Measurement of the dielectric properties of paper

Unlike de resistivities of non-impact printing papers that are routinely me asured, their dielectric properties are seldom considered. The do resistivi ty alone does not predict the behavior of paper very well when paper passes through a transfer nip. If the electric field changes with time, all aniso tropic frequency-dependent electrical properties of paper should be conside red. We present a new method for the measurement of the electrical properti es of paper in the principal directions of the sheet. In the frequency rang e of 20 Hz-1 MHz, the permittivity epsilon' and dielectric loss factor epsi lon" or electrical conductivity cr can be measured. Measurements on coated and uncoated commercial papers showed some interesting characteristics. In spite of the low moisture content of coated paper, its dielectric constant increased with increasing relative humidity at the same rate as in copy pap er. The dielectric constant of copy paper measured in the machine direction (MD) was much higher than what was measured in the thickness direction (ZD ). For coated paper the difference between MD and ZD dielectric constants w as small. For both types of paper the dielectric loss was much greater in m achine direction than in the thickness direction. Dielectric loss increased with increasing relative humidity more in machine direction than in z-dire ction. Differences in the MD and ZD dielectric constant were attributed to the anisotropy of paper.