Non-magnetic monocomponent negatively chargeable spherical toner and full color image forming apparatus using the same

A full color image forming apparatus comprising:

• non-magnetic monocomponent negatively chargeable spherical toners;
• a latent image carrier;
• a plurality of developing units each for developing an electrostatic latent image, without contacting the latent image carrier, by using the non-magnetic monocomponent negatively chargeable spherical toners so as to form toner images sequentially on the latent image carrier;
• an intermediate transfer medium to which the toner images are transferred sequentially so as to form a full color toner image;
• a recording material to which the full color toner image is transferred and fixed,
• wherein each of the non-magnetic monocomponent negatively chargeable spherical toners comprising:
• a toner mother particle comprising a binder resin and a colorant;
• and
• an external additive comprising a hydrophobic inorganic fine particle having a number average primary particle size of 7 to 50 nm and a hydrophobic monodisperse spherical silica particle having a number average primary particle size of 70 to 130 nm,
• wherein each of the non-magnetic monocomponent negatively chargeable spherical toners has a mechanical strength of from 7 to 19 MPa, provided that the mechanical strength is determined from a 10% displacement load of a compression-displacement curve obtained in a microcompression test,
• wherein the hydrophobic monodisperse spherical silica particle has a work function (ΦS) smaller than a work function (ΦTB) of the toner mother particle, and the intermediate transfer medium has a work function (ΦTM) smaller than that of a work function (ΦT) of each of the non-magnetic monocomponent negatively chargeable spherical toners.