Static electricity charge dissipation

Surface Resistivity. Resistance is the measure of the conductivity of a material. Conductive materials have the abiUty to dissipate static charges. Therefore, resistance is a good measure of the abiUty of an object to dissipate a static charge. Since static electricity is primarily a surface phenomenon, the resistance along the surface of the object is of most interest. 

IEC 61340-2-1,2002. Electrostatics - Measurement methods - Ability of materials and products to dissipate static electric charge. 

An fuel-air mixture explosion can be initiated by a sudden discharge of static electricity. Yet, while flowing in systems, a fluid develops an electrical charge which will take as long to dissipate as the fluid is a poor conductor. The natural electrical conductivity of jet fuel is very low, on the order of a few picosiemens per meter, and it decreases further at low temperature. 

Flammable liquids are considered particularly static-prone if their elec trical conductivity is within the range of 0.1 to 10 pS/m. If no particulates or immiscible liquid are present, these prodlic ts are considered safe when their conductivity has been raised to 50 pS/m or higher. Blending operations or other two-phase mixing may cause such a high rate of charging that a conductivity of at least 1000 pS/m is needed for safe charge dissipation (British Standard 5958, part 1, Control of Undesirable Static Electricity, para. 8, 1991). 

Provisions to dissipate accumulated static electricity charges. 

Severe hydroprocessing required to produce ultra-low sulfur fuel removes most of the polar organic compounds from the fuel. The resulting product is virtually unable to effectively dissipate static charge as it builds in moving fuel. Instead, electrical discharge into pockets of gas within a pipeline or into air and vapors as fuel is being dispensed can occur. An explosion may result. Low levels of a fuel-soluble electrical conductivity improver to ultra-low sulfur fuel can help restore fuel electrical conductivity to safe levels. 

Polyurethanes with antistatic properties are suitable for use where static electrical charges must be dissipated. The polyurethane must be compounded to provide the antistatic properties throughout the whole bulk of the material. The antistatic agent, if a liquid, must not migrate to the surface readily. Surface coating the item is not desirable, as the effect is only temporary. 

Anti-Static Laminates. The conventional laminates are relatively good electrical insulators and in consequence in dry conditions can accumulate static electric charges these can present problems—as when discharges take place suddenly in the course of manufacture or assembly of electronic components. It is particularly important for applications such as computer furniture and benches intended for electronic assembly that static charges will be dissipated at controlled rates. 

These additives perform by migrating to the surface of the film, reducing the surface resistivity, and enabling a more rapid dissipation of static electrical charges. Many of them work best in conditions of high humidity, presumably through the absorption of moisture and a consequent increase in conductivity at the surface. The efficiency of such an additive may be measured by the rapidity with which static charges decay. 

Figure 4.27 A blend of Nomex, Kevlar, and a core of pitch-based carbon fiber, P-140, (a) The carbon fiber forms the core, (b) Static charges on the polyamide fabric induce opposite charges on the carbon core, (c) When the induced charges on the carbon core build to a high energy level, surrounding air molecules ionize. Positive and negative ions neutralize charges on the polyamide fabric and the carl n core, thus dissipating the static electricity.

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