A Powerful Way to Increase Dielectric Constant

When we put a plastic insulator in an electric field, if it contains elements besides carbon and hydrogen, it has polar bonds, and these tend to orient in the electric field. If they are free enough to do so, this gives a high dielectric constant. In an alternating field, the polar groups try to follow the field. As we increase the frequency, the field alternates so fast that the polar groups lose the ability to follow it. In this transition region - dielectric loss - electricity turns into heat.

For handling electric power, we want low dielectric constant and low loss. For design of capacitors, we want high dielectric constant. For microwave heating, we want high dielectric loss. We can control these properties by the structure and composition of the plastic material.

Polarity is the basic structural feature. When carbon and hydrogen are attached to more negative atoms, this produces polar bonds, which try to respond to an electric field.

Mobility of the polar group is the second structural feature. Rigid polymer molecules are frozen in place, and the polar groups may be frozen as well, so the dielectric constant remains fairly low. Flexible polymer molecules permit the polar groups to move, so they are able to orient in an electric field, and the dielectric constant increases. Adding plasticizer gives the polymer molecules more mobility, so the dielectric constant increases. With increasing molecular flexibility, polar groups are able to respond at higher frequencies, so the transition region - dielectric loss - moves to higher frequencies.

Cross-linking of thermoset plastics immobilizes the molecules, and eventually even the polar groups, so it decreases dielectric constant and loss. Incidentally, the decrease in dielectric loss offers us an excellent non-destructive instantaneous measure of the progress of the cure reaction.

Adding inorganic fillers, especially metallic fillers, is a powerful way of increasing dielectric constant and microwave heating.

Conversely, air molecules are too mobile to orient in an electrical field, so the dielectric constant and loss are very low. When we foam a plastic material, the lower the density, the more air we build into the foam, the lower the dielectric constant and loss. These reach values far lower than would be possible in any solid plastic.