Powder paints: particle size
Many aluminum producers, for example, aluminum profiles, or have their own equipment for powder coating, or color them "on the side". In both cases, they have questions:
- What is good powder paint?
- What is a good powder paint manufacturer?
- How to distinguish coating defects, caused by poor paint quality, from those defects, the cause of which is the violation of the dyeing technology?
Below is an overview of a typical powder paint technology.. Critical production stages highlighted, which especially affect the quality of powder paint produced. The role of the distribution of particle size of the powder on the quality of the powder coating is shown..
Exact composition, rigorous technology and close monitoring
General technology scheme
The manufacturing technology of industrial powder paints consists of four clear stages [1, 2] (drawing 1):
- Weighing the original components of the paint, their pre-mixing and grinding.
- Melting and extruding the resulting mass, extrudate cooling and coarse crushing.
- Grinding the crushed extrudate into powder.
- Adding components to the finished powder (if necessary), packaging and storage.
Figure 1 - Scheme of a typical technology for the production of powder paint 
At each stage, the quality of the intermediate product is subject to strict control.. It is very important, as, when powder paint is completely made, it cannot be corrected or corrected in any way. Quality of raw materials, their accuracy and strict adherence to technology are very important for obtaining high-quality powder paint.
Weighing, mixing and grinding
Raw materials typically include resins., hardeners, pigments, fillers and additives, for example, to improve fluidity and facilitate degassing. Each source material undergoes its individual input quality control..
The proportion of each component is carefully weighed with the required degree of accuracy.. For some components, this accuracy may be 0,0001 r . All components, measured in quantity according to the recipe for this brand of paint, loaded into pre-mix container – mixer (drawing 2).
Figure 2 – Weigh and load
source materials 
Then this container is installed on the mixer.. Here, all the starting materials are thoroughly mixed using special cutting blades for a given amount of time.. In this case, all source materials are crushed to the maximum, in order to facilitate the mixing of the melt in the next steps (figure 3) .
Figure 3 - Premixing of raw materials 
A sample of this “batch” of raw materials is checked for uniformity and passed through small laboratory extruders and millstones.. The resulting powder is then applied to the control panel., the resulting coating is cured in an oven and subjected to the following tests:
- Colour, surface spreading and gloss
- Mechanical properties
- Duration of curing (polymerization)
If any corrections are required based on the results of these tests, then the mixing process and quality control are repeated . They do it until, until the powder obtained meets the established technical requirements.
After this stage of production, no further powder composition modifications are already possible..
Extruding, cooling and crushing
Next, the mixture of raw materials is fed using a dosing system to the extruder. The preset temperature is maintained in the extruder (from 70 to 120 ºС depending on the type of paint). This is to ensure, so that the resin is sufficiently melted to mix well with the other components as the worm screw of the extruder passes through (Figure 4).
Figure 4 – Extruding, cooling and crushing 
As a result, at the exit from the extruder, all components are crushed and wetted with resin to form a homogeneous composite. The dispenser feed rate and the screw speed are balanced in this way., to ensure full loading of the extruder. This ensures good mixing of all paint components..
After the extruder, the resulting molten mass is fed to a conveyor cooler for cooling.. Then the material in the form of flat pieces enters the crusher (Figure 4), where it is broken and crushed into “chips” of size convenient for grinding them at the next production stage 5-10 mm (figure 5).
Figure 4 - Semi-finished powder paint after the extruder 
Figure 5 - Coarsely crushed powder paint extrudate 
At this stage, test the samples obtained "chips". On a laboratory millstone they are crushed into powder. This powder is then applied to the control panel.. The resulting coating is checked for quality according to the following criteria:
- Colour, gloss, appearance and fluidity
- Mechanical and chemical properties
- Electrostatic ability
Too high temperature in the extruder will not only result in low melt viscosity, low shear forces and insufficient pigment dispersion, but will also cause low gloss powder coating. The resin and hardener in the mixture may also begin to react in the extruder, which will have a detrimental effect on the characteristics of the powder paint .
At this stage of production it is no longer possible to make changes in the composition of the paint. Cheaper to send off-the-shelf extruded chips for recycling now, than later – ready-made powder after grinding these chips .
Grinding the extrudate into finished powder
Crushed extrudate ("chips") is loaded into the unit, which is called a mill. In it, the extrudate is ground into powder. To obtain the optimal particle size distribution (RFD), the powder goes through several processing stages: cycloning, separation, filtering and screening. In modern plants, sieved particles that are too large are automatically sent back to the feed in the grinding unit.
Figure 6 - Grinding crushed extrudate into a finished powder 
Finished powder paint is subject to strict quality control., to make sure, that it meets all established requirements, including, specified requirements for RFC.
Packaging and storage
In order to meet the special requirements of the customer or the special conditions for the use of powder coatings, additives may be introduced at this stage, which are mixed in an already prepared powder.
The powder can normally be stored in unopened industrial packaging (cardboard box, bag or container) in a dry, cool place (30 ° C) to 12 months. Higher temperatures and longer storage may result in moisture absorption.. Storage conditions may vary for some paints., therefore, always follow the directions on the paint packaging, as well as in a passport or certificate.
It is recommended to check the paint after 6 months of storage, to make sure, that she does not have any quality problems.
Standard particles, small and large
These are particles, which the paint manufacturer wants to pack in boxes and send to the customer. The size range of these particles is very strictly controlled.. Standard particles provide the formation of a powder coating with the highest possible quality and confident fulfillment of all requirements for mechanical and other properties.
These are very small particles., which may cause staining problems, such as a defect "frame", when a thicker coating forms at the edge of the product. Fine powder particles settle on the product at a different speed, than standard particles, especially on the side and back surfaces of the product, resulting in excessive variation in coating thickness. Excessive amounts of fine particles in powder paint can also lead to clogging of guns and hoses., as small particles liquefy more easily, than standard particles .
These particles are too large., to get through the last separation screen. Some manufacturers are re-grinding these large particles., to get good powder out of them, some of them just throw them away. it happens, that these large particles are mistaken or due to malfunctioning separation equipment in the box to be sent to the customer. With electrostatic spraying, they will not be able to get the necessary electric charge., as standard particles usually do, and most likely just fall to the bottom of the spray booth. This leads to a decrease in the efficiency of the primary spraying of the powder on the product. Moreover, seen, that an excessive amount of large particles in powder paint can lead to powder shedding on some areas of the product surface during its movement along the paint line with the formation of spots with a reduced coating thickness .
Powder paint in box
A certain amount of small and large powder particles can fall into the paint box, regardless of the applied powder paint manufacturing technology (picture 7). Each powder paint manufacturer has a dedicated Particle Size Distribution Analysis (PMD) machine. This machine tests paint samples and shows how large the difference is between powder particle sizes.. There should not be an excessive amount of large or small particles in a normal powder box, which can lead to problems for powder coating.
Figure 7 - Typical type of powder particles
in various powder paints 
What does particle size affect?
The following steps in the deposition process are particularly sensitive to the particle size distribution of the powder :
- Powder transport by coating system
- Electrostatic charging of powder particles and their penetration into "Faraday cages"
- The formation of a uniform cloud of powder in the spray booth
- Deposition and growth rate of coating thickness on the surface of the product
- Primary Powder Efficiency
Thermosetting powders are applied electrostatically to products.. Therefore, powder particles must be able to charge in an electrostatic field or as a result of friction with a gun. This charge should be sufficient, to attract particles to the surface and edges of the product. However, sedimentation of the first powder particles should not lead to electrical insulation of the surface, so it will prevent the formation of a coating of a given thickness .
The fluidity of the powder must comply with all internal and external requirements for its transportation. Particle size to a large extent determines the ability of the powder to flow freely.. This fluidity depends not only on the powder material, but also on the shape and size of the particles .
Most industrial powder paints have a particle size of 10 to 100 m. For the electrostatic method of powder spraying, powder particles with sizes ranging from 15 to 75 m, and ideal particles - from 25 to 45 μm .
In the drawing graph 8 typical RFC for industrial pigmented epoxy powder paint presented. The graph shows, that some particles are slightly larger, and some are slightly smaller than the average particle size. The sharper the peak of this curve, the more stable the staining process and the more uniform the powder coating itself.
Figure 8 - Typical particle size distribution of powder paint 
Particle Size Distribution of Finished Powder, largely determines the quality of the powder coating.
The quality of powder paint is influenced by a large number of production factors:
- quality of raw materials
- precision technology
- state of technological equipment
- effective quality control.
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