First Published in EOS/ESD Technology
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 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
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
|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.
|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).
|Figures 5 and 6: These figures
show the minimum average body voltage generation and body
voltage decay, respectively, for floor sample D tested with
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
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 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
|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.
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.
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
1. ASTM F150, "Standard Test Method for Electrical
Resistance of Conductive Resilient Flooring."
2. AATCC/ANSI 134, "Electrostatic Propensity
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
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