The effect of electric field strength on the induction charge of freely levitating particles

Many industrial processes such as electrostatic separation, fluidization, and coating rely upon induction charging of fine particles. This paper considers the effects of electric field strength on the magnitude of the induction charge on freely levitating particles. The charging time and charge on a freely levitating particle depend on a number of properties, mainly the electric field strength, particle size, density, and resistivity. A charging model showing the dependence upon the electric field strength is presented and analyzed, along with a model of the levitation process. A high-speed digital imaging system was used to measure individual particle motion during levitation. Using these data along with the developed models, it was possible to determine the charge on the particle. Semiconductive particles with a mass mean diameter (MMD) of 156 /spl mu/m were used in these experiments and tested at electric fields of 6.8, 8.5, 15, and 21 kV/cm, respectively. In addition, some experiments using particles 97-/spl mu/m and 412-/spl mu/m MMD at an electric field of 15 kV/cm were carried out to confirm the results obtained for the 156-/spl mu/m particles. It was found that the particle charge was dependent upon both the charging time and electric field strength. In particular, for high electric fields the particle did not achieve its saturation charge before liftoff occurred. This shows that higher electric field strength is not necessarily the optimum condition for levitation of semiconductive particles.     

Bipolar charging of poly-disperse polymer powders in fluidized beds

It is well known that many common industrial powder handling operations such as pneumatic transport, mixing, and fluidization may produce a net charge on the powder particles. However, it is less well known that the net charge is often the result of a bipolar charge distribution in which the smaller particles acquire charges of a polarity opposite to those on the larger. This suggests contact charging between particles having the same chemical makeup. Very little quantitative data exist in the literature concerning this observation and no acceptable explanation currently exists. The purpose of this paper is to review the previous published work and to describe some results showing bipolar charging using polymer powders in fluidized beds. A new measurement system is described for measuring the bipolar charge distribution. This consists of a vertical array of seven Faraday pail sensors, which can selectively detect different charge components based upon particle size (gravity segregation) and charge (space-charge repulsion). For the experiments reported here the charge and mass values were measured for each sensor allowing the calculation of charge-to-mass ratio (Q/M). In addition, size distribution and surface analyses were carried out for representative samples of the powder components. Data are presented for several types of polymer powders (surface area mean diameter <100 /spl mu/m). The results show that, for a given powder, even though the net charge may be positive or negative, the smaller particles show a negative charge and the coarser particles positive. These results are compared under several possible hypotheses. Each of these possibilities is examined using the measurements of the Q/M for different size fractions and the results of surface analysis and particle size distributions of these fractions.     

Thermal stability of polyethylene terephthalate (PET): oligomer distribution and formation of volatiles

Two ovenable PET (polyethylene terephthalate) samples were investigated under severe heating conditions and oligomers and volatile substances were analysed as potential migrants into foods. The samples were tested for migration into water, 3% acetic acid and 15% ethanol solution for 1 hour at 95°C. Overall migration and the specific migration of terephthalic acid, ethylene glycol and diethylene glycol were all very low. The plastics were heated at 150°C, 260°C and 270°C, for 5 minutes 30 minutes and 60 minutes. Oligomer analysis by LC/MS (liquid chromatography‐MS) showed that the concentration of the second series alicyclic oligomers increased up to 15‐fold on heating whereas the major oligomer fraction, the cyclic trimer, tetramer, pentamer and hexamer showed only minor concentration changes with heating. Volatiles evolved by the samples were trapped on a Tenax trap and identified by GC/MS (gas chromatography‐MS). They were few in number and low in concentration and none merited migration tests. It is concluded that even when tested up to melting point, PET plastics of this type have good temperature stability and are well suited for high‐temperature food contact applications. Copyright © 1999 John Wiley & Sons, Ltd.     

Studies on functional barriers to migration. 3. Migration of benzophenone and model ink components from cartonboard to food during frozen storage and microwave heating

Studies of the migration of benzophenone from inks and of model ink components incorporated deliberately into cartonboard have been carried out at low temperature and during microwave heating. Benzophenone was found in the packaging of four of seven frozen foods examined and this was attributed to its use as a photoinitiator for UV‐cured ink. Benzophenone was found to migrate to the packaged food even from polyethylene‐coated board, and this was attributed in part to the fact that polyethylene is rather permeable to low molecular weight substances and is not a good barrier to migration. Additional studies were conducted with two other frozen foods by incorporating model ink components into their cartonboard packaging. The substances were benzophenone, benzylbutyl phthalate, butyl benzoate, chlorodecane and dimethyl phthalate. Migration was readily detected after storing the food at −20°C for 1 week in the impregnated cartonboard. Migration levels in the food increased only slowly thereafter, up to 1 year, except for the most volatile substances used. Subsequent heating of the frozen food using a microwave oven decreased the concentration of some migrants by volatilization, but increased the concentration of other, less volatile substances by heat‐accelerated migration. It is concluded that for inks used to print food contact materials, if the content of low molecular weight volatiles is controlled and if transfer of higher molecular weight components via set‐off is controlled also, then migration levels could be kept low in these conditions. Copyright © 2000 Crown Copyright. Reproduced with the permission of Her Majesty's Stationery Office. Published by John Wiley & Sons, Ltd.     

In-Flight Absolute Radiometric Calibration of the Thematic Mapper

Ground spectral reflectance and atmospheric spectral optical depth measurements made at White Sands, New Mexico on January 3, 1983, were used with an atmospheric radiative transfer program to determine the spectral radiance at the entrance pupil of the Landsat-4 Thematic Mapper (TM). A comparison with the output digital counts of the TM, when imaging the measured ground area, provided an absolute calibration for five detectors in TM bands 2, 3, and 4. By reference to an adjacent, larger uniform area, the calibration was extended to all 16 detectors in each of the three bands. Pre-flight calibration results agreed with these inflight measurements to 6.6, 2.4, and 12.9 percent in bands 2, 3, and 4, respectively.     

The effect of microwave energy on specific migration from food contact plastics.

Experiments have been carried out with (a) microwave treatment of plastics followed by migration testing using the food simulant, olive oil, and (b) microwave treatment of plastics in direct contact with an organic extractant (iso-octane). In neither of these complementary approaches was there evidence of any difference in migration from plastics that had been microwave-treated compared with plastics that had received an equivalent thermal treatment. Five plastics commonly employed in microwave applications were tested and oligomers, plasticizer, antioxidant and volatile contaminants were monitored as representatives of typical migration species.     

Generation of bipolar electric fields during industrial handling of powders

This paper presents the results of some field tests that were carried out to determine the electric fields generated due to bipolar charging that occurs during the filling of a container with conveyed powders in an industrial setting. Approximately 1000 kg of pneumatically conveyed powder was allowed to fall into a cubically shaped container made out of woven fibreglass cloth that was placed on a metallic base electrically connected to ground. During the filling operation, the electric fields at three points close to the outside surface of the container were measured with a calibrated electrostatic probe. The results show that as the container is being filled, the mass of powder that accumulates in the lower part generates outward directed positive electric fields. In contrast, the shrinking, unfilled, upper space in the container generates considerably stronger outward directed negative electric field at the surface. At the level of the powder filling the container, the outward directed electric field was approximately zero. As the container fills, the zero electric field level moves upwards and the negative electric field disappears leaving the entire surface with a positive electric field. The net charge to mass ratio of the conveyed powder was measured in the order of 1.5 E-10 C/g, while falling into the container. Some of the generated negative electric fields at the surface were as high as 30 kV/cm. These results confirm previous measurements with other powders that showed that in a poly-disperse mixture, fine particles tend to charge negatively whereas coarser particles charge positively.