Analysis of common problems in the use of coating (3)

Flaking is usually caused by poor surface treatment, dirt in the substrate or coating, such as dust, dirt, grease or chemicals, etc., the curing time between the two coatings is too long, or powdering occurs Peeling is similar to peeling, with the exception that the peeling coating is hard and brittle and can be removed from the substrate Once the coating cracks, the edge will roll up from the base material, which may cause peeling The peeling of the coating is due to aging, weathering, etc., which can be judged according to ISO 4628-5 6 cracking in many cases, cracking is caused by the formulation of the coating itself, aging and weathering of the coating The cracking of the coating can be divided into fine cracking, cracking and crazing Fine crack is a small crack on the surface of the film This is a surface phenomenon that has not penetrated the entire coating depth Some cracks are hard to find with the naked eye and can only be seen under the magnifying glass Fine cracks are usually caused by pressure on the coating surface, and in many cases by formulation design In most cases, the generation of fine cracks is due to the improper combination of resin and pigment Or when the film is dry, the surface hardness is high and the brittleness is large, so the surface stress is produced When it develops to a certain extent, it will produce fine cracks Although the fine cracks do not penetrate into the base material, the climate change (cold and hot weather, etc.) will lead to further deterioration of the surface fine cracks When the coating cracks, there are two kinds of conditions, that is, the cracks in the coating and the cracks reaching the substrate When the shrinkage rate of the coating surface is much faster than that of the body, that is to say, under the action of surface stress, the coating will crack If the hard and tough coating is applied on the soft coating, the cracking will occur A typical example is the coal tar pitch coating When exposed to the sun, the surface becomes hard, while the lower part is still a soft coating, which is easy to crack In the same way, if other hard coating is applied on the asphalt coating, when the surface coating shrinks and hardens, it is enough to cause the asphalt coating to crack, and at the same time, it also drives the top coating to crack together Therefore, it is not allowed to coat the soft primer with oxidation polymerization or chemical curing hard coating