The challenges of powder-in-powder technology

Technique offers tremendous benefits, but requires a great deal of development expertise in terms of raw material and application process interaction.

Powder-in-Powder coating process

More and more powder coating manufacturers are developing powder-in-powder technology solutions. This new coating technique offers many advantages, but also places stringent requirements on application technology and powder coating formulation.

Powder-in-powder technology offers numerous advantages. The absence of baking and cooling processes reduces energy consumption and production times, which consequently increases productivity. In addition, the technique incurs lower investment and maintenance costs thanks to the absence of conveyor systems, ovens and cooling zones, meaning that less production space is needed. The powder-in-powder technique also provides a higher level of corrosion protection in comparison to single-coat powder coating. Currently the technology is primarily applied in the coating of heavy construction and agricultural machinery. This is where productivity increases and energy savings through the elimination of baking and cooling processes are the highest.

Repulsion due to excess charge

In terms of application technology, ideally the equipment should charge the powder coating without an excess charge. Therefore, the component being coated must be properly grounded. Excessive charging of the primer and/or top coat will result in a repelling effect, which can cause both coats to mix.

The amount of air entering the gun should be set as low as possible (’soft cloud’), otherwise excessive coating may blow off the primer and also cause a mixing effect. Ensuring a uniform coating is particularly important when using tribo applications. Powder-in-powder applications can be carried out using four different procedures.

Because like-charged particles repel each other, one would think that the opposite charging of the primer and top coat would be the more generally accepted market norm. On the contrary: charging both the same has become established in practice. Although opposite charging can deposit the top coat with a minimum of repulsion, discharging occurs as opposite charges cancel each other out. This leads to a reduction in the primary adhesion of the powder coating onto the substrate, whereby the powder coating is more likely to run.

The Tribo Masterlance offers the following advantages compared to conventional tribo guns:

  • Significantly higher charging potential, even using minimum air volumes
  • Greater coverage performance (at least in comparison to corona guns) due to increased exit volumes of charged powder
  • The lance is telescopic to enable manual coating of large machines
  • Compact construction and nozzle head angle adjustment capability

Minimise mixing effects

To minimise mixing effects when overcoating, there should be no significant difference in grading between the primer and top coat. Crosslinking of the powder coatings prevents migration of the primer to the surface, which consequently impairs light resistance. Furthermore, this also improves intermediate adhesion, in particular during overbaking processes.
Both the primer and top coat must have a mutually compatible charging behaviour. The primer should have a high charging potential in order to achieve a good degree of deposition on the substrate without running. At the same time, it must be able to be coated with a top coat that also has an equally high degree of deposition that is devoid of running and mixing effects. The charging behaviour is set via the powder coating formulation. In this respect, the selection of binding agents, pigments, fillers and additives has a considerable influence on the charging behaviour and consequently has to be precisely aligned to the respective process.

Edge coverage for enhanced protection against corrosion

Pulver-in-Pulver corrosion protectionThe primer should have the highest corrosion protection level possible, particularly on the edges. To achieve this, primer edge coverage needs to be improved by lowering the flow properties. If the primer flows too well, then during the melt phase the top coat and primer will run away from the edge. When using a primer with excellent edge coverage, the thickness of the primer should not be overly high – max. 80 µm – as the flow will otherwise be significantly poorer. As such, the use of an intensive edge-covering primer means the scope for coating is considerably tighter and difficult to maintain in practice when it comes to complex components. In such cases, a reasonable compromise between flow and edge coverage needs to be found. To prevent edge corrosion, the edges of the component being coated also need to undergo sufficient mechanical processing or sandblasting.

Different coating technologies on the market

Similar to the application technology, depending on the powder coating manufacturer and the user, different solution approaches are also being pursued regarding the coating structure.
In order to perform a visual layer thickness check, the primer and top coat need to have different colours. That means that the top coat can generally not be run within a circuit as the primer would be carried into the top coat circuit while overcoating the primer. This would lead to stippling of the top coat.
If primer and top coat have the same colour, in the case of an epoxy or hybrid primer, the top coat still has to be operated at a loss because the light resistance of the top coat will be reduced significantly due to the presence of the primer powder. This financial disadvantage must be offset against productivity gains and energy savings to determine whether powder-in-powder technology makes sense in individual application cases. The top coat can consequently only be run within a circuit with a polyester/polyester structure of the same colour, where the mixing effects would not be visible during coating.

Summary

Powder-in-powder technology is a promising process, particularly for heavy components, and is finding an increasing number of users in the market. The successful use of this innovative technology requires the exact coordination of the coating process with the utilised powder coating system. In turn, development of the commensurate powder coating systems requires developers to have an in-depth understanding of the interaction between individual raw materials and their impact on the coating itself. It remains to be seen which application technology and powder coating systems will prevail and which new application fields can be developed as a result.
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Further information of powder-in-powder