The coating film of highly decorative coatings, such as the surface of automobile topcoat, will appear that the reflection image is not clear enough, as if covered by a layer of fog . This is because the reflected light incident on the coating film scatters to both sides of the reflection direction. The reflected image is not clear, which is called poor sharpness. Generally, it can be measured by haze gloss meter. The haze gloss meter has a statistical function and can provide single-angle glossiness, double- angle glossiness, automatic angle selection, and display of haze values. Through the rapid measurement of the instrument, it can be judged whether the haze value of the coating film meets the requirements of the standard sample. At present, there is an index of distinctness of image (expressed by PGD value) in the standard of automobile top paint, which is determined by comparing it with the standard contrast of glass mirror. Taking the clarity of image of the glass mirror as the standard, the value is 1, and the clarity of image of the paint film is generally as high as 0.9 (for luxury car topcoat), and the clarity of image of the truck topcoat should also be above 0.4. Therefore, if the distinct image value of the coating film is lower than the standard, it is regarded as pathological.
The sharpness of image is mainly directly related to the leveling property of the coating film after coating. After coating a high-gloss surface coating, various pathological conditions (such as orange peel, micropores, loss of gloss, etc.) appear on the coating film surface. Pilling, wrinkling, etc.) will cause haze on the surface of the coating film, resulting in poor sharpness of the coating film. Secondly, it is related to the roughness and surface finish of the workpiece substrate and the curvature of the workpiece.
The preventive measures are mainly to control the intrinsic quality of the coating product. In the process of application, firstly, good leveling is required. Avoid film defects. The sharpness of image has a lot to do with the substrate treatment and the surface state of the matching primer. It has been proved by practice that if the surface state of the supporting primer and the flatness of the substrate are strictly controlled, a satisfactory sharpness of image will be obtained.
