JP4882231B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus using an image carrier capable of obtaining a good image without fogging.

Electrophotographic electrophotographic apparatuses are used in electrophotographic apparatuses such as copying machines and laser beam printers because they can perform high-quality printing at high speed.
As the image carrier used in such an electrophotographic apparatus, an organic image carrier using a photoconductive organic material and functionally separating the structure of the image carrier into a charge generation layer and a charge transport layer is mainly used. It has become.
On the other hand, as a transfer means used in an electrophotographic apparatus, a method of directly transferring a toner image formed on an image carrier such as a photosensitive drum to a transfer medium (recording medium) conveyed by a belt-shaped conveyance body; The toner image formed on the image bearing member is temporarily transferred onto an intermediate transfer member formed of a drum member or an endless film belt member, and then the toner image on the intermediate transfer member is newly transferred onto a transfer medium. A method of obtaining a copy image by transferring next is used.
When an intermediate transfer medium is used in the transfer process, the amount of toner to be discharged is increased and the transfer efficiency is lowered as compared with the case of direct transfer to a storage medium. When an electrophotographic apparatus that directly transfers to a storage medium is used, toner is transferred without waste and an excellent image can be obtained.

In recent years, an electrophotographic apparatus in which an acceptor compound is added to a photosensitive layer in order to maintain a stable transfer potential for a long period of time and to improve a transfer image has been proposed (see, for example, Patent Document 1).
However, when the means for directly transferring the latent image on the image carrier to a storage medium such as paper is used, the image can be displayed when the latent image area is designed to be wide and compatible with various storage medium sizes. Fog due to poor chargeability may occur at the end of the carrier. The use of an auxiliary charging device improves the fogging, but it requires a lot of power and costs, and the environmental load increases due to the generation of ozone and discharge products. Development of technology to improve was desired.
JP-A-9-244276

  An object of the present invention is to provide an image forming apparatus capable of obtaining an excellent image without fog corresponding to various storage media.

The cause of the fog phenomenon that occurs when storage media of various sizes are used has not been clarified, but is presumed as follows.
If the normally used storage medium is sufficiently smaller than the width of the latent image area of the image carrier, an area that does not contact the storage medium is generated. Since a constant current is applied to the entire latent image area of the image carrier, the transfer current flows more concentratedly in the area where the storage medium does not pass than in the area where the storage medium passes. For this reason, the influence of the opposite polarity becomes remarkable at the time of transfer, and a history is generated in an area where the storage medium does not pass. Due to this history, a charging failure occurs at the edge of the image carrier in the next charging, and fogging occurs when a storage medium having a width wider than the previous history is used.

Further, the relationship between the fog and the volume resistance value of the undercoat layer is not clear, but is estimated as follows.
When an undercoat layer having a volume resistance value equal to or greater than a prescribed value is used on the image carrier, the surface potential of the image carrier has a reverse polarity after the transfer step in a region where the transfer medium is not in contact. If used continuously, the chargeability deteriorates due to the transfer history. Accordingly, when the data is output to the large size storage medium subsequently to the small size storage medium, the history of the non-contact area between the small size transfer medium and the image carrier appears as a fog during the transfer process.
If a subbing layer with a volume resistance value less than the specified value is used, the cause is not clear, but in the area where the transfer medium and the image carrier do not contact, ions having a polarity opposite to that at the time of charging are localized after the transfer process. Therefore, it is considered that fogging occurs due to a decrease in chargeability due to the history.

As a result of intensive studies based on the above situation, the present inventors have conducted conductive support in an image forming apparatus having a mechanism for directly transferring at least a toner image formed on the image carrier to a storage medium such as paper. An image bearing member containing a conductive metal oxide and an acceptor compound on the body and provided with an undercoat layer with a controlled volume resistance value is used, and a region where the transfer medium is in contact with the image bearing member is not in contact with A. When the region is B, the image carrier is charged, exposed, developed, and transferred in order, and the potential (V) of the image carrier in the A region immediately after the next charging is printed is V ₁ , B region. It has been found that an excellent electrophotographic apparatus that does not cause fogging can be obtained by setting | V ₁ −V ₂ | to a predetermined value when the potential (V) of the image bearing member in V is V _2. The present invention has been shown.

<1> An image carrier, a charging device that charges the image carrier, an exposure device that exposes the image carrier to form an electrostatic latent image, and a toner obtained by developing the electrostatic latent image with toner An image forming apparatus comprising: a developing device that forms an image; and a transfer device that directly transfers the toner image to a transfer medium,
The image carrier comprises, on a conductive support , an undercoat layer and a charge generation layer containing at least a conductive metal oxide and at least one compound selected from the group consisting of alizarin, quinizarin, antalphine and pullpurin , And a charge transport layer are sequentially laminated,
The volume resistance of the undercoat layer is 10 ¹⁰ Ω · cm or more and 10 ¹³ Ω · cm or less when an electric field of 10 ⁶ V / m is applied to the undercoat layer at a temperature of 10 ° C. and a humidity of 15% RH.
When the area where the transfer medium contacts the image carrier is A, and the area where B does not contact is B, the image carrier is charged, exposed, developed, and transferred by the charging device, the exposure device, the developing device, and the transfer device. Immediately after the transfer, one image is printed and immediately after the next charging, the potential (V) of the image carrier in the area A is V ₁ , and the potential (V) of the image carrier in the area B is V ₂ . In this case, the image forming apparatus satisfies the following formula (i).
Formula (i); 0 <| V ₁ −V ₂ | <150

< 2 > The image forming apparatus according to <1> , further including an auxiliary charging step after the transfer step, wherein a current value I (μA) in the auxiliary charging step satisfies the following formula (ii).
Formula (ii); | I | <500

< 3 > The image forming apparatus according to <1> or < 2 >, wherein an auxiliary charging step is not provided after the transfer step.

< 4 > The image forming apparatus according to any one of <1> to < 3 >, wherein the undercoat layer contains zinc oxide.

  An electrophotographic apparatus capable of obtaining a good image without fogging corresponding to various storage media can be provided.

First, a preferred embodiment of the image forming apparatus of the present invention will be described.
FIG. 1 is a cross-sectional view schematically showing a basic configuration of an image forming apparatus of the present invention. An image forming apparatus 100 illustrated in FIG. 1 includes an image carrier 10, a charging device 11 that charges the image carrier 10, a power source 12 connected to the charging device 11, and an image carrier 10 that is charged by the charging device 11. Formed by an exposure device 13 that forms an electrostatic latent image by exposing the toner, a developing device 14 that develops the electrostatic latent image formed by the exposure device 13 with toner and forms a toner image, and a developing device 14. The transfer device 15 that transfers the toner image to the transfer medium (image output medium, recording medium) 1, the auxiliary charging device 16 that charges the residual toner after transfer, the cleaning device 17, and the residual charge on the image carrier 10 An erase device 18 for erasing and a fixing device 19 are provided. These apparatuses can be appropriately selected according to the purpose.
The cleaning device 17 and the erase device 18 are provided as necessary, and may be omitted.

  The charging device 11 is not particularly limited. For example, a contact-type charger using a conductive or semiconductive roller, brush, film, rubber blade, or the like, a scorotron charger using a corona discharge, a corotron charger, or the like. Examples of the chargers known per se.

  The exposure device 13 is not particularly limited, and examples thereof include an optical system device that can expose a light source such as semiconductor laser light, LED light, and liquid crystal shutter light on the surface of the image carrier in a desired image-like manner.

  There is no restriction | limiting in particular as the image development apparatus 14, For example, the well-known developing device etc. which use the 1 component, 2 component normal or reversal developer, etc. are mentioned.

  The transfer device 15 is not particularly limited. For example, a contact transfer charger using a belt, a roller, a film, a rubber blade, etc., a scorotron transfer charger using a corona discharge, a corotron transfer charger, etc. are known per se. And a transfer charger.

  There is no restriction | limiting in particular about the structure, shape, material, etc. as a transfer member, A well-known thing can be used. Examples of the structure include a structure in which an elastic layer formed of at least rubber, elastomer, resin or the like and at least one coating layer are provided on a conductive support. Examples of the shape include a roller shape and a belt shape. Examples of the material include polyurethane resin, polyester resin, polystyrene resin, polyolefin resin, polybutadiene resin, polyamide resin, polyvinyl chloride resin, polyethylene resin, and fluorine resin. And conductive carbon particles, metal powders and the like dispersed and mixed.

The auxiliary charging device 16 is not particularly limited. For example, a contact-type charger using a conductive or semiconductive roller, brush, film, rubber blade, etc., a scorotron charger using a corona discharge, or a corotron charger. Examples of such chargers are known per se.
Constant current control is used for the auxiliary charging device 16, and this auxiliary charging current value is preferably within a predetermined range. Further, the auxiliary charging current needs to have the same polarity as that in the charging step. When the auxiliary charging current and the charging current are the same polarity, charging is facilitated in the subsequent charging in which the image carrier is sequentially charged, exposed, developed, transferred, and auxiliary charged. However, if the auxiliary charging current is larger than a predetermined value, the reason is not clear, but the cleaning becomes defective and fogging occurs. Therefore, it is preferable that the auxiliary charging current (μA) satisfies the following formula (ii).
Formula (ii); | I | <500
Note that the auxiliary charging device 16 may be omitted because the image forming apparatus of the present invention has excellent chargeability.

  2 to 5 are cross-sectional views of the image carrier 10. The image carrier includes a conductive substrate 20, an undercoat layer 21 formed on the conductive substrate 20, and a photosensitive layer 22 formed on the undercoat layer 21. Further, the photosensitive layer 22 may have a two-layer structure of a charge generation layer 24 and a charge transport layer 25, and a protective layer 23 may be provided on the photosensitive layer 22.

  In the image carrier 10 of the present invention, the conductive substrate 20 includes a metal drum such as aluminum, copper, iron, nickel, and a cylindrical substrate made of glass, plastic, aluminum, copper, gold, silver, platinum, Palladium, titanium, nickel-chromium, stainless steel, drum-conducting drum by depositing metal such as indium, depositing conductive metal compounds such as indium oxide and tin oxide, or laminating metal foil A well-known material can be used.

  The undercoat layer 21 of the present invention is formed by containing at least a conductive metal oxide and an acceptor compound.

The conductive metal oxide used in the present invention requires a resistance of about 10 ² to 10 ¹¹ Ω · cm. The resistance of the conductive metal oxide is preferably 10 ⁴ to 10 ¹⁰ Ω · cm, more preferably 10 ⁷ to 10 ¹⁰ Ω · cm. Since an appropriate resistance is required for the undercoat layer in order to acquire leakage resistance, it is preferable to use a conductive oxide having the resistance value.
Among them, it is preferable to use conductive metal oxides such as tin oxide, titanium oxide, and zinc oxide having the above resistance values. In particular, zinc oxide is preferably used. In addition, if the resistance value of the conductive metal oxide is lower than the lower limit of the above range, sufficient leakage resistance cannot be obtained, and if it is higher than the upper limit of the range, there is a concern that the residual potential is increased.
The conductive metal oxide is preferably fine particles in order to obtain desired properties of the coating solution for the undercoat layer. The primary average particle diameter is preferably 500 nm or less, more preferably 10 to 200 nm. If it is above a predetermined value, the desired undercoat layer coating solution cannot be obtained because the dispersibility decreases, and if it is below the predetermined value, the undercoat layer coating solution will gel, making it unsuitable as a coating solution. .

In addition, the conductive metal oxide fine particles may be used in a mixture of two or more types such as those having different surface treatments or particles having different particle diameters.
Further, if the volume resistance of the undercoat layer is within a range of 10 ¹⁰ Ω · cm to 10 ¹³ Ω · cm when an electric field of 10 ⁶ V / m is applied at a temperature of 10 ° C. and a humidity of 15% RH, The content of the conductive metal oxide is not limited. Preferably it is 10-90 vol%, More preferably, it is 20-70 vol%.

  The conductive metal oxide can be subjected to a surface treatment. The surface treatment agent can be selected from known materials as long as desired properties such as a silane coupling agent, a titanate coupling agent, an aluminum coupling agent, and a surfactant can be obtained. In particular, silane coupling agents are preferably used because they give good electrophotographic characteristics. Furthermore, a silane coupling agent having an amino group is preferably used because it gives a good blocking property to the undercoat layer.

Any silane coupling agent having an amino group can be used as long as the desired image carrier characteristics can be obtained. Specific examples include γ-aminopropyltriethoxysilane, N-β- ( Aminoethyl) -γ-aminopropyltrimethoxysilane, N-β- (aminoethyl) -γ-aminopropylmethylmethoxysilane, N, N-bis (β-hydroxyethyl) -γ-aminopropyltriethoxysilane, and the like. Although it is mentioned, it is not limited to these. Moreover, 2 or more types of silane coupling agents can also be mixed and used. Examples of silane coupling agents that can be used in combination with the silane coupling agent having an amino group include vinyltrimethoxysilane, γ-methacryloxypropyl-tris (β-methoxyethoxy) silane, β- (3, 4-epoxycyclohexyl) ethyltrimethoxysilane, γ-glycidoxypropyltrimethoxysilane, vinyltriacetoxysilane, γ-mercaptopropyltrimethoxysilane, γ-aminopropyltriethoxysilane, N-β- (aminoethyl)- γ-aminopropyltrimethoxysilane, N-β- (aminoethyl) -γ-aminopropylmethylmethoxysilane, N, N-bis (β-hydroxyethyl) -γ-aminopropyltriethoxysilane, γ-chloropropyltri Examples include methoxysilane. The present invention is not limited to, et al.
As the surface treatment agent, a silane coupling agent, a titanate coupling agent, and an aluminum coupling agent can be used alone, or two or more kinds can be used in combination.

As the surface treatment method, any known method can be used, but a dry method or a wet method can be used.
When surface treatment is performed by a dry method, while stirring conductive metal oxide fine particles with a mixer having a large shearing force or the like, a silane coupling agent dissolved directly or in an organic solvent is added dropwise to dry air or nitrogen gas. It is processed uniformly by spraying together. The addition or spraying is preferably performed at a temperature below the boiling point of the solvent. Spraying at a temperature equal to or higher than the boiling point of the solvent is not preferred because it causes the solvent to evaporate before being stirred uniformly, causing the silane coupling agent to locally accumulate and making uniform processing difficult. After addition or spraying, baking can be performed at 100 ° C. or higher. Baking can be carried out in an arbitrary range as long as the temperature and time allow the desired electrophotographic characteristics to be obtained.
As the wet method, the conductive metal oxide fine particles are stirred in a solvent, dispersed using ultrasonic waves, a sand mill, an attritor, a ball mill, etc., added with a silane coupling agent solution and stirred or dispersed, and then the solvent is removed. It is processed uniformly. The solvent removal method is distilled off by filtration or distillation. After removing the solvent, baking can be performed at 100 ° C. or higher. Baking can be carried out in an arbitrary range as long as the temperature and time allow the desired electrophotographic characteristics to be obtained. In the wet method, water containing conductive metal oxide fine particles can be removed before adding the surface treatment agent. For example, a method of removing with stirring and heating in the solvent used for the surface treatment, azeotroping with the solvent. It is also possible to use a removal method.
As mentioned above, although the surface treatment method in a silane coupling agent was described, surface treatment of a conductive metal oxide can be performed by the same method also about surface treatment agents, such as a titanate type coupling agent and an aluminum type coupling agent. .

  The amount of the silane coupling agent, titanate coupling agent and aluminum coupling agent relative to the conductive metal oxide in the undercoat layer 21 can be arbitrarily set as long as desired electrophotographic characteristics can be obtained.

In the present invention, an acceptor compound is a compound that has an electron-withdrawing structure and has the property of extracting electrons from holes and electrons generated in a charge-generating pigment by light irradiation.
As the acceptor compound, have use an acceptor compound having an anthraquinone structure having a hydroxyl group, specifically alizarin, quinizarin, Ann Tsentralnyi fins, Ru purpurin der.

  The content of the acceptor compound in the undercoat layer used in the present invention can be arbitrarily set as long as desired properties are obtained, but preferably 0.01 to 20% by mass with respect to the conductive metal oxide. Used in a range. More preferably, it is used in the range of 0.05 to 10% by mass with respect to the conductive metal oxide. If it is 0.01% by mass or less, sufficient acceptor property that contributes to improvement of charge accumulation in the undercoat layer cannot be imparted, so that it tends to cause deterioration in sustainability such as increase in residual potential during repeated use. Further, when the content is 20% by mass or more, aggregation of metal oxides is likely to occur, and therefore, when the undercoat layer is formed, the metal oxide cannot form a good conductive path in the undercoat layer, and the residual potential is reduced during repeated use. Not only is it likely to cause deterioration of maintainability such as an increase, but it also tends to cause image quality defects such as black spots.

As the binder resin contained in the undercoat layer 21, any known resin can be used as long as it can form a good film and can obtain desired characteristics. For example, acetal such as polyvinyl butyral can be used. Resin, polyvinyl alcohol resin, casein, polyamide resin, cellulose resin, gelatin, polyurethane resin, polyester resin, methacrylic resin, acrylic resin, polyvinyl chloride resin, polyvinyl acetate resin, vinyl chloride-vinyl acetate-maleic anhydride resin, silicone resin , Using known polymer resin compounds such as silicone-alkyd resin, phenol resin, phenol-formaldehyde resin, melamine resin, urethane resin, charge transport resin having a charge transport group, or conductive resin such as polyaniline, etc. Can . Of these, resins insoluble in the upper coating solvent are preferably used, and phenol resins, phenol-formaldehyde resins, melamine resins, urethane resins, epoxy resins, and the like are particularly preferably used.
The ratio of the acceptor compound, the conductive metal oxide, and the binder resin in the coating solution for forming the undercoat layer 21 can be arbitrarily set within a range in which desired image carrier characteristics can be obtained.

Various additives can be used in the coating liquid for forming the undercoat layer 21 in order to improve electrical characteristics, environmental stability, and image quality.
As additives, quinone compounds such as chloranil and bromoanil, tetracyanoquinodimethane compounds, fluorenone compounds such as 2,4,7-trinitrofluorenone, 2,4,5,7-tetranitro-9-fluorenone, 2- (4-biphenyl) -5- (4-t-butylphenyl) -1,3,4-oxadiazole, 2,5-bis (4-naphthyl) -1,3,4-oxadiazole, Oxadiazole compounds such as 2,5-bis (4-diethylaminophenyl) 1,3,4 oxadiazole, xanthone compounds, thiophene compounds, 3,3 ′, 5,5′tetra-t-butyldiphenoxy Electron transport materials such as non-diphenoquinone compounds such as non-electron transport pigments such as polycyclic condensation systems and azo systems, zirconium chelate compounds, titanium chelate compounds, Rumi hexafluorophosphate chelate compounds, titanium alkoxide compounds, organic titanium compounds, there can be used a known material such as a silane coupling agent. The silane coupling agent is used for the surface treatment of zinc oxide, but it can also be used as an additive added to the coating solution.

  Specific examples of the silane coupling agent used here include vinyltrimethoxysilane, γ-methacryloxypropyl-tris (β-methoxyethoxy) silane, β- (3,4-epoxycyclohexyl) ethyltrimethoxysilane, γ- Glycidoxypropyltrimethoxysilane, vinyltriacetoxysilane, γ-mercaptopropyltrimethoxysilane, γ-aminopropyltriethoxysilane, N-β- (aminoethyl) -γ-aminopropyltrimethoxysilane, N-β- (Aminoethyl) -γ-aminopropylmethylmethoxysilane, N, N-bis (β-hydroxyethyl) -γ-aminopropyltriethoxysilane, γ-chloropropyltrimethoxysilane and the like.

  Examples of zirconium chelate compounds include zirconium butoxide, zirconium zirconium acetoacetate, zirconium triethanolamine, acetylacetonate zirconium butoxide, ethyl zirconium acetoacetate, zirconium acetate, zirconium oxalate, zirconium lactate, zirconium phosphonate, zirconium octoate, naphthene. Zirconate, zirconium laurate, zirconium stearate, zirconium isostearate, methacrylate zirconium butoxide, stearate zirconium butoxide, isostearate zirconium butoxide and the like.

  Examples of titanium chelate compounds include tetraisopropyl titanate, tetranormal butyl titanate, butyl titanate dimer, tetra (2-ethylhexyl) titanate, titanium acetylacetonate, polytitanium acetylacetonate, titanium octylene glycolate, titanium lactate ammonium salt, Examples thereof include titanium lactate, titanium lactate ethyl ester, titanium triethanolamate, and polyhydroxytitanium stearate.

Examples of the aluminum chelate compound include aluminum isopropylate, monobutoxy aluminum diisopropylate, aluminum butyrate, diethyl acetoacetate aluminum diisopropylate, aluminum tris (ethyl acetoacetate) and the like.
These compounds can be used alone or as a mixture or polycondensate of a plurality of compounds.

As a solvent for adjusting the coating solution for forming the undercoat layer, any known organic solvent such as alcohol, aromatic, halogenated hydrocarbon, ketone, ketone alcohol, ether, ester, etc. You can choose. For example, methanol, ethanol, n-propanol, iso-propanol, n-butanol, benzyl alcohol, methyl cellosolve, ethyl cellosolve, acetone, methyl ethyl ketone, cyclohexanone, methyl acetate, ethyl acetate, n-butyl acetate, dioxane, tetrahydrofuran, Usual organic solvents such as methylene chloride, chloroform, chlorobenzene, and toluene can be used.
Moreover, the solvent used for these dispersion | distribution can be used individually or in mixture of 2 or more types. When mixing, any solvent can be used as long as it can dissolve the binder resin as the mixed solvent.

The coating solution for forming the undercoat layer 21 is prepared by dissolving a binder in the above solvent and further adding and dispersing a conductive metal oxide and an acceptor compound.
As a method for dispersing the conductive metal oxide, known methods such as a roll mill, a ball mill, a vibrating ball mill, an attritor, a sand mill, a colloid mill, and a paint shaker can be used. Further, as the coating method used when the undercoat layer 21 is provided, conventional methods such as a blade coating method, a wire bar coating method, a spray coating method, a dip coating method, a bead coating method, an air knife coating method, and a curtain coating method are used. Can be used.

  The undercoat layer 21 is formed on the conductive substrate 20 using the coating solution for forming the undercoat layer 21 obtained in this way.

  The thickness of the undercoat layer 21 can be arbitrarily set within a range where desired characteristics are obtained, but is preferably 15 μm or more, more preferably 20 μm or more and 50 μm or less. When the thickness of the undercoat layer 21 is less than 15 μm, sufficient leakage resistance cannot be obtained, and when it is 50 μm or more, a residual potential tends to remain during long-term use, so that it tends to cause an abnormal image density. is there.

Further, the surface roughness of the undercoat layer 21 is adjusted to ¼ n (n is the refractive index of the upper layer) to ½λ of the exposure laser wavelength λ used in order to prevent moire images. In order to adjust the surface roughness, particles such as a resin can be added to the undercoat layer 21. As the resin particles, silicone resin particles, cross-linked PMMA resin particles, and the like can be used.
In addition, the undercoat layer 21 can be polished to adjust the surface roughness. As a polishing method, buffing, sand blasting, wet honing, grinding, or the like can be used.

The volume resistivity of the undercoat layer is 10 ¹⁰ Ω · cm or more and 10 ¹³ Ω · cm or less when an electric field of 10 ⁶ V / m is applied at a temperature of 10 ° C. and a humidity of 15% RH. In order to achieve such volume resistivity, the type and amount of conductive metal oxide, the type and amount of acceptor compound, the thickness of the undercoat layer, the addition of organometallic oxide, etc. Although it is determined by the dispersion state of the layer coating liquid and the like and can be adjusted by a combination thereof, it is not limited to the above method.
The volume resistance of the undercoat layer when an electric field of 10 ⁶ V / m was applied at a temperature of 10 ° C. and a humidity of 15% RH was measured by bringing a gold electrode with a diameter of 1 mm as a counter electrode and contacting the image carrier. This is performed by applying an electric field of 10 ⁶ V / m at a temperature of 15 ° C. and a humidity of 15%.
By using such an undercoat layer, the image carrier region that is not in contact with the storage medium is not easily affected by reverse polarity or + ions in the transfer process. Further, since the charging property is excellent, a sufficient surface potential can be maintained in the subsequent charging step, and a good image quality without fogging can be obtained.

  Next, the charge generation layer 24 will be described. The charge generation layer 24 is composed of a known charge generation material and a binder resin. The charge generating material is preferably a metal phthalocyanine pigment, particularly hydroxygallium phthalocyanine having a specific crystal described below. As disclosed in JP-A-5-263007 and JP-A-5-279591, hydroxygallium phthalocyanine used in the present invention is obtained by reacting a chlorogallium phthalocyanine crystal produced by a known method in an acid or alkaline solution. Hydroxy gallium phthalocyanine crystals are synthesized by hydrolyzing or acid pasting and directly solvent treatment, or the hydroxy gallium phthalocyanine crystals obtained by synthesis are mixed with a solvent in a ball mill, mortar, sand mill, kneader. It can manufacture by carrying out a wet-grinding process using etc., or performing a solvent process after performing a dry-type grinding process without using a solvent. Solvents used in the above treatment are aromatics (toluene, chlorobenzene, etc.), amides (dimethylformamide, N-methylpyrrolidone, etc.), aliphatic alcohols (methanol, ethanol, butanol, etc.), aliphatic polyvalents. Alcohols (ethylene glycol, glycerin, polyethylene glycol, etc.), aromatic alcohols (benzyl alcohol, phenethyl alcohol, etc.), esters (acetic ester, butyl acetate, etc.), ketones (acetone, methyl ethyl ketone, etc.), dimethyl sulfoxide Ethers (diethyl ether, tetrahydrofuran, etc.), several mixed systems, mixed systems of water and these organic solvents, and the like. The solvent used is used in an amount of 1 to 200 parts, preferably 10 to 100 parts, based on hydroxygallium phthalocyanine. The treatment temperature is 0 to 150 ° C, preferably room temperature to 100 ° C. In addition, grinding aids such as sodium chloride and sodium nitrate can be used during grinding. The grinding aid is used 0.5 to 20 times, preferably 1 to 10 times the pigment.

  The binder resin can be selected from a wide range of insulating resins, and can also be selected from organic photoconductive polymers such as poly-N-vinylcarbazole, polyvinylanthracene, polyvinylpyrene, and polysilane. Preferred binder resins include polyvinyl butyral resin, polyarylate resin (polycondensate of bisphenol A and phthalic acid, etc.), polycarbonate resin, polyester resin, phenoxy resin, vinyl chloride-vinyl acetate copolymer, polyamide resin, acrylic resin. Insulating resin such as polyacrylamide resin, polyvinyl pyridine resin, cellulose resin, urethane resin, epoxy resin, casein, polyvinyl alcohol resin, polyvinyl pyrrolidone resin can be used, but is not limited thereto. These binder resins can be used alone or in admixture of two or more.

  The blending ratio of the charge generation material and the binder resin (weight ratio) is preferably in the range of 10: 1 to 1:10. In addition, as a method for dispersing these, a usual method such as a ball mill dispersion method, an attritor dispersion method, a sand mill dispersion method, or the like can be used, but in this case, a condition that the crystal type does not change by dispersion is required. . Incidentally, it has been confirmed that the crystal form is not changed before dispersion in any of the dispersion methods implemented in the present invention. Further, at the time of this dispersion, it is effective to make the particles have a particle size of 0.5 μm or less, preferably 0.3 μm or less, more preferably 0.15 μm or less. Moreover, as a solvent used for these dispersions, methanol, ethanol, n-propanol, n-butanol, benzyl alcohol, methyl cellosolve, ethyl cellosolve, acetone, methyl ethyl ketone, cyclohexanone, methyl acetate, n-butyl acetate, dioxane, tetrahydrofuran Ordinary organic solvents such as methylene chloride, chloroform, chlorobenzene and toluene can be used alone or in admixture of two or more.

  The thickness of the charge generating layer 24 used in the present invention is generally 0.1 to 5 μm, preferably 0.2 to 2.0 μm. As a coating method used when the charge generation layer 24 is provided, a normal method such as a blade coating method, a Meyer bar coating method, a spray coating method, a dip coating method, a bead coating method, an air knife coating method, or a curtain coating method is used. be able to. Further, a pigment treated for the purpose of increasing the dispersion stability of the pigment, photosensitivity, or stabilizing electrical characteristics may be used.

  Next, the charge transport layer 25 will be described. As the charge transport layer, a layer formed by a known technique can be used. These charge transport layers are formed containing a charge transport material and a binder resin, or are formed containing a polymer charge transport material.

In the present invention, a charge transport layer containing a fluororesin can be used. In the charge transport layer containing the fluororesin of the present invention, the content of the fluororesin in the charge transport layer is suitably from 0.1 to 40% by weight, particularly from 1 to 30% by weight, based on the total amount of the charge transport layer. preferable. If the content is less than 1% by mass, the modification effect due to the dispersion of the fluororesin particles is not sufficient. On the other hand, if the content exceeds 40% by mass, the light transmission property decreases, and the residual potential increases due to repeated use. .
Examples of the fluorine resin particles used in the present invention include a tetrafluoroethylene resin, a trifluorinated ethylene resin, a hexafluoropropylene resin, a vinyl fluoride resin, a vinylidene fluoride resin, a difluorodiethylene chloride resin, and their It is desirable to appropriately select one or two or more types from among the copolymers, but tetrafluoroethylene resin and vinylidene fluoride resin are particularly preferable.
The primary particle size of the fluororesin is preferably 0.05-1 μm, more preferably 0.1-0.5 μm. When the primary particle size is less than 0.05 μm, aggregation during dispersion tends to proceed. If it exceeds 1 μm, image quality defects are likely to occur.

  Examples of the charge transport material used in the present invention include oxadiazole derivatives such as 2,5-bis (p-diethylaminophenyl) -1,3,4-oxadiazole, 1,3,5-triphenyl- Pyrazoline, pyrazoline derivatives such as 1- [pyridyl- (2)]-3- (p-diethylaminostyryl) -5- (p-diethylaminostyryl) pyrazoline, triphenylamine, tri (p-methylphenyl) aminyl-4- Aromatic tertiary amino compounds such as amine and dibenzylaniline, aromatic tertiary diamino compounds such as N, N′-bis (3-methylphenyl) -N, N′-diphenylbenzidine, 3- (4 ′ -1,2,4-triazine derivatives such as -dimethylaminophenyl) -5,6-di- (4'-methoxyphenyl) -1,2,4-triazine, 4- Hydrazone derivatives such as ethylaminobenzaldehyde-1,1-diphenylhydrazone, quinazoline derivatives such as 2-phenyl-4-styryl-quinazoline, benzofuran derivatives such as 6-hydroxy-2,3-di (p-methoxyphenyl) benzofuran, Hole transport of α-stilbene derivatives such as p- (2,2-diphenylvinyl) -N, N-diphenylaniline, enamine derivatives, carbazole derivatives such as N-ethylcarbazole, poly-N-vinylcarbazole and derivatives thereof Substances, quinone compounds such as chloranil and broanthraquinone, tetraanoquinodimethane compounds, fluorenone compounds such as 2,4,7-trinitrofluorenone, 2,4,5,7-tetranitro-9-fluorenone, xanthone Compounds, thiophene compounds, etc. Examples thereof include a child transport material and a polymer having a group consisting of the above-described compounds in the main chain or side chain. These charge transport materials can be used alone or in combination of two or more.

Examples of the binder resin used in the present invention include acrylic resin, polyarylate, polyester resin, polycarbonate resin such as bisphenol A type or bisphenol Z type, polystyrene, acrylonitrile-styrene copolymer, acrylonitrile-butadiene copolymer, Examples thereof include insulating resins such as polyvinyl butyral, polyvinyl formal, polysulfone, polyacrylamide, polyamide, and chlorine rubber, and organic photoconductive polymers such as polyvinyl carbazole, polyvinyl anthracene, and polyvinyl pyrene. The charge transport layer can be formed by applying and drying a solution in which the charge transport material and the binder resin shown above are dissolved in an appropriate solvent. Examples of the solvent used for forming the charge transport layer include aromatic hydrocarbon solvents such as toluene and chlorobenzene, aliphatic alcohol solvents such as methanol, ethanol, and n-butanol, acetone, cyclohexanone, and 2-butanone. Ketone solvents, halogenated aliphatic hydrocarbon solvents such as methylene chloride, chloroform, and ethylene chloride, cyclic or linear ether solvents such as tetrahydrofuran, dioxane, ethylene glycol, and diethyl ether, or a mixed solvent thereof. be able to.
The blending ratio of the charge transport material and the binder resin is preferably 10: 1 to 1: 5. As a method for dispersing the fluorine-based resin in the charge transport layer 24 of the present invention, a roll mill, a ball mill, a vibrating ball mill, an attritor, a sand mill, a high-pressure homogenizer, an ultrasonic disperser, a colloid mill, a collision-type medialess disperser, and a penetration A method such as a type medialess disperser can be used.

Examples of the dispersion of the coating liquid for forming the charge transport layer of the present invention include a method in which fluorine-based resin particles are dispersed in a solution such as a binder resin or a charge transport material dissolved in a solvent.
As a method of controlling the temperature of the coating liquid in the coating liquid manufacturing process to 0 ° C. to 50 ° C., cool with water, cool with wind, cool with refrigerant, adjust the room temperature of the manufacturing process, warm with hot water, with hot air You can use methods such as warming, heating with a heater, creating a coating liquid manufacturing facility with a material that does not easily generate heat, creating a coating liquid manufacturing facility with a material that easily dissipates heat, or creating a coating liquid manufacturing facility with a material that easily stores heat. . As a premixing method for the coating liquid, methods such as a stirrer, stirring with a stirring blade, a roll mill, a sand mill, an attritor, a ball mill, a high pressure homogenizer, and an ultrasonic disperser can be used. As a dispersion method, a sand mill, an attritor, a ball mill, a high-pressure homogenizer, an ultrasonic disperser, a roll mill, or the like can be used.

In the present invention, it is also effective to add a small amount of a dispersion aid in order to improve the dispersion stability of the dispersion and to prevent aggregation during the formation of the coating film. Examples of the dispersion aid include fluorine-based surfactants, fluorine-based polymers, silicone-based polymers, and silicone oils.
The film thickness of the charge transport layer 25 is generally 5 to 50 μm, preferably 10 to 40 μm. As a coating method, a normal method such as a blade coating method, a Meyer bar coating method, a spray coating method, a dip coating method, a bead coating method, an air knife coating method, or a curtain coating method can be used. Furthermore, as a solvent used when providing a charge transport layer, aromatic hydrocarbons such as benzene, toluene, xylene and chlorobenzene, ketones such as acetone and 2-butanone, and halogenation such as methylene chloride, chloroform and ethylene chloride Ordinary organic solvents such as aliphatic hydrocarbons, cyclic ethers such as tetrahydrofuran and ethyl ether and linear ethers can be used alone or in admixture of two or more. In addition, additives such as antioxidants, light stabilizers, and heat stabilizers are added to the photosensitive layer for the purpose of preventing deterioration of the image carrier due to ozone, oxidizing gas, or light and heat generated in the copying machine. Can be added. For example, examples of the antioxidant include hindered phenol, hindered amine, paraphenylenediamine, arylalkane, hydroquinone, spirochroman, spiroidanone and derivatives thereof, organic sulfur compounds, and organic phosphorus compounds. Examples of the light stabilizer include derivatives such as benzophenone, benzotriazole, dithiocarbamate, and tetramethylpiperidine. In addition, at least one kind of electron accepting substance can be contained for the purpose of improving sensitivity, reducing residual potential, and reducing fatigue during repeated use. Examples of the electron acceptor usable for the image carrier of the present invention include succinic anhydride, maleic anhydride, dibromomaleic anhydride, phthalic anhydride, tetrabromophthalic anhydride, tetracyanoethylene, tetracyanoquinodimethane, Examples include o-dinitrobenzene, m-dinitrobenzene, chloranil, dinitroanthraquinone, trinitrofluorenone, picric acid, o-nitrobenzoic acid, p-nitrobenzoic acid, and phthalic acid. Of these, benzene derivatives having electron-withdrawing substituents such as fluorenone, quinone and Cl, CN, NO ₂ are particularly preferred.

A protective layer 23 may be further provided on the charge transport layer 25. In the image carrier 10 having a laminated structure, the protective layer 23 is used to prevent chemical change of the charge transport layer during charging and to further improve the mechanical strength of the photosensitive layer.
The protective layer 23 includes a curable resin, a cured resin film having a charge transporting compound, a film formed by containing a conductive material in a suitable binder resin, and the like. Any curable resin can be used as long as it is a public resin. Examples thereof include phenol resin, polyurethane resin, melamine resin, diallyl phthalate resin, and siloxane resin. As the charge transport resin cured film, any resin cured film having an organic machine derived from a known charge transport compound can be used. Examples of the conductive material include conductive metal oxides such as zinc oxide, tin oxide, and titanium oxide.

  In order to facilitate application, the protective layer can be used with a solvent added as necessary. Specifically, water, methanol, ethanol, n-propanol, i-propanol, n-butanol, benzyl alcohol, methyl cellosolve, ethyl cellosolve, acetone, methyl ethyl ketone, cyclohexanone, methyl acetate, n-butyl acetate, dioxane, tetrahydrofuran, Usual organic solvents such as methylene chloride, chloroform, dimethyl ether, dibutyl ether and the like can be used alone or in admixture of two or more.

In the formation of the protective layer, as a coating method, a normal method such as a blade coating method, a Meyer bar coating method, a spray coating method, a dip coating method, a bead coating method, an air knife coating method, or a curtain coating method may be used. it can.
The protective layer preferably has a thickness of 0.5 to 20 μm, particularly 2 to 10 μm.
In the image carrier obtained by the present invention, the thickness of the functional layer above the charge generation layer for obtaining high resolution is preferably 50 μm or less, preferably 40 μm or less. When the functional layer is a thin film, the combination of the particle-dispersed undercoat layer of the present invention and a high-strength protective layer is particularly effectively used.

FIG. 6 is a schematic diagram showing a state where an area where the image carrier 10 and the transfer medium 1 are in contact with each other and an area where they are not in contact exist in the transfer process.
In FIG. 6, an area where the transfer medium 1 is in contact is A, and an area where the transfer medium 1 is not in contact is B. Using the charging device, the exposure device, the developing device, and the transfer device, the image carrier is charged, exposed (latent image formation), developed, and transferred in sequence, immediately after the next charging after printing one sheet. When the potential of the image carrier in the region A is V ₁ and the potential of the image carrier in the region B is V ₂ , a satisfactory image without fogging can be obtained when the following formula (i) is satisfied.
Formula (i); 0 <| V ₁ −V ₂ | <150

When the difference between V ₁ and V ₂ is 150 or more, the uniformity of the surface potential on the image carrier is not maintained, and the region not in contact with the transfer medium is not sufficiently charged. is important. Preferably, the difference between V ₁ and V ₂ is 0-80 (V), more preferably 0-50 (V).

The measurement method of V ₁ and V ₂ is that a surface electrometer (Model 555P-4: manufactured by TREC) is attached at the development position, and the image bearing member and the contact portion between the sheet and the non-contact portion on the image bearing member are printed. Measure the surface potential.

  In order for the image carrier potential to satisfy the formula (i) immediately after the next charging step after printing one sheet, the image carrier according to the present invention is used, the current of the transfer device is adjusted, the transfer device and the image It is preferable to appropriately select the type of the composition such as the binder resin in the carrier, adjust the dispersion state of the composition, and adjust the contact area between the transfer device and the image carrier. Examples of the adjustment of the contact area include, but are not limited to, adjustment of the nip width, setting of the curvature of the roll, adjustment of the rotation speed of the transfer device and the image carrier.

That is, in order to obtain a good image without fogging, the image forming apparatus of the present invention requires at least the following three conditions.
(1) using an undercoat layer containing at least a conductive metal oxide and at least one compound selected from the group consisting of alizarin, quinizarin, anthalphine and pullpurin ,
(2) When an electric field of 10 ⁶ V / m is applied to the undercoat layer at a temperature of 10 ° C. and a humidity of 15% RH, the volume resistance of the undercoat layer is 10 ¹⁰ Ω · cm or more and 10 ¹³ Ω · cm or less. To be prepared,
(3) When the area of the image carrier that contacts the transfer medium is A and the area that does not contact is B, the image carrier is charged, exposed, developed, and transferred in sequence, immediately after the next charge on which one sheet is printed. When the potential (V) of A is V ₁ and the potential (V) of B is V ₂ , the above equation (i) is satisfied.

  By satisfying the above conditions, it is possible to provide an image forming apparatus capable of obtaining an excellent image without fog corresponding to various transfer media.

  EXAMPLES The present invention will be specifically described below with reference to examples, but the present invention is not limited to these examples.

<Example 1>
Zinc oxide: (average particle diameter: 70 nm, prototype product manufactured by Teika: specific surface area value: 15 m ² / g) 100 parts by weight are stirred and mixed with 480 parts by weight of toluene, and a silane coupling agent (KBM603: manufactured by Shin-Etsu Chemical Co., Ltd.) 1 .25 parts by weight were added and stirred for 2 hours. Thereafter, toluene was distilled off under reduced pressure and baked at 120 ° C. for 3 hours to obtain a silane coupling agent surface-treated zinc oxide pigment.
60 parts by weight of zinc oxide subjected to the surface treatment, 0.7 parts by weight of alizarin, a curing agent {blocked isocyanate Sumijour 3175 (manufactured by Sumitomo Bayern Urethane Co., Ltd.)} and 13.5 parts by weight of butyral resin {BM-1 (Sekisui (Made by Chemical Co., Ltd.)} 40 parts by weight of a solution obtained by dissolving 15 parts by weight of 85 parts by weight of methyl ethyl ketone and 25 parts by weight of methyl ethyl ketone are mixed, and light transmittance is 40% in a sand mill using 1 mmφ glass beads. Was dispersed.
As a catalyst, 0.005 part by weight of dioctyltin dilaurate and 4.0 parts by weight of silicone resin particles {Tospearl 145 (manufactured by GE Toshiba Silicone)} were added as a catalyst to obtain an undercoat layer coating liquid. This coating solution was applied on an aluminum substrate having a diameter of 84 mm, a length of 340 mm, and a thickness of 1 mm by a dip coating method, followed by drying and curing at 170 ° C. for 40 minutes to obtain an undercoat layer having a thickness of 25 μm.

At this time, a volume resistance when an electric field of 10 ⁶ V / m was applied was measured using a gold electrode of φ1 mm as a counter electrode under a temperature of 10 ° C. and a humidity of 15% RH.

Next, a photosensitive layer was formed on the undercoat layer. First, X-ray diffraction spectrum Bragg angles (2θ ± 0.2 °) using Cukα as a charge generating material are at least 7.3 °, 16.0 °, 24.9 °, and 28.0 °. A mixture of 15 parts of hydroxygallium phthalocyanine having a diffraction peak at 10 parts, 10 parts of vinyl chloride / vinyl acetate copolymer resin (VMCH, manufactured by Union Carbide) as a binder resin, and 200 parts of n-butyl acetate was added to a 1 mmφ mixture. The glass beads were used for dispersion for 4 hours in a sand mill.
To the obtained dispersion, 175 parts of n-butyl acetate and 180 parts of methyl ethyl ketone were added and stirred to obtain a coating solution for a charge generation layer. This charge generation layer coating solution was dip-coated on the undercoat layer and dried at room temperature to form a charge generation layer having a thickness of 0.2 μm.

  Furthermore, 4 parts of N, N′-diphenyl-N, N′-bis (3-methylphenyl)-[1,1 ′] biphenyl-4,4′-diamine and 6 weight of bisphenol Z polycarbonate resin (molecular weight 40,000) A coating solution prepared by adding 80 parts by weight of tetrahydrofuran is dissolved on the charge generation layer and dried at 115 ° C. for 40 minutes to form a charge transport layer having a thickness of 30 μm. Got the body.

Using the image carrier produced as described above, the image carrier surface potential after charging was modified with a DocuCentre 705 having a direct transfer system and an auxiliary charging device.
The “image carrier surface potential in the initial state” in Table 1 is the surface potential V ₁ and V ₂ of the image carrier immediately after the next charging after transporting the A4 sideways 30 mm cut sheet as the first sheet and printing one sheet. Was measured. The evaluation of fogging was carried out by conveying A4 sideways 30 mm cut paper as the first sheet, conveying A4 sheet in the horizontal direction as the second sheet, and outputting it, and visually evaluating the second printed image.
The “image carrier surface potential after printing 10,000 sheets” in Table 1 indicates that, after printing 10,000 sheets of A4 paper in the horizontal direction, a sheet of 30 mm cut paper of A4 is conveyed in the horizontal direction (that is, 10,001 sheets). Eye) The surface potentials V ₁ and V ₂ of the image carrier immediately after the next charging after printing one sheet were measured. To evaluate the fog, after 10,000 sheets of A4 paper were printed in the horizontal direction, one A4 sideways 30 mm cut sheet was transported (that is, the 10,001th sheet), and then A4 paper was transported in the horizontal direction. The printed image of A4 paper that was output (10,002 sheets) and output in the horizontal direction at the end (10,002 sheets) was visually evaluated.
The results of this example are shown in Table 1.

<Example 2>
In Example 1, it carried out by the same method except not outputting auxiliary charging.

<Examples 3 and 4, Comparative Examples 1 and 2>
In Example 1, electrophotographic image support was carried out in the same manner except that 60 parts by weight of surface-treated zinc oxide and 0.7 parts by weight of alizarin were used and the addition amounts thereof were changed as shown in Table 1. A body was prepared and evaluated in the same manner as in Example 1.

<Comparative Example 3>
The same operation was performed except that the coating solution for the undercoat layer dispersed in Example 1 so that the light transmittance was 90% was used.
<Comparative example 4>
In Example 1, it carried out by the same method except not containing alizarin.

<Comparative Example 5>
In Example 2, it carried out by the same method except not containing alizarin.

<Comparative Example 6>
(Create an undercoat layer)
To 170 parts by weight of n-butyl alcohol in which 4 parts by weight of polyvinyl butyral resin (S-REC BM-S, manufactured by Sekisui Chemical Co., Ltd.) is dissolved, 30 parts by weight of an organic zirconium compound (acetylacetone zirconium butyrate) and an organic silane compound (γ-amino) 3 parts by weight of propyltrimethoxysilane) was added, mixed and stirred to obtain a coating solution for forming an undercoat layer. This coating solution is dip-coated on a 30 mmφ aluminum support roughened by a honing treatment, air-dried at room temperature for 5 minutes, and then the support is heated to 50 ° C. over 10 minutes. And 85% RH (dew point 47 ° C.) in a constant temperature and humidity chamber, and subjected to a humidification hardening acceleration treatment for 20 minutes. Then, it put into the hot air dryer and dried for 10 minutes at 170 degreeC.
Evaluation was performed in the same manner as in Example 1 except that this undercoat layer was used.

<Comparative Example 7>
In Example 2, it implemented by the same method except having made the undercoat layer into the comparative example 5.
The results of this example are shown in Table 1.

As shown in the results of Table 1, an undercoat layer containing at least a conductive metal oxide and an acceptor compound is used, and an electric field of 10 ⁶ V / m is applied to the undercoat layer at a temperature of 10 ° C. and a humidity of 15% RH. When applied, the volume resistance of the undercoat layer is 10 ¹⁰ Ω · cm or more and 10 ¹³ Ω · cm or less, and the image carrier is charged, exposed, developed, and transferred in order, and immediately after the next charge printed on one sheet. When the difference between the potential V ₁ of the contact portion of the transfer medium in the image carrier and the potential V ₂ of the non-contact portion satisfies a certain range, a good image with no fogging at the edge of the sheet is obtained. It was.
Further, when such a condition is satisfied, even when a small-sized transfer medium is conveyed after printing 10,000 sheets, and a transfer medium having a larger size is further conveyed and printed, A history of conveying a small-sized transfer medium does not remain on the image carrier, and a good image with no fogging at the edge of the sheet can be obtained.

1 is a schematic view of an electrophotographic apparatus of the present invention. It is an example of sectional drawing of an image carrier. It is another example of sectional drawing of an image carrier. It is another example of sectional drawing of an image carrier. It is another example of sectional drawing of an image carrier. FIG. 4 is a schematic diagram illustrating a state of latent image transfer onto a transfer medium in an image forming apparatus having a wide latent image possible area.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Transfer medium 10 Image carrier 11 Charging device 12 Power supply 13 Exposure device 14 Development device 15 Transfer device 16 Auxiliary charging device 17 Cleaning device 18 Erase device 19 Fixing device 20 Conductive base material 21 Undercoat layer 22 Photosensitive layer 23 Protective layer 24 charge generation layer 25 charge transport layer 100 image forming apparatus

Claims (4)

An image carrier, a charging device for charging the image carrier, an exposure device for exposing the image carrier to form an electrostatic latent image, and developing the electrostatic latent image with toner to form a toner image An image forming apparatus comprising: a developing device that transfers; and a transfer device that directly transfers the toner image to a transfer medium,
The image carrier comprises, on a conductive support , an undercoat layer and a charge generation layer containing at least a conductive metal oxide and at least one compound selected from the group consisting of alizarin, quinizarin, antalphine and pullpurin , And a charge transport layer are sequentially laminated,
The volume resistance of the undercoat layer is 10 ¹⁰ Ω · cm or more and 10 ¹³ Ω · cm or less when an electric field of 10 ⁶ V / m is applied to the undercoat layer at a temperature of 10 ° C. and a humidity of 15% RH.
When the area where the transfer medium contacts the image carrier is A, and the area where B does not contact is B, the image carrier is charged, exposed, developed, and transferred by the charging device, the exposure device, the developing device, and the transfer device. Immediately after the transfer, one image is printed and immediately after the next charging, the potential (V) of the image carrier in the area A is V ₁ , and the potential (V) of the image carrier in the area B is V ₂ . In this case, the image forming apparatus satisfies the following formula (i).
Formula (i); 0 <| V ₁ −V ₂ | <150 The image forming apparatus according to claim 1 , further comprising an auxiliary charging step after the transfer step, wherein a current value I (μA) in the auxiliary charging step satisfies the following formula (ii).
Formula (ii); | I | <500 The image forming apparatus according to claim 1 or claim 2, characterized in that no auxiliary charging step after said transferring step. The undercoat layer is, the image forming apparatus according to any one of claims 1 to 3, characterized by containing zinc oxide.

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