A.G. Bailey

The science and technology of electrostatic powder spraying, transport and coating

Abstract The electrostatic powder coating process is reviewed. Three basic regions of a powder coating system are considered. The charged powder source region embraces corona and triboelectric guns. These are reviewed and particle charging processes considered. The second region where charged powder transport to the workpiece occurs is one in which aerodynamic, space charge, and gravitational forces act to determine particle trajectories. With corona sources ions generally outnumber charged particles by 10 to 1 so that the space charge due to charged particles is negligible compared to that of ions. Charged particle deposition onto the workpiece, the third region, is affected by the electric field of particles already deposited. Back ionisation may occur if ions are present and lead to poor coating finish. The adhesion of charged particles to a workpiece is often greater than expected by consideration of image forces alone.

Charging of Solids and Powders

Abstract The electrostatic charging of materials such as webs, films and powders due to contact and triboelectric phenomena is reviewed. Some of the problems that may arise in industry due to material charging are discussed. Particular operations such as charging of webs moving over rollers, pneumatic transfer of powder along pipes and into a silo, and the charging of single polymer particles impacting onto a metal surface are reviewed. Some of the problems that arise due to charge accumulation on the surface of materials are considered. Powder flow may be affected and electrical discharges which cause fires and explosions are examples of some of these problems.

Drug delivery by inhalation of charged particles

Abstract All solid and liquid particles produced naturally or by energetic industrial processes are electrically charged. Although the natural levels of charge are normally insufficient to influence the deposition of inhaled particles in the lung it is possible to increase charge levels so that a significant increase in lung deposition is caused. By careful control of breathing, particle size and charge it is possible to target specific regions of the lung. Predictions of targeted deposition using the Southampton lung model are presented and a brief description of complementary clinical studies is given.

The charging of insulator surfaces

Abstract The first recorded investigations of triboelectric charging of materials occurred over 2000 years ago. Since the 18th Century a greater understanding of charging processes and the manner by which insulators hold charge has developed. A brief review of some aspects of insulator/insulator and insulator/metal charging is presented.

Study of the three-dimensional geometry of the central conducting airways in man using computed tomographic (CT) images

Clinical research on the deposition of inhaled substances (e.g. inhaled medications, airborne contaminants, fumes) in the lungs necessitates anatomical models of the airways. Current conducting airway models lack three‐dimensional (3D) reality as little information is available in the literature on the distribution of the airways in space. This is a limitation to the assessment or predictions of the particle deposition in relation to the subject’s anatomy. Detailed information on the full topology and morphology of the airways is thus required to model the airway tree realistically. This paper presents the length, diameter, gravity, coronal and sagittal angles that together describe completely the airways in 3D space. The angle at which the airways branch out from their parent (branching angle) and the rotation angle between successive bifurcation planes are also included. These data are from the study of two sets of airways computed tomography (CT) images. One CT scan was performed on a human tracheobronchial tree cast and the other on a healthy male volunteer. The airways in the first nine generations of the cast and in the first six conducting generations of the volunteer were measured using a computer‐based algorithm. The data contribute to the knowledge of the lung anatomy. In particular, the spatial structure of the airways is shown to be strongly defined by the central airways with clear angular lobar patterns. Such patterns tend to disappear with a mean gravity, coronal and sagittal angles of 90° in each generation higher than 13–15. The mean branching angle per generation appears independent of the lobe to which the airways belong. Non‐planar geometry at bifurcation is observed with the mean (± SD) bifurcation plane rotation angle of 79 ± 41° (n = 229). This angle appears constant over the generations studied. The data are useful for improving the 3D realism of the conducting airway structure modelling as well as for studying aerosol deposition, flow and biological significance of non‐planar airway trees using analytical and computational flow dynamics modelling.

The inhalation and deposition of charged particles within the human lung

Abstract When particles are inhaled they deposit within the respiratory system depending on particle size, shape and charge together with breathing conditions such as inhalation flow rate, breath-holding pause and exhalation flow rate. In addition further control of particle deposition may be achieved by pulsing the aerosol particles into the lung during only part of the inhalation phase. We have developed a mathematical model of the human lung which is implemented on a personal computer. It is of value for studies of airborne pollutant particle hazards, the administration of drugs and for the diagnosis of lung disease. When small particles of about 1 μm are considered the importance of electrical charge on particle deposition is demonstrated. Increases in deposition occur above a threshold charge level as charge is increased. The model may be used to demonstrate how particle deposition within a diseased lung may be controlled by means of aerosol pulses. It is possible by means of adjusting pulse volume and pulse delay time to target particular regions within the respiratory system. Clinical studies using radio labelled, monodisperse aerosols and also polydisperse aerosols from nebulizers are under way. These studies are with both healthy human volunteers and mildly asthmatic subjects. Clinical data is in the form of images obtained by means of a Gamma camera. Much of the present work is directed at transforming the observed experimental data which is in spatial form to the airway branching structure. Success has been achieved with the correlation of clinical and predicted data.

Electrostatic Charging Characteristics of Polyethylene Powder During Pneumatic Conveying

A full-scale experimental rig has been set up to study the electrostatic charging characteristics and possible ignition hazards during pneumatic conveying and storage of powders. The rig consists of two large silos ( 55 m3 and 60 m3) which are connected via approximately 35 m length of steel pipe work. The powder conveying parameters such as velocity, relative humidity, dilution, etc., can be accurately controlled. The rig has been used to study the charging characteristics of polyethylene powder. Bipolar charging has been detected on the powder¿the fine particles charge negatively while the coarse particles charge positively. Electric field strengths up to 150 kV/m have been measured during the silo filling operation. Detailed measurements have been carried out on the charging characteristics of polyethylene as a function of different conveying parameters. The experiments and results are reported.

The influence of breathing patterns on particle deposition in a nasal replicate cast

Abstract The purpose of these experiments was to measure the differences in total deposition in the nose arising from differences in the breathing patterns. Monodisperse droplets (from 1.7 to 10 μm ) were deposited in an artificial nasal cavity (cast) using different human breathing patterns as well as under constant flow rate conditions. The human breathing patterns were recorded from a volunteer and then reproduced by a breathing simulator. The nasal cast was made from Magnetic Resonance images of the same volunteer, which were digitised and milled into consecutive plates to form a cavity. The results for small particles (1.7 and 3 μm ) show much higher deposition at high constant flow rates than at fast human breathing. The difference becomes less pronounced with increasing particle size, but is still significant at high flow rates. This suggests that it might not be sufficient to take the average flow rate of the breathing pattern for comparison with constant flow. Therefore, the breathing patterns were partitioned into small segments and deposition was calculated for each segment. Adding deposition of each segment gives a theoretical predicted total deposition caused by the particular breathing pattern. However, the theoretical deposition is higher than the measured deposition at high flow rates, and again this is more pronounced for small particles. An explanation was given for this behaviour.

Changes in the Size Distribution of a Water-in-Oil Emulsion Due to Electric Field Induced Coalescence

A knowledge of the droplet size distribution of a water-in-oil emulsion subjected to an electric field provides useful information regarding electrostatic coalescence which can aid the formulation of coalescence models and the design of commercial electrostatic separators. Water droplet size-distribution measurements made using a laser light-scattering technique during the electrostatic resolution of a low-water-content water-in-oil emulsion are reported. A qualitative explanation of the results is presented.

Electrostatic measurements on a miniaturized fluidized bed

In the pharmaceutical industry fluidization is often used in drying, coating and granulation processes. During fluidization powder particles contact other particles as well as the walls of the vessel and this leads to generation of electric charges. These charges may result in sparks, dust explosions, fires, reduced process efficiency and particle accumulation on the walls. However, the mechanism of the charging is not well understood. In this paper, an inductive method for charge generation measurement is presented. The system consists of an electrostatic ring probe, which allows charge scanning across the miniaturized fluidized bed without disturbing the fluidization process. The charge coupling from the system to the probe has been modelled and the experimental data have been simulated using an advanced field solving software. Experiments on lactose monohydrate, microcrystalline cellulose and glass beads have been performed and these results are presented.