Antistatic, conductive, or static dissipative?

Knowing the type of static control
required of a textile is essential.

Imagine this scenario, as described by Bud Weisbart, vice president of A & R Tarpaulins, the parent company of AR Tech, Fontana, Calif. Mechanics are assembling an aircraft. Large fabric tubes transfer fresh, cool air inside where they’re working. If the fabric tubes are not antistatic, the workers’ tools, and friction of any kind, could create static electricity, which could cause them to get “zapped.” The workers would not likely be in any danger, but they could drop a tool, or some other object that would need to be retrieved, or it could jeopardize the integrity of the aircraft. Additionally, there are epoxies and other flammable materials inside the aircraft. A small spark could, conceivably, cause a fire. In other words, static dissipative textiles serve an important function in this—and many other—industrial environments.

Know your static dissipation levels

When Chad Twombly, business manager at Herculite Products Inc., York, Pa., gets a call from a customer who needs an antistatic material, his first question is, “What level of static dissipation do you need?” Often, there’s silence on the other end of the line. “Too many people don’t understand what they need,” says Twombly. “The majority of the people don’t understand the difference between surface electrical charges and electrical current. What we’re trying to address with our products is surface electricity,” he says.

What they need is an ohms test to find out. “Ohms testing is really important,” he says. “It gives them a quantifiable way to pick the right solution. That will help them to determine what level of protection they need. Somebody knows, and they need to go back to that person and find out.”

For instance, a very conductive fabric is required if microchips are involved. If the microchips are components in explosives, such as a weapons system, there is no room for error. A much lower level of dissipation is needed in basic health care situations, for example, to control static electricity in fabrics, which can occur when a patient is transferred to a gurney.

Both of these market areas—health care and industries that use microchip technologies—continue to be strong. There are other industrial settings where static dissipative textiles are essential. Lots of products loaded into containers can generate static electricity. “Any place where there is surface or static electricity, they need to be cautious,” he says. Recently, Twombly was asked to provide an antistatic fabric for a client that was transferring resin into railcars. The customer was trying to dissipate the static electricity that was gathering from the movement of the resin through the chute; the collection of static energy was causing the resin to “stick” to the chute.

Electricity 101

If your level of expertise in the area of electricity and static electric charge doesn’t extend further than high school physics, basic information that Twombly outlined in his blog post is a good starting point. To understand the difference between antistatic, dissipative and conductive as it relates to electricity and fabrics, it is helpful to first understand the difference between the terms “insulation” and “conductive” as it relates to electricity.

Conductors are materials that permit the flow of electric charges in one or more directions.  Metals are particularly conductive, which is why they’re used to move electricity in the form of electric wiring, for example.

Electric insulators prevent or reduce the passage of heat, electricity or sound—just the opposite of electric conductors. In these materials, internal electric charges do not flow freely, and so they limit the flow of electricity. The PVC and paper used to wrap electrical wire are examples. The insulators on an extension cord obviously prevent the charge from passing through them so you don’t get shocked.

As a rule, PVC makes a good insulator, but there are things that can be done to make PVC engineered textiles more conductive. The level of manipulation to the material to change its conductive properties will put it into one of three classifications: antistatic, static dissipative or conductive.

Here’s how the MIL-HDBK-773A DOD [U.S. Military] Handbook classifies these three:  

• Antistatic refers to the property of a material that inhibits triboelectic (static electricity) charge generation effects. 
• Static Dissipative materials rapidly dissipate electrostatic charges over their surface or volume, having a resistivity range between conductive and insulative.
• Conductive materials are either surface or volume conductive and may be either metal or impregnated with metal, carbon particles or other conductive ingredients, or may have a surface that has been treated with such materials through a process of lacquering, plating, metallizing or printing.

To determine if materials meet one of these three classifications, testing is done for surface resistivity that is measured in ohms/square. 

When designing a product solution it is important to understand the demands of the specific application. When dealing with the engineers or designers, it is recommended that you ask for the ohms level required to make sure the materials chosen are the right ones.

Laundering and money

Doug Hager is sales vice president, Strauss Knitting, St. Croix Falls, Wis., whose products are used primarily for garments and garment trim, such as cuffs. Hager stresses two key considerations when choosing the best fabric for an antistatic application: how it will be laundered and how much conductive material is needed.

If it’s a garment that needs to be sterilized, it must withstand autoclave cleaning, which is at more than 212 F and under pressure. Polyester can be used for antistatic fabrics that must withstand sterilization.

Stauss’ X-static, a polyester or nylon that has a silver coating, is an extremely conductive material and offers antimicrobial protection. Its conductivity will withstand many washings, but it won’t hold up to autoclave laundering because silver is volatile and can flake off. With silver at $182 a pound as this article was written, the client would want to be sure about how the garment would be laundered.

With carbon-based fibers, such as the company’s Resistat and Beltran brands, the entire fiber is conductive and not a coating, so it’s much more durable—and considerably less expensive. With these materials, the conductive portion is the core, so it takes a higher voltage to instigate the dissipative action, Hager says.

Hager also stresses the importance of determining exactly how much conductive material is needed. “It’s so expensive that you don’t want to use more than is necessary,” he says. But having enough is equally critical. “Our biggest market for ESD (electrostatic discharge) textiles is for cleanrooms—most often in electronic assembly, where even a small ‘shock’ could ruin an expensive microchip,” he says.

Under the radar

Textiles are engineered with a range of properties, depending on the performance capability that’s required. Textiles that contain metallic materials can block electronic information, such as radar, and are termed electromagnetic interference barriers. That’s an application fairly easy to imagine, at least on a more superficial level.

But some potential projects may be a little harder to see. Weisbart says the first job his company had in the early 1980s was for a laminator flow table for a company that made eye glasses. “Particles were getting into the glass, which is soft at that point, and ruining it, so we made a flow table and enclosure out of an antistatic fabric,” he says.

“We do a lot of blankets for satellites when they launch,” Weisbart says. The blankets have carbon in them imbedded into Teflon®, and are multi-purpose. Besides their antistatic properties, they mitigate the effects of vibration, and their stainless steel filters prevent particles from coming through the blanket into the payload area.

Truck curtains may also require an antistatic material, depending on the load the truck is hauling. Weisbart said they provided a custom curtain to protect solid rocket fuel during an over-the-road transfer. That project required keeping the load from heating up in the sun, as well.

“That’s conceptually where we need to grow as an industry,” Weisbart says, “to understand the extent to which fabrics can be applied … to be able to address whatever problem there is to solve.”