Coatings Clinic
By Clifford K. Schoff, Schoff Associates
Dielectric Constants of Paints and Coatings
In an earlier article (JCT CoatingsTech, June
2010), I presented information on dielectric analysis
(DEA) of coatings, which involves monitoring changes
in dielectric properties such as resistivity or ion
mobility with temperature or time in response to the
application of an electric field. Transition or index
temperatures such as melting point, glass transition, and onset of crosslinking can be measured.
The dielectric constant (relative permittivity), ε, is
another parameter that can be used in such analysis, as well as for other purposes. It is a measure of
a substance’s ability to insulate charges from each
other and is defined as the ratio of the capacitance
of a capacitor using that material as a dielectric compared to a similar capacitor with a vacuum as the
dielectric. Because it is a ratio, it is a dimensionless
number. A low value indicates that the material is
an insulator, and dielectric constant measurements
have been used to compare the insulating ability of
coatings. The dielectric constant also indicates how
polar the material is. It has been used as a polarity
index for solvents (polar solvents ε > 20, nonpolar
solvents ε < 20). I have seen measurement of it
used to evaluate solvents and paints for electrostatic
spray. Table 1 lists dielectric constant values for a
few coatings-related materials.
The measurement of the dielectric constant
requires specialized equipment. There are a number of
methods for solids, virtually all of them aimed at electrical insulating materials.1–4 The most practical device
Table 1—Dielectric Constants
Material
Dielectric Constant, ε
Vacuum
1 (by definition)
n-Hexane
1.9
PTFE
2.0 @ 60 Hz
Polypropylene
2.2–2.3 @ 1 kHz
Xylene2.4
Epoxies~3
PMMA
3.6 @ 50 Hz
Alkyds~4
n-Butyl acetate
5
Neoprene rubber 6.5–8.1 @ 1 kHz
Talc
~8
MAK15
n-Butanol17.8
Ethanol24
Methanol33
Carbon black
~ 60
DI water
80
~ 100
R900 TiO2 92
November/December 2014
COATINGSTECH
�The measurement of the
dielectric constant requires
specialized equipment... The
most practical device for
coatings probably is the
parallel plate capacitor.�
for coatings probably is the parallel plate capacitor. A
free film (it is sometimes necessary to stack several)
is placed between the plates of the capacitor and the
capacitance is measured. A second run is then done
without the specimen. The ratio of the two values is the
dielectric constant. Measurement of ε values for liquids
is simpler. One method involves inserting an openended coaxial cylinder probe into the liquid and measuring the current between the inner and outer cylinders
of the probe.5 There are other coaxial cylinder probe
methods as well.3, 4, 6 The dielectric constant varies with
temperature and the frequency at which the device is
operating. Most literature values have been measured
at 20–25°C, but the frequency often is not listed.
The dielectric properties of composite dielectrics
generally are a weighted average of the individual
component properties. Therefore, it is possible to
calculate dielectric constants for liquid paints from
their formulas and knowledge of the ε values of the
components. I have done the calculations for a few
paints, but never had an opportunity to actually measure the dielectric constants for them to see how the
results compared. The calculation method is an alternative for times when no measuring device is available, and it certainly provides at least an estimate of
the degree of polarity of the paint. CT
References
1. ASTM D150, “Standard Test Methods for AC Loss
Characteristics and Permittivity (Dielectric Constant)
of Solid Electrical Insulation.”
2. www.intertek.com/polymers/testlopedia
3. http://cp.literature.agilent.com/litweb/pdf/
5989-2589EN.pdf
4. http://literature.cdn.keysight.com/litweb/pdf/
5991-1843EN.pdf
5. www.brookhaveninstruments.com/products/zeta_
potential/p_ZP_BI-870.html
6. www.youtube.com/watch?v=2BbrvIXnCbQ