Dan A. Hays

Small particle adhesion: measurement and control

Abstract A number of technologies, including electrophotographic printing, require an understanding of particle–surface adhesion forces. At Xerox, we have developed and applied new techniques in order to understand the dependence of particle adhesion on physical properties, such as particle charge, shape and surface roughness. Atomic force microscopy and computer modeling were used to investigate the effects of surface roughness, external additives and applied electric fields on the adhesion of single particles. Centrifugal detachment was used to measure the adhesion force distribution of several hundred particles simultaneously and to determine its sensitivity to particle charge and size. Electric field detachment has enabled rapid characterizations of the adhesion of particle layers, providing insight into the roles of particle–surface contact area and nonuniform particle charging on the adhesion of ensembles of particles. The addition of digital photography to the detachment techniques has allowed in-situ visualization of particle detachment, enabling us to probe interparticle effects on adhesion.

Contact electrification between mercury and polyethylene: Effect of surface oxidation

The contact electrification between mercury and polyethylene films has been measured under a variety of conditions. For freshly prepared films, the mercury contact charging of polyethylene is low, ≈1 nC/cm2. For films exposed to the laboratory air for a few hours, the contact charging increases to ≈20 nC/cm2. A brief exposure of the polyethylene to air near a corona, or ozone from a generator, also increases the contact charging. The infrared spectra of unsaturated polyethylene crystals display the formation of ozonides when the crystals are exposed to ozone or the laboratory air. Since there is a strong correlation between the appearance of ozonides and an increase in contact charging, we believe that a definite charge exchange site on polymers has been identified for the first time. To substantiate our observation, we find that ozonization of saturated polyethylene does not significantly increase the contact charge exchange. Analogous results have been obtained for lower molecular weight hydrocarbons. T...

Adhesion of charged particles

The adhesion properties of charged particles are of considerable importance in the electrophotographic process. Measurements on irregularly-shaped, pigmented particles, called toner in the electrophotographic industry, show that adhesion increases with toner charge but that the magnitude is much larger than expected from a simplified electrostatic image force model. An enhanced electrostatic adhesion is also seen in electric field detachment measurements on spherical charged particles. In both cases, this unexpected large adhesion can be attributed to a nonuniform distribution of charge on the surfaces of the particles.

Electric field detachment of a nonuniformly charged dielectric sphere on a dielectric coated electrode

Abstract In the electrophotographic process, charged toner particles are transferred from one surface to another with an electric field. To enable electric field transfer of toner, the externally applied field strength must be greater than a threshold value so that the Coulomb force can overcome the toner adhesion force at the residing surface. Toner particles in electrophotography are often charged by the phenomenon of triboelectricity. Triboelectrically charged particles tend to have nonuniform surface charge distributions which may significantly influence the electrostatic adhesion force and threshold field strength for detachment. In this work, the threshold detachment field strength and the electrostatic adhesion force associated with a nonuniformly charged dielectric sphere are determined efficiently by using the Galerkin finite element method to simultaneously solve the Laplace equation for the field distribution and an overall constraint equation for the force balance. Problems with various complicated geometric configurations can be treated within this computational framework. For illustrative purposes, however, we consider the axisymmetric problem of electric field detachment of a charged sphere on a dielectric coated donor electrode. Our analysis is particularly focused on the effects of a nonuniform charge distribution expressed in terms of Legendre functions on the spherical particle surface. The electrostatic adhesion force appears to vary linearly with the coefficient for each Legendre function. Depending on the orientation of the particle relative to the residing surface, the nonuniformity in the particle surface charge distribution may either enhance or reduce the electrostatic adhesion force.

Role of Electrostatics in Adhesion

The phenomenon of adhesion is the attraction of one material to another in either a gaseous or liquid medium. There are many mechanisms which can contribute to the adhesion between materials. The discussion in this chapter is limited to adhesion mechanisms due to electrostatic forces between dissimilar solid materials in a gaseous medium. The effect of electrostatic forces on adhesion is widely known and observed not only today but as early as 500 BC when the Greeks attracted pieces of straw to amber that had been charged by rubbing with another material. In spite of early observations and the ubiquitous nature of adhesion and triboelectric charging between materials, our present understanding is not satisfactory. Our understanding of electrostatic attraction is satisfactory when materials are separated. But when the materials are in intimate contact, the importance of electrostatic effects relative to other adhesion mechanisms becomes uncertain.

Paper documents via the electrostatic control of particles

Abstract Paper documents continue to be a user-friendly medium for disseminating and archiving information. Although the vast majority of documents are printed with offset technology, a growing percentage (∼30% in US) is produced by digital copiers and printers based on the electrostatic control of charged particles. Electrophotography represents the dominant electrostatic marking technology. However, alternative marking concepts based on different types of electrostatic imaging physics have been proposed in recent years to improve the cost and image quality of electrostatically produced paper documents. To provide a framework for discussion, the marking concepts are grouped into one of three system approaches. The system approaches are (1) indirect marking by developing a latent electrostatic image with charged particles that are subsequently transferred to paper (exemplified by electrophotography), (2) direct marking of charged particles onto an intermediate or paper, and (3) rewritable electronic paper displays enabled by electrostatically controlling the rotation or translation of encapsulated charged particles coated on a paper substrate. The different system approaches are illustrated by examples of current and proposed concepts for printers and displays.

A finite-element analysis of the electrostatic force on a uniformly charged dielectric sphere resting on a dielectric-coated electrode in a detaching electric field

In the electrophotographic process, charged toner particles are transferred from one surface to another with an electric field. To enable electric field transfer of toner, the externally applied field strength must be greater than a threshold value, so that the Coulomb force can overcome the toner adhesion force at the supporting surface. In this paper, the threshold field strength to detach a charged dielectric particle is determined efficiently by using the Galerkin finite-element method to simultaneously solve the Laplace equation for the field distribution and an overall constraint equation for the force balance. This computational method also enables calculation of the electrostatic adhesion force and is applicable to various particle-electrode configurations. For illustrative purposes, however, we consider the axisymmetric problem of electric field detachment of a dielectric sphere with uniform surface charge resting on a planar dielectric-coated electrode. The analysis is particularly focused on the dependence of the electrostatic force upon the dielectric overcoating thickness and spacing between parallel plate electrodes. The electrostatic force on a uniformly charged particle in contact with a surface is found to be influenced significantly by the thickness of dielectric overcoating and the spacing between electrodes when either becomes less than five times the particle radius.

Electric Field Detachment of Charged Particles

Electric field detachment of charged toner particles in the xerographic process is of central importance in the image development and transfer steps. The theoretical concepts for electric field detachment of single charged particles and particle layers are reviewed. Limitations on the applied force imposed by dielectric polarization and air electrical breakdown are considered. Measurements on the electric field detachment of toner layers in vacuum with different surface coverage are presented. The results for 21 µm toner suggest that the adhesion is described by an electrostatic image force model due to a nonuniform charge distribution on irregularly shaped particles.

ROLE OF PARTICLES AND DISPERSIONS IN ELECTROPHOTOGRAPHY

ABSTRACT The genesis of a 30 billion dollar a year electrophotographic industry can be traced to the sole effort of Chester Carlson who demonstrated in 1938 that dry images could be produced by a new process involving the attraction of charged pigmented particles to an electrostatic image formed on a photoconducting film. Carlsons invention, a particular form of electrophotography, is known as xerography. Most electrophotographic processes are based on the deposition of charged particles in either an air or liquid medium. The marking particles are usually pigments dispersed in a polymer matrix. The various electrophotographic processes are critically dependent on the electrical and mechanical properties of particles and dispersions. We examine the role of particles and dispersions in electrophotographic processes in general, and xerographic processes in particular.

Electrical Properties of Conductive Two-Component Xerographic Developer

Xerographic development with a conductive two-component magnetic brush depends on the electrical properties of a developer mixture consisting of insulative toner particles and conductive carrier beads. The electric field driving toner deposition onto an image receiver is affected by the carrier bead conductivity, toner concentration, magnetic field strength, developer thickness, and developer agitation due to the relative motion between the developer and image receiver. To elucidate the dependence of the electrical properties on these parameters, the measurements on an electroded cell of developer under conditions simulating magnetic brush development systems are described. The measurements provide insight into the solid area and line development process with conductive xerographic developer.