Measurement of the dielectric properties of paper

Unlike dc resistivities of non-impact printing papers that are routinely measured, their dielectric properties are seldom considered. The dc resistivity 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 anisotropic frequency-dependent electrical properties of paper should be considered. We present a new method for the measurement of the electrical properties of paper in the principal directions of the sheet. In the frequency range of 20 Hz-1 MHz, the permittivity e' and dielectric loss factor e' or electrical conductivity σ can be measured. Measurements on coated and uncoated commercial papers showed some interesting characteristics. In spite of the low moisture content ofcoated paper, its dielectric constant increased with increasing relative humidity at the same rate as in copy paper. 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 was small. For both types of paper the dielectric loss was much greater in machine direction than in the thickness direction. Dielectric loss increased with increasing relative humidity more in machine direction than in z-direction. Differences in the MD and ZD dielectric constant were attributed to the anisotropy of paper.