Measurement Methods
The most widely adopted method for the measurement of volume and surface resistivities in the paper industry is the method based on ASTM D257-93 1 known as the Keithley method. It uses two circular electrodes and a ring electrode around one circular electrode, as shown in Figure 1. The applied voltage is usually 90–400 V. Resistance is given by the applied voltage divided by the measured current. Resistivity can then be calculated from geometry and the measured resistance. Because of the very small current, proper shielding is necessary to minimise external noise. The measurement is simple, and commercial devices are available. The circular geometry eliminates in-plane anisotropy from the results.
The dc measurement of paper resistivity described above has several problems:
- Resistance is time-dependent. It can take several minutes before it becomes stable.
- Resistance depends on applied voltage.
- The electrode material affects the results (charge injection).
- The surface properties of paper influence charge injection, contact area and contact resistance.
- The contact resistance and therefore the measured resistance depend on applied pressure and electrode finishing.
Figure 1. Keithley method for measuring volume resistivity (a) and surface resistivity (b) 2.
For these reasons, comparing values found in the literature is difficult. Some improvements have been suggested to the standard, including different electrodes, two- or four-point contacts, needle beds and pulsed methods. Since the listed problems arise from the dc measurement, it is not clear that any of the proposed modifications would substantially improve the situation.
An ac measurement of paper resistivity would have several benefits. Most importantly, it would not have the problems of contact resistance and time-dependent results. The ac electric field would couple to the specimen capacitively without direct mechanical contact. The main drawback of ac measurement is the difficult interpretation of the frequency-dependent permittivity. The ac measurement of paper resistivity and permittivity is still under development.
Certain electrostatic properties of paper, such as the charging capacity of paper and the rate of charge decay, mimic the behaviour of paper in electrophotographic printers and other applications where the static electricity of paper is a problem. In the static electricity meter shown in Figure 2, charge is deposited on a paper surface with high-voltage corona wires. Paper is polarised by the deposited charge and the potential difference to the corona wires. After charging, the corona wires are rapidly moved aside, and the decay of the potential of paper is measured with a potentiometer, as shown in Figure 3.
Figure 2. Static electricity meter.
Figure 3. Charge decay curves for several papers at 23 °C and 20% RH: A = newsprint, B = supercalendered paper, C = lightweight coated paper, D–F = copy papers 3.
The static electricity parameters of interest are the maximum potential, Umax, and the time constant of charge decay, τ. If only one type of charge carrier were present in paper, the measured potential should decay exponentially in time with the time constant
(1)
Measured curves such as in Figure 3 do not follow the simple exponential decay. One possible reason is that there are several decay constants corresponding to different charge carriers.
