JP7349889B2 - External additives for toner, toner and image forming equipment - Google Patents

Description

The present invention relates to an external additive for toner, a toner, and an image forming apparatus.

In an electrophotographic image forming method, a uniform charge is applied to the surface of a photoreceptor, and then an electrostatic latent image corresponding to the image is formed by exposing the photoreceptor to light according to the image. A toner with a predetermined charge is attached to this electrostatic latent image to make it visible (developed), and the toner image formed on the photoreceptor is transferred to paper, etc. via an intermediate transfer body as necessary. An image is formed by transferring the image to an image forming support and fixing it on the image forming support using heat or the like. Examples of image forming apparatuses using the above image forming method include copying machines and printers.

The photoreceptors used in the image forming apparatus are classified into negatively chargeable photoreceptors and positively chargeable photoreceptors from the viewpoint of uniform charge characteristics imparted to the surface. In addition, from the viewpoint of the material constituting the photoreceptor, the photoreceptor is classified into inorganic photoreceptors using selenium, amorphous silicon, etc., and organic photoreceptors made of organic materials.

Negatively chargeable organic photoreceptors are produced by forming an undercoat layer on a substrate made of aluminum, for example, for the purpose of imparting adhesion and controlling charge transfer, as necessary, and containing a charge-generating substance on top of the undercoat layer. Generally, it has a laminated structure in which a charge generation layer is formed, and a charge transport layer is further formed thereon.

Positively charging organic photoreceptors are effective because they can achieve high image quality and can suppress the production of ozone. In this positively charging organic photoreceptor, since it is necessary to form a charge generation layer on the surface layer, a charge transport layer and a charge generation layer are laminated in this order on a substrate made of, for example, aluminum. There is also a method in which a charge-generating substance and a charge-transporting substance are mixed and a single layer is formed on the substrate.

As inorganic photoreceptors, photoreceptors using materials such as selenium have been replaced by photoreceptors using amorphous silicon. Amorphous silicon is suitable because it has very high hardness and can achieve a long life.

In recent years, since digital data can be easily used, there has been a demand for printing large amounts of variable information. So-called industrial printers are used that provide valuable printed matter by printing variable information rather than mere copying. In this case, since the printed matter has value, it is necessary that the image quality be stable over a long period of time.

For example, Patent Document 1 proposes an image forming apparatus in which image fading in a high humidity environment is suppressed when a photoreceptor having a protective layer containing a crosslinked polymer is used. In the image forming apparatus, a cleaning effect is exhibited by supplying a lubricant, and furthermore, a lubricant removing means is provided to prevent the lubricant from remaining.

Patent Document 2 is a technique that solves the image deletion problem in an amorphous silicon photoreceptor by using an external additive having a specific aggregation diameter. Specifically, the toner for developing an electrostatic latent image includes toner base particles and conductive fine particles as an external additive, wherein the conductive fine particles have an average primary particle diameter of 90 nm or less, and the toner base particles have an average primary particle diameter of 90 nm or less, and It is described that aggregates having an average aggregate particle diameter of 0.5 to 2.0 μm and a BET specific surface area of 15 m ² /g or more are formed on the surface of the particles. Titanium oxide is disclosed as the conductive fine particles.

Patent Document 3 discloses that an external additive externally added to a non-magnetic one-component developing toner has a crystallite diameter of 12.0 to 16.0 nm, is hydrophobically treated, and has a true specific gravity of 2.5 to 3. A technique has been shown that can suppress the occurrence of fog and achieve excellent color reproducibility by using metatitanic acid particles having a particle size of .3 g/cm ³ . Furthermore, Patent Document 4 describes, as external additives to be added to toner, silica particles having a volume average particle size of 5 nm or more and 30 nm or less, and silica particles having a volume average particle size of 50 nm or more and 120 nm or less and an aspect ratio of 3 or more and less than 10. A technology that prevents the occurrence of white streaks in a toner image by including at least two types of metatitanic acid particles treated with a silane compound, and by setting the respective coverage ratios of the silica particles and the metatitanic acid particles to the toner particles. It is shown.

Japanese Patent Application Publication No. 2015-114647 JP2009-180890A JP2016-062082A Japanese Patent Application Publication No. 2011-154278

In Patent Documents 1 and 2, image deletion is suppressed by increasing the cleaning effect, but it seems difficult to exhibit a good cleaning effect over a long period of time. Further, Patent Document 3 does not show that excellent images can be achieved even when the image forming apparatus is used repeatedly over a long period of time.

The present invention has been made in view of the above circumstances, and its purpose is to stably produce images of excellent image quality even when an image forming apparatus is repeatedly used over a long period of time under various environmental changes. An object of the present invention is to provide an external additive used in a toner of an image forming apparatus capable of forming an image, a toner using the external additive, and the above image forming apparatus using the toner.

Aspect 1 of the present invention is a toner comprising, on the surface of metatitanic acid fine particles having a number average primary particle diameter of 10 to 300 nm, a hydrophobized substance created using a hydrophobizing agent containing an amino-based silane coupling agent. It is an external additive.

Aspect 2 of the present invention is the external additive for toner according to aspect 1, wherein the hydrophobizing agent further contains modified silicone oil.

Aspect 3 of the present invention is the external additive for toner according to aspect 2, wherein the modified silicone oil is an amino-modified silicone oil.

Aspect 4 of the present invention is the external additive for toner according to aspect 2 or 3, wherein the mass ratio of the amino-based silane coupling agent to the modified silicone oil is 20:1 to 1:1.

Aspect 5 of the present invention is the external additive for toner according to any one of aspects 1 to 4, wherein the proportion of the hydrophobizing agent is 1 to 40% by mass of the content of metatitanic acid fine particles in the external additive. It is.

Aspect 6 of the present invention is the external additive for toner according to any one of aspects 1 to 5, which is positively charged.

Aspect 7 of the present invention is the external additive for toner according to any one of Aspects 1 to 6, which has a degree of hydrophobicity of 5 to 60%.

Aspect 8 of the present invention is a toner comprising colored resin particles containing at least a colorant and a resin, and the external additive for toner according to any one of Aspects 1 to 7.

Aspect 9 of the present invention is the toner according to aspect 8, wherein the proportion of the external additive for toner is 0.05 to 2.0% by mass of the content of colored resin particles in the toner.

Aspect 10 of the present invention is the toner according to aspect 8 or 9, wherein the colored resin particles further contain a release agent and a charge control agent.

Aspect 11 of the present invention is the toner according to any one of aspects 8 to 10, further comprising small inorganic fine particles having a number average primary particle diameter of 5 to 20 nm as an additional external additive.

Aspect 12 of the present invention is the toner according to any one of Aspects 8 to 11, further comprising large inorganic fine particles having a number average primary particle diameter of 25 to 1000 nm as an additional external additive.

Aspect 13 of the present invention is an image forming apparatus having at least a positively charging photoreceptor, a charging means, an exposure means, a developing means, a transfer means, and a cleaning means,
The image forming apparatus is provided with at least the toner according to any one of aspects 8 to 12 as the developing means.

A fourteenth aspect of the present invention is the image forming apparatus according to the thirteenth aspect, wherein the positively charging photoreceptor is an amorphous silicon photoreceptor.

Aspect 15 of the present invention is the image forming apparatus according to aspect 13, wherein the positively chargeable photoreceptor is a positively chargeable organic photoreceptor.

Aspect 16 of the present invention is the image forming apparatus according to aspect 13, wherein the positively chargeable photoreceptor is a laminated positively chargeable organic photoreceptor having a protective layer on its surface.

According to the present invention, a toner used in an image forming apparatus that can stably form images of excellent image quality even when the image forming apparatus is used repeatedly over a long period of time under various environmental changes. It is possible to provide an external additive, a toner using the external additive, and the above image forming apparatus using the toner.

FIG. 1 is a micrograph showing an example of the external additive and toner of the present invention in an example.

In an image forming apparatus, after the image forming process, that is, after transferring the toner image formed on the photoreceptor, the remaining toner, paper dust, and other residues are removed, and the surface of the photoreceptor is cleaned in order to form the next image. is cleaned using a urethane blade, fur brush, etc. The image forming apparatus is required to sufficiently remove the above-mentioned residue through the cleaning. However, during image formation, charging is performed using corona discharge, a roller charging device, etc. in order to uniformly charge the surface of the photoreceptor, but due to the presence of water during this discharge, the residual As a material, there is a problem in that extremely minute oxides are likely to be formed.

By the way, in offices, for example, there is a need to reduce the running costs of printers and the like. In order to reduce the cost, it is effective to lengthen the maintenance cycle such as parts replacement as much as possible. To this end, the most effective method is to suppress deterioration of the photoreceptor, and in particular, to suppress abrasion of the photoreceptor film, which is the most important factor. In order to suppress wear, for example, a laminated type photoreceptor having a hard protective layer formed on its surface or an amorphous silicon photoreceptor is used.

However, as mentioned above, if the surface of the photoreceptor is highly hard, the oxides cannot be removed sufficiently by cleaning, and the surface of the photoreceptor becomes conductive, causing the surface charge to diffuse to the surface layer, resulting in what is called "image deletion." There's a problem. This problem becomes more noticeable when the image forming apparatus is used for a long period of time and images are formed repeatedly. Further, even when a positively charged organic photoreceptor is used, a similar problem of charge diffusion occurs.

In view of the above circumstances, the present inventors have discovered that the residues containing the oxides can be sufficiently removed in the cleaning step (hereinafter, this effect may be referred to as "high cleaning performance"), and that the residues can be removed. This high cleaning performance can be maintained even when the image forming device is used repeatedly over a long period of time under various environmental changes, and as a result, images of excellent quality can be produced. In order to obtain an image forming apparatus that can stably form images, we conducted intensive research on toners used in the image forming apparatuses and external additives used in the toners.

As a result, the external additive used in the above toner was applied to the surface of metatitanic acid fine particles with a number average primary particle size of 10 to 300 nm, and a hydrophobized product was created using a hydrophobizing agent containing at least an amino-based silane coupling agent. It has been found that if the photoreceptor has the following properties, the formation of oxides on the surface of the photoreceptor can be suppressed and the above-mentioned effects can be achieved. In particular, in image forming apparatuses that use amorphous silicon photoreceptors or organic photoreceptors with hard protective layers, which are compatible with long lifespans, as photoreceptors, the image forming apparatus is repeatedly operated over a long period of time under various environmental changes. The first discovery was that even when used, high cleaning performance can be maintained and images of excellent quality can be formed over a long period of time.

Hereinafter, the external additive for toner, toner, and image forming apparatus of the present invention will be explained in order.

(External additive of the present invention)
[Metatitanic acid fine particles]
First, the external additive for toner of the present invention uses metatitanic acid fine particles having a number average primary particle diameter of 10 to 300 nm as base particles. In the present invention, by using metatitanic acid fine particles as base particles, it is possible to suppress the generation of oxides formed on the surface of the photoreceptor, and it is believed that the occurrence of image defects can be suppressed. The reason for this is not clear, but it is thought to be as follows.

That is, titanium oxide is generally said to have the function of suppressing the effects of environmental changes and stabilizing the state. The so-called vacant orbitals that exist in titanium oxide make it easy to transfer electrons, and as a result, it suppresses the dissipation of charge into the air, or exhibits a trapping function, suppressing the effects of environmental changes and stabilizing the state. It is presumed that it is possible.

However, as explained below, titanium oxide is considered unable to fully exhibit its effects under more severe environmental changes. In particular, when a positively charged organic photoreceptor is used, a charge generation layer exists near the surface and electrons are generated. Therefore, charge leakage is more likely to occur, and it is presumed that titanium oxide has an insufficient function of suppressing the fluctuation in such a situation. On the other hand, metatitanic acid has a crystal structure with slightly distorted crystallinity, and as a result, it is presumed that the electron transfer function is sufficiently effective.

Furthermore, the distorted crystal structure can easily trap peroxides such as ozone generated during charging, and as a result, for example, the effect of suppressing oxide generation on the surface of an organic photoreceptor can be sufficiently exerted. Conceivable.

The metatitanic acid fine particles of the present invention have a number average primary particle diameter of 10 to 300 nm. The number average primary particle diameter is preferably 25 nm or more, more preferably 50 nm or more, preferably 175 nm or less, more preferably 150 nm or less, and still more preferably less than 120 nm. The number average primary particle diameter indicates the average value of the Feret horizontal particle diameter of 100 particles observed using a transmission electron microscope. The Feret horizontal particle diameter refers to the length of a side parallel to the x-axis when considering a rectangle circumscribing a particle on the xy plane.

[Manufacture of metatitanic acid fine particles]
In the present invention, the method for producing the metatitanic acid fine particles is not particularly limited. For example, it can be produced by a method of hydrolyzing titanyl sulfate. Furthermore, air flow pulverization can be used as a means for controlling the number average primary particle diameter of metatitanic acid fine particles within the range of 10 to 300 nm.

[Hydrophobizing agent]
The external additive of the present invention has, on the surface of the metatitanic acid fine particles, a hydrophobized material prepared using a hydrophobizing agent containing at least an amino-based silane coupling agent. It is considered that the formation of oxides is sufficiently suppressed by treating the metatitanic acid fine particles with an amino-based silane coupling agent. Furthermore, by using an amino-based silane coupling agent, it is possible to realize a positively charged external additive and toner that are compatible with positively charged photoreceptors that can form images of excellent quality.

Examples of the amino-based silane coupling agent include N-2-(aminoethyl)-3-aminopropylmethyldimethoxysilane, N-2-(aminoethyl)-3-aminopropyltrimethoxysilane, and 3-aminopropyltrimethoxysilane. Methoxysilane, 3-aminopropyltriethoxysilane, 3-triethoxysilyl-N-(1,3-dimethyl-butylidene)propylamine, N-phenyl-3-aminopropyltrimethoxysilane, N-2-(amino Examples include ethyl)-8-aminooctyltrimethoxysilane, N-(vinylbenzyl)-2-aminoethyl-3-aminopropyltrimethoxysilane, and the like. One or more of these can be used.

The hydrophobizing agent may further contain modified silicone oil. By using the modified silicone oil in combination, it is possible to exhibit the effect of improving the charging rise property. It is preferable to use amino-modified silicone oil as the modified silicone oil. By using the amino-modified silicone oil together with the amino-based silane coupling agent, the external additive and toner can be sufficiently positively charged. Examples of amino-modified silicone oils include general-purpose products such as amino-modified organopolysiloxane, such as dimethyl(aminoethylaminopropyl)methylsiloxane, dimethyl(aminoethylaminopropyl)ethylsiloxane, diaminopropyltetramethyldisiloxane, and dimethyl(aminoethylaminopropyl)methylsiloxane. Examples include polymers and copolymers such as propyl)methylcyclosiloxane.

The proportion of the hydrophobizing agent is preferably 1 to 40% by mass based on the content of metatitanic acid fine particles in the external additive of the present invention. By setting the proportion of the hydrophobizing agent to 1% by mass or more with respect to the content of the metatitanic acid fine particles, the above-mentioned effects can be further enhanced. The proportion of the hydrophobizing agent is more preferably 5% by mass or more, and even more preferably 8% by mass or more, based on the content of the metatitanic acid fine particles. On the other hand, excessive coating can be prevented by suppressing the proportion of the hydrophobizing agent to 40% by mass or less based on the content of metatitanic acid fine particles. The proportion of the hydrophobizing agent is more preferably 15% by mass or less based on the content of the metatitanic acid fine particles. When using multiple types of hydrophobizing agents, the above ratio is determined using the total amount of the multiple types of hydrophobizing agents.

When the amino-based silane coupling agent and the modified silicone oil are used together, the mass ratio of the amino-based silane coupling agent and the modified silicone oil is preferably 20:1 to 1:1. By using the amino-based silane coupling agent in a mass ratio of at least one time that of the modified silicone oil, the effects of the amino-based silane coupling agent described above can be fully exhibited. Preferably, the mass ratio of the amino-based silane coupling agent is 1.5 times or more that of the modified silicone oil. On the other hand, from the viewpoint of exhibiting the effect of the modified silicone oil described above, the amount of the amino-based silane coupling agent is preferably 20 times or less, more preferably 10 times or less, and even more preferably 10 times or less, by mass ratio of the modified silicone oil. It is 5 times or less, more preferably 3 times or less.

The external additive of the present invention is preferably positively charged. Further, the external additive of the present invention preferably has a degree of hydrophobicity of 5 to 60%.

[Manufacture of external additive of the present invention]
The present invention is not limited to the method for producing the external additive of the present invention. For example, while stirring a solution containing metatitanic acid fine particles, a hydrophobizing agent is added dropwise or sprayed to treat the surface of metatitanic acid fine particles, or a method in which a hydrophobizing agent is dissolved in an organic solvent and the organic solvent is stirred , a method in which surface treatment is performed by adding metatitanic acid fine particles, and the like. In the former method, the hydrophobizing agent may be diluted with an organic solvent or the like. After adding the hydrophobizing agent, it may be maintained at 60 to 100°C for 0.5 to 2.0 hours. Thereafter, solid-liquid separation is performed by filtration and drying to obtain metatitanic acid fine particles having a hydrophobized product prepared using a hydrophobizing agent, that is, the external additive of the present invention.

(toner)
(1) External Additive of the Present Invention In the toner of the present invention, the external additive of the present invention is added to colored resin particles (hereinafter sometimes simply referred to as "colored resin particles") containing at least a colorant and a resin. It is assumed that it is attached externally. As a result, as described above, the toner of the present invention is easily removed when cleaning the photoreceptor, maintains good cleaning performance over a long period of time, and can stably obtain images with excellent image quality.

The proportion of the external additive of the present invention in the toner is preferably 0.05 to 2.0% by mass based on the content of the colored resin particles. It is preferable to set the proportion of the external additive to 0.05% by mass or more because the above-mentioned effects of the external additive can be fully exhibited. The proportion of the external additive is more preferably 0.1% by mass or more. On the other hand, even if the external additive is contained in excess, the effect will be saturated, so the upper limit is preferably 2.0% by mass. The proportion of the external additive is more preferably 1.5% by mass or less.

(2) Colored resin particles The colored resin particles contain at least a colorant and a resin, and may further contain a release agent (wax) and a charge control agent as internal additives, if necessary. Each component will be explained below.

〔resin〕
Examples of the resins contained in the toner particles constituting the toner of the present invention include styrene resins, acrylic resins, styrene-acrylic resins, vinyl resins such as olefin resins, polyester resins, polyamide resins, polycarbonate resins, and polyether resins. , polyvinyl acetate resin, polysulfone, epoxy resin, polyurethane resin, urea resin, and other known resins. One type of these can be used alone or two or more types can be used in combination. Particularly, one or more of styrene-acrylic resin and polyester resin is preferable because a toner with excellent low-temperature fixing properties can be obtained. The resin preferably has a glass transition point of 35 to 70°C, more preferably 45 to 60°C. Further, the softening point (T1/2) measured by a Koka type flow tester is preferably 80 to 140°C, more preferably 90 to 135°C.

When using a styrene resin, (meth)acrylic resin, or a copolymer resin thereof as a binder resin, the mass average molecular weight Mw is 20,000 or more and 100,000 or less, and the number average molecular weight Mn is 2,000 or more and 30,000 or less. It is preferable to use a range of . On the other hand, when using a polyester resin as a binder resin, it is preferable to use one with a mass average molecular weight Mw of 5,000 to 40,000 and a number average molecular weight Mn of 2,000 to 10,000. preferable. The glass transition temperature of the binder resin is preferably in the range of 40°C or higher and 80°C or lower. When the glass transition temperature is within the above range, the minimum fixing temperature can be easily maintained. Further, the softening point (T1/2 measured by Koka flow tester) of the resin is 105 to 160°C, preferably 110 to 140°C.

[Colorant]
As the colorant contained in the toner particles according to the present invention, commonly known dyes and pigments can be used. As a colorant for obtaining a black toner, various known colorants may be used, including carbon black such as furnace black and channel black, magnetic substances such as magnetite and ferrite, dyes, and inorganic pigments including non-magnetic iron oxide. can be used.

As the coloring agent for obtaining colored toners, any known coloring agent such as dyes and organic pigments can be used. Specifically, examples of organic pigments include C.I. I. Pigment Red 5, 48:1, 53:1, 57:1, 81:4, 122, 139, 144, 149, 166, 177, 178, 222, 238 , 269, C. I. Pigment Yellow 14, 17, 74, 93, 94, 138, 155, 180, 185, C. I. Pigment Orange 31, Pigment Orange 43, C.I. I. Pigment Blue 15;3, Pigment Blue 60, Pigment Blue 76, etc., and examples of dyes include C.I. I. Solvent Red 1, 49, 52, 58, 68, 11, 122, C. I. Solvent Yellow 19, 44, 77, 79, 81, 82, 93, 98, 103, 104, 112, 162, C. I. Examples include Solvent Blue 25, Solvent Blue 36, Solvent Blue 69, Solvent Blue 70, Solvent Blue 93, and Solvent Blue 95. The coloring agents for obtaining toners of each color can be used singly or in combination of two or more.

The content of the colorant is preferably 1 to 10% by mass, more preferably 2 to 8% by mass, based on the content of the resin.

The colored resin particles may further contain a release agent (wax) and a charge control agent as internal additives.

〔Release agent〕
A known wax can be used as a mold release agent. For example, hydrocarbon waxes such as low molecular weight polyethylene wax, low molecular weight polypropylene wax, Fischer-Tropsch wax, microcrystalline wax, and paraffin wax, ester waxes such as carnauba wax, pentaerythritol behenate, behenyl behenate, and behenyl citrate. etc. can be used. One type of these can be used alone or two or more types can be used in combination. The content of the mold release agent is preferably 2 to 20% by mass, more preferably 3 to 10% by mass based on the resin.

[Charge control agent]
As the charge control agent, a known charge control agent can be used. For example, nigrosine dyes, metal salts of naphthenic acid or higher fatty acids, alkoxylated amines, quaternary ammonium salt compounds, azo metal complexes, salicylic acid metal salts or metal complexes thereof, calixarene compounds, and the like can be used. These can be used alone or in combination of two or more. The content of the charge control agent is preferably 0.1 to 3.0% by mass, more preferably 0.1 to 1.0% by mass based on the resin.

Furthermore, a resin-type charge control agent (charge control resin) may be used in which the resin itself has a substituent group and has charge imparting ability.

(Method for producing colored resin particles)
Examples of methods for producing the resin particles include a kneading and pulverizing method, a suspension polymerization method, an emulsion aggregation method, a dissolution suspension method, an ester extension polymerization method, a dispersion polymerization method, and the like. Furthermore, there are also those that have a core made of resin particles obtained by kneading and pulverizing colorants and resins, and have a coating layer with functions such as heat resistance formed on the surface, and those that have resin particles formed by suspension polymerization that are coated with monomers for coating materials. It may be a so-called microcapsule type toner, such as one in which a coating layer is formed by adding agglomerated polymer particles, or a toner in which a coating layer is formed around a core of particles made of agglomerated polymer particles, and another resin particle is formed around the core.

The volume average particle size of the colored resin particles (toner particles) to which the external additives and the like of the present invention are externally added is preferably 3 to 15 μm, more preferably 5 to 10 μm. This size is approximately the same as the colored resin particles (toner base particles) before adding the external additive.

(3) Additional external additives In addition to the case where the external additive is only the external additive of the present invention, the toner of the present invention may also contain the following additional external additives in addition to the external additive of the present invention. Agents can be used.

(Small size inorganic particles)
In addition to the external additive of the present invention, the toner of the present invention contains small inorganic fine particles with a number average primary particle size of 5 to 20 nm as an additional external additive. It is preferable that it be attached.

Various types of small-sized inorganic particles include silicon carbide, boron carbide, titanium carbide, zirconium carbide, hafnium carbide, vanadium carbide, tantalum carbide, niobium carbide, tungsten carbide, chromium carbide, molybdenum carbide, calcium carbide, diamond carbon lactam, etc. Carbides, various nitrides such as boron nitride, titanium nitride, and zirconium nitride, borides such as zirconium boride, various oxidations such as iron oxide, chromium oxide, calcium oxide, magnesium oxide, zinc oxide, copper oxide, aluminum oxide, and silica. compound oxides such as silica-titania-alumina, calcium titanate, strontium titanate, and magnesium titanate, sulfides such as molybdenum disulfide, fluorides such as magnesium fluoride and carbon fluoride, aluminum stearate, and stearate. Examples include fine particles made of various metal soaps such as calcium phosphate, zinc stearate, and magnesium stearate, and various nonmagnetic inorganic substances such as talc and bentonite. These can be used alone or in combination of two or more.

Among these, it is particularly preferable to use one type or a combination of two or more types of small-sized inorganic fine particles formed of silica, alumina, and a composite oxide of silica-titania-alumina.

In addition, these small-sized inorganic particles can be used with conventionally used hydrophobizing agents such as silane coupling agents, titanate coupling agents, silicone oil, and silicone varnish, fluorine-based silane coupling agents, or fluorine-based silicone oils. It is preferable that the surface be treated by a known method with a treatment agent such as , a coupling agent containing an amino group/quaternary ammonium salt, or a modified silicone oil.

Examples of hydrophobizing agents include hexamethyldisilazane, trimethylsilane, trimethylchlorosilane, dimethyldichlorosilane, methyltrichlorosilane, allyldimethylchlorosilane, benzyldimethylchlorosilane, methyltrimethoxysilane, methyltriethoxysilane, isobutyltrimethoxy Silane, dimethyldimethoxysilane, dimethyldiethoxysilane, trimethylmethoxysilane, hydroxypropyltrimethoxysilane, phenyltrimethoxysilane, n-butyltrimethoxysilane, n-hexadecyltrimethoxysilane, n-octadecyltrimethoxysilane, vinyltrimethoxysilane Silane coupling agents such as methoxysilane, vinyltriethoxysilane, γ-methacryloxypropyltrimethoxysilane, and vinyltriacetoxysilane; silicones such as dimethylpolysiloxane, methylhydrogenpolysiloxane, methylphenylpolysiloxane, dimethylsilicone oil, etc. oil; etc., and one or more of these can be used. Further, as the amine-based hydrophobizing agent, the above-mentioned amino-based silane coupling agent and amino-modified silicone oil used in the production of the external additive of the present invention can be used.

When small inorganic fine particles are externally added to the toner, the proportion of the small inorganic fine particles is preferably 0.1 to 2.0% by mass relative to the content of the colored resin particles. From the viewpoint of fully exhibiting the effect of imparting fluidity and the like, which is the effect of the small-sized inorganic fine particles mentioned above, the content is preferably 0.1% by mass or more, more preferably 0.5% by mass or more. On the other hand, from the viewpoint of suppressing image defects caused by adhesion to photoreceptors etc. due to the excessive presence of small-sized inorganic fine particles, the content is preferably 2.0% by mass or less, more preferably 1.5% by mass. % or less. When using multiple types of small-sized inorganic fine particles, the above ratio is determined using the total amount of the multiple types of small-sized inorganic fine particles.

(Large size inorganic particles)
Furthermore, by adding large-sized inorganic particles with a number average primary particle size of 25 to 1000 nm to the resin particles consisting of at least a colorant and a resin, embedding of small-sized inorganic particles into the surface of the colored resin particles is suppressed. can. As the large-sized inorganic fine particles, the same material as the above-mentioned small-sized inorganic fine particles can be used.

Further, as the large-sized inorganic fine particles, fine particles exceeding 100 nm have abrasive properties, and can be expected to have the effect of polishing the surface of the photoreceptor and cleaning the photoreceptor.

In addition, when the large-sized inorganic fine particles are externally added, the proportion thereof is preferably 0.1 to 2.0% by mass based on the content of the colored resin particles. More preferably, it is 0.2 to 1.0% by mass. By setting the proportion of large-sized inorganic fine particles to 0.1% by mass or more, the effects of the particles can be exhibited. Further, by controlling the proportion of large-sized inorganic fine particles to 2.0% by mass or less, damage to the photoreceptor due to excessive large-sized inorganic fine particles and image defects that may occur as a result can be suppressed.

The toner of the present invention can be used as a magnetic or non-magnetic one-component developer, or may be mixed with a carrier and used as a two-component developer. When the toner is used as a two-component developer, magnetic particles made of conventionally known materials such as metals such as iron, ferrite, and magnetite, and alloys of these metals with metals such as aluminum and lead are used as carriers. ferrite particles are particularly preferred. Further, as the carrier, a coated carrier in which the surface of magnetic particles is coated with a coating agent such as a resin, a dispersed carrier in which magnetic fine powder is dispersed in a binder resin, etc. may be used. As the resin forming the coating material and the binder resin, conventionally used resins can be used. The volume average particle diameter of the carrier is preferably 20 to 100 μm, more preferably 25 to 80 μm.

(Toner manufacturing method)
The toner of the present invention has the above-mentioned colored resin particles added with the external additive of the present invention and, if necessary, additional external additives such as the above-mentioned small-sized inorganic fine particles and large-sized inorganic fine particles. You can get toner. As a method for mixing these, a known mixing device such as a Henschel mixer or a V-type mixer can be used.

(Image forming device)
The image forming apparatus of the present invention includes at least a positively chargeable photoreceptor, a charging means, an exposure means, a developing means, a transfer means, and a cleaning means, and the above-mentioned external additive of the present invention is externally added as the developing means. At least they have toner.

The image forming apparatus of the present invention includes a positively charging photoreceptor. As the positively charging photoreceptor, an amorphous silicon photoreceptor, a positively charging organic photoreceptor, or a laminated positively charging organic photoreceptor having at least a protective layer on the surface is preferably used. These photoreceptors will be explained below.

(Amorphous silicon photoreceptor)
Examples of amorphous silicon (a-Si) photoreceptors include those in which an amorphous silicon photosensitive layer (a-Si photosensitive layer) is formed on the surface of a conductive substrate formed into a predetermined shape such as a drum shape. . In particular, those having a thin a-Si photosensitive layer having a thickness of 30 μm or less are preferable because they have excellent productivity and can form images with high resolution. In addition to a single layer or two or more layers that actually function as a photosensitive layer, the a-Si photosensitive layer may have a carrier blocking layer, a surface protection layer, etc., which will be described in detail below. However, when it has a plurality of layers, it is preferable that the total thickness of the layers is 30 μm or less. Furthermore, an a-Si photosensitive layer containing H or a halogen element, or containing elements such as C, N, and O may be formed on the surface of the conductive substrate. That is, materials constituting the a-Si photosensitive layer include, in addition to a-Si, a-SiC, a-SiO, a-SiON, and the like.

[Carrier blocking layer]
A carrier blocking layer may be interposed between the a-Si photosensitive layer and the conductive substrate. As such a carrier blocking layer, an inorganic insulating layer such as a-SiC or an organic insulating layer such as polyethylene terephthalate can be used, and the thickness thereof can be within the range of 0.01 to 5 μm. can.

[Surface protective layer]
The surface protective layer may be made of a conventionally used material, such as an organic or inorganic insulating material. For example, an inorganic insulating layer made of a-Si added with C, O, N, etc. is preferable. used.

(Positively charged organic photoreceptor)
In a positively charging organic photoreceptor, an undercoat layer is formed as necessary on the surface of a conductive substrate, and a photosensitive layer is formed by laminating a charge transport layer and a charge generation layer in this order on top of the undercoat layer. . Alternatively, an undercoat layer may be formed on the surface of the conductive substrate, if necessary, and a photosensitive layer containing a charge-generating substance and a charge-transporting substance mixed in the same layer may be further formed thereon. .

Examples of conductive substrates include metal plates such as aluminum, stainless steel, and iron; flexible supports such as paper and plastic films with metal layers such as aluminum, palladium, and gold provided by lamination or vapor deposition on the surface; and paper. A flexible support such as a plastic film or the like may be coated or vapor-deposited with a layer containing a conductive compound such as a conductive polymer, indium oxide, or tin oxide.

The undercoat layer formed as necessary is used when it is necessary to supplement the adhesion between the conductive substrate and the photosensitive layer. The undercoat layer may be made of a conventionally used material, and its thickness may be 0.05 to 20 μm.

The charge transport layer is a layer containing a charge transport substance that transports charges generated in the charge generation layer. The charge transport material is not particularly limited, and conventionally used materials can be used. The thickness of the charge transport layer can be 5 to 50 μm, preferably 10 to 30 μm.

The charge generation layer is a layer containing a charge generation substance. The charge generating substance is not particularly limited, and conventionally used substances can be used. The thickness of the charge generation layer can be 0.5 to 15 μm, preferably 0.7 to 10 μm.

When a photosensitive layer containing a charge generating substance and a charge transporting substance is formed, the charge generating substance and the charge transporting substance are dispersed or dissolved in a resin. As the resin used here, conventionally used resins can be used. The thickness of the photosensitive layer in which a charge generating substance and a charge transporting substance are mixed can be 5 to 60 μm, preferably 10 to 40 μm.

(protective layer)
As the positively charging organic photoreceptor, a laminated type positively charging organic photoreceptor having at least a protective layer on the surface can also be used. Having the above-mentioned protective layer further increases the durability of the photoreceptor. As mentioned above, highly durable photoreceptors tend to have poor cleaning performance, but by using the toner containing the external additive of the present invention, a polishing effect is exhibited and the occurrence of filming is prevented, and as a result, Images can be stably output for a longer period than before.

The protective layer may be made of any conventionally used binder resin, such as a heat or photocurable resin as the binder resin, metal oxide fine particles, lubricant particles, an antioxidant, or a binder resin other than the binder resin, if necessary. A material containing resin or the like can be used. The thickness of the protective layer is preferably 0.2 to 10 μm, more preferably 0.5 to 5 μm.

(Metal oxide fine particles)
The protective layer contains, for example, alumina (Al ₂ O ₃ ), tin oxide (SnO ₂ ), titanium dioxide (TiO ₂ ), etc., with a number average primary particle size of 1 to 300 nm, for the purpose of imparting higher durability. metal oxide fine particles may be contained.

The image forming apparatus of the present invention includes at least a charging means, an exposure means, a developing means, a transfer means, and a cleaning means in addition to the positively charging photoreceptor, and the developing means is provided with at least the toner of the present invention. . Conventionally known means can be used as the charging means, exposure means, developing means other than the toner of the present invention, such as a mag roll or toner supply, transfer means, and cleaning means.

As an image forming method using the image forming apparatus, for example, a uniform charge is applied to the surface of the photoreceptor, and then an electrostatic latent image corresponding to the image is formed by exposing the photoreceptor to light according to the image. A toner with a predetermined charge is attached to this electrostatic latent image to make it visible (developed), and the toner image formed on the photoreceptor is transferred to paper, etc. via an intermediate transfer body as necessary. It is possible to transfer the image to an image forming support and fix it on the image forming support using heat or the like. The toner of the present invention is preferably used for developing such an electrostatic latent image, and is also preferably used as a positively charged toner used together with a positively chargeable photoreceptor.

After the transfer, the untransferred toner on the surface of the photoreceptor is neutralized and removed by cleaning. According to the present invention, during this cleaning, the toner to which the external additive of the present invention has been added is easily removed from the photoreceptor, so that residuals such as oxides are difficult to form, and as a result, the amount of moisture in the atmosphere is reduced. Images of excellent image quality can be stably formed over a long period of time even under various environmental fluctuations.

Hereinafter, the present invention will be explained in more detail with reference to Examples. The present invention is not limited by the following examples, and can be implemented with appropriate changes within the scope that can meet the spirit described above and below, and all of these are within the technical scope of the present invention. included in

In this example, the evaluation items, such as the number average primary particle diameter of the metatitanic acid fine particles, the degree of hydrophobicity of the external additive, and the amount of charge, were measured as follows.

(Number average primary particle diameter)
By photographing metatitanic acid powder with a transmission electron microscope (TEM) at a magnification of 10,000 times and performing image analysis on 100 particles on the photograph, the average value of Feret's horizontal particle diameter, that is, Feret's The average particle diameter in the horizontal direction was determined, and this was taken as the number average primary particle diameter.

(Hydrophobicity)
A plurality of ethanol-water mixtures in which the volume ratio of ethanol/water was varied in steps of 5 between 0/100 and 100/0 were prepared in an amount of 50 mL each. While stirring with a magnetic stirrer, 0.20 g of hydrophobized metatitanic acid fine particles were added, and the volume ratio of ethanol in the mixture when the entire amount was wetted in 5 minutes was defined as the degree of hydrophobization of the sample. Note that in consideration of environmental stability, the degree of hydrophobicity is preferably 5% or more. By setting the degree of hydrophobicity to 5% or more, it is possible to suppress the leakage of the charge of the toner and suppress the decrease in the amount of charge, especially during long-term storage under high temperature and high humidity. The upper limit of the degree of hydrophobicity can be, for example, 60%.

(Charge amount)
19.5 g of iron powder carrier and 0.5 g of external additive were placed in a 20 mL glass container, and the container was stored in a normal temperature and normal humidity environment at 20° C. and 50% RH for 24 hours. After taking out the glass container, it was covered with a lid and shaken in a paint shaker for 30 minutes. After shaking, 1 g of the mixed sample was taken, and the sample was blown with nitrogen for 10 seconds using a blow-off charge measuring device (Model 230TO, manufactured by Trek Japan Co., Ltd.), and the value obtained was taken as the charge amount.

[Evaluation using first image forming device]
(Preparation of metatitanic acid fine particles)
A titanyl sulfate aqueous solution (titanyl sulfate concentration 10% by mass) was heated to 130° C. and thermally hydrolyzed to obtain a sulfuric acid aqueous solution slurry of metatitanic acid. The obtained slurry of metatitanic acid in sulfuric acid aqueous solution was subjected to solid-liquid separation by filtration. Then, the filter cake obtained by washing with water was dried at a temperature of 120° C. for 40 hours. The obtained dried product was pulverized with a hammer mill to obtain metatitanic acid fine particles. Here, the heating conditions and the like were adjusted to obtain metatitanic acid fine particles having various number average primary particle diameters. Table 1 shows the obtained metatitanic acid fine particles.

As comparative example fine particles, titanium oxide fine particles were prepared as follows. That is, a titanyl sulfate aqueous solution (titanyl sulfate concentration: 10% by mass) was heated to 130° C. and thermally hydrolyzed to obtain a sulfuric acid aqueous solution slurry of metatitanic acid. The obtained slurry of metatitanic acid in sulfuric acid aqueous solution was subjected to solid-liquid separation by filtration. Then, the filter cake obtained by washing with water was dried at a temperature of 120°C for 40 hours, and further dried at a temperature of 500°C for 4 hours. The obtained dried product was pulverized with a hammer mill to obtain titanium oxide fine particles having a number average primary particle size of 30 nm. The number average primary particle diameter was measured by the method described above.

(Manufacture of external additive of the present invention)
The above various metatitanic acid fine particles were treated with various hydrophobizing agents shown in Table 2 (hydrophobic treatment). The details are as follows.

First, external additives 1 to 12 of the present invention shown in Table 2 were obtained as follows. That is, the amino-based silane coupling agent shown in Table 2 was added as a hydrophobizing agent to a slurry in which the metatitanic acid fine particles were dispersed in water at a solid concentration of 10% by mass, and the content of the metatitanic acid fine particles was as shown in Table 2. They were added in the proportions shown and heated to 80°C. After reaching 80°C, the temperature is maintained for 1 hour, followed by solid-liquid separation by filtration and drying to obtain metatitanic acid fine particles having a hydrophobized product prepared using a hydrophobizing agent, that is, the particles of the present invention. External additives (external additives 1 to 12 of the present invention) were obtained.

In Table 2, as 3-aminopropyltriethoxysilane, amino-based silane coupling agent KBE-903 manufactured by Shin-Etsu Chemical Co., Ltd. was used, and as N-2-(aminoethyl)-3-aminopropyltrimethoxysilane, KBM-603 was used.

In addition, external additives 13 to 15 of the present invention shown in Table 2 were obtained as follows. That is, amino-modified silicone oil was added as a hydrophobizing agent together with the amino-based silane coupling agent to a slurry in which the metatitanic acid fine particles were dispersed in water at a solid content concentration of 10% by mass. In this case, the amino-based silane coupling agent was added to the slurry as described above, and after solid-liquid separation, amino-modified silicone oil was kneaded into the filter cake before drying using a kneader. In detail, as the amino-modified silicone oil, amino-modified silicone oil KF-862 (main component: amino-modified organopolysiloxane) manufactured by Shin-Etsu Chemical Co., Ltd. was used at the ratio shown in Table 2 with respect to the content of metatitanic acid fine particles. The mixture was added to the mixture in such a manner that the mixture was mixed, stirred for 1 hour at room temperature using a pressure kneader, and then subjected to surface treatment.

(Manufacture of external additive in comparative example)
The titanium oxide fine particles were surface-treated with an amino-based silane coupling agent as a hydrophobizing agent in the same manner as external additive 1 of the present invention to obtain an external additive of comparison 1. Further, the external additive of Comparative 2 was prepared by using the titanium oxide fine particles as they were without surface treatment.

The degree of hydrophobicity and amount of charge of the obtained external additive were measured as described above. The results are also listed in Table 2.

(Manufacture of colored resin particles)
Colored resin particles for use in toner were manufactured. The resin, colorant, mold release agent, and charge control agent shown in Table 3 were weighed in the amounts shown in Table 3, premixed dry, and then melted and kneaded in a twin-screw extruder. Then, the mixture was pulverized with a mechanical pulverizer and classified with a stopper classifier to obtain colored resin particles (colored resin particles No. 1 to 5) having the volume average particle diameter shown in Table 3.

In Table 3, the styrene-acrylic resin has a Tg of 56°C and a softening point of 126°C, and the polyester resin has a Tg of 55°C and a softening point of 120°C. Raven 330 manufactured by Cabot was used as carbon black. The paraffin wax in Table 3 has a melting point of 98°C. Further, as the quaternary amine charge control agent, TP-415 manufactured by Hodogaya Chemical Co., Ltd. was used, and as the azo iron complex, T-77 manufactured by Hodogaya Chemical Co., Ltd. was used.

(Manufacture of toner)
The colored resin particles obtained as described above, external additives 1 to 15 of the present invention shown in Table 2, comparative external additives 1 and 2, and additional external additives shown in Table 4 (small inorganic fine particles, large particle and inorganic fine particles) were mixed in a Henschel mixer in the proportions shown in Table 4 (all proportions relative to colored resin particles) to obtain toners (invention toners 1 to 18 and comparative toners 1 to 10). Hydrophobic silica RA-200H (number average primary particle size: 12 nm) manufactured by Nippon Aerosil Co., Ltd. and RX200 (number average primary particle size: 12 nm) manufactured by Nippon Aerosil Co., Ltd. were used as small particle size inorganic fine particles. In addition, as large-sized inorganic fine particles, NA-50H (number average primary particle size is 30 nm) manufactured by Nippon Aerosil Co., Ltd. and NAX-50 (number average primary particle size is 30 nm) manufactured by Nippon Aerosil Co., Ltd. were used.
As an example of the obtained toner of the present invention, a micrograph (magnification: 5,000 times) observed with a scanning electron microscope is shown in FIG. In FIG. 1, it can be seen that the external additive of the present invention is uniformly dispersed on the surface of the toner base particles.

(Toner characteristic evaluation)
Toners 1 to 15 of the present invention and comparative toners 1 and 2 were used as a first image forming device in a multifunction machine (TASKalfa 620 (62-sheet machine) manufactured by Kyocera Document Solutions) equipped with an amorphous silicon photoreceptor, and In a humid environment (temperature: 35° C., relative humidity: 80% RH), 300,000 sheets were printed in an intermittent mode at a pixel ratio of 5%, and the image density and fog of the 300,000th print were evaluated. The image density is the average value of the image density obtained by measuring the reflection density of a solid black image at 10 locations and subtracting the reflection density of the paper, and if this image density is less than 1.20, it is not practical. A case where the above concentration was 1.20 or more was judged to be practical and a pass. The above-mentioned fog is the average value of the image density obtained by measuring the reflection density at 10 locations on a blank paper portion and subtracting the reflection density of the paper. If the fog density was less than 0.005, it was determined to be at a practical level and passed, and if the fog density was 0.005 or more, it was determined to be not at a practical level and rejected.

Furthermore, after printing 300,000 sheets, the first printed sheet was left to stand for one day and night, and the presence or absence of image deletion was evaluated as image density, fog, and image quality. The image deletion was evaluated by printing a 30% halftone image and visually determining the reproducibility of the halftone. When unevenness occurred on the halftone, it was determined that there was image deletion, and when no unevenness occurred on the halftone, it was determined that there was no image deletion. These results are shown in Table 5.

Note that the above-mentioned "leaving overnight" is a necessary operation in evaluating the presence or absence of image deletion (image defect). As mentioned above, a photoreceptor with high hardness has low abrasiveness, and an oxide film is likely to be formed on the surface of the photoreceptor due to repeated use. The oxide film absorbs moisture from the air, and the surface of the photoreceptor, which used to be insulating, becomes conductive, and the charges formed on the surface of the photoreceptor are diffused, causing image blurring. (image defects) are likely to occur. In this evaluation, in order to evaluate the presence or absence of this image blurring, an operation of "leaving overnight" is performed to encourage moisture absorption.

From the results in Table 5, it is clear that the toner of the present invention has no image density, no fogging, and no image bleeding even when used for a long period of time, and good images can be stably obtained over a long period of time. I understand.

[Evaluation using second image forming device]
The second image forming device was Fuji Xerox's Color 1000 Press (100-sheet machine), which uses a layered organic photoreceptor with a surface layer formed with a crosslinked structure and a protective layer containing a charge transport material with polyfunctional reactivity. )It was used. In this image forming apparatus, 800,000 sheets were printed in an intermittent mode at a pixel rate of 5% in a high temperature and high humidity environment (temperature: 35°C, relative humidity 80% RH) using the toner shown in Table 6. The image density and fog of the 800,000th print were evaluated. In addition, the image density, fog, and image quality (presence or absence of image bleeding) of the first printed matter printed after printing 800,000 copies and leaving it for one day and night were evaluated. The results are shown in Table 6.

From the results in Table 6, it is clear that the toner of the present invention has good image density, no fogging, and no image smearing, even in image forming apparatuses using laminated organic photoreceptors, even in long-term use. Images could be stably obtained over a long period of time. On the other hand, with the toner of the comparative example, image density, fogging, and image smearing occur during long-term use of an image forming apparatus using a laminated organic photoreceptor, making it difficult to obtain good images over a long period of time. I couldn't.

[Evaluation using third image forming device]
As the third image forming apparatus, a full color printer HL-L9310CDW (31 sheets) manufactured by Brother Industries, Ltd. using a positively charging organic photoreceptor was used. In this image forming apparatus, toner of the present invention 1 (black), toner of the present invention 22 (cyan), toner of the present invention 23 (magenta), and toner of the present invention 24 (yellow) were used in combination. Also used were the combinations of Comparative Toner 1 and Comparative Toners 5 to 7, and the combinations of Comparative Toner 1 and Comparative Toners 8 to 10. Using a full-color evaluation pattern with 5% pixels each for yellow, magenta, cyan, and black, intermittent printing was performed on 30,000 sheets at 33°C and 80% RH under high temperature and high humidity conditions. Image density and fog were evaluated at the initial stage and after 30,000 copies. In addition, the image density, fog, and image quality (presence or absence of image bleeding) of the first printed material, which was printed after printing 30,000 copies and leaving it for one day and night, were evaluated. The results are shown in Table 7. Note that the image density was determined by measuring the density of a solid black image.

From the results in Table 7, it is clear that even when multiple toners are used together, the toner of the present invention provides good image density, no fogging, and no image smearing even when used for a long period of time. was obtained over a long period of time. Further, the results of experiments using the toner of the present invention showed that there was almost no change in the color gamut of color images from the initial stage to after 30,000 sheets were printed. On the other hand, when the comparative toners were used, there was a change in the color gamut, and the quality of full-color images was greatly reduced.

Claims (14)

For use in toner, the surface of metatitanic acid fine particles having a number average primary particle diameter of 10 to 300 nm has a hydrophobized material treated with a hydrophobizing agent containing an amino-based silane coupling agent and an amino-modified silicone oil . External additives. The external additive for toner according to claim 1 , wherein the mass ratio of the amino-based silane coupling agent to the amino-modified silicone oil is 20:1 to 1:1. The external additive for toner according to claim 1 or 2 , wherein the proportion of the hydrophobizing agent is 1 to 40% by mass of the content of metatitanic acid fine particles in the external additive. The external additive for toner according to any one of claims 1 to 3 , which is positively charged. The external additive for toner according to any one of claims 1 to 4 , having a degree of hydrophobicity of 5 to 60%. A toner comprising colored resin particles containing at least a colorant and a resin, and the external additive for toner according to any one of claims 1 to 5 . The toner according to claim 6 , wherein the proportion of the external additive for toner is 0.05 to 2.0% by mass of the content of colored resin particles in the toner. The toner according to claim 6 or 7 , wherein the colored resin particles further contain a release agent and a charge control agent. The toner according to any one of claims 6 to 8 , further comprising small inorganic fine particles having a number average primary particle size of 5 to 20 nm as an additional external additive. The toner according to any one of claims 6 to 9 , further comprising large inorganic fine particles having a number average primary particle diameter of 25 to 1000 nm as an additional external additive. An image forming apparatus comprising at least a positively charging photoreceptor, a charging means, an exposure means, a developing means, a transfer means, and a cleaning means,
An image forming apparatus comprising at least the toner according to claim 6 as said developing means. The image forming apparatus according to claim 11 , wherein the positively charged photoreceptor is an amorphous silicon photoreceptor. The image forming apparatus according to claim 11 , wherein the positively chargeable photoreceptor is a positively chargeable organic photoreceptor. The image forming apparatus according to claim 11 , wherein the positively chargeable photoreceptor is a laminated positively chargeable organic photoreceptor having a protective layer on its surface.