Today, ESD demands more and more attention, not only in classic sensitive areas like electronics production and explosives manufacturing, but also in clean-room applications such as in the pharmaceutical and optical industries. Obviously, controlling ESD is of prime importance.
For these industries, there are a variety of products on the market that control ESD. One example is dissipative or conductive flooring. Besides electrical resistance, an important feature of a floor covering is the level of triboelectric voltage generated when a person walks over the floor or when a cart is pushed on it.
Human-body voltage generation already has been examined, although no internationally accepted standards have been developed for measuring it. But there is much less research done about static generation caused by pushcarts widely used in the industry.
Here is one approach to get a better appreciation of the voltage-generation characteristics of various combinations of floor materials and rollers. This method does not give exact measurements. Additional development of the test procedure and equipment might lead to more precise measurements.
Measuring Equipment and Procedure
The castor-wheel test is well known in the floor-covering business: Standardized wheels are pressed against a rotating floor sample and, after approximately 100 rotations, the wear of the floor covering is observed and measured. Our modified castor-wheel in-house test machine served the purpose for measuring the voltages. A Plexiglas separator disk was applied on the rotating metal plate (Figure 1). Samples of floor-covering materials were fixed on the Plexiglas by a conductive, biadhesive sheet, imitating a conductive installation. From under the floor sample, a copper tape led to the metal plate that was connected to a permanent ground (Figure 2a).
Rollers made of steel, polyamide, polyurethane, and rubber normally used for cart wheels were mounted on metallic axles, which were grounded. Their weight was equalized by exercising a controllable, pneumatic pressure on the roller-holder fork (Figure 2b).
With the rollers pressed on the surface of the floor sample, the plate made 100 rotations at a speed of 24 rpm, corresponding to a linear speed of approximately 3.6 km/h or a fairly normal walking pace of a person pushing a cart. The static-charge level was measured indirectly by a JCI 140 Digital Field Meter. Readings were taken, stored, and then evaluated on a PC.
Between two test runs, the residual charges were neutralized by a high-voltage ionizer bar located over the floor-sample surface. The rollers were discharged each time by wrapping them in tin foil connected to ground. Tests were made at 18% and 40% relative humidity (RH).
Conductive and dissipative resilient floor covering materials produced with four different technologies were tested:
Traditional, calendered (pressed together and adjusted for thickness) sheet PVC.
Sheets/tiles produced by continuous pressing (two steel bands pressed with a force of 10 g for approximately 1 s).
Tiles made from a static press procedure (pressed for approximately 25 minutes).
A coated PVC sheet (this kind of material generally is not considered an industrial floor covering).
Most of the voltage readings were between 10 V and 50 V. About 10% exceeded 100 V and, in some extreme cases, reached as high as 270 V.
Some wheels made of conductive materials (steel, for example) generated more static potential than the insulating ones. The conductive rubber wheel generated higher voltages in 60% of the cases than its insulating version. Triboelectricity was the suspected cause.
Ambient conditions (RH) had different effects depending on the specific sample under test. One-fourth of the test runs with the same floor-wheel combination gave higher readings at 18% to 50% RH—once more, in spite of the old rules that suggested that higher humidity meant less static generation.
Wear and tear seemed to influence the static behavior of the different floor-covering types. Test runs repeated over the same samples, even after charge neutralization, gave higher readings.
Floors with a low resistance to ground, in some cases, produced higher voltages than those with a high resistance to ground.
Among the different types of floorings, the group of pressed tiles showed more consistency in their behavior when tested with the different wheels.
The Ancient Greeks described static electricity as being generated by amber, an insulating material. It still is a general belief that a conductive material will not generate static.
We often learn the contrary through hard lessons. Undoubtedly, there is a relationship between the electrical resistance and the capability of a material to generate more or less static charges. But this relationship is far from being linear and needs more investigation.
It always is the combination of two materials—like the floor and the wheels or the floor and shoe soles—that plays the most important role. Testing continues to confirm this conclusion. As a result, a floor material alone cannot be judged as static free. Its performance might drastically change depending on the nature of the object moving on it. The choice of a certain floor covering requires a careful matching of the shoes and wheels of the cart.
Let’s not forget that the choice of the floor covering itself is not simple and deserves careful attention. Besides the ESD-control features, several other properties are of equal importance in high-tech industrial areas: mechanical/chemical resistance, low outgassing, and ease of maintenance.
Choices cannot be based solely on commercial brochures. For example, manufacturers today do not offer floors and wheels in an optimal combination. On the other hand, the whole subject of triboelectrification still needs investigation. The existing theories are not always easy to translate into practical terms.
Standardization also has a long way to go. Until then, using proven basics and gathering and understanding additional experience can help in everyday applications.
About the Authors
Elvio Manso is a member of the research and development department at Forbo-Giubiasco. Before joining the company in 1989, he was chief technician of the polymer research and development department at Ciba-Geigy in Basle, Switzerland, for more than 20 years. Mr. Manso studied chemistry in Italy and Switzerland.
László Kende has been employed by Forbo-Giubiasco since 1991 and presently is product manager of conductive and dissipative floor tiles. He has a degree in electronics engineering from the University of Rome.
Forbo-Giubiasco SA, Via Industrie 16, 6512 Giubiasco, Switzerland, (011) 41 91 850 0111.
Copyright 1998 Nelson Publishing Inc.