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4.7 Discussion If the area charged is known or can be estimated, then the ‘capacitance loading’ can be expressed as a capacitance. If this capacitance is divided by the charge decay time then one has a value that is an effective ‘resistance’ inhibiting charge movement on the surface. It must be noted that this is NOT the same as the conventional ‘resistivity’ that is obtained by applying a voltage to contacting electrodes and measuring a current. The ‘effective resistance’ is a combination of the rate of charge decay and the capacitance loading as experienced by the charge on the surface. It is a convenient ‘Figure of Merit’ to describe the performance of materials. To arrive at this figure it is necessary to use appropriate methods of measurement – such as are described in the present paper. 5. CONCLUSIONS 5.1 The charge decay time is commonly used to judge if problems and risks are likely to arise from static charge retained on materials. A decay time of ˝s is often quoted as acceptable. The present studies have shown, however, that quite high transient local surface voltages can be generated by tribocharging actions even when charge decay times are as short as 0.2s. Decay time target values hence need to be reviewed and reduced. 5.2 Measurement of the quantity of charge involved in tribocharging and corona charging studies gives a basis for more meaningful comparisons between different tests and between different operators. It provides opportunity for calculation of capacitance loading. 5.3 Capacitance loading values enable the local surface voltages, and hence risks, to be calculated from the charge likely to be available in practical situations 5.4 Comparable results are obtained with tribo and corona charging of a variety of materials. There is thus opportunity that the suitability of materials can be fairly assessed using compact, easy to use test equipment based on corona charging and involving simple test protocols. 5.5 It is proposed: - that materials be assessed by measurement both of capacitance loading and charge decay time. - that these measurements be made by methods as described. - that for general applications the charge decay time shall be below 0.2s and/or the capacitance loading shall be greater than about 100. - that a ‘Figure of Merit’ for materials be calculated from the decay time divided by the capacitance loading expressed as a capacitance.
ACKNOWLEDGEMENTS The author wishes to thank Paul Holdstock, British Textile Technology Group (BTTG), for discussions and opportunity to use his low humidity test conditions and to Mick Dyer, consultant to Dupont Fibres, for supply of some test fabrics.
REFERENCES [1] D. E. Swenson "The effect of resistance to ground on human body ESD" 'Electrostatics 1999', Cambridge 28-31 March, 1999 Inst Phys Confr Series 163 p351 [2] J. N. Chubb "Dependence of charge decay characteristics on charging parameters" 'Electrostatics 1995', York April 3-5, 1995 Inst Phys Confr Series 143 p103 [3] J. N. Chubb, P. Malinverni "Experimental comparison of methods of charge decay measurements for a variety of materials" EOS/ESD Symposium 1992 p5A.5.1 [4] J. N. Chubb "Instrumentation and standards for testing static control materials" IEEE Trans Ind. Appl. 26 (6) Nov/Dec 1990 p1182. [5] J. N. Chubb "The assessment of materials by tribo and corona charging and charge decay measurements" 'Electrostatics 1999' Univ Cambridge, March 1999 Inst Phys Confr Series 163 p329 [6] J. N. Chubb "Measurement of tribo and corona charging features of materials for assessment of risks from static electricity" IEEE-IAS Meeting Phoenix, Arizona Oct 1999 [7] J. N. Chubb "Corona charging of practical materials for charge decay measurements" J. Electrostatics 37 1996 p53 [8] R. Gompf, P. Holdstock, J. N. Chubb "Electrostatic test methods compared" EOS/ESD Symposium, Sept 26-30 1999 [9] R. Gompf "Standard test method for evaluating triboelectric charge generation and decay" NASA Report MMA-1985-79 Rev 2, July 1988 [10] N. Wilson "The incendiary nature of spark discharges from textile surfaces" J. Electrostatics 16 1985 p231-245 [11] BS 7506: Part 2: 1996 "Methods for measurements in electrostatics" British Standards Institution [12] IEC 61340-5-1: 1998 "Electrostatics – Part 5-1: Protection of electronic devices from electrostatic phenomena – General requirements" (Technical Report) International Electrotechnical Commission |