Reasons for bad curing of powder coating film
In powder spraying, the reason for bad curing of the coating film is twofold, one is the formulation design of powder coating is not reasonable, the coating film performance can not meet the requirements, the second is the performance of the powder coating itself is not a problem, but in powder spraying, the coating process control is not reasonable, resulting in the coating film performance failed to meet the technical specifications.
Considering from powder coating, if the matching of resin and curing agent in the formula design is unreasonable, such as the choice of reactivity of resin or the variety and amount of curing agent is unreasonable; the mass percentage content of filler or volume concentration is too large, etc. These problems will affect the physical and mechanical properties of the coating film.
Considering from the powder coating process, the reasons that affect the poor curing of the coating film are as follows.
1. The baking temperature of the oven fails to reach the required baking temperature of the powder coating. Although the control temperature of the baking oven is easy to reach the design temperature or the baking temperature required by the powder coating when there is no load, but under full load, due to the lack of total heat supply, sometimes it may not reach the design temperature or the baking temperature required by the powder coating, especially in northern areas where the workshop temperature is low in winter, if the insulation condition of the baking oven is poor, the temperature of the coated material before entering the baking oven is low, and the coated material is When the wall is thick and the heat capacity is large, it is more likely to have this problem. When the temperature of the baking oven does not reach the curing temperature of powder coating baking, it is more likely to have this problem. When the oven temperature does not reach the powder coating baking and curing temperature, on the one hand, by increasing the heating power to solve; on the other hand, can also be solved by extending the baking and curing time and other measures.
2. In the baking oven, the baking curing time does not reach the baking time required for powder coatings. In the powder coating baking conditions, the baking time refers to the baking time calculated after the coated material reaches the baking temperature, not the time calculated after the coated material is put into the baking oven. When the coated material is thicker and more pieces are hung, then the heat capacity of the coated material is bigger and the time needed for the coated material to warm up to the control temperature is longer. In a certain length of baking oven, when the speed of the conveyor chain is constant, the effective baking time will be shorter. If the effective baking time is not enough, the coating film will not be cured completely and the physical and mechanical properties of the coating film will not reach the requirements. Therefore, in powder coating, it is best to use the oven temperature tracking meter to determine the temperature change of the coated material in the baking oven under the full load condition to determine the reasonable baking and curing temperature and time, so as to ensure the complete curing of the coating film, and also to ensure the physical and mechanical properties of the coating film.
3. The surface of the coated material is not well treated. In the coated surface oil is not clean, oxidation or rust is not clean, or phosphate film quality is not good, in the surface stains many phosphate residues, etc., in this case, powder coating after the adhesion of the coating film and the substrate is not good, while leading to the impact resistance of the coating film is also bad. This problem is solved by improving the quality of surface treatment of the coated material.
4. In terms of coating film performance inspection, it should be carried out strictly in accordance with the test temperature and coating film thickness specified in the product standard. Generally speaking, the film thickness is thicker than specified; when the test room temperature is lower than the specified temperature, the film's impact strength, flexibility, cupping test and adhesion and other properties are poor and not easy to pass; but the film thickness is thin, the test temperature is higher than the specified temperature, these physical and mechanical properties are easy to pass. If you want to really accurate determination of coating film performance, you must test under the conditions specified in the standard or close to the conditions to be better. From the point of view of ensuring product quality, try to test the coating film performance indicators under more demanding conditions in order to ensure the quality of the coating product without fail.
5. Here also take into account the baking curing temperature is too high, baking time is too long, due to the thermal aging of the coating film so that the color of the coating film changes, while the film flexibility, impact strength and other physical and mechanical properties of the phenomenon of decline. Therefore, the baking and curing temperature is too high and the time is too long, which is also unfavorable to the physical and mechanical properties of the coating film.
In powder coating, the baking and curing temperature and time of powder coating play a decisive role in the physical and mechanical properties of the coating film. Therefore, it is necessary to go through the condition test and reasonably classify the coated objects to determine the number and arrangement of pegs, as well as the corresponding baking temperature and time, in order to ensure the quality of coating products.