What are the factors that affect the particle size and distribution of powder coatings?

Electrostatic spraying of powder coatings has entered the field of application since the 1960s, and has become more and more popular.

As a 100% solid powder coating, it has the characteristics of high coating efficiency, good protection and decoration, etc.

With the development of the industry, the downstream manufacturers of powder coatings have higher and higher requirements for powder coatings in the spraying industry

This puts forward higher requirements for the production process of powder coatings, in which particle size and particle size distribution are extremely important quality indicators, which directly affect the fluidity, stability, electrification, coating appearance, spraying efficiency and powder The utilization rate directly determines the "good use" of powder coatings

The particle size of powder coating refers to the average diameter of irregular-shaped particles.

1) Affect the powdering performance of the powder: if the powder is too coarse or too fine, the powder will be difficult to be powdered.

2) It will affect the thickness and leveling of the coating: Generally speaking, the smaller the powder particle size is, the better the leveling of the coating film is.

3) Affecting powder stability: the larger the amount of fine powder (below 10 μm), the worse the powder stability, that is, the powder is easy to absorb moisture, agglomerate, etc.

4) Affect the utilization rate of the powder: when the average particle size of the powder is too fine, or the content of the fine powder (below 10 μm) is too high, the fine powder is not easy to be recycled;

5) Affect the texture size of some textured powders: For example, the fineness of the sand textured powder is directly related to the particle size of the powder, and the texture size of some wrinkle, moiré or reticulated varieties with added texture agent is also affected by the particle size.

It can be seen that the particle size and distribution state of powder coatings directly affect the chargeability, powder rate, stability, leveling, film thickness and texture size of powder coatings, which are important factors affecting their coating construction performance.

Below, we analyze and discuss the factors affecting the particle size and distribution of powder coatings from the perspectives of powder coating formulation, main and auxiliary mill rotating speed, feeding speed, mill temperature, induced draft size, cyclone separator, and particle shape

01

Grinding screening parameter control

Mill pulverization is an important process in the production process of powder coatings.

Whether the particle size and particle size distribution of powder coatings can meet the requirements first depends on the crushing equipment used

Its structure diagram is shown in Figure 1.

1.
ACM main and auxiliary grinding parameters

The grinding disc of the main mill is equipped with grinding columns, and the tablets are hit by the high-speed grinding columns in the grinding body, and collide with each other, and then are crushed into powder.
Coupled with the action of bottom-up airflow, it is transferred to the upper part of the grinding disc and brought into the classifier blade for particle size classification
.

During the classification process, the large particles are subjected to a large centrifugal force, and are thrown to the inner wall of the mill for continued pulverization
.

The speed of the linear speed of the grinding disc will directly affect the particle size of the powder.
If the linear speed is too fast, the force and times of the impact of the powder particles will be large, and the particle size of the powder will be too fine; The particle size of the powder will be too coarse, which will also increase the resistance of the mill
.

The size of the gap between the auxiliary mill blade and the grinding body cover will also affect the particle size of the powder.
If the gap is large, the powder particles are easy to short-circuit during the classification process, so there will be more coarse powder; There are many, so we need to control the gap, generally 2-3mm is more suitable
.

The speed of the auxiliary mill will directly affect the particle size of the powder.
Due to the influence of centrifugal force, the auxiliary mill rotates quickly, and the powder particles with slightly coarse particle size are not easy to be classified upwards, and the particle size of the powder is fine.
The diameter is thick
.
Therefore, in the production process, we must adjust the corresponding auxiliary mill speed according to the particle size requirements of different customers and powder types
.

2.
Feeding speed

The feeding speed directly affects the amount of material in the mill at the same time, and also directly affects the residence time of the tablet in the mill.
Control of particle size distribution plays a decisive role
.

3.
Mill temperature

Cyclone separator: The powder particles are transported to the cyclone separator for gas-solid separation by airflow, and a vortex airflow is formed in the cyclone separator.
Also known as air lock valve) is input to the rotary sieve for screening, and finally the finished product with suitable particle size is obtained
.

The ultra-fine powder particles are separated and brought into the micro-powder collector.
This part of the powder is what we call ultra-fine powder
.
The tightness of the rotary valve has a great influence on the efficiency of the cyclone separator.
Excessive air leakage will destroy the gas flow of the cyclone separator, affect the separation efficiency, and cause the powder particle size to be finer and the ultrafine powder content to increase significantly
.

Induced fan: The fine crushing of powder is carried by wind, so the air volume will directly affect the effect of powder crushing
.
If the air volume is too large, there will be more coarse powder; if it is too small, there will be more fine powder, and the powder temperature will be high
.

The air valve adjustment plate of the induced draft fan can adjust the air volume.
We adjust it according to the speed of feeding, the particle size of the powder and the current load of the induced draft fan
.
When the damper is closed, the current of the induced draft fan is the smallest; when the damper is opened, the current will increase, and the larger the air leakage of the mill system, the current of the fan will increase accordingly
.
Usually, we can judge whether the mill system is leaking according to the current of the induced draft fan to ensure that the system is in a normal and stable state
.

When any part above the rotary valve (including the rotary valve) leaks air, the pulse bag is blocked, the filter at the air inlet of the mill is blocked, etc.
, the air volume will be reduced, the load of the mill will be increased, the powder particle size range will be too wide, and the ultra-fine powder will increase
.
Therefore, when the rotary valve, cyclone separator, air duct joint and other parts have serious air leakage, we will find that there are a lot of fine powder, so we should observe it in time during the production process to ensure that no air leakage occurs in all parts
.

02

Powder coating formulation composition

Powder coating formulations are generally composed of film formers, pigments, fillers and auxiliaries.
In actual production, it is found that the type of film formers and the content of fillers have a great influence on the particle size and distribution of the powder produced.
The type of film formers and fillers The content can directly affect the toughness of the melt-extruded material at room temperature.
The greater the toughness, the less likely it is to be broken, which is commonly referred to as "not easy to grind".
"Good grinding" tablet, the less fine powder (<10μm) in the powder with similar average particle size obtained:

1.
Film former

At present, the more common film-forming materials are pure polyester type, epoxy polyester type, and pure epoxy type.
The material after melt extrusion has different flexibility (plasticity) at room temperature.
Generally, pure epoxy type It is more brittle, followed by epoxy polyester, and pure polyester has the best flexibility, and different types of polyesters in the same TGIC-cured pure polyester system also have different flexibility
.

2.
Filler content

The amount of filler also directly affects the brittleness of the tablet.
The larger the amount of filler, the more brittle the tablet is, the easier it is to break, and the less likely it is to obtain powder with a concentrated particle size distribution
.
As the amount of filler increases, the tablet is more easily broken and the resulting particle size distribution is less concentrated, and the fines content in it increases significantly with the increase of filler
.
Analysis believes that the addition of filler reduces the flexibility of the tablet, and the tablet is particularly easy to be crushed when it is stressed
.

3.
Sheet temperature

The tablet temperature also directly affects the grinding efficiency and particle size distribution.
During production, the actual temperature of the tablet is generally required to be below 30°C, which is beneficial to the grinding efficiency, as well as the particle size distribution and the shape of the powder particles to be spherical
.
Grinding when the temperature is too high will cause "jagged edges", etc.
, and it is easy to cause current overload
.

In summary, particle size and particle size distribution have a great influence on the workability and performance of powder coatings, which can be adjusted by grinding parameters and coating formulation composition in actual production
.

In terms of grinding parameters, the particle size and distribution can be adjusted by adjusting the rotational speed, air pressure and air volume of the main and auxiliary mills, thereby ensuring the balance of production efficiency and particle size distribution
.

In terms of formulation, polyesters with different flexibility can be selected and the amount of filler added can be adjusted to achieve a balance between particle size distribution, performance and cost
.