Page 106 - 20250602 EEA QM Edition 6.0

Quality Requirements Edition 6.0 European Enamel Association

8.4.3 Testing of powder adhesion 8.4.4. Testing of the electrical resistance of the powder

For the determination of the powder adhesion a special impact apparatus has been The electrical resistance of enamel powder can be determined by a simple principle.
developed (see fig.). A 200 x 200 mm stainless steel sheet, covered with about 20 g The powder (80 g) is placed between two electrodes formed as tank and stamp (see
enamel pow- der on one side, is fixed on the pendulum arm C by a screw. After fig.). By applying an electrical voltage, a current begins to flow; its intensity is a measure
removing the positioning bolt D the pendulum arm bounces on the rack E. By this for the resistance of the enclosed powder.
impact a part of the powder drops. The steel sheet is then removed from its fixture
and the remaining powder is reweighed. The quotient of the powder on the steel The reliability of the test result depends essentially on the amount of powder, the
sheet in g before and after the impact, multiplied by 100 gives the value of adhesion surface and geometry of the electrodes and the pressure of the stamp onto the powder.
in %. Standardized measuring conditions can be achieved in a standard cell (see fig.) with
integrated resistance device. The graduation allows readings up to 10 Ω. Between
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Because the powder adhesion drops with the time between powder coating and the electrodes in the tank and the stamp, two paths for the current are available:
impact, an interval of 15 min. has been defined for standard applications. Under these
conditions the powder adhesion should have a minimum value of 60 %. a. the path of high resistance (> 10 Ω) throughout the enamel particles
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b. the path of minor resistance along the surface of the particles.
Impact Apparatus (sketch)
As the current always chooses the way of the lowest resistance, it flows principally
A – fixed test plate
along the surface. This means, that the determined Ω-value is identical to the surface
B – adjusting bench for pendulum arm
resistance of the powder. Multiplication of this value with the quotient of the electrode
C – pendulum arm
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area (stamp) in cm and the thickness of the powder layer gives the specific surface
D – positioning bolt
resistance in the dimension Ω • cm. These values have been reported mostly. As the
E – rack
specific resistance is a constant property of a solid material - according to the volume
resistance – this parameter should not be used for the characterization of powders of
low density and surface conductivity. Therefore, the surface resistance in Ω • cm is best
suited for the expression of the electrical resistance of powders. Despite the fact, that
the figures are only relative, the measuring results achieved under standard conditions
allow for an effective comparison of different electrostatic powders.

Correlation between surface resistance and powder adhesion
A good powder adhesion requires that the enamel particles spread onto the sheet

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