Toner Charge and Environmental Interactions with Toner Adhesion

Understanding charged particle adhesion forces is a critical step in the understanding and modeling of electrophotographic printing processes. Electrostatic and mechanical (Van der Waals) forces are both significant contributors to toner adhesion to substrates, and previous work has shown these to have roughly equivalent magnitudes in modern printer designs. Measuring distributions of toner adhesion as a function of multiple parameters including environmental and toner charge variation has revealed that there are additional interactions beyond Coulombic Attraction and dipole induced London-Van der Waals’ forces which are significant contributors to system performance. A model for toner adhesion is presented, including a term which describes the increased adhesion resulting from particle deformation at higher temperatures. Experimental results show good correlation to the model. Introduction There has been controversy as to the relative roles of Coulombic and dipole induced mechanical attraction in the development and transfer of toner [1]. Measurements of toner adhesion have shown that toner, not covered with surface additives to reduce adhesion (EPAs), were highly adhered to a variety of substrates, more varied in their adhesion and less dependent on toner charge [2,3]. The size of toner also has an impact on toner adhesion with smaller toner more critically impacted by non-charge dependent forces [1,4]. Understanding toner adhesion has been hindered by the difficulty in measuring a distribution of toner adhesion for actual printing systems. The recent development of a toner adhesion measurement tool based on an air jet to remove toner and optical measurements to quantify that removal distribution has allowed for a new level of testing of the factors contributing to toner adhesion [5]. The tool has been used to quantify toner adhesion in actual printing systems including on photoconductors and transfer belts, and under a variety of actual printing conditions. The initial results of this tool coupled with scanning electron microscope analysis of high and low adhesion toners confirmed that contact area, either reduced due to EPA loading or increased due to toner damage in transfer, had a significant impact on toner adhesion and therefore toner performance. Additionally, the range of adhesion forces for toner on a transfer belt in current electrophotographic printers ranged from less than 10nN to over 1000nN within one sample. Within those samples the toner mid to low adhesion forces were quite predictable when particle charge was known. Actual printing systems provide a variety of challenges to the development community as they are required to perform well under a wide range of environmental and other operating conditions. Measuring key parameters, such as toner adhesion, under actual printing conditions yields an understanding of actual process mechanisms. That understanding, in turn, should lead to better product design and performance. The Two-Term Model The controversy surrounding toner adhesion has centered on the assumption that there are two key mechanisms contributing to toner adhesion to any substrate. These two forces are the long range Coulomb attraction/repulsion forces and the close range dipole forces; also known as Van der Waals forces, London-Van der Waals forces or dispersion forces [1]. These forces act both between particles and between particles and substrates, and are complicated by the application of external fields for development and transfer and by the non-uniformity of charge on the toner [3,6,7] The first of these mechanisms is the long range Coulomb attraction/repulsion forces. The attraction force between two charged particles at a distance is known to be a function of the charges on the two particles and the distance between them. Known as Coulomb’s law or Coulomb’s inverse square law, it suggests that at a reasonable distance the force between two particles not on a substrate would be: F=ke (q1 q2)/r 2 (1)