Kurt H. Hartwig
Dielectric Corp.
Menomonee Falls, Wis.
Edited by Jean M. Hoffman
www.dielectriccorp.com|
Special fillers and fibers make resins such as ABS, acetal, polypropylene, polycarbonate, and Ultem dissipate static. Stock shapes such as rod, sheet, and slabs made by Westlake Plastics Co., Lenni, Pa., are machined into various components shown here. |
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Surface resistivity is the common measurement of a plastic material's ability to dissipate an electrostatic charge across its surface. It is equal to resistance times the perimeter of the electrodes divided by the gap distance, yielding units of Ω/sq. |
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Pomalux SD-A from Westlake Plastics Co., Lenni, Pa., is a noncarbon filled static dissipative acetal. It provides good heat and wear resistance, high lubricity, and is easily machined. Pomalux SD-A is used in disk-drive assemblies depicted here. It is also used for telecommunications hardware and in automotive applications. |
Manufacturers of electronic devices ideally want an electrostatic-free environment for components. Certain plastics can prevent electrostatic discharge (ESD). ESD is the transfer of charge between bodies at different electrical potentials. The main goal with ESD materials is controlling the charge during an ESD event. The best materials for the job are conductive with a high dielectric constant.
Ordinary plastics are insulators, materials that prevent the flow of electrons across their surfaces or through their volumes. This contrasts with conductors such as metal which readily support electron flow.
Special static-control plastics have electrical resistance properties between those of insulators and conductors. They dissipate static in a controlled manner through the addition of conductive fillers or fibers made from stainless steel or carbon. The efficiency with which these plastics dissipate static is significantly higher than that of insulators, but less than conductors.
Static-control plastics are designed such that they have specific properties that manage the process of charge leaking off. These properties are dielectric strength, capacitance, dielectric constant, and resistivity.
Dielectric strength is the degree to which a material will not breakdown (and become a conductor) under the application of voltage. It is commonly referred to as the strength of an insulator. Capacitance is the degree to which a material can store charge. The dielectric constant is a measure of how much electrostatic energy the material stores for a given voltage potential. Another important property for static-control plastics is resistivity.
Statically dissipative plastics have a surface resistivity greater than 10 5 but not greater than 10 9 Ω/sq. Electron flow across or through the dissipative material is controlled by the surface resistance or volume resistance of the material.
Antistatic plastics have a surface resistivity from 10 10 to 10 12 Ω/sq. They reduce the amount of charge generated triboelectrically — charges generated by the contact and separation of materials. Electrons transfer from the surface of one material to the surface of the other. Which material loses electrons and which gains depends on the properties of the two materials. The material that loses electrons becomes positively charged, while the material that gains electrons becomes negatively charged.
Insulative materials have a surface resistivity greater than 10 12 Ω/sq. and don't conduct electricity. Insulators often tribocharge to high levels because electrons can't easily flow across the material's surface and may remain stationary until neutralized through ionization or some other mechanism.
Conductive materials have a maximum surface resistivity of 10 5 Ω/sq. Electrons flow easily across surfaces or volumetrically. Surface charge distribute uniformly. A charged conductive material brought into contact with another conductor easily transfers electrons between the two materials. Grounding the second conductor to earth neutralizes excess charge.
Composition
Static-control or ESD-safe plastics are loaded with special fillers and alloys. These static dissipative and nonconductive alloy/fillers are categorized as noncarbon alloys (SD-A), carbon powders (CN-P), carbon fibers (CN-F), and stainless-steel fibers (CN-SS).
Noncarbon alloys (SD-A) are the chemical combination of two or more separate components. One such alloy combines a nonconductive polymer plus a noncarbon nylon derivative. Nylon by itself is not a conductor. But because nylon absorbs water and water conducts electricity the material will provide a conductive path for the electrical charge. Static control alloys work by providing a moderately conductive path for electrical charge. Nonalloyed plastics act as insulators.
Fillers, are added, not chemically bonded, to the nonconductive polymer. Static control fillers provide a conductive path for the electric charge.
Carbon powders (CN-P) are inherently conductive. With enough loading, particles touch each and form a conductive path through the nonconductive plastic matrix.
Carbon fibers (CN-F) stretch to make a conductive path and dissipate electric charge more efficiently than carbon powder.
Stainless-steel fibers (strands of stainless steel, CN-SS) also stretch and touch other strands. They dissipate electric charge best.
The first step in preventing ESD build-up is, however, to reduce friction and rubbing and eliminate un-necessary activities and materials that create static charges. For example, carts carrying sensitive components across a floor should be equipped with antistatic casters. Likewise, covering all work areas with grounded dissipative materials and floor mats also quells static build-up.
Another means of preventing ESD is to shield components from discharges created by electro-magnetic interference/radio frequency interference (EMI/RFI). EMI/RFI is electrical energy created by electromagnetic fields that are radiated or absorbed by electrical components. Sources of EMI/RFI come from computer circuits, radio transmitters, electric motors, and lighting.
| Insulators | Partial insulators/partial conductors | Conductors | ||
| INSULATIVE |
STATIC DISSIPATIVE
| CONDUCTIVE PLASTIC | CONDUCTIVE | |
(antistatic) | ||||
| Charge encounters high resistance | Charge encounters high impedance | Charge encounters intermediate impedance | Charge encounters low impedance | Charge encounters minimal impedance |
| Unfilled plastic or unfilled rubber | Plastic filled with noncarbon alloy (SD-A) | Plastic filled with carbon powder or carbon fiber (CN-P or CN-F) | Plastic filled with stainless steel (CN-SS) | Pure metal copper, aluminum, steel, or gold |
| High flow blockage 1015 Ω | 1012 to 1010 Ω | 109 to 106 Ω | 105 to 105 Ω | No flow blockage 10– 3 Ω |
WHAT IS ESD
Static electricity is generated whenever two materials touch then separate. Friction between the two materials causes heat which, in turn, excites electrons. When objects separate, there is a transfer of electrons from one object to another. One material charges positively and the other negatively during separation The result is static build-up.
This is referred to as triboelectric generation and is the most common form of uncontrolled transfer of static charge. However, an electrically conductive path to ground will harmlessly dissipate charge from the material. Electrical charge also dissipates into the air, a phenomena that increases with humidity.
The amount and rate of charge build-up is influenced by factors such as type of materials, material cleanliness, speed of separation, material texture, friction, and relative humidity. The simple act of walking across an untreated vinyl floor generates a static charge up to 12 kV, yet a charge of less than 10 V can destroy an electronically sensitive device.