Getting polymers in shape – microwave permittivity characterization
Polymers play an important role in a variety of applications, from household equipment to cars and aircrafts. A key step in processing them is forming the appropriate shape, e.g. bending a pipe. The standard way to realize this is using hot air or water vapor. However, this heats the whole processing equipment and not only the polymer part. It is much more attractive to employ heating by high-frequency electric fields, which can be focused such that only the relevant parts of the polymer are affected and thus energy consumption is minimized. But this requires that dielectric loss is high enough to allow efficient heating. In general, polymers absorb energy from electromagnetic fields due to inelastic oscillation of the polymer chain and, at higher temperatures, due to the breaking of these chains. However, this effect varies greatly, depending on the type of material, the frequency and also the temperature. To this end, only few information is available so far, particularly for the microwave frequency range and for elevated temperatures approaching the melting point.
In the framework of a ProFIT Project1 and in cooperation with mobitec GmbH, the Ferdinand-Braun-Institut is developing a dedicated measurement setup to characterize dielectric constant and loss tangent of plastic materials at low GHz frequencies. The special feature is that the samples can be analyzed and heated contactless by microwaves and thus complex permittivity can be determined as a function of temperature. Fig. 2 shows the measurement chamber, a microwave TE011 resonator where a pipe sample (black) is inserted in the center. The resonator has two coupling inputs for a network analyzer and for a high power generator. The generator heats the sample and the analyzer measures the resonance curve continuously. Fig. 3 illustrates the change in the resonance curve when varying the temperature from 20 °C to 200 °C, using a fully automated program in LabVIEW. Using electromagnetic simulations, one can obtain the relationship between the change of the resonance frequency and the Q-factor on one side and the complex permittivity expressed as epsilon (relative, real) and tan(d) on the other side. This measuring method is simple, sensitive and flexible. It can be applied for many types of materials and captures the full range of temperatures up to values close to the melting point.
This work is funded by IBB Berlin within the 1ProFIT Project 10167421 KuBiMikE.