Two-component, dry electrographic developer compositions containing hard magnetic carrier particles and method for using the same

An electrographic, two-component dry developer composition comprising charged toner particles and oppositely charged binder-free carrier particles which (a) comprise a hard magnetic material exhibiting a coercivity of at least 300 gauss when magnetically saturated and (b) exhibit an induced magnetic moment of at least 20 EMU/gm of carrier when in an applied field of 1000 gauss.

An electrographic, two-component dry developer composition comprising charged toner particles and oppositely charged composite carrier particles which (a) comprise a binder and a plurality of magnetic particles dispersed in said binder composed of a hard magnetic material exhibiting a coercivity of at least 300 gauss when magnetically saturated and (b) exhibit an induced magnetic moment of at least 20 EMU/gm of carrier when in an applied field of 1000 gauss.

A method for developing an electrostatic image comprising contacting the image with at least one magnetic brush comprising (a) a rotating magnetic core of a preselected magnetic field strength, (b) an outer nonmagnetic shell and (c) an electrographic, two-component dry developer composition comprising charged toner particles and oppositely charged carrier particles which (i) comprise a hard magnetic material exhibiting a coercivity of at least 300 gauss when magnetically saturated and (ii) exhibit an induced magnetic moment of at least 20 EMU/gm when in an externally applied field of 1000 gauss, and which magnetic moment is sufficient to prevent said carrier from transferring to said electrostatic image.

A method for developing an electrostatic image comprising contacting the image with at least one magnetic brush comprising (a) a rotating magnetic core of a preselected magnetic field strength, (b) an outer nonmagnetic shell and (c) an electrographic, two-component dry developer composition comprising charged toner particles and oppositely charged binder-free hard ferrite carrier particles exhibiting (i) a coercivity of at least 500 gauss when magnetically saturated sufficient to cause said developer to flow circumferentially on said shell in a direction opposite the direction of magnetic core rotation and (ii) an induced magnetic moment of at least 25 EMU/gm when in an externally applied field of 1000 gauss, and which magnetic moment is sufficient to prevent said carrier from transferring to said electrostatic image, said toner and carrier particles in said developer having a triboelectric force of attraction which is greater than the magnetic force of attraction between carrier particles in said developer.

A method for developing an electrostatic image comprising contacting the image with at least one magnetic brush comprising (a) a rotating magnetic core which rotates at a speed from about 1000 to about 3000 revolutions per minute and has a preselected magnetic field strength (b) an outer non-magnetic shell and (c) an electrographic, two-component dry developer composition comprising charged toner particles and oppositely charged carrier particles which (i) comprise a hard magnetic material exhibiting a coercivity of at least 300 gauss when magnetically saturated and (ii) exhibit an induced magnetic moment of at least 20 EMU/gm when in an externally applied field of 1000 gauss, and which magnetic moment is sufficient to prevent said carrier from transferring to said electrostatic image.