Surface and volume resistivity

The concept of volume resistivity p implies that the resistance R of a bar is proportional to its length L and inversely proportional to its cross section A 

It also implies that a constant applied voltage produces a constant current. However, for polymers with resistivities in the range 10 to lO ilm, the extremely small currents decay with time after a voltage step is applied. Consequently, a time, often 1 min, is specified for the cvurent measurement. 

The composition of polymer surface layers often differs from the bulk, due to the migration of organic antistatic additives. The surface resistivity of the polymer can dominate the insulation resistance. The concept of sruface resistivity implies the existence of a surface layer on top of an insulating substrate. The surface resistivity ps is related to the layer thickness t and volume resistivity p by 

It is also equal to the resistance in ohms between the opposite sides of a square of any size on the surface of the product. 

The mobility, the ratio of the carrier velocity to the electric field, is of the order of 10 m s in polymers. Hence, one carrier, with an electronic charge q of 1.60 x 10 C, for every 10 monomer units causes the polymer resistivity to be 10 flm. Ionic impurity levels of this magnitude have no effect on other physical properties. 

The volume resistivity (log, ohm, cm) of a material is its resistance to leakage of electrical current through itself. Specifically, it is the ratio of the potential gradient in the direction of the current to the current density. It is dependent upon moisture content and temperature. The values given in Table 5.2 are typical room temperature values. 

Nedjar [26] have reported on the measurement volume of resistivity cross-linked polyethylene grades used in high-voltage cables and the effect of thermal aging on the electrical properties of the cable. 

Volume resistivities are listed in Table 5.2. They range from as low as 2 ohm.cm for epoxy resins to as high as 16-18 ohm.cm for high-density polyethylene, polyether ether ketone, polystyrene, polymethylpentene, polyethylene terephthalate, polyarylates, polyphenylene oxide, polyamide imide, polyimides, polyurethane, polytetrafluoro-ethylene, perfluoroalkoxy ethylene, fluorinated ethylene-propylene copolymer, ultra-high-molecular-weight polyethylene, polysulfones, and polyethersulfones. 

The volume resistivity, permittivity, and dielectric loss factor of nanostructured interpenetrating polymer networks based on natural rubber/polystyrene have been found to increase as a function of blend composition, reaching a maximum of 10 -10 Hz dielectric loss factor [27]. Measurements of volume resistivity have also been reported on epoxy resin-polyaniline blends resulting in the establishment of a correlation between a shoulder on the 1583 cm band with the degree of volume resistivity [31]. 

Niak andMishra [13] found that the surface resistivity of natural fiber high-density polyethylene composites decreased with an increase of fiber content of the composite, whereas the volume resistivity increased. 

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See 2-3.1. Electrical conduction through solids takes place both through the bulk material and over the surface. In most cases surfaces have different physical and chemical properties than the bulk, for example due to contamination or moisture. Volume and surface resistivity can be separately measured for solid materials such as antistatic plastic sheet. Powders represent a special case since although both surface and bulk conduction occur, their contributions cannot be individually measured and the volume or bulk resistivity of a powder includes surface effects. 

Insulation resistance is the ratio of direct voltage applied to the electrodes to the total current between them dependent upon both volume and surface resistance of the specimen. In materials used to insulate and support components of an electrical network, it is generally desirable to have insulation resistance as high as possible. 

Surface resistivity is the ratio of the potential gradient parallel to the current along its surface to the current per unit width of the surface. Knowing the volume and surface resistivity of an insulating material makes it possible to design an insulator for a specific application. 

Surface resistance is the ratio of the direct voltage applied to the electrodes to that portion of the current between them that is in a thin layer of moisture or other semiconducting material which may be deposited on the surface. High volume and surface resistance are desirable in order to limit the current leakage of the conductor that is being insulated. 

The usual standard methods for volume and surface resistivity both use the same test piece and electrode geometry and essentially the same measuring circuit. There are no ISO or IEC methods for rubbers but, where national standards specifically for polymers exist, they are usually adaptations of the IEC Publication 600931 for insulating materials in general. The relevant British Standards for rubbers are BS 903-C12 for surface resistivity and BS 903-C23 for volume resistivity. ASTM has an equivalent to the IEC standard for insulating materials generally, D2574, but no standard specifically for rubbers. 

Volume and surface resistivity tests for conductive and antistatic materials include ... 

Properties of fluoropolymers that have led to applications include chemical resistance, thermal stability, cryogenic properties, low coefficient of friction, low surface energy, low dielectric constant, high volume and surface resistivity, and flame resistance. Fluoropolymers are used as liners (process surface) because of their resistance to chemical attack. They provide durable, low maintenance and economical alternatives to exotic metals for use at high temperatures without introducing impurities. Electrical properties make fluoropolymers highly valuable in electronic and electrical applications as insulation, e.g., FEP in data communications. 

The size of the fluorine atom allows the formation of a uniform and continuous sheath around the carbon-carbon bonds and protects them from attack, thus imparting chemical resistance and stability to the molecule. The fluorine sheath is also responsible for the low surface energy (18 dynes/cm)[ i and low coefficient of friction (0.05-0.08, static)[ i of PTFE. Another attribute of the imiform fluorine sheath is the electrical inertness (or non-polarity) of the PTFE molecule. Electrical fields impart only slight polarization to this molecule, so volume and surface resistivity are high. Table 1.1 summarizes the fundamental properties of PTFE, which represents the ultimate polymer among all fluoroplastics. 

Holm (1967) identifies the contact resistance between particles of clean metal to be the result of current constriction at the point of contact. This geometric constriction together with the volume and surface resistivities integrated over the remaining volume and surface of a particle constitute the total resistance measured between two contacts located at the poles of the particle. In addition, if a thin film exists between the particle contacts, the tunnel effect provides a current independent of the film resistivity. 

IEC 93, Recommended methods of test for volume and surface resistivities of electrical insulating materials, the International Electrotechnical Commission, Geneve (1958). 

The theory of electrical conduction in polymeric materials is extremely complex, and the phenomena are still incompletely understood. The apparent current between electrodes separated by a polymeric material is neither constant in time nor proportional to the applied potential. Most aspects of the subject are discussed in a review paper 2). This review also presents an extensive bibliography. From the practical point of view, there arc several published standards dealing with both volume and surface resistivity [3 5) in addition, several standards deal with specialized materials from the viewpoint of electrostatics (see the section on electrostatics). Some of the latter may well be withdrawn as the lEC ISO situation on electrostatic matters is rationalized. 

The insulation resistance is a measure of the resistance a plastic provides to current flow when two terminals are placed in contact with it It is the ratio of the applied DC voltage on the electrodes to the total current between them. Insulation resistance is often reported as the ohms resistance generated between the two electrodes but is, in fact, a composite of volume and surface resistivity. 

Electrical properties discussed include the measurement of volume and surface resistivity, dielectric strength and surface arc and tracking resistance. Optical properties and light stability includes a discussion of stress optical analysis and the effects of light and other influences on stabilised and unstabilised polymers. 

Volume and surface resistivity Teraohmeter Model M510E Q cm (volume) Q (surface) ASTM D257-99 [147] DIN lEC 60093 [145] DIN lEC 60167 [146]... 

Volume and surface resistivity Teraohmeter fl cm (volume) ASTM D257-99 [4]... 

The insulation resistance between two conductors or plated holes is the ratio of the voltage to the total current between the conductors. Two measures of electrical resistance are volume and surface resistivities. Since these properties can vary with temperature and humidity, testing is normally performed at two standardized environmental conditions, one involving humidity conditioning, the other involving elevated temperature. Humidity conditioning subjects the sample to 90 percent relative humidity and 35°C for 96 hours (96/35/90).The elevated temperature conditioning normally subjects the sample to 125°C for 24 hours (24/125). 

All measurements are performed by applying 500 volts direct current.The voltage needs to be applied to the samples for 60 sec. before taking the resistance reading to allow the test structures to stabilize. The surface resistance is determined between the outer ring electrode and the inner solid electrode.The volume resistance is determined between the solid front and back electrodes after changing the connecting cables appropriately. The values for volume and surface resistivity can be calculated from the measured resistance values as shown in Eqs. 12.5 and 12.6. 

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