JP2009025748A - Two-component developer and image forming device using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a two-component developer with which the charging quantity of toner can be controlled and the occurrence of foggings or decrease of image density can be prevented without influence of coverage of an image to be printed. <P>SOLUTION: The two-component developer is composed of toner and a carrier. The volume resistivity of external additive contained in the toner is 1×10<SP>12</SP>Ωcm to 5×10<SP>15</SP>Ωcm, and the volume resistivity of the carrier is 2×10<SP>10</SP>Ωcm to 5×10<SP>12</SP>Ωcm. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a two-component developer used for visualizing a latent image such as an electrostatic latent image in an image forming method such as electrophotography, and an image forming apparatus using the same.

  When an image is formed on a recording medium by an image forming apparatus using an electrophotographic method, the image is formed on the recording medium as follows. First, the surface of the photoreceptor to which the charging unit is driven to rotate is uniformly charged. Next, the exposure unit irradiates the surface of the photoconductor with laser light to form an electrostatic latent image on the surface of the photoconductor. Next, the developing means supplies a developer containing charged fine particles called toner to the electrostatic latent image formed on the surface of the photoconductor to form a visible toner image on the surface of the photoconductor. Next, the transfer means transfers the toner image to the recording medium. Thereafter, the fixing unit fixes the toner image transferred to the recording medium, thereby forming an image on the recording medium. In addition, the cleaning unit removes the transfer residual toner remaining on the surface of the photosensitive member, and the neutralizing unit neutralizes the residual charge on the surface of the photosensitive member to prepare for the next image formation.

  There are two types of developers for developing the electrostatic latent image formed on the surface of the photosensitive member: a one-component developer composed only of toner and a two-component developer composed of toner and particles called magnetic carrier. Are known. Since a one-component developer does not use a carrier and does not require a stirring mechanism for uniformly mixing the toner and the carrier, it has an advantage that the structure of the developing unit is simplified. Have drawbacks such as being difficult to stabilize. Since the two-component developer requires a stirring mechanism for uniformly mixing the toner and the carrier, there is a disadvantage that the configuration of the developing unit becomes complicated, but a stable charge amount is imparted to the toner. Therefore, it is excellent in adaptability to a high-speed image forming apparatus that performs image formation at high speed and a color image forming apparatus that forms a color image.

  In the high-speed image forming apparatus and the color image forming apparatus, there is a strong demand from users for downsizing the apparatus. For this reason, the developing apparatus that contains the developer is also required to be downsized. The toner of the two-component developer used in the miniaturized developing device has a performance (fast charge rising property) that can obtain a toner charge amount optimum for development in a short time by contacting with the carrier in the developing tank. It is required to have a performance (charging stability) in which the toner charge amount hardly changes in the developing tank. In order to achieve such fast charge rising property and charge stability, a toner containing a charge control agent is used. Patent Document 1 discloses a toner containing, for example, a boron compound represented by the following structural formula (2) as a charge control agent. Since the boron compound represented by the structural formula (2) has high negative chargeability, the toner has excellent charge stability, and even when continuously printing images with high coverage, A stable image density free from fogging can be obtained. Further, since the boron compound is a colorless substance, it is suitable as a charge control agent for color toners.

  In recent image forming apparatuses using an electrophotographic system, there are an increasing number of multifunction peripherals equipped with a facsimile function in addition to a copying machine and a printer function. For users who mainly use copiers and printer functions, the average coverage of printed images is 5% to 10%, but for users who frequently use functions such as fax machines, the coverage is 3% or less. There are cases where a large number of images (images with low toner consumption) are continuously printed.

Japanese Patent Publication No.7-13765

  In a two-component developer using a toner containing a boron compound disclosed in Patent Document 1, a large number of images with low toner consumption per sheet, such as an image with a coverage of 3% or less, are continuously printed. As a result, the charge amount of the toner decreases, fogging and image density decrease.

  Accordingly, an object of the present invention is to provide a two-component development capable of preventing the occurrence of fogging and the reduction in image density by suppressing the reduction in the toner charge amount without being affected by the coverage of the image to be printed. The present invention provides an agent and an image forming apparatus using the agent.

The present invention is a two-component developer comprising a toner for visualizing an electrostatic latent image and a carrier having magnetism,
The toner has at least the following general formula (1)

(Wherein R ₁ and R ₄ represent a hydrogen atom, an alkyl group, a substituted or unsubstituted aromatic ring (including a condensed ring), and R ₂ and R ₃ represent a substituted or unsubstituted aromatic ring (including a condensed ring). And X ⁺ represents a cation.) Colored resin particles formed from a boron compound represented by the following formula:
An external additive externally added to the surface of the colored resin particles,
The external additive is adjusted so that its volume resistivity is 1 × 10 ¹² Ω · cm or more and 5 × 10 ¹⁵ Ω · cm or less,
The carrier is a two-component developer having a volume resistivity of 2 × 10 ¹⁰ Ω · cm to 5 × 10 ¹² Ω · cm.

In the present invention, the carrier includes core particles and a coating layer that covers the core particles.
The coating layer includes a conductive agent having conductivity and a silicone resin.

Further, the present invention is characterized in that the core particle is formed of particles made of ferrite having a volume resistivity of 1 × 10 ⁸ Ω · cm to 5 × 10 ¹⁰ Ω · cm.

In the present invention, the thickness of the coating layer is 0.5 μm or more and 5 μm or less.
In the present invention, the conductive agent contained in the coating layer is carbon black having an oil absorption of 90 ml / 100 g or more and 170 ml / 100 g or less.

  In the present invention, the external additive is a fine particle composed of silica having a surface hydrophobized and a primary particle diameter of 5 nm to 15 nm.

Further, in the present invention, the colored resin particles have a volume average particle diameter of 5 · upper and lower than 7 · net, and a BET specific surface area of 1.5 m ² / g or more and 1.9 m ² / g or less. It is characterized by.

The present invention also provides a photoreceptor carrying an electrostatic latent image, a charging unit for charging the surface of the photoreceptor, an exposure unit for forming an electrostatic latent image on the surface of the photoreceptor, and an electrostatic formed on the surface of the photoreceptor. A developing means for supplying a developer to the latent image to form a visible image; a transferring means for transferring the visible image to a recording medium; a cleaning means for removing the developer remaining on the surface of the photoreceptor; and a recording medium. An image forming apparatus including a fixing means for fixing the transferred visible image,
An image forming apparatus, wherein the developer is the two-component developer.

According to the present invention, the toner of the two-component developer includes colored resin particles formed from a boron compound, a binder resin, and a colorant, and a volume resistivity of 1 × 10 ¹² Ω · cm to 5 × 10 ¹⁵ Ω · cm or less of external additives. Since the volume resistivity of the external additive is adjusted to be 1 × 10 ¹² Ω · cm or more, the external additive can be used even when a large number of high-coverage images that increase toner consumption are continuously printed. Can be prevented from leaking to the colored resin particles, and the external additive can hold a sufficient amount of charge. Therefore, the toner can maintain a fast charge rising property and can prevent fogging and a decrease in image density. Further, since the volume resistivity of the external additive is adjusted to be 5 × 10 ¹⁵ Ω · cm or less, the external additive can be produced relatively easily.

Further, when continuously printing images with low toner consumption and low toner coverage and low coverage, the time during which the toner is stirred in the developing tank becomes long, and the external additive may be buried in the colored resin particles. is there. When the external additive is buried in the colored resin particles as described above, the carrier and the boron compound included in the colored resin particles are likely to come into contact with each other, and the charge held by the boron compound may leak to the carrier. In the present invention, the carrier of the two-component developer is adjusted so that the volume resistivity is 2 × 10 ¹⁰ Ω · cm or more and 5 × 10 ¹² Ω · cm or less. Even in this case, it is possible to prevent the carrier and the boron compound from coming into contact with each other, and thus the charge held by the boron compound can be prevented from leaking to the carrier, and the charge amount of the toner can be prevented from being lowered. Therefore, it is possible to prevent the occurrence of fogging and the decrease in image density.

  According to the invention, the carrier includes core particles and a coating layer that covers the core particles, and the coating layer includes a conductive agent having conductivity and a silicone resin. Silicone resins have the property that boron compounds do not easily adhere. Since the coating layer containing the silicone resin having such properties is formed on the carrier surface, it is possible to prevent the charge held by the boron compound from leaking to the carrier. In addition, since the conductive layer is included in the coating layer, the volume resistivity of the carrier can be controlled to be in a desired range by adjusting the addition amount of the conductive agent.

According to the invention, the core particle is formed of particles made of ferrite having a volume resistivity of 1 × 10 ⁸ Ω · cm to 5 × 10 ¹⁰ Ω · cm. A carrier made of ferrite having a volume resistivity adjusted to the above range is a carrier excellent in electrical insulation and the ability to remove counter charges remaining on the carrier surface. Therefore, it is possible to prevent an edge effect and a decrease in image density at the periphery of the solid image of the obtained image.

  Moreover, according to this invention, the thickness of the coating layer which coat | covers a core particle is 0.5 micrometer or more and 5 micrometers or less. By adjusting the thickness of the coating layer to 0.5 μm or more, when developing an electrostatic latent image carried on the photoconductor, it is possible to prevent the charge on the surface of the photoconductor from moving to the carrier, and to obtain an image obtained It is possible to prevent the occurrence of brush streaks. Further, by adjusting the thickness of the coating layer to 5 μm or less, the counter charge remaining on the carrier after the toner is developed can be quickly eliminated.

  According to the invention, the conductive agent contained in the coating layer is carbon black having an oil absorption of 90 ml / 100 g or more and 170 ml / 100 g or less. This makes it possible to easily form a coating layer in which the conductive agent is uniformly dispersed in the coating layer, and to further prevent the charge on the surface of the photoreceptor from moving to the carrier.

  According to the invention, the external additive is fine particles made of silica having a surface hydrophobized and a primary particle diameter of 5 nm to 15 nm. By hydrophobizing the surface of the external additive in this way, it is possible to prevent the external additive from absorbing moisture and to prevent the toner charge amount from becoming unstable even in a high humidity environment. it can. Further, since the external additive is an external additive having a small primary particle diameter of 5 nm or more and 15 nm or less, it is possible to improve the fluidity of the toner with a small amount of addition and to improve the charge rising property of the toner. Thus, the toner replenished in the developing tank can be quickly charged.

According to the invention, the colored resin particles have a volume average particle diameter of 5 μm or more and 7 μm or less, and a BET specific surface area of 1.5 m ² / g or more and 1.9 m ² / g or less. Therefore, the unevenness of the colored resin particle surface is reduced, the external additive can be prevented from entering the concave portion, and the external additive can be uniformly attached to the colored resin particle surface. As a result, sufficient fluidity is imparted to the toner, and the spacer effect of the external additive (the effect of preventing leakage of the charge held by the toner) can be sufficiently obtained, thereby preventing fogging of the resulting image. In addition, toner scattering can be prevented.

  According to the invention, the developer for forming an image by the image forming apparatus is the two-component developer. Since an image is formed using a two-component developer in which the toner charge amount is suppressed from being reduced without being affected by the coverage of the image to be printed, an image with low coverage and an image with high coverage are continuously printed. Even in this case, it is possible to prevent the toner charge amount from being lowered and to prevent the occurrence of fogging and the image density from being lowered.

  The two-component developer of the present invention includes a toner for visualizing an electrostatic latent image and a magnetic carrier.

[toner]
The toner included in the two-component developer of the present invention includes at least colored resin particles formed from a boron compound, a binder resin, and a colorant, and an external additive externally added to the surface of the colored resin particles. Is done.

(Boron compound)
The boron compound has the following general formula (1)

(Wherein R ₁ and R ₄ represent a hydrogen atom, an alkyl group, a substituted or unsubstituted aromatic ring (including a condensed ring), and R ₂ and R ₃ represent a substituted or unsubstituted aromatic ring (including a condensed ring). X ⁺ represents a cation.) And has high negative chargeability and charge stability. Therefore, by adding a boron compound to the toner, it is possible to impart fast charge rising property and charge stability. As a result, when a toner containing a boron compound is used as a developer of the image forming apparatus, occurrence of fog can be prevented.

The compound represented by the general formula (1) will be described in detail. Examples of the alkyl group of R ₁ and R ₄ include a methyl group, an ethyl group, an n-butyl group, an iso-amyl group, an n-dodecyl group, an n-octadecyl group, a cyclohexyl group, and the like, and R ₁ , R ₂ , Examples of the aromatic ring of R ₃ and R ₄ include a benzene ring and a naphthalene ring, and examples of the substituent include an alkyl group, an alkoxy group, a halogen atom, an aryl group, an aralkyl group, a nitro group, and a cyan group. As the cation, various inorganic cations and organic cations can be used. Inorganic cations include hydrogen ions and metal ions, and monovalent and divalent metal ions include Li ⁺ , Na ⁺ , K ⁺ , Mg ²⁺ , Ca ²⁺ , Zn ^{2+ and the} like. Examples of the organic cation include ammonium ion, iminium ion, and sulfonium ion. Specific examples of such boron compounds are shown in Table 1 below.

  The addition amount of the boron compound is preferably 0.5 parts by weight or more and 3 parts by weight or less, and more preferably 1 part by weight or more and 2 parts by weight or less with respect to 100 parts by weight of the binder resin described later.

(Binder resin)
As the binder resin, those commonly used as toner binder resins can be used. For example, polyester resins, styrene-acrylic resins such as polystyrene-acrylic acid ester copolymers, styrene resins such as polystyrene, ) Acrylic resin, vinyl chloride resin, phenol resin, epoxy resin, polyurethane resin, polyvinyl butyral resin and the like. Among these, linear or non-linear polyester resins are preferable. Polyester resins are excellent in achieving both mechanical strength (difficult to generate fine powder due to wear, etc.), fixability (hard to peel off from paper after fixing), and hot offset resistance.

  The polyester resin is obtained by polymerizing a monomer composition composed of a dihydric or higher polyhydric alcohol and a polybasic acid and, if necessary, a trihydric or higher polyhydric alcohol or polybasic acid. Examples of the divalent alcohol used for polymerization of the polyester resin include ethylene glycol, diethylene glycol, triethylene glycol, 1,2-propylene glycol, 1,3-propylene glycol, 1,4-butanediol, neopentyl glycol, 1 Diols such as 1,4-butenediol, 1,5-pentanediol, 1,6-hexanediol, bisphenol A alkylene such as bisphenol A, hydrogenated bisphenol A, polyoxyethylenated bisphenol A, polyoxypropylenated bisphenol A And oxide adducts.

  Examples of the trihydric or higher polyhydric alcohol include sorbitol, 1,2,3,6-hexanetetrol, 1,4-sorbitan, pentaerythritol, dipentaerythritol, tripentaerythritol, sucrose, 1,2,4- Butanetriol, 1,2,5-pentanetriol, glycerol, 2-methylpropanetriol, 2-methyl-1,2,4-butanetriol, trimethylolethane, trimethylolpropane, 1,3,5-trihydroxymethyl Benzene is mentioned.

  Examples of the divalent polybasic acid include maleic acid, fumaric acid, citraconic acid, itaconic acid, glutaconic acid, phthalic acid, isophthalic acid, terephthalic acid, cyclohexanedicarboxylic acid, succinic acid, adipic acid, sebacic acid, azelaic acid, Examples include malonic acid, anhydrides of these acids, lower alkyl esters, or alkenyl succinic acids or alkyl succinic acids such as n-dodecenyl succinic acid and n-dodecyl succinic acid.

  Examples of the tribasic or higher polybasic acid include 1,2,4-benzenetricarboxylic acid, 1,2,5-benzenetricarboxylic acid, 1,2,4-cyclohexanetricarboxylic acid, and 2,5,7-naphthalenetricarboxylic acid. 1,2,4-naphthalenetricarboxylic acid, 1,2,5-hexanetricarboxylic acid, 1,3-dicarboxyl-2-methyl-2-methylenecarboxypropane, tetra (methylenecarboxyl) methane, 1,2,7 , 8-octanetetracarboxylic acid, and anhydrides thereof.

(Coloring agent)
As the colorant, pigments and dyes commonly used as toner colorants can be used. Examples of the colorant for black toner include carbon black and magnetite. Examples of the colorant for yellow toner include C.I. I. Acetoacetic acid arylamide monoazo yellow pigments such as C.I. Pigment Yellow 1, 3, 74, 97 and 98; I. Pigment Yellow 12, 13, 13, 14 and the like, acetoacetic acid arylamide disazo yellow pigments, C.I. I. Condensed monoazo yellow pigments such as CI Pigment Yellow 93 and 155; I. Other yellow pigments such as C.I. Pigment Yellow 180, 150 and 185, C.I. I. Solvent Yellow 19, 77, 79, C.I. I. And yellow dyes such as Disperse Yellow 164.

Examples of the colorant for magenta toner include C.I. I. Pigment Red 48, 49: 1, 53: 1, 57, 57: 1, 81, 122, 5, 146, 184, 238, C.I. I. Red or red pigments such as CI Pigment Violet 19; I. And red dyes such as Solvent Red 49, 52, 58 and 8.
Examples of the colorant for cyan toner include C.I. I. A blue dye / pigment of copper phthalocyanine and its derivatives such as CI Pigment Blue 15: 3 and 15: 4; I. And green pigments such as CI Pigment Green 7 and 36 (phthalocyanine green). The addition amount of the colorant is preferably 1 part by weight or more and 15 parts by weight or less, and more preferably 2 parts by weight or more and 10 parts by weight or less with respect to 100 parts by weight of the binder resin.

  The colored resin particles are formed of at least the boron compound, binder resin and colorant described above, and the volume average particle diameter is preferably 3 μm or more and 15 μm or less, and it is excellent in dot reproducibility that it is 5 μm or more and 7 μm or less. More preferable in terms. The volume average particle diameter was measured with a Coulter counter manufactured by Coulter, Inc., using a 100 μm aperture.

The colored resin particles preferably have a BET specific surface area of 1.5 m ² / g or more and 1.9 m ² / g or less. When the BET specific surface area exceeds 1.9 m ² / g, the colored resin particle surface becomes uneven, and external additives added to the toner surface, which will be described later, enter the concave portions, and the external additive is evenly distributed on the toner surface. It becomes difficult to adhere to. As a result, the roller effect (effect of improving toner fluidity) and the spacer effect (effect of preventing charge leakage) of the external additive cannot be obtained sufficiently, and fog and toner scattering are likely to occur. On the other hand, when the BET specific surface area is less than 1.5 m ² / g, the surface of the colored resin particles becomes too smooth, so that a cleaning failure occurs and fog is likely to occur. Colored resin particles having a desired BET specific surface area can be obtained by forming particles with no corners. As the method, for example, a method of rotating at high speed with colored resin particles in a cylindrical pipe, a sfufusion system that instantaneously melts toner in a hot air stream, and the like can be mentioned. The BET specific surface area was measured by a three-point measurement method using a BET specific surface area measuring device Gemini 2360 (manufactured by Shimadzu Corporation).

  The colored resin particles can be produced by a known method such as a kneading pulverization method or a polymerization method. When producing colored resin particles, an additive such as a release agent may be added in addition to the boron compound, binder resin and colorant described above. By adding the release agent, it is possible to improve the releasability of the toner with respect to the fixing roller or the fixing belt, and it is possible to prevent high temperature / low temperature offset during fixing. The addition amount of the release agent is not particularly limited, but is 1 part by weight or more and 5 parts by weight or less with respect to 100 parts by weight of the binder resin. Release agents include synthetic waxes such as polypropylene and polyethylene, paraffin waxes and derivatives thereof, petroleum waxes such as microcrystalline wax and derivatives thereof, and modified waxes thereof, carnauba wax, rice wax, candelilla wax, and other plant systems. Wax etc. are mentioned.

  When producing colored resin particles by a kneading and pulverizing method, first, a boron compound, a binder resin, a colorant, a release agent and other additives are mixed from a Henschel mixer, a super mixer, a mechano mill, a Q-type mixer, etc. Mix by machine. Next, the obtained mixture is melt-kneaded at a temperature of about 70 ° C. or higher and 180 ° C. or lower by a kneader selected from a twin-screw kneader, a single-screw kneader, and the like. Thereafter, the obtained kneaded product is cooled and solidified, and the solidified product is pulverized by an air pulverizer such as a jet mill, and particle size adjustment such as classification is performed as necessary to produce colored resin particles.

(External additive)
The toner of the present invention includes an external additive. The external additive is externally added to the surface of the colored resin particles, imparts fluidity to the toner, and improves toner transportability. The volume resistivity of the external additive is adjusted to 1 × 10 ¹² Ω · cm or more and 5 × 10 ¹⁵ Ω · cm or less. The volume resistivity of the external additive can be adjusted, for example, depending on the type of the substrate constituting the external additive. Examples of the substrate constituting the external additive include inorganic materials such as silica, alumina, and titanium oxide. Moreover, although mentioned later for details, it can also adjust by changing the kind and processing amount of the surface treating agent which process the surface of an external additive.

As described above, since the volume resistivity of the external additive is adjusted to be 1 × 10 ¹² Ω · cm or more, the positive charge held by the external additive is negatively charged contained in the colored resin particles. Leakage into the boron compound can be prevented, and the external additive can retain a sufficient amount of charge. For this reason, the toner can maintain a fast charge rising property, and even when a large number of images with high coverage that consume a large amount of toner are continuously printed, the occurrence of fog and the decrease in image density are prevented. be able to. Further, since the volume resistivity of the external additive is adjusted to be 5 × 10 ¹⁵ Ω · cm or less, the external additive can be produced relatively easily.

  The external additive preferably has a primary particle size of 5 nm or more and 15 nm or less. An external additive having a small primary particle size of 5 nm or more and 15 nm or less can improve the fluidity of the toner when added in a small amount and improves the charge rising property of the toner and is replenished into the developing tank. Toner can be charged quickly. The external additive is preferably fine particles made of silica whose surface has been subjected to a hydrophobic treatment. Thus, by hydrophobizing the surface of the external additive, it is possible to prevent the external additive from absorbing moisture and to prevent the toner charge amount from becoming unstable. In addition, the surface of the external additive can be hydrophobized to prevent the negative charge held by the boron compound contained in the colored resin particles from leaking to the positively charged external additive. The agent can retain a positive charge over a long period of time. Therefore, when the external additive holding a positive charge comes into contact with the carrier, the positive charge can be quickly transferred to the carrier, and the quick charge rising property of the toner can be maintained.

  The volume resistivity of the external additive can be adjusted by changing the type and amount of the surface treatment agent to be used. Examples of the hydrophobizing surface treatment agent include silane coupling agents, titanium coupling agents, and silicone oils. Among these, it is preferable to use hexamethyldisilazane, which is a silane coupling agent having excellent hydrophobicity and preventing the toner charge amount from becoming unstable, as a surface treatment agent. The amount of the external additive added is preferably 0.5 to 5 parts by weight with respect to 100 parts by weight of the colored resin particles. If the addition amount is too small, it is difficult to obtain the effect of the external additive, and conversely if the addition amount is too large, the toner fixability is lowered. Further, the external additive is preferably added so that the coverage of the colored resin particle surface is 20% or more and 80% or less. The external additive can be externally added to the colored resin particles using an air flow mixer such as a Henschel mixer.

The primary particle diameter of the external additive was measured by calculating the number average value measured using a scanning electron microscope. Further, the volume resistivity of the external additive was measured by a compression method performed in the following procedure. First, an external additive left for 24 hours in an environment of temperature 20 ° C. and humidity 65% is sandwiched between ^two copper plate electrodes and pressed at a pressure of 100 kg / cm ² , and the distance between the copper plate electrodes is 8 mm or more and 10 mm or less. A sample was prepared. Next, a voltage with an electric field strength of 500 V / cm was applied to the sample, and the resistance 15 seconds after the voltage was applied was measured to calculate the volume resistivity of the external additive.

[Career]
The carrier is a material having magnetism. As the saturation magnetization of the carrier, the lower the saturation magnetization, the softer the magnetic brush in contact with the photoreceptor, so that an image faithful to the electrostatic latent image can be obtained. However, if the saturation magnetization is too low, the carrier adheres to the surface of the photoreceptor. However, white spots are likely to occur in the obtained image, and if the saturation magnetization is too high, it becomes difficult to obtain an image faithful to the electrostatic latent image due to the stiffening of the magnetic brush. Therefore, it is preferable to adjust the saturation magnetization of the carrier to a range of 30 emu / g or more and 100 emu / g or less.

The carrier contained in the two-component developer of the present invention is adjusted so that the volume resistivity is 2 × 10 ¹⁰ Ω · cm or more and 5 × 10 ¹² Ω · cm or less. When continuously printing images with low toner consumption and low toner coverage and low coverage, the time during which the toner is stirred in the developing tank becomes long, and the external additive may be buried in the colored resin particles. When the external additive is buried in the colored resin particles as described above, the carrier and the boron compound included in the colored resin particles are likely to come into contact with each other, and the charge held by the boron compound may leak to the carrier. In the present invention, the carrier of the two-component developer is adjusted so that its volume resistivity is 2 × 10 ¹⁰ Ω · cm or more. Therefore, even when the carrier and the boron compound are in contact with each other, The retained charge can be prevented from leaking to the carrier, and the toner charge amount can be prevented from decreasing. Therefore, even when images with low coverage are continuously printed, it is possible to prevent the occurrence of fogging and the decrease in image density. In addition, since the volume resistivity of the carrier is adjusted to be 5 × 10 ¹² Ω · cm or less, it is possible to prevent the effect of preventing the accumulation of positive charges of the carrier (charging leakage property) from being reduced, and When developing the electrostatic latent image carried on the body, it is possible to prevent carriers remaining after development from accumulating positive charges.

  The volume resistivity of the carrier can be adjusted, for example, depending on the type of core particles described later, which is a substrate constituting the carrier. Moreover, although mentioned later for details, it can also adjust with the kind and addition amount of the electrically conductive agent added in the coating layer which a carrier has.

  The volume resistivity of the carrier was measured by a bridge method performed by the following procedure. First, in an environment of temperature 20 ° C. and humidity 65%, 0.2 g of carrier was filled between two copper plate electrodes 30 mm wide and 10 mm high installed with a gap of 6.5 mm. Next, a voltage of 500 V is applied in a state where a bridge formed by carriers is formed by magnetic lines of force of two magnets (100 mT) arranged outside each copper plate electrode so that the N pole and the S pole face each other. The volume resistivity of the carrier was calculated by measuring the resistance 15 seconds after application of.

  Moreover, it is preferable that the volume average particle diameter of a carrier is 30 micrometers or more and 100 micrometers or less. When the volume average particle diameter of the carrier is too small, the carrier easily moves from the developing roller to the photoreceptor during development, and white spots occur in the obtained image. If the volume average particle diameter of the carrier is too large, the dot reproducibility is deteriorated and the obtained image becomes rough. The volume average particle size of the carrier was measured using a dry dispersion device RODOS (manufactured by SYMPATEC) in a laser diffraction particle size distribution measuring device HELOS (manufactured by SYMPATEC) under a dispersion pressure of 3.0 bar. .

  The carrier contained in the two-component developer of the present invention includes magnetic core particles and a coating layer that covers the core particles.

(Core particles)
As the core particles, known magnetic particles can be used, but ferrite particles are preferable in terms of chargeability and durability. Known ferrite particles can be used, such as zinc ferrite, nickel ferrite, copper ferrite, nickel-zinc ferrite, manganese-magnesium ferrite, copper-magnesium ferrite, manganese-zinc ferrite, manganese. -Copper-zinc ferrite etc. are mentioned. These ferrite particles can be produced by a known method. For example, first, ferrite raw materials such as iron oxide (Fe ₂ O ₃ ) and magnesium hydroxide (Mg (OH) ₂ ) are mixed, and this mixed powder is heated in a heating furnace and calcined. Next, after cooling the obtained calcined product, it is pulverized by a vibration mill so as to become particles of about 1 μm, and a dispersant and water are added to the pulverized powder to prepare a slurry. Thereafter, the obtained slurry is wet pulverized with a wet ball mill, and the resulting suspension is granulated and dried with a spray dryer, whereby ferrite particles can be produced. The volume resistivity of the ferrite particles is preferably adjusted to 1 × 10 ⁸ Ω · cm or more and 5 × 10 ¹⁰ Ω · cm or less. A carrier having ferrite particles whose volume resistivity is adjusted to the above range is a carrier excellent in electrical insulation and the ability to remove counter charges remaining on the carrier surface. Therefore, it is possible to prevent fogging, the edge effect at the periphery of the solid image, and the decrease in image density.

(Coating layer)
As the material constituting the coating layer, a known resin material can be used, and examples thereof include an acrylic resin and a silicone resin. Among these, a silicone resin is preferable. The reason is that the carrier having a coating layer made of a silicone resin is hard to adhere a boron compound to the surface thereof, and can maintain the toner's charge imparting ability over a long period of time.

  Known silicone resins can be used, such as silicone varnishes (trade names: TSR115, TSR114, TSR102, TSR103, YR3061, TSR110, TSR116, TSR117, TSR108, TSR109, TSR180, TSR181, TSR187, TSR144, TSR165, etc. All manufactured by Shin-Etsu Chemical Co., Ltd., KR271, KR272, KR275, KR280, KR282, KR267, KR269, KR211, KR212, etc., all manufactured by Toshiba Corporation, alkyd modified silicone varnishes (trade names: TSR184, TSR185, etc.) Manufactured by Toshiba), epoxy-modified silicone varnish (trade names: TSR194, YS54, etc., all manufactured by Toshiba Corporation), polyester-modified Ricone varnish (trade name: TSR187, manufactured by Toshiba Corporation), acrylic modified silicone varnish (trade name: TSR170, TSR171, etc., all manufactured by Toshiba Corporation), urethane modified silicone varnish (trade name: TSR175, etc., Toshiba Corporation) And a reactive silicone resin (trade names: KA1008, KBE1003, KBC1003, KBM303, KBM403, KBM503, KBM602, KBM603, etc., all manufactured by Shin-Etsu Chemical Co., Ltd.).

  A conductive agent is added to the coating layer in order to adjust the volume resistivity of the carrier. Examples of the conductive agent include silicon oxide, alumina, carbon black, graphite, zinc oxide, titanium black, iron oxide, titanium oxide, tin oxide, potassium titanate, calcium titanate, aluminum borate, magnesium oxide, barium sulfate, Examples include calcium carbonate. Among these, carbon black is preferable from the viewpoints of production stability, cost, and low electric resistance. The type of carbon black is not particularly limited, but those having a DBP (dibutyl phthalate) oil absorption in the range of 90 ml / 100 g or more and 170 ml / 100 g or less are preferable from the viewpoint of excellent production stability. In addition, those having a primary particle diameter of 50 nm or less are particularly preferable because of excellent dispersibility. A conductive agent can be used individually by 1 type, and can also be used in combination of 2 or more type. The addition amount of the conductive agent to the coating layer is 0.1 part by weight or more and 20 parts by weight or less with respect to 100 parts by weight of the resin material constituting the coating layer.

  As a method for forming the coating layer on the core particles, a known method can be adopted. For example, first, a resin material and a conductive agent constituting the coating layer are dissolved in an organic solvent to prepare a coating resin solution. Next, a coating layer is formed on the surface of the core particles using the coating resin solution. The method includes an immersion method in which the core particles are immersed in the coating resin solution, a spray method in which the coating resin solution is sprayed onto the core particles, a fluidized bed method in which the coating resin solution is sprayed onto the core particles suspended by flowing air, Examples thereof include a kneader coater method in which core particles and a coating resin solution are mixed in a kneader coater. A specific method for forming a coating layer on the surface of the core particle by the dipping method will be described below. In the following description, ferrite particles are used as the core particles, and thermosetting silicone resin is used as the resin material constituting the coating layer. First, a coating resin solution prepared by dissolving a thermosetting silicone resin in toluene and ferrite particles are put in a container of a stirrer at a predetermined ratio and stirred for a predetermined time. At this time, a coating film is formed on the surface of the ferrite particles. Next, the ferrite particles on which the coating film is formed are loaded into a fixed heating device and heated to about 240 ° C. At this time, the silicone resin is thermoset on the surface of the ferrite particles to form a coating layer.

  The thickness of the coating layer is preferably adjusted to 0.5 μm or more and 5 μm or less. If the thickness of the coating layer is too thin, the charge on the surface of the photoreceptor moves to the carrier, and brush streaks occur in the resulting image. On the other hand, if the coating layer is too thick, the counter charge remaining on the carrier cannot be quickly lost after the toner is developed, resulting in an edged image. The thickness of the coating layer can be measured by observing a thin slice of the carrier with a transmission electron microscope. When the coating layer is formed, the particle diameter specific gravity of the core particles and the coating resin It can be calculated based on the application amount and specific gravity of the solution.

  The two-component developer of the present invention can be produced by mixing the above-described toner and carrier with a mixer such as a Nauta mixer. The toner and the carrier are mixed so that the amount of the toner is 3 to 15 parts by weight with respect to 100 parts by weight of the carrier.

  FIG. 1 is a diagram illustrating a configuration of an image forming apparatus 31 according to an embodiment of the present invention. The image forming apparatus 31 is a digital copying machine, and can select a copy mode and a print mode. In the copy mode, a copy of the document is generated according to the image information of the document read by the scanner unit 29 described later. In the print mode, an image corresponding to the image information from an external device connected to the image forming apparatus 31 via a network can be printed.

  In the image forming apparatus 31, the developer for forming an image is the above-described two-component developer. In the image forming apparatus 31, an image is formed by using a two-component developer in which the charge amount of the toner is suppressed without being affected by the coverage of the image to be printed. Even when a high image is continuously printed, it is possible to prevent the toner charge amount from being lowered and to prevent the occurrence of fogging and the image density from being lowered.

  The image forming apparatus 31 includes a photoconductor 20, a charging unit 21, an exposure unit 22, a developing unit 1, a transfer unit 23, a fixing unit 25, a cleaning unit 24, a paper feed tray 28, and a scanner unit 29. And a paper discharge tray 30.

  The photoconductor 20 is a roller-like member that is supported by a driving unit (not shown) so as to be rotatable about an axis, and that has a photosensitive film on the surface of which an electrostatic latent image and thus a toner image is formed. For the photoreceptor 20, for example, a roller-shaped member including a conductive base (not shown) and a photosensitive film (not shown) formed on the surface of the conductive base can be used. As the conductive substrate, a conductive substrate having a cylindrical shape, a columnar shape, a sheet shape or the like can be used, and among them, the cylindrical conductive substrate is preferable. Examples of the photosensitive film include an organic photosensitive film and an inorganic photosensitive film. As an organic photosensitive film, a layered photoreceptor including a charge generation layer that is a resin layer containing a charge generation material and a charge transport layer that is a resin layer containing a charge transport material, and the charge generation material and the charge transport in one resin layer And a single layer photoreceptor containing the substance. Examples of the inorganic photosensitive film include films containing one or more selected from zinc oxide, selenium, amorphous silicon and the like. A base film may be interposed between the conductive substrate and the photosensitive film, and a surface film (protective film) for mainly protecting the photosensitive film may be provided on the surface of the photosensitive film.

  The charging means 21 is, for example, a saw-tooth type charger that performs corona discharge on the photoconductor 20. A power source (not shown) is connected to the charging unit 21 to apply a voltage to the charging unit 21. The charging unit 21 receives a voltage from a power source and performs corona discharge to charge the surface of the photoconductor 20 to a predetermined polarity and potential. The charging means 21 is not limited to this, and a charger such as a charging brush type charger, a roller charger, an ion generator, or a magnetic brush can be used.

  The exposure means 22 is, for example, a laser scanning device including a light source. The laser scanning device is a device that combines a light source, a polygon mirror, an fθ lens, a reflection mirror, and the like. As the light source, for example, a semiconductor laser, an LED array, an electroluminescence (EL) element, or the like can be used. The exposure unit 22 receives image information of a document read by a scanner unit 29 to be described later or image information from an external device, and irradiates the surface of the charged photoreceptor 20 with signal light corresponding to the image information. As a result, an electrostatic latent image corresponding to the image information is formed on the surface of the photoreceptor 20.

  The developing unit 1 supplies the two-component developer of the present invention to the electrostatic latent image carried on the photoreceptor 20 to form a visible image. FIG. 2 is a cross-sectional view showing the configuration of the developing unit 1. The developing unit 1 includes a developing tank 2, a developing roller 3, a first stirring member 4, a second stirring member 5, a conveying member 6, a regulating member 7, a regulating member support 8, and a sink plate 9. The magnetic member 10, the magnetic member support 11, and the toner concentration detection sensor 12 are included. The developing tank 2 is a substantially prismatic container member having an internal space, and rotatably supports the developing roller 3, the first stirring member 4, the second stirring member 5, and the conveying member 6, a regulating member 7, and a sink plate 9 or the like is supported directly or indirectly, and the two-component developer of the present invention is accommodated. Further, the developing tank 2 has an opening 2a formed on a side surface facing the photoconductor 20, and a developer is supplied from the opening 2a toward an electrostatic latent image formed on the surface of the photoconductor 20. A toner replenishing port 2 b that is an opening for replenishing toner is formed on the upper surface of the developing tank 2.

  A toner cartridge and a toner hopper (not shown) are provided above the developing tank 2 in the vertical direction. More specifically, the toner cartridge, the toner hopper, and the developing tank 2 are provided in this order from the top to the bottom in the vertical direction. The toner cartridge accommodates toner in its internal space and is detachably attached to the wall surface of the image forming apparatus 31. The toner accommodated in the toner cartridge is dropped toward the toner hopper from the opening formed in the toner cartridge when the toner cartridge is rotated around the axis by driving means (not shown), and is supplied to the toner hopper. The toner hopper is provided with a toner discharge port, which is an opening through which toner is discharged from the toner hopper toward the developing tank 2, so as to communicate with the toner supply port 2 b formed in the developing tank 2 in the vertical direction. Further, the toner hopper is provided with a toner supply roller 19 vertically above the toner discharge port. The toner supply roller 19 is rotatably supported by a toner hopper and is driven to rotate by a driving unit (not shown). The rotational driving of the toner replenishing roller 19 is controlled by a control means (not shown) provided in the image forming apparatus in accordance with the detection result of the toner concentration in the developing tank 2 by the toner concentration detection sensor 12. As the toner replenishing roller 19 is driven to rotate, toner is replenished from the toner hopper into the developing tank 2 through the toner replenishing port 2b.

  The developing roller 3 is a roller-like member that is at least partially supported by the developing tank 2 so as to be rotatable and is driven to rotate about an axis by a driving unit (not shown). The developing roller 3 is disposed so as to face the photoconductor 20 through the opening 2 a of the developing tank 2. The developing roller 3 is provided so as to be separated from the photoreceptor 20 with a gap, and the closest portion becomes a developing nip portion. In the developing nip portion, toner is supplied to the electrostatic latent image on the surface of the photoreceptor 20 from a developer layer (not shown) on the surface of the developing roller 3. In the developing nip portion, a developing bias voltage is applied to the developing roller 3 from a power source (not shown) connected to the developing roller 3, and the toner from the developer layer on the surface of the developing roller 3 to the electrostatic latent image on the surface of the photoreceptor 20. The transition proceeds smoothly. The developing roller 3 includes a magnet roller 13 and a sleeve 14. The magnet roller 13 is supported by the developing tank wall of the developing tank 2 at both ends in the longitudinal direction, and magnetic poles N1, N2, and N3 that are a plurality of bar magnets having a rectangular cross-sectional shape at the circumferential position of the developing roller 3. S1, S2, S3, and S4 are multi-pole magnetized magnet rollers that are spaced apart from each other and arranged radially in the radial direction of the developing roller 3. The magnetic poles are arranged in the order of magnetic poles N1, S1, N2, S2, N3, N4, and S3 in the direction opposite to the rotation direction of the developing roller 3 (sleeve 14).

  In the magnet roller 13, the magnetic pole N3 is preferably provided in a specific angle range on the upstream side in the rotation direction of the developing roller 3 from the magnetic pole S2 (hereinafter, simply referred to as “upstream side” unless otherwise specified). Here, the angle range includes the radius in the cross section of the developing roller 3 where the magnetic pole S2 is disposed (hereinafter referred to as “radius S2”) and the radius in the cross section of the developing roller 3 where the magnetic pole N3 is disposed (hereinafter referred to as “ The radius N3 "). The angle range is 33 ° or more, and an angle formed by a radius S2 and a straight line connecting an axis of the developing roller 3 and an axis of a first stirring member 4 described later (hereinafter referred to as “first stirring member 4”). It is preferable that the angle is set as follows. If the angle range is less than 33 °, image defects such as uneven image density tend to occur. If the installation angle of the first stirring member 4 is exceeded, the extension line of the radius N3 extends below the axis of the first stirring member 4 in the vertical direction. As a result, the ability of the first stirring member 4 to pump the developer to the developing roller 3 is reduced, and image defects such as a reduction in image density and uneven image density are likely to occur.

  In the present embodiment, the arrangement of the magnetic poles in the magnet roller 13 is as follows, and FIG. 3 is a diagram showing the magnetic force distribution of the magnetic poles in the magnet roller 13 included in the developing roller 3. The magnetic pole N1 (peak value = 105.8 mT) is provided on a straight line connecting the axis of the developing roller 3 and the axis of the photoconductor 20 at a position facing the photoconductor 20. The magnetic pole S1 (peak value = −87.0 mT) is provided at a position of 50.55 ° upstream from the magnetic pole N1. This angle is an angle formed by the radius S1 in the cross section of the developing roller 3 where the magnetic pole S1 is disposed and the radius N1 in the cross section of the developing roller 3 where the magnetic pole N1 is disposed. The same shall apply hereinafter. The magnetic pole N2 (peak value = 39.3 mT) is provided at a position 111.33 ° upstream from the magnetic pole N1. The magnetic pole S2 (peak value = −46.9 mT) is provided at a position 153.73 ° upstream from the magnetic pole N1. The magnetic pole N3 (peak value = 52.8 mT) is provided at a position 199.40 ° upstream from the magnetic pole N1 and 45.67 ° upstream from the magnetic pole S2. The magnetic pole N4 (peak value = 46.8 mT) is provided at a position 272.40 ° upstream from the magnetic pole N1. The magnetic pole S3 (peak value = −83.2 mT) is provided at a position 314.80 ° upstream from the magnetic pole N1.

  The sleeve 14 is a cylindrical member that is externally fitted to the magnet roller 13, is rotatably supported by the developing tank 2 and a support member (not shown), and is rotatably provided by a drive unit (not shown). The sleeve 14 is formed using a nonmagnetic material. In the present embodiment, the sleeve 14 rotates counterclockwise, and the photoconductor 20 rotates clockwise. Accordingly, the sleeve 14 and the photoconductor 20 rotate in the opposite directions at the developing nip portion.

  Each of the first stirring member 4 and the second stirring member 5 is a roller-like member that is rotatably supported by the developing tank 2 and that can be driven to rotate around an axis by a driving unit (not shown). In the present embodiment, the first stirring member 4 rotates counterclockwise, and the second stirring member 4 rotates clockwise. The first stirring member 4 is provided at a position facing the photoconductor 20 via the developing roller 3 and being vertically lower than the developing roller 3. In the present embodiment, the installation angle of the first stirring member 4 that is an angle formed by the radius S2 and a straight line connecting the axis of the developing roller 3 and the axis of the first stirring member 4 is 54 °. . The second agitating member 5 is provided at a position facing the developing roller 3 with the first agitating member 4 interposed therebetween and lower than the developing roller 3 in the vertical direction. The first agitating member 4 and the second agitating member 5 agitate the developer stored in the developing tank 2 to give a uniform charge to the toner, and pump up the charged developer to Send to the surroundings.

  The conveyance member 6 is a roller-like member that is rotatably supported by the developing tank 2 and is rotatably driven by a driving unit (not shown). The conveying member 6 faces the first stirring member 4 through the second stirring member 5 and is provided below the toner supply port 2b in the vertical direction. The conveying member 6 conveys the toner replenished into the developing tank 2 through the toner replenishing port 2 b around the second stirring member 5.

  The regulating member 7 is a plate-like member extending parallel to the axial direction of the developing roller 3, and one end in the short direction is supported by the developing tank 2 and the regulating member support 8 above the developing roller 3 in the vertical direction. And the other end is provided so as to be separated from the surface of the developing roller 3 with a gap. In the present embodiment, the regulating member 7 is provided on the radial direction of the developing roller 3 (extension line of the radius of the developing roller 3), and the radius N1 in the cross section of the developing roller 3 where the extension line and the magnetic pole N1 are disposed. The angle formed by and is set to be 90 °. The regulating member 7 is made of, for example, a nonmagnetic metal having elasticity such as stainless steel or aluminum, a synthetic resin, or the like. In the present embodiment, a thin plate-like stainless steel is used for the regulating member 7. The regulating member support 8 supports the regulating member 7 together with the developing tank 2. Specifically, one end of the regulating member 7 in the short direction and the vicinity thereof are supported so as to be sandwiched between the regulating member support 8 and the developing tank 2. The restricting member support 8 is formed of a material such as synthetic resin or metal. In the present embodiment, the regulating member support 8 is formed from a synthetic resin. The regulating member 7 adjusts the transport amount of the developer by removing excess developer from the developer layer carried on the surface of the developing roller 3 and regulating the thickness of the developer layer to be constant. Further, the regulating member 7 imparts electric charge to the developer that is insufficiently charged in the developer layer by rubbing between the other end in the short side direction and the developer layer, and the developer contained in the developer layer. Is sufficiently charged.

  The flow plate 9 is a plate-like member provided in the developing tank 2 upstream of the regulating member 7 in the rotation direction of the developing roller 3 and above the first stirring member 4 and the second stirring member 5. It is. The sink plate 9 has one end in the short direction facing the surface of the developing roller 3 and is separated with a gap, and the other end extends in a direction away from the developing roller 3. In the present embodiment, the upper surface of the flow plate 9 is parallel to the horizontal direction at the end of the flow plate 9 on the developing roller 3 side in the short direction and in the vicinity thereof, and at the other portions, the developing roller 3 is parallel to the horizontal direction. It is provided so as to descend downward in the vertical direction as it moves away. The sink plate 9 is supported by a support member 9a that is formed so as to penetrate in the longitudinal direction of the sink plate 9 in the lower part in the vertical direction and is inserted into the through hole. By providing the flow plate 9, a smooth flow of the developer in the developing tank 2 is generated, and the occurrence of uneven charging of the toner, blocking of the toner and the like is prevented. Specifically, the developer removed from the surface of the developing roller 3 by the regulating member 7 temporarily stays in the space above the developing roller 3, but when the amount increases, the developer flows upward in the direction away from the developing roller 3. Begin to have. The developer flows along the upper surface of the sink plate 9, and falls toward the second stirring member 5 from the end of the sink plate 9 opposite to the developing roller 3 side in the short direction. The dropped developer is uniformly mixed with the other developer and newly supplied toner by the first stirring member 4 and the second stirring member 5, and is conveyed toward the developing roller 3.

  The dimensions of the first agitating member 4, the second agitating member 5, the conveying member 6, the regulating member 7, the flow plate 9 and the magnetic member 10 are appropriately determined from an appropriate range according to the dimensions of the developing roller 3. . For example, the toner concentration detection sensor 12 is mounted on the bottom surface of the developing tank 2 below the second stirring member 5 in the vertical direction, and is provided so that the sensor surface is exposed to the inside of the developing tank 2. The toner concentration detection sensor is electrically connected to control means (not shown). The control means controls to rotate the toner cartridge according to the detection result by the toner concentration detection sensor 12 and replenish the toner into the developing tank 2 through the toner hopper. That is, when it is determined that the detection result by the toner density detection sensor 12 is lower than the toner density setting value, a control signal is sent to the driving means for driving the toner cartridge to rotate, and the toner cartridge is driven to rotate. As the toner concentration detection sensor 12, a general toner concentration detection sensor can be used, and examples thereof include a transmitted light detection sensor, a reflected light detection sensor, and a magnetic permeability detection sensor. Among these, a magnetic permeability detection sensor is preferable.

  When a magnetic permeability detection sensor is used as the toner concentration detection sensor 12, a power source (not shown) is connected to the toner concentration detection sensor 12. The power supply applies to the toner concentration detection sensor 12 a drive voltage for driving the toner concentration detection sensor 12 and a control voltage for outputting a detection result of the toner concentration to the control means. The application of voltage to the toner concentration detection sensor 12 by the power source is controlled by the control means. The toner concentration detection sensor 12 is a type of sensor that receives the application of a control voltage and outputs the detection result of the toner concentration as an output voltage value. Basically, the sensitivity near the median value of the output voltage is good. A control voltage is applied so that an output voltage can be obtained. A magnetic permeability detection sensor that is such a type of toner concentration detection sensor 12 is commercially available, and examples thereof include TS-L, TS-A, and TS-K (all are trade names, manufactured by TDK Corporation). .

  According to the developing unit 1, the developer stored in the developing tank 2 is transported vertically above the first stirring member 4 by the rotation of the first stirring member 4 and the second stirring member 5, where the developer is stored by the magnetic member 10. It is lifted and supplied to the surface of the developing roller 3. The developing roller 3 carries a developer layer on its surface and rotates. After the developer layer is regulated by the regulating member 7 and the developer is charged, the developing roller 3 is electrostatically charged on the photoreceptor 20 at the development nip. The latent image is developed with toner. After the development is completed, the developing roller 3 further rotates and receives the developer supply again. On the other hand, the developer removed from the surface of the developing roller 3 by the regulating member 7 flows in a direction away from the developing roller 3 on the upper surface of the flow plate 9 and is returned between the second stirring member 5 and the conveying member 6. Then, it is mixed again with another developer and conveyed toward the developing roller 3. In the developing tank 2, the developer circulation as described above occurs. Further, the transport member 6 transports the toner replenished into the developing tank 2 around the second stirring member 5 according to the detection result by the toner concentration detection sensor 12.

  The transfer unit 23 transfers the toner image on the surface of the photoreceptor 20 to a recording medium. The transfer unit 23 is a roller-like member that is rotatably supported by a support member (not shown), is rotatably provided by a drive unit (not shown), and is in pressure contact with the photoreceptor 20. For the transfer means 23, for example, a roller-shaped member including a metal core having a diameter of 8 mm to 10 mm and a conductive elastic layer formed on the surface of the metal core is used. As the metal forming the metal core, stainless steel, aluminum, or the like can be used. As the conductive elastic layer, a rubber material in which a conductive agent such as carbon black is mixed with a rubber material such as ethylene-propylene rubber (EPDM), foamed EPDM, or foamed urethane can be used. In synchronism with the toner image being conveyed by the rotation of the photoconductor 20 to the pressure contact portion (transfer nip portion) between the photoconductor 20 and the transfer means 23, the paper feed tray 28 passes through a pickup roller and a registration roller (not shown). Recording media are supplied one by one. As the recording medium passes through the transfer nip portion, the toner image on the surface of the photoreceptor 20 is transferred to the recording medium. A power source (not shown) is connected to the transfer means 23, and when transferring the toner image to the recording medium, a voltage having a polarity opposite to the charging polarity of the toner constituting the toner image is applied to the transfer means 23. As a result, the toner image is smoothly transferred to the recording medium.

  The fixing unit 25 fixes the toner image on the recording medium by passing the recording medium onto which the toner image has been transferred through the fixing nip, melting the toner constituting the toner image, and pressing the toner on the recording medium. The fixing unit 25 includes a fixing roller 26 and a pressure roller 27. The fixing roller 26 is a roller-like member that is rotatably supported by a support member (not shown) and is rotatable around an axis line by a drive unit (not shown). The fixing roller 26 has a heating member (not shown) inside, and heats the toner constituting the unfixed toner image carried on the recording medium conveyed from the transfer nip portion to melt the toner and fix it on the recording medium. Let As the fixing roller 26, for example, a roller-shaped member including a cored bar and an elastic layer is used. The core metal is formed of a metal such as iron, stainless steel, or aluminum. The elastic layer is formed of an elastic material such as silicone rubber or fluorine rubber, for example. The heating member receives heat from a power source (not shown) and generates heat. A halogen lamp, an infrared lamp, etc. can be used for a heating member.

  The pressure roller 27 is a roller-like member that is rotatably supported and is provided in pressure contact with the fixing roller 26 by a pressure member (not shown). The pressure roller 27 rotates following the rotation of the fixing roller 26. A pressure contact portion between the fixing roller 26 and the pressure roller 27 is a fixing nip portion. The pressure roller 27 promotes fixing of the toner image to the recording medium by pressing the toner in a molten state against the recording medium when the fixing roller 26 heats and fixes the toner image to the recording medium. A roller-like member having the same configuration as that of the fixing roller 26 can be used for the pressure roller 27. A heating member may also be provided inside the pressure roller 27. As the heating member, the same heating member as that in the fixing roller 26 can be used. The recording medium on which the toner image is fixed is discharged to a discharge tray 30 (to be described later) by a conveying unit (not shown) and stacked on the discharge tray 30.

  The cleaning unit 24 cleans the surface of the photoconductor 20 after the toner image is transferred. The cleaning unit 24 includes a cleaning blade (not shown) and a toner storage tank (not shown). The cleaning blade is a plate-like member provided so as to extend in parallel with the longitudinal direction of the photoconductor 20 and so that one end in the short side direction is in contact with the surface of the photoconductor 20. The cleaning blade removes toner, paper powder, and the like remaining on the surface of the photoconductor 20 after the toner image is transferred to the recording medium from the surface of the photoconductor 20. The toner storage tank is a container-like member having an internal space, and temporarily stores the toner removed by the cleaning blade.

  The paper feed tray 28 is a tray that stores recording media such as plain paper, coated paper, color copy paper, and OHP film. A plurality of paper feed trays 28 are provided, and recording media having different sizes are accommodated in the respective paper feed trays 28. The size of the recording medium includes A3, A4, B5, and B4. Further, a plurality of paper feed trays 28 may contain the same size recording medium. A recording medium is fed one by one in synchronization with the toner image on the surface of the photoreceptor 20 being conveyed to the transfer nip portion by a pickup roller (not shown), a conveyance roller, and a registration roller.

  The scanner unit 29 is provided with a document setting tray (not shown), an automatic reversing document feeder (RADF) and the like, and a document reading device (not shown). The automatic reversing document conveying device conveys a document placed on a document set tray to a document placing table of a document reading device. The document reading device includes a document placement table, a document scanning device, a reflecting member, a photoelectric conversion element (hereinafter referred to as “CCD”) line sensor, and the like, and an image of a document placed on the document placement table. Information is read every plural lines, for example every ten lines. The document placing table is a glass plate-like member for placing a document for reading image information.

  The document scanning device includes a light source (not shown) and a first reflecting mirror, and reciprocates at a constant speed V in parallel along the lower surface in the vertical direction of the document placement table, thereby forming an image of the document placed on the document placement table. Irradiate the surface with light. A reflected light image is obtained by light irradiation. The light source is a light source for irradiating a document placed on the document table. The first reflecting mirror reflects the reflected light image toward the reflecting member. The reflecting member includes a second reflecting mirror, a third reflecting mirror, and an optical lens (not shown), and forms a reflected light image obtained by the document scanning device on the CCD line sensor. The reflecting member reciprocates at a speed of V / 2 following the reciprocating movement of the document scanning device. The second and third reflection mirrors reflect the reflected light image so that the reflected light image faces the optical lens. The optical lens forms a reflected light image on the CCD line sensor. The CCD line sensor includes a CCD circuit (not shown) that photoelectrically converts a reflected light image formed by the optical lens into an electric signal, and outputs an electric signal as image information to an image processing unit in the control means. The image processing unit converts image information input from an external device such as a document reading device or a personal computer into an electrical signal and outputs the electrical signal to the exposure unit 22.

  The image forming apparatus 31 may include a control unit that individually or comprehensively controls operations of the developing unit 1, the charging unit 21, the exposure unit 22, the transfer unit 23, the cleaning unit 24, the fixing unit 25, and the like. The control means includes a storage unit, a calculation unit, and a control unit. Detection results, setting values, image information, table data, programs, and the like by various sensors such as the toner concentration detection sensor 12 are written in the storage unit. As the storage unit, those commonly used in this field can be used, and examples thereof include a read only memory (ROM), a random access memory (RAM), and a hard disk drive (HDD). The arithmetic unit takes out various data (print command, detection result, image information, etc.) input to the storage unit and a program for executing various controls, and performs various detections and / or determinations. The control unit sends a control signal to the corresponding unit according to the determination result in the calculation unit, and performs operation control. A control part and a calculating part are processing circuits implement | achieved by the microcomputer provided with a central processing unit (CPU, Central Processing Unit), a microprocessor, etc., for example.

(Example)
Hereinafter, the present invention will be specifically described with reference to Examples and Comparative Examples. First, the two-component developers of Examples and Comparative Examples were produced by the following method.

[Production of colored resin particles]
The colored resin particles of Examples and Comparative Examples were produced using the materials shown below.

Binder resin (polyester resin obtained by polycondensation using bisphenol A propylene oxide, terephthalic acid and trimellitic anhydride as monomers: glass transition temperature 60 ° C., softening temperature 130 ° C.): 100 parts by weight Boron compound (compound number) B1): 2 parts by weight Carbon black (manufactured by Mitsubishi Chemical: MA-100): 5 parts by weight Polypropylene wax (manufactured by Sanyo Chemical Co., Ltd .: Viscol 550P): 2 parts by weight The above materials are mixed with an airflow (Mitsui Mining Co., Ltd .: Henschel mixer) For 10 minutes. The mixture was melt-kneaded with a kneading and dispersing apparatus (Mitsui Mining Co., Ltd .: Needix MOS140-800), the kneaded product was cooled, and then coarsely pulverized with a cutting mill. The coarsely pulverized product is finely pulverized by a fine pulverizer (Mitsui Mining Co., Ltd .: CGS), and classified by using an air classifier (Hosokawa Micron Corp .: TSP separator), so that the volume average particle diameter is 6.5 μm, BET Colored resin particles (J1) having a specific surface area of 1.8 m ² / g were produced. The volume average particle size was measured with Coulter Multisizer II (manufactured by Beckman Coulter, Inc.).

The volume average particle diameter is 6.5 μm and the BET specific surface area is 1 except that the boron compound of compound number B2 is used instead of the boron compound of compound number B1. Colored resin particles (J2) of .8 m ² / g were produced. Moreover, it is colored with a volume average particle diameter of 6.5 μm and a BET specific surface area of 1.8 m ² / g by the same production method as the colored resin particles (J1) except that the boron compound of Compound No. B1 is not used. Resin particles (J3) were produced.

[Manufacture of external additives]
Table 2 shows external additives used in Examples and Comparative Examples. An external additive (G1) was produced by subjecting silica fine particles (BET specific surface area of about 140 m ² / g) having a primary particle diameter of 12 nm to surface treatment with hexamethyldisilazane. Also, external additives G2, G3 and G4 having different volume resistivity were prepared by the same production method as that of the external additive (G1) except that the amount of hexamethyldisilazane was different. Further, an external additive (G5) was produced by the same production method as the external additive (G1) except that titanium oxide fine particles (BET specific surface area of about 140 m ² / g) were used instead of the silica fine particles.

[Manufacture of toner]
As toners of Examples and Comparative Examples, the external additives (G1 to G5) shown in Table 2 were externally added to the colored resin particles (J1 to J3) to prepare toners (T1 to T11) shown in Table 3. As for the external addition method, 100 parts by weight of the colored resin particles and 1.2 parts by weight of the external additive were introduced into an airflow mixer (Mitsui Mining Co., Ltd .: Henschel mixer), and the tip speed of the stirring blade was set to 15 m / second. Mixed for 2 minutes. The coverage of the external additive in the obtained toner was in the range of 30% to 50%.

[Carrier production]
The carriers of Examples and Comparative Examples were produced by the following method. After mixing the ferrite raw material in a ball mill, the calcined powder was calcined at 900 ° C. in a rotary kiln, and the obtained calcined powder was finely pulverized to a mean particle size of 2 μm or less using a steel ball as a grinding medium by a wet pulverizer. . The obtained ferrite fine powder was granulated by a spray drying method, and the granulated product was fired at 1300 ° C. After firing, the mixture was pulverized using a crusher to obtain core particles composed of a ferrite component having a volume average particle diameter of about 50 μm and a volume resistivity of 1 × 10 ⁹ Ω · cm. Next, as a coating solution for coating the core particles, 100 parts by weight of a silicone resin (trade name: TSR115, manufactured by Shin-Etsu Chemical Co., Ltd.) and 1 part by weight of carbon black (primary particle diameter 25 nm, oil absorption 150 ml / 100 g) Were dissolved and dispersed in toluene to prepare a coating solution for coating. The coating liquid for coating was sprayed on the core particles by a spray coating apparatus to coat the core particles. Toluene was completely removed by evaporation to prepare a carrier (C1) having a volume average particle diameter of 50 μm, a silicone resin film thickness of 1 μm, a volume resistivity of 5 × 10 ¹¹ Ω · cm, and a saturation magnetization of 65 emu / g.

  Further, in the production of the carrier, carriers (C2 to C9) having the volume resistivity shown in Table 4 are produced by the same production method as that for the carrier (C1) except that the amount of carbon black added to the coating liquid is changed. did.

[Manufacture of two-component developer]
As two-component developers of Examples and Comparative Examples, the two components of Examples 1 to 14 and Comparative Examples 1 to 13 shown in Table 5 are mixed by mixing toners (T1 to T11) and carriers (C1 to C9). A developer was prepared. As for the mixing method of the two-component developer, 6 parts by weight of toner and 94 parts by weight of carrier are put into a Nauta mixer (trade name: VL-0, manufactured by Hosokawa Micron Corporation) and mixed by stirring for 20 minutes. And the two-component developer of the comparative example was produced.

A continuous print test was performed by the image forming apparatus 31 using the produced two-component developer. The developing conditions of the image forming apparatus 31 are: the peripheral speed of the photosensitive member 20 is 400 mm / second, the peripheral speed of the developing roller 3 is 560 mm / second, the gap between the photosensitive member 20 and the developing roller 3 is 0.42 mm, and the developing roller 3 and the regulating blade. Of the photosensitive member under the conditions that the toner adhesion amount on the paper in a solid image (100% density) is 0.5 mg / cm ² and the toner adhesion amount in the non-image portion is the smallest. The surface potential and development bias of 20 were adjusted. A4 size electrophotographic paper (multi-receiver: manufactured by Sharp Document System) was used as the test paper. A text image print test was performed in which the coverage of the print image recorded on the test paper was 10% and 3%, respectively. Table 5 also shows the evaluation results for the charge amount, the image density, and the fog density when the two-component developers of Examples and Comparative Examples are used. The evaluation method is as shown below.

<Charge amount>
The toner charge amount was measured using a suction type small charge amount measuring device (manufactured by Trek Japan Co., Ltd .: 210HS-2A).

<Image density>
The image density was evaluated by printing a solid image (100% density) with a side of 3 cm and measuring the image density using a reflection densitometer (Macbeth: RD918). The evaluation criteria are as follows.
○: Good. The image density is 1.3 or more, and the fibers of the test paper are completely covered with toner.
Δ: Slightly poor The image density is 1.2 or more and less than 1.3.
X: Defect. The image density is less than 1.2, and the test paper fibers are incompletely covered with toner.

<Fog density>
The fog density was evaluated by the fog density which is the density of the non-image area (0% density) in the test paper printed by the image forming apparatus 31. The fog density measurement method is as follows. First, using a whiteness meter (Nippon Denshoku Kogyo Co., Ltd .: Z-Σ90 COLOR MEASURING SYSTEM), the whiteness of the test paper before printing was measured in advance. Next, the whiteness in the non-image area of the test paper after printing was measured using the whiteness meter, and the difference from the whiteness of the test paper before printing was determined to obtain the fog density. The evaluation criteria are as follows.
○: Good. The fog density is less than 0.6, and the fog is hardly visible to the naked eye.
Δ: Slightly poor The fog density is 0.6 or more and less than 1.0.
X: Defect. The fog density is 1.0 or more, and the fog is clearly visible with the naked eye.

  As shown in Table 5, in the continuous print test using the two-component developer of the present invention shown in Examples 1 to 14, an image with high coverage (10% coverage) and an image with low coverage (3% coverage) Even when 2000 sheets were continuously printed, the charge amount of the toner was stable, an image with high image density and no fog was obtained. On the other hand, in Comparative Example 1 (developer using toner that does not contain a boron compound), when 2000 images of 10% coverage are continuously printed, an image with low image density fogging is formed, and further, the image forming apparatus is caused by toner scattering. Dirt occurred inside. In addition, when the two-component developer including the external additive having a low volume resistivity of Comparative Examples 2, 3, 8, and 9 is used, the toner charge amount decreases when 2000 images of 10% coverage are continuously printed. The image was fogged, and the toner was scattered due to toner scattering. Further, when the two-component developer containing a carrier having a high volume resistivity of Comparative Examples 4, 5, 10, and 11 is used, if 2000 images of 3% coverage are continuously printed, the toner charge amount increases, and the image The image has a low density and an edge. Further, when the two-component developer containing a carrier having a low volume resistivity of Comparative Examples 6, 7, 12, and 13 is used, if 2000 images of 3% coverage are continuously printed, the toner charge amount is reduced and fogging is caused. In addition, the image forming apparatus was smudged due to toner scattering.

1 is a diagram illustrating a configuration of an image forming apparatus 31 according to an embodiment of the present invention. 2 is a cross-sectional view showing a configuration of developing means 1. FIG. It is a figure which shows magnetic force distribution of each magnetic pole in the magnet roller 13 which the developing roller 3 has.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Developing means 20 Photoconductor 21 Charging means 22 Exposure means 23 Transfer means 24 Cleaning means 25 Fixing means 31 Image forming apparatus

Claims (8)

A two-component developer comprising a toner for visualizing an electrostatic latent image and a carrier having magnetism,
The toner has at least the following general formula (1)

(Wherein R ₁ and R ₄ represent a hydrogen atom, an alkyl group, a substituted or unsubstituted aromatic ring (including a condensed ring), and R ₂ and R ₃ represent a substituted or unsubstituted aromatic ring (including a condensed ring). And X ⁺ represents a cation.) Colored resin particles formed from a boron compound represented by the following formula:
An external additive externally added to the surface of the colored resin particles,
The external additive is adjusted so that its volume resistivity is 1 × 10 ¹² Ω · cm or more and 5 × 10 ¹⁵ Ω · cm or less,
The two-component developer, wherein the carrier has a volume resistivity of 2 × 10 ¹⁰ Ω · cm to 5 × 10 ¹² Ω · cm. The carrier includes core particles and a coating layer that covers the core particles,
The two-component developer according to claim 1, wherein the coating layer contains a conductive agent having conductivity and a silicone resin. 3. The two-component developer according to claim 2, wherein the core particle is formed of a ferrite particle having a volume resistivity of 1 × 10 ⁸ Ω · cm to 5 × 10 ¹⁰ Ω · cm.   The two-component developer according to claim 2 or 3, wherein the coating layer has a thickness of 0.5 µm or more and 5 µm or less.   5. The two-component developer according to claim 2, wherein the conductive agent contained in the coating layer is carbon black having an oil absorption of 90 ml / 100 g to 170 ml / 100 g.   The two-component developer according to any one of claims 1 to 5, wherein the external additive is a fine particle composed of silica having a hydrophobic surface and a primary particle diameter of 5 nm to 15 nm. . The colored resin particles have a volume average particle diameter of 5 μm or more and 7 μm or less and a BET specific surface area of 1.5 m ² / g or more and 1.9 m ² / g or less. The two-component developer according to any one of the above. A photosensitive member carrying an electrostatic latent image, a charging unit for charging the surface of the photosensitive member, an exposure unit for forming an electrostatic latent image on the surface of the photosensitive member, and a developer for the electrostatic latent image formed on the surface of the photosensitive member A developing means for forming a visible image by supplying the toner, a transferring means for transferring the visible image to the recording medium, a cleaning means for removing the developer remaining on the surface of the photoreceptor, and the visible image transferred to the recording medium. An image forming apparatus including a fixing unit that fixes an image,
An image forming apparatus, wherein the developer is the two-component developer according to claim 1.

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