Volume resistivity, polymer electricity

Poly(vinyl chloride) has a good resistance to hydrocarbons but some plasticisers, particularly the less polar ones such as dibutyl sebacate, are extracted by materials such as iso-octane. The polymer is also resistant to most aqueous solutions, including those of alkalis and dilute mineral acids. Below the second order transition temperature, poly(vinyl chloride) compounds are reasonably good electrical insulators over a wide range of frequencies but above the second order transition temperature their value as an insulator is limited to low-frequency applications. The more plasticiser present, the lower the volume resistivity. 

The dielectric constant of a polymer (K) (which we also refer to as relative electric permittivity or electric inductive capacity) is a measure of its interaction with an electrical field in which it is placed. It is inversely related to volume resistivity. The dielectric constant depends strongly on the polarizability of molecules tvithin the polymer. In polymers with negligible dipole moments, the dielectric constant is low and it is essentially independent of temperature and the frequency of an alternating electric field. Polymers with polar constituents have higher dielectric constants. When we place such polymers in an electrical field, their dipoles attempt... 

The volume resistivity is the electrical resistance of a polymer sample of unit area and unit thickness when electrodes placed on two opposite faces apply an electrical potential across it. The volume resistivity is expressed in ohm.cm. 

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. 

There are several important electrical properties. Insulation resistance is the resistance of polyurethane to the flow of electricity. The insulation resistance is composed of two main components. The "volume resistivity" is the resistance to flow through the bulk of the material and is a function of the composition of the polymer. The second component is the "surface resistivity,"... 

The measurement of many other polymer characteristics are of practical importance. Among these are various electrical properties such as volume resistivity, surface resistivity, dielectric constant, arc resistance, etc. Also, solubility properties, polymer-polymer compatibility [22], chemical resistance, resistance to radiations, etc. are measured on polymers (see e.g. [1]). 

Step 16. Calculate the key electrical properties. The dielectric constant at room temperature can be calculated from equations 9.11 and 9.12. For polystyrene, e(298K)=2.57 is predicted. The predicted e(298K) can be inserted into Equation 9.7, to predict the volume resistivity at room temperature. The prediction for polystyrene is 7.2 1017 ohnvcm. For polar polymers, these two properties can only be predicted at T=298K with the present correlations. 

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. 

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. 

Electrical Reoulrements. The pottant should contain no plasticizer because plasticizer can reduce the volume resistivity of a polymer drastically. It reduces the resistivity of PVB by 5 orders of magnitude In some formulations. PVB with 40 dlester plasticizer measures only 10 ohm-cm In laminated form at room temperature whereas It measures 10 ohm-cm with the plasticizer driven out. Volume resistivities of 10 2 ohm-cm or less will conduct small amounts of current fairly readily, albeit slowly. (For example, a resolved 5 line palr/mm charge Image has been observed to blur within the first few seconds when placed on the surface of a film or Immersed In a llould of lO I-IO ohm-cm resistivity. The same Image on or In 101 ohm-cm material will not blur for several hours. On a ohm-cm material an Image will last unblurred from weeks to months.)... 

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... 

To function as electrical conductors, epoxies and other polymer resins, because they are inherently insulators, must be filled with electrically conductive particles such as metals. The selection of electrically conductive or insulative adhesives is based largely on their conductivities or, conversely, on their volume resistivities. Electrically conductive adhesives should have low resistivities initially and retain those values on aging, moisture exposure, thermal cycling, and other operating and test conditions. The resistivities of metal-filled epoxy adhesives can range from... 

To obtain the lowest electrical volume resistivities (in the 10 to 10 ohm-cm range) and the lowest contact resistances, polymer resins must be highly Ailed with 70-80% by weight (25-30% by volume) of metal particles and must be thoroughly cured. Silver and silver alloys are the most widely used fillers but, in specific applications, other metal fillers including gold, copper, nickel, and even carbon are used. Carbon-filled polymers are often used to dissipate static electricity or to protect from electromagnetic interference (EMI). 

In adding conductive filler to an insulating resin, the volume resistivity changes slowly until a critical level of filler is reached, called the percolation point. The percolation point occurs when the resistivity drops abruptly, then continues to drop slowly (Fig. 2.11). " Almost continuous linkage of metal particles occurs at the percolation point where typical filler volumes for silver flakes are 25-30%. According to the percolation theory, there is a minimum critical volume of filler required for electrical conductivity in a polymer at which each filler particle must contact two other particles. A misconception in the use of silver flakes is that increasing the number of contacts lowers volume resistivity. Actually, the converse is true because, once the percolation point has been reached, each additional contact adds resistance. Thus, increasing the particle size can increase conductivity since the total number of contacts for a fixed volume decreases." ... 

PPS is a good electrical insulator, even at high temperatures. The volume resistivity and the dielectric strength remain high up to 200°C. The polymers have a low dielectric constant and a low loss factor throughout a wide temperature range. 

Polypropylene materials (PP), because of their electric properties (such as surface resistivity Ps, volume resistivity pv, dielectric loss factor tg8, permittivity e), mechanical properties and resistance to noxious agents (resistance to acids, bases, salts and organic solvents) are used in various industries. Polypropylene materials characterise, also, with the lowest specific density among widely used polymers. Those properties predispose polypropylene to be used as a substrate for composite protective screens shielding people and electric or electronic devices against noxious activity of electromagnetic (EM) fields. Composite shields... 

Typical volume conductivities of rubbers are around 10 -10 Qcm. The electric conductivity of rubber is caused by the presence of impurities which transfer ions and thus electric current. On transition from glassy to elastic state voltrme resistivity decreases by several orders of magnitude because of an increase in ion mobihty. Also plasticization of mbbers decreases their volume resistivity because of the increased ion mobility. Classification of various rubbers according to their electrical conductivity was published elsewhere. As data in Table 10.6 show, polymers are excellent electrical insulators but they can be modified to become good conductors having volume resistance of 10 Qcm or less. ... 

The mechanical and electrical properties of acrylates are similar to those for many other polymer types. Tensile strengths range from approximately 1,300 psi to 4,500 psi and shear strengths of PCB lap samples range from 424 to 566 psi. Aluminum-to-aluminum lap-shear strengths are reported at approximately 3,000 psi. Electrical volume resistivities under room ambient conditions ranged from 1 x ohm-cm to 1.4 x 10 ohm-cm. dielectric constants from 3.15 to 3.43 at 10 MHz and dissipation factors... 

The ability of a polymer to flow or to oppose an electrical current under the application of an external electric field is known as its electrical property. In a good conductor, electrons or ions are able to flow easily through the material. If the material resists the flow, it is described as an insulator. Most polymers are insulators. The volume resistance of a polymer is the resistance of an electric field (current) between the opposite faces of a unit... 

The Hostatec has low water absorption. Dielectrie properties of films from polyesteresterketone Hostatec are high. This amorphous polymer has the electric inductivity 3, 6, loss factor 10 and the speeifie volume resistance 10 Ohm cm, these values remain still up to 60 °C. 

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