Study on the Importance of Particle Size Control to Powder Coatings

Abstract: By introducing the influence of air classifier mill (ACM) on the particle size distribution of powder coatings in the process of use, the advantages and disadvantages of particle size analysis methods, the influence of powder coating particle size on various aspects of powder coatings (such as coating film) The appearance of the powder, the chargeability of the powder, the storage stability of the product, the thickness of the coating, the fluidization effect, the recovery rate, the angle of repose, the outflow test, the bulk density, etc.
), and systematically expound the importance of particle size to powder coatings
.

introduction 

With the promulgation of the "Soil Pollution Prevention and Control Action Plan", "Water Pollution Prevention and Control Action Plan", and "Air Pollution Prevention and Control Action Plan" regulations, the direction of oil to water and paint to powder tends to be obvious
.
Especially in terms of VOC control, the demand for powder coatings has risen sharply .
Coatings Online coatingol.
com
.

At present, powder coatings are mainly used in the treatment of substrates in the fields of home appliances, furniture, automobiles, building materials, outdoor facilities, pipes, etc.

With the increase in demand, product quality stability has gradually been paid attention to, such as formula stability, production process stability, equipment stability, product batch stability, storage stability, construction stability etc.

Generally, the color, luster or texture of the product will be different from the standard model when purchasing or purchasing for the second time.

Conventional products need to be routinely tested (such as horizontal flow or oblique flow test, gel time, particle size analysis, adhesion, impact, bending, color difference, gloss, film thickness and other physical properties) to ensure product quality and quality.

Each test plays an important role in the stability of the product batch

For example, the product's electrification, appearance, partial performance, storage characteristics, construction method, coating thickness,

Except for some particles, most of the particles are irregular, and there is no real particle size value in the strict sense

1 Particle size analysis method

The methods of particle size analysis (only the laser particle size analyzer is introduced) are divided into dry analysis and wet analysis (the test results will be different between the two test methods due to different dispersion media)

1.
1 Dry analysis

Dry analysis uses air as the dispersion medium and uses the principle of turbulent dispersion to fully disperse the sample particles, and then introduce them into the optical path system for analysis
.

Its advantages include:

(1) Processed and synthesized in a dry state, dry analysis is more representative than wet analysis;

(2) Dry analysis is the best choice for some materials that will dissolve in dispersants or exhibit hydration after dispersing (the particle size will change after encountering water);

(3) Dry analysis can complete the measurement in a short time with high efficiency
.
The disadvantage is that the dispersion mechanism of the dry method is destructive and is not suitable for extremely fragile samples
.

1.
2 Wet analysis

Wet analysis is to directly add the sample into a dispersion medium such as water or ethanol for dispersion, and then pass through the optical path system for particle size analysis
.

Its characteristics are: (1) Small or viscous samples are more suitable for wet measurement, because surfactants or other dispersants may be required to achieve complete dispersion of the sample; (2) The test time is long, and bubbles are easily generated during the measurement process.
, resulting in unstable analysis results and errors
.

2 The main factors affecting the particle size distribution of powder coatings

In the process of powder coating production, air classifier mill (ACM) is commonly used to obtain the ideal powder particle size.
focus of attention
.

At the same time, the formula of the powder coating (such as the difference in raw materials, the difference in the pigment-to-base ratio, etc.
), the relative humidity of the environment, and the temperature are also important factors that affect the powder particle size parameters
.

2.
1 ACM parameters

There are 4 adjustable parameters in the production process: feed speed, classifier speed, main mill speed, and induced air volume; 2 control values: grinding temperature and particle size value; the grinding process is actually powered by airflow, The particle size distribution and grinding temperature are the working standards, and the process of coordination and cooperation of the feed volume, the induced air volume, the main grinding speed and the classifier speed is carried out
.

At the heart of this process is the force relationship (the centrifugal force created by the rotation of the powder particles and the suction created by the airflow)
.

The centrifugal force of powder particles is related to the kinetic energy of the particles.
The greater the moving speed of the particles, the greater the kinetic energy.
The fineness or coarseness of the powder particles can be controlled by adjusting the speed of the classifier.
The specific value depends on the analysis results of the particle size analyzer
.

2.
2 Main mill and classifier

After entering the main grinding disc of the pulverizer, the broken flakes will collide with the grinding column of the main mill through the centrifugal force effect, and be broken into powder state, and will be transferred to the cyclone separator by the airflow on the inner wall of the grinding body for classification
.

Compared with the main mill, the classifier has the greatest influence on the particle size of the powder, but the relationship between them is that the larger the rotation speed, the finer the particle size, the lower the rotation speed, the coarser the particle size (other conditions are constant)
.

2.
3 Feeding speed

When other parameters are constant, the faster the feeding speed, the coarser the powder particle size, the slower the feeding speed, the finer the particle size
.

2.
4 Air volume

For ACM mills, a large air volume is conducive to cooling; but for a cyclone, the size of the air volume will directly affect the particle size of the powder: the larger the air volume, the coarser the particle size
.

The air volume is small and the particle size is fine
.
When adjusting the air volume, be sure to consider the temperature of the mill
.
Usually, the air volume is large and the temperature inside the mill is low
.

Of course, by appropriately reducing the amount of feed, the internal temperature of the mill can also be reduced (note: different formulations or varieties have different hardness of the flakes, and under the same parameter conditions, the particle size of the material to be pulverized is also different)
.

3 Several parameters of powder coating and their influencing factors

3.
1 Bulk density

The bulk density of powder coatings reflects how tightly the powder particles are packed
.
Since the powder coating will not change its stacking state after electrostatic adsorption, the stacking density also reflects the density of the particle arrangement in the dry powder layer to a certain extent (see Figures 1 to 4)
.

The size of the bulk density value depends on the particle size, shape, surface roughness, etc.
of the powder particles (constant temperature and humidity)
.

Commonly used powder coatings have particle sizes ranging from 30 to 60 μm
.
If the content of the ultrafine powder is too high, it will seriously affect the bulk density of the powder.
At this time, the powder is in a fluffy state and exhibits adhesion when grasping
.

This kind of powder fluidization is prone to local bubbling and serious flying, and it cannot provide a stable and uniform powder airflow during spraying, which is easy to cause coating defects
.
Irregularly shaped powders are loosely packed in the coating film
.

A large number of pointed powder particles, with uneven charge distribution and strong electrostatic adsorption orientation, will cause the powder in the coating film to decrease in compactness, and affect the appearance and stacking state after film formation (in constant humidity, temperature, formulation system, apparent density, average particle size, etc.
)
.

3.
2 The angle of repose

The angle of repose refers to the minimum angle formed with the horizontal surface when the object placed on the inclined plane is in a critical state of sliding along the inclined plane, that is, as the inclination angle increases, the object on the inclined plane will slide more easily; critical state angle
.

Through the measurement of the angle of repose, the fluidity of the powder coating can be preliminarily determined, because there are too many factors affecting its value.
Therefore, under the same conditions, it can be considered that within a certain range of values, the smaller the angle, the higher the fluidity of the powder coating.
Good, otherwise the worse
.

The factors that affect the angle of repose include temperature, relative humidity, particle size, shape, and roughness.
Under the condition of constant temperature, relative humidity, and surface roughness, the size and distribution of particle size will directly affect the measured value of the angle of repose
.
The above-mentioned samples 1# to 7# were respectively measured for the angle of repose under the condition that the above information is constant, and the results are shown in Figure 5
.

It can be seen from Figure 5 that when the particle size of the powder coating is within a certain range, the angle of repose (when the temperature, relative humidity, apparent density, etc.
are constant) is inversely proportional to the particle size
.
When the particle size of the powder exceeds a certain value, the angle of repose will gradually become larger, which is due to the excessive particle size of the powder and the surface roughness
.

It can be seen that the reasonable control of the particle size of the powder coating has a great influence on the measurement of the angle of repose of the powder coating
.
That is to say, if the particle size is too large or too small, it will affect the angle of repose, so the reasonable control of the particle size range plays an important role in the measurement of the angle of repose
.

From the above data, it can be widely considered that the angle of repose test has the best effect when the D50 is between 36 and 43 μm
.

3.
3 Outflow Test

Flow test is one of the methods to check the fluidity of powder coatings, which can simply simulate the degree of fluidization of powder in the fluidization cylinder
.
The powder coating with good outflow test data has high powder bulkiness, good transmission effect, good atomization effect and convenient construction
.

Powder coatings with poor flow test data, prone to agglomeration, poor or poor powder application (this should be viewed from a different perspective), poor storage stability of the product (long-term storage and extrusion, the powder will become hard and agglomerated), or even Agglomeration occurs during product packaging
.

The fluidity of the powder should be specifically set according to different products and different constructions, or a critical setting, because the fluidity is too good or not good will directly lead to the uneven distribution of the powder
.

For example, too high fluidity will lead to powder stratification, and too poor fluidity will lead to undesirable phenomena such as powder agglomeration.
Therefore, reasonable control of powder fluidity is an important factor to be considered in the construction process of powder coatings
.

The influencing factors of the outflow test are closely related to the specific gravity, particle size, temperature, relative humidity, etc.
of the powder (the outflow test results cannot represent the pros and cons of powder coatings)
.
Under the conditions of constant specific gravity, temperature, and relative humidity of the powder coating, the numerical relationship between particle size and outflow test is shown in Figure 6
.

It can be seen from Figure 6 that under the condition that other conditions are constant, the outflow test data is proportional to the powder particle size, but when the particle size exceeds a certain value, the outflow test data is inversely proportional to the powder particle size
.

This is due to the quality of the particles themselves and the roughness of the surface
.
That is to say, if the particle size is too large or too small, it will affect the outflow test, so the reasonable control of the particle size range plays an important role in the outflow test
.
From the above data, it can be widely considered that the outflow test effect is the best when the D50 is between 34 and 43 μm
.

4 Influence of particle size on the quality and construction of powder coatings

4.
1 The effect of particle size on the appearance of the coating film

The control of particle size of powder coatings is one of the most important links in the production process of powder coatings.
Powder coatings with different appearance >
.
If the particle size distribution of the powder coating is not uniform, too coarse or too fine, it will seriously affect the appearance of the coating film
.

Within a certain particle size range, it is generally considered that a flat product, with a smaller particle size, has better surface leveling and is less likely to appear particles
.
The particle size of art paint should be determined according to the specific appearance effect requirements
.

4.
2 Influence of particle size on the charging effect of powder coatings

The adhesion of powder coating particles to metal substrates is related to the chargeability of the powder
.
If the particle size is too large or too small, it will seriously affect the powdering rate of the powder
.

Generally speaking, the particle size of powder coating particles is too large, and the chargeability will be relatively better, but when the mass of the particles is greater than the charge amount of the powder and the power of the air, due to gravity, the powder coating rate and coating The effect will decrease
.

The particle size is too small, the chargeability is relatively low, and the powdering effect will be correspondingly worse, resulting in lower construction efficiency, see Table 1
.

From the data in Table 1, if other requirements (state or film thickness, etc.
) are not considered, when the particle size is 30-40 μm, the adhesion of powder particles to the metal substrate is the best
.

4.
3 Influence of particle size on powder fluidization effect

The particle size of the powder coating is too fine, and the fluidization effect in the fluidization cylinder is poor (the particle size is too fine, and it is easy to cause agglomeration), and it is not easy to disperse
.

Small bags are easily formed on the surface of the sprayed workpiece (the particle size of the powder coating is too fine, resulting in the phenomenon of "spitting powder" during the spraying process), which affects the appearance of the coating
.
The particle size of the powder coating is too coarse, and it is not easy to fluidize in the fluidization cylinder, and it is also prone to bad phenomena such as poor powder coating
.

4.
4 Effect of particle size on storage stability and recovery rate of powder coatings

The storage stability of powder coatings is affected by many factors, such as temperature, relative humidity, particle size of powder coatings, and glass transition temperature of powder coatings
.

For powder coatings with a small particle size, the probability of the powder being affected by moisture and difficult to dissipate heat will increase the probability of agglomeration, and will also reduce the recovery rate of the powder
.
The powder coating with larger particle size has much better anti-caking resistance than the powder coating with smaller particle size (same formula, different particle size), but if there is too much coarse powder, the recovery rate will decrease
.

4.
5 Effect of particle size on the thickness of powder coatings

The average particle size of powder coatings is generally controlled at 30-60μm, and the coating thickness is generally between 50-200μm, see Table 2
.

The specific construction thickness also depends on the application characteristics of the product (the thickness of the coating determines the sprayed area of ​​the powder coating per unit mass)
.
Referring to the characteristics of powder coatings, combined with the data in Table 2, when the particle size is 30~40μm, the best effect of the coating can be matched
.

 5 Conclusion

Through the elaboration of the above aspects, the factors affecting the particle size of powder coatings, and the influence of particle size of powder coatings on the quality, storage, construction, testing and other aspects of powder coatings are emphatically introduced, and the importance of particle size to powder coatings is explained.
meaning
.

At present, some high-end powder coating construction parties have begun to pay attention to the particle size analysis of powder coatings, and have restricted the particle size range of powder coatings processors
.
Reasonable control of the particle size index will not only be an element of the production link of the processing plant, but also a monitoring standard for the construction party to ensure the constructability and stability of the product
.