Filler dielectric constant

Electrical Resistance—Conductivity. Most fillers are composed of nonconducting substances that should, therefore, provide electrical resistance properties comparable to the plastics in which they are used. However, some fillers contain adsorbed water or other conductive species that can gready reduce their electrical resistance. Standard tests for electrical resistance of filled plastics include dielectric strength, dielectric constant, arc resistance, and d-c resistance. 

Fig. 4-4 Example of how additives or fillers provide a wide dielectric constant range.

Fig. 6.7 (a) The variation of electrical conductivity of PVA-EG hybrid with increasing graphene content. Inset shows the dependence of dielectric constant for the hybrid, (b) The variation of conductivity of the polystyrene-graphene hybrid with filler content. Inset shows the four probe setup for in-plane and transverse measurements and the computed distributions of the current density for in-plane condition (reference [8]). 

In contrast to metals and semiconductors, the valence electrons in polymers are localized in covalent bonds.The small current that flows through polymers upon the application of an electric field arises mainly from structural defects and impurities. Additives, such as fillers, antioxidants, plasticizers, and processing aids of flame retardants, cause an increase of charge carriers, which results in a decrease of their volume resistivity. In radiation cross-linking electrons may produce radiation defects in the material the higher the absorbed dose, the greater the number of defects. As a result, the resistivity of a radiation cross-linked polymer may decrease. Volume resistivities and dielectric constants of some polymers used as insulations are in Table 8.3. It can be seen that the values of dielectric constants of cross-linked polymers are slightly lower than those of polymers not cross-linked. 

Fig. 29 Effect of filler loading on a AC conductivity and b dielectric constant. EVA-EG, EVA-T, and EVA-F represent EVA-based nanocomposites reinforced with EG, MWCNTs, and CNFs, respectively...

This behavior becomes more transparent in Fig. 30a,b, where the a.c.-con-ductivity a and relative dielectric constant (permittivity e ), respectively, for a series of less polar S-SBR-samples filled with various amounts of the coarse black N550 are show at 20 °C in a broader frequency range up to 107 Hz. For filler concentrations below the percolation threshold (O<0.15), the conductivity behaves essentially as that of an isolator and increases almost linearly with frequency. Above the percolation threshold (5>>0.2), it shows a characteristic conductivity plateau in the small frequency regime. Since at low frequencies the value of the conductivity a agrees fairly well with the d.c.-con-ductivity, the plateau value exhibits the characteristic percolation behavior considered above. In the high frequency regime the conductivity depicted in... 

Another important distinction is based on (1) filled and reinforced plastics, and (2) foams. (1) When any family of polymers is combined with particulate inorganic fillers, this produces major increase in density, modulus, dimensional stability, heat transfer, dielectric constant, and opacity, and frequently a decrease in cost. When the fillers are reinforcing fibers, they can further produce a great increase in strength, impact resistance, and dimensional stability. Thus, these properties may depend more upon the use of fillers and fibers, than upon the choice of the particular polymer in which they are used. 

Methyl silicone rubber also shares the excellent electrical properties of the resins and oil. A molded sample with silica filler had a dielectric constant of 3.0 at room temperature over a range of 60 to 1010 cycles. The loss factor remains at 0.004 from 60 to 107 cycles and then rises rapidly to 0.037 at 109 cycles and 0.055 at 1010 cycles. At 102° C. the values remain the same except for a small decrease in dielectric constant (caused by a decrease in density) and a slight indication of enhanced d-c conductivity. The rubber does not seem to be affected by ozone. 

Epoxies are excellent electrical insulators. Electrical properties are reduced on increasing the polarity of the molecules. Addition of metallic fillers, metallic wools and carbon black convert the non-conductive epoxy formulation into an electrically conductive system. Non-conductive fillers increase the arc resistance and to some extent increase the dielectric constant. 

Filler Resistivity D-cm Dielectric constant Dielectric strength V/cm Loss tangent... 

A dataset containing the dielectric constants of 61 polymers measured at room temperature was prepared by careful comparison and combination of the data provided by many sources [3,14-20]. For polar polymers, special care was exercised to select values of e which represented, whenever possible, the "intrinsic" properties of the polymers, rather than the effects of the additives and fillers used. It will be seen in Section 9.D that the dielectric constants of typical commercial grades of many polar polymers, which contain significant amounts of additives, are considerably higher than the "intrinsic" values used in this section. 

Table 9.4. Measurement frequency v in Hertz, dielectric dissipation factor tan 8 and dielectric constant e observed at room temperature at the specified frequency of measurement, and the fitted value of tan 8 calculated as described in the text, for 206 data points covering a wide variety of polymers. Most measurements were made on commercial samples containing various plasticizers, fillers and other additives, so that the (exp) values listed for many polar polymers differ significantly from (i.e., exceed) the more "intrinsic" e(exp) values listed in Table 9.1. 

Closed-form expressions from composite theory are also useful in correlating and predicting the transport properties (dielectric constant, electrical conductivity, magnetic susceptibility, thermal conductivity, gas diffusivity and gas permeability) of multiphase materials. The models lor these properties often utilize mathematical treatments [54,55] which are similar to those used for the thermoelastic properties, once the appropriate mathematical analogies [56,57] are made. Such analogies and the resulting composite models have been pursued quite extensively for both particulate-reinforced and fiber-reinforced composites where the filler phase consists of discrete entities dispersed within a continuous polymeric matrix. 

With regard to scaling Vx for a given filler medium, the dominant variables are insulator thickness and relative dielectric constant. Equation... 

In the case of isolated spheres, the equations are quite straightforward, and bring out clearly the effects of field frequency co and filler volume fraction complex dielectric constant e are ... 

Electrical properties. Fillers and additives significantly increase the porosity of polytetrafluoroethylene compounds. Electrical properties are affected by the void content as well as the filler characteristics. Dielectric strength drops while dielectric constant and dissipation factor rise. Metals, carbon, and graphite increase the thermal conductivity of PTFE compounds. Tables 3.19 and 3.20 present electrical properties of a few common compounds. 

Nearly every polymeric system absorbs some moisture under normal atmospheric conditions from the air. This can be a difficult to detect, very small amount as for polyethylene or a few percent as measured for nylons. The sensitivity for moisture increases if a polymer is used in a composite system i.e. as a polymeric matrix with filler particles or fibres dispersed in it. Hater absorption can occur then into the interfacial regions of filler/fibre and matrix [19]. Certain polymeric systems, like coatings and cable insulation, are for longer or shorter periods immersed in water during application. After water absorption, the dielectric constant of polymers will increase due to the relative high dielectric constant of water (80). The dielectric losses will also increase while the volume resistivity decreases due to absorbed moisture. Thus, the water sensitivity of a polymer is an important product parameter in connection with the polymer s electrical properties. The mechanical properties of polymers are like the electrical properties influenced by absorption of moisture. The water sensitivity of a polymer is therefore in Chapter 7 indicated as one of the key-parameters of a polymeric system. 

A number of approaches have been explored for increasing the dielectric constant of elastomers for DEs. The most common approach involves the addition of high dielectric constant filler materials to an elastomer host. Silicone is of particular interest for this type of approach as it possesses good actuation properties to begin with, is readily available in gel form, and has a low dielectric constant. Results thus far do not appear particularly promising increases in dielectric constant have been met with concomitant increases in dielectric loss and reductions in dielectric breakdown strength [184—186]. It has also been shown that the elastic modulus is affected by the addition of filler [187]. 

For many applications of filled polymers, knowledge of properties such as permeability, thermal and electrical conductivities, coefficients of thermal expansion, and density is important. In comparison with the effects of fillers on mechanical behavior, much less attention has been given to such properties of polymeric composites. Fortunately, the laws of transport phenomena for electrical and thermal conductivity, magnetic permeability, and dielectric constants often are similar in form, so that with appropriate changes in nomenclature and allowance for intrinsic differences in detail, a general solution can often be used as a basis for characterizing several types of transport behavior. Useful treatments also exist for density and thermal expansion. 

Figure 9.19 Frequency dependence of (a) dielectric constant (cO and real permeability ip. ), (b) dielectric loss (e")> and magnetic loss (p") of PANI-MWCNT/PS nanocomposites with increasing loading (10, 20, and 30 wt.%) of PANl-MWCNT filler. Reprinted from Ref [11] with permission from Elsevier.

Polymer Filler Volume resistivity (Ohm- cm) Dielectric strength (mV/m) Dielectric constant (1 kHz) Dissipating factor (1 MHz) Surface arc resistance (Ohm) Tracking resistance (Ohm) Volume resistivity (Ohm/m) Dielectric strength (mV/m) Dielectric constant (IkHz Ohm) Dissipation factor (1 MHz) Surface arc resistance (Ohm) Tracking resistance (Ohm)... 

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