Coating powders production

Discover the step-by-step process of coating powders production. From weighing and pre-mixing to grinding and packaging, learn how to create high-quality powder paint.

Mixing

The process

Raw materials for coating powders production include:

• resins,
• hardeners,
• pigments,
• fillers, and
• additives.

The proportion of each component is carefully weighed with the required degree of accuracy. For some components, this accuracy may be 0,0001 g [2]. All components, measured in quantity according to the recipe for this brand of paint, loaded into pre-mix container – mixer (Figure 1).Figure 1– Weigh and load raw coating powder materials [2]

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 2) [2].

Figure 2 – Premixing of raw materials [2]

Quality control

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

The process

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 3).

Figure 3 – Extruding, cooling and crushing [2]

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 (Figure 5).

Figure 4 – Semi-finished coating powder material after the extruder [3]

Figure 5 – Coarsely crushed coating powder extrudate [3]

Quality control

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 [2].

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 [2].

Grinding

The process

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, 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 coating powder [2]

Quality control

Finished powder coating is subject to strict quality control to make sure, that it meets all established requirements, including, specified requirements for the optimal particle size distribution.

What does particle size affect?

The following steps in the deposition process are particularly sensitive to the particle size distribution of the powder [2]:

• 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

Most industrial powder paints have a particle size of 10 to 100 micrometers. For the electrostatic method of powder spraying, powder particles with sizes ranging from 15 to 75 micrometers, and ideal particles – from 25 to 45 micrometers [6].

Sources:

1. The Review of Powder Coatings //Journal of Materials Science and Chemical Engineering, 2016, 4, 54-59
2. Complete Guide to Powder Coatings – Akzo Nobel – 1999
3. TCI Powder Coatings Manufacturing  – 2020
4. What Coaters Need To Know About Powder Coating Particle Size – 2017
5. TCI Powder Coating Troubleshooting Guide – 2020  
6. Патент №US8377346B2 – Akzo Nobel – 2013.
7. Powder Coatings: Chemistry and Technology / Emmanouil Spyrou – 2012.