IMAGE FORMING APPARATUS, AND METHOD OF PRODUCING BELT MEMBER USED IN THE APPARATUS

An image forming apparatus includes an image bearer; an image developer developing a latent image formed on the image bearer with a toner to form a toner image thereon; a ring-shaped intermediate transfer belt rotatably stretched by plural tension rollers, and the toner image is first transferred on; a curving roller contacting an outer surface of the intermediate transfer belt the toner image is transferred on and curving the belt inside; and a transferer secondly transferring the toner image on the intermediate transfer belt to a recording medium. The intermediate transfer belt has a double-layered structure formed of an inner layer and an outer layer each including a same resin.

DETAILED DESCRIPTION

The present invention provides an image forming apparatus capable of reducing curls to inside and outside of the intermediate transfer belt while being downsized.

FIG. 2is a schematic view illustrating an embodiment of the image forming apparatus of the present invention.FIG. 3is a schematic view illustrating a main composition of a transferer60in the embodiment of the image forming apparatus of the present invention.

FIG. 2is an embodiment of typified tandem image forming apparatus having an intermediate transfer belt, and the present invention is not limited to the following composition.

An image forming apparatus1includes an automatic document feeder (ADF)5automatically feeding documents loaded thereon, a scanner (reader)4reading the documents, an image former3forming a toner image, and a paper feeder2including and feeding a recording paper6below.

The paper feeder2includes a feed paper cassette80containing the recording paper6, a paper feed roller82feeding the recording paper6contained in the feed paper cassette80toward a transfer route79, and a separation roller81separating the recording paper6fed. A recording paper6fed by the paper feed roller82and separated one by one by the separation roller81is transferred by a transfer roller83in the transfer route79.

A pair of registration rollers84are located at downstream side of a recording paper transfer direction of the transfer route79. The pair of registration rollers84sandwich the recording paper6fed by the transfer roller83in the transfer route79and feed the recording paper6to a second transfer nip at a predetermined timing.

The image former3is located at the center of the image forming apparatus1. Almost at the center of the image former3, image forming units10as process cartridges for each yellow (Y), magenta (M), cyan (C) and black (K) color toner are parallelly located in a horizontal direction to from a tandem image former20.

Above the four image forming units10Y,10C,10M and10K, an irradiator12irradiating the surface of each charged photoreceptor11, based on image information read by the scanner4, to form1latent image thereon.

Below the four image forming units10Y,10C,10M and10K, a transferer60is located rotatably holding an intermediate transfer belt61while hung around a drive roller652, a driven roller651and a support roller653. The intermediate transfer belt61is a ring-shaped belt member formed of a substrate having a medium resistivity, formed of a heat-resistant material such as polyimide and polyamideimide.

Each of the image forming units has the same composition and Y, C, M and K are omitted when difference of colors do not mean.

The image forming units10Y,10C,10M and10K have photoreceptors11Y,11C,11M and11K, respectively. Around each of the photoreceptors11, a charging roller21, an image developer30, a lubricator (unillustrated) and a cleaner40are located.

The charging roller21charges the surface of the photoreceptor. The image developer30develops a latent image formed on the surface of the photoreceptor with a toner to form a toner image thereon. The lubricator applies a lubricant to the surface of the photoreceptor. The cleaner40cleans the surface of the photoreceptor with a cleaning blade after the toner image is transferred therefrom.

The charging roller21is formed of an electroconductive core metal coated with an elastic layer having a medium resistivity. The charging roller21is connected to an unillustrated electric source and applied with a predetermined DC or AC voltage.

The charging roller21is formed of an elastic resin roller, and may include an inorganic electroconductive material such as carbon black or an ionic electroconductive material to control electrical resistance thereof.

The charging roller21is located with a fine gap from the photoreceptor11. The fine gap is formed by winding a spacer member having a specific thickness around non-image forming areas at both ends of the charging roller21and contacting the surface of the spacer member to the surface of the photoreceptor11.

The charging roller21discharges at a part close to the photoreceptor11to charge the photoreceptor11. When the charging roller21does not contact the photoreceptor11, adherence of an untransferred toner to the charging roller21from the photoreceptor11is prevented. Therefore, the charging roller21is not contaminated with the untransferred toner. The charging roller21may contact the photoreceptor11.

The charging roller21includes an unillustrated cleaner roller cleaning the surface of the charging roller21while contacting thereto. Even when a toner floating in the apparatus adheres to the surface of the charging roller21, the cleaner roller cleans the surface of the charging roller21so as not be contaminated.

The image developer30includes an unillustrated developing sleeve including a magnetic field generator at a position facing the photoreceptor11.

A stirring and feeding screw mixing a toner placed from an unillustrated toner bottle with a developer and pumping the developer to the developing sleeve while stirring the developer is located below the developing sleeve.

A two-component developer formed of a toner and a magnetic carrier, fed by the developing sleeve is regulated by a regulation member to form a layer having a predetermined thickness, and which is borne by the developing sleeve. The developing sleeve bearing the developer feeds a toner to the photoreceptor11while travelling in a same direction at a position facing the photoreceptor11.

A toner cartridge for each color containing unused toner is detachably installed in a space above the photoreceptor11. An unillustrated toner feeder such as a mohno pump and an air pump feeds a toner to each of the image developers30when necessary. The toner cartridge for black toner consumed much may have larger capacity.

The cleaner40is formed of a cleaning blade and a holder holding the blade, and contacts the blade to the photoreceptor11with pressure to remove a residual toner therefrom. Further, the cleaner40is equipped with a mechanism contacting and separating the cleaning blade to and from the photoreceptor11, and which is controlled by a controller of the image forming apparatus1as desired. The cleaning blade contacts the photoreceptor11in a counter direction in respect of traveling direction thereof to remove a toner remaining thereon and additives such as talc, kaolin and calcium carbonate of a recording medium, which adhere thereto. The removed toner is collected by an unillustrated waster toner collection coil and reserved in an unillustrated waster toner container.

Alternatively, the removed toner is collected by a service man or fed to the image developer to be used for development as a recycle toner.

The transferer60includes the intermediate transfer belt61a toner image is layered on, a first transfer roller62transferring and layering the toner image on the photoreceptor11to and on the intermediate transfer belt61and a second transfer roller63transferring the layered toner image onto a recording paper6.

Further, the transferer60includes a support roller653on the inside of the intermediate transfer belt61at a position facing the second transfer roller63. In addition, a tension roller657pressing the intermediate transfer belt61from outside to impart a tension thereto is located on the outside thereof. The first transfer roller62first transferring the toner image formed on the photoreceptor11onto the intermediate transfer belt61is located at a position facing the photoreceptor11across the intermediate transfer belt61.

The first transfer roller62is connected to an unillustrated electric source and applied with a predetermined DC or AC voltage.

The voltage applied has a polarity reverse to that of a toner, and the toner image is first transferred when first transfer roller62is drawn from the photoreceptor11to the intermediate transfer belt61.

The first transfer roller62preferably includes an inorganic electroconductive material such as carbon black or an ionic electroconductive material to control electrical resistance thereof such that the first transfer roller62is semiconductive.

Even when the first transfer roller62differs in resistivity, transfer efficiency scarcely changes. However, when an image areal ratio differs, the transfer efficiency largely differs and is not stably maintained. This is because a current preferentially runs through a part of a transfer nip where a toner is not present, and a transfer voltage becomes low and an electric field needed to transfer is not sufficiently formed when the image areal ratio is small.

Particularly when the first transfer roller62has a low resistivity, the influence of the resistivity of the toner present at a transfer site becomes large. The lower the resistivity of the first transfer roller62, the larger the influence.

When a constant current control is used, the first transfer roller62preferably has a high resistivity. However, when the resistivity is higher than 5×108[Ω], the current may leak to disturb a toner image. Therefore, the first transfer roller62preferably has a resistivity of from 1×105to 5×108[Ω].

A current preferentially runs through a part where a toner is not present is not only because of the resistivity. This is partly because a transfer current is likely to run to a larger potential difference since a potential difference between a first transfer voltage applied to the central core metal of the first transfer roller62and the photoreceptor11at a position where a toner is not developed is larger than a position where the toner is developed.

This occurs in the image forming apparatus1in which the toner image has the same polarity as that of the photoreceptor11and a toner is developed on a discharged part of the photoreceptor11having received imagewise light to form a toner image thereon.

The photoreceptor has a high potential at a part where a toner image is not formed and a low potential at a part where a toner image is formed. However, the transfer potential has a polarity reverse to that of a potential of the photoreceptor, and a difference between a first transfer voltage and potential of the photoreceptor is larger at a part where a toner image is not formed than a part where a toner image is formed.

In this case, the first transfer roller62preferably has a resistivity of from 5×107to 5×108[Ω].

An optical sensor90is located a position facing the drive roller652through the intermediate transfer belt61with a predetermined gap from an outer surface thereof. The optical sensor90is a reflective photosensor emitting light from an unillustrated light emitting element to be reflected on the surface of the intermediate transfer belt61or on a toner image thereon and detecting a reflective light amount with an unillustrated light receiving element.

Based on an output voltage value from the optical sensor90, an unillustrated controller detects a toner image on the intermediate transfer belt61or an image density (adherence amount per unit area) thereof.

A toner image layered on the intermediate transfer belt61is secondly transferred onto the recording paper6at a second transfer part formed by contact between the second transfer roller63and the intermediate transfer belt61. The second transfer roller63is connected to an unillustrated electric source as the first transfer roller62and applied with a predetermined DC or AC voltage. The voltage applied has a polarity reverse to that of a toner, and the toner image is secondly transferred when the second transfer roller63is drawn from the intermediate transfer belt61to the recording paper6.

The second transfer roller63is formed of a cylindrical core metal, an elastic layer overlying an outer circumferential surface of the core metal, and a surface layer formed of a resin overlying an outer circumferential surface of the elastic layer.

Specific examples of metals forming the core metal include, but are not limited to, stainless and aluminum. The elastic layer is typically a rubber layer. This is because the second transfer roller63is required to have elasticity such that it is deformed to form a second transfer nip. Therefore, the elastic layer preferably has a JIS-A hardness not greater than 70°.

Further, a cleaning blade22is used to clean the second transfer roller63. When the elastic layer is too soft, the cleaning blade22unstably contacts the second transfer roller63, resulting in unsuitable cleaning angle. Therefore, the elastic layer preferably has a JIS-A hardness not less than 40°.

The elastic layer preferably includes a foamed resin imparted with electroconductivity because the second transfer roller63transfers a toner image onto a recording paper, and has a thickness of from 2 to 10 mm. Specific examples of materials imparting electroconductivity include, but are not limited to, EPDM or Si rubber in which carbon black is dispersed, NBR having ionic conductivity, and urethane rubber.

Many of the foamed resins used in the elastic layer have high chemical affinity with a toner and large friction coefficient. Therefore, the surface layer the cleaning blade22contacts includes a fluorine-containing resin and a resistivity controller because of needing low friction coefficient and releasability.

The second transfer roller63rotates contacting the intermediate transfer belt61, and a small difference in linear speed therebetween influences upon drive of the intermediate transfer belt61. Therefore, the elastic layer of the second transfer roller63is required to have slidability with the intermediate transfer belt61, and the outermost surface of the surface layer preferably has a friction coefficient not greater than 0.4.

In addition, the intermediate transfer belt61includes a belt cleaner64cleaning the surface of the intermediate transfer belt61.

The support roller653has a mechanism of contacting and separating itself to and from the intermediate transfer belt61, and which is controlled by a controller of the image forming apparatus1as desired.

A lubricant applicator67applying a lubricant to the intermediate transfer belt61may be installed when necessary.

The lubricant applicator67includes a solid lubricant contained in a fixed case, a brush roller contacting the solid lubricant to scrape the lubricant off and apply it to the intermediate transfer belt61, and a lubricant application blade leveling off the lubricant applied by the brush roller. The solid lubricant has the shape of a cuboid and is biased to the brush roller by a pressure spring. The solid lubricant is scraped off by the brush roller and consumed. The solid lubricant decreases in thickness as time passes and constantly contacts the brush roller because it is pressurized by the pressure spring. The brush roller applies the scraped lubricant to the surface of the intermediate transfer belt61while rotating. A lubricant having the same functions may be installed for the photoreceptor11.

In this embodiment, an unillustrated lubricant application blade as a lubricant leveler contacts the surface of the intermediate transfer belt61at downstream side relative to a position where the lubricant is applied by the brush roller.

The lubricant application blade is formed of an elastic rubber, and has function as a cleaner as well and contacts the intermediate transfer belt61in a counter direction in respect of travelling direction thereof.

A dry solid hydrophobic lubricant can be used as the solid lubricant. Besides zinc stearate, metal compounds having a fatty acid group such as stearic acid, oleic acid and palmitic acid can be used. Further, waxes such as candelilla wax, carnauba wax, rice wax, Japan wax, jojoba oil, honey wax and lanoline.

Below the transferer60, a fixer70fixing a toner image on a recording paper thereon is installed.

The recording paper6a toner image is transferred on at the second transfer nip is fed to the fixer70by a recording paper feed belt66hung with tension by tension rollers65and6b.

The fixer70is mainly formed of a fixing roller71including an unillustrated halogen heater and a pressure roller72facing and contacting the fixing roller71with pressure.

The fixer70is controlled by an unillustrated controller to have optimum fixing conditions according to a printed matter, i.e., whether it is a full-color image or a monochrome image, one-side printing or duplex printing, or a kind of recording medium.

When the duplex printing mode is selected, a switching claw851feeds the recording paper6an image is fixed on one side thereof to a recording paper reverser89. Plural rollers and unillustrated guide members in a reverse route87reciprocates the recording paper thereon to be turned over. When the recording paper is turned over, a switching claw852returns the recording paper6to the transfer position again. After an image is transferred and fixed on the backside of the recording paper6, it is finally discharged by a paper discharge roller85on a discharged paper tray86.

When the recoding paper6is one, it is turned over and passed through the reverse route87of the recording paper reverser89. The pair of registration rollers84feed the recording paper6after an image is formed on one side thereof by the image forming unit10to the transfer position where an image is transferred on the recording paper6. After an image is transferred and fixed on the backside of the recording paper6, it is finally discharged by a paper discharge roller85on a discharged paper tray86.

When there are plural recording papers6, a predetermined number thereof on each of which a toner image is formed are contained in a recording paper reverser container88in the recording paper reverser89once. Next, the recording papers6are fed from the recording paper reverser container88by the paper feed roller82and separated from one by one by the separation roller81. The recoding paper6is fed by the feed roller83to the pair of registration rollers84, and they feed the recording paper6after an image is formed on one side thereof by the image forming unit10to the transfer position where an image is transferred on the recording paper6. After an image is transferred and fixed on the backside of the recording paper6, it is finally discharged by a paper discharge roller85on a discharged paper tray86.

InFIG. 2, the intermediate transfer belt61is hung around the drive roller652and the driven roller651.

Below the drive roller652, between the support roller653and the driven roller651, a tension roller657curving the belt from outside to inside is located. The tension roller657forms a space below the curving part of the intermediate transfer belt61. The fixer70is located in the space to effectively use spaces in the image forming apparatus in a vertical direction. Thus, a size of the image forming apparatus in a vertical direction is shortened, and the apparatus is downsized.

FIG. 1is a schematic view illustrating a layer composition of an embodiment of the intermediate transfer belt61for use in the present invention.

The intermediate transfer belt61has a double-layered structure including an inner (base) layer61aformed of a resin and an outer (surface) layer61bformed of the same resin overlying the inner (base) layer61a. The resin is preferably a polyimide resin or a polyamideimide resin having high strength and high elasticity.

The resin includes a filler or an additive regulating electrical resistance, i.e., an electrical resistance regulator such as metal oxides and carbon black.

Specific examples of the metal oxides include, but are not limited to, zinc oxide, tin oxide, titanium oxide, zirconium oxide, aluminum oxide, and silicon oxide. Other examples thereof include products obtained by subjecting the above metal oxide to a surface treatment for improving dispersibility thereof.

Specific examples the carbon black include, but are not limited to, ketjen black, furnace black, acetylene black, thermal black and gas black. They are preferably dispersible in polyimide.

The electrical resistance regulators in this embodiment are not limited to the above. A coating liquid including at least a resin for preparing a seamless belt as the intermediate transfer belt61of this embodiment may further include an additive such as a dispersion auxiliary, a reinforcing agent, a lubricant, a heat-transfer agent and an antioxidant.

Next, the polyimide resin used in this embodiment is explained.

Aromatic polyimide is obtained through a polyamic acid (polyimide precursor) produced from a reaction between aromatic polycarboxylic acid anhydrides (or their derivatives) and aromatic diamines.

The aromatic polyimide is insoluble in solvents because of its rigid main chain structure, and does not melt. First, aromatic polycarboxylic acid anhydrides and aromatic diamines are reacted with each other to synthesize a polyimide precursor (polyamic acid). The polyamic acid is heated or chemically dehydrated to obtain cyclized (imidized) polyimide. Aromatic polyimide is obtained by subjecting almost same moles of the following aromatic polycarboxylic acid anhydrides and aromatic diamines to polymerization reaction in an organic polar solvent:

wherein Ar1is a tetravalent aromatic residue containing at least one aromatic ring.

Thus, the polyimide precursor (polyamic acid) is obtained, and then the polyamic acid is dehydrated to be cyclized and imidized. Methods of preparing the polyamic acid are specifically explained.

Specific examples of the organic polar solvent used in the polymerization reaction to prepare the polyamic acid include, but are not limited to, a sulfoxide-based solvent such as dimethylsulfoxide, and diethyl sulfoxide; a formamide-based solvent such as N,N-dimethylformamide, and N,N-dimethylformamide; an acetoamide-based solvent such as N,N-dimethylacetamide, and N,N-diethylacetamide; a pyrrolidone-based solvent such as N-methyl-2-pyrrolidone, and N-vinyl-2-pyrrolidone; and a phenol-based solvent such as phenol, o-, m-, or p-cresol, xylenol, halogenated phenol, and catechol; an ether-based solvent such as tetrahydrofuran, dioxane, and dioxolane; an alcohol-based solvent such as methanol, ethanol, and butanol; a cellosolve-based solvent such as butyl cellosolve; hexamethylphosphoramide; and γ-butyrolactone. These can be used alone or in combination.

The organic polar solvents are not particularly limited if they dissolve the polyamic acid, but γ-butyrolactone and N-methyl-2-pyrrolidone are preferably used.

An example of preparing the polyimide precursor, first, one or more diamine is dissolved in the organic solvent, or dispersed therein to form a slurry in an atmosphere of inactive gas such as argon or nitrogen.

To the resultant solution, an aromatic polycarboxylic anhydride (or a derivative thereof) is added (which may be in the state of a solid, a solution being dissolved in an organic solvent, or a slurry) to thereby proceed to a ring-opening polyaddition reaction with generation of heat. As a result, the solution suddenly increases its viscosity, to thereby prepare a high-molecular-weight polyamic acid solution. The reaction temperature is preferably from −20 to 100° C., and more preferably in the range of −20 to 60° C. The reaction time is about from 30 min to 12 hrs.

The example described above is one example. Conversely to the order of addition above, first, aromatic tetracarboxylic dianhydride or a derivative thereof is dissolved or dispersed in an organic solvent in advance, and to the resultant solution, the aromatic diamine(diamine) may be added. The diamine may be added in the state of a solid, or a solution prepared by dissolving the aromatic diamine compound in an organic solvent, or a slurry. Namely, the order for adding the aromatic tetracarboxylic dianhydride and the diamine is not limited. Further, aromatic tetracarboxylic dianhydride and the diamine compound may be simultaneously added to the organic polar solvent to proceed to a reaction.

In the manner as described above, an equimolar aromatic polycarboxylic anhydride or a derivative thereof, and an equimolar aromatic diamine compound are subjected to a polymerization reaction in an organic polar solvent, to thereby prepare a polyamic acid (polyimide precursor) solution in the state that polyamic acid is uniformly dissolved in the organic polar solvent.

The polyamic acid can be imidized by a (1) heating method, or a (2) chemical method. The (1) heating method is a method for transforming (imidizing) the polyamic acid to polyimide by heating the polyamic acid to 200° C. to 350° C., and is a simple and practical method for attaining polyimide (a polyimide resin). The (2) chemical method is a method in which after reacting the polyamic acid with a cyclodehydration reagent (e.g., a mixture of carboxylic anhydride and tertiary amine), the resultant is heated to thereby completely imidize the polyamide acid, and a complicated and costly method compared to the (1) heating method. From this reason, generally, the (1) heating method is commonly used.

The polyimide is preferably heated at not less than a glass transition to be imidized such that the polyimide exert its original performance. Marketed polyimides can be used in this embodiment such as general-purpose polyamides from Du Pont-Toray Co., Ltd., Ube Industries, Ltd., New Japan Chemical Co., Ltd., JSR, Unitika Ltd., IST, Hitachi Chemical Co., Ltd., etc.

The polyamideimide is a resin having a rigid imide group and a flexibility-imparting amide group in its molecular skeleton. Known polyamideimide can be used in this embodiment.

Typically, the following methods of synthesizing polyamideimide resins are known.

(a) Japanese published examined application No. JP-S42-15637-B discloses an acid chloride method of reacting a derivative halide of tricarboxylic acid having an acid anhydride group, typified by a chloride compound of the derivative, and diamine in a solvent.

(b) Japanese published examined application No. JP-S44-19274-B discloses an isocyanate method of reacting a trivalent including an acid anhydride group and a carboxylic acid having, and an aromatic isocyanate in a solvent.

Specific examples of the derivative halide of tricarboxylic acid having an acid anhydride group include compounds having the following formulae:

wherein X represents a halogen atom, and

wherein X represents a halogen atom; and Y represents —CH2—, —CO—, —SO2— or —O—.

Specific examples of the diamine include, but are not limited to, aromatic diamines, aliphatic diamines and alicyclic diamines. The aromatic diamines are preferably used.

Silicone-modified polyamideimide can be obtained when siloxane compounds having an amino group at both ends as diamine are used. Specific examples of the siloxane compounds having an amino group at both ends as diamine include 1,3-bis(3-aminopropyl)-1,1,3,3-tetramethyldisiloxane, α, Ω-bis-(3-aminopropyl)polydimethylsiloxane, 1,3-bis(3-aminophenoxymethyl)-1,1,3,3-tetramethyldisiloxane, α,Ω-bis-(3-aminophenoxymethyl)polydimethylsiloxane, 1,3-bis(2-(3-aminophenoxy)ethyl)-1,1,3,3-tetramethyldisiloxane, α,Ω-bis-(2-(3-aminophenoxy)ethyl))polydimethylsiloxane, 1,3-bis(3-(3-aminophenoxy)propyl)-1,1,3,3-tetramethyldisiloxane, and α, Ω-bis-(3-(3-aminophenoxy)propyllpolydimethylsiloxane.

To obtain polyamideimide in the embodiment by the acid chloride method, as the polyimide resin is prepared, a derivative halide of tricarboxylic acid having an acid anhydride group and diamine are dissolved in an organic polar solvent, and reacted therein at a low temperature of form 0 to 30° C. to prepare a polyamideimide precursor (polyamide-amic acid).

Specific examples of the organic polar solvent include, but are not limited to, a sulfoxide-based solvent such as dimethylsulfoxide, and diethyl sulfoxide; a formamide-based solvent such as N,N-dimethylfomamide, and N,N-diethylformamide; an acetoamide-based solvent such as N,N-dimethylacetamide, and N,N-diethylacetoamide; a pyrrolidone-based solvent such as N-methyl-2-pyrrolidone, and N-vinyl-2-pyrrolidone; and a phenol-based solvent such as phenol, o-, m-, or p-cresol, xylenol, halogenated phenol, and catechol; an ether-based solvent such as tetrahydrofuran, dioxane, and dioxolane; an alcohol-based solvent such as methanol, ethanol, and butanol; a cellosolve-based solvent such as butyl cellosolve; hexamethylphosphoramide; and γ-butyrolactone. These can be used alone or in combination.

The organic polar solvents are not particularly limited if they dissolve the polyamideimide precursor, but γ-butyrolactone and N-methyl-2-pyrrolidone are preferably used.

The polyamideimide is typically used alone, but can be combined with compatible polyamideimide or modified groups such as silicone and fluorine. Marketed polyamideimide varnish from Hitachi Chemical Co., Toyobo Co., Ltd., and Arakawa Chemical Industries, Ltd can also be used.

The other components are appropriately selected depending on the intended purpose without any limitation, and examples thereof include an electrical resistance regulator, an ion conductive agent, a dispersion auxiliary, a reinforcing agent, a lubricant, a heat-transfer agent and an antioxidant.

The intermediate transfer belt61in the image forming apparatus of the embodiment has a double-layered structure formed of the inner layer61aand the outer layer61beach formed of a same resin to prevent the belt from curling inside or outside and improve adhesiveness between the inner layer61aand the outer layer61b.

Examples of the electrical resistance regulator include metal oxide, carbon black, an ion conductive agent, and an electric conductive polymer material. Examples of the metal oxide include zinc oxide, tin oxide, titanium oxide, zirconium oxide, aluminum oxide, and silicon oxide. Other examples thereof include products obtained by subjecting the above metal oxide to a surface treatment for improving dispersibility thereof. Among these, carbon black is preferably used because it is easy to disperse and difficult to deteriorate in strength.

Examples of the carbon black include ketjen black, furnace black, acetylene black, thermal black and gas black.

Examples of the ion conductive agent include a tetra alkyl ammonium salt, a trialkylbenzyl ammonium salt, an alkylsulfonic acid salt, an alkylbenzenesulfonic acid salt, alkyl sulfate, glycerin fatty acid ester, sorbitan fatty acid ester, polyoxyethylenealkylamine, ester of polyoxyethylene aliphatic alcohol, alkyl betaine, and lithium perchlorate. Examples of electric conductive polymer material include polyaniline, polythiophene, and polypyrrole. These can be used alone or in combination.

The electrical resistance regulators in this embodiment are not limited to the above. A coating liquid including at least a resin for preparing a seamless belt of this embodiment may further include an additive such as a dispersion auxiliary, a reinforcing agent, a lubricant, a heat-transfer agent and an antioxidant.

Each of the inner layer61aand the outer layer61bpreferably includes a similar amount of the electrical resistance regulator such as carbon black such that they have similar contraction when heated. The outer layer61bpreferably includes the electrical resistance regulator in an amount of ±5% by weight of that in the inner layer61ato prevent the belt from curling.

When a seamless belt is used as the intermediate transfer belt61, carbon black is included in its layers such that the electric resistance thereof is 1×108Ω/□ to 1×1015Ω/□ in the surface resistance when 500 V is applied thereto, and 1×108Ω·cm to 1×1014Ω·cm in the volume resistance when 100 V is applied thereto. However, in terms of mechanical strength, carbon black is included in the layers in such an amount as they are not fragile and easily cracked.

Namely, a coating liquid including the resin (a polyimide resin precursor or a polyamideimide resin precursor) and the electrical resistance regulator in suitable amounts, respectively is preferably used to prepare a seamless belt having a good balance between electrical properties (surface resistivity and volume resistivity) and mechanical strength.

When the electrical resistance is the carbon black, the content thereof is preferably from 10 to 25% by weight, and more preferably from 15 to 20% by weight. When the electrical resistance is the metal oxide, the content thereof is preferably from 1 to 50% by weight, more preferably from 10 to 30% by weight. When the content is too low, the resistance is difficult to have uniformity and largely varies relative to arbitrary potentials. When too much, the intermediate transfer belt61deteriorates in mechanical strength for practical use.

The average thickness of the inner layer61aand the outer layer61bis appropriately selected depending on the intended purpose without any limitation, but the outer layer61bpreferably has a thickness of from 40 to 60% based on total thickness of the belt because of preventing the belt from curling and adhesiveness of the inner layer61aand the outer layer61b, and the belt preferably has a thickness of from 40 to 150 μm, and more preferably from 50 to 90 μm.

When less than 40 μm, ends of the belt are likely to cut. When greater than 150 μm, the intermediate transfer belt61is likely to crack due to a curve formed between the belt and a roller.

The average thickness is an average value of the values of the thickness measured at arbitrarily selected 10 spots. The thickness can be measured by typical needle-indicating or eddy-current thickness meters, for example, by an electric micrometer manufactured by Anritsu Corporation. The thickness of each of the inner layer61aand the outer layer61bcan be measured by measuring a cross-section at an arbitrary point of the belt by a scanning electron microscope (SEM).

Methods of forming the inner layer61aand the outer layer61bof the intermediate transfer belt61are appropriately selected depending on the intended purpose without any limitation. Examples thereof include a method containing: preparing a coating liquid in which the aforementioned other components such as the electrical resistance regulator optionally dispersed in the polyimide precursor solution (polyamic acid solution); applying the coating liquid onto a substrate; and transforming (imdizing) polyamic acid, which is polyimide precursor, into polyimide, as well as forming the coating liquid into a layer by a processing, such as heating.

The substrate is appropriately selected depending on the intended purpose without any limitation, and examples thereof include a cylindrical metal mold.

An example of preparing the intermediate transfer belt61is specifically explained. An outer surface coating method preferably used in the embodiment is explained.

A coating liquid containing polyimide precursor is applied onto a cylindrical mold, such as a cylindrical metal mold, by a liquid supplying device, such as a nozzle and a dispenser, while slowly rotating the cylindrical mold, so as to uniformly coat the outer surface of the cylindrical mold with the coating liquid, to thereby perform flow casting (forming a coating film). Thereafter, the rotational speed is increased to a predetermined speed. Once the rotational speed reaches the predetermined speed, the rotational speed is maintained constant, and the rotation is continued for a predetermined period. Then, the temperature is gradually elevated while rotating the cylindrical mold, to thereby evaporate the solvent in the coating film at the temperature of 80° C. to 150° C. It is preferred that the vapor (e.g., the evaporated solvent) in the atmosphere be efficiently circulated and removed.

Once a self-supporting film is formed, the temperature of the furnace is increased stepwise, and eventually a high temperature heat treatment (baking) is performed at the temperature ranging from about 200° C. to about 350° C., to thereby imidize the polyimide precursor to some extent to form the inner layer61a. When the polyimide precursor is completely imidized, adhesiveness with polyimide in the outer layer61bbecomes poor. After fully cooled, an outer layer coating liquid is coated and dried as the inner layer coating liquid, and then the polyimide precursor sufficiently imidized. The inner layer and the outer layer are applied with different heat energies such that even the same resin has a difference in contraction when imidized to prevent the inner and outer layers form curling.

The metal mold is not dried with hot air applied from outside by marketed driers or heating furnaces, and preferably heated from the inside. Any heaters may be used, and specifically a halogen heater and an IH heater can be used.

When an outer surface of the belt is coated and the metal mold is heated from the inside (backside), a curl angle from the surface of the belt is larger than a curl angle from the backside of the belt, i.e., 85° or more.

The inner layer61aand the outer layer61bare preferably heated from the inside of the metal mold to be dried. They are preferably heated from the inside of the metal mold as well when coated inside. The inside-coated belt is likely to separate from the metal mold when heated to perform polyimide contraction, and tends to curl more than the outside-coated belt.

This is because the surface is imidized but not imidized well inside when they are heated from outside of the metal mold, and the resultant belt tends to curl more.

Next, a method of measuring a curl angle of the intermediate transfer belt61is explained.FIG. 4is a schematic view illustrating a curl applicator50curling the belt.FIGS. 5A and 5Bare schematic views for explaining a curl angle, in which a sample52placed on a horizontal table for 60 min is viewed from directly above.

A cut sample 125 mm (circumferential direction)×15 mm (axial direction) is hung on a metallic bar51having a diameter of 14 mm asFIG. 4shows, one side of the sample is fixed with a clip53and a weight54weighing 2.25 N is hung on the other side. Then, the sample is left for 14 days in an environment of 45° C. and 90% RH.

Then, the clip53and the weight54are taken off from the sample52, and the sample is placed on the horizontal table with an end in the circumferential direction down and left for 60 min in an environment of 23° C. and 50% RH. Then, an angle of a curl on the sample52is measured asFIGS. 5A and 5Bshow.

A curl angle Ai of the backside (inner layer61a) of the belt is a curl angle when the backside (inner layer side) of the intermediate transfer belt61is wound around the metallic bar51while contacting thereto. A curl angle Ao of the surface (outer layer61b) of the belt is a curl angle when the surface (outer layer side) of the intermediate transfer belt61is wound around the metallic bar51while contacting thereto.

The curl angle Ai and the curl angle Ao are preferably not less than 85° for practical use. When less than 85°, the belt waves, and the belt is damaged and may be cracked or broken in the worst case.

Further, a transfer pressure changes and transfer amount becomes less at the curled part, resulting in uneven image density.

When the belt has a single-layered structure, either the curl angle Ai or the curl angle Ao can be not less than 85°, and it is difficult for them both to be not less than 85°.

In an image forming apparatus without a pressure member (tension roller), the intermediate transfer belt61only curves from inside to outside. Therefore, there is no problem in practical use if only the curl angle Ai is not less than 85°.

However, in the image forming apparatus of the embodiment, the intermediate transfer belt61not only curves from inside to outside, but also from outside to inside. Therefore, not only the curl to inside (backside) but also the curl to outside (surface side) need to be considered.

When a curled part enters the first transfer nip, a contact pressure changes. The image density is high at a part where the contact pressure is high and low at a part where the contact pressure is low. As a result, the image has uneven image density in a travel direction of the belt, i.e., lateral band unevenness.

Not only the intermediate transfer belt61but also other belts in an image forming apparatus have problems when curled. For example, when a curled recording paper feed belt feeds the recording paper6to the second transfer position bearing the paper on the curled part, the resultant image may have the same uneven image density due to changes of contact pressure.

Japanese published unexamined application No. JP-2002-182488-A discloses a method of including a fibrous conductive agent and an inorganic filler in a polyimide resin to improve creep (curl). However, the inorganic filler is difficult to disperse in polyimide, uniformity of image density is insufficient. Further, the conductive agent reduces adhesiveness of the resin layer and the layer is likely to peel off. Although creep is improved in an environment of normal temperature, curl remains same in an environment of high temperature and high humidity.

Japanese published unexamined application No. JP-2004-126068-A discloses a method of including a flaky particulate powder in a polyimide resin to improve creep. However, the flaky particles are easy to aggregate and the resultant images do not have sufficient quality. Further, the method does not prevent the belt from curing in an environment of high temperature and high humidity.

A residual solvent largely influences upon properties of the belt. When too much, the belt absorbs moisture in an environment of high temperature and high humidity, and changes in sizes and lowers in resistivity, resulting in variation of image density and defective running. When too little, the belt deteriorates in flexibility, resulting in crack and break thereof. Therefore, the residual solvent is preferably from 5 to 2,000 ppm at an arbitrary point when the layers are layered.

An amount of the residual solvent is controlled by drying temperature and time. When a coating liquid is coated on the inner surface of the metal mold, a solvent vapor is likely to remain when heated and the residual solvent in the belt is likely to increase. Therefore, the coating liquid is preferably coated on the outer surface of the metal mold.

When the belt has a single-layered structure, the residual solvent needs to be 3 ppm or less such that the curl angles Ai and Ao are both not less than 85°. However, in this case, the drying time is so long that the cost largely increases. Further, the belt deteriorates in flexibility and is likely to crack. Namely, the belt does not have sufficient durability for practical use.

Methods of measuring the residual solvent in the belt include analyzing a part randomly cut out from the intermediate transfer belt61by thermal extraction gas chromatograph mass analysis (GC-MS) method. Marketed products of the GC-MS apparatus include GCMS-QP2010 from Shimadzu Corp.

EXAMPLES

An amount of the residual solvent N-methyl-2-pyrrolidone (NMP) in the seamless belt was determined by the following formula, using the method of analyzing a part randomly cut out from the belt by thermal extraction gas chromatograph mass analyzer GCMS-QP2010 from Shimadzu Corp.

Measured Amount of N-methyl-2-pyrrolidone (μg)/Weight of Belt Sample (g)

A curl angle was measured by hanging a cut sample 125 mm (circumferential direction)×15 mm (axial direction) on a metallic bar51having a diameter of 14 mm asFIG. 4shows, one side of the sample is fixed with a clip53and a weight54weighing 2.25 N was hung on the other side. Then, the sample was left for 14 days in an environment of 45° C. and 90% RH. Then, the clip53and the weight54were taken off from the sample52, and the sample was placed on a horizontal table with an end in the circumferential direction down and left for 60 min in an environment of 25° C. and 60% RH. Then, an angle of a curl on the belt along the contact surface of the metallic bar51was measured asFIGS. 5A and 5Bshow.

A curl angle Ai of the backside (inner layer61a) of the belt was a curl angle when the backside (inner layer side) of the intermediate transfer belt61was wound around the metallic bar51while contacting thereto. A curl angle Ao of the surface (outer layer61b) of the belt was a curl angle when the surface (outer layer side) of the intermediate transfer belt61was wound around the metallic bar51while contacting thereto.

Preparation of Polyimide Coating Liquid A

In a mixture including U-varnish A and U-varnish S from Ube Industries, Ltd. at a solid content ratio of 50/50, which are polyimide varnishes including a polyimide resin precursor as a main component, a dispersion including N-methyl-2-pyrrolidone and carbon black (Special Black 4 from Evonik Degussa GmbH) dispersed by beads mill therein was mixed and stirred such that the content of the carbon black was 17% by weight based on total weight of the solid contents to prepare a polyimide coating liquid A.

(Preparation of Seamless Belt A)

The polyimide coating liquid A was uniformly coated by a dispenser on a blasted (roughened) outer surface of a metallic cylindrical mold having an outer diameter of 375 mm and a length of 340 mm while rotated at 50 rpm.

After the polyimide coating liquid A was uniformly coated, the cylindrical mold was placed in a drier while rotated at 100 rpm. A halogen heater was placed in the center of the metallic mold and the metallic mold was gradually heated up to have a temperature of 110° C. for 60 min.

Further, the metallic mold was heated up to have a temperature of 250° C. for 30 min, and then the rotation thereof was stopped. Then, the cylindrical mold a film was formed on was gradually cooled and taken out from the drier.

Next, the polyimide coating liquid A was uniformly coated by a dispenser on the cylindrical mold while rotated at 50 rpm.

After the polyimide coating liquid A was uniformly coated, the cylindrical mold was placed in a drier while rotated at 100 rpm. A halogen heater was placed in the center of the metallic mold and the metallic mold was gradually heated up to have a temperature of 110° C. for 60 min.

Further, the metallic mold was heated (burned) up to have a temperature of 340° C. for 60 min such that the polyimide coating liquid A was imidized. Then, the metallic mold was gradually cooled and taken out to prepare a seamless belt A.

The procedure for preparation of the seamless belt in Example 1 was repeated except that the drier did not heat the metallic mold from the inside thereof with a halogen heater but generated and circulate hot air from outside of the metallic mold to prepare a seamless belt B.

The procedure for preparation of the seamless belt in Example 1 was repeated except for changing the thickness of the outer layer and the inner layer to prepare a seamless belt C.

The procedure for preparation of the seamless belt in Example 1 was repeated except for changing the thickness of the outer layer and the inner layer to prepare a seamless belt D.

The procedure for preparation of the seamless belt in Example 1 was repeated except for changing the thickness of the outer layer and the inner layer to prepare a seamless belt E.

The procedure for preparation of the seamless belt in Example 1 was repeated except for changing the thickness of the outer layer and the inner layer to prepare a seamless belt F.

The procedure for preparation of the seamless belt in Example 1 was repeated except for burning the outer layer at 340° C. for 120 min to prepare a seamless belt G.

The procedure for preparation of the seamless belt in Example 1 was repeated except for burning the outer layer at 300° C. for 30 min to prepare a seamless belt H.

The polyimide coating liquid A was uniformly coated on a mirrored inner surface treated with a release agent of a metallic cylindrical mold having an outer diameter of 375 mm and a length of 340 mm while rotated at 50 rpm.

After the polyimide coating liquid A was uniformly coated, the cylindrical mold was placed in a drier generating and circulating hot air from outside of the metallic mold while rotated at 100 rpm, and was gradually heated up to have a temperature of 110° C. for 60 min.

Further, the metallic mold was heated up to have a temperature of 250° C. for 30 min, and then the rotation thereof was stopped. Then, the cylindrical mold a film was formed on was gradually cooled and taken out from the drier.

Next, the polyimide coating liquid A was uniformly coated by a dispenser on the cylindrical mold while rotated at 50 rpm.

After the polyimide coating liquid A was uniformly coated, the cylindrical mold was placed in a drier while rotated at 100 rpm. A halogen heater was placed in the center of the metallic mold and the metallic mold was gradually heated up to have a temperature of 110° C. for 60 min.

Further, the metallic mold was heated (burned) up to have a temperature of 340° C. for 60 min. Then, the metallic mold was gradually cooled and taken out to prepare a seamless belt I.

In Vylomax HR-16NN from Toyobo Co., Ltd, which is polyamideimide varnish including a polyamideimide resin precursor as a main component, a dispersion including N-methyl-2-pyrrolidone and carbon black (MA77 from Mitsubishi Chemical Corp.) dispersed by beads mill therein was mixed and stirred such that the content of the carbon black was 23% by weight based on total weight of the solid contents to prepare a polyamideimide coating liquid.

The polyamideimide coating liquid was uniformly coated by a dispenser on a blasted (roughened) outer surface of a metallic cylindrical mold having an outer diameter of 375 mm and a length of 340 mm while rotated at 50 rpm.

After the polyamideimide coating liquid was uniformly coated, the cylindrical mold was placed in a drier while rotated at 100 rpm. A halogen heater was placed in the center of the metallic mold and the metallic mold was gradually heated up to have a temperature of 110° C. for 60 min.

Further, the metallic mold was heated up to have a temperature of 190° C. for 30 min, and then the rotation thereof was stopped. Then, the cylindrical mold a film was formed on was gradually cooled and taken out from the drier.

Next, the polyamideimide coating liquid was uniformly coated by a dispenser on the cylindrical mold while rotated at 50 rpm.

After the polyamideimide coating liquid was uniformly coated, the cylindrical mold was placed in a drier while rotated at 100 rpm. A halogen heater was placed in the center of the metallic mold and the metallic mold was gradually heated up to have a temperature of 110° C. for 60 min.

Further, the metallic mold was heated (burned) up to have a temperature of 260° C. for 60 min such that the polyimide coating liquid A was imidized. Then, the metallic mold was gradually cooled and taken out to prepare a seamless belt J.

Comparative Example 1

The procedure for preparation of the seamless belt in Example 1 was repeated except for not forming the inner layer to prepare a single-layered seamless belt K having a thickness of 60 μm.

Comparative Example 2

The procedure for preparation of the seamless belt in Comparative Example 1 was repeated except for burning the layer at 370° C. for 150 min to prepare a seamless belt L.

Comparative Example 3

The procedure for preparation of the seamless belt in Example 9 was repeated except for not forming the inner layer to prepare a single-layered seamless belt M having a thickness of 60 μm.

Properties of the Seamless Belts A to M are Shown in Tables 1-1 to 1-2

The seamless belt of each Example had an Ai and an Ao not less than 85°, respectively.

When the inner layer61ais formed before the outer layer61b, the inner layer61ais dried twice in the process of forming the inner layer61aand the outer layer61b. The outer layer61bis dried once in the process of forming the same. When the outer layer61bis formed before the inner layer61a, the outer layer61bis dried twice in the process of forming the outer layer61band the inner layer61a. The inner layer61ais dried once in the process of forming the same.

Therefore, the inner layer61aand the outer layer61bhave different properties each other even formed of a same resin. It is thought a difference of heat expansion coefficient therebetween generates a force offsetting curls to inside and outside of the belt, resulting in reduction thereof.

Each of the seamless belts A to M was installed in the image forming apparatus inFIG. 2. After left in an environment of 40° C. and 85% RH for 24 hrs, 1,000 cyan and magenta two-color solid images were produced on plain papers TYPE 6200 from Ricoh Company, Ltd. thereby. This was one cycle and repeated for plural times to visually observe image quality.

Excellent: No uneven image density sample

Good: 1 to 5 pieces of uneven image density samples

Fair: 6 to 10 pieces of uneven image density samples

Poor: 11 or more pieces of uneven image density samples

In addition, after left in an environment of 40° C. and 85% RH for 24 hrs, 100,000 images were continuously produced thereby to evaluate durability (break), appearance (damage) and waving of the belt, and jamming (JAM) of papers due to poor runnability.

Appearance: Excellent: No damageGood: Slight damage, but no influence on image qualityPoor: Abnormal images were produced due to damage

The belt broken on the way was evaluated then.

The results are shown in Tables 2-1 to 2-2.

As Tables 2-1 and 2-2 show, each of the seamless belts of Examples provides an intermediate transfer belt having high durability and an image forming apparatus producing high-quality images without uneven image density even after left in an environment of high temperature and high humidity.