Variation in Static Decay Time With Surface Resistivity

The static decay time and the surface electrical resistivity are commonly used to specify the static performance of plastic films and sheets. While the surface electrical resistivity is easier to measure, the time needed for static to dissipate in a typical application is not easily found given the surface resistivity. The static decay time measures directly the time needed for static to dissipate from a sheet in a specified geometry. Measurements on seven samples reveal that, while the static decay time increases approximately linearly with the surface resistivity, two samples with the same surface resistivities have significantly different static decay times. The static dissipation time as a function of the electrical surface resistivity is found by modeling the standard test geometry using a rectangular boundary value problem. While the model results are only approximate, three important insights are obtained. The potential at the center of the sample decreases approximately exponentially with time. For ohmic conduction, the static decay time is proportional to the surface electrical resistivity. Nonohmic conduction shortens the measured static dissipation time. The model results are consistent with measurements.