Page 103 - QRedition5.0 062021

Quality Requirements Edition 5.0 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. The
developed (see fig.). A 200 x 200 mm stainless steel sheet, covered with about 20 g enamel powder (80 g) is placed between two electrodes formed as tank and stamp (see fig.). By
pow- der on one side, is fixed on the pendulum arm C by a screw. After removing the applying an electrical voltage a current begins to flow; its intensity is a measure for the
positioning bolt D the pendulum arm bounces on the rack E. By this impact a part of the resistance of the enclosed powder.
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 sheet in g before and after the impact, The reliability of the test result depends essentially on the amount of powder, the surface
multiplied by and geometry of the electrodes and the pressure of the stamp onto the powder. Standard-
100 gives the value of adhesion in %. ized measuring conditions can be achieved in a standard cell (see fig.) with integrated
resistance device. The graduation allows readings up to 10 Ω. Between the electrodes in
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Because the powder adhesion drops with the time between powder coating and impact, an the tank and the stamp, two paths for the current are available:

interval of 15 min. has been defined for standard applications. Under these conditions the
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powder adhesion should have a minimum value of 60 %. a. the path of high resistance (> 10 Ω ) throughout the enamel particles
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 along

A – fixed test plate the surface. This means, that the determined Ω-value is identical to the surface resistance
B – adjusting bench for pendulum arm of the powder. Multiplication of this value with the quotient of the electrode area (stamp)
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C – pendulum arm in cm and the thickness of the powder layer gives the specific surface resistance in the
D – positioning bolt dimension Ω • cm. These values have been reported mostly. As the specific resistance is a
E – rack constant property of a solid material - according to the volume resistance – this parameter
should not 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 a effective comparison of different
electrostatic powders.

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

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