Fowler Associates Labs

 

 

Static Fire Stories Articles & Technical Papers Current News

First Published in EOS/ESD Technology Oct/Nov 1991

A Proposed Test Methodology
For Floor Materials: Part 2

Testing various characteristics provides important information
for proper selection of floor materials.

Michael T. Brandt
President, Marketing Resources Ltd.
305 Euclid Avenue, Sheboygan, WI

Stephen A. Halperin
President, Stephen Halperin & Associates
P.O. Box 1225, Elmhurst, IL 60126

This is the second part of a two-article series on floor testing. Part 1 (EOS/ESD Technology, August/September 1991, p. 86) covered various procedures and methods for testing floor materials. The test methodology covered electrical resistance, body voltage generation, body voltage decay, voltage gradient, and voltage suppression characteristics.

The following article discusses the results of the tests described in Part 1 of the series.

To thoroughly examine the electrostatic properties of ESD floor materials, electrical resistance, body voltage gradient, and voltage suppression characteristics should be considered. The test methods and procedures discussed in Part 1 of this series simulate the conditions of use and include the human body for measurements of voltage generation and decay.

Six types of footwear usually worn in conjunction with the floor materials were evaluated in the test methodology since static generation and dissipation characteristics are a function of the interaction between floor and footwear.

The types of floor samples tested included a variety of materials such as epoxy floor coatings and resilient floor coverings, with and without floor finishes. Floors A, B, D, E, F, and G represented different ESD floor materials while Floor C was the uncontrolled test bed.

The following information details the results and observations of the floor testing.

Electrical Resistance

Electrical resistance is a basic material attribute and not a measure of indicated or actual performance. The resistance measurements do, however, characterize basic differences in material properties across a broad range of materials (Table 1). Floors A and E represent typical conductive floor materials with resistances in the 104 to 106 range. Floors B, D, F, and G represent typical dissipative floor materials with resistances in the 106 to 1010 range. The test bed (Floor C) measured greater than 1012 .

Table 1: This table shows the difference in average electrical resistance for the various floor sample.

As expected, the level or applied voltage had a strong effect on the measured resistance of the samples. The lower voltages resulted in higher resistance values. Similarly, the conductive elastomer electrode surfaces tended to give lower resistance values than the foil-covered electrodes. The measurements made at lower voltages and with the conductive elastomer electrodes also tended to show less variability and better reproducibility. These observations are consistent with earlier work on worksurfaces and floor materials for EOS/ESD Association standards subcommittees. Based largely on this earlier work, it appears that the use of conductive elastomer electrode surfaces and applied voltages of 10V or 100V are the preferred alternatives for measuring the resistance of floor materials.

Body Voltage Generation and Decay

Unlike resistance, body voltage generation and decay represent actual performance characteristics of the floor materials. In a controlled test environment, the proposed procedures provide indicators of material performance in actual use. Body voltage generation and decay are the two most critical characteristics used to evaluate the performance of ESD floor materials.

Table 2: The measurements for body voltage generation and decay shown here vary depending on the type of floor material and footwear combination.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The test data indicate significant differences in body voltage generation and decay characteristics among the various ESD floor material and footwear combinations (Table 2) and provide several insights into material differences.

Figure 1

Figure 2

Figures 1 and 2: The body voltage generation and body voltage decay test results, shown in Figures 1 and 2 respectively, vary depending on the floor material

Floors and Footwear. As expected, voltage generation and decay vary among floor materials with given footwear (Figs, 1,2). However, the decay rates for the test bed floor material (Floor C) with controlled footwear may be partially a suppressive effect rather than total decay.

Figure 3: This figure shows the minimum average body voltage generation measurements for conductive/dissipative floors and various types of footwear. Figure 4: The min. avg. body voltage generation for dissipative floors have similar resistances but different voltage generations.

 

Material Resistance. Body voltage generation is not a function of material resistance. Floor G is three orders of magnitude higher in resistance than Floors A and E, yet Floor G generates significantly lower body voltage (Fig. 3). Floors B, D, F, and G are similar in resistance, but significantly different in voltage generation (Fig. 4).

Figure 5 Figure 6
Figures 5 and 6: These figures show the minimum average body voltage generation and body voltage decay, respectively, for floor sample D tested with varied footwear.

 

Voltage Generation, Decay Time and Footwear. Voltage generation and decay times vary with the footwear. For example, Figures 5 and 6 show the differences in voltage generation and decay times for Floor D with different footwear. Not surprisingly, voltage generation levels and decay times were the highest with uncontrolled footwear (insulated or leather soles) and lowest with the conductive footwear.

In environments where devices with ESD sensitivities less than 200V are handled, insulated shoe soles used with any of the floor materials tested would create unacceptable levels of static generation. Leather-soled shoes also showed problems with most of the floor materials.

Figure 7 and 8: These figures show the minimum (left) and maximum (right) body voltage generation probabilities for the Floor D and dissipative tennis shoe combination.

Probability Analysis. A probability analysis can help predict the performance of various floor and footwear combinations, to aid in material selection. For example, the combination of dissipative floor material D and dissipative tennis shoes has less than 0.1% probability of generating a charge in excess of 70V with both feet on the floor (Fig. 7). The same combination has less than 0.1% probability of generating in excess of 450V while a person walks through the work area (Fig. 8).

Figures 9 and 10: Figure 9 shows the probability of minimum body voltage generation and Figure 10 shows the probability for maximum body voltage generation for Floor D combined with various types of footwear.

Other combinations of the same floor material with different footwear show different probabilities of specific body voltages being generated (Figs. 9, 10). The right combination of floor material and footwear depends on the requirements of the specific environment (Table 2).

Voltage Suppression

Voltage suppression characteristics of floor materials provide additional data to evaluate the various alternatives. Although there are no specific criteria to base an analysis of this characteristic, the various materials tested here exhibit significant differences in voltage suppression.

Table 3: The average voltage gradient measurements for the various floor samples shown here can be an important evaluation criteria.
Table 4: The data from the voltage suppression tests show that some floor samples have no suppressive effects while others have varying residual voltages.

 

Two of the specimens, Floors A and B, showed no suppressive effects at all (Table 4). The charge on the metal plate was fully dissipated by contact with the floor material, and no residual charge remained when the plate was removed. The other four materials tested (Floors C, D, E, and G) showed residual voltages of varying magnitude. Floors E and G had previously shown the best generation and decay characteristics, but retained residual voltages of greater than 100V.

Voltage suppression and voltage decay can be significant in environments where people and materials move from unprotected areas to protected ones. For example, people will generate a static charge as they walk about on an uncontrolled floor in an aisleway. As they step on a floormat at a workstation, the decay and suppression properties will determine how quickly and completely that charge dissipates though the mat to ground.

Voltage Gradient

The final parameter of evaluation is voltage gradient. According to the tests, it appears that resistance is not the determinant of the differences between materials (Tables 1 and 3). The material design and installation are the more likely influencing factors. Those materials (such as Floor G) with a conductive pathway across their surface instead of into the bulk of the material (such as Floor A) are more likely to show higher levels of voltage. This is because the current travels across the surface to the sensing electrode.

When establishing the qualification criteria for evaluating floor materials on this characteristic, it must be determined whether the environment needs protection from exposure to voltages moving across the surface of the floor material. In some situations, where unprotected parts are stored or put on the floor, this can become a important parameter.

Summary

The proposed test methodology and evaluation of the supportive data lead us to a number of conclusions:

1. The proposed methodology demonstrates material attribute and material performance differences among the various materials.

2. Electrical resistance characteristics do not define material performance.

3. Body voltage generation and decay are critical evaluation parameters, but the levels of performance depend on the combination of floor material and footwear.

4. The proper footwear is critical to the performance of ESD floors.

5. Performance on one parameter is not necessarily an indicator of performance on others.

Evaluation of ESD floor surfaces should be performed on parameters other than electrical resistance. The test methodology proposed here allows evaluation across a wide range of electrostatic properties: static generation, static decay, voltage suppression, voltage gradient, and electrical resistance.

The test procedures are designed to specifically evaluate and compare flooring materials intended for use in sensitive electronic manufacturing environments. Combining the performance characteristics of the materials with appropriate statistical analysis can lead to a prediction of performance in the field and can lead to better selection of the most appropriate floor material and footwear combinations for the specified applications.

Copyright 1990, Stephen Halperin & Associates

References

1. ASTM F150, "Standard Test Method for Electrical Resistance of Conductive Resilient Flooring."

2. AATCC/ANSI 134, "Electrostatic Propensity of Carpets."

3. Chase, E. W., and Unger, B. A., "Triboelectric Charging of Personnel from Walking on Tile Floors," 1986 EOS/ESD Symposium Proceedings.

4. Crowley, Joseph M., and Halperin, Stephen A., "Resistance Testing of Static Dissipative Worksurfaces," 1988 EOS/ESD Symposium Proceedings."

5. Crowley, Joseph M., "Floor Covering Resistances Measured with Persons and Electrodes," and "Resistance Testing of Carpets and Other Floor Coverings," test reports prepared for the EOS/ESD Association Standards Subcommittee on Floor Materials, 1989.

6. EOS/ESD Association, "Standard for Protection of Electrostatic Discharge Susceptible Items: Worksurfaces."

7. Halperin, Stephen A., "How to Select Flooring," EOS/ESD Technology, February/March 1988.

8. Kolyer, John J., and Cullop, Dale M., "Methodology for Evaluation of Static-Limiting Floor Finished," 1986 EOS/ESD Symposium Proceedings.

9. Kolyer, John M., Watson, Donald E., Anderson, William E., and Cullop, Cale M., "Controlling Voltage on Personnel," 1989 EOS/ESD Symposium Proceedings.

10. Lingousky, J.E., and Holt, V.E., "Analysis of Electrostatic Charge Propensity of Floor Finished," 1983 EOS/ESD Symposium Proceedings.

11. NFPA 99, "Health Care Facilities."

12. Shah, B.M., Martinez, P.L., and Unger, B.A., "Test Methods to Characterize Triboelectric Properties of Materials," 1988 EOS/ESD Symposium Proceedings.

13. EOS/ESD Draft Standard 7.1 (Proposed), "Floor Materials- Resistive Characterization of Materials," June 7, 1990.

The ESD Journal is not affiliated with any trade organization, Association or Society

ESD Journal & esdjournal.com are Trademarks of Fowler Associates, Inc. - All Rights Reserved

The content & Look of the ESD Journal & esdjournal.com are Copyrighted by Fowler Associates, Inc. - All Rights Reserved Copyright 2011

The YouTube name and logo are copyright of YouTube, LLC.