Enhanced magnetic moment of monocomponent toner particles by surface coating

Because of advantages in machine simplicity and consistent image quality, dry monocomponent toners will play an increasingly important role as consumables for nonimpact electrostatic and magnetic imaging systems. Most currently available monocomponent toners have the ferromagnetic component, generally magnetite, present as uniformly dispersed particles in a matrix of thermoplastic resin. It is shown that, by incorporating the ferromagnetic material as a surface coating, the same magnetic moment can be obtained at given field strength with a reduced proportion of ferromagnetic material. With the ferromagnetic material in particulate form, the enhancement in magnetic moment over the case of a uniform distribution is relatively modest. For example, if magnetite particles are incorporated in a thin, dense outer shell, a toner particle with 40 wt-pct magnetite develops the same magnetic moment for applied fields up to 500 oersteds as a toner particle with 60 wt-pct magnetite uniformly distributed. A higher degree of magnetic moment enhancement requires applying a continuous outer shell of ferromagnetic material.