Many processes require the engineered control of electrostatic discharges (ESD) to prevent damage to electrically sensitive equipment and analytic processes, as well as to prevent fire or explosion when handling flammable liquids, powders, and gases. Static charges are the result of triboelectric charging of dissimilar materials such as clothing against skin and shoe soles on the floor surface. In addition, insulating materials such as plastic binders, document sleeves, and even innocuous items such as wastebasket liners can create substantial charges. Reduction of these charges by effective grounding is an important factor in maintaining a safe environment and protecting test equipment. An engineered ESD flooring material provides an economical and effective means to ground personnel without having to rely on hard point grounds such as wrist straps.
What type of work are you doing?
The selection of an ESD control flooring surface depends primarily on the intended use of the facility. The selection of ESD control materials should begin with a thorough evaluation of the type of activity to be performed. In addition to the electrical characteristics, considerations for chemical resistance, load density, and cleaning should be made. Because ESD flooring represents a significant capital investment, any evaluation should carefully consider possible future requirements or the possibility that currently applicable standards may be changed in the future.
How do you choose the appropriate system?
ESD control in any workplace is a standards-driven industry. There are two primary types of standards: those that handle sensitive electronic devices and equipment and those that control hazardous environments. In addition, the practical requirements of the flooring system, such as wear and chemical resistance, need to be matched with the electrical requirements.
The primary standard for the handling of electronic devices is ANSI ESD S20.20. This standard defines an ESD control program that applies to facilities of any type that “manufacture, process, assemble, install, package, label, service, test, inspect, transport or otherwise handle electrical or electronic parts, assemblies and equipment susceptible to damage by electrostatic discharges greater than or equal to 100 volts HBM (Human Body Model).” The standard also recognizes and addresses the fact that there are situations where devices and processes may be sensitive to potentials lower than 100 volts and contains a “tailoring” clause for the program administrator to lower the requirement as required.
The test methods contained within ANSI S20.20 evaluate personnel within the ESD sensitive arena by measuring the electrical resistance between the grounding system, the flooring, and the person wearing ESD footwear. There is a linear relationship between the flooring-to-footwear resistance and the ability of the person to build an electrostatic field on his body. External factors such as apparel, design humidification, and other factors will affect the ultimate body voltage generation (BVG).
Flammable and hazardous materials
Static charges in environments where the intended use is the handling and processing of flammable liquids, powders, and gases pose a significant threat to the safety of the personnel within the facility and, of course, the facility itself. National Fire Protection standard NFPA 77-14 Recommended Practice on Static Electricity details the safe conditions and procedures for handling hazardous materials in respect to electrostatic discharges and effective grounding. The ESD flooring and personnel footwear should be thoroughly evaluated in conjunction with the building ground system as part of any general safety program.
In years past the terms static “dissipative” and “conductive” became, through common usage, the two de facto ranges of resistance by which static control flooring materials were specified and marketed. In current usage they have become almost meaningless in terms of evaluating effective static control in today’s workplace. The term static “dissipative” was generally accepted to represent surfaces that had a resistance of 1.0 x 106 to 109 ohms when measured by the typical method. “Conductive” surfaces indicated resistance to ground of 2.5 x 104 ohms to an upper limit of 1.0 x 106 ohms. These ranges were essentially arbitrary in nature and represented the understanding of ESD phenomenon at the time. ANSI S20.20 has established standard requirements for flooring/footwear resistance which recognize the primary importance of reducing body voltage generation.
ANSI S20.20 indicates by omission that there is no recognized lower limit of resistance in flooring materials. Because there is a linear relationship between flooring and footwear resistance and body voltage generation, it would presumably be ideal to have the lowest possible resistance value. The generally accepted lower limit, however, has historically been 2.5 x 104 ohms of resistance to ground. This lower limit represents what is considered the lower safe limit in respect to potentially hazardous currents that could result in involuntary muscle reactions that would prevent a person from pulling back from a ground fault condition. In industrial environments where exposure to higher voltage could be possible, it would be prudent to raise this minimum to at least 1.0 x 105 ohms and ensure the use of GFCI utility devices.
What solutions are available?
There is a wide variety of products for ESD control floor surfaces. Careful consideration should be given to present and future requirements. Each type of material will have advantages and disadvantages in different environments and intended uses.
Resilient ESD tile and sheet goods are products that will give good general service in environments where there is no danger of spillage of liquids or the presence of heavy point loads. They are relatively economical and can present a variety of colors and finishes to compliment the architectural finish schedule.
ESD carpet is available in a wide variety of textures and colors to add aesthetic value as well as ergonomic and sound reduction qualities. Conductive fiber and backing provide effective grounding of potentially damaging charges. Non-ESD carpet is a major source of ESD charges in the administrative areas of the facility where carpet would be typically used. The use of ESD carpet eliminates this problem. ESD carpet finishes would be reserved for administrative areas.
Two component ESD epoxy floor paints are a very economical finish for larger areas that require ESD protection. They are often water-based products that are applied in relatively thin films. They represent an economical alternative for flexible spaces where the intended use may change in a relatively short period or very large areas that are subject to light or moderate levels of use. Because of their thin layer application they may be inappropriate for some applications where heavy physical abuse is expected.
ESD waxes and other topical treatments are temporary solutions to ESD control and should not be expected to provide compliant performance for long periods of time. These products would typically be used only as a stop-gap measure when necessary.
Polymer ESD coatings have been in use for decades as an alternative to ESD VCT, sheet goods, and other ESD floor treatments. They are typically formulated from very high solids (very low VOC) epoxy polymers and contain conductive elements to effect grounding from personnel wearing ESD footwear as well as furniture and equipment placed on them. Polymer ESD coatings provide a seamless surface resistant to liquid spills. Modifications of the resin system, typically epoxy, can provide chemical resistance to the most aggressive reagents. Various surface textures are available to address concerns of slip resistance where required.
Because epoxy polymer ESD floor coatings, when properly applied, bond tenaciously to the concrete, they are suited for applications where heavy loads are expected. ESD vinyl tile and sheet goods may fail under heavy point loads at the joint line.
The most advanced of polymer ESD floor coatings utilize a metallic oxide particulate dispersion. These products are superior to carbon fiber or aggregate systems as the particulate is evenly dispersed throughout the resin matrix producing a very uniform, electrically reactive layer with excellent wear resistance without affecting the surface electrical resistance.
Summary
The selection of an effective ESD floor covering system for the controlled environments should always begin with a thorough evaluation of the intended use and possible future uses. Intended use should include the evaluation of the devices or processes that are most sensitive to ESD events, chemical resistance, and aesthetic requirements. Understanding the need for and importance of ESD compliant footwear is critical in the performance of the ESD flooring chosen. Always remember that personnel grounding requires effective ESD footwear that is appropriate to the application, properly worn, and in good repair. In addition, ESD awareness training is invaluable for increasing the effectiveness of any ESD mitigation system.
Erik S. Van Anglen is Senior Technical ESD Specialist with Sika Industrial Flooring. [email protected]; http://usa.sika.com.
This article appeared in the January 2014 issue of Controlled Environments.
