ã€China Aluminum Industry Network】 The powder coating film formation depends on the flow of the resin in the molten state and adheres to each other. The melt has a high viscosity, which limits its free flow. The leveling effect is somewhat poor, and slight orange peel is produced. To improve the leveling property of powder coatings and form a good-looking coating film has always been the focus of the powder coating industry. Therefore, it is of great practical significance to explore the factors that affect the leveling property of powder coatings, optimize the formulation design, improve the equipment and construction process, and improve the appearance of powder coatings. 1 leveling mechanism Powder coating is a solvent-free homogeneous system. It must be melted before it adheres to the surface of the object to be coated. After leveling and curing, it usually takes three processes [1]: (1) from a single powder particle, aggregate into a continuous, uneven film, this process is called the aggregation process; (2) Form a relatively smooth and even surface from the continuous uneven surface, ie the leveling process; (3) The melted coating solution undergoes cross-linking reaction, and the viscosity is continuously increased, and later solidified into a hard coating film, which is called a curing process. The time required for the leveling process can be expressed using the Nix equation and the Orchand relation [2]: A, melting phase: t = f (η × Re / r) (1) Where: t - time; η - the melt viscosity of the coating; Re - the average radius of curvature of coating particles; r - surface tension; B. Leveling stage: In the formula: Δt—leveling time; λ—distance between two peaks; η—melting viscosity; an/at—ratio of peak height before and after leveling; r—surface tension; x—coating thickness. From the formulas (1) and (2), it can be seen that the factors affecting the leveling of the powder coating are mainly the melt viscosity, surface tension, particle size, coating thickness, curing temperature and time of the resin, and the following factors are performed. discuss. 2 Factors affecting the leveling properties of powder coatings 2.1 Coating resin The glass transition temperature and melting temperature of the powder coating resin determine the flow properties and storage stability of the powder coating, which is related to the relative molecular mass and its distribution. To obtain a better coating, the narrower the relative molecular mass distribution of the resin, the better. An excessively wide distribution of relative molecular masses means that there is a resin with a too small or large relative molecular mass that cannot be well blended with the other components of the coating. In such conditions, high-temperature baking results in a mixture of coatings. Badly, the surface tension of the coating is not balanced, the force between the particles will produce a repulsive or shrinking motion, and the melt flow time is limited, there can be no sufficient leveling time between them, with the coating The curing of the layer, the coating surface will appear shrinkage, orange peel and other defects. Therefore, the resins with low melting viscosity, long gelation time of powder coatings, and high melt flowability should be selected; however, the mechanical properties of coating films are shorter, and the cross-link curing time is better. Should take into account. In addition, the polyester powder product is easy to agglomerate during storage, and sagging, orange peel, pinhole, etc. are prone to occur during spraying, which is due to the unsuitability of the glass transition temperature (Tg) of the polyester resin. Production practice test, polyester resin Tg controlled in the range of 50 ~ 65 °C, both to ensure that the coating is full, there is a good leveling, but also to make powder coatings stored below 40 °C 4 to 6 months no caking, Has a certain degree of stability. 2.2 Leveling agent The leveling agent is an ingredient that is indispensable for obtaining the appearance of a flat coating film. The leveling agent is evenly dispersed in the powder coating. When the powder is electrostatically sprayed and baked to the workpiece, it can reduce the viscosity and the surface tension of the coating system, so that the molten coating can increase the flowability before the uncured, and during the melt flow of the coating. After the surface migration, a monomolecular layer is gradually formed from the interior of the molecule to the surface of the coating film, so that the density of the orientated molecular structure is increased throughout the entire surface, and the surface of the coating film is smooth and smooth, and the formation of shrinkage pores during film formation is avoided. The effect of the level of acrylate leveling agent on the appearance of the A90162SF75 powder coating is shown in Table 1. Table 1 Effect of Leveling Agent on Leveling of Coating Film (Visual Inspection Method) As can be seen from Table 1, when the leveling agent or leveling agent is used in a small amount in the formulation, the leveling property of the coating film is poor, and the coating film shrinks or separates the shrinkage of the entire plate. When the amount is over 110% of the resin, it can be obtained. Smoothing film without shrinkage. However, excessive addition of a leveling agent will reduce the storage stability of the powder coating, and the leveling agent will easily float to the surface of the coating film to reduce the contamination resistance and gloss of the coating film. The amount of the leveling agent is 1.0 for the total mass of the resin and the curing agent. %~1.3% is more suitable.