ELECTRO-CONDUCTIVE COATINGS FOR EMC-EMI-ESD

Electro-magnetic fields

The worldwide diffusion of electric and electronic equipments and the huge
increase of telecommunication technologies in the last decades have carried
out, in addition to sure benefits from the communication and computation
point of view, also some undesired effects.
In fact, the energy consumption and the emissions of polluting agents have
increased accordingly. Among all the new pollution sources, recently the
electromagnetic pollution or “electrosmog” is becoming of great importance,
due to the large technological development in the fields of information
technology, power production and distribution and telecommunication.
The electro-smog phenomenon means the emission in the environment of
electro-magnetic waves by electric and electronic equipments and every
equipment used for telecommunication purposes.
On the one hand the telecommunication systems found their proper operation
on the transmission of electromagnetic signals, on the other hand many
equipments and devices that make use of electric power or electronic
technologies generate electromagnetic emissions that are not directly
functional to the purpose for which they are designed.
Furthermore, many important studies and researches, carried on by the
international scientific community, aim to establish the possible long-term
effects of the exposure to electromagnetic fields, also of low intensity, on the
human health. In such a scenario, the normative process1 has by now
established precise limits of exposure for the protection from the acute effects
on the human body. These effects are broadly shown by scientific evidence,
for exposures to electromagnetic fields of high intensity in the whole field of
frequencies of the non-ionizing radiations (0 Hz - 300 GHz).
With the scientific and regulatory development it is foreseen that exposure
limits and conditions, measurement and prevention methods will be
progressively improved2.
It is also important to underline that the phenomenon of electromagnetic
interference between equipments has been broadly regulated and controlled
for decades by the discipline called “Electromagnetic Compatibility”: several
European Directives3 (that have correspondence in other international
standards) establish, since more than twenty years, precise limits of
electromagnetic emission for all the equipments and the electric and
electronic systems with the purpose to limit their mutual interferences.
The wide spreading of fixed installations for telecommunications and of
devices for wireless telephony has stimulated, in the last times, the attention
of the public about the electromagnetic pollution. A sensible increase of the
electromagnetic fields level in the environment in which we live and work
every day, in comparison to the natural electromagnetic background (it
should be remembered that the electromagnetic fields exist in nature,
independently from the artificial sources) is undeniable. These circumstances
create sometimes eccessive alarm, sometimes underestimation of the
danger.
Before the international scientific community can finally give sure answers
regarding the real significance of the possible long-term effects of the
electromagnetic low intensity long-term exposure, it seems anyway
reasonable to adopt the best available technologies in order to the reduce the
electromagnetic emissions from all the electric and electronic equipments.

1. DPCM 23/04/1992, DM 381/98
2. Guidelines ICNIRP, ENV 50166, Legge Quadro, prEN 12198
3. 89/336/CEE and following amendments, 93/42/CE, 89/392/CE

Solutions from IRIS Vernici

IRIS Vernici, for a long time active in Research & Development of special
coatings, has just completed a comprehensive research work on coating
materials that can be used to cover surfaces (enclosures for electronic
equipments, walls of buildings, etc.) for the partial screening and attenuation
of electromagnetic fields.

The use of shielding paints has been examined under three different aspects:

a) Electro-magnetic shielding of non-conductive enclosures for the
attenuation of electromagnetic fields incoming from sources external to the
enclosure
b) Electro-magnetic shielding of non-conductive enclosures for the
attenuation of electromagnetic fields originated internally (i.e., limitation of
the electromagnetic emissions of the electronic equipments)
c) Electro-magnetic shielding of large surfaces (e.g., walls of buildings) for
the attenuation of external electromagnetic fields (electrosmog)
d) Reduction of the build-up of electric charges over large insulating surfaces
(anti-static coatings for floorings and walls)

The products intended for the uses of points a) and b) find application in the
field of the electromagnetic compatibility, i.e. the discipline that studies and
controls the phenomenon of electromagnetic emission and susceptibility from
the electric and electronic equipments.
The products to be used for the local shielding of large surfaces (point c) find
an application in the field known as electrosmog or electromagnetic pollution
prevention, for the reduction of the electromagnetic fields generated by
external sources to the buildings, such as power lines and installations for the
diffusion of radio-telecommunication signals and for cellular telephony.
In consideration of the physics of the electromagnetic waves propagation in
the space and in different materials, when choosing a shielding coating one
has to consider, first of all, the purpose of the product application. Coatings
with good performance for the attenuation of Radio Frequency fields incoming
over walls of buildings (electrosmog remediation) may have poor
performance when used to achieve the shielding of enclosures of electronic
equipments for the limitation of the electromagnetic emissions.
The research work, recently concluded by IRIS Vernici, has led to to the
formulation and optimization of some shielding products with different
performances, each one optimized according to the end use. Particularly, the
following products have been developed:

R & D

The development work has been conducted with the support of various research
corporate bodies of international fame, such as the Polytechnic of Turin, the
laboratories of TecnoParco del Lago Maggiore and the Technological Scientific
Park for Telecommunications in Valle Scrivia, with the contribution of qualified
researchers experienced in the field of Electromagnetic Compatibility and
Electrosmog.
Due to the character of innovation of the research, some methods of
development and testing have been developed on purpose.
Testing methods and instruments developed during the research are now part of
IRIS Vernici’s know-how and laboratory and currently used in production quality
control and further improvement of the technology.
The research has been developed through the following steps:

Tests of “dry” resistivity of fillers

The selection of suitable conductive materials to be used as fillers into the
coatings has been carried out, in the first step, on conductive pigments powders.
In this step the methods for characterization of powders for their electric
conductivity have been developed. This parameter is fundamental for the
optimization of the performance of coatings in terms of electromagnetic field
shielding. The electrical conductivity of powders is strongly affected by the
intrinsic electrical conductivity of the material, but also by particle size and shape
and by their surface properties. This method is also used for incoming goods
quality control.

Surface resistivity testing

The effectiveness of coatings is then verified with a further testing technique:
surface conductivity measurement on the final product. To the purpose a dry
coating film drawn on a proper substrate and dried is placed on a bench in
contact with special electrodes and submitted to voltage and current
measurements.
The value of surface resistivity (Ω/ ) is the specification parameter for ESD
products (antistatics), but also give important information about shielding
performance (at least for conductive-shield systems at medium to high
frequencies). Therefore this method is generally used for production quality
control.

Shielding effectiveness testing

Different methods have been used for testing shielding effectiveness:

· 2 antennas method in an anechoic chamber (in figure), based on
measurements on a coated panel of 1m x 1m size.
· SEMS (Shielding Effectiveness Measurement System) method, where a coated
panel is placed as the closure of a small anechoic chamber within which is placed
the signal source.
· TEM cell method.

For each of these methods a single measurement is performed, or a scan over a
wide range of frequencies. This type of tests evaluate the actual shielding
effectiveness of a coating film and can be applied both to conductive systems
and to other materials.

Shielding electromagnetic fields

For better understanding the physical phenomenon of the propagation of
electromagnetic fields, it is useful to refer to an electromagnetic radiation of
which we have the perception: the light.
By analogy with the propagation of light, we are able to explain the concept of
screening the electromagnetic fields. Consider the dark lenses of normal
sunglasses: they represent a partial screen for light, in that they attenuate the
intensity of the rays of light that our eyes perceive.
Let’s imagine we want to build a completely dark room in the middle of a sunny
square. We should use non-transparent materials (e.g., bricks walls) and we
should have care to close completely any small crack: a small entrance for the
sunlight would be enough to break our " total dark " and our eyes would soon get
used to see something. If we don’t want to completely eliminate the light, but
only reduce it, we could make some small holes in the walls or put a glass
window (glass is almost totally transparent to the visible light). Increasing the
number of holes we could get the level of lighting that we desire, even if this
would not be uniform in the whole room. If we wanted to get a more uniform
distribution of lighting, instead of creating patways for the external light we
should build walls with a given level of attenuation of the light (e.g., darkened
glasses). The darker the glass, the darker the room will be and uniformly in its
volume.
The electromagnetic waves behave like light rays: they can be both reflected or
absorbed by some materials (as the brick walls for light), attenuated by some
other materials (as the darkened glasses for light) or left unchanged from others
(as the transparent glass for light). It is self evident that there is no
correspondence between the materials that reflect, screen or attenuate visible
light and those that do so with the electromagnetic fields: a bricks wall or a
wooden wall don't have the proper physical characteristics to attenuate the
radio-frequency electromagnetic waves incoming over them, as happens instead
for the light radiation. Equally the X rays are screened only by some types of
materials.
To screen or to attenuate an electromagnetic field directed on a surface means
therefore to use materials that have characteristics suitable to the purpose.
The numerical representation of the shielding effectiveness of materials has to be
carefully considered. The following figure shows the difference between the
different methods of representation of the attenuation of the electromagnetic
fields.
It is evident that some values of attenuation that are apparently low when
expressed in dB (that is a logarithmic unit), can be more than enough for certain
applications when we consider them in absolute value.

Clic for details

Consider, for instance, the problem of the attenuation of an electric field
incoming on the walls of a building placed in the bundle of irradiation of a station
for cellular telephony. The treshold value of field inside the buildings is
established from the Italian national regulations in 6 V/m; the attenuation given
by the walls to an inward field is, as a rule, around 3 dB. An inward electric field
of 10 V/m outside produces inside the building a field of about 7 V/m. This value
is still higher than the allowed limits.
Coating the wall facing the emitting antenna with a layer of a conductive coating
(e.g., 2 coats that is approx. 400 g/m2 of IRIShield MB 285) gives a further
reduction of 20 dB (at 900 MHz). This corresponds to a reduction of 90 % of
incoming field. The field inside will be 0,7 V/m, far lower than the regulation’s
threshold and the most prudential recommended values.
Even neglecting the attenuation effect of the wall, this coating layer would
achieve the value of 6 V/m inside (treshold limit) in the presence of an incoming
field of 60 V/m outside.

Test results