Surface resistivities

The corresponding Ohmic relationship to that of Equation 3.1 relating the surface current density Js (A/m), electric field strength E (V/m), and the surface resistivity yis, 

The surface resistivity of clean glass in air can be very high, of the order 1014 Ohm/square or larger (Morey, 1954). In the case of glass it is possible to increase the surface resistivity with a water repellent which serves to prevent the formation of a continuous layer of water (Holland, 1966). With powders this a common practice using silicone compounds. 

In contrast to surface resistivity, the volume resistivity of glass and ceramics is controlled largely by its composition. For example, the conductivity of sodium silicate glass increases in direct proportion to the sodium ion concentration (Kingery et. al., 1976). Electronic conduction is also possible in 

The most widely accepted test for determining conductivity of plastics is ASTM D257, which details procedures for measuring surface and volume resistivity. Where the intention is to find out dissipation of electrostatic charges, the more meaningful is surface resistivity. Volume resistivity measurements are useful in indicating the dispersion of a conductive additive throughout the matrix. 

Resistivity is the inverse measure of conductivity of a material as it increases, so conductivity decreases. It should not be confused with resistance. Resistivity is calculated from measurement of resistance, using an equation that takes into account the geometry of the sample and electrodes. Under ASTM D257, surface resistivity is determined from measurement of surface resistance between two electrodes forming opposite sides of a square. Values are stated in per square. 

Volume resistivity (also termed bulk resistivity) is taken from the volume resistance between opposite faces of a 1 cm cube of material, and values are reported in cm. 

---

Fig. 9. Uptake curves for N2 in two samples of carbon molecular sieve showing conformity with diffusion model (eq. 24) for sample 1 (A), and with surface resistance model (eq. 26) for example 2 (0)j LDF = linear driving force. Data from ref. 18.

Electrica.1 Properties. Because of excellent electrical properties, FEP is a valuable and versatile electrical insulator. Within the recommended service temperature range, PTFE and EEP have identical properties as electrical insulators. Volume resistivity, which is >10 H/cm, remains unchanged even after prolonged soaking in water surface resistivity is >10 H/sq. 

Electrical Properties. Poly(methyl methacrylate) has specific electrical properties that make it unique (Table 4). The surface resistivity of poly(methyl methacrylate) is higher than that of most plastic materials. Weathering and moisture affect poly(methyl methacrylate) only to a minor degree. High resistance and nontracking characteristics have resulted in its use in high voltage appHcations, and its excellent weather resistance has promoted the use of poly(methyl methacrylates) for outdoor electrical appHcations (22). 

Drop breakage occurs when surrounding fluid stresses exceed the surface resistance of drops. Drops are first elongated as a result of pressure fluctuations and then spHt into small drops with a possibiUty of additional smaller fragments (Fig. 19). Two types of fluid stresses cause dispersions, viscous shear and turbulence. In considering viscous shear effects, it is assumed that the drop size is smaller than the Kohnogoroff microscale, Tj. 

Sihcone-based coatings are well suited for high temperature and high speed appHcations. They are flexible, tough, and resistant to thermal and oxidative deterioration. They have good surface resistance and are fungus- and flame resistant. However, they possess a high coefficient of thermal expansion and have poor adhesion. 

Fig. 2. Measuring static charge, (a) Field voltage measurement (b) Faraday cage (c) surface resistivity measurement and (d) static decay test.

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. 

Surface resistance is measured by placing two electrodes on the surface of the object to be tested. A d-c voltage is appHed to one electrode and an ammeter is attached to the other electrode. The current travels from one electrode to the other by traveling across the surface of the object. When the current is measured and the appHed voltage is known, resistance can be calculated by the formula resistance = voltage/current. 

The terms resistance and resistivity are both used when referring to the resistance of an object to current flow. Surface resistance is the measure of electrical resistance along the surface of an object. However, the current flow is not limited to the surface of the object. Some of the current passes through the bulk of the object from one electrode to the other electrode. Surface resistivity includes the dimensions of the object in its measurement (Eig. 2c). 

Materials that have the abiUty to dissipate a charged formed, by any means including tribocharging and induction, on the material are referred to as static dissipative. There is a correlation between static dissipation and surface resistance. EIA-541 currently defines the static dissipative range as 10 to 10 ... 

Film composition Surface resistivity,Q/sq Triboelectric voltage... 

Fig. 5. Effect of surfactant type on surface resistivity, (a) Concentration of surface-active compound in low density polyethylene (LDPE) requked to achieve 10 Q/sq surface resistivity and (b) effect on surface resistivity of an acrylic polymer. Concentration of surface-active compound is 0.3%.

Internal antistats are considered permanent antistats. This permanence is based on the concept that most plastic products are disposable, so that the antistat is not required to last long. The antistatic effectiveness of an internal antistat can decrease over time. One study showed large increases in surface resistivity on antistatic bags stored at 71 °C for six months. Antistatic bags stored at room temperature showed only a small increase in surface resistivity (137). Loss of antistatic effectiveness is attributed to the volatility of the antistatic agent. The antistat does not easily wear off the plastic, but it can be removed with solvents and/or repeated wear. 

Surface Resistance Measurement of Static Dissipative Planar MaterialsD SW.W, EOS/ESD Association, 1991. 

The evidence is that the coefficients kc and ko and the interfacial area depend differently upon operating variables. For purposes of design, therefore, it is ultimately necessary to have separate information on the quantities kc, ko, and a, .. The role of an additional surface resistance is emphasized by the studies of Kishinevsld and Moehalova [Zh. Prikl Khim.,. 3.3, 2049 (I960)]. 

---