Electrophotographic photoreceptor, image forming apparatus and process cartridge for image forming apparatus

1. An electrophotographic photoreceptor, comprising:

an electroconductive substrate;

a photosensitive layer, overlying the substrate; and

a cured protection layer, overlying the photosensitive layer,

wherein the cured protection layer comprises a cured material of a tri- or more functional radical polymerizable compound and a filler exposed from the surface of the cured protection layer which comprises a bump along the surface of the filler, and

wherein the cured protection layer has a thickness (T) larger than a diameter (2r) of the filler therein and the following relationships (a) is satisfied:

(the number of the fillers present to a depth of T/ 2 from a free surface of the cured protection layer/the total number of the fillers in the cured protection layer)×100≧7%  (a).

2. The electrophotographic photoreceptor of claim 1 , wherein the bump satisfies the following relationship:

0.015 ×r/ 2 ≦h 1≦0.5 ×h 2  (b)

wherein r is a particle diameter of the filler, h 1 is a maximum height of the bump covering the filler from the bottom of the bump, and h 2 is a distance from the bottom thereof to the highest point of the exposed part of the filler.

3. The electrophotographic photoreceptor of claim 1 , wherein the cured protection layer satisfies the following relationship:

0.10 ≦S 1/( S 1 +S 2)≦0.50  (c)

wherein S 1 is a projection area of the filler having a part which is not covered by the cured protection layer and S 2 is a projection area of a part where the filler is not present.

4. The electrophotographic photoreceptor of claim 1 , wherein the cured protection layer further comprises a filler covered thereby.

5. The electrophotographic photoreceptor of claim 4 , wherein the filler covered by the cured protection layer is an alumina filler.

6. The electrophotographic photoreceptor of claim 1 , wherein the filler exposed from the surface of the cured protection layer comprises a first filler and a second filler formed of a material different from that of the first filler.

7. The electrophotographic photoreceptor of claim 6 , wherein the first filler is an organic filler comprising a silicon atom or a fluorine atom, and the second filler is a metal oxide filler.

8. The electrophotographic photoreceptor of claim 7 , wherein the cured protection layer satisfies the following relationship:

0.3 ≦Sa /( Sa+Sb )≦0.7  (d)

wherein Sa is a projection area of the organic filler comprising a silicon or a fluorine atom and Sb is a projection area of at least one metal oxide filler when the cured protection layer is projected from above.

9. The electrophotographic photoreceptor of claim 1 , wherein the cured protection layer further comprises a cured material of a radical polymerizable compound having a charge transport structure and a tri- or more radical polymerizable compound.

10. The electrophotographic photoreceptor of claim 1 , wherein the cured protection layer is formed by applying a cured protection layer coating liquid, coating a filler, and curing the liquid.

11. A method of preparing the electrophotographic photoreceptor of claim 1 , comprising:

forming a photosensitive layer on an electroconductive substrate;

applying a cured protection layer coating liquid on the photosensitive layer;

coating a filler on the cured protection layer; and

curing the cured protection layer coating liquid.

12. An image forming apparatus, comprising:

the electrophotographic photoreceptor according to claim 1 ;

a charger configured to charge the electrophotographic photoreceptor;

an irradiator configured to irradiate the electrophotographic photoreceptor to form an electrostatic latent image thereon;

an image developer configured to develop the electrostatic latent image with a toner to form a toner image; and

a transferer configured to transfer the toner image onto a transfer paper.

13. A process cartridge for image forming apparatus, comprising:

the electrophotographic photoreceptor according to claim 1 ; and

at least one of a charger, an irradiator, an image developer, a transferer and a cleaner.