What to know about ESD and Material Selection
Static electricity develops when electrons transfer between materials as they contact, rub, slide, or separate. On insulating materials such as plastics and fiberglass composites, that electrical charge can remain trapped until it moves to another object.
In electronics, semiconductor, aerospace, chemical-processing, and other static-sensitive environments, an uncontrolled discharge can damage components, interrupt manufacturing processes, create a painful shock, or ignite combustible materials.
If a finished component is going to interactive with electrical charge, selecting the right composite is critical. Depending on the application, a material may need to provide electrical insulation, reduce charge generation, dissipate charge gradually, or conduct it to ground.
What Is Electrostatic Discharge?
Electrostatic discharge, commonly abbreviated as ESD, occurs when accumulated electrical charge moves rapidly between objects at different electrical potentials.
When static develops on an insulating surface, the charge may remain concentrated near the point where it was generated. Because the material does not provide an easy path for electrons to move, the charge can remain in place until the material approaches or contacts another object with a sufficiently different electrical potential.
At that point, the charge may discharge as a spark or arc.
A person may experience ESD as anything from a mild sensation to a painful shock. Sensitive electronic components, however, can be damaged by a discharge that is too small for a person to feel.
Static discharge can be particularly hazardous in environments containing combustible dust, flammable liquids, gases, solvents, propellants, or other ignition-sensitive materials. These environments may include electronics manufacturing, aerospace and defense assembly, chemical processing, medical-equipment production, and explosive-device handling.
ESD should not be confused with arc flash. Although both involve electrical discharge, arc flash is generally associated with a high-energy fault in an energized electrical system and presents a substantially different type and level of hazard.
Why Plastics and Composites Retain Static Charge
Electrical conductors allow electrons to move relatively freely. When properly incorporated into a grounding system, a conductive material can provide a path for accumulated charge to move away from a component.
Insulating materials behave differently.
Many plastics and traditional thermoset composite laminates have high electrical resistance. Electrons cannot easily travel across their surfaces or through their internal structure, so static charge may remain localized for extended periods.
Two properties are commonly used to evaluate this behavior:
Surface resistivity describes how strongly a material resists electrical current moving across its surface.
Volume resistivity describes how strongly a material resists electrical current moving through its internal structure.
These measurements help engineers evaluate whether a material will retain, dissipate, or conduct electrical charge.
Insulative, Anti-Static, Dissipative, and Conductive Materials
Static-control materials are commonly described as insulative, anti-static, static-dissipative, or conductive.
Although resistance ranges are often used to differentiate these categories, classifications can vary based on the standard, test method, material thickness, environmental conditions, and whether surface or volume properties are being measured.
Material specifications should therefore be evaluated using the test method and performance requirements that apply to the finished component.
Insulative Materials
Insulative materials strongly restrict the movement of electrons across their surfaces and through their internal structure.
Many engineering plastics and thermoset composite laminates fall into this category. Their high electrical resistance makes them valuable for electrical isolation, dielectric barriers, terminal boards, switchgear components, electrical supports, and other insulation applications.
Their insulating behavior, however, means they are not automatically suitable for static-sensitive handling applications.
Electrical insulation and electrostatic control are related but different design requirements. A material may be highly effective at preventing current from passing through a component while still allowing static charge to accumulate on its surface.
Anti-Static Materials
Anti-static materials are designed primarily to reduce or suppress the initial generation of static charge.
They may work by reducing friction-related charging, attracting a small amount of moisture to the surface, or lowering surface resistance. Anti-static properties can be introduced through coatings, surface treatments, additives, or fillers distributed throughout the material.
Traditional examples include topical treatments applied to textiles and polymer surfaces.
Anti-static materials can help limit excessive charging, but they do not necessarily provide a dependable path to ground.
Static-Dissipative Materials
Static-dissipative materials allow electrical charge to move in a controlled manner. Instead of allowing charge to remain trapped or release suddenly, they permit it to decay more gradually.
This controlled movement can reduce the risk of sudden discharge near sensitive electronic components.
Static-dissipative composites are commonly used for semiconductor fixtures, test sockets, assembly tooling, electronic work surfaces, material-handling components, and equipment used near sensitive circuitry.
In some electronics-handling applications, a dissipative material may be preferred over a highly conductive one because it allows charge to decay more gradually.
Dissipative properties may be created using surface coatings or additives distributed throughout the material. Materials filled throughout generally maintain more consistent performance after machining or wear because their static-control characteristics are not limited to the original outer surface.
We examine this middle range in more detail in our article on how dissipative materials protect people, products, and processes.
Conductive Materials
Conductive materials have comparatively low electrical resistance. Electrons can move readily across their surfaces or through their internal structure.
When properly incorporated into a grounding system, a conductive material can provide a path for accumulated charge to move away from the component.
A conductive material that is not properly grounded, however, may still retain charge or transfer it rapidly to another object.
Conductive plastics and composites are often produced using carbon-based fillers such as carbon powder, carbon fiber, graphite, carbon nanotubes, or other electrically conductive additives. These fillers create interconnected pathways through which electrical charge can travel.
Conductive materials may be appropriate for grounding components, shielding applications, fixtures, enclosures, and other situations where charge must be transferred quickly.
How Composite Materials Are Engineered for Static Control
Composite materials can be engineered to provide a wide range of electrical resistance.
A traditional glass-reinforced thermoset laminate may function as an effective electrical insulator. By modifying the reinforcement, resin system, filler content, or surface treatment, manufacturers can create materials with anti-static, dissipative, or conductive characteristics.
Common approaches include:
- Adding carbon or graphite fillers to the resin
- Using conductive fibers or fabrics
- Applying a static-dissipative surface coating
- Incorporating additives that attract moisture
- Designing layered structures with insulating and conductive regions
The appropriate construction depends on the intended function of the finished part.
An electrical barrier may require high dielectric strength and insulation resistance. A fixture used to handle semiconductor components may instead require controlled charge dissipation. An enclosure may require conductivity for grounding or electromagnetic shielding.
There is no single electrical-resistance range that is appropriate for every application.
Surface Treatments Versus Filled Materials
Static-control properties can be applied at the surface or incorporated throughout the material.
A surface-treated material may be a cost-effective option when the component will experience limited machining, abrasion, cleaning, or wear. However, cutting, sanding, cleaning, or repeated handling may remove or reduce the effectiveness of the treatment.
A material filled throughout maintains its electrical characteristics beneath the original surface. This can provide more consistent performance when parts are machined, drilled, routed, exposed to abrasion, cleaned with aggressive chemicals, or required to perform throughout a long service life.
The appropriate option depends on the application, manufacturing process, expected wear, and required level of electrical control.
How Machining Can Affect Static-Control Performance
Machining can materially affect the electrical behavior of a static-control composite.
Removing a treated surface may reduce or eliminate the intended property. Cutting a filled composite can expose new dissipative or conductive surfaces. Feature geometry, reinforcement orientation, surface finish, cleaning methods, and filler distribution may also influence measured resistance.
For machined components, the specified electrical performance should apply to the finished part, not only to the original sheet, rod, tube, or molded stock.
Engineers should consider:
- Whether machining will remove a coating or treated layer
- Whether electrical properties are consistent throughout the material
- How newly machined surfaces will behave
- Whether conductive pathways could be interrupted
- How cleaning, sanding, or finishing may affect resistance
- Whether the completed part will be tested after machining
This makes coordination between the material supplier, machinist, designer, and end user especially important.
Environmental and Testing Conditions Matter
Electrical-resistance measurements can change with temperature, humidity, surface contamination, and material conditioning.
Humidity is particularly important. Moisture on the surface of a polymer can make it easier for electrical charge to move. A material may therefore appear more dissipative in a humid environment and more insulative under dry conditions.
Other variables may include material thickness, electrode configuration, applied test voltage, surface cleanliness, machining residue, reinforcement orientation, filler concentration, and test duration.
Published resistivity values should always be reviewed alongside the applicable test method and the environmental conditions in which the finished component will operate.
Selecting a Composite for an ESD-Sensitive Application
Choosing a material for an ESD-sensitive application involves more than selecting a resistance range from a data sheet.
Engineers should first define the function the material must perform:
- Prevent the generation of static charge
- Allow charge to dissipate gradually
- Provide a conductive path to ground
- Maintain electrical insulation
- Combine electrical isolation with localized conductive features
The application should then be evaluated for additional requirements, including:
- The sensitivity of nearby electronics
- The presence of combustible dust, gases, liquids, or solvents
- The grounding method
- Expected humidity and temperature
- Machining and finishing requirements
- Mechanical strength and dimensional stability
- Chemical, abrasion, and wear resistance
- Applicable ESD, electrical, and safety standards
Material selection alone does not create an effective ESD-control system. Grounding methods, personnel controls, packaging, work surfaces, equipment design, and verification procedures must also work together.
The electrical properties of the material must be balanced with the mechanical, thermal, chemical, and manufacturing demands placed on the finished part.
Balancing Static Control with Structural Performance
Static electricity is sometimes treated as little more than a nuisance. In sensitive environments, however, uncontrolled charge can become a product-quality, equipment-reliability, process-control, or personnel-safety concern.
Traditional insulating composites remain the right choice for many applications requiring electrical isolation. Other applications may benefit from materials engineered to suppress, dissipate, or conduct electrical charge.
The key is to define how the finished component is expected to interact with electricity throughout manufacturing and service.
Selecting the right static-control composite requires balancing electrical resistance with strength, temperature exposure, wear, chemical compatibility, machinability, dimensional requirements, and finished-part geometry.
Atlas Fibre can help evaluate material options and machine composite components for applications requiring electrical insulation, controlled static dissipation, or conductivity.