SPECIFIC VOLUME RESISTIVITY

It is therefore essential to identify polymers that do not change their electrical properties within a broad range of temperatures. When electrical conductivity turns out to be desirable in a polymer, in some cases this can be obtained by using conductive fillers (carbon black or metallic powders). In these cases, the resistivity rises with temperature, while a sharp increase occurs around transition temperatures, such as T, . This phenomenon may be utilized in novel switching and control devices (PTC - positive temperature coefficient). Polyacetylene serves as an example of a conductive polymer. 

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The membranes also have a certain, though small ionic conductivity. The electrical resistance of membranes when referred to unit surface area is 10 to 10 Q-cm, which when allowing for the small membrane thickness (about 10 nm) corresponds to the rather high value of specific (volume) resistance of 10 to 10 Q-cm. 

Ffg. 24. Dependence of dielectric perme ability e (1), tangent of dielectric loss angle (2), specific volume resistance (3) and water absorption (4) on the curvii period for epoxide foams (7 = 35 kg m ) at 98% relative humidity... 

This polarisability now causes that the charge increase from Q to Q needs a clearly measurable amount of time. Thus, a charging current (Ic) decreasing with time, superimposed on I(dc), flows if a voltage (U) is applied over the plate/ polymer/plate capacitor. The proper I(dc) value necessary to calculate the specific volume resistivity, can be measured only if Ic has become zero. 

The specific volume resistivity/temperature or electrical field-strength relation is measured during what usually are called DC measurements. A specific method was developed to determine the volume resistivity of a polymer as a function of the temperature and/or the electrical field strength [2] without the need to wait until the Ic current component has become zero. This method is schematically given in Figure 5.1. 

I(dc) is inserted in equation 5.2 to calculate the specific volume resistivity at that temperature and electrical field strength. This measuring procedure is completely automated, see 5.1.4. The volume resistivity determination is described in the ASTM D257 (US), BS 202A (UK), DIN 53596/51953 (BRD) and in ISO 93 [3]. The time dependency of Ic is mentioned in all these methods. Besides, all methods indicate that I(dc) is determined usually after a standard charging time of 60 seconds. It will be clear that if one assumes I(dc) = Ic(60 seconds), this current nearly always contains a time dependent charging current contribution. 

The automated volume resistivity/dielectric properties measuring system is schematically drawn in Figure 5.2. The whole system is controlled by the HP9000/300 computer which runs either the specific volume resistivity measuring program or the dielectric properties measuring program. 

If the Ic-value after 60 seconds is used as I(dc) -value, for both experiments shown in the Figures 5.4 and 5.5, this l(dc) value contains a time dependent contribution i.e. it is too high. Thus, the calculated resistivity value will be lower than the real dc resistivity value. The results of all specific volume resistivity measurements (including the Ic[60 seconds] values) are plotted in Figure 5.6. The curves show that the difference between the I(dc)-data and the ic(60)-data is mainly present if the sample is in its glassy phase. For temperatures higher than 29°C the time dependent part of the charging current has vanished within 60 seconds i.e. Ic(60) and Idc are equal. 

Specific volume resistivity/T(-1) relation of a liquid epoxy/polyamide resin system... 

The Ki-value determination of PVC cable compounds The specific volume resistivity is the most important electrical property of an electrical grade PVC. It is measured on a heavily plasticised product, the cable compound, pressed to a 2 mm. thick sample sheet. Cable manufactures usually test the resistivity of these compounds on cable samples and express the results in a so-called Ki-value. The Ki-value is in fact a volume resistivity value (see below) but measured on a cable sample with tapwater as low potential measuring electrode. A series of Ki-value determinations was performed to investigate the different parameters influencing this quantity. 

The (in this way) calculated Ki-value in Mega-ohm.km has in fact the same dimensions as the specific volume resistivity which is expressed in Ohm.m. The derivation of Ki starting from equation 5.2 is straight forward the resistance of a rectangular piece of material between two flat metal electrodes is given according to equation 5.2 by ... 

The DC properties of anti-static epoxy GFR pipes The use of glass-fibre reinforced (GFR) epoxy resin pipes is, especially in tankers, hampered by the bad electrostatic properties of these pipes. The possibility to decrease the volume resistivity to an acceptable level using only a small amount of carbon black (see 5.3.2) resulted in the development of the WAVIMAR anti-static GFR pipe system by Wavin BV. This pipe system is based on a liquid DGEBA/MDA (100/27) and cured for two hours at 120°C, containing about 1.5 %wt. (on the resin phase) of Ketjen black EC-2000 carbon black. The specific volume resistivity of a sample of such a pipe was measured as a function of the direction, the field strength and the pipe wall thickness. 

Specific volume resistivity determination of an epoxy resin based, GFR pipe... 

SPECIFIC VOLUME RESISTIVITY OF WAVIMAR CLASS FIBRE REINFORCED. CARBON BLACK FILLEO EPOXY PIPE IN THE AXIAL DIRECTION AS A FUNCTION OF THE ELECTRICAL FIELO STRENGTH ANO THE PIPE WALL THICKNESS... 

The volume resistivity determination of PK terpolymer The specific volume resistivity was measured between -40°C and 140°C during a third, step-wise heating scan. The sample/ sample cell combination used for the dielectric measurements was connected for this purpose with the automated resistivity measuring system described in 5.1.4. 

The results of the specific volume resistivity measurements on PK terpolymer are listed in Table 9.12 and plotted as a function of the reciprocal, absolute temperature in Figure 9.19. The p-dc value of PK terpolymer decreases about five decades (from about 1E14 Ohm.m to 1E9 Ohm.m) due to the change in the amorphous phase from a glassy into a rubbery state. The measured resistivity level is too high to be responsible for the low frequency background losses as measured in the 0.1 kHz. curve of Figure 9.17. 

PK terpolymer specific volume resistivity absolute temperature relation (dried, compression moulded sample)... 

I = measured current, Ampere A = electrode area, m2 d = electrode distance, m p = a specific volume resistivity value, Ohm.m... 

Electric conductivity exerts a perceptible effect on crumbling of both polymer and inhibitor particles in the fluidized bed. It should be underlined that electrostatic deposition of inhibitor powders, related mainly to semiconductors, differs a little from that of thermoplastics. In the case that the specific volume resistance is pv < 10 fl- m, there is a strong probability of crumbling of... 

The insulation resistance of a dielectric is represented by its volume resistivity and surface resistivity. The volume resistivity (also known as specific volume resistance) is defined as the resistance between two electrodes covering opposite faces of a centimeter cube. The rang of volume resistivities of different materials including plastics is shown in Figure 3.54. Values for plastics range from approximately lo ohm-cm for a typical cellulose acetate to abut 10 ohm-cm for a high-performance polystyrene. 

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