Lubricant for electrophotography, lubricant applying unit, process cartridge, and image forming apparatus

Info

Publication number: 20050164108
Type: Application
Filed: Dec 10, 2004
Publication Date: Jul 28, 2005
Inventors: Eisaku Murakami (Tokyo), Masami Tomita (Shizuoka), Ken Amemiya (Tokyo), Masato Yanagida (Tokyo), Naohiro Kumagai (Kanagawa), Takeshi Shintani (Kanagawa), Takaaki Tawada (Kanagawa), Yuji Arai (Kanagawa), Toshio Koike (Kanagawa), Takeshi Tabuchi (Saitama), Yutaka Takahashi (Tokyo), Takeshi Uchitani (Kanagawa), Masanori Kawasumi (Kanagawa)
Application Number: 11/008,516

Abstract

An lubricant for electrophotography is applied to a latent image carrier that is supplied with toner having a sphericity of 0.94 or more. The lubricant for electrophotography is added with an inorganic additive having the following relationship: 2Y/1000≦X≦Y/10 where Y is a toner particle size (micrometer), and X is an inorganic additive particle size (micrometer).

Description

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

The present document incorporates by reference the entire contents of Japanese priority document, 2003-414090 filed in Japan on Dec. 12, 2003.

BACKGROUND OF THE INVENTION

1) Field of the Invention

The present invention relates to a lubricant for electrophotography used in an electrostatic duplicating process for development by supplying toner to a latent image carrier that carries a latent image, and to a lubricant applying unit, a process cartridge, and an image forming apparatus.

2) Description of the Related Art

Electrophotographic color image forming apparatuses are widespread recently, and digitized images are easily available. Under these circumstances, higher definition is required for printed images. Higher resolution and gradation of images are studied, and improvement of toner used to visualize a latent image is also studied. In order to form a high-definition image, it is also studied to obtain a higher degree of sphericity and a smaller particle size of toner.

For example, pulverized spherical toner particles having a specified particle size distribution are described in Japanese Patent Application Laid-Open Publication No. Hei 11-12253, Japanese Patent Application Laid-Open Publication No. Hei 2-284158, Japanese Patent Application Laid-Open Publication No. Hei 3-181952, and Japanese Patent Application Laid-Open Publication No. Hei 4-162048. Japanese Patent Application Laid-Open Publication No. Hei 5-72808 discloses a method of obtaining spherical toner having a small particle size through suspension polymerization. Japanese Patent Application Laid-Open Publication No. Hei 9-15902 discloses a method of obtaining spherical toner having a small particle size by mixing a,binder resin with a colorant in a solvent without water therein and dispersing the mixture into an aqueous solvent containing a dispersion stabilizer. Japanese Patent Application Laid-Open Publication No. Hei 11-133668 discloses a method of obtaining spherical toner having a small particle size by mixing a binder resin partially containing modified resin with a colorant in an organic solvent, and dispersing the mixture into an aqueous solvent to allow polyaddition reaction of the modified resin to be performed. By using such toner, improved image quality and improved fluidity are obtained.

To improve image quality, it is necessary to remove toner remaining on the surface of a photosensitive element as much as possible after a toner image is transferred. Particularly, since spherical toner is easy to roll, the toner enters between a cleaning blade and the photosensitive element upon cleaning of the photosensitive element (latent image carrier). Some toner passes under the cleaning blade and remains on the photosensitive element, which may cause an abnormal image such as surface fog to occur. Various examinations to solve this problem are carried out.

For example, Japanese Patent Application Laid-Open Publication No. Hei 11-184340 discloses a method of forming an electrophotographic image using a cleaning member for cleaning toner remaining on a photosensitive element with an elastic rubber blade. In the method, by making zinc stearate as a lubricant contain in toner by a range from 0.01% to 0.5% to the weight of the toner, a frictional coefficient of the photosensitive element is reduced and cleaning efficiency with the elastic rubber blade is increased.

Japanese Patent Application Laid-Open Publication No. 2002-287567 discloses an image forming device that forms an electrostatic latent image on a photosensitive element, visualizes the electrostatic latent image as a toner image with toner containing a mold release agent, transfers the toner image to a recording medium directly or through an intermediate transfer element, and fixes the toner image on the recording medium. In the image forming device, by applying zinc stearate as a lubricant to the photosensitive element, a frictional coefficient of the photosensitive element is reduced and cleaning efficiency with the elastic rubber blade is increased.

However, in the invention of Japanese Patent Application Laid-Open Publication No. Hei 11-184340, if zinc stearate is added to the toner, the zinc stearate may be unevenly distributed over the photosensitive element depending on the status of an image to be developed. In a portion with a small amount of zinc stearate, some toner particles pass under the elastic rubber blade to remain on the photosensitive element. In the invention of Japanese Patent Application Laid-Open Publication No.,2002-287567, if an image with a large image area therein is formed in image mode, the zinc stearate adheres to the toner particles, and a large amount of zinc stearate are taken away. Thereby, the distribution of the zinc stearate on the photosensitive element becomes uneven, and in a portion with a small amount of the zinc stearate, some of the toner particles pass under the cleaning blade to remain on the photosensitive element. There is a problem, which is common to the inventions of Japanese Patent Application Laid-Open Publication No. Hei 11-184340 and 2, such that in an initial stage of use of a new photosensitive element, only a small amount of zinc stearate is present on the photosensitive element, which causes insufficient removal of toner remaining on the photosensitive element.

SUMMARY OF THE INVENTION

It is an object of the present invention to solve at least the above problems in the conventional technology.

A lubricant for electrophotography according to one aspect of the present invention is applied to a latent image carrier to which toner having a sphericity of equal to or more than 0.94 is supplied. An inorganic additive is added to the lubricant, and the inorganic additive satisfies
2Y/1000≦X≦Y/10
where Y is a particle size of the toner in micrometer, and X is a particle size of the inorganic additive in micrometer.

A lubricant applying unit according to another aspect of the present invention, which applies a lubricant for electrophotography to a latent image carrier, includes a brush roller that rotates around a rotational axis, and is in contact with the latent image carrier. Toner having a sphericity of equal to or more than 0.94 is supplied to the latent image carrier. An inorganic additive is added to the lubricant, and the inorganic additive satisfies
2Y/1000≦X≦Y/10
where Y is a particle size of the toner in micrometer, and X is a particle size of the inorganic additive in micrometer. The lubricant is a molded lubricant that is molded as a solid block, and the brush roller slides along, and scrapes off the molded lubricant so that the lubricant is applied to the latent image carrier.

A process cartridge according to still anther aspect of the present invention includes a latent image carrier; at least one of

- a charging unit that causes a charging member to contact or to be close to the latent image carrier to charge the latent image carrier, a developing unit that deposits toner on a latent image on the latent image carrier to develop the latent image with the toner, and a cleaning unit that cleans the toner remaining on the latent image carrier with a cleaning blade; and a lubricant applying unit that applies a lubricant for electrophotography to the latent image carrier. The lubricant applying unit includes a brush roller that rotates around a rotational axis, and is in contact with the latent image carrier. Toner having a sphericity of equal to or more than 0.94 is supplied to the latent image carrier. An inorganic additive is added to the lubricant, and the inorganic additive satisfies
  2Y/1000≦X≦Y/10
  where Y is a particle size of the toner in micrometer, and X is a particle size of the inorganic additive in micrometer. The lubricant is a molded lubricant that is molded as a solid block, and the brush roller slides along, and scrapes off the molded lubricant so that the lubricant is applied to the latent image carrier.

An image forming apparatus according to still another aspect of the present invention includes a process cartridge that includes a latent image carrier; at least one of a charging unit that causes a charging member to contact or to be close to the latent image carrier to charge the latent image carrier, a developing unit that deposits toner on a latent image on the latent image carrier to develop the latent image with the toner, and a cleaning unit that cleans the toner remaining on the latent image carrier with a cleaning blade; and a lubricant applying unit that applies a lubricant for electrophotography to the latent image carrier. The lubricant applying unit includes a brush roller that rotates around a rotational axis, and is in contact with the latent image carrier. Toner having a sphericity of equal to or more than 0.94 is supplied to the latent image carrier. An inorganic additive is added to the lubricant, and the inorganic additive satisfies
2Y/1000≦X≦Y/10
where Y is a particle size of the toner in micrometer, and X is a particle size of the inorganic additive in micrometer. The lubricant is a molded lubricant that is molded as a solid block, and the brush roller slides along, and scrapes off the molded lubricant so that the lubricant is applied to the latent image carrier.

An image forming apparatus according to still another aspect of the present invention includes a lubricant applying unit that applies a lubricant for electrophotography to a latent image carrier. The lubricant applying unit includes a brush roller that rotates around a rotational axis, and is in contact with the latent image carrier. Toner having a sphericity of equal to or more than 0.94 is supplied to the latent image carrier. An inorganic additive is added to the lubricant, and the inorganic additive satisfies
2Y/1000≦X≦Y/10
where Y is a particle size of the toner in micrometer, and X is a particle size of the inorganic additive in micrometer. The lubricant is a molded lubricant that is molded as a solid block, and the brush roller slides along, and scrapes off the molded lubricant so that the lubricant is applied to the latent image carrier.

The other objects, features, and. advantages of the present invention are specifically set forth in or will become apparent from the following detailed description of the invention when read in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an enlarged cross section of a main part near a contact portion between a cleaning blade and a photosensitive element when a lubricant for electrophotography is applied thereto;

FIG. 2 is an enlarged cross section of the main part when the lubricant for electrophotography is not applied thereto;

FIG. 3 is a schematic diagram of the whole of a compact-size full color printer;

FIG. 4 is a schematic diagram of a configuration of a process cartridge;

FIG. 5 is a flowchart of an operation of a new-latent-image-carrier detector;

FIG. 6A and FIG. 6B are schematic diagrams of shapes of toner, in which FIG. 6A is a diagram for explaining a shape factor SF-1 and FIG. 6B is a diagram for explaining a shape factor SF-2;

FIG. 7A and FIG. 7B are schematic diagrams of outline shapes of toner, in which FIG. 7A is a diagram of external appearance of the toner and FIG. 7B is a cross section of the toner;

FIG. 8 is a schematic diagram of toner;

FIG. 9 is a diagram of a relationship between the particle size of an additive added to a molded lubricant and the soil of a charging roller;

FIG. 10 is a diagram of changes in the frictional coefficient of the photosensitive element caused by application of the lubricant thereto when sheets of paper are actually passed;

FIG. 11 is a diagram for explaining how to measure the frictional coefficient of the photosensitive element; and

FIG. 12 is a diagram of an application time of the lubricant and a change with time in the frictional coefficient of the photosensitive element.

DETAILED DESCRIPTION

Exemplary embodiments of a lubricant for electrophotography, a lubricant applying unit, a process cartridge, and an image forming apparatus according to the present invention are explained in detail below with reference to the accompanying drawings.

In an embodiment of the present invention, silica is added to a lubricant for electrophotography (hereinafter, “lubricant”) as an inorganic additive that satisfies the following relationship.
2Y/1000≦X≦Y/10 (1)
where Y is a volume-average particle size of toner (μm), and X is a particle size of silica (μm)

As the inorganic additive to be added, an inorganic substance such as silica, titania, alumina, magnesia, zirconia, ferrite, and magnetite can be used alone or in combination if the particle size satisfies the relationship of expression (1).

An amount of addition of the inorganic additive to a main component of the lubricant ranges preferably from 1 to 30 weight percent (wt. %). If the amount of addition exceeds 30 wt. %, it is difficult to mold the lubricant as a solid block, i.e., to form a molded lubricant, which is not preferable. Furthermore, if the amount of addition is less than 1 wt. %, adequate cleaning performance cannot be obtained, which is also not preferable.

As the lubricant used for the molded lubricant, a lubricant obtained in the following manner is used. The lubricant is obtained by adding the inorganic additive explained later to zinc stearate that is a main component and melting and solidifying the substance obtained by addition. Examples of a main component of the lubricant are aliphatic acid metal salts which are preferably melted and solidified, such as lead oleate, zinc oleate, copper oleate, zinc stearate, cobalt stearate, iron stearate, copper stearate, zinc palmitate, copper palmitate, and zinc linoleate. Furthermore, the examples are fluorine resins which are preferably molded, such as polytetrafluoroethylene, polychloro-trifluoroethylene, polyvinylidene fluoride, polytrifluoro chloro ethylene, dichloro-difluoroethylene, tetrafl-uoroethylene-ethylene copolymer, and tetrafluoroethylene-oxafluoropropylene copolymer. Particularly, metal stearate that is most effective in reduction in friction of the photosensitive element is preferably used, and zinc stearate is more preferably used.

The shape of toner for use is set to 0.94 or more in sphericity. The sphericity is defined that the circumferential length of a circle having area the same as the area of a projected image of a particle is divided by the circumferential length of the projected image of the particle. Therefore, if toner is close to perfect sphericity, the sphericity reaches a value near unity. The degree of sphericity of the toner having such a degree of sphericity is controlled by being strongly stirred in the process of removing organic solvent in the method of manufacturing toner, which is explained later. It is also possible to obtain toner having high sphericity by subjecting the toner manufactured through dry milling to spherical processing. The spherical processing of the toner manufactured by the dry milling is roughly divided into a thermal method and a mechanical method. The thermal method includes a method of performing the spherical processing by atomizing toner particles with thermal current by an atomizer. The mechanical method includes a method of performing the spherical processing by charging toner together with a mixing medium, such as glass having light specific gravity, into a mixer such as a ball mill, and stirring them. However, in the thermal method, toner particles agglomerate with one another to produce toner particles having a large particle size, while in the mechanical method, fine powder is produced. Therefore, both of the methods require the process of re-classification.

By adding the inorganic additive to the lubricant, stable cleaning performance is obtained. This mechanism can be explained as follows as a result of observing the mechanism with a microscope. FIG. 1 is an enlarged cross section of a main part near a contact portion between a cleaning blade and a photosensitive element when the lubricant according to the embodiment is applied. FIG. 2 is an enlarged cross section of the main part near the contact portion between the cleaning blade and the photosensitive element when the lubricant according to the embodiment is not applied. By referring to FIG. 1, a cleaning blade 15a is in contact with the surface of a photosensitive element 5, and cleaning is performed by scraping toner particles T remaining on the photosensitive element 5 from the surface thereof. The lubricant is supplied by a brush roller (not shown) to the photosensitive element 5 in the upstream side in a direction of rotation thereof from the contact portion between the cleaning blade 15a and the photosensitive element 5. The cleaning blade 15a is curved downwardly near the contact portion, and a space between the photosensitive element 5 and the cleaning blade 15a is getting narrower toward the contact portion. In the space, a portion that is narrower than the size of the toner particle T is defined as a region C.

If the inorganic additive is not added to the lubricant, as shown in FIG. 2, the toner particle T enters the region C and passes under the cleaning blade 15a as if the toner particle T is pulled toward the direction of rotation of the photosensitive element 5. In this case, the toner particles T remain on the surface of the photosensitive element 5 without being cleaned. On the other hand, if the inorganic additive is added to the lubricant, as shown in FIG. 1, inorganic additive particles having a smaller particle size than that of the toner particle T enter the region C. The region C is filled with the inorganic additive particles, and the toner particles T are thereby prevented from entering the region C. The region C filled with the inorganic additive particles in the above manner serves as a roll of a “bank” for stopping the toner particles T. In this case, almost all of the toner particles T is removed from the photosensitive element 5 with the cleaning blade 15a (see Example 1).

The image forming apparatus using the lubricant according to the embodiment is explained below with reference to the attached drawings. A four-drum tandem system and a direct transfer system such that a transfer paper and the photosensitive element (latent image carrier) are in direct contact with each other is exemplified in this embodiment. However, any imaging system such as an intermediate transfer belt system and a revolver type development system may be employed if the imaging system is such that a lubricant is applied to the photosensitive element and the lubricant is the one explained above.

FIG. 3 is a schematic diagram of the whole of a compact-size full color printer. The main body of the image forming apparatus 1 (hereinafter, “main body 1”) includes process cartridges 2A, 2B, 2C, and 2D that have the photosensitive elements 5 being four image carriers, and that are detachably attached to the main body 1, respectively. A transfer device 3 is arranged in a substantially center of the main body 1. The transfer device 3 includes a transfer belt 3a wound around between a plurality of rollers so as to be rotatable in the direction of arrow A. The photosensitive elements 5 respectively provided in the process cartridges 2A, 2B, 2C, and 2D are arranged along the upper surface of the transfer belt 3a so as to be in contact with the transfer belt 3a. Developing devices 10A, 10B, 10C, and 10D each of which includes toner of a different color are arranged corresponding to the process cartridges 2A, 2B, 2C, and 2D, respectively. A writing unit 6 is provided above the process cartridges 2A, 2B, 2C, and 2D, and a double-sided unit 7 is provided under the transfer belt 3a . A reversing unit 8 is provided in the left side of the main body 1. The reversing unit 8 reverses the transfer paper P after an image is formed thereon to discharge the transfer paper P reversed or to convey the transfer paper P reversed to the double-sided unit 7. A fixing device 9 that fixes the image on the transfer paper P is provided between the transfer belt 3a and the reversing unit 8. A paper reverse discharging path 20 is branched on the downstream side of the fixing device 9 in a transfer-paper conveyance direction. The transfer paper P conveyed to the paper reverse discharging path 20 is discharged onto a paper discharge tray 26 by a discharging roller pair 25. A paper feed portion including paper feed cassettes 11 and 12 is arranged in the lower side of the main body 1. The paper feed cassettes 11 and 12 are vertically arranged in two stages and have transfer paper P of different sizes. A manual feed tray 13 is provided in the right side of the main body 1 so as to be open in the direction of arrow B. By opening the manual feed tray 13, a paper can be fed manually therethrough.

The developing devices 10A, 10B, 10C, and 10D have the same configuration as one another but only colors of toner to be used are different from one another. More specifically, the developing device 10A uses toner of magenta, the developing device 10B uses toner of cyan, the developing device 10C uses toner of yellow, and the developing device 10D uses toner of black. Each of the developing devices 10A, 10B, 10C, and 10D includes a developing roller provided opposite to the photosensitive element 5, a screw that conveys and stirs developer, and a toner density sensor. The developing roller includes a sleeve that is provided outside the developing roller and can freely rotate, and a magnet fixed to the inside of the developing roller. Toner is supplied to the developing device by a toner supply device according to the output of the toner density sensor. In the embodiment, a two-component developer consisting of toner and carrier is used as the developer. Carrier itself as a core material or carrier with a coating layer coated on a core material is usually used as the carrier. A material used as the core material includes ferrite and magnetite. The particle size of the core material ranges from 20 to 65 micrometers, and preferably from about 30 to about 60 micrometers. Resin used for forming the coating layer includes styrene resin, acrylic resin, fluororesin, silicone resin, a mixture thereof, or copolymer thereof. The coating layer can be formed in known methods. For example, it can be formed by coating the surface of the core material with resin in an atomizing method or a dipping method.

The writing unit 6 includes four laser-diode light sources prepared for the colors, a pair of polygon scanners having a six-facet polygon mirror and a polygon motor, fθ lenses arranged in light paths of the respective light sources, lenses such as a long-local-length cylindrical lens, and mirrors. A laser beam emitted from the laser diode is polarized and scanned with the polygon scanner and is radiated onto the photosensitive element 5.

The double-sided unit 7 includes a pair of conveying guide plates 45a and 45b, a plurality of conveying roller pairs 46 (four pairs in this case). In a double-sided image forming mode in which images are formed on both surfaces of the transfer paper P, an image is formed on one of the surfaces, the transfer paper P with the image thereon is conveyed to a paper reverse conveying path 54 of the reversing unit 8, switched back therein, and conveyed to the double-sided unit 7. The double-sided unit 7 receives the transfer paper P (reversed) switched back, and conveys it toward the paper feed portion.

The reversing unit 8 includes a plurality of conveying roller pairs 60, and the paper reverse conveying path 54 formed with a plurality of conveying guide plate pairs 61. As explained above, the reversing unit 8 reverses the transfer paper P and sends the transfer paper P reversed to the double-sided unit 7 for forming images on both surfaces of the transfer paper P, or discharges the transfer paper P with an image formed, in the direction as it is, to the outside of the machine or reverses the transfer paper P and discharges the transfer paper P reversed to the outside of the machine.

The paper feed portion includes the paper feed cassettes 11 and 12, and paper separating units 55 and 56 for separating sheets of the transfer paper P one by one for feeding are provided near paper feeding ports, respectively.

The process cartridges 2A, 2B, 2C, and 2D are units having the same configuration as one another. The process cartridge 2A forms an image corresponding to magenta, the process cartridge 2B forms an image corresponding to cyan, the process cartridge 2C forms an image corresponding to yellow, and the process cartridge 2D forms an image corresponding to black. FIG. 4 is a schematic diagram of a configuration of the process cartridge. Each of the process cartridges 2A, 2B, 2C, and 2D includes a charging unit 14, a cleaning unit 15, the photosensitive element 5, and a lubricant applying unit 17. The charging unit 14 makes a charging roller 14a as a charging member come in contact with the photosensitive element 5 and charges it. The cleaning unit 15 includes a cleaning blade 15a for cleaning toner remaining on the photosensitive element 5 by contacting the photosensitive element 5. The photosensitive element 5 rotates and has an electrostatic latent image formed thereon. The lubricant applying unit 17 includes a brush roller 17a that is made to rotate around a rotational axis thereof arranged in parallel to the rotational axis of the photosensitive element 5 and applies the lubricant onto the photosensitive element 5.

The process cartridge includes at least one of the charging unit, the developing device that develops the latent image on the photosensitive element 5 with toner, and the cleaning unit. The process cartridge also includes the photosensitive element 5 and the lubricant applying unit 17. The process cartridge is not limited to the one according to the embodiment if it is detachably attached to the main body 1. By configuring the process cartridge in such a manner as explained above, the life of the photosensitive element 5 accommodated in the process cartridge can be prolonged. For maintenance, the process cartridge is simply replaced with a new one, which allows improved convenience.

The charging roller 14a of the charging unit 14 is conductive, and charges the photosensitive element 5 by applying a DC and/or an AC voltage thereto. A charging-roller cleaning brush 14b is provided in contact with the charging roller 14a so as to clean the surface of the charging roller 14a.

The cleaning unit 15 moves toner scraped off from the surface of the photosensitive element 5 with the cleaning blade 15a , to a toner conveying auger 15d through rotation of the brush roller 17a. Waste toner recovered to the toner conveying auger 15d is conveyed to a waste toner container 18 as shown in FIG. 3.

The lubricant applying unit 17 is included in each of the process cartridges 2A, 2B, 2C, and 2D. The lubricant applying unit 17 includes a molded lubricant 17b obtained by molding the lubricant to an elongated rectangle, and the brush roller 17a for scraping the lubricant by coming in contact with the molded lubricant 17b and applying it to the surface of the photosensitive element 5. The lubricant applying unit 17 also includes a brush roller scraper 17c for removing the toner deposited on the brush roller 17a, a pressure spring 17d that pushes the molded lubricant 17b against the brush roller 17a at predetermined pressure, and a new-latent-image-carrier detector (not shown) that detects whether a latent image carrier or the photosensitive element 5 is used for the first time (whether the photosensitive element 5 is new). Although the lubricant applying unit 17 is included in each of the process cartridges 2A, 2B, 2C, and 2D, it may be directly included in the main body 1.

The brush roller 17a has a shape of extending in the direction of the rotational axis of the photosensitive element 5. The pressure spring 17d biases the brush roller 17a toward the molded lubricant 17b so as to run out of almost all the molded lubricant 17b . The brush roller 17a scrapes off the molded lubricant 17b , so that the thickness of the molded lubricant 17b is reduced with time. However, the molded lubricant 17b is biased by the pressure spring 17d , and is thereby kept in contact with the brush roller 17a. The lubricant is scraped from the molded lubricant 17b , and the lubricant scraped is applied to the photosensitive element 5.

The molded lubricant 17b is pushed against the brush roller 17a by the pressure spring 17d at a pressure of 200 meter-newtons or more including the dead weight. With the increase in the pushing force, the lubricant scraped from the molded lubricant 17b by the brush roller 17a increases, and the amount of the lubricant applied to the photosensitive element 5 is increased, which allows the frictional coefficient of the photosensitive element 5 to be efficiently reduced.

The brush roller 17a is made to rotate in the direction of rotation of the photosensitive element 5 at a portion in contact with the photosensitive element 5. A peripheral speed ratio (peripheral speed of photosensitive element/peripheral speed of brush roller) between the brush roller 17a and the photosensitive element 5 is set to 1.0. By making the brush roller 17a to rotate following rotation of photosensitive element 5, the lubricant stuck to the brush roller 17a can be applied to the photosensitive element 5 without any impact. In other words, the lubricant does not cover the photosensitive element 5 right after the application of the lubricant with the brush roller 17a. However, the lubricant is spread by the pushing force of the cleaning blade 15a when the photosensitive element 5 is passing under the cleaning blade 15a , and the lubricant covers the photosensitive element 5. Therefore, the brush roller 17a is made to rotate in the direction of rotation of the photosensitive element 5 in order to apply the lubricant thereto without any impact. Furthermore, the peripheral speed ratio (peripheral speed of photosensitive element/peripheral speed of brush roller) between the brush roller 17a and the photosensitive element 5 ranges preferably from 0.8 to 1.2. If the peripheral speed ratio is less than 0.8, the amount of supply of the lubricant is reduced, but if the peripheral speed ratio exceeds 1.2, the photosensitive element 5 may be damaged by the impact, which may cause the life of the photosensitive element 5 to be reduced. Furthermore, in order to supply the lubricant from the brush roller 17a to the photosensitive element 5 with less impact, the peripheral speed ratio between the brush roller 17a and the photosensitive element 5 ranges more preferably from 1.0 to 1.1.

The new-latent-image-carrier detector includes a storage unit such as an integrated circuit (IC) chip that stores the use frequency of the photosensitive element 5. The new-latent-image-carrier detector makes the brush roller 17a rotate for a fixed time with respect to the photosensitive element 5 when the use frequency stored is zero (upon the first time use), and issues an instruction to the brush roller 17a to apply the lubricant to a new photosensitive element 5. Although the new-latent-image-carrier detector is configured to store the use frequency of the photosensitive element 5 in the IC chip, a member that is engaged with the main body 1 and disengaged therefrom only upon the first time use may be provided in the process cartridge 2A, and the first time use is detected by checking if the member is engaged with the main body 1. Alternatively, the new-latent-image-carrier detector may be configured as software that is stored in the lubricant applying unit 17.

Upon the first time use, the lubricant is applied to the photosensitive element 5 by the instruction of the new-latent-image-carrier detector, and the frictional coefficient of the photosensitive element 5 is set to 0.4 or less, preferably 0.2 or less. By setting the frictional coefficient of the photosensitive element 5 to 0.4 or less, interaction between the photosensitive element 5 and toner is reduced, which allows the toner to be easily separated from the photosensitive element 5 and transfer efficiency to be enhanced. This also allows the friction between the cleaning blade 15a and the photosensitive element 5 to be suppressed and cleaning efficiency to be enhanced. As for higher spherical toner in particular, since the toner becomes easy to roll on the photosensitive element 5, occurrence of cleaning failure can be suppressed. By increasing transfer efficiency and reducing the amount of toner to be cleaned, occurrence of cleaning failure due to long-term use can be reduced. However, if the frictional coefficient becomes less than 0. 1, the toner rolls too easily on the photosensitive element 5 and the amount of toner passing under the cleaning blade 15a increases. This is not preferable because cleaning failure may occur (see Example 1).

Upon the first time use, an application time of the lubricant by the instruction of the new-latent-image-carrier detector, i.e., the time for rotating the brush roller 17a is set to 100 seconds or more. By setting the application time to 100 seconds or more, the frictional coefficient of the new photosensitive element 5 is reduced to obtain its saturated state (0.2 or less) (see Example 2).

Upon the first time use, a charging bias and a developing bias may be in an off state when the lubricant is applied by the instruction of the new-latent-image-carrier detector. However, in order to more efficiently apply the lubricant to the photosensitive element 5 without deposition of toner on the brush roller 17a , a surface potential is applied to the photosensitive element 5 and the developing sleeve, thus, preventing occurrence of the surface stain. Therefore, the toner due to the surface stain is prevented from entering the brush roller 17a during application of the lubricant.

As explained above, upon the first time use, the lubricant is applied, by the instruction of the new-latent-image-carrier detector, to the new photosensitive element 5 to which no lubricant has been applied, and therefore, a sufficient amount of the lubricant can be applied. In such a manner, the lubricant is applied to the photosensitive element 5 upon the first time use, and the frictional coefficient of the photosensitive element 5 is thereby reduced to become the saturated state. This allows the lubricant to be supplied to the photosensitive element 5 by the brush roller 17a because a low frictional coefficient can be maintained even if toner is stuck to the brush roller 17a. Since the molded lubricant 17b is soft and loose, a new molded lubricant 17b is provided with a coating layer slightly harder than that of an internal lubricant. However, by applying the lubricant upon the first time use, the coating layer can be efficiently removed. If the toner is stuck to the brush roller 17a , application efficiency of the lubricant from the molded lubricant 17b to the photosensitive element 5 is reduced, which causes nonuniform application of the lubricant to occur on a portion with toner deposited and a portion without toner on the photosensitive element 5. However, as explained in the embodiment, because the lubricant is applied to the photosensitive element 5 upon the first time use, uniform application of the lubricant thereto can be performed. Therefore, the frictional coefficient of the photosensitive element 5 becomes uniform, and a rate of transfer of the toner image on the photosensitive element 5 to the transfer paper P becomes uniform, which allows an image with uniform density to be obtained. Furthermore, by applying the lubricant upon the first time use and reducing the frictional coefficient of the photosensitive element 5, even the toner having a small particle size and high sphericity can be cleaned without reduction in cleaning efficiency.

The operation for image formation of the image forming apparatus is explained below with reference to FIG. 3 and FIG. 4. Starting the operation for image formation causes each of the photosensitive elements 5 to rotate in clockwise, respectively. The surface of each photosensitive element 5 is uniformly charged by the charging roller 14a . The writing unit 6 radiates a laser beam corresponding to an image of magenta onto the photosensitive element 5 of the process cartridge 2A, a laser beam corresponding to an image of cyan onto the photosensitive element 5 of the process cartridge 2B, a laser beam corresponding to an image of yellow onto the photosensitive element 5 of the process cartridge 2C, and a laser beam corresponding to an image of black onto the photosensitive element 5 of the process cartridge 2D. Latent images respectively corresponding to image data of the colors are formed. When the latent images reach the respective positions of the developing devices 10A, 10B, 10C, and 10D through rotation of each photosensitive element 5, the latent images are developed with toners of magenta, cyan, yellow, and black, respectively, to form a four-color toner image.

On the other hand, the transfer paper P is fed from one of the paper feed cassette 11 or 12 of the paper feed portion, and is conveyed by a registration roller pair 59 at a timing at which the transfer paper P adequately meets each toner image formed on the photosensitive elements 5. The registration roller pair 59 is provided right before the transfer belt 3a. The transfer paper P is charged to a positive polarity by a paper attracting roller 58 provided near the entrance of the transfer belt 3a, and is electrostatically attracted to the surface of the transfer belt 3a. The toner images of magenta, cyan, yellow, and black are sequentially transferred to the transfer paper P while the transfer paper P attracted to the transfer belt 3a is conveyed thereby, and a full color toner image with the four colors thereon is formed. At this time, some of the toner is not transferred and remains on the photosensitive elements 5. The transfer paper P is applied with heat and pressure by the fixing device 9, and the toner image on the transfer paper P is fused into place. Thereafter, the transfer paper P with the image fixed passes through a paper discharging system according to a mode specified, and is reversed and discharged onto the paper discharge tray 26 on the upper side of the main body 1, or passes through the reversing unit 8 from the fixing device 9 to be directly discharged. Alternatively, if the double-sided image forming mode is selected, the transfer paper P is sent into the paper reverse conveying path 54 of the reversing unit 8 and switched back to be conveyed to the double-sided unit 7, supplied again to an imaging portion provided with the process cartridges 2A, 2B, 2C, and 2D where an image is formed on the rear surface of the transfer paper P, and is discharged.

Each of the photosensitive elements 5 that separate from the transfer belt 3a continues rotation as it is, and the brush roller 17a scrapes off the lubricant from the molded lubricant 17b and applies it to the photosensitive element 5. The lubricant applied is pressed against the photosensitive element 5 when the toner remaining on the photosensitive element 5 is cleaned with the cleaning blade 15a, and thereby a coat is formed on the photosensitive element 5.

Thereafter, in the image formation, the processes of image formation are repeated. However, the coat of the lubricant formed on the photosensitive element 5 is very thin, and therefore, charging by the charging unit 14 is not inhibited. After the processes, the toner image developed again on the photosensitive element 5 is transferred to the transfer paper P attracted to the transfer belt 3a.

The operation of the new-latent-image-carrier detector is explained below with reference to FIG. 5. FIG. 5 is a flowchart of the operation of the new-latent-image-carrier detector. When power to the main body 1 is turned on (step SI), the new-latent-image-carrier detector refers to the use frequency of the photosensitive element 5 stored in the IC chip, and determines whether this is first time use of the photosensitive element 5 (use frequency is zero), i.e., whether the photosensitive element 5 is a new one (step S2 ). At step S2, when it is determined that the photosensitive element 5 is a new one (step S2, Yes (Y)), the process is branched to step S3, and the new-latent-image-carrier detector issues an instruction to the main body 1 to apply the lubricant to the new photosensitive element 5. The main body 1 receives the instruction and drives the photosensitive element 5 for a fixed time (100 seconds or more) to rotate the brush roller 17a, and the lubricant is thereby scraped from the molded lubricant 17b to be applied to the photosensitive element 5. With the application, the frictional coefficient of the photosensitive element 5 is reduced to 0.4 or less (step S3). At step S3, it is desirable to operate the developing roller and the transfer belt 3a at the same linear velocity as that of the photosensitive element 5, or to separate the two from the photosensitive element 5. When the application of the lubricant is finished, the process proceeds to step S4 where an ordinary warm-up operation is conducted. At step S2, if it is determined that this is not the first time use of the photosensitive element 5 (step S2, No (N)), the process skips step S3, and the ordinary warm-up operation is conducted (step S4).

If a smaller volume-average particle size Dv of toner is used for the image forming apparatus according to the embodiment, reproduction of a thin line is improved. Therefore, toner having a particle size of 8 micrometers or less at largest is used. However, if the particle size is too small, developing capability and cleaning capability are reduced, and therefore, toner having a particle size of 3 micrometers or more at smallest is preferable. Furthermore, if it is less than 3 micrometers, carrier or toner having a micro particle size that is not easy to be developed is increased on the surface of the developing roller. Therefore, contact/friction of toner other than the toner with/against carrier becomes insufficient or the contact/friction thereof with/against the developing roller becomes also insufficient, and reversely charged toner is thereby increased. Thus, an abnormal image such as surface fog is formed, which is not preferable.

A particle size distribution expressed by a ratio (Dv/Dn) between the volume-average particle size Dv and a number-average particle size Dn ranges preferably from 1.00 to 1.40. By making the particle size distribution sharp, the distribution of an amount of charge on toner particles can be made uniform. If Dv/Dn exceeds 1.40, the distribution is widened and reversely charged toner is increased, and therefore, it is difficult to obtain high quality images. If Dv/Dn is less than 1.00, manufacture thereof is difficult, which is not practical. The toner particle size is obtained by measuring an average particle size of 50,000 toner particles using a Coulter Counter Multisizer (manufactured by Coulter Co.) and selectively using an aperture having a size of a hole for measurement of 50 micrometers corresponding to the particle size of toner to be measured.

The shape factor SF-1 of toner ranges preferably from 100 to 180, the shape factor SF-2 thereof ranges preferably from 100 to 180. FIG. 6A and FIG. 6B are schematic diagrams of toner shapes. FIG. 6A is a diagram for explaining the shape factor SF-1 and FIG. 6B is a diagram for explaining the shape factor SF-2. The shape factor SF-1 indicates the degree of sphericity of toner shape, and is expressed by the following expression (2). The shape factor SF-1 is a value obtained by dividing the square of a maximum length MXLNG of a shape formed by projecting toner onto a two-dimensional plane, by a shape area AREA, and by multiplying the result by 100Π/4.
SF−1={(MXLNG)²/AREA}×(100Π/4) (2)
If the value of SF-1 is 100, the shape of toner becomes perfect sphericity, and if the value of SF-1 is greater, the shape is more irregular.

The shape factor SF-2 indicates the degree of irregularity of toner shape, and is expressed by the following expression (3). The shape factor SF-2 is a value obtained by dividing the square of a peripheral length PERI of a shape formed by projecting toner onto a two-dimensional plane, by the shape area AREA, and by multiplying the result by 100Π/4.
SF−2={(PERI)²/AREA}×(100Π/4) (3)
If the value of SF-2 is 100, the surface of toner has no irregularity, and if the value of SF-2 is greater, the irregularity on the surface of the toner is more significant. The shape factor is measured specifically by photographing toner with a scanning electron microscope (S-800: manufactured by Hitachi Ltd.), introducing the photograph into an image analyzer (LUZEX3: manufactured by Nireco Corp.), and analyzing and calculating it.

If the shape of toner is more spherical, a contact between toner and toner or between toner and the photosensitive element 5 becomes a point contact. Therefore, the attracting force between toner particles gets weak, and as a result, fluidity becomes high. The attracting force between toner and the photosensitive element 5 gets weak, and as a result, a transfer ratio becomes high, and reversely charged toner can be easily recovered by a temporary container.

The shape factors SF-1 and SF-2 of toner are preferably 100 or more. If SF-1 and SF-2 increase, the reversely charged toner increases, and the distribution of charge amount of toner is widened, which causes the load on the temporary container to increase. Therefore, it is preferable that SF-1 do not exceed 180, and it is also preferable that SF-2 do not exceed 180.

FIG. 7A and FIG. 7B are schematic diagrams of outline shapes of toner. FIG. 7A is a diagram of external appearance of the toner and FIG. 7B is a cross section of the toner. As shown in FIG. 7A, the x axis indicates a long axis r1 of the longest axis of the toner, the y axis indicates a short axis r2 of the second longest axis, and the z axis indicates a thickness r3 of the shortest axis, and there is a relation among them as follows: long axis r1>short axis r2>thickness r3.

The toner has a substantially spherical shape indicated by a ratio (r2/r1) between the long axis and the short axis that ranges from 0.5 to 1.0, and by a ratio (r3/r2) between the thickness and the short axis that ranges from 0.7 to 1.0. If the ratio (r2/r1) between the long axis and the short axis is less than 0.5, the shape is close to an irregular shape, and the distribution of charge amount is widened, which is not preferable. If the ratio (r3/r2) between the thickness and the short axis is less than 0.7, the shape is close to an irregular shape, and the distribution of charge amount is widened, which is not preferable. Particular1y, if the ratio (r3/r2) between the thickness and the short axis is 1.0, the toner has a substantially spherical shape, and the distribution of charge amount is narrowed, which is preferable. The size of toner so far was measured by observing the toner with a scanning electron microscope (SEM) while changing an angle of a visual field.

The shape of toner can be controlled by manufacturing methods. For example, the toner obtained by a dry milling method has irregularities also on the surface of the toner, and the toner shape is not fixed. Even if the toner is obtained by the dry milling method, by applying mechanical or thermal processing thereto, the shape of the toner can be made almost perfect sphericity. In the other methods of manufacturing toner, droplets are formed using a suspension polymerization method and an emulsion polymerization method. The toner made by these methods often has a smooth surface and a shape close to perfect sphericity. Furthermore, by stirring toner particles during reaction in the solvent and applying a shearing force thereto, the shape of toner can be formed in an oval.

The toner adequately used in the image forming apparatus according to the embodiment is obtained by allowing toner material solution to undergo crosslinking reaction and/or extension reaction in an aqueous solvent. The toner material solution is obtained by dispersing polyester prepolymer having at least a functional group containing nitrogen atoms, polyester, a colorant, and a mold release agent, in an organic solvent. A schematic structure of the toner manufactured in the above manner is shown in FIG. 8. FIG. 8 is the schematic diagram of the toner. The toner consists of a binder resin and a colorant. The binder resin consists of polyester prepolymer and polyester cross-linked to each other. An external additive imparting fluidity is externally added to the surface of the toner. However, in addition to the external additive, the toner may contain a charge control agent for controlling the chargeability of the toner and a mold release agent for improving releasability for the fixing device. A material and a method of manufacturing toner are explained below.

Polyester is obtained through a polycondensation reaction of a polyhydric alcohol compound with a polyvalent carboxylic acid compound.

Polyhydric alcohol compounds (PO) include dihydric alcohol, (DIO) and polyhydric alcohols being not less than a trihydric alcohol (TO), and dihydric alcohol (DIO) alone or a mixture of dihydric alcohol (DIO) with a small amount of trihydric alcohol (TO) is preferable. Dihydric alcohol (DIO) includes alkylene glycol (e.g. ethylene glycol, 1,2-propylene glycol, 1,3-propylene glycol, 1,4-butanediol, and 1,6-hexanediol), alkylene ether glycols (e.g. diethylene glycol, triethylene glycol, dipropylene glycol, polyethylene glycol, polypropylene glycol, and polytetramethylene ether glycol), alicyclic diols (e.g. 1,4-cyclohexane dimethanol, and hydrogenated bisphenol A), bisphenols (e.g. bisphenol A, bisphenol F, and bisphenol S), adducts of alkylene oxide of the alicyclic diols (e.g. ethylene oxide, propylene oxide, and butylene oxide), and adducts of alkylene oxide of the bisphenols (e.g. ethylene oxide, propylene oxide, and butylene oxide). Among these, alkylene glycol having a carbon number from 2 to 12 and the adducts of alkylene oxides of the bisphenols are preferable. The adducts of alkylene oxides of the bisphenols and a combination of the adducts of alkylene oxides of the bisphenols and alkylene glycol having a carbon number from 2 to 12 are particularly preferable. Polyhydric alcohols being not less than trihydric alcohols (TO) include polyhydric aliphatic alcohols ranging from trihydric to octahydric alcohols and above (e.g. glycerol, trimethylolethane, trimethylolpropane, pentaerythritol, and sorbitol), phenols being not less than trivalent phenols (e.g. trisphenol PA, phenol novolak, and cresol novolak), and adducts of alkylene oxides of the polyphenols being not less than trivalent polyphenols.

Polyvalent carboxylic acid (PC) includes divalent carboxylic acid (DIC) and polyvalent carboxylic acid being not less than trivalent carboxylic acid (TC). The divalent carboxylic acid (DIC) alone and a mixture of the divalent carboxylic acid (DIC) and a small amount of the polyvalent carboxylic acid (TC) are preferable. The examples of divalent carboxylic acids (DlC) are alkylene dicarboxylic acids (e.g. succinic acid, adipic acid, and sebacic acid), alkenylene dicarboxylic acid (e.g. maleic acid and fumaric acid), and aromatic dicarboxylic acids (e.g. phthalic acid, isophthalic acid, terephthalic acid, and naphthalene dicarboxylic acid). Among these, the alkenylene dicarboxylic acid having a carbon number from 4 to 20 and the aromatic dicarboxylic acids having a carbon number from 8 to 20 are preferable. The examples of polyvalent carboxylic acids being not less than trivalent carboxylic acid (TC) are aromatic polyvalent carboxylic acids having a carbon number from 9 to 20 (e.g. trimellitic acid and pyromellitic acid). The acid anhydrides or lower alkyl esters of these (e.g. methyl ester, ethyl ester, and isopropyl ester) may be used as polyvalent carboxylic acid (PC) and allowed to react with the polyhydric alcohol (PO).

The ratio between the polyhydric alcohol (PO) and the polyvalent carboxylic acid (PC) is an equivalent ratio [OH]/[COOH] between a hydroxyl group [OH] and a carboxyl group [COOH] and is generally from 2/1 to 1/1, preferably from 1.5/1 to 1/1, more preferably from 1.3/1 to 1.02/1.

The polycondensation reaction of polyhydric alcohol (PO) and polyvalent carboxylic acid (PC) is performed by heating them to 150° C. to 280° C. in the presence of a known esterification catalyst such as tetrabutoxytitanate and dibutyltin oxide, and by distilling water generated while pressure is reduced if required, and polyester having the hydroxyl group is obtained. A valence of the hydroxyl group of polyester is preferably 5 or more, and an acid value of polyester generally ranges from 1 to 30, preferably 5 to 20. By making polyester have the acid value, it is easy to be negatively charged, and affinity between a recording paper and toner becomes excellent when the toner is fixed on the recording paper. Thus, low-temperature fixability is improved. However, if the acid value exceeds 30, polyester has a deteriorating tendency of charge stability, particularly of environmental fluctuations.

The weight average molecular weight ranges from 10,000 to 400,000, and preferably, 20,000 to 200,000. If the weight average molecular weight is less than 10,000, offset resistance is worsened, which is not preferable. If the weight average molecular weight exceeds 400,000, the low-temperature fixability gets worse, which is not preferable.

It is preferable to contain urea-modified polyester in polyester, in addition to unmodified polyester obtained through the polycondensation reaction. The urea-modified polyester is obtained by reacting a carboxyl group or a hydroxyl group at the end of polyester obtained through the polycondensation reaction with a polyvalent isocyanate compound (PIC) to obtain polyester prepolymer (A) having an isocyanate group, and by reacting the polyester prepolymer (A) with amine group to crosslink and/or extend a molecular chain.

Examples of polyvalent isocyanate compounds (PIC) are aliphatic polyvalent isocyanates (e.g. tetramethylene diisocyanate, hexamethylene diisocyanate, and 2,6-diisocyanate methyl caproate), alicyclic polyisocyanates (e.g. isophorone diisocyanate and cyclohexylmethane diisocyanate), aromatic diisocyanates (e.g. tolylene diisocyanate and diphenylmethane diisocyanate), aromatic aliphatic diisocyanates (e.g. α,α,α′,α′-tetramethylxylylene diisocyanate), isocyanates, compounds formed by blocking of these polyisocyanates by a phenol derivative, an oxime, and a caprolactam, and a combination of at least two of these.

A ratio of the polyvalent isocyanate compound (PIC) is an equivalent ratio [NCO]/[OH] of an isocyanate group [NCO] and a hydroxyl group [OH] of a polyester and is generally ranges from 5/1 to 1/1, preferably from 4/1 to 1.2/1, and more preferably from 2.5/1 to 1.5/1. If the ratio of [NCO]/[OH] exceeds 5, the low-temperature fixability gets worse. If the mole ratio of [NCO] is less than 1, when urea unmodified polyester is used, the urea content in the ester becomes low, thereby affecting hot offset resistance.

The content of the polyvalent isocyanate compound (PIC) in the polyester prepolymer (A) having an isocyanate group ranges generally from 0.5 wt.% to 40 wt. %, preferably from 1 wt. % to 30 wt. %, and more preferably from 2 wt. % to 20 wt. %. If the content of the polyvalent isocyanate compound is less than 0.5 wt. %, the hot offset resistance deteriorates and it is unfavorable from the viewpoint of compatibility of heat resistant preservability and low-temperature fixability. On the other hand, if the content of the polyvalent isocyanate compound exceeds 40 wt. %, the low-temperature fixability gets worse.

The number of isocyanate groups contained per molecule in the polyester prepolymer (A) having the isocyanate group is generally at least 1, preferably, an average ranging from 1.5 to 3, and more preferably, an average ranging from 1.8 to 2.5. If the isocyanate group per molecule is less than 1, then the molecular weight of the urea-modified polyester becomes low and the hot offset resistance deteriorates.

Further, examples of amines (B) caused to react with the polyester prepolymer (A) are a divalent amine compound (B1), a polyvalent amine compound (B2) being not less than trivalent amines, aminoalcohol (B3), aminomercaptan (B4), amino acid (B5), and a compound (B6) in which the amino groups from the B1 to the B5 are blocked.

Examples of the divalent amine compound (B1) are aromatic diamines (e.g. phenylene diamine, diethylene diamine, and 4,4′-diaminodiphenyl methane), alicyclic diamines (e.g. 4,4′-diamino-3,3′-dimethyidicyclohexylmethane, diamine cyclohexane, and isophorone diamine), and aliphatic diamines (e.g. ethylene diamine, tetramethylene diamine, and hexamethylene diamine). Examples of the polyvalent amine compound (B2) being not less than trivalent amines are diethylene triamine and triethylene tetramine. Examples of the aminoalcohol (B3) are ethanolamine and hydroxyethylaniline. Examples of the aminomercaptan (B4) are aminoethyl mercaptan and aminopropyl mercaptan. Examples of the amino acid (B5) are aminopropionic acid and aminocaproic acid. Examples of the compound (B6) in which the amino groups from the B1 to the B5 are blocked are ketimine compounds obtained from the amines from the B1 to the B5. ketones (e.g. acetone, methyl ethyl ketone, and methyl isobutyl ketone), and oxazolidine compounds. The preferable amines among the amines (B) are the B1 and a mixture of the B1 with a small amount of the B2.

A ratio of amines (B) is an equivalent ratio [NCO]/[NHx] of an isocyanate group [NCO] in the polyester prepolymer (A) having an isocyanate group and an amine group [NHx] in the amines (B), and ranges generally form 1/2 to 2/1, preferably from 1.5/1 to 1/1.5, and more preferably from 1.2/1 to 1/1.2. If the ratio of [NCO]/[NHx] is greater than 2 or less than 1/2, the molecular weight of the urea-modified polyester decreases and the hot offset resistance deteriorates.

Moreover, an urethane bond may be contained together with an urea bond in the urea-modified polyester. A mole ratio of the urea bond content and the urethane bond content ranges generally from 100/0 to 10/90, preferably from 80/20 to 20/80, and more preferably from 60/40 to 30/70. If the mole ratio of the urea bond is less than 10%, the hot offset resistance deteriorates.

The urea-modified polyester is manufactured by a method like a one-shot method. A polyhydric alcohol (PO) and a polyvalent carboxylic acid (PC) are heated up to 150° C. to 280° C. in the presence of a known esterification catalyst such as tetrabutoxy titanate and dibutyltin oxide. The pressure is reduced if necessary and the water generated is removed by evaporation to obtain polyester having a hydroxyl group. Further, polyvalent isocyanate (PIC) is allowed to react with the polyester at a temperature of 40° C. to 140° C. to obtain polyester prepolymer (A) having an isocyanate group. Furthermore, amines (B) are allowed to react with this polyester prepolymer (A) at a temperature of 0° C. to 140° C. to obtain an urea-modified polyester.

If necessary, a solvent can be used for reaction of the polyvalent isocyanate (PIC) and reaction of the polyester prepolymer (A) with the amines (B). Examples of the solvent that can be used are aromatic solvents (e.g. toluene and xylene), ketones (e.g. acetone, methyl ethyl ketone, and methyl isobutyl ketone), esters (e.g. ethyl acetate), amides (e.g. dimethyl formamide, and dimethyl acetoamide), and ethers (e.g. tetrahydrofuran) that are inert to the polyvalent isocyanate (PIC).

For crosslinking reaction and/or extension reaction of polyester prepolymer (A) with amines (B), a reaction terminator can be used if necessary to adjust the molecular weight of the urea-modified polyester that is obtained. Examples of the reaction terminator are monoamines (e.g. diethylamine, dibutylamine, butylamine, and laurylamine) and compounds in which these are blocked (ketimine compounds).

The weight average molecular weight of the urea-modified polyester is generally not less than 10,000, preferably ranges from 20,000 to 10,000,000, and more preferably from 30,000 to 1,000,000. If the weight average molecular weight is less than 10,000, the hot offset resistance deteriorates. The number average molecular weight of the urea-modified polyester is not particularly restricted when the unmodified polyester is used, and may be a number average molecular weight that is suitable to obtain the weight average molecular weight. If the urea-modified polyester is used alone, the number average molecular weight ranges generally from 2,000 to 15,000, preferably from 2,000 to 10,000, and more preferably from 2,000 to 8,000. If the number average molecular weight is greater than 20,000, the low-temperature fixability and the gloss, if the urea-modified polyester is used for a full color unit, are worsened.

By using both the unmodified polyester and the urea-modified polyester, the low-temperature fixability and the gloss, if both of them are used for the full color unit, are improved. Therefore, it is preferable to use them together rather than using the urea-modified polyester alone. The unmodified polyester may contain polyester modified by a chemical bond other than the urea bond.

It is preferable that the unmodified polyester and the urea-modified polyester be at least partly compatible from the viewpoint of the low-temperature fixability and the hot offset resistance. Therefore, it is preferable that the unmodified polyester and the urea-modified polyester have similar composition.

A weight ratio of the unmodified polyester and the urea-modified polyester ranges generally from 20/80 to 95/5, preferably from 70/30 to 95/5, and more preferably from 75/25 to 95/5. The most preferable weight ratio ranges from 80/20 to 93/7. If the weight ratio of the urea-modified polyesters is less than 5 %, the hot offset resistance deteriorates, and compatibility of heat resistant preservability and low-temperature fixability gets worse.

A glass transition point (Tg) of a binder resin containing unmodified polyester and urea-modified polyester ranges generally from 45° C. to 65° C., preferably from 45° C. to 60° C. If the glass transition point is below 45° C., then the heat resistance of the toner deteriorates, while if the glass transition point is above 65° C., the low-temperature fixability becomes insufficient.

The urea-modified polyester tends to be present on the surface of toner base particles obtained, and therefore, even if the glass transition point is lower as compared with that of the known polyester based toner, it has a tendency to have good heat resistant preservability.

For colorant, known dyes and pigments can be used. For example, followings and mixtures thereof can be used: carbon black, Nigrosine dye, ion black, Naphthol Yellow S, Hansa Yellow (10G, 5G, G), cadmium yellow, yellow iron oxide, yellow ocher, chrome yellow, titanium yellow, polyazo yellow, oil yellow, Hansa Yellow (GR, A, RN, R), pigment yellow L, Benzidine Yellow (G, GR), Permanent Yellow (NCG), Vulcan Fast Yellow (5G, R), Tartrazine Lake, Quinoline Yellow Lake, Anthrazane Yellow BGL, Isoindolinone Yellow, red ion oxide, minium, red lead, Cadmium Red, Cadmium Mercury Red, Antimony Vermilion, Permanent Red 4R, Para Red, Fire Red, parachloro-ortho-nitroaniline red, Lithol Fast Scarlet G, Brilliant Fast Scar1et, Brilliant Carmine BS, Permanent Red (F2R, F4R, FRL, FRLL, F4RH), Fast Scar1et VD, Vulcan Fast Rubin B, Brilliant Scarlet G, Lithol Rubin GX, Permanent Red F5R, Brilliant Carmine 6B, Pigment Scarlet 3B, Bordeaux 5B, Toluidine Maroon, Permanent Bordeaux F2K, Helio Bordeaux BL, Bordeaux 10B, BON Maroon Light, BON Maroon Medium, Eosin Lake, Rhodamine Lake B, Rhodamine Lake Y, Alizarin Lake, Thioindigo Red B, Thioindigo Maroon, Oil Red, Quinacridone Red, Pyrazolone Red, Polyazo Red, Chrome Vermilion, Benzidine Orange, Perinone Orange, Oil Orange, Cobalt Blue, Cerulean Blue, Alkali Blue Lake, Peacock Blue Lake, Victoria Blue Lake, metal-free Phthalocyanine Blue, Phthalocyanine Blue, Fast Sky Blue, Indanthrene Blue (RS, BC), indigo, ultramarine blue, Prussian blue, Anthraquinone Blue, Fast Violet B, Methyl Violet Lake, Cobalt Violet, Manganese Violet, Dioxane Violet, Anthraquinone Violet, Chrome Green, Zinc Green, chrome oxide, pyridian, Emerald Green, Pigment Green B, Naphthol Green B, Green Gold, Acid Green Lake, Malachite Green Lake, Phthalocyanine Green, Anthraquinone Green, titanium oxide, zinc white, and lithopone. The content of the colorant is generally from 1 wt. % to 15 wt. %, preferably from 3 wt. % to 10 wt. %, based on total weight of the toner.

The colorant can be also used as a master batch which is prepared by combining the colorant with a resin. Examples of the binder resin for manufacturing the master batch or being kneaded with the master batch are styrene polymers and substituted styrene polymers such as polystyrene, poly-p-chlorostyrene, and polyvinyltoluene; copolymers of one of these and a vinyl compound; and polymethyl methacrylate, polybutyl methacrylate, polyvinyl chloride, polyvinyl acetate, polyethylene, polypropylene, polyester, epoxy resin, epoxy polyol resin, polyurethane, polyamide, polyvinyl butyral, polyacrylic resin, rosin, modified rosin, terpene resin, aliphatic or alicyclic hydrocarbon resin, aromatic petroleum resin, chlorinated paraffin, and paraffin waxes. These resins can be used alone or in combination.

Known charge control agents can be used as a charge control agent. Examples of the charge control agent are Nigrosine based dyes, triphenylmethane based dyes, chromium-containing complex dyes, chelate molybdate pigments, Rhodamine based dyes, alkoxy amines, and quaternary ammonium salts (including fluorine modified quaternary ammonium salts), alkylamides, simple substances of phosphorous or compounds thereof, simple substances of tungsten or compounds thereof, fluorine-based active agents, salicylic acid metal salts, and metal salts of salicylic acid derivatives. More specific examples of the charge control agent are Bontron 03 as a Nigrosine based dye, Bontron P-51 as a quaternary ammonium salt, Bontron S-34 as a metal containing azo dye, E-82 as an oxynaphthoe acid based metal complex, E-84 as a salicylic acid based metal complex, E-89 as a phenol based condensate (these are manufactured by Orient Chemical Industries, Ltd.), TP-302 and TP-415 as quaternary ammonium salt molybdenum complexes (manufactured by Hodogaya Chemical Industries, Ltd.), Copy Charge PSY VP2038 as a quaternary ammonium salt, Copy Blue PR as a triphenylmethane derivative, Copy Charge NEG VP2036 and Copy Charge NX VP434 as quaternary ammonium salts (these are manufactured by Hoechst Co., Ltd.), LRA-901 and LR-147 as a boron complex (manufactured by Japan Carlit Co., Ltd.), copper phthalocyanine, perylene, quinacridone, azo based pigments, and polymer compounds having a functional group such as a sulfonic acid group, a carboxyl group, and quaternary ammonium salt. Among these, materials that control toner negatively are particularly preferable.

The amount of use of the charge control agent is determined depending on the type of binder resins, presence or absence of additives to be used as required, and a method of manufacturing toner including a dispersion method, and therefore, it is not uniquely restricted. However, the charge control agent is used in a range from 0.1 to 10 parts by weight (wt. parts), preferably from 0.2 to 5 wt. parts per 100 wt. parts of the binder resin. If it exceeds 10 wt. parts, the toner is charged too highly, effects of the main charge control agent are decreased, and electrostatic attracting force for a developing roller is increased, which causes fluidity of the developer to be lowered and image density to be reduced.

A wax having a low melting point in a range from 50° C. to 120° C. functions effectively between the fixing roller and the surface of toner as a mold release agent during dispersion with a binder resin. Due to this function of wax, there is no need to apply a mold release agent like oil to the fixing roller to improve high temperature offset. Examples of waxes are vegetable based wax such as carnauba wax, cotton wax, Japan tallow, rice wax; animal based wax such as bees wax and lanolin; mineral based waxes such as ozokerite and cercine; and petroleum based wax such as paraffin, micro crystalline, and petrolatum. Other examples of wax apart from these natural waxes are synthetic hydrocarbon wax such as Fischer Tropsch wax, polyethylene wax; and synthetic wax such as esters, ketones, and ethers. Furthermore, the followings can also used: fatty acid amides of 12-hydroxy stearic acid amides, stearic acid amides, phthalic anhydride imide, and chlorinated hydrocarbon; and crystalline high polymers having a long alkyl group in a side chain such as homopolymers or copolymers (e.g. copolymers of n-stearyl acrylate-ethyl methacrylate) of polyacrylates such as poly-n-stearyl methacrylate, poly-n-lauryl methacrylate, that are crystalline high polymer resins having a low molecular weight.

These charge control agent and the mold release agent can be melted and kneaded with master batch and binder resin, or may be added to an organic solvent when it is dissolved or dispersed.

The external additive is used for helping fluidity, development, and charging of toner particles, and inorganic particles are preferably used as the external additive. The primary particle size of the inorganic particles is preferably from 5×10−3 to 2 micrometers, more preferably from 5×10−3 to 0.5 micrometer. A specific surface area based on the BET method is preferably from 20 m2/g to 500 m2/g. A proportion of the inorganic particles to be used is preferably from 0.01 wt. % to 5 wt. %, more preferably from 0.01 wt. % to 2.0 wt. % of the total weight of the toner.

Specific examples of the inorganic particles are silica, alumina, titanium oxide, barium titanate, magnesium titanate, calcium titanate, strontium titanate, zinc oxide, tin oxide, quartz sand, clay, mica, wollastonite, silious earth, chrome oxide, cerium oxide, red oxide, antimony trioxide, magnesium oxide, zirconium oxide, barium sulfate, barium carbonate, calcium carbonate, silicon carbide, and silicon nitride. Among these, as a fluidity imparting agent, a combination of a hydrophobic silica particle and a hydrophobic titanium oxide particle is preferable. In particular, if a combination of these each of which has an average particle size of not greater than 5×10⁻²micrometer is used and mixed, electrostatic force and Van der Waals force with toner are significantly improved. Therefore, even if toner is mixed and agitated in a developing device to obtain a desired charged level, the fluidity imparting agent is not separated from the toner, which allows excellent image quality without image defects such as “white spots” to be obtained. In addition, toner particles remaining on an image carrier even after an toner image is transferred can be reduced.

Titanium oxide particles are excellent in environmental stability and image density stability. On the other hand, charge rise characteristic tends to be worse. Therefore, if the amount of addition of titanium oxide particles is greater than that of silica particles, influence of the side effect becomes more significant. However, if the amount of addition of hydrophobic silica particles and hydrophobic titanium oxide particles is in a range from 0.3 wt. % to 1.5 wt. %, the charge rise characteristic is not lost so much, and desired charge rise characteristic is obtained. Thus, a stable image quality is obtained even if copying is repeatedly carried out.

A method of manufacturing toner is explained below. The method explained here is a preferable method, but the method is not limited thereto.

1) A toner material solution is prepared by dispersing a colorant, unmodified polyester, polyester prepolymer having an isocyanate group, and a mold releasing agent in an organic solvent.

The organic solvent has preferably a volatile organic solvent having a boiling point below 100° C. because it is easy to be removed after toner base particles are formed. Specific examples of the organic solvent are toluene, xylene, benzene, carbon tetrachloride, methylene chloride, 1,2-dichloroethane, 1,1,2-trichloroethane, trichloroethylene, chloroform, monochlorobenzene, dichloroethylidene, methyl acetate, ethyl acetate, methyl ethyl ketone, methyl isobutyl ketone. These can be used alone or a combination of at least two of them. Particularly, aromatic solvents such as toluene and xylene, and halogen hydrocarbons such as methylene chloride, 1,2-dichloroethane, chloroform, and carbon tetrachloride are preferable. The amount of use of the organic solvent ranges generally from 0 to 300 wt. parts per 100 wt. parts of the polyester prepolymer, preferably from 0 to 100 wt. parts, and more preferably from 25 to 70 wt. parts.

2) The toner material solution is emulsified in an aqueous medium in the presence of a surfactant and fine particles of resin.

The aqueous medium may be water alone or may contain an organic solvent such as an alcohol (e.g. methanol, isopropyl alcohol, and ethylene glycol), dimethyl formamide, tetrahydrofuran, a cellosolve group (e.g. methyl cellosolve), and a lower ketone group (e.g. acetone, and methyl ethyl ketone).

The amount of use of the aqueous medium per 100 wt. parts of the toner material solution ranges generally from 50 to 2,000 wt. parts, preferably from 100 to 1,000 wt. parts. If the amount is less than 50 wt. parts, this affects the dispersion of the toner material solution, and toner particles of a predetermined particle size cannot be obtained. It is not economical to use the aqueous medium if the amount exceeds 2,000 wt. parts.

Further, to improve the dispersion in the aqueous medium, a dispersing agent such as a surfactant and fine particles of resin is added as required. Examples of the surfactant are anionic surfactants such as alkyl benzene sulfonate, (α-olefin sulfonate, and ester phosphate; amine salts such as alkyl amine salts, aminoalcohol fatty acid derivatives, polyamine fatty acid derivatives, and imidazoline; cationic surfactants of quaternary ammonium salt types such as alkyl trimethyl ammonium salts, dialkyl dimethyl ammonium salts, alkyl dimethyl benzyl ammonium salts, pyridinium salts, alkyl isoquinolinium salts, and benzethonium chloride; nonionic surfactants such as fatty acid amide derivatives and polyhydric alcohol derivatives; and ampholytic surfactants such as alanine, dodecyl di (aminoethyl)glycine, di (octylaminoethyl)glycine, N-alkyl-N, and N-dimethyl ammonium betaine.

Furthermore, by using a surfactant having a fluoroalkyl group, a desired effect can be achieved with a very small amount thereof. Preferable examples of anionic surfactants having a fluoroalkyl group are fluoroalkyl carboxylic acids having a carbon number from 2 to 10 and their metal salts, disodium perfluorooctane sulfonyl glutamate, sodium 3-[ω-fluoroalkyl (C6 to C11)oxy]-1-alkyl (C3 to C4)sulfonate, sodium 3-[ω-fluoroalkanoyl (C6 to C8)-N-ethylamino]-1-propane sulfonate, fluoroalkyl (C11 to C20) carboxylic acid and its metal salts, perfluoroalkyl carboxylic acid (C7 to C13) and its metal salts, perfluoroalkyl (C4 to C12)sulfonic acid and its metal salts, perfluorooctane sulfonic acid diethanolamide, N-propyl-N-(2-hydroxyethyl)perfluorooctane sulfonamide, perfluoroalkyl (C6 to C10)sulfonamidepropyl trimethyl ammonium salts, perfluoroalkyl (C6 to C10)-N-ethylsulfonyl glycine salts, monoperfluoroalkyl (C6 to C16)ethyl phosphoric acid esters.

Examples of trade names are SURFLON S-111, S-112, S113 (manufactured by Asahi Glass Co., Ltd.), FLUORAD FC-93, FC-95, FC-98, FC-129 (manufactured by Sumitomo 3M Co., Ltd.), UNIDINE DS-101, DS-102 (manufactured by Daikin Industries, Ltd.), MEGAFACE F-110, F-120, F-113, F-191, F-812, F-833 (manufactured by Dainippon Ink & Chemicals, Inc.), EKTOP EF-102, 103, 104, 105, 112, 123A, 123B, 306A, 501, 201, 204 (manufactured by Tochem Products Co., Ltd.), and FTERGENT F-100 and F150 (manufactured by Neos Co., Ltd.).

Examples of cationic surfactants are primary aliphatic, secondary aliphatic, or secondary amino acid having a fluoroalkyl group; quaternary aliphatic ammonium salts such as perfluoroalkyl (C6 to C10)sulfonamidepropyl trimethyl ammonium salt; benzalkonium salts, benzethonium chloride, pyridinium salts, and imidazolinium salts. The examples of trade names thereof are SURFLON S-121 (manufactured by Asahi Glass Co., Ltd.), FLUORAD FC-135 (manufactured by Sumitomo 3 M Co., Ltd.), UNIDINE DS-202 (manufactured by Daikin Industries, Ltd.), MEGAFACE F-150 and F-824 (manufactured by Dainippon Ink & Chemicals, Inc.), EKTOP EF-132 (manufactured by Tochem Products Co., Ltd.), and FTERGENT F-300 (manufactured by Neos Co., Ltd.).

The fine particles of resin are added to stabilize toner base particles that are formed in the aqueous medium. Therefore, it is preferable to add the fine particles of resin so that a coverage of the fine particles over the surface of the toner base particles is in a range of 10 % to 90 %. Examples of the fine particles are fine particles of poly methyl methacrylate having a particle size of 1 micrometer and 3 micrometers; fine particles of polystyrene having a particle size of 0.5 micrometer and 2 micrometers; and fine particles of poly (styrene-acrylonitrile) having a particle size of 1 micrometer. Examples of trade names are PB-200H (manufactured by Kao Corp.), SGP (manufactured by Soken Co., Ltd.), TECHPOLYMER-SB (manufactured by Sekisui Plastics Co., Ltd.), SGP-3G (manufactured by Soken Co., Ltd.), and MICROPEARL (manufactured by Sekisui Fine Chemical Co. Ltd.). Moreover, inorganic dispersing agents such as calcium phosphate tribasic, calcium carbonate, titanium oxide, colloidal silica, and hydroxyapatite can also be used.

Dispersion droplets may be stabilized by a high polymer protective colloid as a dispersing agent that can be used with both fine particles of resin and an inorganic dispersing agent. Examples are acids such as acrylic acid, methacrylic acid, (α-cyanoacrylic acid, α-cyanomethacrylic acid, itaconic acid, crotonic acid, fumaric acid, maleic acid, or maleic anhydride; or (metha)acrylic monomers containing a hydroxyl group such as β-hydroxyethyl acrylate, β-hydroxyethyl methacrylate, β-hydroxypropyl acrylate, β-hydroxypropyl methacrylate, γ-hydroxypropyl acrylate, γ65-hydroxypropyl methacrylate, 3-chloro 2-hydroxypropyl acrylate, 3-chloro 2-hydroxypropyl methacrylate, diethylene glycol monoacrylic ester, diethylene glycol monomethacrylic ester, glycerol monoacrylic ester, glycerol monomethacrylic ester, N-methylol acrylamide, N-methylol methacrylamide; vinyl alcohol or ethers of vinyl alcohol such as vinyl methyl ether, vinyl ethyl ether, vinyl propyl ether; or esters of compounds that contains a vinyl alcohol and a carboxyl group such as vinyl acetate, vinyl propionate, vinyl butyrate; acrylamide, methacrylamide, diacetone acrylamide or their methylol compounds; acid chlorides such as acryloyl chloride and methacryloyl chloride; homopolymers or copolymers of a compound having a nitrogen atom or heterocyclic ring thereof such as vinylpyridine, vinylpyrrolidone, vinylimidazole, and ethyleneimine; polyoxyethylene compounds such as polyoxyethylene, polyoxypropylene, polyoxyethylene alkyl amine, polyoxypropylene alkyl amine, polyoxyethylene alkyl amide, polyoxypropylene alkyl amide, polyoxyethylene nonyl phenyl ether, polyoxyethylene lauryl phenyl ether, polyoxyethylene stearyl phenyl ester, and polyoxyethylene nonyl phenyl ester; and a cellulose group such as methyl cellulose, hydroxyethyl cellulose, and hydroxypropyl cellulose.

A dispersion method is not particularly limited, and it is possible to use known facilities of a low-speed shearing type, a high-speed shearing type, a friction type, a high-pressure jetting type, and an ultrasonic dispersion type. Among these, to allow the dispersed particles to have an average particle size ranging from 2 to 20 micrometers, the high-speed shearing type is preferred. When a high-speed shearing type dispersing machine is used, the number of revolutions is not particularly limited and is generally from 1,000 revolutions per minute (rpm) to 30,000 rpm, preferably from 5,000 rpm to 20,000 rpm. The dispersion time is not particularly limited and is generally from 0.1 to 5 minutes in a batch system. The dispersing temperature is generally from 0° C. to 150° C. (under a pressure), preferably from 40° C. to 98° C.

3) While preparing an emulsified liquid, amines (B) are added and are allowed to react with polyester prepolymer (A) having an isocyanate group.

This reaction is also performed with cross linkage and/or extension of a molecular chain. The reaction time is selected according to the reactivity of amines (B) with an isocyanate group structure of the polyester prepolymer (A), and is generally in a range from 10 minutes to 40 hours, preferably in a range from 2 hours to 24 hours. The reaction temperature ranges generally from 0° C. to 150° C., preferably from 40° C. to 98° C. Moreover, a known catalyst can be used if necessity. Specific examples of the catalyst are dibutyl tin laurate and dioctyl tin laurate.

4) After completion of the reaction, the organic solvent is removed from a mixture emulsified and dispersed (reaction compound), is washed, and dried to obtain the toner base particles.

To remove the organic solvent therefrom, the whole system is gradually heated up while laminar flow is stirred. The mixture is stirred vigorously at around a particular temperature, a solvent is removed from the mixture, and then fusiform toner base particles are prepared. Further, if a compound like calcium phosphate salt that dissolves in an acid or an alkali is used an a dispersion stabilizer, after the calcium phosphate salt is dissolved in an acid like hydrochloric acid, the calcium phosphate salt is removed from the toner base particles by a method of cleaning. In addition, the calcium phosphate salt may be decomposed by an enzyme and removed.

5) A charge controlling agent is implanted into the toner base particles thus obtained, and inorganic fine particles such as those of silica and titanium oxide are added externally to obtain the toner.

The implantation of the charge controlling agent and the external addition of the inorganic fine particles are carried out by a known method using a mixer and so on. Thus, toner having a small particle size and a sharp particle size distribution can be obtained easily. Moreover, by vigorously stirring in the process of removing the organic solvent, the shape of particles between a perfectly spherical shape and a rugby ball-like shape can be controlled. Furthermore, the morphology of the surface can also be controlled between the smooth and the rough.

FIG. 9 is a diagram of the result of experiments in which stable cleaning performance can be obtained by adding the inorganic additive to the lubricant. Toner manufactured by the method of manufacturing toner and having a volume-average particle size of 5 micrometers is used. The molded lubricant 17b is added with 30 wt. % of silica for zinc stearate as a main component, and it is evaluated how the charging roller 14a gets soiled by changing the particle size of silica to various particle sizes. The soil of the charging roller 14a represents the amount of toner remaining on the surface of the photosensitive element 5 without being cleaned by the cleaning blade 15a, and this amount is thought as a typical property of the cleaning performance of the image forming apparatus. Soil rank 5 indicates that no soil is found on the charging roller 14a, and the soil of the charging roller 14a is getting worse as the rank is lowered. In this evaluation, if the rank is 4 or higher (ranks 4 and 5), no defects occur in an image to be transferred to a transfer paper P. Therefore, it is understood from FIG. 9 that no defects occurs if a particle size of silica to be added ranges from 0.01 to 0.5 micrometer. The similar experiments were conducted using toner having different particle sizes. By adding silica having a particle size that satisfies the expression (1) to the molded lubricant 17b , stable cleaning performance is obtained.

FIG. 10 is a graph of a relationship between the number of sheets of A4-sized transfer paper P passed through a space with a new photosensitive element 5 and a frictional coefficient of the photosensitive element 5. The transfer paper P has an image with an image area of 5% formed thereon and passed through the space with the new photosensitive element 5 to which the lubricant is not applied. According to the graph, the frictional coefficient of the new photosensitive element 5 is 0.4 or more, but is gradually decreasing with an increase in the number of sheets to which images are transferred. The frictional coefficient decreases to 0.2 or less when images are transferred to about 100 sheets, where the frictional coefficient is saturated. It is understood from this that the frictional coefficient of the photosensitive element 5, after the lubricant is applied thereto by the instruction of the new-latent-image-carrier detector, is preferably 0.4 or less, more preferably 0.2 or less.

The frictional coefficient of the photosensitive element 5 was measured by an oiler belt system in the following manner. FIG. 11 is a diagram for explaining how to measure the frictional coefficient of the photosensitive element 5. In this case, a belt 70 was stretched on drum circumference ¼ of the photosensitive element 5 so that a fine paper having an intermediate thickness was directed in a longitudinal direction. A load 71 of, for example, 0.98 N (100 g) was hung at one end of the belt 70, and a force gage 72 was placed on the other end and stretched. The load was read at a point in time when the belt 70 was moved, and a value read was substituted in the following expression: frictional coefficient μs=2/Π×ln (F/0.98) (where p: static frictional coefficient, and F: value measured) to calculate the frictional coefficient.

FIG. 12 is a graph of a relationship between the rotation time of the photosensitive element 5 and the frictional coefficients (at three points) of the photosensitive element 5 when the new photosensitive element 5 is made to rotate alone under the following condition and the lubricant is applied thereto by the brush roller 17a.

Conditions:

Photosensitive element Diameter: 30 Linear velocity: 125 mm/s Molded lubricant Material: Zinc stearate Spring pressure force: 1000 mN Brush roller Material: Conductive nylon Density: 30 K F/inch² Thickness: 10 D Engaging amount against photosensitive element: 1.0 mm

According to FIG. 12, by setting the application time to 100 seconds or more, it is possible to reduce the frictional coefficient of the photosensitive element 5 to obtain its saturated state (0.2 or less).

Since the present invention is configured as explained above, it is possible to provide the lubricant capable of significantly reducing the amount of toner, as compared with the conventional technology, that passes under the cleaning blade and remains on the photosensitive element irrespective of states of an image formed even if spherical toner is used, and also to provide the lubricant applying unit, the process cartridge, and the image forming apparatus.

Furthermore, it is possible to provide the lubricant applying unit, the process cartridge, and the image forming apparatus capable of maintaining stable cleaning performance by supplying the sufficient amount of lubricant to the photosensitive element even if it is the first time use of the photosensitive element.

Although the invention has been described with respect to a specific embodiment for a complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modifications and alternative constructions that may occur to one skilled in the art which fairly fall within the basic teaching herein set forth.

Claims

1. A lubricant for electrophotography applied to a latent image carrier to which toner having a sphericity of equal to or more than 0.94 is supplied, wherein

an inorganic additive is added to the lubricant, and

the inorganic additive satisfies

2Y/1000≦X≦Y/10

where Y is a particle size of the toner in micrometer, and X is a particle size of the inorganic additive in micrometer.

2. The lubricant according to claim 1, wherein the inorganic additive is a substance containing at least one of silica, titania, alumina, magnesia, zirconia, ferrite, and magnetite.

3. The lubricant according to claim 1, wherein a main component of the lubricant is either of fatty acid metal salt and fluorine particles.

4. The lubricant according to claim 3, wherein an amount of addition of the inorganic additive ranges from 1 weight percent to 30 weight percent with respect to the main component.

5. A lubricant applying unit that applies a lubricant for electrophotography to a latent image carrier, the lubricant applying unit comprising a brush roller that rotates around a rotational axis, and is in contact with the latent image carrier, wherein

toner having a sphericity of equal to or more than 0.94 is supplied to the latent image carrier,

an inorganic additive is added to the lubricant,

the inorganic additive satisfies

2Y/1000≦X≦Y/10

where Y is a particle size of the toner in micrometer, and X is a particle size of the inorganic additive in micrometer,

the lubricant is a molded lubricant that is molded as a solid block, and

the brush roller slides along, and scrapes off the molded lubricant so that the lubricant is applied to the latent image carrier.

6. The lubricant applying unit according to claim 5, further comprising a new-latent-image-carrier detector that detects whether the latent image carrier is used for a first time, wherein

when it is detected that the latent image carrier is used for the first time, the lubricant is applied to the latent image carrier.

7. The lubricant applying unit according to claim 6, wherein a frictional coefficient of the latent image carrier after applying the lubricant is equal to or less than 0.4.

8. The lubricant applying unit according to claim 5, further comprising a pushing unit that pushes the molded lubricant against the brush roller by a force of equal to or more than 200 meter-Newtons.

9. The lubricant applying unit according to claim 5, wherein

the brush roller rotates in a direction of rotation of the latent image carrier, and

a peripheral speed ratio of the brush roller to the latent image carrier ranges from 0.8 to 1.2.

10. A process cartridge comprising:

a latent image carrier;

at least one of a charging unit that causes a charging member to contact or to be close to the latent image carrier to charge the latent image carrier; a developing unit that deposits toner on a latent image on the latent image carrier to develop the latent image with the toner; and a cleaning unit that cleans the toner remaining on the latent image carrier with a cleaning blade; and

a lubricant applying unit that applies a lubricant for electrophotography to the latent image carrier, the lubricant applying unit including a brush roller that rotates around a rotational axis, and is in contact with the latent image carrier, wherein

toner having a sphericity of equal to or more than 0.94 is supplied to the latent image carrier,

an inorganic additive is added to the lubricant,

the inorganic additive satisfies

2Y/1000≦X≦Y/10

where Y is a particle size of the toner in micrometer, and X is a particle size of the inorganic additive in micrometer,

the lubricant is a molded lubricant that is molded as a solid block, and

the brush roller slides along, and scrapes off the molded lubricant so that the lubricant is applied to the latent image carrier.

11. An image forming apparatus comprising a process cartridge that includes

a latent image carrier;

at least one of a charging unit that causes a charging member to contact or to be close to the latent image carrier to charge the latent image carrier; a developing unit that deposits toner on a latent image on the latent image carrier to develop the latent image with the toner; and a cleaning unit that cleans the toner remaining on the latent image carrier with a cleaning blade; and

a lubricant applying unit that applies a lubricant for electrophotography to the latent image carrier, the lubricant applying unit including a brush roller that rotates around a rotational axis, and is in contact with the latent image carrier, wherein

toner having a sphericity of equal to or more than 0.94 is supplied to the latent image carrier,

an inorganic additive is added to the lubricant,

the inorganic additive satisfies

2Y/1000≦X≦Y/10

where Y is a particle size of the toner in micrometer, and X is a particle size of the inorganic additive in micrometer,

the lubricant is a molded lubricant that is molded as a solid block, and

the brush roller slides along, and scrapes off the molded lubricant so that the lubricant is applied to the latent image carrier.

12. The image forming apparatus according to claim 11, wherein the toner has

a volume-average particle size ranging from 3 micrometers to 8 micrometers; and

a ratio of a volume-average particle size to a number-average particle size ranging from 1.00 to 1.40.

13. The image forming apparatus according to claim 11, wherein the toner has

a shape factor SF-1 ranging from 100 to 180; and

a shape factor SF-2 ranging from 100 to 180.

14. The image forming apparatus according to claim 11, wherein the toner is substantially spherical.

15. The image forming apparatus according to claim 11, wherein

the toner has a shape defined by a long axis r1, a short axis r2, and a thickness r3, where r1≧r2≧r3,

a ratio r2/r1 ranges from 0.5 to 1.0, and

a ratio r3/r2 ranges from 0.7 to 1.0.

16. The image forming apparatus according to claim 11, wherein the toner is obtained by subjecting a toner material solution to either one or both of crosslinking reaction and extension reaction in an aqueous medium, the toner material solution being obtained by dispersing at least polyester prepolymer having a functional group containing a nitrogen atom, polyester, a colorant, and a mold release agent into an organic solvent.

17. An image forming apparatus comprising a lubricant applying unit that applies a lubricant for electrophotography to a latent image carrier, wherein

the lubricant applying unit includes a brush roller that rotates around a rotational axis, and is in contact with the latent image carrier,

toner having a sphericity of equal to or more than 0.94 is supplied to the latent image carrier,

an inorganic additive is added to the lubricant,

the inorganic additive satisfies

2Y/1000≦X≦Y/10

where Y is a particle size of the toner in micrometer, and X is a particle size of the inorganic additive in micrometer,

the lubricant is a molded lubricant that is molded as a solid block, and

the brush roller slides along, and scrapes off the molded lubricant so that the lubricant is applied to the latent image carrier.

18. The image forming apparatus according to claim 17, wherein the toner has

a volume-average particle size ranging from 3 micrometers to 8 micrometers; and

a ratio of a volume-average particle size to a number-average particle size ranging from 1.00 to 1.40.

19. The image forming apparatus according to claim 17, wherein the toner has

a shape factor SF-1 ranging from 100 to 180; and

a shape factor SF-2 ranging from 100 to 180.

20. The image forming apparatus according to claim 17, wherein the toner is substantially spherical.

21. The image forming apparatus according to claim 17, wherein the toner has a shape defined by a long axis r1, a short axis r2, and a thickness r3, where r1≧r2≧r3,

a ratio r2/r1 ranges from 0.5 to 1.0, and

a ratio r3/r2 ranges from 0.7 to 1.0.

22. The image forming apparatus according to claim 17, wherein the toner is obtained by subjecting a toner material solution to either one or both of crosslinking reaction and extension reaction in an aqueous medium, the toner material solution being obtained by dispersing at least polyester prepolymer having a functional group containing a nitrogen atom, polyester, a colorant, and a mold release agent into an organic solvent.