What’s (Not) So Shocking About Electrostatic Discharge Coatings?
Posted on Tuesday, July 23, 2019
ESD coatings are partially conductive, which means also partially resistive to current flow, and can safely usher static electricity to an attached ground and prevent sparking. By being only partially conductive, built up charges don’t tend to jump to the coated surface all at once, as can happen with fully conductive materials like metals. In other words, the rate of charge transfer is modified (slowed) by the ESD coating. However, ESD coatings are conductive enough that static electricity can’t build up on them - the charges are dissipated. Voltage is directly proportional to current times resistance (Ohm’s law). At a given voltage difference between two surfaces coming into contact, a higher resistance coating decreases the current which flows between them. Some small amount of conductivity must be present so the charges can leave the scene.
Voltage differences up to at least 250 V can be accommodated by typical ESD coatings. Higher voltage uses are not uncommon with the understanding that testing is integral to a successful outcome, and it may be found that alternative safety measures are required above some critical voltage difference. Each coating will be a bit different. Even the most robust coating will eventually succumb to dielectric breakdown, but the voltage difference at which this happens is best determined experimentally at the intended coating thickness. This is a concern in applications involving particularly high voltage differences.
Take Charge of Your Static Electricity:
A few situations that benefit from the ability to dissipate charge include:
Integrated circuits are notoriously prone to shock damage. This is why they arrive packaged in antistatic bags. An ESD-coated workbench can prevent a lot of unintended damage. Companies that assemble integrated circuits into other parts depend on such workbenches.
Equipment housings, particularly for electronics or high voltage parts.
° Semiconductor test equipment, for example, is all housed in ESD-coated cabinets to prevent stray charges from damaging the delicate semiconductor parts being tested. The cabinets also protect the instruments themselves from static charge or high voltage in the environment. Entire cleanrooms and all the equipment housed in them benefit from ESD coatings.
° High voltage equipment poses a shock hazard. Housing such equipment in ESD-painted cabinets provides a safety measure by allowing charge trying to transfer to a lower voltage to make its way to ground rather than finding its way through a nearby human.
Floors which rubber tires roll over, especially in environments where dust or flammable materials may be found.
Forklift tires can build up a strong electrostatic charge as they roll and can dissipate those charges to a nearby ground such as a metal rack in the form of a spark. This is an explosion risk in dusty environments or those in which flammable vapors may be found. A warehouse or mezzanine floor coated in ESD coatings can dissipate the charge as the tires turn, removing the spark hazard. Powder coaters may use an ESD low-cure coating to coat MDF panels attached to an electronic ground that are used to construct the floor.
Companies that assemble integrated circuits into other parts depend on special ESD-coated workbenches to mitigate static electricity.
Garment or other factories that use racks of plastic bags.
In the absence of charge dissipation, the bags can become so charged that they cannot be separated from one another. ESD coatings on work surfaces (which are grounded so that charge can flow away) can reduce the static cling of the plastic, leading to easier part separation and less of a shock hazard for the workers who must handle the parts.
Electromagnetic interference (EMI) shielding.
Some circuits produce signals in radio or shorter wavelengths which can interfere with other electronic circuits. Enclosing such circuits in a conductive or ESD (partially conductive) enclosure can, through the Faraday effect, reduce the signal leakage to the point of no longer interfering with other circuits in the area. Conversely, if a sensitive circuit is surrounded by noisy electromagnetic signal sources it can be protected from them by an ESD-coated enclosure. Device manufacturers design and coat such enclosures and can use ESD coatings to accomplish EMI shielding.
In the absence of charge dissipation, garment bags can become so charged that they cannot be separated from one another.
Anywhere with dry winter weather.
As the humidity of the air drops with temperature the potential for static buildup increases. During warmer weather, humidity can moderate charge buildup, and in non-electronics-related processes reduce it to the point that it is not noticeable. However, cold air has a reduced carrying capacity for water and the humidity drops. The moderating effect of water in the air is thus reduced and charge buildup and static shocks can increase, even in processes that don’t show this behavior during other parts of the year. ESD-coated furniture, equipment cases, handles, and other parts can make the seasonal change less painful.
The goal of ESD-coated surfaces is to prevent sparks and shorts, sudden discharge that can damage electronics or lead to an explosion, and the buildup of static electricity on parts that makes them dangerous or hard to work with. The coating should be in contact with the potentially charged part in order to do its job to the best effect. ESD coatings themselves are designed to be only partially conductive to slow the dissipation process down and move charge away to ground without allowing a spark to form. The thickness of the coating does play a role, and it is generally true that a thicker ESD coating will demonstrate higher resistivity. However, this is not strictly linear and if a specific resistivity value is desired, testing with the chosen coating will reveal how thick the coating needs to be to achieve it. The ESD coating must be in contact with a proper ground to do its job.
Where the Rubber Meets the (ESD-coated) Road.
ESD coatings can be found in many forms. These include powder coatings, waterborne, solvent borne, and UV-cure. Ranges of resistance typically fall within 106 to 1011 ohms, and an individual coating will usually have a slightly tighter specification than the whole range. The components which make the coating appropriate for ESD use are conductive and are loaded only to the degree needed to make the overall coating “conductive enough” for the specified range. Many of these conductive components are black (such as particular carbon black types, carbon fibers, or graphene-based material). Metallic pigments can sometimes play a role in ESD coatings but have some challenges. Metallics can form non-conductive oxide layers or be poor choices for the environment the coating will be used in. Aluminum pigments have been used to make gray ESD floor coats, for example. Other pigments are white or appropriate for clear coats such as mica or other minerals coated in conductive inorganic materials (there are a few types). Coatings incorporating these types of conductive components can be a variety of colors since the conductive part is not making the coating black or gray by default. Finally, some resins (binders) are themselves conductive and can be used without the use of special pigments to create an ESD coating.
Any powder coater that can apply thermoset powder coatings is able to apply ESD powder coatings; no special application equipment is needed. The typical cure schedules are those of standard powders since the ESD components are added to standard systems. The weatherability (UV and humidity and temperature resistance) of an ESD coating can be designed to be similar to that of standard powder coatings, although specific testing is required to validate that an ESD coating holds up the way one would expect given the resin system. A highly filled ESD coating may have interconnected pathways of conductive particles (which is what gives the coating its conductive properties), but without proper care these can lead to providing potential pathways for moisture and salts to worm through to the surface. However, many ESD coatings on the market have undergone weatherability testing and prove to be perfectly acceptable for use in exterior applications. No ESD coating should be top-coated, as this will disrupt the charge pathways to ground and reduce or eliminate the ESD properties of the coating system.
Testing the final ESD properties, as one would do for development or quality control, requires a special resistivity meter. The meter will have either conductive rails on the back have area-to-area resistivity measurements (instead of point-to-point as one would get with narrow probes) due to the erratic nature of the conductive pathways. That is, you get a more consistent answer if you measure area-to-area. ESD components cannot be post-added and still get the required effect so if the test results are not meeting requirements it is necessary to communicate with the powder manufacturer in order to get the correct range of resistivity at the thickness of coating needed for a given application.
ESD powder coatings still retain the benefits of powder coating overall, including great durability and toughness. The physical properties (such as impact resistance or flexibility) will still be dominated by the resins and pigment filling percentage. The ESD additives do not really affect these outcomes.The cost of the conductive components varies widely so ESD coatings of different appearances can have very different price points. What you pay for is what you get, just as is true of every other coating type. Generally, the black and gray tone materials are most cost effective. However, the technology is continuing to evolve, allowing an expansion of the color palette. Once upon a time ESD coatings were black or gray and provided as liquid paint only. Now, it is possible to achieve most colors and finishes and powder coating has established itself as a viable ESD coating option.
Without an ESD-coated surface, forklift tires can build up a strong electrostatic charge as they roll, causing a spark hazard.
Dr. Sasha Tavenner Kruger, Ph.D. is research chemist for Forrest Technical Coatings.