The need to control, and in some cases to take advantage of, static electricity, requires a better understanding of the surface characteristics of materials. Static electricity arises when materials in contact, separate, and the charge on one or both surfaces is trapped because it cannot move to the last point of contact in a time that is short in comparison to the separation time. The quantities of charge and the surface voltages arising depend on the materials involved and on the speed and pressure of the rubbing contact. Central to assessing the suitability of materials, is the charge decay time.

The traditional way of assessing materials has been by measurement of surface resistivity. This is simple, but actually does not show the real practical performance of materials. This is particularly true for modern materials such as garments made of fabrics that include patterns of core-conductive thread. The practical requirement for static control is that the surface voltage remain low despite the largest quantity of static charge likely to be transferred in practical contact or rubbing actions. If the surface voltage is below about 300V, there will be no air static discharges. Voltages below this can of course create risks by inducing charge on nearby items with consequent risks in the microelectronics industry via ‘charge device model’ events. Permissible voltage levels need to be related to maximum device sensitivity.

An approach has been developed that enables meaningful measurements to be made of the characteristics of materials based on tribocharging.1 The approach developed is fine for research type studies (including inhabited garments2), but is really not suitable for industrial assessment work. Charging by a high voltage corona discharge provides the basis for more practical instrumentation. Studies have shown that there is good correspondence between the performance of materials observed with tribocharging and when charged with a high voltage corona discharge.1,2 Thus, there is opportunity to predict the performance of garments from measurements on fabric samples with compact, self-contained instrumentation that is easy to use by non-specialist staff.

Assessment of materials is based on measurement of two main performance features:

* How quickly charge dissipates over the surface of a material

* What capacitance is experienced by static charge on the material surface

If we can know the relevant electrostatic characteristics of materials then there can be confidence in the selection and use of these. This capability should be of benefit to garment makers in their selection of material and to material manufacturers. Without appropriate measurements, it is guesswork.

References:

1 J. N. Chubb. “New Approaches for Testing Materials,” Presented at ESA Annual meeting, Brock University, Niagara Falls, (June 18-21, 2000), J. Electrostatics, Vol. 54, (March 2002) p. 233.

2 J. N. Chubb, P. Holdstock, M. Dyer. “Predicting Maximum Surface Voltages on Inhabited Cleanroom Garments in Practical Use” Paper presented at ESTECH 2003, Contamination Control Division, Phoenix, Arizona, (May, 18-21, 2003) (To be published in IEST Journal).