Through the test of solvent-free polyurethane coatings in metal pipelines, the higher the pipeline temperature, the faster the curing speed. Note: The maximum temperature of the coating refers to the maximum temperature at which the paint is sprayed onto the surface of the pipe and an exothermic reaction occurs; the cooling of the coating to room temperature refers to the cooling from the maximum temperature of the coating to room temperature. The curing speed of the coating accelerates with the increase of the tube temperature; when the temperature is higher, the change in the hardness of the coating decreases with time, indicating that the higher the temperature, the deeper the reaction of the coating in a shorter time. When the temperature reaches 80, the hardness change is not obvious, therefore, it is more appropriate to control the pipeline temperature at 80.
The effect of pipeline temperature on the appearance of the coating: while isocyanate reacts with polyol, it also reacts with the moisture in the air to form CO2. If the temperature of the pipeline is lower, the reaction speed is slower, and the coating is prone to bulging and bulging Experiments have found that when the temperature of the pipeline is appropriate (determined as 90 according to the diameter of the pipeline), the coating will not blister.
The effect of pressure on the coating: pressure directly affects the atomization effect of the coating, and the quality of the atomization directly affects the appearance of the coating (the spiral phenomenon), and also affects the service life of the filter screen seal. During the experiment, it was found that the higher the pressure, the better the atomization, but the filter seal life is short; the lower the pressure, the longer the filter seal life, but the atomization effect is poor. Considering the overall effect, when the pressure is 2500Pa, the atomization is uniform, the coating has no spiral, and the filter screen has a longer sealing life.
Effect of pipeline surface condition on the coating: If the surface of the pipeline has defects such as heavy skin, burrs, iron beans, etc., which causes the local coating to be too thin, electric leakage will occur when conducting the EDM leak test. The surface of the pipeline that has not been subjected to shot blasting often has impurities such as rust spots, oil stains, and asphalt. These impurities cause the surface tension of the coating and the substrate to be different. In addition, the structure of the rust is loose and can store more air. At the same time, the combined water of the rust itself is difficult to decompose, and it is easy to react with the polyurethane coating to generate carbon dioxide gas, which causes bubbles and other defects in the coating.
Therefore, the pipeline must be shot blasted before the polyurethane coating is sprayed (blasting standard: BS7079, Part A1 Sa2.5). The pipeline should be sprayed within 4 hours after the shot blasting. If it exceeds 4 hours, the shot blasting must be repeated.
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