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3.5.7.3 Application
Application consists in first converting the powder primer surfacers to an aerosol (a powder
dispersed in air). To this end, the powder is fluidised in a feed container by compressed air. The
aerosol is fed to an electrostatic spray gun, where the powder particles are electrically charged.
There are two ways to do this. The first takes the form of either corona charging, in which the
aerosol is fed along electrodes, which may be acicular or cylindrical capacitors in the spray gun
(internal charging), or charging the powder particles as they pass electrodes outside the spray
gun (external charging). The applied potential depends on the method and ranges from 10 to
100 kV/cm. The second consists in feeding the aerosol through a pipe made from special plastic
materials; charging takes place by friction of the particles inside the pipe. The method is called
tribo-charging, and it can be boosted by incorporating additives into the powder. The charged
particles are distributed as they exit the gun from rotating application bells, much in the manner
of liquid materials (see Chapter 3.3). The charged particles follow the electrical high voltage field
and are deposited on the object, which is earthed. On account of their charge, the particles adhere
to the objects very well, and so transportation of the object to the stoving oven does not pose a
problem. Film forming in the oven takes the form of melting and crosslinking. Figure 3.5.20 shows
the principle behind the application of powder primer surfacers.
Powder primer surfacers are usually applied in film thicknesses of 60 to 70 µm. That is much higher
than the film thicknesses of liquid primer surfacer materials, which are 30 to 40 µm. The reason is
that such film thicknesses are necessary for providing primer surfacer layers with adequate level-
ling. High film thicknesses are an advantage as they create an optimum barrier effect and boost – if
sufficiently smooth – topcoat holdout. High film thicknesses of powder coatings are free from film
defects, such as blisters. However, the disadvantages are the higher weight of thick layers and higher
material consumption (higher costs). Consequently, a great deal of effort is being expended on devel-
Figure 3.5.20: Principle behind the application of powder primer surfacers
Primer surfacers
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oping powder primer surfacers that are applied at lower film thicknesses yet retain adequate level-
ling and smoothness. The main reason for the need to apply powder primer surfacers in a film layer
greater than 60 µm is the particle size distribution. Since powders contain particles with diameters
around 50 µm, optimum film forming to yield smooth surfaces requires higher film thicknesses than
that. Thus, these studies have focussed on reducing particle size and producing a narrower particle
size distribution. The processes which have been developed are very complex and expensive. How-
ever, there are limitations to this approach. Smaller particles are less amenable to fluidisation; they
do not form mobile aerosols due to strong particle-particle interaction. Furthermore, additives that
boost fluidisation do not work perfectly as they impair other film properties.
Unlike electrostatically applied liquid primer surfacers, electrostatically sprayed powder primer sur-
facers offer a primary material transfer efficiency of just 50 % by weight (the figure for other methods
is up to 85 %). This means that only half of the powder is applied to the object during spraying. How-
ever, again unlike liquid primer surfacers, the overspray can be recycled and returned to the appli-
cation. The resultant transfer efficiency for the whole application process, including recycling is as
much as 95 % by weight, which exceeds that of all other application methods. However, recycling
poses a risk of contamination. Once again, unlike liquid paints, powder coatings are not made in a
way that lends itself to removal of contamination. To circumvent this problem, special application
methods have been developed for powder primer surfacers. As with other types of coating, there are
two application steps. In the first, the powder is sprayed onto the lower part of the car body (bottom
group). In the second, the upper parts, which are more visible, are coated. Fresh powder is used for
the second step only. All powder collected
for recycling is fed to the first step, i.e. the
coating of the lower parts of the car body.
If a potential contamination means that
the primer surfacer film is not so perfect
in that area, this is not visible. The under-
lying principle is shown in Figure 3.5.21.
Coloured powder primer surfacers are also
available. It must be borne in mind that
changing the colours of powder coatings
in the application line is much more com-
plicated than with wet paints. For a better
impression of the application of a powder
primer surfacer, see Figure 3.5.22
[105]
.
Figure 3.5.21: Two-stage application method for powder primer surfacers
Figure 3.5.22: Car body in powder primer surfacer spray
booth.
Automotive OEM coatings
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