Surface conductivities, moisture

Antistatic Agents. Most polymers, because they are poor conductors of current, build up a charge of static electricity. Antistatic agents attract moisture from the air to the plastic surface, improving its surface conductivity and reducing the likelihood of a spark or a discharge. 

For powders and granular solids, there are two types of antistatic agents surface- and volume-active additives. Surface-active agents, which increase the surface conductivity of individual particles, are effective because triboelectric charge is always situated on the surfaces of individual particles. Most if not all surface-active agents are hygroscopic and thus attract a thin film of water to the surface it is this moisture that is responsible forthe increased surface conduction (van Drumpt, 1991). The effective bulk resistivity of the particles % -may be estimated by assuming that the particles are spherical and of radius R (Jones, 1995). 

Many local regulatory agencies require the ESP to be energized whenever the FCC main blower is in operation. Some units have found that steam/moisture injection is useful at temperatures lower than 400°F as a form of resistivity modification. This is because the unit is in the surface conduction phase of the resistivity curve. At this temperature, the moisture improves surface conduction of the particles. 

Free amino acids act as a moisture holding factor in SC,17 and their contents are correlated with skin surface conductance.13 The amino acid levels in both AD and HD patients who had low skin surface conductance were less than half of the control levels. The conversion ratio of ornithine to citrulline (Cit/(Orn+Cit) is related to the degree of cornification disorder and is negatively correlated with the epidermal proliferation rate.12 As Table 9.2 shows, conversion ratios were similar in HD patients and the controls, but showed a significant difference (p < 0.05). On the other hand, the conversion... 

Electronic moisture meters usually operate on a dielectric principle and/or kernel surface conductance with compensation for sample temperature and density. Thus, electronic moisture meters measure electrical properties that are calibrated to oven moisture measurements. The typical air-oven reference methods used for whole soybeans are the AOCS Method Ac 2-41, ASABE Standard S352.2, and AACC Method 44-15a. 

The electrical properties of macromolecular semiconductors are generally characterized by the conductivity, the activation energy of the conductivity, the free radical concentration, and the thermal electromotive force (thermal emf). Since the polymers are usually in the form of amorphous powders, they are compressed into tablets. The contacts are either metal electrodes pressed into the surface or conductive pastes. The samples may not have any ionic conductivity (due to impurities) or surface conductivity and must be free of moisture, otherwise the conductivities will be too high. 

It is well known (see Section 10) that surface-adsorbed layers of different substances may change the molecular interaction. Although adsorbed layers do not have any great effect on the electrical forces created at the expense of double-layer charge (see Section 15), nevertheless, if these layers make the surface conductive, we may expect a decrease in Coulomb interaction as time elapses (see Section 16). Moisture on the surface promotes capillary condensation in the gap between the contiguous bodies (see Section 17). 

Surface conductance n. The direct-current conductance (A/V) between two electrodes in contact with a specimen of sohd insulating material when the current is passing only through a thin film of moisture on the surface of the specimen. 

Antistatic additives can be classified by application method, as internal and external, and by chemistry, as anionic, cationic, and non-ionic. Internal agents are normally compounded at 0.1-3.0% by weight and have a slight compatibility with the polymer, but the molecule has a hydrophilic head forcing it to migrate to the surface and attract moisture from the environment, which increases the surface conductivity. These are easy to use and have low addition rates, often also providing other benefits such as improved processability and mould release. 

The rolling operations that foUow take place first on hot (95°C) differential-speed roUs which dry and coUoid the paste and convert it iato sheet form, and then on even-speed roUs which produce smoothly surfaced propellant sheets ia which all iagredients have been uniformly iacorporated. The roU gap ia the differeatial roUs is adjustable to produce sheets of various thicknesses, and rolling is continued until the moisture is reduced to a predetermined level, usually less than 0.5%. The sheet is then cut off the roU. Differential rolling is potentially hazardous, and fires are not uncommon, although detonations are not apt to occur. Operations are conducted by remote control. 

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