Choosing the Right ESD Flooring
(continued from prior page)
Understanding Charge Generation
Walking body voltage or “triboelectric compatibility” is not determined by measuring the conductive properties of a material. Conductivity is a separate parameter involving a flooring material’s intrinsic ability to discharge stationary and moving conductors. In the case of flooring, conductivity influences charge accumulation only when moving persons interact with the flooring surface while wearing special conductive footwear. In the absence of conductive footwear, the resistance to ground of a floor has little or no impact on static charge accumulation on people. Our tests were conducted to determine the range of expected voltages on people wearing normal footwear.
The generation of static electricity from contact and separation is a well documented phenomenon. The literature on this subject is common knowledge: any time two materials interact and then separate – there is an exchange of electrons between the two materials. If one or both of the materials are either nonconductive or insulated from ground there is a distinct possibility of charge retention on either or both materials. In the case of people and floors, we know that shoe soles interact with the flooring surface and either strip or deposit electrons during walking. When we detect a negative static charge, that material or in this case person has accumulated a negative charge from the floor. A positive charge means that a person has a deficiency of electrons from a similar scenario but, likely involving different material compositions from the first scenario. Certain materials demonstrate strong, repeatable tendencies and always charge to a particular polarity (positive or negative) when they are frictioned by other materials. This proclivity to charge to a specific polarity is reflected in the attached “triboelectric series.”
Test Method And Samples
Triboelectric compatibility or walking body voltage was determined by measuring and evaluating accumulated static charges on test subjects as they walked across 48” by 48” grounded floor surfaces on access floor (raised flooring) panels. The walking pattern was similar to that described in Figure 1 in ESD STM97.2-1999. All floor surfaces were mounted and grounded per manufacturer’s installation. Each subject wore a wrist strap connected to an ungrounded floating metal plate attached to a calibrated Monroe Model-268 Charge Plate Analyzer. No grounded footwear was used. At the end of each walking session, test subjects were grounded to insure that all sessions began at zero volts. Five types of shoe soles were tested: 2 rubber, 1 leather and 2 synthetic composition soles. All measurements were collected using a recently calibrated analyzer set to “peak voltage” mode. Only peak voltages were collected. All graphs and data reflect kilovolts unless noted otherwise.
Triboelectric Series and Work function Theory
Please note the location of PVC and hard rubber on the triboelectric series. Hard rubber is positioned very close to the center of the chart while PVC is one of the last materials near the bottom. The center of the chart is considered a neutral zone while the two extremes represent materials that charge and accumulate charge more easily and within more material combinations. The most potent material interactions should occur when a material from one extreme is frictioned against a material from the opposite extreme. Additionally, materials that reside in the neutral zone tend to produce the least amount of static electricity when frictioned against other materials at either extreme. Our observations involving rubber and PVC conformed to these predictions.
The tribocharging properties of conductive carpet are not explainable based upon positioning on the triboelectric series. All tested carpets were composed of nylon yarn, an insulator and known generator of static electricity. Nylon is an extreme material in the triboelectric series. Conductive carpet’s low charging propensity may be attributable to thousands of conductive filaments on the surface. These filaments are the last points of contact when shoe soles separate from the carpet surface. The poor performance of the PVC carpet tile may be attributed to crushed (no longer part of the upper surface of the carpet) conductive fibers from chair caster abuse.
Testing Results:
Test results for rubber and PVC were consistent with the predictability mentioned in the previous paragraphs. Dissipative PVC flooring produced static voltages significantly greater than rubber flooring in the same interactions. The mean voltage measured on people walking on conductive rubber flooring was .34 kilovolts or 340 volts. The mean voltage measured for the dissipative PVC interaction was 3.13 kilovolts or 3,130 volts. Since the PVC testing yielded a graph that appears bimodal, at least for the shoe sampling used in this testing, the mean voltage should not be used as the comparative statistic in this evaluation. However, the lowest voltage interaction measured on PVC, was between rubber soled shoes and the floor; the mean of those interactions was 2000 volts. Since all means for all rubber floor interactions were below 500 volts there is still a dramatic differential (4:1 minimum and over 10:1 maximum) between the two flooring materials.
All ESD carpet tiles generated lower walking body voltages than dissipative PVC tile. Triboelectric generating properties of the 50cm PVC tiles were significantly impacted by the chair caster abuse test. Damage from chair caster abuse raised the resistance to ground per ESD STM7.1 of the PVC backed carpet tile from acceptable (106 to 108) to unacceptable (over 1010.) There were no measurable consequences from 100,000 cycles of chair caster rolling over the thermoplastic backed conductive carpet tiles.
