ing process. However, since the work-piece wood often does not have a sufficient electrical
conductivity, the electrostatic procedures only are applied in special cases. The electrostaticcoating of chairs and windows is known. The chairs are coated electrostatically in so-called
omega loops by means of high speed rotational discs. In practice, however, the components
have to be finished manually depending on the geometry of the components. The not always
homogeneous wood moisture of ≥ 8 % as well as the continuous assurance of the ground con-
tact during the
necessary voltage by friction of the powder at the spray units (see
Chapter 4.2.2). The currents generated during application are in the µA range and are
Safety, Environmental Protection and Health
624 BASF Handbook Basics of Coating Technology
non-critical. However, extensive charging may occur if all live parts are not earthed.
The same also applies to the operators who must therefore wear conductive footwear
and operate on conductive floors.
Explosion and Fire Protection
There is a risk of explosion in electrostaticcoating lines if the following
. Consequently, influences ari-
sing from fluctuations in materials under different ambient conditions of temperature or
relative humidity are hard to compensate for. A further disadvantage is the only moderate
output performance of approx. 1.5 g/min per 1 cm edge length.
Figure 4.2.56: Low-speed bells and discs
BASF Handbook Basics of Coating Technology 527
To exploit the advantages of high paint yield at higher material output, equipment for
electrostaticcoating was developed which regulated the process via material dispensing.
to 10 –8 S/cm. Polyurethane coatings also
fall within this range. The electrostaticcoating processes can be divided into three cate-
gories, depending on the atomization and electrostatic charging mechanisms: the electro-
static compressed air and the hydraulic (airless) process, the electrostatic atomizing pro-
cess, and the high-speed rotation process. 
Electrostatic spray guns atomize the coating material by the same mechanism as
conventional spray guns, but the coating droplets receive an electrostatic charge as well.
Nearly all coatings can be sprayed
Most European coatings lines for exterior plastics, however, use the standardized hard-
ener from the clear coat also to crosslink the hydroprimer, yet with lower crosslink den-
sity. Most water-borne primers in Europe also contained small amounts of N-methyl pyrro-
lidone (NMP) as co-solvent improving wetting and adhesion. Recent changes  in labelling
rules for mixtures containing such co-solvent led to a phase out of NMP and reformulation
of respective hydroprimers.
Conductive primers are required for the electrostaticcoating of plastic components
electrically conducting varnishes: They have the function to provide
a nonconductive substrate with an electrically conducting layer. This
need depends on different technological requirements. A good exam-
ple is the application of conductive primers for plastics. They contain
conductive pigments, like carbon black or micas, coated with a tin/
antimony oxide layer, and are used to make possible the electrostaticcoating of non-conductive plastic work pieces.
electrochemical series: Classification system, mainly used for metals,