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In the actual application of powder coatings, customers usually pay attention to a question: What is the spray area of powder coatings? In fact, the powder coatings that can be sprayed on a large area have good fluidity characteristics, which means that the powder coatings are loose and easy to fluidize during use, and the transfer from the powder supply tank to the spray gun is light, and the powder from the spray gun is atomized.
Well, there is no clumping or spitting.
Flowability is the basic characteristic of powder coatings, and the improvement of flowability can effectively increase the utilization rate of powder coatings, and it is also an important part of the quality management of the powder production process.
We discuss the influence on fluidity from the raw materials used in powder coatings, particle size distribution, and production technology.
1.
The method to control the free flow of the finished powder 1.
The method to control the free flow of the finished powder 1.
1 The main component of the low-temperature pulverized powder coating is resin.
The molecular weight of the resin is low, generally only a few thousand, and the softening point is generally not more than 115°C.
The softening point of epoxy resin is only about 90°C.
The glass transition temperature of powder coating resin is lower, generally around 58°C.
At present, the grinding and crushing of powder coatings adopts ACM mill, and the crushing and powder classification are carried out simultaneously.
In the actual production process, if the ACM mill works for a long time or in the hot summer, the temperature of the powder coming out of the screen is too high.
If the powder is packaged immediately, it will quickly agglomerate.
Therefore, we install a refrigeration device in the air inlet pipe of the ACM mill to improve the coating performance of the powder.1.
2 Post-mixing flow aid In order to improve the fluidity of powder coatings, we generally add fumed silica while crushing the extruded sheet.
Fumed silica is an extremely fine amorphous silica obtained by flame hydrolysis of silicon tetrachloride.
It is composed of spherical particles with an average particle size of 7-40nm.
Observed under the electron microscope, the powder coating made by this method shows that there is no adhesion between the powder particles, and the graininess is strong.
The reason is that the powder particles float or flow between the particles of finer particle size and smaller specific gravity colloidal silica particles.
These particles play the role of a ball bearing and effectively prevent the powder coating from agglomerating.
Commonly used fumed silicas are: M-5 and EH-5 from CABOT in the United States; Aerosil200 and Aerosil 972 from DEGUSSA in Germany; H-15 and N- from WACKER in Germany 20.
Fumed silica suitable for powder coatings can be divided into hydrophilic and hydrophobic types in terms of chemical structure.
Hydrophobic silica improves the charging performance of powder coatings better than hydrophilic ones, and powder is usually added during pulverization.
1‰~2‰ of the total amount has an ideal effect.
Such a small amount of fumed silica has almost no adverse effect on the film-forming properties of the powder coating and the flow properties during curing, and sometimes it can increase the edge covering ability of the coating film.
However, excessive addition of fumed silica will bring negative effects.
From the physical properties of fumed silica, colloidal fumed silica is very fine, has strong dispersibility, has a particularly large specific surface area, and is easy to float.
It is very difficult to accurately dose fumed silica.
For this, equipment suppliers have developed special equipment, which can accurately add meteorological silica according to actual working conditions.
In the actual production process, we use the following two adding methods:
A: Add a feed port in the feed section of the pulverizer, and use a special feeder to quantitatively add fumed silica to complete the co-pulverization with the extruded semi-finished product.
B: A feed port is added to the rotary sieve port of the ACM mill, and the fumed silica is fed from the feeder by mechanical feeding, and it is crushed and sieved together with the finished product.
2.
Control powder particle size distribution 2.
Control powder particle size distributionThe ACM mill has the function of adjusting the particle size of the powder.
When different types of powder flakes are crushed under different environmental conditions (temperature, season, humidity, etc.
), the input volume of the flakes, the speed of the main and auxiliary mills, and the air volume of the induced draft fan can be adjusted.
And so on to control the particle size, and use a laser particle size analyzer 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 for the flow performance of powder coatings.
Average particle size: generally expressed as Dv50, which means greater than and less than the The particle size of powder particles each account for 50%, 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 subdivided content of ordinary flat powder coatings Should be controlled below 10%.
Coarse powder content: refers to the percentage of powder particles larger than 90μm in the total volume of powder particles.
Powder particles that are too coarse (such as >115µm) should be avoided.
According to actual 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 smaller than 10µm<8%, coarse powder larger 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 3.
Spraying environment.
Powder coatings are very sensitive to moisture absorption, and the powder coatings after moisture absorption are extremely harmful to their fluidity.
The hygroscopicity (moisture content) of the powder coating directly affects the dielectric constant of the powder.
Slight moisture absorption will affect its charging performance, reduce the powder's powdering rate, affect the powder's fluidity, film-forming performance, etc.
, so that the coating film is not smooth.
It is even difficult to adsorb on the workpiece, and the coating film will produce bubbles, pinholes, and spray gun clogging; the powder will agglomerate if it is severely hygroscopic and cannot be electrostatically sprayed.
Therefore, during the powder coating process, the humidity during spraying and storage should be strictly controlled.
Well, there is no clumping or spitting.
Flowability is the basic characteristic of powder coatings, and the improvement of flowability can effectively increase the utilization rate of powder coatings, and it is also an important part of the quality management of the powder production process.
We discuss the influence on fluidity from the raw materials used in powder coatings, particle size distribution, and production technology.
1.
The method to control the free flow of the finished powder 1.
The method to control the free flow of the finished powder 1.
1 The main component of the low-temperature pulverized powder coating is resin.
The molecular weight of the resin is low, generally only a few thousand, and the softening point is generally not more than 115°C.
The softening point of epoxy resin is only about 90°C.
The glass transition temperature of powder coating resin is lower, generally around 58°C.
At present, the grinding and crushing of powder coatings adopts ACM mill, and the crushing and powder classification are carried out simultaneously.
In the actual production process, if the ACM mill works for a long time or in the hot summer, the temperature of the powder coming out of the screen is too high.
If the powder is packaged immediately, it will quickly agglomerate.
Therefore, we install a refrigeration device in the air inlet pipe of the ACM mill to improve the coating performance of the powder.1.
2 Post-mixing flow aid In order to improve the fluidity of powder coatings, we generally add fumed silica while crushing the extruded sheet.
Fumed silica is an extremely fine amorphous silica obtained by flame hydrolysis of silicon tetrachloride.
It is composed of spherical particles with an average particle size of 7-40nm.
Observed under the electron microscope, the powder coating made by this method shows that there is no adhesion between the powder particles, and the graininess is strong.
The reason is that the powder particles float or flow between the particles of finer particle size and smaller specific gravity colloidal silica particles.
These particles play the role of a ball bearing and effectively prevent the powder coating from agglomerating.
Commonly used fumed silicas are: M-5 and EH-5 from CABOT in the United States; Aerosil200 and Aerosil 972 from DEGUSSA in Germany; H-15 and N- from WACKER in Germany 20.
Fumed silica suitable for powder coatings can be divided into hydrophilic and hydrophobic types in terms of chemical structure.
Hydrophobic silica improves the charging performance of powder coatings better than hydrophilic ones, and powder is usually added during pulverization.
1‰~2‰ of the total amount has an ideal effect.
Such a small amount of fumed silica has almost no adverse effect on the film-forming properties of the powder coating and the flow properties during curing, and sometimes it can increase the edge covering ability of the coating film.
However, excessive addition of fumed silica will bring negative effects.
From the physical properties of fumed silica, colloidal fumed silica is very fine, has strong dispersibility, has a particularly large specific surface area, and is easy to float.
It is very difficult to accurately dose fumed silica.
For this, equipment suppliers have developed special equipment, which can accurately add meteorological silica according to actual working conditions.
In the actual production process, we use the following two adding methods:
A: Add a feed port in the feed section of the pulverizer, and use a special feeder to quantitatively add fumed silica to complete the co-pulverization with the extruded semi-finished product.
B: A feed port is added to the rotary sieve port of the ACM mill, and the fumed silica is fed from the feeder by mechanical feeding, and it is crushed and sieved together with the finished product.
2.
Control powder particle size distribution 2.
Control powder particle size distributionThe ACM mill has the function of adjusting the particle size of the powder.
When different types of powder flakes are crushed under different environmental conditions (temperature, season, humidity, etc.
), the input volume of the flakes, the speed of the main and auxiliary mills, and the air volume of the induced draft fan can be adjusted.
And so on to control the particle size, and use a laser particle size analyzer 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 for the flow performance of powder coatings.
Average particle size: generally expressed as Dv50, which means greater than and less than the The particle size of powder particles each account for 50%, 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 subdivided content of ordinary flat powder coatings Should be controlled below 10%.
Coarse powder content: refers to the percentage of powder particles larger than 90μm in the total volume of powder particles.
Powder particles that are too coarse (such as >115µm) should be avoided.
According to actual 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 smaller than 10µm<8%, coarse powder larger 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 3.
Spraying environment.
Powder coatings are very sensitive to moisture absorption, and the powder coatings after moisture absorption are extremely harmful to their fluidity.
The hygroscopicity (moisture content) of the powder coating directly affects the dielectric constant of the powder.
Slight moisture absorption will affect its charging performance, reduce the powder's powdering rate, affect the powder's fluidity, film-forming performance, etc.
, so that the coating film is not smooth.
It is even difficult to adsorb on the workpiece, and the coating film will produce bubbles, pinholes, and spray gun clogging; the powder will agglomerate if it is severely hygroscopic and cannot be electrostatically sprayed.
Therefore, during the powder coating process, the humidity during spraying and storage should be strictly controlled.
