Practical relevance of electrical properties
The electrical properties of paper are crucial to image quality and runnability in electrophotographic copiers and laser printers, since the image is transferred from a photoconductor drum to paper with an electric field. The transfer field is created with a corona wire or a bias roll when paper is in contact with the toner layer. The transfer efficiency is high when the transfer field is strong. This depends on the resistivity and dielectric thickness of paper, Dε = d/ε´, where d is paper thickness. Resistivity affects the charge decay time, as shown in /LINK: 03.09.03.04.01 /. This should be longer than the transit time of paper in the nip. If this is true, then transfer efficiency is insensitive to variations in resistivity and maximum transfer results when the dielectric thickness, Dε, is as small as possible. This corresponds to a high dielectric constant of paper. The structure of paper has an effect on the transfer electric field, which originates from the distribution of highly dielectric fillers, from porosity variations, and in particular from the surface roughness. Since the surface z(x) may have long-range variations originating e.g. from flocculation, the transfer efficiency and image quality can also vary substantially over 1,21–2 mm.
The effect on runnability arises from static electricity that can lead to double feeds, misfeeds, paper jams and even sparks. This happens if resistivity is too high or the charge decay time is too long. Paper performance in electrophotography can be maximised only through optimisation of the electrical properties of paper according to machine speed and design.
Capacitor paper plays the the role of the dielectric in high-voltage capacitors. For this purpose, all the voids in paper are impregnated with oil or epoxy. Paper capacitors are inexpensive and reliable, but they suffer from a limited operating temperature range of -40 °C – +70 °C, large losses, and large size. Paper cannot be made as thin as plastic films, and plastic has therefore partly replaced paper. Capacitor paper should have high dielectric constant, low dielectric losses, and high dielectric breakdown strength. This requires high chemical and mechanical purity, no voids (proper impregnation), and high density. Chemicals may be added to increase the dielectric constant.
An interesting application of paper capacitors is the electromagnetic interference suppression or EMI capacitor. It is typically used in a simple LC circuit between the equipment and its power source to protect it from sudden power surges. A paper-based EMI capacitor contains a metal-coated, epoxy-impregnated paper. It is excellent in withstanding current surges and recovering from local dielectric breakdowns.
If paper is properly impregnated, it has no cavities and it is very resistant to ionisation — a destructive process for EMI capacitors. Paper-based EMI capacitors fulfil all safety requirements in standards and applications.