The film formation of powder coating depends on the flow of resin in the molten state and adhesion with each other. The high viscosity of the melt limits its free flow. The leveling effect is slightly poor, with slight orange peel. To improve the leveling property of powder coating and form a good appearance film has always been the focus of the powder coating industry. Therefore, it is of great practical significance to explore the factors affecting the leveling of powder coatings, optimize the formulation design, improve the equipment and construction process, and improve the appearance of powder coatings.

Leveling mechanism of   1

   powder coating is a solvent-free uniform system, which can only be attached to the coating after melting. After leveling, the coating can be solidified into a film, which generally requires three processes [1]:

                      ;

     (2) flow from the continuous and uneven surface to form a smooth and flat surface, that is, the leveling process;

                      (3) the.

The description of time needed for leveling process can be expressed by Nix equation and orchand relation [2]:

   a, fusion stage: T = f (η × re / R) (1)

Where: T-Time; η - melting viscosity of coating; re - average radius of curvature of coating particles; r-surface tension;

   B. leveling stage:

,

In the formula, △ T - 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.

                     it can be seen from formula (1) and formula (2): the factors that affect the leveling of powder coating are mainly the.

Factors affecting the leveling of powder coating

   2.1 coating resin

The glass transition temperature and melting temperature of powder coating resin determine the flow performance and storage stability of powder coating, which is related to the relative molecular weight and its distribution. In order to obtain a better coating, the narrower the molecular weight distribution of the resin is, the better. Too wide distribution of relative molecular weight means that there are resins with too small or too large relative molecular weight, which can't blend well with other components of the coating. Under such conditions, when baking at high temperature, the miscibility between the coatings is not good, and the surface tension of the coating can't reach equilibrium. The force between the particles will produce repulsive or contraction movement, while the melting flow time is limited, they It is impossible to have sufficient leveling time between them. With the curing of the coating, shrinkage cavity, orange peel and other defects will appear on the surface of the coating. Therefore, the resin with low melting viscosity, long gelation time and high melting level fluidity should be selected, but the mechanical properties of the film with short leveling time and long curing time should be considered. In addition, polyester powder products are easy to agglomerate in the storage process, and are prone to sagging, orange peel, pinholes and other phenomena during spraying, which is due to the unsuitable glass transition temperature (TG) of polyester resin. The test of production practice shows that the Tg of polyester resin is controlled in the range of 50-65 ℃, which can not only ensure the fullness and good leveling of the coating, but also make the powder coating not agglomerate for 4-6 months under 40 ℃, which has certain stability.

   2.2 leveling agent

Leveling agent is an indispensable component for obtaining a smooth film appearance. The leveling agent is evenly dispersed in the powder coating. When the powder is electrostatic sprayed to the workpiece for baking and curing, it can reduce the viscosity and surface tension of the coating system, increase the fluidity of the molten coating before curing, and gradually migrate from the inner part of the molecule to the surface of the coating through the surface migration during the melting process of the coating, forming a single molecular layer, which increases the directional component of the whole surface The density of the substructure makes the surface of the film flat and smooth, avoiding the formation of shrinkage cavity in the film forming process. Effect of the amount of acrylate leveling agent on the appearance of a90162sf75 powder coating

    when there is no leveling agent in the formula or the amount of leveling agent is small, the film has poor leveling performance. When the amount of leveling agent is more than 110% of resin, the film can be flat without shrinkage. However, too much leveling agent will reduce the storage stability of the powder coating, and the leveling agent is easy to float to the surface of the coating film, reducing the pollution resistance and luster of the coating film. The amount of leveling agent is 1.0% - 1.3% of the total mass of resin and curing agent.