Experimental Apparatus & Procedures
Unfortunately, standardized test specifications and procedures available for characterization of ESD flooring materials have not been a topic of concern until recent years.
Staticworx® , superior in performance.Of those specifications available, many are ill-defined or unrelated to ESD flooring, but adapted by the ESD industry while development of specific ESD flooring procedures are being formulated.
Because of this, prototype test specifications were developed in cases where none currently exist, or modifications to traditionally accepted approaches of testing made when deemed appropriate. The critical aspect of this study was consistency. All floors were tested using identical test methodology, equipment & personnel. Test results should be considered on a comparative basis. The following tests were performed or criteria reviewed during the course of the study:
Body Voltage Generation - There is no correlation between surface resistivity of a material and its ability to prevent charge generation. The abilities of a floor to inhibit charge generation and dissipate any accumulated charge are two properties that should always be considered during testing & evaluation. Charge generation caused by rubbing the sole of a shoe on various flooring had to be determined using a variety of shoes and personnel. This was accomplished using a variation of standard AATCC-134 methodology.
Using a charge plate monitor connected to a chart recorder, a designated test operator performed testing on each of the test sites, wearing both ESD footwear and street footwear (Neolite). Measurements were taken with both feet in contact with the flooring and with one foot elevated, a worst case scenario. The difference in resulting voltage levels is due to the direct effect of body capacitance.
The formula for parallel plate capacitance is represented by:
|
Where, C = Capacitance |
Based on this relationship, capacitance (C) is minimized when one foot is raised from the floor surface, resulting in higher voltage levels. As capacitance (C) increases, voltages (V) will decrease. Another concern is the sole material, frictional force and body capacitance. For these reasons, the same operators performed each of the tests during the length of the study. The ESD footwear were tested for compliance to ANSI Type I specifications prior to starting each test. The footwear were then worn for 10 minutes before beginning each test to insure integrity. Readings were taken every other week for two years.
Surface Resistivity – As per ASTM D257, EOS/ESD-DS7.1 and NFPA 99, the point-to-point surface resistivity was checked using (2) 5 pound conductive rubber electrodes connected to a voltmeter, spaced 3 feet apart using a 100 volt source (see Figure#2). In addition, readings were taken & noted using a concentric ring apparatus.
When using the concentric ring fixturing, resistivity was calculated by using the following expression:
|
Where, |
Measurements were taken from various points on the test surface, including the center of the test site and at the edges where there was very little traffic. The electrodes were cleaned, as well as the test patch, using isopropyl alcohol and water between each reading. All equipment used in testing was calibrated and verified according to manufacturer’s recommendations. Readings were taken on a biweekly basis for the first two months, and once per month thereafter.
Installation Requirements – Each type of flooring had a specific method of installation and floor preparation. Comparisons were made, analyzing length of installation, necessary equipment, chemicals or solvents, fumes, amount of necessary floor preparation, etc. based on the manufacturers’ specifications and actual installation.
Appearance – The initial appearance, as well as weekly checks were performed. Color change, loss of finish, lifting, scuffing (see “scuffing”) bubbling, and dirt retention was checked and each test site rated on a 1 – 10 scale with 1 being best. During the entire evaluation, there as no major maintenance performed on any of the test sites, other than a once per week buffing at 5000 RPM using a water and ammonia cleaning solution.
Odor – For obvious reasons, the product was expected to have no offensive odor, as per ASTM D4078.
Resistance to Ground – Each of the floors were grounded and verified using a 100 volt source voltmeter and a five pound conductive rubber electrode, per NFPA 99 and EOS/ESD DS7.1 (see Figure #3).
The necessity of this particular test would depend upon the application. ESD flooring installed in an office area would not typically require grounding. However, in many manufacturing and assembly applications, the use of proper grounding would be essential if the operator were relying on the footwear and flooring as a source to ground.
The test equipment was calibrated and verified every 3 months during the study. Isopropyl alcohol and water was used to clean the test site and test apparatus prior to each reading. Readings were taken once per month over a two year period.
Static Decay – Static Decay tests were performed according to Fed Test Standard 101C, Method 4046, both with and without the addition of the human body. Current Federal test standards do not specify the introduction of a human body during testing. However, in order to properly analyze the floor’s ability to dissipate a charge on a human being, the introduction of an operator with and without ESD footwear was included in this study. A 5000 volt charge was generated on each of the test site surfaces, in addition to a separate test using an operator, with the decay time necessary to drain the charge to 0 volts noted.
|
Scuff Resistance – Black marking resistance, as described in ASTM D 3714, is the ability of a floor to resist black marks usually caused by the impact of heels and soles of footwear, along with various type of wheels rolled across the surface. In addition to using a sheet of 400-A Carborundum, paper and ladder-pendulum test fixtures, real life application testing occurred over the course of the study. The test site was chosen as representative of a typical manufacturing environment with medium traffic, as well as push carts and buggy traffic through the aisles. With this in mind, black marking/scuffing was checked at each of the test sites. Again, as in appearance, they were rated on a 1 – 10 scale, with 1 being best resistance to marking.
Wear - AT&T, like most other companies, strives for flooring that requires very little maintenance or replacement. With the cost of flooring being particularly high, the necessity for replacement or recoating (in the case of “poured” and “rolled-on” flooring) was considered.
Required Maintenance – Most manufacturers of ESD flooring report very little required maintenance, such as an occasional sweeping and wet buffing with a water-and-cleaning mixture to get out ground-in dirt. One goal of this study was to verify what type of maintenance was necessary and the cost of that maintenance over the life of the floor.
Slip-Resistance – For safety reasons, all floors must be UL certifiably slip resistant. This was accomplished using a portable Broom Grapper Machine, in lieu of laboratory testing, which entails the use of a James Machine or equivalent. The portable tester was used was used in order to do on-site evaluations during the first two weeks of the study according to the Underwriter’s Laboratory specifications, adopted by the American Society of testing and Materials in ASTM Standard D2047-69. The American Disability Act (ADS), was also considered, verifying that the floor surfaces were “stable, firm & slip resistant.”
Cost – Pricing was requested for each of the floor types.
Electrical Safety – In order to insure the safety of operators & maintenance personnel working on electrical while standing on the floor surface, the flooring was checked in accordance with UL Std 779. As standard practice, however, anyone working on electrical should always isolate themselves from the ESD flooring in order to avoid shock or other serious injury.
Sales Support – Sales support is an intricate and important part of any on-going or major purchase arrangement. Each of the companies involved in the study were contacted at various intervals and their response times, as well as levels of satisfaction with their responses noted.
Availability - National and International availability was noted for each of the floor types. Some manufacturers contacted were unable to provide the product to certain areas of the world or perhaps not outside the United States, while others had manufacturing or distribution locations world-wide.
Warranty – Although not a rule, warranties sometimes reflect the quality and level of confidence a manufacturer has with their product. It also serves as an insurance policy for the buyer reducing the risk of loss during the period of coverage.
Most floor “types” had similar warranties on mechanical & electrical properties, while others had superior coverage. Warranty information will be outlined and compared accordingly.
Abrasion – A breakdown of electrical characteristics may occur with a minimal amount of wear on certain types of flooring. Testing that determines the amount of wear a floor will exhibit over a period of time was examined per ASTM D-1044 using a CS-10-F wheel, 500Gm of maximum gauge loss of 0.40% at 2500 cycles and again at 1.6% at 10,000 cycles.
Fowler, S.L., “Triboelectricity and Surface Resistivity Do No Correlate”, EOS/ESD Symposium Proceedings, 1998.
Brandt, M. T, and Halperin, S.A., “A proposed Test Methodology for Floor Materials,” EOS/ESD Technology Int’l, August/September, 1991 & October/November, 1991.
Introduction | Experiment Overview | Procedures | Results | Economics | Summary | Conclusion | Credits
