Causes of clumping in powder coatings and precautions
Powder coatings may exhibit clumping or lumping during application, which is due to high temperatures and humidity during production, storage, and transportation. The glass transition temperature of the powder coating is approximately 40 degrees Celsius. To prevent clumping and lumping of powder coatings during application, the following should be noted:
1、When selecting resins, choose resins with high glass transition temperatures (TG). Common epoxy and polyester resins have TG values around 50°C, which can meet general requirements. When storage conditions are demanding, reduce the use of alcohols during synthesis to lower the resin's glass transition temperature, and use amine hardeners for epoxy resins.
2. In the formulation design of powder coatings, reduce the use of low-glass transition temperature polymers, such as leveling agents and light stabilizers, with a glass transition temperature of approximately 30°C. A small reduction in these additives is acceptable. Adding plasticizers reduces the rigidity of the resin, lowering its Tg. Excessive use of TGIC also lowers the Tg of the resin.
3. The auxiliary additives include internal, dry powder flow agents and external anti-caking agents, such as carbon black. The dry powder flow agents primarily consist of wax-based organic materials that prevent adhesion. Anti-caking agents, primarily belonging to the silicate class, are inorganic materials that are mixed with flakes and then ground and sieved for use. This prevents the adhesion and clumping of powders. External carbon black, produced using the gas-phase method, is lightweight and readily absorbs moisture. Therefore, it must be dispersed and kept dry to prevent moisture absorption and the appearance of white spots.
4、The production process primarily focuses on controlling two key stages: extrusion and pulverization. During extrusion, a long conveyor belt and a fan are used to physically cool the extruded material, effectively reducing its temperature. once the material has cooled, it is then pulverized. During pulverization, the feed rate can be reduced, and the airflow can be increased appropriately. If possible, a cold air unit can be installed at the inlet to lower the temperature within the pipe. The finished powder should be cooled to below room temperature before packaging. Some powder plants divide a box of powder into two bags for packaging, which helps to prevent powder accumulation and, to some extent, prevents powder clumping.
5、Powder coatings should be stored in a dry, well-ventilated workshop. Some high-gloss and textured products that are prone to clumping should be stored in an air-conditioned warehouse or wrapped in aluminum foil to provide insulation. Transportation should be carried out using refrigerated trucks or trucks covered with tarpaulins. Avoid using box-type trucks. Also, avoid stacking heavy objects.