A prickly issue
Chubb fig 1
Figure 1: Set-up for testing inhabited garments. The test subject stands on a charge measurement plate with their arm on an insulated support, while the test operator tribo-charges area of sleeve directly in front of JCI 140 static monitor
John Chubb, of John Chubb Instrumentation, argues there is a fallacy in the current assessment of garment materials for control of static electricity.
Static electricity can cause serious problems in cleanrooms. For example, a discharge may damage sensitive products such as microchips, disc drives or sensitive microelectronic measuring devices. In industries where powders are handled, such as food and pharma, spark discharges of static electricity could cause explosions. Polyester cleanroom garments produce static electrical charge as the fabric rubs together or rubs against clothing worn under the cleanroom apparel. Thus cleanroom garments need to meet international Standards for static dissipation.
There is a fallacy in the standard approach for assessing whether materials and garments will show low levels of surface voltages, and hence associated low local electric fields, when these are contacted or rubbed in practical use.
The standard method for testing the suitability of materials1,2 is measurement of surface resistivity. Experimental studies in 20083,4 showed that there is no correlation between surface resistivity and the surface voltages that may arise. The experimental tests involved standard cleanroom garments that included conductive threads in standard grid and stripe patterns with thread spacings of between 2.5mm and 20mm.
The conductive threads were of the two main types used in practice surface conductive and core conductive. The tests involved a test subject clothed in a test garment standing on a charge measuring baseplate with an insulating frame from the ground providing support to stabilise the position of the upper arm and body. An electrostatic fieldmeter was supported from this same frame to measure the local voltage on an area of the garment on the arm to be charged by rubbing.
A test involved rubbing the area of the garment in front of the fieldmeter with a single swipe of a wooden spoon covered with a wool sock. Observations were recorded of the charge transferred to the test subject by the rubbing action and of the peak surface voltage created at the rubbed area and how this decayed with time afterwards. The experimental set-up is shown in Figure 1.
The study outcomes were as follows: The fabrics of cleanroom garments with the surface conductive threads showed, as expected, very acceptable values of surface resistivity. The fabrics with core conductive threads showed, again as expected, very high values of surface resistivity well above the level deemed acceptable in existing electrostatic Standards. The times for charge dissipation on all fabrics was long (several tens of seconds) so the peak surface voltages created by rubbing were not affected by charge dissipation The peak values of surface voltage per unit quantity of charge transferred (volts per nanocoulomb) were smaller for fabrics with the smaller spacing of conductive threads Voltages that may arise on garment surfaces with large thread spacings can be quite large a few hundred volts for likely practical quantities of charge transfer at rubbing actions. Effective limitation of surface voltage requires that the garment fabrics have a small spacing of conductive threads i.e. 5mm or 2.5mm grid patterns.
It was clear from the experiment that what was relevant to limiting surface voltages was not the surface resistivity shown by surface conductive threads but the close spacing of the threads whether surface or core conductive.
These experimental results call into question whether it is appropriate to assess the electrostatic suitability of cleanroom garment fabrics and similar materials, on the basis of resistivity values. It is clearly necessary to take into account the influence of the capacitance experienced by charge on the garment surface.
Practical assessment of the suitability of materials does need to take into account both the influence of the capacitance experienced by charge on the surface of a material and how quickly the charge can dissipate. If charge can dissipate quickly, within a few tenths of a second, then there will be no problem from retained electrostatic charge but note that the rate of dissipation is not measured by resistivity.
Fast charge dissipation is not easy to achieve with cleanroom garment fabrics, because these are basically polyester. If dissipation is slow, then if the surface charge experiences a high capacitance, the surface voltages can be low and problems from static electricity avoided. It needs to be noted that one cannot rely solely on capacitance effects to limit surface voltage if charge cannot leak away in the time between charging events then the surface voltage can build up and problems may arise.5
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