Views: 15 Author: Site Editor Publish Time: 2020-07-06 Origin: Site
Discussion on Factors Influencing the Flowability of Powder Coatings for Electrostatic Spraying of Aluminum Profiles
Powder coating is a type of coating that is dispersed and painted with air as a carrier. It is a typical "4E" (efficient, economic, ecological, and energy) product, which grows at a rate of more than 10% every year. Especially in terms of aluminum construction materials, compared with anodizing and electrophoretic coating surface treatment methods, the degree of water and air pollution and energy consumption are significantly reduced, but the mechanical properties of the coating film such as hardness, wear resistance, acid resistance and other indicators are substantially Increased, the service life is double that of ordinary anodized aluminum material, and the color is rich, which can reflect the diversity and individuality of the building. Therefore, aluminum building materials sprayed with powder coatings are well received by users for their excellent durability, decoration and formability. At present, powder electrostatic spraying has become a hot spot in the surface treatment of aluminum profiles in China. In the electrostatic spraying process of powder coatings, the powder with good fluidity, like water boiling, looks very fluffy and has the effect of running water. From the powder supply barrel to the spray gun, the powder is transported easily, and the powder comes out of the nozzle and atomizes well to overcome the powder Coating agglomeration causes abnormal production phenomena such as spray gun blocking or "spitting powder", avoids the occurrence of pinholes, shrinkage holes, shallow pits, etc. on the surface of the sprayed product due to improper use of powder coating, and improves labor productivity. Discussing the influencing factors of powder coating fluidity has certain guiding significance for the production and use of powder coatings.
１ Liquidity test
The fluidity of the powder refers to the degree to which the powder coating is fluidized by air in the fluidized bed, and it is an important basis for evaluating the construction performance of the powder coating. The flowability of powder coatings can usually be measured by the angle of repose or the coefficient of fluidity. The angle of repose is a direct detection method for powder fluidity. The smaller the angle of repose, the better the fluidity. The fluidity of the powder coatings produced by a company is determined by the Seiler fluidity test method. Enterprise standards require that the fluidization coefficient of the powder coatings to be tested is about 140, which has good fluidity and sprayability. The relationship between fluidity coefficient and fluidization performance of powder coatings is shown in Table 1.
2 Analysis of factors affecting liquidity
2.1 Selection of powder coating resin
The Tg of the powder coating resin determines the flow properties and storage stability of the powder coating. Tg is the transition temperature of an amorphous polymer from a glass state to a highly elastic state. The relationship between the three polymer mechanics states and the temperature transition  is shown in Figure 1.
As can be seen from Figure 1, the Tg of the polymer increases, the stability of the powder increases, the agglomeration decreases, and the fluidity of the powder coating improves. Tg increases with increasing molecular weight. When the molecular weight increases to a certain extent, the glass transition temperature of the polymer does not change significantly. If Tg is too high, the number average molecular weight of the resin is large, the melt viscosity is large, the fluidity is poor, and the mechanical properties of the powder coating film decrease. In the production, the molecular weight of the resin is generally selected from 2000 to 5000, and it is more suitable to control the Tg within the range of 50 to 65°C.
2.2 Powder coating particle shape
The shape of the powder coating particles has a great influence on the powder fluidity . Generally, the shape of the powder particles is divided into three types: spherical, angular and whisker. The relationship between powder fluidity and particle shape is shown in Table 2. Irregular angular powder has better fluidity, and whisker powder has the worst fluidity. At present, most of the production processes of powder coatings are produced by mechanical pulverization, and most of the powder coating particles are amorphous irregular bodies.
2.3 Particle size and distribution of powder coating
The surface appearance of the coating film is also affected by the powder particle size and distribution. The smaller the particles, the lower the heat capacity, and the less time it takes to melt, so that they can coalesce into a film faster and level, resulting in a better appearance; while larger particles require less time to melt, and longer particles, The greater the chance of an orange peel effect. Powder particle size is also an extremely important quality indicator of powder coatings, which affects the appearance quality of products, powder powder rate , powder stability and mechanical properties of coating films. Therefore, the control of powder particle size is the key to the quality of powder coatings of various powder manufacturers. Fig. 2 is the particle size distribution diagram of the pure polyester A9016SF75 aluminum profile powder coating produced by a company, which is detected by a laser particle size distribution analyzer.
It can be seen from Figure 2 that: powder with a particle size of less than 10 μm (superfine powder) <10% (mass percentage, the same text throughout), ultrafine powder has poor fluidity and chargeability; powder with a particle size greater than 90 μm <3%, The particle size is too large, the appearance quality of the coating film is reduced, and the orange peel is serious. The particle size of the powder coating is controlled at 20 to 80 μm, which is more suitable for the requirements of electrostatic spraying. The particle size and particle shape of the powder coating are carried out in the ACM milling sieve. Therefore, adjusting the speed, feeding speed, cyclone separator, and mesh size of the ACM mill to control the process parameters and produce qualified powder coatings.
2. 4 Fluidized air
The flow state of the powder coating is related to the fluidizing air pressure. Properly increase the air pressure of fluidizing air to improve the fluidization quality of powder coatings. The air pressure of the fluidizing gas is determined by the volume of the powder barrel and the quantity of powder. Under a certain pressure, the gas supply amount is adjusted to make the powder in the fluidized barrel reach a stable fluidized state before coating. In order to improve the fluidization quality of the gas-solid fluidized bed, the bottom of the powder barrel is provided with a gas distribution plate to distribute the introduced air evenly. During the production process, the pressure of the fluidized air in the powder barrel is generally 0.01 to 0.1 MPa, and the auxiliary air flow rate is 4 to 5 m3/h.
2. 5 spraying environment
Powder coatings are very sensitive to the absorption of moisture, and the powder coatings after moisture absorption are extremely harmful to their fluidity. The hygroscopicity (water content) of the powder coating directly affects the dielectric constant of the powder. Every 30% change in relative humidity is equivalent to two orders of magnitude change in the resistivity of the powder , as shown in Figure 3.
Slight moisture absorption will affect its charging performance, reduce the powder loading rate, affect the powder's fluidity, film formation performance, etc., so that the coating film is not smooth, even difficult to adsorb on the workpiece, the coating film will produce bubbles and pinholes and spray guns Disadvantages such as blocking the gun; the powder will be clumped if it is seriously hygroscopic, and it cannot be electrostatically sprayed. Therefore, in the process of powder coating, the humidity of spraying and storage is strictly controlled. The fluidized air purification system, after passing through a secondary filtering and moisture absorption device, makes the mass concentration (water content) of compressed air water less than 15×10-6 mg/L, and installs a dehumidification air conditioner in the powder spraying room in areas with relatively high humidity. The relative humidity is controlled at about 60%, which is ideal for painting.
2. 6 Fluidization additives
Fumed silica is a porous material with strong hygroscopicity, which can prevent the moisture absorption of powder coatings, improve the fluidity, anti-caking properties and powder tube conveyance of powder coatings, often in the fine crushing stage of powder coating production process Add to. Fumed silica can coat the surface of powder coating particles to form a separation layer between powder coating particles, reduce mutual attraction and friction between powder coating particles, prevent mutual contact between particles, and improve the dry powder coating powder The fluidity is usually 0.1% to 0.4% of the total mass of the powder coating. Too much addition will affect the appearance and mechanical properties of the coating film. If the addition is too little, the effect of fluidization is not obvious.
The fluidity of powder coatings is the result of a combination of factors. In addition to the main factors discussed above, it is also related to other factors, such as commonly used pigments, filler properties and added amount , spraying process conditions, and angle of repose detection methods, etc., which require scientific and technological personnel to continuously explore in production. Discussing the fluidity of powder coatings, improving powder coating formulations and spraying production process control parameters are of great significance for guiding production practice.