STATIC ELECTRICITY is voltage difference between two surfaces. When an uncontrolled transfer of electrons occurs, this discharge known as ELECTROSTATIC DISCHARGE (ESD), is commonly called STATIC ELECTRICITY. Voltage difference or tribioelectric charging can occur when 2 insulating materials slide or move or rub against each other or when they are separated from each other or when a fluid or gas flows over the surface or can be induced by electrostatic fields. The voltage build-up tends to remain in a localized area of contact. Whether a material remains charged or not depends on:
- its ability to dissipate the static charge, AND
- the existence of a path to ground
The induced or STATIC CHARGE can be a few volts or up to 40,000 volts. When a charged, insulating material comes close to an object of different potential, such a person, a computer chip, a stick of dynamite, a can of gasoline, etc. ELECTROSTATIC DISCHARGE occurs through a spark or arc; possibly with disastrous results.
The importance in controlling and dissipating of static charges on POLYMERS has been realized for a long time. Sparks, in environments such as hospital operating rooms and the assembly of explosive devices where flammable gases, liquids or solids are found or in the dusty environment of grain storage bins, have occurred with deadly results. More recently, arcing in the presence of sensitive electronic equipment, now found everywhere, has led to erroneous readings or irrepairable damage.
Since STATIC ELECTRICITY is a surface phenomenon, measurements of SURFACE RESISTIVITY provide a method of determining a material's ability to dissipate or "bleed-off" a static charge along the surface. Resistivity is the inverse of conductivity. The higher the value for resistivity, the more insulating a material. Another important point is the material's ability to eliminate the charge to air or ground quickly, called STATIC DECAY. U.S. Military standard requires a 5000-volt charge to decay completely at less than 15% relative humidity within 2 seconds. A fire code standard requires 90% decay to 50% relative humidity within 0.5 second.
The following chart indicates the range of SURFACE RESISTIVITY found in various POLYMERS:
POLYMERS that can conduct and dissipate a charge are not new. In the 1950's a SILVER POWDER filled PTFE was available in small diameter rods. In the past few years, other metal powders such as ALUMINUM and STAINLESS STEEL have been used to produce conductive materials. The most recent technologies involve the addition of chemical anti-static agents such as NEOALKOXY TITANITES; blending or alloying with an inherently conductive polymer such as STAT-RITE®, a BF Goodrich POLYETHER elastomer; blending with VERSICON®, Monsanto's insoluble, inherently conductive polymer POLYANILINE; the addition of specific conductive carbon fiber or carbon powder materials or metal fibres. Some materials, like STAT-RITE® require a certain relative humidity in order to be effective.
Within a few years, a large range of resins have become available offering products covering dissipative, conductive and ESD shielding. Now some of these are being produced in standard rods and sheets.