and factory waste gas, often containing water soluble acid compounds, these acids can also cause serious erosion of color.
Weatherability colorant contains light fastness. But does not cover the light fastness of weatherability. Some varieties of colorant shows very good light fastness, but the weather is not good enough. This is because the climate factors affecting the weatherability of colorant in addition to sunshine and humidity, atmospheric composition, and time and humidity is usually the most important atmospheric parameters.
Light fastness index evaluation for Grade 8, Grade 8, grade 1 of the worst.
The weather resistance index is 5 grade system, the 5 level system is the best, and the 1 level is the worst.

Lightfastness (when) the importance of its use varies, sometimes is extremely important, such as outdoor buildings with boards, billboards and taillights. In these products and hope that the products in at least ten years of full sunlight exposure conditions remain stable.

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