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2. NEW APPROACH TO ASSESS MATERIALS A new approach has been devised and examined for assessing materials. The approach is to measure both the time for charge decay and the level of surface voltage created by known quantities of charge on the surface [5,6]. The quantity of charge to achieve a level of surface voltage gives a ‘capacitance loading’ factor. If the surface voltage arising for the largest quantities of charge likely to occur in practice is sufficiently low, then the material should be considered suitable. The decay time divided by the capacitance loading provides a single convenient ‘figure of merit’ for judging materials. Acceptance requires a low value for the ‘Figure of Merit’. This may be achieved either by a short decay time and/or by a high capacitance loading.
3. EXPERIMENTAL STUDIES 3.1 Introduction Experimental studies have been made by tribocharging and corona charging a variety of materials with measurement of the initial peak voltages generated on the surface, the charge transferred to the surface and the charge decay time characteristics [5,6]. Tribocharging studies have involved 'scuffing' the centre region of a 200mm diameter stretched area of sample with an initially charge-neutral Teflon rod. The electric field is measured at a nearby fast response fieldmeter during and after impact. The charge transferred to the surface is measured by the charge left on the Teflon rod. Corona charging studies have used a commercial unit in which a short pulse of corona discharge puts a local patch of charge on the sample and a fast response fieldmeter is used to measure the decay of surface voltage after the corona discharge electrodes have been moved quickly away. The charge received by the sample is measured directly in the sample mounting arrangement [6]. In the above studies ‘capacitance loading’ is calculated from the readings per unit charge without the material compared to those with the material. Charge decay is measured from the rate of decay of fieldmeter readings.
3.2 Arrangements for tribocharging studies In practical situations electrostatic charge arises on materials by contact or rubbing against other materials. Methods to assess materials need to be based on triboelectric charging or be shown to relate to it. A simple experimental arrangement has been devised [6] (shown in Fig. 1) for simultaneous measurement of the charge transferred by rubbing a stretched sample of material, the electric field at a fieldmeter nearby and the rate at which this field decreases as the charge dissipates.
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