Toner for developing electrostatic image, two-component developer and image forming apparatus

1. A toner for developing an electrostatic image, comprising:

toner base particles each comprising a binder resin and a releasing agent; and

inorganic fine particles,

wherein the toner comprises the inorganic fine particles as an external additive on a surface of the toner base particle,

wherein the toner base particles have a BET specific surface area of 2.5 m 2 /g to 5.0 m 2 /g, and

wherein the inorganic fine particles comprise inorganic fine silica particles (A) which are each a secondary particle where a plurality of primary particles are coalesced together by chemically bonding,

wherein the inorganic fine silica particles (A) have Db 50 /Db 10 of 1.20 or less, where Db 50 is a particle diameter at which a cumulative percentage of a secondary particle diameter Db of the inorganic fine silica particles (A) measured from a side of smaller particles is 50% by number, and Db 10 is a particle diameter at which the cumulative percentage measured from the side of smaller particles is 10% by number.

2. The toner for developing an electrostatic image according to claim 1 ,

wherein the BET specific surface area of the toner base particles is 3.5 m 2 /g to 5.0 m 2 /g.

3. The toner for developing an electrostatic image according to claim 1 ,

wherein the inorganic fine particles (A) have an average of degrees of coalescence G of 1.5 to 4.0, where each of the degrees of coalescence is defined as a ratio (Db/Da) with Db being the secondary particle diameter of the inorganic fine particles (A) and Da being an average primary particle diameter of the plurality of primary particles forming the inorganic fine particles (A).

4. The toner for developing an electrostatic image according to claim 1 ,

wherein a content of the inorganic fine particles (A) having the degree of coalescence G of less than 1.3 in the inorganic fine particles (A) is 10% by number or less.

5. The toner for developing an electrostatic image according to claim 1 ,

wherein the inorganic fine particles (A) have an average secondary particle diameter Dba of 80 nm to 200 nm.

6. The toner for developing an electrostatic image according to claim 1 ,

wherein the toner is granulated in an aqueous medium.

7. The toner for developing an electrostatic image according to claim 1 ,

wherein the toner is obtained by: dispersing an oil phase in an aqueous medium to prepare an emulsified dispersion, the oil phase being obtained by dissolving or dispersing, in an organic solvent, toner materials comprising a polyester resin, a colorant and a releasing agent; and removing the organic solvent from the emulsified dispersion.

8. A two-component developer, comprising:

a toner for developing an electrostatic image, and

a carrier,

wherein the toner comprises:

toner base particles each comprising a binder resin and a releasing agent, and

inorganic fine particles,

wherein the toner comprises the inorganic fine particles as an external additive on a surface of the toner base particle,

wherein the toner base particles have a BET specific surface area of 2.5 m 2 /g to 5.0 m 2 /g, and

wherein the inorganic fine particles comprise inorganic fine silica particles (A) which are each a secondary particle where a plurality of primary particles are coalesced together by chemically bonding,

wherein the inorganic fine silica particles (A) have Db 50 /Db 10 of 1.20 or less, where Db 50 is a particle diameter at which a cumulative percentage of a secondary particle diameter Db of the inorganic fine silica particles (A) measured from a side of smaller particles is 50% by number, and Db 10 is a particle diameter at which the cumulative percentage measured from the side of smaller particles is 10% by number.

9. An image forming apparatus, comprising:

an electrostatic latent image bearing member;

an electrostatic latent image forming unit which forms an electrostatic latent image on the electrostatic latent image bearing member;

a developing unit which comprises a toner for developing an electrostatic image and which forms a visible image by developing the electrostatic latent image with the toner;

a transfer unit which transfers the visible image on the electrostatic latent image bearing member to a recording medium;

a fixing unit which fixes the visible image transferred on the recording medium; and

a cleaning unit which removes the toner remaining on the image bearing member,

wherein the toner comprises:

toner base particles each comprising a binder resin and a releasing agent, and

inorganic fine particles,

wherein the toner comprises the inorganic fine particles as an external additive on a surface of the toner base particle,

wherein the toner base particles have a BET specific surface area of 2.5 m 2 /g to 5.0 m 2 /g, and

wherein the inorganic fine particles comprise inorganic fine silica particles (A) which are each a secondary particle where a plurality of primary particles are coalesced together by chemically bonding,

wherein the inorganic fine silica particles (A) have Db 50 /Db 10 of 1.20 or less, where Db 50 is a particle diameter at which a cumulative percentage of a secondary particle diameter Db of the inorganic fine silica particles (A) measured from a side of smaller particles is 50% by number, and Db 10 is a particle diameter at which the cumulative percentage measured from the side of smaller particles is 10% by number.

10. The toner for developing an electrostatic image according to claim 1 , wherein the inorganic fine silica particles (A) are a plurality of primary particles coalesced together by chemically bonding using a silane-based treating agent or an epoxy-based treating agent.