Developer, development device, image forming device and method of forming developer

1. A negatively chargeable developer, comprising:

negatively chargeable toner mother particles including at least binding resin and colorant;

a first external additive having positive chargeability that is externally added to a surface of the toner mother particles, the first external additive including polymethyl methacrylate added in a range from approximately 0.4 parts by weight to approximately 0.8 parts by weight inclusive per 100 parts by weight of the toner mother particles; and

a second external additive that is externally added to the surface of the toner mother particles, the second external additive being a member selected from the group consisting of silica, oxidized titanium and mixtures thereof and being added in a range from approximately 2.2 parts by weight to approximately 5.0 parts by weight inclusive per 100 parts by weight of the toner mother particles.

2. The developer of claim 1 , wherein

the toner mother particles have a circularity degree within a range from approximately 0.94 to approximately 0.97 inclusive that is calculated by dividing a circumferential length of a circle having the same area as an area of a projected image of a particle, by the length of the circumferential length of the projected image of the particle.

3. The developer of claim 1 , wherein

the toner mother particles are pulverized toner mother particles, and

the toner mother particles have a circularity degree within a range from approximately 0.92 to approximately 0.97 inclusive that is calculated by dividing a circumferential length of a circle having the same area as an area of a projected image of a particle by the length of the circumferential length of the projected image of the particle.

4. The developer of claim 1 , wherein

the toner mother particles are emulsion-polymerized toner mother particles, and

the toner mother particles have a circularity degree within a range from approximately 0.94 to approximately 0.98 inclusive that is calculated by dividing a circumferential length of a circle having the same area as an area of a projected image of a particle by the length of the circumferential length of the projected image of the particle.

5. A development device that develops an image using the developer of claim 1 .

6. An image forming device, comprising:

the development device of claim 5 .

7. The developer of claim 1 , wherein the polymethyl methacrylate has an average particle size of 0.15-2.0 μm.

8. The developer of claim 1 , wherein the second additive is only silica that is added in a range from approximately 2.2 parts by weight to approximately 5.0 parts by weight inclusive per 100 parts by weight of the toner mother particles.

9. The developer of claim 1 , wherein the second additive is a mixture of silica and oxidized titanium, the silica is in a range from approximately 2.0 parts by weight to approximately 4.5 parts by weight inclusive per 100 parts by weight of the toner mother particles, and the oxidized titanium is in a range from approximately 0.2 parts by weight to approximately 0.5 parts by weight inclusive per 100 parts by weight of the toner mother particles.

10. A method of forming a negatively chargeable developer, comprising:

producing negatively chargeable toner mother particles including at least binding resin and colorant; and

externally adding first and second external additives to a surface of the toner mother particles, wherein

the first external additive includes polymethyl methacrylate that is added within a range from approximately 0.4 parts by weight to approximately 0.8 parts by weight inclusive per 100 parts by weight of the toner mother particles and that has positive chargeability, and

the second external additive includes a member selected from the group consisting of silica, oxidized titanium and a mixture thereof that is added within a range from approximately 2.2 parts by weight to approximately 5.0 parts by weight inclusive per 100 parts by weight of the toner mother particles.

11. The method of claim 10 , wherein

the toner mother particles have a circularity degree within a range from approximately 0.94 to approximately 0.97 inclusive that is calculated by dividing a circumferential length of a circle having the same area as an area of a projected image of a particle, by the length of the circumferential length of the projected image of the particle.

12. The method of claim 10 , wherein

producing the toner mother particles comprises producing the toner mother particles by a pulverization method, and

the toner mother particles have a circularity degree within a range from approximately 0.92 to approximately 0.97 inclusive that is calculated by dividing a circumferential length of a circle having the same area as an area of a projected image of a particle by the length of the circumferential length of the projected image of the particle.

13. The method of claim 12 , wherein

the pulverization method includes:

mixing, melting and kneading the binding resin and the colorant to form a mixture,

cracking the mixture, and

pulverization the cracked mixture.

14. The method of claim 10 , wherein

producing the toner mother particles comprises producing the toner mother particles by an emulsion polymerization method, and

the toner mother particles have a circularity degree within a range from approximately 0.94 to approximately 0.98 inclusive that is calculated by dividing a circumferential length of a circle having the same area as an area of a projected image of a particle by the length of the circumferential length of the projected image of the particle.

15. The method of claim 14 , wherein the emulsion polymerization method includes:

forming the binding resin in a water solvent,

mixing emulsified colorant in the water solvent,

agglomerating the mixture, and

drying the agglomerated mixture.

16. The method of claim 10 , wherein the polymethyl methacrylate has an average particle size of 0.15-2.0 μm.

17. The method of claim 10 , wherein the second additive is only silica that is added in a range from approximately 2.2 parts by weight to approximately 5.0 parts by weight inclusive per 100 parts by weight of the toner mother particles.

18. The method of claim 10 , wherein the second additive is a mixture of silica and oxidized titanium, the silica is in a range from approximately 2.0 parts by weight to approximately 4.5 parts by weight inclusive per 100 parts by weight of the toner mother particles, and the oxidized titanium is in a range from approximately 0.2 parts by weight to approximately 0.5 parts by weight inclusive per 100 parts by weight of the toner mother particles.