Fixing belt, fixing device, and image forming apparatus

1. A fixing belt comprising:

an annular polyimide resin base layer having a lower imidization ratio in an outer peripheral surface than in a central portion in a thickness direction;

a metal layer provided on an outer peripheral surface of the polyimide resin base layer; and

an elastic layer provided on an outer peripheral surface of the metal layer,

wherein the annular polyimide base layer is obtained by:

forming an annular polyimide resin film, and

modifying an imidization ratio of an outer peripheral surface of the polyimide resin film to obtain the polyimide resin base layer;

wherein the step of modifying the imidization ratio of the outer peripheral surface of the polyimide resin film increases the number of polar groups present on the outer peripheral surface of the polyimide resin film; and

wherein the polar groups comprise amide groups and/or carboxyl groups.

2. The fixing belt according to claim 1 , wherein the imidization ratio in the outer peripheral surface of the polyimide resin base layer is 50% or more and 95% or less.

3. The fixing belt according to claim 2 , wherein the imidization ratio in the outer peripheral surface of the polyimide resin base layer is 60% or more and 90% or less.

4. The fixing belt according to claim 3 , wherein the imidization ratio in the outer peripheral surface of the polyimide resin base layer is 60% or more and 80% or less.

5. The fixing belt according to claim 1 , wherein an absolute value of a difference between the imidization ratio in the central portion and the imidization ratio in the outer peripheral surface in the thickness direction of the polyimide resin base layer is 5% or more and 50% or less.

6. A fixing belt comprising:

an annular polyimide resin base layer having a lower imidization ratio in an outer peripheral surface than in a central portion in a thickness direction;

a metal layer provided on an outer peripheral surface of the polyimide resin base layer; and

an elastic layer provided on an outer peripheral surface of the metal layer,

wherein the imidization ratio in the outer peripheral surface of the polyimide resin base layer is 60% or more and 80% or less, and

wherein the absolute value of a difference between the imidization ratio in the central portion and the imidization ratio in the outer peripheral surface in the thickness direction of the polyimide resin base layer is 10% or more and 40% or less.

7. The fixing belt according to claim 6 , wherein the absolute value of a difference between the imidization ratio in the central portion and the imidization ratio in the outer peripheral surface in the thickness direction of the polyimide resin base layer is 20% or more and 40% or less.

8. The fixing belt according to claim 1 , wherein the polyimide resin base layer is a base layer containing an aromatic polyimide resin.

9. The fixing belt according to claim 8 , wherein the aromatic polyimide resin is a polyimide resin having a structural unit represented by general formula (PI1) below,

(in the general formula, R P1 represents a phenyl group or a biphenyl group, and R P2 represents a divalent aromatic group).

10. The fixing belt according to claim 1 , wherein the metal layer has an underlaying metal layer provided on the outer peripheral surface of the polyimide resin base layer, a metal heat-generating layer provided on the outer peripheral surface of the underlaying metal layer, and a metal protection layer provided on the outer peripheral surface of the metal heat-generating layer.

11. A fixing device comprising:

the fixing belt according to claim 1 ;

a pressure member that presses an outer peripheral surface of the fixing belt; and

an electromagnetic induction device that allows at least a portion of a metal layer of the fixing belt to generate heat by electromagnetic induction,

wherein a recording medium with an unfixed toner image formed on a surface thereof is held between the fixing belt and the pressure member, thereby fixing the toner image to the recording medium.

12. An image forming apparatus comprising:

an image holding member;

a charging device that charges a surface of the image holding member;

an electrostatic latent image forming device that forms an electrostatic latent image on the charged surface of the image holding member;

a developing device that develops the electrostatic latent image formed on the surface of the image holding member with a toner to form a toner image;

a transfer device that transfers the toner image formed on the surface of the image holding member to a recording medium; and

the fixing device according to claim 11 that fixes the toner image to the recording medium.

13. The fixing belt according to claim 1 , wherein the modifying comprises treating the outer peripheral surface of the polyimide resin film with an alkali solution.

14. The fixing belt according to claim 13 , wherein the alkali solution has a pH in a range of from 12 to 13.2.

15. The fixing belt according to claim 1 , wherein the modifying comprises immersing the polyimide resin film in an alkali solution.

16. The fixing belt according to claim 1 , wherein the modifying comprises immersing an entirety of the polyimide resin film in an alkali solution.

17. The fixing belt according to claim 1 , wherein the modifying comprises treating the outer peripheral surface of the annular polyimide resin film with electron beams, ultraviolet light, an excimer laser, or plasma.

18. The fixing belt according to claim 1 , wherein the annular polyimide resin base layer has the lower imidization ratio over an entirety of the outer peripheral surface.

19. The fixing belt according to claim 1 , wherein the modifying lowers the imidization ratio of the outer peripheral surface of the polyimide resin film.

20. The fixing belt according to claim 1 , wherein the modifying lowers the imidization ratio of an entirety of the outer peripheral surface of the polyimide resin film.

21. The fixing belt according to claim 1 , wherein the metal layer is a metal plating layer.