Subject surface resistance

To measure the surface resistivity of paper, a variant of a four-point probe method proposed by Cronch 15 was used. This approach has proven to be reliable, avoiding the effects of contact resistance by employing an electrostatic voltmeter (utilizing contactless probes) to measure the surface potential of paper subject to a constant current. 

In the frequency response method, first applied to the study of zeolitic diffusion by Yasuda [29] and further developed by Rees and coworkers [2,30-33], the volume of a system containing a widely dispersed sample of adsorbent, under a known pressure of sorbate, is subjected to a periodic (usually sinusoidal) perturbation. If there is no mass transfer or if mass transfer is infinitely rapid so that gas-solid mass-transfer equilibrium is always maintained, the pressure in the system should follow the volume perturbation with no phase difference. The effect of a finite resistance to mass transfer is to cause a phase shift so that the pressure response lags behind the volume perturbation. Measuring the in-phase and out-of-phase responses over a range of frequencies yields the characteristic frequency response spectrum, which may be matched to the spectrum derived from the theoretical model in order to determine the time constant of the mass-transfer process. As with other methods the response may be influenced by heat-transfer resistance, so to obtain reliable results, it is essential to carry out sufficient experimental checks to eliminate such effects or to allow for them in the theoretical model. The form of the frequency response spectrum depends on the nature of the dominant mass-transfer resistance and can therefore be helpful in distinguishing between diffusion-controlled and surface-resistance-controlled processes. 

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. 

Resistance to abrasion is defined as the ability of a material to withstand mechanical action that tends to progressively remove material from its surface. Abrasion resistance of polymeric materials is a complex subject. The resistance to abrasion is closely related to other factors such as hardness, resiliency and the type and amount of added fillers and additives. Resistance to abrasion depends on factors such as test conditions, type of abradant and development and dissipation of heat during the test cycle. This all makes abrasion a difficult mechanical property to define as well as to measure adequately. 

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). 

The lowest surface resistance of plastics used for hook-up wire, even when subject to fungus growth, is relatively high and affects performance only of very high impedance circuits these are rarely encountered. 

For constant coefficients, the temperature profiles for various geometries such as flat plates, infinite cylinders, and spheres are given in terms of infinite series. For example, for flat plates with surface resistance to heat transfer the equation to be solved is the one-dimensional heat transfer equation subject to the following boundary conditions ... 

Surface resistivity (Table 11.3) is especially important when the fabricated article may be subjected to a high humidity, which can alter the surface physically or chemically to give a lower resistance than the bulk of the material. Circuit boards require a high surface resistivity. Containers for transporting charge-sensitive materials such as computer chips require high surface conductivity. 

Thermoduric, Thermophilic, andPsychrophihc Bacteria. Thermoduric bacteria survive but do not grow at pasteurization temperatures. They are largely non spore-forming, heat-resistant types that develop on surfaces of unclean equipment. These bacteria are determined by subjecting a sample to laboratory pasteurization and examining it by the agar plate method. 

Water resistance test methods include AATCC 127 (hydrostatic pressure test), AATCC 42 (impact penetration test), and AATCC 35 (rain test). In the hydrostatic pressure test, a sample is subjected to a column of increasing water pressure until leakage occurs. The impact penetration test requires water to be sprayed on the taut surface of a fabric sample from a height of two feet. The fabric is backed by a blotter of predeterrnined weight, which is reweighed after water penetration. The rain test is similar in principle to the impact penetration test. 

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