Image forming apparatus Number:6,800,410 from the United States Patent and Trademark Office (PTO) owispatent An image forming apparatus has at least a photoconductor, a charger, and an irradiator for irradiating a light for optically writing in on the photoconductor to form a latent electrostatic image using a process for electrophotography in which a resolution of the optical writing in operation is 1200 dpi or more. Further, the optical writing operation is performed using a laser beam having a diameter of 35 .mu.m or less. The photoconductor is provided with at least a charge generating layer containing a charge generating substance and a charge transporting layer containing a charge transporting substance. The charge transporting layer has a carrier mobility of 1.times.10.sup.-5 cm.sup.2.multidot.V.sup.-1.multidot.sec.sup.-1 or more under an electric field of 3.times.10.sup.5 V.multidot.cm.sup.-1.<H apparatus, forming, Image, forming, a, 6800410.html, Patent, pto, 6800410

Title: Image forming apparatus

Abstract: An image forming apparatus has at least a photoconductor, a charger, and an irradiator for irradiating a light for optically writing in on the photoconductor to form a latent electrostatic image using a process for electrophotography in which a resolution of the optical writing in operation is 1200 dpi or more. Further, the optical writing operation is performed using a laser beam having a diameter of 35 .mu.m or less. The photoconductor is provided with at least a charge generating layer containing a charge generating substance and a charge transporting layer containing a charge transporting substance. The charge transporting layer has a carrier mobility of 1.times.10.sup.-5 cm.sup.2.multidot.V.sup.-1.multidot.sec.sup.-1 or more under an electric field of 3.times.10.sup.5 V.multidot.cm.sup.-1.

Patent Number: 6,800,410 Issued on 10/05/2004 to Yasutomi, et al.

Inventors: Yasutomi; Kei (Yokohama, JP), Suzuki; Yasuo (Fuji, JP), Tamoto; Nozomu (Numazu, JP)
Assignee: Ricoh Company, Ltd. (Tokyo, JP)

Appl. No.: 10/260,275

Filed: October 1, 2002

Foreign Application Priority Data


Oct 02, 2001 [JP]			2001-306801
Nov 05, 2001 [JP]			2001-340055

Current U.S. Class: 430/35 ; 399/320; 430/55; 430/58.05

Field of Search: 430/35,55,58.05 399/320

References Cited [Referenced By]

U.S. Patent Documents


5028502	July 1991	Yuh et al.
5100453	March 1992	Richards
5286588	February 1994	Suzuki
5561016	October 1996	Suzuki et al.
5578405	November 1996	Ikegami et al.
5665500	September 1997	Suzuki
5677096	October 1997	Suzuki
5789128	August 1998	Adachi et al.
5834145	November 1998	Yoshinaga et al.
5846680	December 1998	Adachi et al.
5910561	June 1999	Adachi et al.
5928828	July 1999	Suzuki
5942363	August 1999	Tanaka et al.
5999773	December 1999	Yasutomi et al.
6026262	February 2000	Kinoshita et al.
6030736	February 2000	Ikegami et al.
6069224	May 2000	Adachi et al.
6136483	October 2000	Suzuki et al.
6191249	February 2001	Tanaka et al.
6249304	June 2001	Sawayama et al.
6322940	November 2001	Nishigaki et al.
6326112	December 2001	Tamura et al.
6366751	April 2002	Shakuto et al.
6432596	August 2002	Ikuno et al.
6444387	September 2002	Ri et al.
6492079	December 2002	Shimada et al.

Foreign Patent Documents


7-181705	Jul., 1995	JP
8-62862	Mar., 1996	JP
9-304954	Nov., 1997	JP
9-319164	Dec., 1997	JP
11-95462	Apr., 1999	JP

Primary Examiner: Chapman; Mark A.
Attorney, Agent or Firm: Oblon, Spivak, McClelland, Maier & Neustadt, P.C.

Claims

What is claimed is:

1. An image forming apparatus comprising: a photoconductor which comprises a charge generating layer containing a charge generating substance and a charge transporting layer containing a charge transporting substance; and a charger for charging the photoconductor; an irradiator for irradiating a laser beam having a diameter of 35 .mu.m or less to the photoconductor for optically writing in with a resolution of 1200 dpi or more to form a latent electrostatic image; wherein the charge transporting layer has a carrier mobility of 1.times.10.sup.-5 cm.sup.2.multidot.V.sup.1.multidot.sec.sup.-1 or more under an electric field of 3.times.10.sup.5 V.multidot.cm.sup.-1.

2. The image forming apparatus according to claim 1, wherein the charge transporting layer contains a triarylamine structure.

3. The image forming apparatus according to claim 2, wherein the compound having the triarylamine structure is a compound expressed by the following structural formula (A-I): ##STR661## (where R.sub.1, R.sub.3, and R.sub.4 may be the same or different and each independently represents a hydrogen atom, an amino group, an alkoxy group, a thioalkoxy group, an aryloxy group, a methylenedioxy group, a substituted or unsubstituted alkyl group, a halogen atom, or a substituted or unsubstituted aryl group; R.sub.2 represents a hydrogen atom, an alkoxy group, a substituted or unsubstituted alkyl group or halogen, except for a combination in which each of R.sub.1, R.sub.2, R.sub.3, and R.sub.4 is a hydrogen atom; and k, l, m, and n are each independently 1, 2, 3, or 4).

4. The image forming apparatus according to claim 2, wherein the compound having the triarylamine structure is a compound expressed by the following structural formula (A-II): ##STR662## (where Ar.sub.1 and Ar.sub.2 may be the same or different and each independently represents a substituted or unsubstituted aryl group or a substituted or unsubstituted heterocyclic group; R.sub.6, R.sub.7, and R.sub.5 may be the same or different and each independently represents a hydrogen atom, a substituted or unsubstituted alkyl group, a substituted or unsubstituted alkoxy group, a substituted or unsubstituted aryl group, or a substituted or unsubstituted heterocyclic group, of which R.sub.7 and R.sub.6 may be combined to form a ring; and Ar.sub.3 represents a substituted or unsubstituted allylene group).

5. The image forming apparatus according to claim 2, wherein the compound having the triarylamine structure is a compound expressed by the following structural formula (A-III): ##STR663## (where R.sub.10, R.sub.11, and R.sub.12 may be the same or different and each independently represents a hydrogen atom, a halogen atom, a nitro group, a substituted or unsubstituted alkyl group, a substituted or unsubstituted alkoxy group, or a substituted or unsubstituted aryl group; R.sub.8 and R.sub.9 may be the same or different and each independently represents a hydrogen atom, an alkoxycarbonyl group, a substituted or unsubstituted alkyl group, or a substituted or unsubstituted aryl group; W represents a hydrogen atom, a substituted or unsubstituted alkyl group, a phenylthio group, a divalent chain unsaturated hydrocarbon group, a monovalent or divalent and substituted or unsubstituted carbocyclic aromatic group, or a monovalent or divalent and substituted or unsubstituted heterocyclic group; j represents an integer of 1 to 5; f represents an integer of 1 to 4; g represents an integer of 1 or 2; h represents an integer of 1 or 2; and i represents an integer of 1 to 3).

6. The image forming apparatus according to claim 2, wherein the compound having the triarylamine structure is a compound expressed by the following structural formula (A-IV): ##STR664## (where Ar.sub.4 represents a condensed polycyclic hydrocarbon group having 18 or less carbon atoms; and R.sub.13 and R.sub.14 may be the same or different and each independently represents a hydrogen atom, a halogen atom, a substituted or unsubstituted alkyl group, an alkoxy group, or a substituted or unsubstituted phenyl group).

7. The image forming apparatus according to claim 2, wherein the compound having the triarylamine structure is a compound expressed by the following structural formula (A-V): ##STR665## ##STR666## (where R.sub.15 and R.sub.16 may be the same or different and each independently represents a lower alkyl group, a lower alkoxy group, or a halogen atom; p and q each independently represents an integer of 1 to 4; and R.sub.1 and R.sub.18 may be the same or different and each independently represents a hydrogen atom, a lower alkyl group, a lower alkoxy group, or a halogen atom).

8. The image forming apparatus according to claim 2, wherein the compound having the triarylamine structure is a compound expressed by the following structural formula (A-VI): ##STR667## (where R.sub.19, R.sub.20, R.sub.21, and R.sub.22 may be the same or different and each independently represents a hydrogen atom, an alkyl group which may have a substituent, an alkoxy group, an allyl group, an aryl group, or a halogen atom; and R.sub.23 and R.sub.24 may be the same or different and each independently represents a hydrogen atom, an alkyl group, an alkoxy group, a halogen atom, an amino group, an N-substituted amino group, an allyl group, or an aryl group).

9. The image forming apparatus according to claim 1, wherein a content of the charge transporting substance in the charge transporting layer is 40% by weight or more with respect to a total amount of the charge transporting layer.

10. The image forming apparatus according to claim 1, wherein a film thickness of the charge transporting layer is 20 .mu.m or less.

11. The image forming apparatus according to claim 1, wherein the photoconductor has a protection layer.

12. The image forming apparatus according to claim 11, wherein a transmittance of the protection layer with respect to the laser beam is 90% or more and a total film thickness of the charge transporting layer and the protection layer is 20 .mu.m or less.

13. An image forming apparatus comprising: a photoconductor which comprises a charge generating layer containing a charge generating substance and a charge transporting layer containing a charge transporting substance; a charger for charging the photoconductor; an image processor for performing a halftoning operation with respect to an input image; and an irradiator for irradiating a laser beam having a diameter of 35 .mu.m or less to the photoconductor for optically writing in to form a latent electrostatic image based on an image data obtained by the halftoning operation using a number of lines of 200 lpi or more with respect to the input image; wherein the charge transporting layer has a carrier mobility of 1.times.10.sup.-5 cm.sup.2.multidot.V.sup.-1.multidot.sec.sup.-1 or more under an electric field of 3.times.10.sup.5 V.multidot.cm.sup.-1, and the optical writing is based on.

14. The image forming apparatus according to claim 13, wherein a resolution of the optical writing is 1200 dpi or more.

15. The image forming apparatus according to claim 13, wherein the charge transporting layer contains a triarylamine structure.

16. The image forming apparatus according to claim 15, wherein the compound having the triarylamine structure is a compound expressed by the following structural formula (A-I): ##STR668## (where R.sub.1, R.sub.3, and R.sub.4 may be the same or different and each independently represents a hydrogen atom, an amino group, an alkoxy group, a thioalkoxy group, an aryloxy group, a methylenedioxy group, a substituted or unsubstituted alkyl group, a halogen atom, or a substituted or unsubstituted aryl group; R.sub.2 represents a hydrogen atom, an alkoxy group, a substituted or unsubstituted alkyl group or halogen, except for a combination in which each of R.sub.1, R.sub.2, R.sub.3, and R.sub.4 is a hydrogen atom; and k, l, m, and n are each independently 1, 2, 3, or 4).

17. The image forming apparatus according to claim 15, wherein the compound having the triarylamine structure is a compound expressed by the following structural formula (A-II): ##STR669## (where Ar.sub.1 and Ar.sub.2 may be the same or different and each independently represents a substituted or unsubstituted aryl group or a substituted or unsubstituted heterocyclic group; R.sub.6, R.sub.7, and R.sub.5 may be the same or different and each independently represents a hydrogen atom, a substituted or unsubstituted alkyl group, a substituted or unsubstituted alkoxy group, a substituted or unsubstituted aryl group, or a substituted or unsubstituted heterocyclic group, of which R.sub.7 and R.sub.6 may be combined to form a ring; and Ar.sub.3 represents a substituted or unsubstituted allylene group).

18. The image forming apparatus according to claim 15, wherein the compound having the triarylamine structure is a compound expressed by the following structural formula (A-III): ##STR670## (where R.sub.10, R.sub.11, and R.sub.12 may be the same or different and each independently represents a hydrogen atom, a halogen atom, a nitro group, a substituted or unsubstituted alkyl group, a substituted or unsubstituted alkoxy group, or a substituted or unsubstituted aryl group; R.sub.8 and R.sub.9 may be the same or different and each independently represents a hydrogen atom, an alkoxycarbonyl group, a substituted or unsubstituted alkyl group, or a substituted or unsubstituted aryl group; W represents a hydrogen atom, a substituted or unsubstituted alkyl group, a phenylthio group, a divalent chain unsaturated hydrocarbon group, a monovalent or divalent and substituted or unsubstituted carbocyclic aromatic group, or a monovalent or divalent and substituted or unsubstituted heterocyclic group; j represents an integer of 1 to 5; f represents an integer of 1 to 4; g represents an integer of 1 or 2; h represents an integer of 1 or 2; and i represents an integer of 1 to 3).

19. The image forming apparatus according to claim 15, wherein the compound having the triarylamine structure is a compound expressed by the following structural formula (A-IV): ##STR671## (where Ar.sub.4 represents a condensed polycyclic hydrocarbon group having 18 or less carbon atoms; and R.sub.13 and R.sub.14 may be the same or different and each independently represents a hydrogen atom, a halogen atom, a substituted or unsubstituted alkyl group, an alkoxy group, or a substituted or unsubstituted phenyl group).

20. The image forming apparatus according to claim 15, wherein the compound having the triarylamine structure is a compound expressed by the following structural formula (A-V): ##STR672## ##STR673## (where R.sub.15 and R.sub.16 may be the same or different and each independently represents a lower alkyl group, a lower alkoxy group, or a halogen atom; p and q each independently represents an integer of 1 to 4; and R.sub.17 and R.sub.18 may be the same or different and each independently represents a hydrogen atom, a lower alkyl group, a lower alkoxy group, or a halogen atom).

21. The image forming apparatus according to claim 15, wherein the compound having the triarylamine structure is a compound expressed by the following structural formula (A-VI): ##STR674## (where R.sub.19, R.sub.20, R.sub.21, and R.sub.22 may be the same or different and each independently represents a hydrogen atom, an alkyl group which may have a substituent, an alkoxy group, an allyl group, an aryl group, or a halogen atom; and R.sub.23 and R.sub.24 may be the same or different and each independently represents a hydrogen atom, an alkyl group, an alkoxy group, a halogen atom, an amino group, an N-substituted amino group, an allyl group, or an aryl group).

22. The image forming apparatus according to claim 13, wherein a content of the charge transporting substance in the charge transporting layer is 40% by weight or more with respect to a total amount of the charge transporting layer.

23. The image forming apparatus according to claim 13, wherein a film thickness of the charge transporting layer is 20 .mu.m or less.

24. The image forming apparatus according to claim 13, wherein the photoconductor comprises a protection layer.

25. The image forming apparatus according to claim 24, wherein a transmittance of the protection layer with respect to the laser beam is 90% or more and a total film thickness of the charge transporting layer and the protection layer is 20 .mu.m or less.

26. An image forming apparatus comprising: a photoconductor which comprises: a charge generating layer containing a charge generating substance; a charge transporting layer containing a charge transporting substance; and a protection layer disposed closer to a surface of the photoconductor than the charge generating layer and the charge transporting layer and having a transmittance of 90% or more with respect to the laser beam; the charge transporting layer and the protection layer has a total thickness of 20 .mu.m or less; a charger for charging the photoconductor; and an irradiator for irradiating a laser beam having a diameter of 35 .mu.m or less to the photoconductor for optically writing in with a resolution of 1200 dpi or more to form a latent electrostatic image.

27. The image forming apparatus according to claim 26, wherein the protection layer contains at least one of a filler, a charge transporting substance, and a binder resin.

28. The image forming apparatus according to claim 27, wherein the filler has a refractive index in a range of 1.0 to 2.0.

29. The image forming apparatus according to claim 28, wherein the filler is at least one of an inorganic pigment and a metal oxide.

30. The image forming apparatus according to claim 26, wherein the protection layer is formed from a water dispersion containing at least one of an inorganic pigment and a metal oxide dispersed therein, and a pH of the water dispersion is 5 or more.

31. The image forming apparatus according to claim 26, wherein the protection layer contains at least one of an inorganic pigment and a metal oxide, processed with a surface treatment using at least one surface treatment agent.

32. The image forming apparatus according to claim 26, wherein, the protection layer contains a dispersing agent and the dispersing agent is an organic compound having at least one carboxyl group in a structure thereof.

33. The image forming apparatus according to claim 32, wherein the dispersing agent is a polycarboxylic acid derivative.

34. The image forming apparatus according to claim 32, wherein the dispersing agent is an organic compound having an acid value of 10 to 400 (mgKOH/g).

35. The image forming apparatus according to claim 32, wherein the dispersing agent is added in an amount selected from a range satisfying the following expression:

36. The image forming apparatus according to claim 26, wherein a maximum intensity of an electric field applied by the charging means to the charge transporting layer and to the protection layer is -30 V/.mu.m.

37. An image forming apparatus comprising: a photoconductor which comprises: a charge generating layer containing a charge generating substance; a charge transporting layer containing a charge transporting substance; and a protection layer disposed closer to a surface of the photoconductor than the charge generating layer and the charge transporting layer and having a transmittance of 90% or more with respect to the laser beam; the charge transporting layer and the protection layer has a total thickness of 20 .mu.m or less; a charger for charging the photoconductor; an irradiator for irradiating a laser beam having a diameter of 35 .mu.m or less to the photoconductor for optically writing in to form a latent electrostatic image based on an image data obtained by the halftoning operation using a number of lines of 200 lpi or more with respect to an input image; and an image processor for performing a halftoning operation with respect to the input image.

38. The image forming apparatus according to claim 37, wherein a resolution of the optical write operation performed by the optical writing means is 1200 dpi or more.

39. The image forming apparatus according to claim 37, wherein the protection layer contains a filler, a charge transporting substance, and/or a binder resin.

40. The image forming apparatus according to claim 39, wherein the filler has a refractive index in a range of 1.0 to 2.0.

41. The image forming apparatus according to claim 40, wherein the filler is at least one of an inorganic pigment and a metal oxide.

42. The image forming apparatus according to claim 37, wherein the protection layer is formed from a water dispersion containing an inorganic pigment and/or a metal oxide dispersed therein and a pH of the water dispersion is 5 or more.

43. The image forming apparatus according to claim 37, wherein the protection layer contains an inorganic pigment and/or a metal oxide processed with a surface treatment using at least one surface treatment agent.

44. The image forming apparatus according to claim 37, wherein, the protection layer contains a dispersing agent and the dispersing agent is an organic compound having at least one carboxyl group in a structure thereof.

45. The image forming apparatus according to claim 44, wherein the dispersing agent is a polycarboxylic acid derivative.

46. The image forming apparatus according to claim 44, wherein the dispersing agent is an organic compound having an acid value of 10 to 400 (mgKOH/g).

47. The image forming apparatus according to claim 44, wherein the dispersing agent is added in an amount selected from a range satisfying the following expression:

48. The image forming apparatus according to claim 37, wherein a maximum intensity of an electric field applied by the charging means to the charge transporting layer and to the protection layer is -30 V/.mu.m.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an image forming apparatus using an electrophotographic process, such as an electrostatic copier or a laser printer. More particularly, the present invention relates to an image forming apparatus involving a so-called fixing step for fixing a toner image onto a recording sheet such as a paper.

2. Description of the Related Art

A conventional image forming apparatus of this type has a structure as shown in FIG. 1. An image forming apparatus shown in FIG. 1 has a photoconductive drum 1, charging means (charging unit) 2, exposing means 3, developing means 4, transferring means 5, cleaning means 7, and fixing means 8 to form an image on a recording sheet 6. A typical image forming process will be shown below. (1) The charging means 2 charges a surface of the photoconductor 1 to a desired potential. (2) The exposing means 3 exposes the photoconductor 1 to form a latent electrostatic image corresponding to a desired image on the photoconductor. (3) The developing means 4 develops the latent electrostatic image formed by the exposing means 3 with a toner and thereby forms a toner image on the photoconductor 1. (4) The transferring means 5 transfers the toner image from the photoconductor to the recording sheet 6 carried by carrying means not shown. (5) The cleaning means 7 cleans, from the photoconductor, the toner remaining thereon without being transferred to the recording sheet 6. (6) The recording sheet 6 is carried to the fixing means 8. (7) The fixing means 8 heats the toner (recording sheet 6) to fix it onto the recording sheet.

The image forming apparatus forms a desired image on the recording sheet 6 by rotating the photoconductive drum in the direction indicated by the arrow in FIG. 1 and repeating the steps (1) to (7).

In general, the photoconductive drum 1 is formed by coating a photoconductor on a surface of a conductor. The mainstream of the photoconductor has been a so-called organic photoconductor. As the photoconductor, a multilayer type (multilayer organic photoconductor) composed of a so-called charge generating layer and a charge transporting layer stacked on a conductive base has been used in most cases because of the high durability of the charge transporting layer. There has also appeared a photoconductor having a protection layer in addition to the charge generating layer and the charge transporting layer, which is provided to enhance the durability of the charge transporting layer.

There has been known that, to provide such an image forming apparatus capable of image reproduction at even a spatial frequency at which a development field is high, the film thickness of a photoconductor should be reduced (thinned) (Basic and Applied Electrophotographic Techniques, pp.150-151, Corona Publishing Co., Ltd.).

However, a photoconductor small in film thickness is low in durability against cleaning-induced abrasion, flaws, or the like. If a charging step and an exposing step are performed repeatedly, the photoconductor small in film thickness deteriorates fast. Briefly, the durability of a photoconductor is reduced significantly if the film thickness thereof is reduced for the formation of a high-quality image. If the film thickness of the photoconductor is increased for the enhanced durability thereof, on the other hand, only a low-quality image is obtainable. To eliminate the tradeoff, it has been requested to satisfy the two requirements of enhanced durability of a photoconductor and high-quality image formation.

In a conventional multilayer organic photoconductor, polycarbonate has been used commonly as a binder resin in a charge transporting layer. In such a photoconductor, the film thickness of the charge transporting layer has normally been adjusted to a range of 20 .mu.m to 30 .mu.m. This indicates that a higher priority has been given to the high durability of the photoconductive than to image qualities.

SUMMARY OF THE INVENTION

When the present inventors formed an image at a resolution of 1200 dpi or more which had been known to be necessary for the discrimination of character images (font) by using an image forming apparatus having a multilayer photoconductor in which a charge transporting layer has a film thickness of 20 .mu.m to 30 .mu.m, the image forming apparatus could not reproduce an image at a so-called high spatial frequency such as a single isolated dot or a 1-dot line. This indicates that the conventional image forming apparatus cannot perform through outputting of input images such as so-called bit map images due to unsatisfactory reproduction of a single isolated dot or a 1-dot line. In other words, it is not until an input image has undergone complicated image processing steps that an image can be formed.

If the resolution is adjusted to 600 dpi or 400 dpi, a single isolated dot or a 1-dot line can be reproduced. However, since the single isolated dot or 1-dot line is increased in size, only a coarse image is formed. In an image including an oblique line, a reduction in resolution aggravates a so-called jaggy and degrades image qualities.

The present inventors also performed writing of image data processed with a halftoning operation using the number of lines of 200 lpi or more by using an image forming apparatus having a photoconductor including a charge transporting layer with a film thickness of about 20 .mu.m to 30 .mu.m. The resulting image was extremely low in tone. This proved that an image which needs tonic representation, such as a photographic image, could not be formed satisfactorily. This also proved that so-called banding was likely to occur and only an image with much noise was obtained.

When a halftoning operation was performed by using the number of lines less than 200 lpi, a sufficient tone was achieved. However, a dither texture was visually observed and a fine-texture image could not be obtained.

The present invention has been achieved in view of the aforementioned problems. It is therefore an object of the present invention to provide an image forming apparatus capable of providing a high-quality image by adjusting the diameter of a beam for forming a latent image which is applied to the photoconductor to 35 .mu.m or less and combining a condition placed on the carrier mobility of the charge transporting layer of a photoconductor or a condition placed on the transmittance of a protection layer provided on the photoconductor with a condition placed on the film thicknesses of the protection layer and the charge transporting layer.

A first image forming apparatus according to the present invention is an image forming apparatus having at least: a photoconductor; charging means; and optical writing means for performing an optical write operation with respect to the photoconductor to form a latent electrostatic image thereon, the apparatus using an electrophotographic process in which a resolution of the optical write operation is 1200 dpi or more, the optical writing means performing the optical write operation by using a laser beam with a diameter of 35 .mu.m or less, the photoconductor being provided with at least a charge generating layer containing a charge generating substance and a charge transporting layer containing a charge transporting substance, the charge transporting layer having a carrier mobility of 1.times.10.sup.-5 cm.sup.2.multidot.V.sup.-1.multidot.sec.sup.-1 or more under an electric field of 3.times.10.sup.5 V.multidot.cm.sup.-1.

The image forming apparatus having such a structure is capable of reproducing an image at a high spatial frequency such as a single isolated dot or a 1-dot line and performing through outputting of bit map images or the like. Even in an image including an oblique line, therefore, a so-called jaggy does not occur. In character images also, various fonts can be discriminated.

A second image forming apparatus according to the present invention is an image forming apparatus having at least: a photoconductor; charging means; optical writing means for performing an optical write operation with respect to the photoconductor to form a latent electrostatic image thereon; and image processing means for performing a halftoning operation with respect to an input image, the apparatus using an electrophotographic process which allows the optical writing means to perform the optical write operation based on image data obtained by performing the halftoning operation using a number of lines 200 lpi or more with respect to the input image, the optical writing means performing the optical write operation by using a laser beam with a diameter of 35 .mu.m or less, the photoconductor being provided with at least a charge generating layer containing a charge generating substance and a charge transporting layer containing a charge transporting substance, the charge transporting layer having a carrier mobility of 1.times.10.sup.-5 cm.sup.2.multidot.V.sup.-1.multidot.sec.sup.-1 or more under an electric field of 3.times.10.sup.5.multidot.V.multidot.cm.sup.-1.

In the image forming apparatus having such a structure, tone is improved so that even an image which needs tonal representation, such as a photographic image, is reproduced satisfactorily. As a result, it becomes extremely difficult to visually observe a dither texture which occurs with a smaller number of lines of 200 lpi or less. In addition, the occurrence of banding is minimized.

The present inventors found that, if at least one compound having a triarylamine structure is contained in the charge transporting layer, the charge transporting layer is allowed to have the aforementioned carrier mobility. This may be because, since a compound having a triarylamine structure has a high carrier mobility, it imparts the aforementioned property to the charge transporting layer.

It was also proved that at least compounds expressed by the following structural formulae (A-I) to (A-VI) as compounds each containing a triarylamine structure are excellent in miscibility with a binder resin contained in the charge transporting layer and capable of enhancing the resistance of the photoconductor to oxidizing gas and the optical stability thereof. ##STR1##

(where R.sub.1, R.sub.3, and R.sub.4 may be the same or different and each independently represents a hydrogen atom, an amino group, an alkoxy group, a thioalkoxy group, an aryloxy group, a methylenedioxy group, a substituted or unsubstituted alkyl group, a halogen atom, or a substituted or unsubstituted aryl group; R.sub.2 represents a hydrogen atom, an alkoxy group, a substituted or unsubstituted alkyl group or halogen, except for a combination in which each of R.sub.1, R.sub.2, R.sub.3, and R.sub.4 is a hydrogen atom; and k, l, m, and n are each independently 1, 2, 3, or 4). ##STR2##

(where Ar.sub.1 and Ar.sub.2 may be the same or different and each independently represents a substituted or unsubstituted aryl group or a substituted or unsubstituted heterocyclic group; R.sub.6, R.sub.7, and R.sub.5 may be the same or different and each independently represents a hydrogen atom, a substituted or unsubstituted alkyl group, a substituted or unsubstituted alkoxy group, a substituted or unsubstituted aryl group, or a substituted or unsubstituted heterocyclic group, of which R.sub.7 and R.sub.6 may be combined to form a ring; and Ar.sub.3 represents a substituted or unsubstituted allylene group). ##STR3##

(where R.sub.10, R.sub.11, and R.sub.12 may be the same or different and each independently represents a hydrogen atom, a halogen atom, a nitro group, a substituted or unsubstituted alkyl group, a substituted or unsubstituted alkoxy group, or a substituted or unsubstituted aryl group; R.sub.8 and R.sub.9 may be the same or different and each independently represents a hydrogen atom, an alkoxycarbonyl group, a substituted or unsubstituted alkyl group, or a substituted or unsubstituted aryl group; W represents a hydrogen atom, a substituted or unsubstituted alkyl group, a phenylthio group, a divalent chain unsaturated hydrocarbon group, a monovalent or divalent and substituted or unsubstituted carbocyclic aromatic group, or a monovalent or divalent and substituted or unsubstituted heterocyclic group; j represents an integer of 1 to 5; f represents an integer of 1 to 4; g represents an integer of 1 or 2; h represents an integer of 1 or 2; and i represents an integer of 1 to 3). ##STR4##

(where Ar.sub.4 represents a condensed polycyclic hydrocarbon group having 18 or less carbon atoms; and R.sub.13 and R.sub.14 may be the same or different and each independently represents a hydrogen atom, a halogen atom, a substituted or unsubstituted alkyl group, an alkoxy group, or a substituted or unsubstituted phenyl group). ##STR5##

(where R.sub.15 and R.sub.16 may be the same or different and each independently represents a lower alkyl group, a lower alkoxy group, or a halogen atom; p and q each independently represents an integer of 1 to 4; and R.sub.17 and R.sub.18 may be the same or different and each independently represents a hydrogen atom, a lower alkyl group, a lower alkoxy group, or a halogen atom). ##STR6##

(where R.sub.19, R.sub.20, R.sub.21, and R.sub.22 may be the same or different and each independently represents a hydrogen atom, an alkyl group which may have a substituent, an alkoxy group, an allyl group, an aryl group, or a halogen atom; and R.sub.23 and R.sub.24 may be the same or different and each independently represents a hydrogen atom, an alkyl group, an alkoxy group, a halogen atom, an amino group, an N-substituted amino group, an allyl group, or an aryl group).

A content of the charge transporting substance in the charge transporting layer is adjusted appropriately to 40% by weight or more, preferably 50% by weight or more with respect to a total amount of the charge transporting layer. If the content of the charge transporting substance is increased, the carrier mobility of the charge transporting layer is increased. The present inventors found that, if the content of the charge transporting substance was adjusted to 40% by weight or more with respect to a total amount of the charge transporting layer, extremely excellent image qualities were obtained, while the temperature dependence was reduced and degradation of image qualities by environments (low-temperature/low-humidity or high-temperature/high-humidity environments) was reduced. The present inventors also found that, if the content of the charge transporting substance was adjusted to 50% by weight or more, the dependence of the carrier mobility on the intensity of an electric field was reduced.

In the photoconductor, a film thickness of the charge transporting layer is preferably 20 .mu.m or less. It was found that, if the aforementioned structure was used, both high image qualities and the high durability of the photoconductor were achievable and, if the film thickness of the charge transporting layer (CT film thickness) was 20 .mu.m or less, a sufficient durability was obtainable and an extremely high-quality image was obtainable. Since the CT film thickness is small, cost for manufacturing a photoconductive drum can be reduced and a coated film may be excellently uniform. If such a structure is used, the photoconductor is preferably provided with a protection layer for the high durability (for the prevention of a shorter life) of the photoconductive drum.

A third image forming apparatus according to the present invention is an image forming apparatus having: charging means; a photoconductor; and optical writing means to form an image at a resolution of at least 1200 dpi, the optical writing means emitting a laser beam with a diameter of 35 .mu.m or less, the photoconductor having at least a charge generating layer containing a charge generating substance and a charge transporting layer containing a charge transporting substance which are provided on a conductive support, a protection layer being disposed closer to a surface of the photoconductor than the charge generating layer and the charge transporting layer to have a transmittance of 90% or more with respect to the laser beam, a total film thickness of the charge transporting layer and the protection layer being 20 .mu.m or less.

A fourth image forming apparatus according to the present invention is an image forming apparatus having: image processing means for performing a halftoning operation with respect to an input image; charging means; a photoconductor; and optical writing means, the image processing means performing the halftoning operation with respect to at least the input image by using a number of lines of 200 lpi or more, the optical writing means emitting a laser beam with a diameter of 35 .mu.m or less, the photoconductor having at least a charge generating layer containing a charge generating substance and a charge transporting layer containing a charge transporting substance which are provided on a conductive support, a protection layer being disposed closer to a surface of the photoconductor than the charge generating layer and the charge transporting layer to have a transmittance of 90% or more with respect to the laser beam, a total film thickness of the charge transporting layer and the protection layer being 20 .mu.m or less.

Preferably, the protection layer contains a filler, a charge transporting substance, and/or a binder resin.

Preferably, the filler has a refractive index in a range of 1.0 to 2.0 in terms of providing a high transmittance and a satisfactory image.

Preferably, the filler is at least one of an inorganic pigment and a metal oxide.

Preferably, the protection layer is formed from a water dispersion containing an inorganic pigment and/or a metal oxide dispersed therein and a pH of the water dispersion is 5 or more in terms of a high electric insulating property and a lower probability of an image blur.

Preferably, the inorganic pigment and/or metal oxide is processed with a surface treatment using at least one surface treatment agent in terms of increasing dispersibility, reducing a residual potential at the photoconductor, increasing transparency, preventing a defect in the coat film, imparting wear-resistance, and preventing localized abrasion.

Preferably, the surface treatment agent is at least one of a titanate coupling agent, a higher fatty acid, and/or a metal salt of a higher fatty acid in terms of retaining an insulating property.

Preferably, the inorganic pigment and/or the metal oxide is processed with the surface treatment in an amount of 2 to 30% by weight in terms of achieving the effect of the addition of the filler without increasing the residual potential.

Preferably, the protection layer contains a binder resin containing a resin having an acid value of 10 to 400 (mgKOH/g).

Preferably, the protection layer contains, as a dispersing agent, an organic compound having at least one carboxyl group in a structure thereof and the dispersing agent is a polycarboxylic acid derivative.

Preferably, the dispersing agent is an organic compound having an acid value of 10 to 400 (mgKOH/g).

Preferably, the dispersing agent is added in an amount selected from a range satisfying the following expression:

Preferably, a maximum intensity of an electric field applied by the charging means to the charge transporting layer and to the protection layer is -30 V/.mu.m.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing an example of a structure of an image forming apparatus;

FIG. 2 is a view showing an example of a layer structure of a photoconductor according to the present invention;

FIG. 3 is a view showing another example of the layer structure of the photoconductor according to the present invention;

FIG. 4 is a view showing still another example of the layer structure of the photoconductor according to the present invention;

FIG. 5 is a view showing yet another example of the layer structure of the photoconductor according to the present invention;

FIG. 6 shows an example of a structure of an optical writing means in the image forming apparatus of FIG. 1;

FIG. 7 shows a first example of a structure of a corona charger used in the image forming apparatus shown in FIG. 1;

FIG. 8A is a view showing a second example of the structure of the corona charger used in the image forming apparatus shown in FIG. 1 and FIG. 8B is a schematic view of a sawtooth electrode in the corona charger;

FIG. 9 shows a third example of the structure of the corona charger used in the image forming apparatus shown in FIG. 1;

FIG. 10 is a first view illustrating a relationship between input data and lightness L;

FIG. 11 is a second view illustrating a relationship between input data and lightness L;

FIG. 12 is a view showing a relationship between the carrier mobility of a charge transporting layer and tone in Examples A-1 to 17 and COMPARATIVE Examples A-1 to 3; and

FIG. 13 is a view illustrating an example of the definition of tone reproduction stability.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Herein below, a detailed description will be given to the embodiments of an image forming apparatus according to the present invention.

The image forming apparatus can use the same structure as used conventionally. Specifically, there can be used any of well-known structures of an electrophotographic image forming apparatus having at least a photoconductor, charging means, and optical writing means (exposing means) for performing an optical write operation with respect to the photoconductor to form a latent electrostatic image thereon. Accordingly, the structure shown in, FIG. 1 is also usable.

As the image forming apparatus, there is used an apparatus in which the resolution of the optical write operation is 1200 dpi or more and/or which is capable of forming an image from data obtained by performing a halftoning operation using the number of lines of 200 lpi or more with respect to an input image (or processed with the halftone operation).

Although each of image forming apparatus according to the first and second aspects of the present invention uses a structure/construction as described above, it is different from a conventional image forming apparatus in that:

(1) the beam emitted from the optical writing means has a diameter of 35 .mu.m or less; and

(2) a carrier mobility in the charge transporting layer of the photoconductor (electrophotographic photoconductor) is 1.times.10.sup.-5 cm.sup.2.multidot.V.sup.-1 sec.sup.-1 or more under an electric field of 3.times.10.sup.5 V.multidot.cm.sup.-1.

Thus, the image forming apparatus according to the present invention is an image forming apparatus obtained by combining the structure of a writing system satisfying the aforementioned conditions (the resolution of the write operation and the beam diameter) with the structure of the photoconductor satisfying the aforementioned condition (prescription for the charge transporting layer).

Although each of image forming apparatus according to the third and fourth aspects of the present invention uses a structure/construction as described above, it is different from a conventional image forming apparatus in that:

(1) the beam emitted from the optical writing means has a diameter of 35 .mu.m or less; and

(2) a protection layer having a transmittance of 90% or more with respect to the laser beam from the optical writing means is provided and a total film thickness of the charge transporting layer and the protection layer is 20 .mu.m or less.

The beam diameter is defined herein as a diameter at a position where the intensity of the beam decreases to 1/e.sup.2 of a maximum value in a Gaussian distribution of light intensities. The beam diameter was measured using a Beam Scan Model 180-xy/11/5 HZ manufactured by PHONTON, Inc.

Referring to the drawings, the photoconductor will be described in detail.

(Photoconductor)

A photoconductor according to the present embodiment has a charge generating layer 35 and a charge transporting layer 37.

FIGS. 2, 3, 4, and 5 show examples of a layer structure of the photoconductor according to the present embodiment.

The photoconductor shown in FIG. 2 has a multilayer structure composed of the charge generating layer 35 and the charge transporting layer 37 stacked successively on a conductive support 31.

The photoconductor shown in FIG. 3 has a structure obtained by providing an undercoat layer 33 between the conductive support 31 and the charge generating layer 35.

The photoconductors shown in FIGS. 4 and 5 have structures obtained by further forming protection layers 39 on the respective charge transporting layers 37 of the photoconductors shown in FIGS. 2 and 3.

It is to be noted that the layer structures shown in FIGS. 2 to 5 are only exemplary of the layer structure of the photoconductor according to the present embodiment. If necessary, other structures may also be used as appropriate. For example, an intermediate layer may also be provided between photoconductive layers (the charge generating layer 35 and the charge transporting layer 37) and the protection layer 39. In the image forming apparatus according to the third and fourth embodiments, the protection layers are essential components.

(Conductive Support 31)

As the conductive support 31, any of well-known supports for photoconductors can be used but a support having a conductivity of 10.sup.10 .OMEGA..multidot.cm or less is used preferably. As such a support, there can be used: a support composed of a metal such as aluminum, nickel, chrome, nichrome, copper, gold, silver, or platinum or a metal oxide such as tin oxide or indium oxide which is covered with a film-like or cylindrical plastic or paper by vapor deposition or sputtering; a plate composed of aluminum, an aluminum alloy, nickel, or stainless steel; or a pipe obtained by forming such a plate into a primary tube by a technique such as extrusion or pultrusion and performing thereto a surface treatment such as cutting, super finishing, or polishing. It is also possible to use an endless nickel belt or an endless stainless-steel belt disclosed in Japanese Patent Application Laid-Open (JP-A) No. 52-36016 as the conductive support 31.

A base (support) to which a conductive powder dispersed in a proper binder resin has been applied may also be used as the conductive support 31.

Examples of the conductive powder include carbon black, acetylene black, powders of metals such as aluminum, nickel, iron, nichrome, copper, zinc, and silver, and powders of metal oxides such as conductive tin oxides and ITO. Conductive powders may be used either alone or in a mixture of two or more thereof.

Examples of the binder resin (binder resin for forming the conductive support) include thermoplastic resins, thermosetting resins, and photo-setting resins such as polystyrene, styrene-acrylonitrile copolymers, styrene-butadiene copolymers, styrene-maleic anhydride copolymers, polyesters, polyvinyl chloride, vinyl chloride-vinyl acetate copolymers, polyvinyl acetate, polyvinylidene chloride, polyarylate resins, phenoxy resins, polycarbonates, cellulose acetate resins, ethyl cellulose resins, polyvinyl butyral, polyvinyl formal, polyvinyl toluene, poly-N-vinylcarbazole, acrylic resins, silicone resins, epoxy resins, melamine resins, urethane resins, phenol resin, and alkyd resins. These resins may be used either alone or in a mixture of two or more thereof.

The conductive support 31 having a conductive layer provided on the base can be produced by dissolving or dispersing a conductive powder and a binder resin in an appropriate solvent or dispersion medium (such as tetrahydrofuran, dichloromethane, methyl ethyl ketone, or toluene) and coating the resulting solution or fluid dispersion on the base.

A proper base (preferably a cylindrical base) covered with a heat-shrinkable tube containing any of the aforementioned conductive powders may also be used as the conductive support 31.

For the thermo-shrinkable tube, there can be used, e.g., polyvinyl chloride, polypropylene, polyesters, polystyrene, polyvinylidene chloride, polyethylene, chlorinated rubber, or Teflon (Trademark).

If a resin that can be used for a thermo-shrinkable tube is impregnated with any of the aforementioned conductive powders and adhered to a proper base, the resulting structure can be used as the conductive support 31.

(Charge Generating Layer 35)

The charge generating layer 35 is a layer containing a charge generating substance as a main component. In general, the charge generating layer 35 is provided on the conductive support 31 or on the undercoat layer 33.

As the charge generating substance, any of well-known charge generating substances can be used. For example, any of well-known materials including a phthalocyanine pigment such as titanyl phthalocyanine, vanadyl phthalocyanine, copper phthalocyanine, hydroxygallium phthalocyanine, or organic phthalocyanine, an azo pigment such as a monoazo pigment, a disazo pigment, an asymmetric disazo pigment, or a trisazo pigment, a perylene pigment, a perinone pigment, an indigo pigment, a pyrrolopyrrole pigment, an anthraquinone pigment, a quinacridone pigment, a quinone condensed polycyclic compound, and a squarylium pigment can be used. These charge generating substances may be used either alone or in a mixture of two or more thereof.

The charge generating layer 35 is produced as follows:

(1) A charge generating substance is dissolved or dispersed in a proper solvent or dispersion medium together with a binder resin (binder resin for forming the charge generating layer 35), if necessary. Dissolution or dispersion is performed by using a ball mill, an attritor, a sand mill, or an ultrasonic wave; and

(2) The solution or fluid dispersion (coating liquid) obtained in (1) is coated on a specified layer and dried.

As a method for coating the coating liquid, there can be adopted a method such as dip coating, spray coating, bead coating, nozzle coating, spinner coating, or ring coating.

The proper range of the film thickness of the charge generating layer 35 is about 0.01 to 5 .mu.m, preferably 0.1 to 2 .mu.m.

Examples of the binder resin for forming the charge generating layer 35 include polyamide, polyurethane, epoxy resins, polyketone, polycarbonates, silicone resins, acrylic resins, polyvinyl butyral, polyvinyl formal, polyvinyl ketone, polystyrene, polysulfone, poly-N-vinylcarbazole, polyacrylamide, polyvinyl benzal, polyesters, phenoxy resins, vinyl chloride-vinyl acetate copolymers, polyvinyl acetate, polyphenylene oxide, polyamide, polyvinyl pyridine, cellulose resins, casein, polyvinyl alcohol, and polyvinyl pyrolidone. These resins may be used either alone or in a combination of two or more thereof.

The amount of the binder resin is adjusted to a range from 0 parts by weight to 500 parts by weight, preferably from 10 parts by weight to 300 parts by weight relative to 100 parts by weight of charge generating substance. The binder resin may be added to a solvent or a dispersion medium either before or after the charge generating substance is dissolved or dispersed in the solvent or dispersion medium.

As examples of the solvent or dispersion medium for forming the charge generating layer 35, mention may be made of isopropanol, acetone, methyl ethyl ketone, cyclohexanone, tetrahydrofuran, dioxane, ethyl cellosolve, ethyl acetate, methyl acetate, dichloromethane, dichloroethane, monochlorobenzene, cyclohexane, toluene, xylene, and ligroine. In particular, ketone solvents, ester solvents, and ether solvents are used satisfactorily. They may be used either alone or in a combination of two or more thereof.

Although the charge generating layer 35 contains the charge generating substance, the solvent, and the binder resin as main components, another component (additive) may also be contained therein. Examples of the additive include any of well-known additives for the charge generating layer 35 including a sensitizing agent, a dispersing agent, a surface active agent, and silicone oil.

(Charge Transporting Layer 37)

The charge transporting layer 37 is a layer containing a charge transporting substance as a main component.

The charge transporting layer 37 is formed by dissolving or dispersing a binder resin in a proper solvent or a dispersion medium, coating the resulting solution or fluid dispersion on a specified layer, and drying it.

For the charge transporting layer 37, an additive such as a plasticizer, a leveling agent, an antioxidant, or a lubricant may be added as appropriate. As the additive, any of well-known additives for the charge transporting layer may be used.

The charge transporting substance can be subdivided into a hole transporting material and an electron transporting material.

Examples of the electron transporting material include chloranil, bromanil, tetracyanoethylene, tetracyanoquinodimethane, 2,4,7-trinitro-9-fluorenone, 2,4,5,7-tetranitro-9-fluorenone, 2,4,5,7-tetranitroxanthone, 2,4,8-trinitrothioxanthone, 2,6,8-trinitro-4H-indeno [1,2-b]thiophene-4-one, 1,3,7-trinitrodibenzothiophene-5,5-dioxide, and benzoquinone derivatives.

Examples of the hole transporting material include poly-N-vinylcarbazole and derivatives thereof, poly-.gamma.-carbazole ethylglutamate and derivatives thereof, pyrene-formaldehyde condensation product and derivatives thereof, polyvinyl pyrene, polyvinyl phenanthrene, polysilane, oxazole derivatives, oxadiazole derivatives, imidazole derivatives, monoarylamine derivative, diarylamine derivatives, triarylamine derivatives, stilbene derivatives, .alpha.-phenylstilbene derivatives, benzidine derivatives, diarylmethane derivatives, triarylmethane derivatives, 9-styrylanthracene derivatives, pyrazoline derivatives, divinylbenzene derivatives, hydrazone derivatives, indene derivatives, butadiene derivatives, pyrene derivatives, bisstilbene derivatives, enamine derivatives, and other well-known materials. These charge transporting substances may be either alone or in a mixt