When talking in terms of conductivity, both thermal and electrical, plastic materials are considered insulative. Insulative materials do not allow the flow of thermal energy or conduct current through its mass quickly or at all. Sometimes, however, we need our plastic parts to do just that, conduct current or ground a part.
To quantify electrical conductivity, we test either the surface or the volume resistivity of the plastic. Resistivity is the resistance to leakage current through the body (volume) or along the surface of an insulating material. The values are then given in ohms (surface) or ohms-m (volume). The higher the value, the better they are at resisting the conducting of the current or, the more insulating they are. Plastic materials without fillers, additives, etc., to improve conductivity are in the insulating range of the ≥ 1012 ohms resistivity, compared to metals in the conductive range of ≤ 106 ohms.
The Electrostatic Dissipative (ESD) protective range can be broken down into three categories and their corresponding resistivity ranges:
- Anti-static (anti-stat) is 109 to 1012 ohms. In this range, initial electrostatic charges are suppressed, preventing the buildup of static electricity. In plastics, we can achieve this with additives.
- Static dissipative is 106 to 109 ohms. There are low or no initial charges in this range and prevent discharge to and from human contact. It will also ground charges, but much slower than conductive grades. In plastics, we can achieve this with metal fiber reinforcements and other conductive additives.
- Conductive is less than 106 ohms. In this range, there are no initial charges. It provides a path for electrons to flow freely across the surface or through the bulk of these materials, making it easy to ground charges or move them to another conductive object. In plastics, we can achieve this with metal fiber reinforcements and other conductive additives.
So, why do we need ESD protection and parts that offer that protection? Static electricity. Static electricity can build up to as much as 30,000+ volts. Plastics or other insulative materials do not move the charge, and it remains on the surface. Once a person comes into contact with the built-up charge, it will discharge via an arc or spark. The discharge that occurs to that person can range from a mild to painful shock, and in extreme cases, can result in death.
Another reason that we need ESD protection is that electronic parts can be destroyed or damaged by a discharge as little as 20 volts. Discharge that results in sparks can also be dangerous around flammable liquids, solids, or gases, such as in a hospital operating room. In these cases, we would look to utilize plastic materials that have been specially compounded to meet any of the ESD ranges needed to protect against harm or damage.
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