How much do you know about powder coatings?

In the application practice of powder coatings, customers will pay attention to a question, that is, what is the spraying area of powder coatings? Hualan pigments, in fact, powder coating products with a large spray area have a very obvious feature, that is, they have good free flow, which means that the powder coating is loose and easy to fluidize during use, from the powder supply bucket to the spray gun. The transmission is light, and the powder from the spray gun is in a good atomization state without agglomeration or powder spitting.

Influencing factors and control methods of powder coating flow performance

The fluidity of powder coatings is one of the basic characteristics of powder coatings. Improving the fluidity of powders can effectively improve the powder application rate of powder coatings, and is also an important link in the quality management of powder production processes. We systematically discuss and solve the powder fluidity problem according to the comprehensive factor analysis of the materials used in the formulation, the particle size distribution analysis of the powder coating, and the influence of the powder production process on the fluidity.

1. The method of controlling the free flow of finished powder

1.1 The main component of low temperature pulverized powder coating is resin, and the molecular weight of resin is low, generally only a few thousand, the softening point is generally not more than 115 ℃, and the softening point of epoxy resin is about 90 ℃. The glass transition temperature Tg of resin for powder coating is lower, generally around 58℃. At present, the grinding and pulverization of powder coatings adopts ACM mill, and the pulverization and powder classification are carried out at the same time. In the actual production process, when the ACM mill works for a long time or in hot summer, the temperature of the powder coming out of the screen is too high. If the powder is packaged immediately, it will agglomerate quickly. Therefore, we installed a refrigeration device in the air inlet pipe of the ACM mill to improve the coating construction performance of the powder.

Influencing factors and control methods of powder coating flow performance

1.2 Post-mixing flow aid In order to improve the fluidity of the powder coating, we generally add fumed silica while extruding the sheet and pulverizing it. Fumed silica is a fine amorphous silica obtained by flame hydrolysis of silicon tetrachloride. It is composed of spherical particles with an average particle size of 7~40nm. The powder coating thus prepared is observed under an electron microscope, and the powder particles are not sticky and have a strong particle feel, because the colloidal silica particles with finer particle size and smaller specific gravity float or flow between the powder particles. These particles act as ball bearings and effectively prevent the powder coating from agglomerating. Commonly used fumed silicas are: M-5 and EH-5 of CABOT of the United States; Aerosil200 and Aerosil 972 of DEGUSSA of Germany; H-15 and N- of WACKER of Germany 20. The fumed silica suitable for powder coatings is divided into hydrophilic and hydrophobic types in terms of chemical structure. Hydrophobic silica improves the charging performance of powder coatings better than hydrophilic type. Usually, powder is added during pulverization. 1‰~2‰ of the total amount has the ideal effect. Such low levels of fumed silica have little detrimental effect on the powder coating's film-forming properties and flow properties during curing, and sometimes increase the film's edge-covering capabilities. However, adding too much fumed silica will bring negative effects. From the physical properties of fumed silica, colloidal fumed silica is very fine, has strong scattering, has a particularly large specific surface area, and is easy to float, so accurate Dosing fumed silica is difficult. For this purpose, equipment suppliers have developed special equipment to precisely add fumed silica according to actual working conditions. In the actual production process, we use the following two adding methods:

A: Add a feeding port in the feeding section of the pulverizer, and use a special feeding machine to quantitatively add fumed silica to complete the co-pulverization with the extruded semi-finished product.

B: Add a feeding port on the rotary screen of the ACM mill, use mechanical feeding to feed the fumed silica from the feeder, and crush and sieve it together with the finished product.

2. Control the particle size distribution of powder

The ACM mill has the function of adjusting the particle size of the powder. When different types of powder flakes are pulverized under different environmental conditions (temperature, season, humidity, etc.) Particle size control is carried out, and a laser particle size analyzer is used to monitor the distribution of powder particle size in a timely manner. The analysis of the powder particle size distribution test chart focuses on the following aspects:

Influencing factors and control methods of powder coating flow performance

——Average particle size: generally expressed as Dv50, which means that the powder particles larger than and smaller than the particle size account for 50% each, and the Dv50 of ordinary powder coatings is about 30~35μm

——Fine powder content: generally refers to the percentage of powder particles smaller than 10~15μm in the total volume of powder particles, and the fine powder content of ordinary flat powder coatings should be controlled below 10%.

——Coarse powder content: refers to the percentage of powder particles larger than 90 μm in diameter in the total volume of powder particles. Coarse powder particles (eg >115μm) should be avoided.

According to practical experience, the particle size distribution parameters of conventional powders are controlled as follows:

——Powder for high voltage electrostatic spray gun: Dv50=30~35μm, fine powder less than 10μm <8%, coarse powder greater than 90μm <3%.

——Powder for friction gun: Dv50=40~45μm, the content of fine powder less than 10μm is less than 6%, and the total amount of powder particles less than 70μm is more than 90.

3. Spraying environment

Powder coatings are very sensitive to the adsorption of moisture, and the powder coatings after moisture absorption are harmful to their fluidity. The hygroscopicity (water content) of powder coatings directly affects the dielectric constant of the powder. Slight moisture absorption will affect its charging performance, reduce the powder loading rate, affect the fluidity and film-forming properties of the powder, etc., so that the coating film is not smooth. Even if it is adsorbed on the workpiece, the coating film will produce air bubbles and pinholes, and the spray gun will block the gun. Therefore, in the powder coating coating process, the humidity during spraying and storage should be strictly controlled.