The immense possibilities of design and finishes of plastic materials plus the great potential of transformation processes have contributed to the huge growth of these materials in some determined sectors. Nevertheless, we cannot forget an essential property of plastics: their dielectric properties.
Therefore, the insulating characteristics of plastics have been traditionally critical for applications such as cables covers, different electric devices as plugs, multiple connectors, switches, etc. And the use of plastics in electronics has multiplied even further in recent years, thanks to the development of conducting polymers.
Intrinsically conducting polymers, such as polypyrroles or polyaniline, are being suggested as substitutes for silicon chips while keeping flexibility as a distinctive element. Without having high levels of electric conductivity, polymers charged with conductive particles (graphite, nanofillers and hybrid combinations with metal fibers) are specially attractive for uses like electronic-device casing with electrostatic discharge properties (ESD) and electromagnetic interference (EMI).
The use of plastic materials in these fields is based, traditionally, on the application of paints or surface metal treatments. However, any incision or surface scratch can turn the case into a perfect aerial and completely lose the required protective function. This is one of the negative points of metal coatings, whose energetic cost — along with the treatment cost, etc. — has to be added.
Other attractive and huge-technological-impact applications of conductive plastics are their possibilities as electronic sensors, which react with an external stimulus — in this case, pressure or stress (piezoresistive).
The level of conductivities and resistivities of these materials fences in the possible application for the conductive plastic, whether as ESD, EMI or dielectric. So, the characterization of material conductive properties is a critical point to keep in mind.
We have different equipment to determine the electrical resistivities of plastic materials at different levels, including a multimeter and a picoammeter. This latter allows us to work with materials that present low levels of conductivity.
In order to avoid any kind of external interference and improve the contact between electrodes, it is recommended to use a Faraday cage. This cage gives more precise measures, mainly in the case of highly-dielectric materials. Another important characteristic of this technique is the possibility to determine superficial and volumetric conductivies, depending on the final application.
In the field of intelligent materials — specificallysensoring — it is possible to characterize the electrical response of a plastic material when it is subjected to a deformation, or in other words, piezoresistive behavior.
The response can be characterized through a specific assembly, combining a universal trials machine or other mechanical characterization equipment and a multimeter. Consecutive cycles of deformation on the material produce some changes in volumetric conductivity and resistivity, turning it into a sensor.
We can determine here the speed of response, its variability, sensibility, including environmental effects such as humidity and temperature. In the same way, it is possible to develop capacitive sensor systems from the highly-conductive materials that present a characteristic electrical sensibility to the touch when modifying the capacitance of these surfaces. So this last concept for automated developments, safety ones, etc., is interesting.
In the field of electromagnetic shielding, the range of electric conductivity is not the only determining factor. The design and the geometry, the presence of escapes and the case sealing — mainly in holes and apertures — are also critical aspects.
The characterization of electromagnetic shielding is a delicate technique. The necessity of anechoic chambers, the determination of the adequate electromagnetic field range, the type and the orientation of it, etc., are aspects to keep in mind when developing this characterization's methodology.
We are currently working on the optimization of these characterization techniques, offering a complete and wide spectrum of possibilities for the study of electric properties of plastic materials.
