Rethinking the Role of Thresholds in Achieving ESD Compliance When Using Epoxy-Based Coatings – In Compliance

The ESD flooring market continues to grow and evolve as robotics and other electronics are deployed in new ways within various work environments. Mobile robots, for example, which can build up a charge when operating on standard flooring, are increasingly common in warehouses where they increase productivity and allow e-commerce operators to meet customer demands for faster delivery times. In such cases, warehouses that once relied almost exclusively on forklifts will need to be equipped with ESD flooring to support new technology.

These warehouse applications, along with those in data centers and many manufacturing facilities, are best served by covering the floor with an epoxy coating mixed with a conductive additive such as carbon black, tin oxide or carbon nanotubes. The additives create a conductive network that transfers charge to copper strips mounted beneath the floor, effectively dissipating static electricity generated by humans and mobile robotics.

With an epoxy-based flooring, floor aesthetics and performance are difficult to determine until after the floor is installed and has fully cured, making the mixing and installation processes critical to the success of any project. Too little conductive additive within the coating and the floor will not be conductive enough to dissipate charge; too much and the floor becomes so conductive it can introduce a shock hazard.

In addition, at the loading rates required to achieve high levels of conductivity, some flooring additives are more likely to aggregate in certain areas of the floor, creating hot spots that break the conductive network and make the entire floor ineffective. High loading rates of inherently dark additive can also negatively impact floor aesthetics, making it harder to achieve the desired color and finish.

While building operators may reluctantly live with subpar aesthetics, they cannot live with hot spots, which are a common problem for ESD control flooring manufacturers and installers. Even expert installers have fallen victim to hot spots forming post-installation, indicating they are not always the result of improper installation. Manufacturers and installers may then disagree on liability, delaying resolution and frustrating all involved.

Addressing the challenge of hot spots within the framework of compliance is one of the most important issues the ESD flooring industry faces as it seeks to capitalize on the demand resulting from increased penetration of electronics in the workplace.

While there are a number of standards that can apply to ESD control flooring, depending on the application, by far the most commonly used is ANSI/ESD S20.20. This standard was last revised in 2014 at which time significant updates were made, most notably to the qualification method.

Prior to 2014, the standard allowed for qualification based only on resistance. If the total resistance was less than 3.5 x 10^7ohms from a persons hand to ground, the floor was in compliance. A walking voltage test was required only when resistance was greater than 3.5x 10^7ohms and less than 1.0 x 10^9ohms.

In the 2014 revision, the resistance method of qualification was eliminated and a walking voltage test was required for qualification, regardless of floor resistance. Specifically, the standard covers:

The requirements necessary to design, establish, implement and maintain an Electrostatic Discharge (ESD) Control Program forelectrical or electronic parts, assemblies and equipment susceptible to damage by electrostatic discharges greater than or equal to 100 volts Human Body Model (HBM).

As a result of this revision, the focus for compliance shifted from achieving specific resistance levels or thresholds to the ability of the floor to prevent the build-up of charge of 100 volts as determined by a walking voltage test.

However, one issue that was not addressed in the 2014 revision was the distinction between static dissipative and static conductive flooring that was included within the standard. Conductive flooring is generally defined as having resistance below 10^6 ohms, while dissipative flooring is defined as having resistance above 10^6 and below 10^9 ohms.

This was meaningful when compliance was based on floor resistance because flooring that is too conductive can subject workers to shock hazards. In fact, the National Fire Protection Association, prior to the development of ANSI/ESD S20.20, created a standard to prevent injury to workers in ESD flooring applications, a standard which is still referenced today.

That standard required an ESD control flooring system to not be more than 25,000 ohms when tested at 500 volts. Newer testing apparatuses test at 100 volts rather than 500 volts, so that translates into a ceiling of 100,000 (10^5) ohms, the same as is included in the current version of the S20.20 standard. Since the qualification is now based on a walking voltage test, the distinction between dissipative and conductive is irrelevant.

However, that distinction spawned a convenient threshold for specifiers. Wanting to stay on the low end of the dissipative range, many specifiers adopted one megaohm (10^6 ohms) as their guiding specification for ESD control flooring. If they could achieve a resistance level of 10^6 ohms across the floor, they could safely avoid the need for a walking voltage test under the previous version of the standard. Yet, even though the need for a walking voltage test is no longer dependent on floor resistance, the one megaohm threshold has become so well established that specifiers continue to use it today despite the fact it is not directly relevant to compliance.

The EOS/ESD Association, which maintains the S20.20 standard, is currently reviewing the standard with an eye toward addressing the confusion that has been created by maintaining the distinction between conductive and dissipative.

According to David Swenson, president of Affinity Static Consulting and a director of the EOS/ESD Association:

In the 2014 revision, the committee clarified that the requirements for the flooring and footwear system be less than 10^9 ohms with no lower limit. However, the current standards maintain the distinction between dissipative and conductive, which is not relevant in this application, and that may be contributing to confusion among specifiers. This distinction will likely be eliminated in the next revisions of the applicable standardsanything under 10^9 will be considered conductiveto further address this issue. The most important characteristic is the resistance-to-ground of the person standing on the floor and their walking voltage.

As Swenson makes clear, the distinction between dissipative and conductive, while still in the standard, is irrelevant in light of the other changes made in 2014. Yet, continued reliance on one megaohm as a threshold by specifiers is introducing unnecessary complications into compliance with the standard.

One megaohm is problematic as an absolute threshold because it is at the high end of the effective range of conductivity. Because resistance is logarithmic rather than linear, it is extremely difficult to hit a precise target when mixing ESD control flooring.

In addition, temperature and humidity variations across the floor, along with uneven dispersion of some conductive materials used in ESD control flooring, create variations in resistance measurements. When resistance of 10^6 ohms is set as an absolute threshold, its likely actual overall floor resistance will be closer to 10^5 ohmsintroducing higher risk of shock hazard and hot spots.

The impact of one megaohm as a target has even evolved beyond its use as an absolute threshold and contributed to the misconception by some that resistance should be driven as low as possible. Some specifiers now operate on the assumption that if 10^6 ohms is good, 10^5 ohms must be even better. Of course, this is not the case; yet, manufacturers and installers continue to have to educate their customers that a threshold of 10^5 ohms does not enhance protection of equipment and increases the likelihood the floor will accumulate charge, which can present risks to workers.

Rather than seeking to achieve the highest possible conductivity level, OEMs, architects and building owners should be looking to reduce conductivity levels to the point where they can safely and consistently meet the S20.20 requirement of not discharging 100 volts. That level is usually below one megaohm.

This has the added benefit of reducing the amount of additive required in the flooring compound, which reduces the impact of the additive on floor aesthetics and the likelihood of hot spots. However, with additives such as carbon black, relatively high loading rates are still required to achieve resistance levels below 10^9, and the risk of hot spots, while reduced, still exists.

That risk is being addressed through a new generation of conductive additives now being introduced into ESD control flooring. The reason traditional additives require high loading rates is the length of the fibers that compose the additive. Creating a cohesive conductive network requires a relatively dense distribution of fibers across the floor and achieving that level of density requires loading rates as high as 20%. That density is what can lead to hot spots.

New materials such as single-wall carbon nanotubes have a higher length-to-diameter ratio than any materials. Carbon nanotubes have a length-to-diameter ratio that can be as high as high as 132,000,000:1, allowing them, when added to the epoxy coating in a pre-dispersed matrix, to create an effective conductive network that is virtually impervious to hot spots at loading rates as low as 0.1%.

In addition, control over color and finish is not compromised in the way it is with other additives. Not only can manufacturers provide architects and specifiers with greater control over color when coatings dont have high percentages of inherently dark additives, but installers can more easily achieve the desired smooth finish and avoid what is commonly referred to as the orange peel effect.

While the expected changes to the S20.20 standard may help reduce the confusion caused by the current distinction between conductive and dissipative resistance that has led to the one megaohm specification, the convenience of a simple threshold for floor resistance may remain attractive to specifiers.

However, specifiers should also be aware that there are costs associated with this convenience. The ESD flooring industry can only address the challenge of hot spots and their costs to manufacturers, installers and building owners by better understanding the conductivity levels required to safely comply with S20.20 resistance-to-ground values and migrating to newer materials that can achieve compliance without the risk of hot spots.

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Rethinking the Role of Thresholds in Achieving ESD Compliance When Using Epoxy-Based Coatings - In Compliance