Negatively chargeable spherical toner, color image forming apparatus, and process for producing negatively chargeable spherical toner

1. A negatively chargeable spherical toner comprising:

a toner mother particle comprising a binder resin and a colorant, which has: a number average particle size of from 4.5 to 9 μm; a particle size distribution that has an integrated value of particle sizes of 3 μm or less of 1% or less; and an average sphericity of from 0.95 to 0.99; and

an alumina fine particle externally added to the toner mother particle, which has a number average particle size of from 0.1 to 1.0 μm,

wherein a work function (Φ t ) of the toner mother particle is larger than a work function (Φ A ) of the alumina fine particle by at least 0.4 eV.

2. The negatively chargeable spherical toner according to claim 1 ,

wherein the work function (Φ t ) of the toner mother particle is from 5.2 to 5.8 eV; and the work function (Φ A ) of the alumina fine particle is from 4.8 to 5.3 eV.

3. The negatively chargeable spherical toner according to claim 1 ,

wherein the alumina fine particle is an α-type alumina fine particle.

4. The negatively chargeable spherical toner according to claim 1 ,

wherein the toner mother particle is obtained by a solution suspension method.

5. The negatively chargeable spherical toner according to claim 1 ,

wherein the toner mother particle has the colorant on a surface of the toner mother particle.

6. The negatively chargeable spherical toner according to claim 1 , which is a full-color toner.

7. A color image forming apparatus comprising:

negatively chargeable spherical toners;

a latent image carrier;

a plurality of developing units each for developing an electrostatic latent image by using the 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 color toner image; and

a recording material to which the color toner image is transferred and fixed,

wherein each of the negatively chargeable spherical toners comprises:

a toner mother particle comprising a binder resin and a colorant, which has; a number average particle size of from 4.5 to 9 μm; a particle size distribution that has an integrated value of particle sizes of 3 μm or less of 1% or less; and an average sphericity of from 0.95 to 0.99; and

an alumina fine particle externally added to the toner mother particle, which has a number average particle size of from 0.1 to 1.0 μm,

wherein a work function (Φ t ) of the toner mother particle is larger than a work function (Φ A ) of the alumina fine particle by at least 0.4 eV.

8. The color image forming apparatus according to claim 7 ,

wherein the developing units each develops the electrostatic latent image by a non-contact developing method.

9. The color image forming apparatus according to claim 7 ,

wherein the developing units each develops the electrostatic latent image by a 4-cycle type rotary developing method.

10. The color image forming apparatus according to claim 7 ,

wherein the developing units each develops the electrostatic latent image by a tandem type developing method.

11. A process for producing a negatively chargeable spherical toner, which comprises:

externally adding an alumina fine particle having a number average particle size of from 0.1 to 1.0 μm to a toner mother particle comprising a binder resin and a colorant, which has: a number average particle size of from 4.5 to 9 μm; a particle size distribution that has an integrated value of particle sizes of 3 μm or less of 1% or less; and an average sphericity of from 0.95 to 0.99 in a spherical mixing processing tank,

wherein a work function (Φ t ) of the toner mother particle is larger than a work function (Φ A ) of the alumina fine particle by at least 0.4 eV,

wherein the spherical mixing processing tank has:

a bottom having a horizontal disc-shape;

a rotary driving shaft vertically penetrating the center of the bottom;

a stirring blade which upwardly discharges materials including the alumina fine particle and the toner mother particle so that the materials are treated spirally along an inner wall of the spherical mixing processing tank; and

a cylinder-shaped member vertically penetrating a top of the spherical mixing processing tank, which is on an extension of the rotary driving shaft and is arranged so that an edge thereof is located within the spherical mixing processing tank,

wherein the materials which is upwardly discharged are transferred to the top of the spherical mixing processing tank spirally by a rotation of the stirring blade, and are lowered the kinetic energy threreof, and are resupplied to the stirring blade.

12. The process for producing a negatively chargeable spherical toner according to claim 11 ,

wherein the work function (Φ t ) of the toner mother particle is from 5.2 to 5.8 eV; and the work function (Φ A ) of the alumina fine particle is from 4.8 to 5.3 eV.

13. The process for producing a negatively chargeable spherical toner according to claim 11 ,

wherein the toner mother particle is obtained by a solution suspension method.

14. The process for producing a negatively chargeable spherical toner according to claim 11 ,

wherein the toner mother particle has the colorant on a surface of the toner mother particle.

15. The process for producing a negatively chargeable spherical toner according to claim 11 ,

which further comprising externally adding at least one of a hydrophobilized silica and a hydrophobilized titania.

16. The process for producing a negatively chargeable spherical toner according to claim 15 ,

which further comprising externally adding, after externally adding at least one of a hydrophobilized silica and a hydrophobilized titania, a metal soap particle and a fine particle having a polarity reverse to that of the toner mother particle.