The present invention relates to an improved carrier particle for use with a toner in an electrostatic copying process. Such processes are now commonly used by laser printers and photocopy machines.
Electrostatic processes typically use developers that have two components: toner particles and carrier particles. The carrier particles impart a triboelectric charge to the toner particles with a proper polarity and magnitude to insure that the toner particles are preferentially attracted to desired image areas on a latent image field. The magnitude of the triboelectric charge is important. If the charge is too low, the attractive force between the carrier particles and the toner particles will be too weak, resulting in "background," that is, the transfer of too much toner from the carrier. If the charge is too high, not enough toner is transferred from the carrier, resulting in low print density.
Additionally, it is important for the carrier particles to have low surface energy. Low surface energy makes it difficult for the toner particles to permanently adhere to the carrier particles. Permanent adhesion of toner particles to carrier particles impairs the normal triboelectric charging of the remaining toner particles, resulting in decreased output quality and shortened developer life.
Therefore, it is desirable for carrier particles to have a strong triboelectric charge so that toner particles can be attracted and deposited in sufficient quantities to achieve high print density while at the same time resisting the permanent adhesion of toner particles so that developer life is increased and output quality remains stable and good over the life of the developer.
The present invention provides the aforementioned desirable characteristics while avoiding the undesirable characteristics of prior art carrier particles.