dioxide established itself as the leading white
pigment when it went into mass production in 1919.
These developments in coating chemistry were paralleled (finally) by advances in coating technology.
The various methods of brush application and spraying were supplemented by electrodeposition,
electrostaticcoating and powder coating techniques. Ambient air drying was joined by infrared and
radiation drying methods (UV, electron beam), and the automation of coating processes continued
to advance. It is also worth mentioning environmental technologies for the
ciency and the so-called wrap around the work piece (see Chapter 4.3: Primers, Figure 4.4:
Principle of the electrostaticcoating method). For this method conductive substrates are
necessary as well. The conductivity can be achieved by the use of conductive primers as
explained in Chapter. 4.3.
High speed bell atomizer
The equipment manufacturers have further increased operating and transfer efficiencies
by increasing brush speed (up to 1000 mm/s), increased fluid flows (>600 ml/min) and
developed more reliable air bearing turbines so that transfer efficiency can
rather than internal charging (see above) or
• insulating the entire spraying plant, including the coating supply system (a cumbersome proc-
• insulating the spray gun only and supplying restricted quantities of coating by means of ”shut-
tles” or similar vessels which are to be insulated separately (”canister system”)
Spray application processes
without electrostatic assistance
workpiece coating film
with electrostatic assistance
Figure 6.16: Coating droplet deposition without/with electrostaticcoating charging, according to 
the barrier effect against sol-
vents and inward water diffusion under humid climate conditions.
Primers are made conductive by the use of conductive pigments (carbon black, doped mate-
rials). Conductive primers allow the electrostaticcoating of the non-conductive plastic. In
the electrostaticcoating process the coating is electrically charged at the atomizer (pneu-
matic or high speed rotation), the work piece is grounded. The charged paint droplets move
along the electrical field lines to the part where the static is discharged to the ground (see
low contact the powder producer
4. Membrane obstructed 4. Check bottom of the bed
5. Plugged or broken membrane 5. Check the membrane
C Electrostaticcoating operation
C1. Fluctuations in the coating thickness
1. Irregular powder feed 1a.
Clean powder feed hoses, pump venturis
Check humidity of the air supply which can
cause powder compaction
Check powder free flowing properties
2. Inadequate gun distance to the workpiece 2. Readjust the distance between gun and
3. Inadequate conveyer speed in relation to the
high voltage of
agents that can lead to blistering
and delamination of the coating during oven drying.
Increasing the electrical surface conductivity
Surface conductivity (cf. DIN 53 482), e.g. for electrostaticcoating, can be increased by means of
the following, fundamentally different methods:
• reduction of the plastic resistance in the compound
• application of conductive films
• technical equipment (special electrodes, special supports or holding devices for parts) which
allow the charge to be discharged.
Let us briefly consider just one of these: the application of
and in some cases an audible signal in the instrument. Commercial pore
detectors of this type are generally now automated devices with electronic pore meters. Depending
on film thickness, the applied voltage can be varied between 200 V and 15 kV, or applied in pulses
of up to 35 kV in the case of very thick coatings. Thanks to their integral current limiting device,
they are nevertheless perfectly safe to handle (similarly to electrostaticcoating).
Other instruments operate on the basis of the sponge method with conductivity detectors. In this case