In a two-component developer, external additive particles of toner particles include resin particles containing a thermoplastic resin. Coat layers of carrier particles contain a coating resin and barium titanate particles. The coating resin includes a silicone resin. The barium titanate particles have a number average primary particle diameter of 100-500 nm. The barium titanate particles have a content of 4-45 parts by mass relative to 100 parts by mass of the coating resin. The rate of the mass of the coat layers to the mass of the carrier cores is 0.10-4.90% by mass. The average of coverage rates of the carrier cores is at least 80.0% and less than 100.0%.

TECHNICAL FIELD

The present invention relates to a two-component developer.

BACKGROUND ART

Image forming apparatuses for forming images with toner are required to stably charge the toner to a desired charge amount in order to stably form images with desired image density. For example, the positively chargeable cyan developer disclosed in Patent Literature 1 contains a toner and a carrier containing core particles and resin coats provided on the surfaces of the core particles in order to inhibit overcharging of the toner. The surfaces of the core particles have a resin coat coverage rate of 60% to 90%.

CITATION LIST

Patent Literature

SUMMARY OF INVENTION

Technical Problem

However, there is room for the positively chargeable cyan developer disclosed in Patent Literature 1 in terms of improvement on fog resistance, stable formation of images with desired image density, and inhibition of occurrence of image defects resulting from cleaning failure.

The present invention has been made in view of the foregoing and has its object of providing a two-component developer that contributes to excellent fog resistance, stable formation of images with desired image density, and inhibition of occurrence of image defects resulting from cleaning failure.

Solution to Problem

A two-component developer according to the present invention contains a toner containing toner particles and a carrier containing carrier particles. The toner particles each include a toner mother particle and external additive particles provided on a surface of the toner mother particle. The external additive particles include resin particles. The resin particles contain a thermoplastic resin. The carrier particles each include a carrier core and a coat layer covering a surface of the carrier core. The coat layers contain a coating resin and barium titanate particles. The coating resin includes a silicone resin. The barium titanate particles have a number average primary particle diameter of at least 100 nm and no greater than 500 nm. The barium titanate particles have a content of at least 4 parts by mass and no greater than 45 parts by mass relative to 100 parts by mass of the coating resin. A rate of a mass of the coat layers to a mass of the carrier cores is at least 0.10% by mass and no greater than 4.90% by mass. An average of coverage rates of the carrier cores is at least 80.0% and less than 100.0%. Each of the coverage rates is a rate of an area of a covered region of the carrier core covered with the coat layer to an area of the surface of the carrier core.

Advantageous Effects of Invention

According to the present invention, the two-component developer can contribute to excellent fog resistance, stable formation of images with desired image density, and inhibition of occurrence of image defects resulting from cleaning failure.

DESCRIPTION OF EMBODIMENTS

The meanings of the terms used in the present description and measurement methods are described first. A toner is a collection (e.g., a powder) of toner particles. An external additive is a collection (e.g., a powder) of external additive particles. A carrier is a collection (e.g., a powder) of carrier particles. Unless otherwise stated, evaluation results (values indicating shape or physical properties) for a powder (specific examples include a powder of toner particles, a powder of external additive particles, and a powder of carrier particles) are number averages of values as measured for a suitable number of particles selected from the powder. The “main component” of a material means a component most abundant in the material in terms of mass unless otherwise stated. The level of hydrophobicity (or hydrophilicity) can be expressed by a contact angle of a water droplet (ease of getting wet with water), for example. A lager contact angle of a water droplet indicates a higher level of hydrophobicity. The term “-based” may be appended to the name of a chemical compound in order to form a generic name encompassing both the chemical compound itself and derivatives thereof. When the term “-based” is appended to the name of a chemical compound used in the name of a polymer, the term indicates that a repeating unit of the polymer originates from the chemical compound or a derivative thereof. The term “(meth)acryl” may be used as a generic term for both acryl and methacryl. The term “(meth)acrylonitrile” may be used as a generic term for both acrylonitrile and methacrylonitrile. One type of each component described in the present specification may be used independently, or two or more types of the component may be used in combination.

The measurement value for volume median diameter (D50) of a powder is a median diameter of the powder as measured using a laser diffraction/scattering type particle size distribution analyzer (“LA-950”, product of HORIBA, Ltd.) unless otherwise stated. Unless otherwise stated, the number average particle diameter of a powder is a number average value of equivalent circle diameters (Heywood diameters: diameters of circles having the same areas as projected areas of the primary particles) of primary particles of the powder as measured using a scanning electron microscope. The number average primary particle diameter is a number average value of equivalent circle diameters of 100 primary particles, for example. The softening point (Tm) is a value as measured using a capillary rheometer (“CFT-500D”, product of Shimadzu Corporation) unless otherwise stated. On an S-shaped curve (vertical axis: temperature, horizontal axis: stroke) as plotted using the capillary rheometer, the softening point corresponds to the temperature corresponding to a stroke value of “(base line stroke value+maximum stroke value)/2”. The melting point (Mp) is a temperature at a maximum endothermic peak on an endothermic curve (vertical axis: heat flow (DSC signal), horizontal axis: temperature) as plotted using a differential scanning calorimeter (“DSC-6220”, product of Seiko Instruments Inc.) unless otherwise state. The endothermic peak appears due to melting of the crystallization site. The glass transition point (Tg) is a value as measured in accordance with “Japanese Industrial Standard (JIS) K7121-2012” using a differential scanning calorimeter (“DSC-6220”, product of Seiko Instruments Inc.) unless otherwise stated. The glass transition point corresponds to the temperature corresponding to a point of inflection (specifically, an intersection point of an extrapolated baseline and an extrapolated falling line) caused by glass transition on a heat absorption curve (vertical axis: heat flow (DSC signal), horizontal axis: temperature) as plotted using the differential scanning calorimeter. Measurement values for acid value and hydroxyl value are values as measured in accordance with the “Japanese Industrial Standards (JIS) K0070-1992” unless otherwise stated. Measurement values for mass average molecular weight (Mw) are values as measured by gel permeation chromatography unless otherwise stated. The charge amount (unit: μC/g) is a value as measured in an environment at a temperature of 25° C. and a relative humidity of 50% using a compact toner draw-off charge measurement system (“MODEL 212HS”, product of TREK, INC.) unless otherwise stated. Unless otherwise stated, the level of chargeability is the ease of triboelectric charging to a standard carrier provided by The Imaging Society of Japan. For example, a measurement target is stirred together with a standard carrier (anionicity: N-01, cationicity: P-01) provided by The Imaging Society of Japan to triboelectrically charge the measurement target. The charge amount per unit mass of the measurement target is measured before and after triboelectric charging using for example a Q/m meter (“MODEL 212HS”, product of TREK, INC.). A larger change in charge amount per unit mass between before and after triboelectric charging indicates a higher chargeability of the measurement target. The meanings of the terms used in the present description and the measurement methods have been described so far.

The following describes a two-component developer (also referred to below as a developer)1according to an embodiment of the present invention with reference toFIG.1.FIG.1illustrates the developer1according to the present embodiment. Note that a plurality of identical elements are indicated by the same hatching and one of these identical elements is labeled with a reference sign while the other identical elements are indicated with the reference sign omitted.

The developer1contains a toner and a carrier. The toner contains toner particles10. The carrier contains carrier particles20. The toner particles10each include a toner mother particle11and external additive particles12. The external additive particles12are provided on the surface of the toner mother particle11. The external additive particles12include resin particles13. The resin particles13contain a thermoplastic resin. The carrier particles20each include a carrier core21and a coat layer22. The coat layer22covers the surface of the carrier core21. The coat layers22contain a coating resin and barium titanate particles23. The coating resin includes a silicone resin. The barium titanate particles23have a number average primary particle diameter of at least 100 nm and no greater than 500 nm. The barium titanate particles23have a content of at least 4 parts by mass and no greater than 45 parts by mass relative to 100 parts by mass of the coating resin. A rate of the mass of the coat layers22to the mass of the carrier cores21is at least 0.10% by mass and no greater than 4.90% by mass. The average of the coverage rates of the carrier cores21is at least 80.0% and less than 100.0%. Each of the coverage rates of the carrier cores21is a rate of the area of a covered region A1of the carrier core21covered with the coat layer22to the area of the surface of the carrier core21.

In the following, the “rate of the mass of the coat layers22to the mass of the carrier cores21” may be referred to as a “coat layer/core rate”.

As a result of having the above features, the developer1according to the present embodiment can contribute to excellent fog resistance, stable formation of images with desired image density, and inhibition of occurrence of image defects resulting from cleaning failure. Presumably, the reasons therefor are as follows.

The coat layers22of the carrier particles20contain barium titanate particles23in the developer1according to the present embodiment. Since the barium titanate particles23being a ferroelectric have a high specific permittivity, the carrier particles20containing the barium titanate particles23in their coat layers22have high charge retention ability. The carrier particles20with high charge retention ability can provide a sufficient amount of charge to the toner particles10by contact with the toner particles10. Here, where multiple image printing is performed using an image forming apparatus, the toner concentration in the developer1loaded in a development device may vary during printing. However, the carrier particles20with high charge retention ability can provide a sufficient amount of charge to the toner particles10up to the saturation charge of the toner particles10even when the toner concentration in the developer1loaded in the development device increases during printing, thereby increasing the number of charged toner particles10. As a result, variation in charge amount of the toner can be reduced to achieve stable formation of images with desired image density even when the toner concentration in the developer1changes. Furthermore, since the carrier particles20can provide a sufficient amount of charge to the toner particles10, a portion of the toner particles10whose charge amount is less than a desired value and another portion of the toner particles10that are oppositely charged can be reduced, thereby achieving formation of images with less fog.

The barium titanate particles23have a number average primary particle diameter of at least 100 nm and no greater than 500 nm in the developer1according to the present embodiment. When the number average primary particle diameter of the barium titanate particles23is less than 100 nm, the specific permittivity thereof tends to decrease. As a result of the number average primary particle diameter of the barium titanate particles23being set to at least 100 nm, the specific permittivity of the barium titanate particles23is sufficiently high. As a result of including the coat layers22containing the barium titanate particles23with high specific permittivity, the carrier particles20can provide a sufficient amount of charge to the toner particles10. As a result, toner particles10whose charge amount is less than the desired value and toner particles10that are oppositely charged can be reduced, thereby achieving formation of images with less fog. As a result of the number average primary particle diameter of the barium titanate particles23being set to no greater than 500 nm by contrast, the barium titanate particles23will sink into the coat layers22and are hardly detached from the coat layers22. Accordingly, a phenomenon in which the barium titanate particles23become detached to be transported to a gap between a photosensitive drum and a cleaning blade can be inhibited. As a result, cleaning failure and ultimately image defects resulting therefrom will hardly occur.

The barium titanate particles23have a content of at least 4 parts by mass and no greater than 45 parts by mass relative to 100 parts by mass of the coating resin in the developer1according to the present embodiment. As a result of the content of the barium titanate particles23being set to at least 4 parts by mass relative to 100 parts by mass of the coating resin, the amount of the barium titanate particles23in the coat layers22increases to enhance charge retention ability of the carrier particles20. The carrier particles20with high charge retention ability can provide a sufficient amount of charge to the toner particles10by contact with the toner particles10. Therefore, variation in charge amount of the toner can be reduced to achieve stable formation of images with desired image density even when the toner concentration in the developer1changes. Furthermore, since the carrier particles20can provide a sufficient amount of charge to the toner particles10, toner particles10whose charge amount is less than the desired value and toner particles10that are oppositely charged can be reduced, thereby achieving formation of less fog. As a result of the content of the barium titanate particles23being set to no greater than 45 parts by mass relative to 100 parts by mass of the coating resin by contrast, the barium titanate particles23will sink into the coat layers22and are hardly detached from the coat layers22. Accordingly, a phenomenon in which the detached barium titanate particles23inhibit contact between the toner particles10and the carrier particles20will hardly occur. Thus, a sufficient amount of charge can be provided to the toner particles10from the carrier particles20. As a result, variation in charge amount of the toner can be reduced to achieve stable formation of images with desired image density even when the toner concentration in the developer1changes. Furthermore, since the carrier particles20can provide a sufficient amount of charge to the toner particles10, toner particles10whose charge amount is less than the desired value and toner particles10that are oppositely charged can be reduced, thereby achieving formation of less fog.

The carrier particles20have a coat layer/core rate of at least 0.10% by mass and no greater than 4.90% by mass in the developer1according to the present embodiment. As a result of the coat layer/core rate being set at no greater than 4.90% by mass, the coat layers22can be suitably thin. The coating resin contained in the coat layers22is hygroscopic. When the coat layers22are suitably thin, the amount of the coating resin decreases to reduce influence (e.g., influence of decreasing triboelectric charge amount of the toner particles10) on triboelectric charging caused by the coating resin absorbing moisture. Furthermore, as a result of the coat layer/core rate being set to no greater than 4.90% by mass, agglomeration of the carrier particles20can be inhibited in formation of the coat layers22in a later-described carrier formation process. Non-agglomerated or less agglomerated carrier particles20can cause favorable triboelectric charging with a result that the toner particles10can be charged to the desired charge amount. As a result, toner particles10whose charge amount is less than the desired value and toner particles10that are oppositely charged can be reduced, thereby achieving formation of images with less fog. As a result of the coat layer/core rate being set to at least 0.10% by mass by contrast, the coat layers22are not excessively thin. As a result, the toner particles10can be triboelectrically charged to the desired charge amount by contact between the toner particles10and the coat layers22of the carrier particles20. Thus, toner particles10whose charge amount is less than the desired value and toner particles10that are oppositely charged can be reduced, thereby achieving formation of images with less fog.

The average of the coverage rates of the carrier cores21is at least 80.0% and less than 100.0% in the developer1according to the present embodiment. The coverage rates of the carrier cores21are less than 100.0%, which means they are not 100.0%. Therefore, the coat layers22do not completely cover the entire surface of the coat layers22. The coat layers22partially cover the surfaces of the carrier cores21. As illustrated inFIG.1, a carrier particle20has covered regions A1and non-covered regions A2. The covered regions A1each are a region of the surface of the carrier core21that is covered with a coat layer22. The non-covered regions A2each are a region of the surface of the carrier core21that is not covered with the coat layer22. The carrier core21is exposed in the non-covered regions A2without being covered with the coat layer22.

The average of the coverage rates of the carrier cores21is less than 100.0%, which means it is not 100.0%. Therefore, the non-covered regions A2not covered with the coat layers22are present in the carrier cores21. The coating resin contained in the coat layers22is hygroscopic. Presence of the non-covered regions A2can reduce influence (e.g., influence of decreasing triboelectric charge amount of the toner particles10) on triboelectric charging caused by the coating resin absorbing moisture. Furthermore, the non-covered regions A2not covered with the coat layers22containing the coating resin have a low electric resistance, so charges can easily move through the non-covered regions A2. Presence of the non-covered regions A2through which charge easily moves can triboelectrically charge the toner particles10to the desired charge amount within a short period of time by contact with the carrier particles20. Furthermore, presence of the non-covered regions A2through which charge easily moves can make the toner particles10not excessively charged triboelectrically by contact with the carrier particles20. As a result, toner particles10whose charge amount is less than the desired value and toner particles10that are oppositely charged can be reduced, thereby achieving formation of images with less fog.

Here, the non-covered regions A2are present in a dispersed manner in the surfaces of the carrier particles20. Contact of the toner particles10with the covered regions A1present around the non-covered regions A2can triboelectrically charge the toner particles10to the desired charge amount. However, when the average of the coverage rates of the carrier cores21is less than 80.0%, the covered regions A1are too narrow. Therefore, it is difficult to triboelectrically charge the toner particles10to the desired charge amount even upon contact with the carrier particles20. As a result of the average of the coverage rates of the carrier cores21being set to at least 80.0%, the toner particles10can be triboelectrically charged to the desired charge amount by contact with the carrier particles20. Thus, toner particles10whose charge amount is less than the desired value and toner particles10that are oppositely charged can be reduced, thereby achieving formation of images with less fog.

The external additive particles12of the toner particles10include resin particles13that contain a thermoplastic resin in the developer1according to the present embodiment. The coat layers22contain the barium titanate particles32which are hard, and therefore the carrier particles20are relatively hard. The resin particles13of the toner particles10function as a spacer in contact between the toner particles10and the carrier particles20. As such, even when the carrier particles20are relatively hard, the external additive particles12(e.g., optional external additive particles14, and particularly, silica particles contributing to charging) are hardly buried in the surfaces of the toner mother particles11by contact with the carrier particles20, thereby inhibiting the charge amount of the toner particles10from being lower than the desired value. Thus, images with less fog can be formed.

The reasons why the developer1according to the present embodiment can contribute to excellent fog resistance, stable formation of images with desired image density, and inhibition of occurrence of image defects resulting from cleaning failure have been described so far.

In addition to the above advantages, scraping of the coat layers22can be reduced as a result of the coat layers22containing the hard barium titanate particles23, thereby extending lifespan of the carrier particles20in the developer1according to the present embodiment. Furthermore, as a result of the resin particles13containing a thermoplastic resin which softens by heat, the toner particles10can be favorably fixed to recording mediums. Next, the toner and the carrier that are contained in the developer1are described further in detail.

The toner contains toner particles10. The toner particles10each include a toner mother particle11and external additive particles12. The external additive particles12are provided on the surface of the toner mother particle11. The external additive particles12and the toner mother particles11are described below.

The external additive particles12include resin particles13. The external additive particles12may further include external additive particles (also referred to below as optional external additive particles)14other than the resin particles13as necessary. The resin particles13and the optional external additive particles14are described below.

The resin particles13contains a thermoplastic resin. Examples of the thermoplastic resin contained in the resin particles13include polyester resins, styrene-based resins, acrylic acid ester-based resins (specific examples include acrylic acid ester polymers and methacrylic acid ester polymers), olefin-based resins (specific examples include polyethylene resin and polypropylene resin), vinyl resins (specific examples include vinyl chloride resin, polyvinyl alcohol, vinyl ether resin, and N-vinyl resin), polyamide resins, and urethane resins. Any copolymer of these resins, that is, any copolymer (more specific examples include styrene-acrylic resin and styrene-butadiene-based resin) with any repeating unit introduced into any of the resins may be used as the thermoplastic resin contained in the resin particles13.

In order to favorably fix the toner particles10to the recording mediums, the thermoplastic resin contained in the resin particles13is preferably a styrene-acrylic resin. The styrene-acrylic resin is a copolymer of at least one styrene-based monomer and at least one acrylic acid-based monomer. That is, the styrene-acrylic resin includes at least one repeating unit derived form a styrene-based monomer and at least one repeating unit derived from an acrylic acid-based monomer.

Examples of the acrylic acid-based monomer include (meth)acrylic acid, (meth)acrylonitrile, (meth)acryl acid alkyl esters, and (meth)acrylic acid hydroxyalkyl esters. Examples of the (meth)acryl acid alkyl esters include alkyl esters having a carbon number of at least 1 and no greater than 8 of a (meth)acrylic acid, and more specific examples include methyl (meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, iso-propyl (meth)acrylate, n-butyl (meth)acrylate, iso-butyl (meth)acrylate, and 2-ethylhexyl (meth)acrylate. Examples of the (meth)acrylic acid hydroxyalkyl esters include 2-hydroxyethyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, and 4-hydroxybutyl (meth)acrylate. The acrylic acid-based monomer is preferably (meth)acrylic acid alkyl ester, more preferably alkyl ester having a carbon number of at least 1 and no greater than 8 of (meth)acrylic acid, further preferably butyl (meth)acrylate, and further more preferably n-butyl (meth)acrylate or iso-butyl (meth)acrylate.

In order to favorably fix the toner particles10to the recording mediums, the styrene-acrylic resin is preferably a copolymer of styrene and (meth)acrylic acid alkyl ester, more preferably a copolymer of styrene and alkyl ester having a carbon number of at least 1 and no greater than 8 of (meth)acrylic acid, further preferably a copolymer of styrene and butyl(meth)acrylate, and further more preferably a copolymer of styrene and butyl methacrylate.

The glass transition point of the styrene-acrylic resin tends to decrease as the amount (blending ratio) of the acrylic acid-based monomer increases relative to the amount of the styrene-based monomer. In order to favorably fix the toner particles10to the recording mediums, the rate of the amount of the repeating unit derived from the acrylic acid-based monomer to the total amount of the repeating unit derived from the styrene-based monomer and the repeating unit derived from the acrylic acid-based monomer is preferably at least 50% by mol and no greater than 95% by mol, more preferably at least 70% by mol and no greater than 90% by mol, and particularly preferably 80% by mol.

In order to favorably fix the toner particles10to the recording mediums, the thermoplastic resin may not include a repeating unit (crosslinked structure) derived from a crosslinking agent. Examples of the repeating unit derived from a crosslinking agent include repeating units derived from a compound having two or more vinyl groups. Specific examples thereof include repeating units derived from a compound (divinyl compound) having two vinyl groups. Further specific examples thereof include a repeating unit derived from divinylbenzene. In order to favorably fix the toner particles10to the recording mediums, the thermoplastic resin is preferably a styrene-acrylic resin not including a repeating unit derived from a compound having two or more vinyl groups. For the same purpose as above, the thermoplastic resin preferably includes as a repeating unit only a repeating unit derived from a compound having one vinyl group, and further preferably a styrene-acrylic resin including as a repeating unit only a repeating unit derived from a compound having one vinyl group.

In order to enhance the spacer function and in order that the toner has excellent heat-resistance preservability, the resin particles13have a number average primary particle diameter of preferably at least 30 nm and no greater than 120 nm, more preferably at least 40 nm and no greater than 100 nm, and further preferably at least 60 nm and no greater than 80 nm. The number average primary particle diameter of the resin particles13can be measured using a scanning electron microscope, for example.

The number average primary particle diameter of the resin particles13can be adjusted for example by changing the reaction time and the stirring speed in polymerization reaction of the monomers. The longer the reaction time is, the larger the number average primary particle diameter of the resin particles13tends to be. Also, the lower the stirring speed is, the larger the number average primary particle diameter of the resin particles13tends to be. Table 1 shows reaction examples A to E of the polymerization reaction of the monomers. Reaction examples A to E indicate the relationship between the number average primary particle diameter of the resin particles13obtained by the polymerization reaction and the reaction temperature, the reaction time, and the stirring speed in the polymerization reaction of the monomers. In Table 1, “Diameter” indicates the number average primary particle diameter of the resin particles13.

The amount of the resin particles13is preferably at least 0.1 parts by mass and no greater than 10.0 parts by mass relative to 100.0 parts by mass of the toner mother particles11, and more preferably at least 0.3 parts by mass and no greater than 1.0 parts by mass.

Examples of the optional external additive particles14include silica particles, alumina particles, magnesium oxide particles, and zinc oxide particles. The optional external additive particles14may be surface-treated. For example, when silica particles are used as the optional external additive particles14, either or both hydrophobicity and positive chargeability may be imparted to the surfaces of the silica particles with a surface treatment agent. The optional external additive particles14have a number average primary particle diameter of preferably at least 1 nm and no greater than 60 nm, and more preferably at least 5 nm and no greater than 25 nm. The amount of the optional external additive particles14is preferably at least 0.1 parts by mass and no greater than 10.0 parts by mass relative to 100.0 parts by mass of the toner mother particles11, and more preferably at least 1.0 parts by mass and no greater than 2.0 parts by mass.

The toner mother particles11contain at least one selected from the group consisting of a binder resin, a colorant, a charge control agent, and a releasing agent, for example. The following describes the binder resin, the colorant, the charge control agent, and the releasing agent.

In order that the toner has excellent low-temperature fixability, the toner mother particles11preferably contain a thermoplastic resin as the binder resin, and more preferably contain a thermoplastic resin at a rate of at least 85% by mass of the total of the binder resin. Examples of the thermoplastic resin that can be used as the binder resin are the same as the examples of the thermoplastic resin contained in the resin particles13as described previously.

The binder resin is preferably a polyester resin. The polyester resin is a polymer of at least one polyhydric alcohol monomer and at least one polybasic carboxylic acid monomer. Note that a polybasic carboxylic acid derivative (specific examples include an anhydride of polybasic carboxylic acid and a polybasic carboxylic acid halide) may be used instead of the polybasic carboxylic acid monomer.

Examples of the polyhydric alcohol monomer include diol monomers, bisphenol monomers, and tri- or higher-hydric alcohol monomers.

Examples of the bisphenol monomers include bisphenol A, hydrogenated bisphenol A, bisphenol A ethylene oxide adducts, and bisphenol A propylene oxide adducts.

Examples of the polybasic carboxylic acid monomer include dibasic carboxylic acid monomers and tri- or higher-basic carboxylic acid monomers.

Preferably, the polyester resin is a polymer of a bisphenol monomer, a dibasic carboxylic acid monomer, and a tri-basic carboxylic acid monomer. More preferably, the polyester resin is a polymer of a bisphenol A alkylene oxide adduct, a dicarboxylic acid having a carbon number of at least 3 and no greater than 6, and an aryltricarboxylic acid. The polyester resin is further preferably a polymer of a bisphenol A ethylene oxide adduct, a bisphenol A propylene oxide adduct, fumaric acid, and trimellitic acid.

The polyester resin is preferably a non-crystalline polyester resin. For many non-crystalline polyester resins, it is often not possible to determine a clear melting point. As such, a polyester resin for which no clear endothermic peak cannot be determined on an endothermic curve measured using a differential scanning calorimeter can be determined to be a non-crystalline polyester resin.

The polyester resin has a softening point of preferably at least 50° C. and no greater than 200° C., and more preferably at least 80° C. and no greater than 120° C. The polyester resin has a glass transition point of preferably at least 40° C. and no greater than 100° C. and more preferably is at least 40° C. and no greater than 60° C.

The polyester resin has a mass average molecular weight of preferably at least 10,000 and no greater than 50,000, and more preferably at least 20,000 and no greater than 40,000.

The polyester resin has an acid value of preferably at least 1 mgKOH/g and no greater than 30 mgKOH/g, and more preferably at least 10 mgKOH/g and no greater than 20 mgKOH/g. The polyester resin has a hydroxyl value of preferably at least 1 mgKOH/g and no greater than 50 mgKOH/g, and more preferably at least 20 mgKOH/g and no greater than 40 mgKOH/g.

The amount of the binder resin is preferably at least 85 parts by mass and no greater than 95 parts by mass relative to 100 parts by mass of the toner mother particles11.

The colorant can be a known pigment or dye that matches the color of the toner. Examples of the colorant include black colorants, yellow colorants, magenta colorants, and cyan colorants.

Carbon black can for example be used as a black colorant. Alternatively, a black colorant can be used that has been adjusted to a black color using a yellow colorant, a magenta colorant, and a cyan colorant.

Examples of a cyan colorants that can be used include at least one compound selected from the group consisting of a copper phthalocyanine compound, an anthraquinone compound, and a basic dye lake compound. Examples of the cyan colorant include C.I. Pigment Blue (1, 7, 15, 15:1, 15:2, 15:3, 15:4, 60, 62, or 66), Phthalocyanine Blue. C.I. Vat Blue, and C.I. Acid Blue.

The amount of the colorant is preferably at least 1 part by mass and no greater than 20 parts by mass relative to 100 parts by mass of the binder resin.

The charge control agent is used for example for the purpose of improving charge stability and a charge rise characteristic of the toner. The charge rise characteristic of the toner is an indicator as to whether the toner can be charged to a specific charge level in a short period of time. Examples of the charge control agent include positive charge control agents and negative charge control agents. When a positive charge control agent is contained in the toner mother particles11, cationic strength (positive chargeability) of the toner can be increased. When a negative charge control agent is contained in the toner mother particles11, anionic strength (negative chargeability) of the toner can be increased. Examples of the positive charge control agents include pyridine, nigrosine, and quaternary ammonium salts. Examples of the negative charge control agents include metal-containing azo dyes, sulfo group-containing resins, oil-soluble dyes, naphthenic acid metal salts, acetylacetone metal complexes, salicylic acid-based metal complexes, boron compounds, fatty acid soaps, and long-chain alkyl carboxylates. However, the toner mother particle11does not need to contain a charge control agent where sufficient chargeability is ensured in the toner. The amount of the charge control agent is preferably at least 1 part by mass and no greater than 10 parts by mass relative to 100 parts by mass of the binder resin.

The releasing agent is used for example for the purpose of obtaining a toner excellent in hot offset resistance. Examples of the releasing agent include aliphatic hydrocarbon-based waxes, oxides of aliphatic hydrocarbon-based waxes, plant waxes, animal waxes, mineral waxes, waxes having a fatty acid ester as a main component, and waxes in which a fatty acid ester has been partially or fully deoxidized. Examples of the aliphatic hydrocarbon waxes include polyethylene waxes (e.g., low molecular weight polyethylene), polypropylene waxes (e.g., low molecular weight polypropylene), polyolefin copolymers, polyolefin wax, microcrystalline wax, paraffin wax, and Fischer-Tropsch wax. Examples of the oxides of aliphatic hydrocarbon waxes include oxidized polyethylene waxes and block copolymers of oxidized polyethylene waxes. Examples of the plant waxes include candelilla wax, caranuba wax, Japan wax, jojoba wax, and rice wax. Examples of the animal waxes include bee wax, lanolin, and spermaceti. Examples of the mineral waxes include ozokerite, ceresin, and petrolatum. Examples of the waxes having a fatty acid ester as a main component include montanic acid ester wax and castor wax. Examples of the waxes in which a fatty acid ester has been partially or fully deoxidized include deoxidized carnauba wax. The amount of the releasing agent is preferably at least 1 part by mass and no greater than 20 parts by mass relative to 100 parts by mass of the binder resin.

Note that the toner particles10may contain a known additive as necessary. Preferably, the toner particles10have a volume median diameter of at least 4 μm and no greater than 12 μm. The toner mother particles11have a volume median diameter of preferably at least 4 μm and no greater than 12 μm, and more preferably at least 5 μm and no greater than 9 μm. The toner particles10may be a magnetic toner or anon-magnetic toner. When the toner particles10are a magnetic toner, the toner mother particles11further contain a magnetic powder. The amount of the toner in the developer1is preferably at least 1 part by mass and no greater than 15 parts by mass relative to 100 parts by mass of the carrier, and more preferably at least 3 parts by mass and no greater than 10 parts by mass.FIG.1illustrates a non-capsule toner mother particle11for ease of description. However, capsule toner mother particles may be used each of which include the toner mother particle11illustrated inFIG.1as a toner core and a shell layer covering the toner core. The toner has been descried so far.

The carrier contains carrier particles20. The carrier particles20each include a carrier core21and a coat layer22. The coat layer22covers the surface of the carrier core21. The coat layer22is provided on the surface of the carrier core21.

As described previously, the coat layer/core rate is at least 0.10% by mass and no greater than 4.90% by mass. Preferably, the coat layer/core rate is at least 0.11% by mass. The coat layer/core rate is preferably no greater than 4.60% by mass, more preferably no greater than 4.40% by mass, further preferably no greater than 4.00% by mass, still more preferably no greater than 3.00% by mass, still further preferably no greater than 2.00% by mass, further more preferably no greater than 1.40% by mass, still further more preferably no greater than 1.00% by mass, especially preferably no greater than 0.90% by mass, more especially preferably no greater than 0.50% by mass, further especially preferably no greater than 0.25% by mass, particularly preferably no greater than 0.24% by mass, and more particularly preferably no greater than 0.20% by mass.

The rate of the mass of the coating resin to the mass of the carrier cores21is preferably at least 0.05% by mass and no greater than 4.00% by mass. In the following, the “rate of the mass of the coating resin to the mass of the carrier cores21” may be also referred to as “resin/core rate”. Preferably, the resin/core rate is at least 0.10% by mass. The resin/core rate is preferably no greater than 3.00% by mass, more preferably no greater than 2.00% by mass, further preferably no greater than 1.40% by mass, further more preferably no greater than 1.00% by mass, still further preferably no greater than 0.90% by mass, especially preferably no greater than 0.50% by mass, more especially preferably no greater than 0.25% by mass, particularly preferably no greater than 0.24% by mass, and more particularly preferably no greater than 0.20% by mass.

The coverage rates of the carrier cores21each are a rate of the area of the covered regions A1of a carrier core21covered with a coat layer22to the area of the surface of the carrier core21. The coverage rate of the carrier core21is calculated in a manner that from a surface photographed image of a carrier particle20photographed using a scanning electron microscope, an area of the covered regions A1appearing on the surface photographed image and an area of the non-covered regions A2appearing on the surface photographed image are obtained and a coverage rate is calculated using a formula “(coverage rate)=100×(area of covered regions A1)/(area of surface of carrier core21)=100×(area of covered regions A1)/(total area of covered regions A1and non-covered regions A2)”. Note that the method for adjusting the coverage rates of the carrier cores21is described later in <Carrier Formation Process>.

The average of the coverage rates of the carrier cores21is a number average value calculated in a manner that coverage rates of a considerable number of (e.g., 100) carrier cores21contained in the carrier are measured and an average is calculated using a formula “(average of coverage rates)=(total of coverage rates of measured carrier cores21)/(number of measured charrier cores21)”. The average of the coverage rates of the carrier cores21is at least 80.0% and less than 100.0% as described previously. In order to form images with less fog, the average of the coverage rates of the carrier cores21is preferably at least 85.0%, more preferably at least 90.0%, further preferably greater than 90.0%, further more preferably at least 92.0%, still further preferably at least 95.0%, and particularly preferably at least 96.0%. In order to form images with less fog, the average of the coverage rates of the carrier cores21is preferably no greater than 99.0%.

In order to reduce coverage rate dispersion of the carrier cores21, the standard deviation of the coverage rates of the carrier cores21is preferably no greater than 4.0. The lower limit of the standard deviation of the coverage rates of the carrier cores21is not particularly limited, and the standard deviation of the coverage rates of the carrier cores21is at least 0.5, for example.

In order to reduce coverage rate dispersion of the carrier cores21, the coefficient of variation of the coverage rates of the carrier cores21is preferably no greater than 4.7%, more preferably no greater than 4.0%, and further preferably no greater than 3.0%. The lower limit of the coefficient of variation of the coverage rates of the carrier cores21is not particularly limited, and the coefficient of variation of the coverage rates of the carrier cores21is at least 0.5%, for example. The coefficient of variation (unit. %) of the carrier cores21is calculated using a formula “(coefficient of variation)=100×(standard deviation of coverage rates)/(average of coverage rates)”.

In order to charge the toner to the desired charge amount and form images with less fog, the carrier cores21preferably have a BET specific surface area of at least 0.3 m2/g and no greater than 3.5 m2/g. The BET specific surface area of the carrier cores21is obtained from the amount of liquid nitrogen adsorbed on the surfaces of the carrier cores21measured based on the BET method (nitrogen adsorption specific surface area method) using an automatic specific surface area measuring device.

The carrier cores21and the coat layers22of the carrier particles20are described next.

The carrier cores21contain a magnetic material, for example. Examples of the magnetic material contained in the carrier cores21include metal oxides, and more specific examples include magnetite, maghemite, and ferrite. Ferrite has high fluidity and tends to be chemically stable. As such, the carrier cores21preferably contain ferrite in terms of formation of high-quality images over a long period of term. Examples of ferrite include barium ferrite, manganese ferrite (Mn-ferrite). Mn—Zn ferrite, Ni—Zn ferrite, Mn—Mg ferrite, Ca—Mg ferrite, Li ferrite, and Cu—Zn ferrite. The shape of the carrier cores21is not limited particularly and may be irregular or spherical. A commercially available product may be used as the carrier cores21. Alternatively, the carrier cores21may be self-made by crushing and sintering the magnetic material.

The carrier cores21have a volume median diameter of preferably at least 20.0 μm and no greater than 80.0 μm, more preferably at least 20.0 μm and no greater than 65.0 μm, further preferably at least 20.0 μm and no greater than 60.0 μm, further more preferably at least 20.0 μm and no greater than 50.0 μm, still further preferably at least 20.0 μm and no greater than 40.0 μm, and particularly preferably at least 20.0 μm and no greater than 35.0 μm. As a result of the volume median diameter of the carrier cores21being set to at least 20.0 μm, defects (carrier development) resulting from the carrier particles20attaching to the photosensitive drum can hardly occur. Thus, a phenomenon in which the carrier particles20attaching to the photosensitive drum transfers from the photosensitive drum to a transfer section can be inhibited, thereby inhibiting occurrence of image defects such as void. As a result of the volume median diameter of the carrier cores21being set to no greater than 80.0 μm by contrast, a fine magnetic brush of the developer1can be formed on the circumferential surface of a development roller in image formation, thereby achieving formation of fine texture images. The volume median diameter of the carrier cores21is measured by the method described in Examples, for example.

The carrier cores21have a saturation magnetization of preferably at least 65 emu/g and no greater than 90 emu/g, and more preferably at least 70 emu/g and no greater than 85 emu/g. As a result of the saturation magnetization of the carrier cores21being set to at least 65 emu/g, carrier development will hardly occur. As a result of the saturation magnetization of the carrier cores21being set to no greater than 90 emu/g, a fine magnetic brush of the developer1can be formed on the circumferential surface of the development roller in image formation, thereby achieving formation of fine texture images. Where the carrier cores21contain Mn-ferrite, the higher the percentage content of Mn is, the lower the saturation magnetization of the carrier cores21tends to be. Also, where the carrier cores21contain Mn—Mg ferrite, the higher the percentage content of the Mg is, the lower the saturation magnetization of the carrier cores21tends to be. The saturation magnetization of the carrier cores21is measured by the method described in Examples, for example.

The carrier cores21preferably have an apparent density of at least 1.20×103kg/m3and no greater than 2.80×103kg/m3. The carrier cores21preferably have a degree of fluidity of at least 21 sec/50 g and no greater than 50 sec/50 g. The carrier cores21preferably have an electrical resistivity of at least 1-102Ω·m and no greater than 1×107Ω·m. The carrier cores preferably have a residual magnetization of at least 0.4 Am2/kg and no greater than 10.0 Am2/kg. The carrier cores21have a coercive force of at least 5 A/m·103/4π and no greater than 10 A/m·103/4π.

The coat layers22contain a coating resin and barium titanate particles23. Preferably, the coat layers22further contain carbon black particles24. However, the carbon black particles24can be dispensed with. The coat layers22each have coating resin regions25. The coating resin regions25are constituted by the coating resin. The coating resin regions25each are a region that contains only the coating resin. The coat layers22are each constituted by the barium titanate particles23, the carbon black particles24, and the coating resin regions25present therearound. The coating resin, the barium titanate particles23, and the carbon black particles24are described below.

The coating resin includes a silicone resin. As a result of the coating resin including a silicone resin, the toner can be triboelectrically charged to the desired charge amount in a favorably manner. Furthermore, as a result of use of a silicone resin as the coating resin, the coat layers22can be thinner than those made with a resin (e.g., fluororesin) other than the silicone resin. Thus, the amount of the coating resin contained in the coat layers22can be reduced and influence (e.g., influence of decreasing triboelectric charge amount of the toner particles10) on triboelectric charging caused by the coating resin absorbing moisture can be reduced.

Preferable examples of the silicone resin include epoxy resin modified silicone resins and silicone resins having a methyl group. One examples of the silicone resins having a methyl group is a silicone resin having a methyl group and not having a phenyl group. Another example of the silicone resins having a methyl group is a silicone resin (also referred to below as a “methylphenyl silicone resin”) having a methyl group and a phenyl group. The coat layers22may contain only a silicone resin as the coating resin or may further contain a resin other than the silicone resin.

The number of the barium titanate particles23per unit area of a coat layer22in a photographed cross-sectional image of a carrier particle20is preferably at least 6 particles/μm2and no greater than 450 particles/μm2. In the following, the “number of the barium titanate particles23per unit area of the coat layer22in a photographed cross-sectional image of the carrier particle20” may be also referred to as the “BT number”. As a result of the BT number being set to at least 6 particles/μm2and no greater than 450 particles/μm2, images with less fog can be formed and images with desired image density can be stably formed. As a result of the BT number being set to at least 6 particles/μm2by contrast, the coat layers22are hardly scraped. In order to form images with less fog and stably form images with desired image density, the BT number is preferably at least 10 particles/μm2, more preferably at least 20 particles/μm2, and further preferably at least 30 particles/μm2. For the same purpose as above, the BT number is preferably no greater than 400 particles/μm2, more preferably no greater than 350 particles/μm2, and further preferably no greater than 300 particles/μm2.

The BT number is calculated in a manner that from a photographed cross-sectional image of a carrier particle20photographed using a scanning electron microscope, the area of the coat layer22appearing on the photographed cross-sectional image and the number of barium titanate particles23appearing on the photographed cross-sectional image are obtained and a BT umber is calculated using a formula “(BT number)=(number of barium titanate particles23)/(area of coat layer22)”. The BT number can be adjusted for example by changing the mass of the barium titanate particles23relative to the mass of the coating resin and the number average primary particle diameter of the barium titanate particles23.

As described previously, the barium titanate particles23have a number average primary particle diameter of at least 100 nm and no greater than 500 nm. In order to form images with less fog, the number average primary particle diameter of the barium titanate particles23is preferably at least 200 nm. In order to inhibit occurrence of image defects resulting from cleaning failure, the number average primary particle diameter of the barium titanate particles23is preferably no greater than 400 nm. The number average primary particle diameter of the barium titanate particles23is measured by the method described in Examples, for example.

As described previously, the barium titanate particles23have a content of at least 4 parts by mass and no greater than 45 parts by mass relative to 100 parts by mass of the coating resin. The content of the barium titanate particles23is preferably at least 25 parts by mass and no greater than 45 parts by mass. Note that when the coating resin includes two or more resins, 100 parts by mass of the coating resin means the total mass of the two or more resins being 100 parts by mass.

No particular limitations are placed on a method for producing the barium titanate particles23, and the method may be hydrothermal synthesis or the oxalate method, for example. Preferably, the method for producing the barium titanate particles23is the hydrothermal synthesis. That is, the barium titanate particles23are preferably made from a hydrothermal compound. Having voids thereinside, barium titanate particles23produced by the hydrothermal synthesis have a smaller true specific gravity than those produced by the oxalate method. Furthermore, the barium titanate particles23produced by the hydrothermal synthesis have a sharp particle diameter distribution. For these reasons, the barium titanate particles23produced by the hydrothermal synthesis easily disperse uniformly in the coating resin, thereby easily obtaining a carrier with uniform charge imparting ability. As a result, the toner is quickly charged by friction with the carrier and images with further less fog can be formed.

The hydrothermal synthesis includes a hydrothermal reaction process and a thermal treatment process, for example. In the hydrothermal reaction process, a water-soluble barium salt is added to a titanium oxide dispersion in which titanium oxide particles are dispersed and the resultant dispersion is heated to cause a hydrothermal reaction. Barium titanate hydrothermally synthesized particles are obtained in the manner described above. In the thermal treatment process, the barium titanate hydrothermally synthesized particles are heat-treated to obtain the barium titanate particles23. The heating temperature in the hydrothermal reaction process is preferably at least 8° C. The heating temperature in the thermal treatment process is preferably at least 650° C. and no greater than 850° C. The number average primary particle diameter of the barium titanate particles23can be adjusted for example by changing the heating temperature and the time for the hydrothermal reaction in the hydrothermal reaction process. For example, the higher the heating temperature in the hydrothermal reaction process is, the larger the number average primary particle diameter of the barium titanate particles23is. Also, the longer the time for the hydrothermal reaction is, the larger the number average primary particle diameter of the barium titanate particles23is.

The carbon black particles24are conductive. As such, charge can smoothly move from the carrier particles20to the toner particles10as a result of the coat layers22containing the carbon black particles24. Thus, the toner particles10can be charged to the desired charge amount, thereby achieving formation of images with less fog. Furthermore, variation in charge amount of the toner can be reduced to achieve stable formation of images with desired image density even when the toner concentration in the developer1changes.

The carbon black particles24have a number average primary particle diameter of preferably at least 10 nm and no greater than 50 nm, and more preferably at least 20 nm and no greater than 40 nm. The carbon black particles24have a DBP oil absorption of preferably at least 50 cm3/100 g and no greater than 700 cm3/100 g, and more preferably at least 100 cm3/100 g and no greater than 600 cm3/100 g. The carbon black particles24have a BET specific surface area of preferably at least 100 m2/g and no greater than 2000 m2/g, and more preferably at least 100 m2/g and no greater than 200 m2/g or at least 1200 m2/g and no greater than 1500 m2/g.

As a result of the coat layers22containing the barium titanate particles23, the electric resistance of the carrier particles20can be moderately low. As such, the electric resistance of the carrier particles20can be moderately low even w % ben the amount of the carbon black particles24being conductive is small. Since the amount of the carbon black particles24can be reduced, occurrence of color turbidity can be inhibited in images formed using the developer1containing the carrier particles20. The amount of the carbon black particles24is preferably at least 1 part by mass and no greater than 10 parts by mass relative to 100 parts by mass of the coating resin, more preferably at least 3 parts by mass and no greater than 9 parts by mass, and further preferably at least 3 parts by mass and no greater than 6 parts by mass or at least 6 parts by mass and no greater than 9 parts by mass.

Note that the carrier particles20may contain a known additive as necessary. Preferably, the carrier particles20have a volume median diameter of at least 25 μm and no greater than 100 μm. The carrier has been descried so far.

The following describes one example of a method for producing the developer1according to the present embodiment. The method for producing the developer1according to the present embodiment includes a toner formation process, a carrier formation process, and a process of mixing a toner and a carrier, for example.

In the toner formation process, for example, the binder resin, the colorant, the charge control agent, and the releasing agent are mixed to obtain a mixture. The mixture is melt-kneaded to obtain a melt-kneaded product. The melt-knead product is pulverized to obtain a pulverized product. The pulverized product is classified to obtain the toner mother particles11. The toner mother particles11and the external additive particles12(the resin particles13and the optional external additive particles14) are mixed using a mixer. Through mixing, the external additive particles12are attached to the surfaces of the toner mother particles11. Thus, a toner containing the toner particles10is obtained. The external additive particles12are mixed preferably under a condition that the external additive particles12are not entirely buried in the toner mother particles11. The external additive particles12are attached to the surfaces of the toner mother particles11by physical bond (physical force) rather than chemical bond.

In the carrier formation process, the coat layers22are formed on the surfaces of the carrier cores21to obtain a carrier containing the carrier particles20. For example, a coating liquid containing the coating resin, the barium titanate particles23, and the optional carbon black particles24is sprayed on the carrier cores21in a fluid layer. >Next, the carrier cores21on which the coating liquid has been sprayed are heated at a first specific temperature (also referred to below as a specific drying temperature) to dry the coating liquid attached to the surfaces of the carrier cores21, thereby obtaining a dried product. Next, the dried product is heated at a second specific temperature (also referred to below as a specific baking temperature) using an electric furnace to harden the coating resin contained in the coating liquid on the surfaces of the carrier cores21. Thus, the coat layers22are formed on the surfaces of the carrier cores21. Preferably, the specific drying temperature is at least 70° C. and no greater than 80° C. Preferably, the specific baking temperature is at least 200° C. and no greater than 300° C.

The coverage rates of the carrier cores21can be adjusted for example by changing the specific drying temperature and the amount of the coating liquid sprayed on the carrier cores21. A higher specific drying temperature dries the coating liquid before the coating liquid spreads over the entire surface of the carrier cores21. Therefore, at a higher specific drying temperature, the coat layers22are formed locally on the surfaces of the carrier cores21rather than over the entire surface thereof, which tends to reduce the coverage rates of the carrier cores21. Furthermore, the smaller the amount of the coating liquid sprayed on the carrier cores21is, the more the coverage rates tend to reduce.

<Process of Mixing Toner and Carrier>

In the process of mixing a toner and a carrier, the toner and the carrier are mixed using a mixer to obtain the developer1.

EXAMPLES

The following provides more specific description of the present invention through use of examples. However, the present invention is not limited to the scope of the examples.

Carriers (CA-1) to (CA-22) and (CB-1) to (CB-7) were prepared. The compositions of these carriers are shown in Tables 3 to 5 described later. Note that the carriers (CA-1) to (CA-22) and (CB-1) to (CB-7) were used for preparing the developers (A-1) to (A-22) and (B-1) to (B-7), respectively.

(Preparation of Carrier (CA-1))

Using a homomixer, 60.00 g of a silicone resin solution (“KR-255”, product of Shin-Etsu Chemical Co., Ltd., solid concentration: 50% by mass, solid content amount: 30.00 g), 1.50 g of barium titanate (“BT-01”, product of SAKAI CHEMICAL INDUSTRY CO., LTD., barium titanate produced by the hydrothermal synthesis, number average primary particle diameter: 102 nm), 0.90 g of carbon black (“KETJEN BLACK EC-300J”, product of Lion Specialty Chemicals Co., Ltd.), and 240.00 g of toluene were mixed to obtain a coating liquid.

While 5000 g of carrier cores are allowed to flow, the coating liquid was sprayed on the carrier cores using a fluidized bed coating apparatus (“FD-MP-01 D”, product of Powrex Corporation). Thus, carrier cores coated with the coating liquid were obtained. The coating conditions included a supply air temperature (corresponding to the specific drying temperature described in the embodiment) of 75° C., supply air flow rate of 0.3 m3/min, and a rotor rotational speed of 400 rpm. The carrier cores used were manganese ferrite cores (product of DOWA IP CREATION CO., LTD., volume median diameter: 20.3 μm, saturation magnetization: 67 emu/g). The carrier cores coated with the coating liquid were baked at a temperature of 200° C. (corresponding to the specific baking temperature described in the embodiment) for 1 hour using an electric furnace. In the manner described above, coat layers were formed on the surfaces of the carrier cores to obtain a carrier (CA-1).

(Preparation of Carriers (CA-2) to (CA-22) and (CB-1) to (CB-7))

Carriers (CA-2) to (CA-22) and (CB-1) to (CB-7) were prepared according to the same method as that for preparing the carrier (CA-1) in all aspects other than the following changes. That is, the coating resin solutions shown in Tables 3 to 5 were used in amounts to give the solid content amounts shown in Tables 3 to 5. Barium titanates with number average primary particle diameters shown in Tables 3 to 5 produced by the methods shown in Tables 3 to 5 were used in amounts shown in Tables 3 to 5. The carbon blacks shown in Tables 3 to 5 were used in amounts shown in Tables 3 to 5. Carrier cores with volume median diameters shown in Tables 3 to 5 and saturation magnetizations shown in Tables 3 to 5 were used. The supply air temperature as one of the coating conditions was adjusted to give the averages of the coverage rates of the carrier cores shown in Tables 3 to 5. Note that the higher the supply air temperature is, the lower the average of the coverage rates of the carrier cores is.

Details of the coating resins and carbon blacks shown in Tables 3 to 5 are described later in explanation of the terms in Tables 3 to 5. The barium titanates with number average primary particle diameters shown in Tables 3 to 5 produced by the methods shown in Tables 3 to 5 used were those described below. Any types of the carrier cores with the volume median diameters shown in Tables 3 to 5 and the saturation magnetizations shown in Tables 3 to 5 were manganese ferrite cores produced by DOWA IP CREATION CO., LTD.Barium titanate (production method: hydrothermal synthesis, number average primary particle diameter: 102 nm): “BT-01” produced by SAKAI CHEMICAL INDUSTRY CO., LTD.Barium titanate (production method: hydrothermal synthesis, number average primary particle diameter: 304 nm): “BT-03” produced by SAKAI CHEMICAL INDUSTRY CO., LTD.Barium titanate (production method: hydrothermal synthesis, number average primary particle diameter: 495 nm): “BT-05” produced by SAKAI CHEMICAL INDUSTRY CO., LTD.Barium titanate (production method: hydrothermal synthesis, number average primary particle diameter: 76 nm): particle size adjusted product produced by SAKAI CHEMICAL INDUSTRY CO., LTD.Barium titanate (production method: hydrothermal synthesis, number average primary particle diameter: 687 nm): “BT-07” produced by SAKAI CHEMICAL INDUSTRY CO., LTD.Barium titanate (production method: oxalate method, number average primary particle diameter: 304 nm): 0.3-μm product of “PARUSERAMU BT” produced by NIPPON CHEMICAL INDUSTRIAL CO., LTD.

Resin particles (R1) to (R6) for use as external additives of toners were synthesized by the following methods.

(Synthesis of Resin Particles (R1))

A glass-made reaction vessel equipped with a thermometer (thermocouple), a stirring device, a reflux condenser, and a nitrogen gas inlet tube was set in a water bath set at 80° C. A solution was obtained by adding 300 parts by mass of ion exchange water and 1 part by mass of di-tert-butyl peroxide into the reaction vessel. While the resultant solution was kept at a temperature of 80° C. and stirred, 0.2 parts by mass of ammonium persulfate and 60 parts by mass of a monomer mixture were dripped into the solution over 1 hour in a nitrogen gas atmosphere. The monomer mixture was a mixture of 20% by mol of styrene and 80% by mol of butyl methacrylate. Next, a polymerization reaction of the contents of the reaction vessel was caused under stirring. The reaction conditions of the polymerization reaction included a reaction temperature of 100° C., a reaction time of 3 hours, and a stirring speed of 1400 rpm. An emulsion solution obtained by the reaction was dried to obtain resin particles (R1). The resin particles (R1) had a number average primary particle diameter of 30 nm.

(Synthesis of Resin Particles (R2) to (R6))

Resin particles (R2) to (R6) were synthesized according to the same method as that for synthesizing the resin particles (R1) in all aspects other than that the number average primary particle diameter was changed from 30 nm to those shown in Tables 6 to 8 by changing the reaction time and the stirring speed in the polymerization reaction. The reaction time and the stirring speed were set with reference to the method for adjusting the number average primary particle diameter of the resin particles described in the embodiment.

A non-crystalline polyester resin (PS1) for used as a binder resin of toner mother particles of the toners was synthesized by the following method. First, a reaction vessel equipped with a thermometer (thermocouple), a dewatering conduit, a nitrogen inlet tube, and a stirring device (stirring impeller) was set in an oil bath. Into the reaction vessel, 1575 g of a bisphenol A propylene oxide adduct (BPA-PO), 163 g of a bisphenol A ethylene oxide adduct (BPA-EO), 377 g of fumaric acid, and 4 g of a catalyst (dibutyltin oxide) were added. Subsequently, after a nitrogen atmosphere was created in the reaction vessel, the internal temperature of the reaction vessel was raised to 220° C. using the oil bath while stirring the contents thereof. The contents of the reaction vessel were polymerized for 8 hours under conditions of the nitrogen atmosphere and a temperature of 220° C. while by-product water was removed. Subsequently, after the internal pressure of the reaction vessel was reduced, the contents of the reaction solution were further polymerized for 1 hour under conditions of the reduced pressure atmosphere (pressure: 60 mmHg) and a temperature of 220° C. Subsequently, after the internal temperature of the reaction vessel was reduced to 210° C., 336 g of trimellitic anhydride was added into the reaction vessel. Thereafter, the contents of the reaction vessel were caused to react under conditions of the reduced pressure atmosphere (pressure: 60 mmHg) and a temperature of 210° C. The reaction time for the reaction was adjusted so that the non-crystalline polyester resin (PS1) being a reaction product had the following physical properties. Thereafter, the reaction product was taken out of the reaction vessel and cooled to obtain a non-crystalline polyester resin (PS1) with the following physical properties. Note that the resultant polyester resin (PS1) was determined to be non-crystalline because no clear endothermic peak was observed on the endothermic curve plotted using a differential scanning calorimeter and no clear melting point was determined.

Mass average molecular weight (Mw): 30,000

Toners (TA-1) to (TA-22) and (TB-1) to (TB-7) were prepared. The compositions of these toners are shown in Tables 6 to 8 described later. Note that the toners (TA-1) to (TA-22) and (TB-1) to (TB-7) were used for preparing the developers (A-1) to (A-22) and (B-1) to (B-7), respectively. To aid understanding, even toners with the same composition are shown in Tables 6 to 8 as toners with different toner numbers corresponding to the numbers of the developers.

Using an FM mixer (“FM-10B”, product of Nippon Coke & Engineering Co., Ltd.), 100 parts by mass of a binder resin, 4 parts by mass of a colorant, 1 part by mass of a charge control agent, and 5 parts by mass of a releasing agent were mixed to obtain a mixture. The binder resin used was the non-crystalline polyester resin (PS1) obtained in <Synthesis of Non-crystalline Polyester Resin (PS1)> described above. The colorant used was a copper phthalocyanine blue pigment (C.I. Pigment Blue 15:3). The charge control agent used was a quaternary ammonium salt (“BONTRON (registered Japanese trademark) P-51”, product of ORIENT CHEMICAL INDUSTRIES CO., LTD.). The releasing agent used was a caranuba wax (“SPECIAL CARNAUBA WAX No. 1”, product of S. Kato & Co.). The resultant mixture was melt-kneaded using a twin screw extruder (“MODEL PCM-30”, product of Ikegai Corp.) to obtain a melt-kneaded product. The melt-kneading was carried out under conditions of a set temperature of 120° C., a rotational speed of 150 rpm, and a processing amount of 5 kg/hour. The melt-kneaded product was pulverized using a mechanical pulverizer (“TURBO MILL”, product of FREUND-TURBO CORPORATION) to obtain a pulverized product. The pulverized product was classified using a classifying apparatus (“ELBOW-JET”, product of Nittetsu Mining Co., Ltd.). Through the above, toner mother particles in a powder state with a volume median diameter of 6.8 μm were obtained.

Using an FM mixer (“FM-10B”, product of Nippon Coke & Engineering Co., Ltd.), 100.0 parts by mass of the toner mother particles, 1.5 parts by mass of silica particles, and 0.4 parts by mass of the resin particles (R1) were mixed for 5 minutes under a condition of 4,000 rpm. The silica particles used were “AEROSIL (registered Japanese trademark) REA90” (dry silica particles rendered positively chargeable through surface treatment, number average primary particle diameter 20 nm) produced by Nippon Aerosil Co., Ltd. The resultant mixture was sifted using a 200-mesh sieve (opening 75 μm) to obtain a toner (TA-1).

(Preparation of Toners (TA-2) to (TA-22) and (TB-1) to (TB-7))

Toners (TA-2) to (TA-22) and (TB-1) to (TB-7) were prepared according to the same method as that for preparing the toner (TA-1) in all aspects other than that the resin particles shown in Tables 6 to 8 were used in amounts shown in Tables 6 to 8.

The saturation magnetization of each type of the carrier cores was measured under a condition of an external magnetic field of 3000 (unit: Oe) using a high-sensitivity vibrating sample magnetometer (“VSM-P7”, product of Toei Industry Co., Ltd.). The measurement results are shown below in Tables 3 to 5.

The volume median diameter (i.e., median diameter) of each type of the carrier cores was measured using a laser diffraction/scattering type particle size distribution analyzer (“LA-950”, product of HORIBA, Ltd.). The measurement results are shown below in Tables 3 to 5.

<Number Average Primary Particle Diameter Measurement>

The number average primary particle diameters of each type of the barium titanate particles, each type of the silica particles, and each type of the resin particles were measured using a scanning electron microscope (“JSM-7600F”, product of JEOL Ltd., field emission scanning electron microscope). In the number average primary particle diameter measurement, the equivalent circle diameters (Heywood diameters: diameters of circles having the same areas as projected areas of primary particles) of 100 primary particles were measured and a number average thereof was obtained. Tables 3 to 5 show the results of the number average primary particle diameter measurement for the barium titanate particles. Tables 6 to 8 show the results of the number average primary particle diameter measurement for the silica particles and the resin particles.

<Determination of Average and Standard Deviation of Coverage Rates>

(Photographing of Backscattered Electron Image of Surface of Carrier Particle)

Conductive tape was fixed to a SEM sample stage with the adhesive side thereof facing upward. The carrier particles of any of the carriers were sprayed on the adhesive side of the conductive tape. Next, excess carrier particles were removed from the adhesive side thereof by air blowing. Next, the carrier particles were fixed to the conductive tape by covering the adhesive side thereof with powder paper and applying a load to the carrier particles through the powder paper. Next, the powder paper was peeled off from the adhesive side of the conductive tape. Thus, a sample was obtained that included the conductive tape and the carrier particles dispersed on and fixed to the adhesive side of the conductive tape. Using a field emission scanning electron microscope (FE-SEM, “JSM-7600F”, product of JEOL Ltd.), backscattered electron images (surface photographed images) of the surfaces of the carrier particles of the obtained sample were photographed. The FE-SEM setting conditions included the followings.

(FE-SEM Setting Conditions for Coverage Rate Measurement)

The photographed images (surface photographed images of the carrier particles) were analyzed using image analysis software (“WinROOF”, product of MITANI CORPORATION). Specifically, histograms with the number of pixels on the vertical axis and the brightness on the horizontal axis were each created from a corresponding one of the images. Each of the created histograms included a low brightness peak PBG1corresponding to the conductive tape in the image, a medium brightness peak PCL1corresponding to the coating layer in the image, and a high brightness peak PCC1corresponding to the carrier core in the image. Next, the image was binarized using a brightness at the minimum value between the peak PBG1and the peak PCL1as a threshold. Thus, each of the images was divided into a conductive tape region and a combination region of a covered region and a non-covered region. The covered region corresponds to a region indicated by the medium brightness peak PCL1derived from the coat layer. The non-covered region corresponds to a region indicated by the high brightness peak PCC1derived from an exposed part of the carrier core and not covered with the coat layer. Next, area calculation was carried out based on the binarized image to calculate a total area (ACL1+ACC1) of the covered region and the non-covered region of the image. Next, the binarization condition was changed by setting a threshold of the brightness at the minimum value between the peak PCL1and the peak PCC1. Thus, the combination region of the covered region and the non-covered region was divided into the covered region and the non-covered region. Thereafter, an area (ACC1) of the non-covered region and an area (ACL1) of the covered region were calculated. Next, a coverage rate of the carrier core was obtained for the surface photographed image of the one carrier particle based on the measured values using a formula “(coverage rate)=100×(area(ACL1) of covered region)/(area of surface of carrier core)=100×(area(ACL1) of covered region)/(total area (ACL1+ACC1) of covered region and non-covered region)”. With respect to each of the surface photographed images of 100 carrier particles, a coverage rate of the carrier core was obtained. Based on the coverage rates of the 100 carrier cores, an average of the coverage rates (number average of coverage rates) and a standard deviation of the coverage rates were obtained. Furthermore, a coefficient of variation of the coverage rates of the carrier cores was obtained using a formula “(coefficient of variation)=100×(standard deviation of coverage rates)/(average of coverage rates)”. Tables 3 to 5 show the average, standard deviation, and coefficient of variation of the coverage rates of the carrier cores.

(Backscattered Electron Image Photographing of Cross-Section of Carrier Particles)

Observation using a scanning electron microscope (SEM) was carried out by the following method. With respect to each type of the carrier particles, the carrier particles were dispersed in a visible photocurable resin (“ARONIX (registered Japanese trademark) LCR D-800”, product of Toagosei Co., Ltd.), and the resin was then hardened by visible light irradiation to obtain a hardened material. The resultant hardened material was processed using a knife and a file to obtain a sample thin piece in a rectangular plate shape with specific dimensions (length: 1 cm, width: 1 cm, thickness: 3 mm). The sample thin piece was processed using a cross-sectional sample preparation device (“CROSS SECTION POLISHER (registered Japanese trademark) SM-09010”, product of JEOL Ltd., processing method: ion beam) under the conditions described below to obtain cross-sectional images of the carrier particles. The processing conditions included an ion accelerating voltage of 4.0 kV, use of argon (purity: at least 99.9999%, pressure: 0.15 MPa) as a gas, and a processing time of 12 hours. Next, backscattered electron images (cross-sectional image) of the cross-sections of the obtained carrier particles were photographed using a scanning electron microscope (SEM, “JSM-7900F”, product of JEOL Ltd.). The SEM setting conditions included the followings.

(SEM Setting Conditions for BT Number Measurement)

As one example, a photographed cross-sectional image of a carrier particle contained in the carrier (CA-2) is shown inFIG.2. The scale bar inFIG.2indicates a dimension of 100 nm. In the photographed cross-sectional image ofFIG.2, a carrier core21, a plurality of barium titanate particles23, and a coating resin region25were recognized.

The number of barium titanate particles appearing on the photographed image (photographed cross-sectional image of a carrier particle) was counted. The barium titanate particles have high brightness and accordingly appear white in the vicinity of the surface of the carrier core on the photographed cross-sectional image. Note that the carbon black particles can be distinguished from the barium titanate particles because the carbon black particles have lower brightness than the barium titanate particles and appear black on the photographed cross-sectional image.

Next, the photographed image (photographed cross-sectional image of the carrier particle) was analyzed using image analysis software (“WINROOF” product of MITANI CORPORATION) to calculate an area of the coat layer. In detail, a histogram with the number of pixels on the vertical axis and the brightness on the horizontal axis was created from the image first. The created histogram included a first peak Pncorresponding to a region other than the carrier particle, a second peak PCScorresponding to the carbon black particles and the silicone resin, a third peak PCCcorresponding to the carrier core, and a fourth peak PBTcorresponding to the barium titanate particles in the stated order from the low brightness side. Next, the image was binarized using a brightness at the minimum value between the first peak Pnand the second peak PCSas a threshold. Thus, the image was divided into a region other than the carrier particle and a region of the carrier particle. Thereafter, area calculation was carried out based on the binarized image to calculate a total area (ACS+ACC+ABT) of the areas of the region of the carrier particle, that is, a region of the carbon black and the silicone resin, a region of the carrier core, and a region of the barium titanate particles in the image. Next, the binarization conditions were changed and the image was binarized using a brightness at the minimum value between the second peak PCSand the third peak PCCas a threshold. Thus, the region of the carrier particle was divided into the region of the carbon black particles and the silicone resin and regions (the region of the carrier core and the region of the barium titanate particles) other than that. Thereafter, area calculation was carried out based on the binarized image to calculate an area (ACS) of the region of the carbon black particles and the silicone resin in the image. Next, the binarization conditions were changed and the image was binarized using a brightness at the minimum value between the third peak PCCand the fourth peak PBTas a threshold. Thus, the region of the carrier particle was divided into the region of the barium titanate and regions (the region of the carbon black particles and silicone resin and the region of the carrier core) other than that. Thereafter, area calculation was carried out based on the binarized image to calculate an area (ABT) of the region of the barium titanate in the image. Next, the BT number of the one carrier particle was obtained based on the measured values using a formula “(BT number)=(number of barium titanate particles appearing on photographed cross-sectional image)/(area of coat layer appearing on photographed cross-sectional image)=(number of barium titanate particles appearing on photographed cross-sectional image)/[(area (ABT) of region of barium titanate)]+(area (ACS) of region of carbon black particles and silicone resin)]”. The BT number was calculated for each photographed cross-sectional image of 100 carrier particles. The number average of the BT numbers was obtained from the BT numbers of the 100 carrier particles, and the number average was taken to be the BT number of the carrier particles. The BT numbers are as shown in Tables 3 to 5.

<BET Specific Surface Area Measurement>

Among the prepared carriers, the BET specific surface area of each of the carriers (CA-1) to (CA-3), (CA-5) to (CA-7), (CA-10), and (CA-13) were measured as typical examples. Nitrogen was adsorbed onto the surface of a sample (each carrier) using an automatic specific surface area measuring device (“MACSORB MODEL 1208”, product of Mountech Co., Ltd.), and the BET specific surface area of the sample was measured by the flow method (BET single point method). In detail, the mass of an empty cell was measured. Next, 9 g of the sample was loaded in the cell so as not to be attached to the inner wall of the cell. Nitrogen was allowed to flow into the cell loaded with the sample at a temperature of 45° C. for 30 minutes while the flow rate of the nitrogen was adjusted to 25 mL/min using a flow meter. In the manner as above, the sample was degassed. Next, the cell was cooled for 2 minutes and measurement using the automatic specific surface area measuring device was then started. After the measurement start, adsorption was carried by immersing the cell in liquid nitrogen in a Dewar bottle and desorption was then carried out by returning the cell from the Dewar bottle to the atmosphere. Automatic measurement during the desorption measured the actual surface area of the sample. A BET specific surface area (unit: m2/g) of the sample was obtained based on the measured values using a formula “(specific surface area)=(actual surface area of sample)/(mass of sample)”. The measurement results are shown in Table 2.

TABLE 2CarrierBET specific surface area [m2/g]CA-10.5CA-20.9CA-31.1CA-50.3CA-60.5CA-71.4CA-103.5CA-132.8

An evaluation apparatus (prototype produced by KYOCERA Document Solutions Japan Inc.) having the following configuration was used for evaluation of each developer. The developer was loaded into a development device for cyan color of the evaluation apparatus, and a toner for replenishment use was loaded into a toner container for cyan color thereof.

(Configuration of Evaluation Apparatus)

Using the evaluation apparatus, durable printing of printing A4-size image on 100,000 sheets of paper was carried out under the printing conditions (specifically, conditions of a printing environment, a printing mode, and an image printing rate) shown in Table 9.

The printing environments shown in Table 9 were as follows.LL environment: environment at a temperature of 10° C. and a relative humidity of 15%NN environment: environment at a temperature of 22° C. and a relative humidity of 50%HH environment: environment at a temperature of 32.5° C. and a relative humidity of 80%

The printing modes shown in Table 9 were as follows.Consecutive mode: mode for consecutive sheet printing5-sheet intermittent mode: mode for repeating printing pattern of 5-sheet printing and 12-second printing stop

The images with printing rates shown in Table 9 were as follows.2%: character image at a printing rate of 2%5%: character image at a printing rate of 5%20%: character image at a printing rate of 20%50%: character image at a printing rate of 50%

Note that “Start” in Table 9 indicates from which sheet of the 100,000 sheets of paper printing under the corresponding printing conditions starts. Furthermore, “Image evaluation timing” indicates that image evaluation was carried out after image printing on which sheet out of 100,000 sheets of paper has been done. In addition, for changing the printing environment, the evaluation apparatus was left to stand for 24 hours in the changed printing environment and the durable printing was resumed then. The evaluation results of the developers are shown in Tables 6 to 8.

First, a solid image (A4 size) was printed on one sheet of paper using the evaluation apparatus in the NN environment and the printed sheet was taken to be a first evaluation sheet. Next, the aforementioned durable printing was carried out. In the durable printing, a solid image (A4 size) was printed on one sheet of paper with each timing shown in Table 9 using the evaluation apparatus. The printed sheet was taken to be a second evaluation sheet. The image density of each solid image printed on the first evaluation sheet and the second evaluation sheets was measured using a reflectance densitometer (“RD-19I”, product of X-Rite Inc.). Thereafter, a decrease width in image density was calculated using a formula “(decrease width in image density)=(image density of solid image printed on first evaluation sheet)−(image density of solid image printed on second evaluation sheet)”. A decrease width in image density for each of the second evaluation images was calculated. The maximum value of the calculated decrease widths was taken to be an evaluation value. The evaluation value was rated according to the following criteria. A smaller decrease width in image density indicates that images with desired image density can be formed more stably. Cases rated as A, B, or C in the evaluation were considered passed, and cases rated as D were considered failed.

(Evaluation Criteria of Image Density)

A: Decrease width in image density of less than 0.2B: decrease width in image density of at least 0.2 and less than 0.3C: decrease width in image density of at least 0.3 and less than 0.4D (poor): decrease width in image density of 0.4 or more

The aforementioned durable printing was carried out. In the durable printing, a blank image (A4 size) was printed on one sheet of paper with each timing shown in Table 9 using the evaluation apparatus. The printed sheets were each taken to bean evaluation sheet. The reflection density of a blank area of the evaluation sheet was measured using a white light meter (“TC-6DS”, product of Tokyo Denshoku Co., Ltd.). Thereafter, a fog density was calculated using a formula “(fog density)=(reflection density of blank area)−(reflection density of unprinted sheet)”. Fog density of each of the evaluation sheets was calculated. The maximum value of the calculated fog densities was taken to be an evaluation value. The evaluation value was rated according to the following criteria. Cases rated as A or B in the evaluation were considered passed, and cases rated as C were considered failed.

(Evaluation Criteria of Fog Density)

A: fog density of less than 0.010B: fog density of at least 0.010 and less than 0.020C (poor): fog density of 0.020 or more

(Method for Evaluating Inhibition of Occurrence of Carrier Development)

The aforementioned durable printing was carried out. In the durable printing, a blank image (A4 size) was printed on one sheet of paper with each timing shown in Table 9 using the evaluation apparatus. The printed sheet was taken to be an evaluation sheet. The blank image printed on the evaluation sheet was observed using a loupe with a magnification of 25×. The numbers of carriers present in regions each with an area of 10 cm2in the blank image was counted. The number of carriers present in each of 10 regions (specifically, upstream 3 regions, central 4 regions, and downstream 3 regions in terms of a sheet travelling direction) of the blank image printed on each evaluation sheet was counted. Thereafter, the number (unit: occurrences/cm2) of occurrences of carrier development was obtained using a formula “(number of occurrences of carrier development)=(total number of carriers present in 10 regions)/(total area of 10 regions)=(total number of carriers present in 10 regions)/100”. The number of occurrences of carrier development was calculated for each of the evaluation sheets. The maximum value of the calculated numbers of occurrences of carrier development was taken to be an evaluation value. The evaluation value was rated according to the following criteria. Cases rated as A, B, or C in the evaluation were considered passed, and cases rated as D were considered failed.

(Evaluation Criteria of Inhibition of Occurrence of Carrier Development)

A: number of occurrences of carrier development of less than 0.1 occurrences/cm2B: number of occurrences of carrier development of at least 0.1 occurrences/cm2and less than 0.3 occurrences/cm2C: number of occurrences of carrier development of at least 0.3 occurrences/cm2and less than 1.0 occurrences/cm2D (poor): number of occurrences of carrier development of 1.0 occurrences/cm2or more

(Method for Evaluating Inhibition of Decrease in Texture)

First, a halftone image (band-shaped image with a printing rate of 50%) was printed on one sheet of paper using the evaluation apparatus in the NN environment and the printed sheet was taken to be a first evaluation sheet. Next, the aforementioned durable printing was carried out. In the durable printing, a halftone image (band-shaped image with a printing rate of 50%) was printed on one sheet of paper with each timing shown Table 9 using the evaluation apparatus. The printed sheets were each taken to be a second evaluation sheet. The texture of the halftone images printed on the first evaluation sheet and the second evaluation sheet was observed with the naked eye. By doing so, it was confirmed to what extent the texture of the halftone image printed on the second evaluation sheet was decreased compared to the texture of the halftone image printed on the first evaluation sheet. Of all the second evaluation sheets, the evaluation sheet with texture of the halftone image decreased the worst was evaluated according to the following criteria Cases rated as A, B, or C in the evaluation were considered passed, and cases rated as D were considered failed.

(Evaluation Criteria of Inhibition of Decrease in Texture)

A: No decrease in texture occurred at all.B: Decrease in texture occurred to some extent.C: Decrease in texture occurred to the extent that there is no problem involved in actual useD (poor): Decrease in texture occurred that is noticeable to the extent that it involves a problem in actual use.
(Evaluation Method of Inhibition of Occurrence of Image Defects Resulting from Cleaning Failure)

The aforementioned durable printing was carried out. In the durable printing, a character image with a printing rate of 5% was printed on one sheet of paper with each timing shown in Table 9 using the evaluation apparatus. The printed sheets each were taken to be an evaluation sheet. The character image printed on each evaluation sheet was observed with the naked eye to confirm the occurrence or non-occurrence of image defects resulting from cleaning failure. Note that the image defects resulting from cleaning failure are image defects in which a thin line parallel to the sheet travelling direction appears. Of all the evaluation sheets, the evaluation sheet in which image defects resulting from cleaning failure occurred the most was evaluated according to the following criteria. Cases rated as A, B. or C in the evaluation were considered passed, and cases rated as D were considered failed.

(Evaluation Criteria of Inhibition of Occurrence of Image Defects Resulting from Cleaning Failure)A: No image defects resulting from cleaning failure occurred at all.B: Some image defects resulting from cleaning failure occurred.C: Image defects resulting from cleaning failure occurred to the extent that there is no problem involved in actual use.D (poor): Image defects resulting from cleaning failure occurred that are noticeable to the extent that they involve a problem in actual use.

The meanings of the terms used below in Tables 3 to 8 are explained next. The terms in Tables 3 to 8 mean as follows.Core: carrier coresD50: volume median diameterSolid content amount: solid content amount of coating resin. The solid content amount of the coating resin is calculated using a formula “[solid content amount (unit: part by mass) of coating resin]=[amount (unit: part by mass) of silicone resin solution]×[solid concentration (unit: % by mass) of silicone resin solution]/100”.Resin/core: resin/core rate (unit: % by mass) The resin/core rate was calculated using a formula “[resin/core rate (unit: % by mass)]=100 [mass (unit: parts by mass) of coating resin]/[mass (unit: parts by mass) of carrier cores]=100×[solid content amount (unit parts by mass) of silicone resin solution]/[mass (unit: parts by mass) of carrier cores]”.wt %: % by massPart: parts by massBT: barium titanate particlesMethod: barium titanate particle production methodHydrothermal: hydrothermal synthesisOxalate: oxalate methodAmount ratio: content of barium titanate particles to 100 parts by mass of coating resinDiameter: number average primary particle diameterCB: carbon black particlesCoat layer/core: coat layer/core rate The coat layer/core rate is calculated using a formula “[coat layer/core rate (unit: % by mass)]=100×[mass (unit: parts by mass) of coat layers]/[mass (unit: part by mass) of carrier cores]=100×[mass (unit: part by mass) of solid content of coating liquid]/[mass (unit: part by mass) of carrier cores]=100×{[solid content amount (unit: part by mass) of silicone resin solution]+[mass (unit: part by mass) of barium titanate]+[mass (unit: part by mass) of carbon black]}/[mass (unit: part by mass) of carrier cores]”.Average in column Coverage rate: average of coverage rates of carrier coresDeviation in column Coverage rate: standard deviation of coverage rates of carrier coresVariation Coefficient in column Coverage rate: coefficient of variation of coverage rates of carrier coresKR-255: silicone resin solution (“KR-255”, product of Shin-Etsu Chemical Co., Ltd., solid content: methylphenyl silicone resin, solid concentration: 50% by mass)KR-301: silicone resin solution (“KR-301”, product of Shin-Etsu Chemical Co., Ltd., solid content: methylphenyl silicone resin, solid concentration: 40% by mass)ES-1001N: silicone resin solution (“ES-1001N”, product of Shin-Etsu Chemical Co., Ltd., solid content: epoxy resin modified silicone resin, solid concentration: 45% by mass)EC: carbon black (“KETJEN BLACK EC-300J”, product of Lion Specialty Chemicals Co., Ltd., conductive carbon black, DBP oil absorption: 360 cm3/100 g, BET specific surface area: 1270 m2/g, number average primary particle diameter: 39.5 nm)MA: carbon black (“MITSUBISHI (registered Japanese trademark) CARBON BLACK MA100”, product of Mitsubishi Chemical Corporation, DBP oil absorption: 100 cm3/100 g, BET specific surface area: 110 m2/g, number average primary particle diameter: 24 nm)FD: fog densityFog: rating of fog resistanceCarrier development: rating of inhibition of occurrence of carrier developmentTexture: rating of inhibition of decrease in textureImage density: rating of image densityCleaning: rating of inhibition of occurrence of image defects resulting from cleaning failure

As shown in Table 4, the content of the barium titanate particles of the carrier particles contained in the carrier (CB-1) of the developer (B-1) was less than 4 parts by mass relative to 100 parts by mass of the coating resin. As shown in Table 7, both the evaluation result of fog resistance and the evaluation result of image density for the developer (B-1) were rated as poor and were determined to be failed.

As shown in Table 4, the content of the barium titanate particles of the carrier particles contained in the carrier (CB-2) of the developer (B-2) exceeded 45 parts by mass relative to 100 parts by mass of the coating resin. As shown in Table 7, both the evaluation result of fog resistance and the evaluation result of image density for the developer (B-2) were rated as poor and determined to be failed.

As shown in Table 5, the number average primary particle diameter of the barium titanate particles of the carrier particles contained in the carrier (CB-3) of the developer (B-3) was less than 100 nm. As shown in Table 8, the evaluation result of fog resistance for the developer (B-3) were rated as poor and determined to be failed.

As shown in Table 5, the number average primary particle diameter of the barium titanate particles of the carrier particles contained in the carrier (CB-4) of the developer (B-4) exceeded 500 nm. As shown in Table 8, the evaluation result of inhibition of occurrence of image defect resulting from cleaning failure for the developer (B-4) was rated as poor and determined to be failed.

As shown in Table 5, the coat layer/core rate of the carrier particles contained in the carrier (CB-5) of the developer (B-5) exceeded 4.90% by mass and the average of the coverage rates of the carrier cores was 100.0%. As shown in Table 8, the evaluation result of fog resistance for the developer (B-5) was rated as poor and determined to be failed.

As shown in Table 5, the coat layer/core rate of the carrier particles contained in the carrier (CB-6) of the developer (B-6) was less than 0.10/o by mass and the average of the coverage rates of the carrier cores was less than 80.0%. As shown in Table 8, the evaluation result of fog resistance for the developer (B-6) was rated as poor and determined to be failed.

As shown in Table 8, the external additive particles of the toner particles contained in the toner (TB-7) of the developer (B-7) included no resin particles. As shown in Table 8, the evaluation result of fog resistance for the developer (B-7) was rated as poor and determined to be failed.

As shown in Tables 3 to 8, each of the developers (A-1) to (A-22) had the following features. That is, the external additive particles of the toner particles included resin particles containing a thermoplastic resin. The coat layers of the carrier particles contained barium titanate particles and a coating resin including a silicone resin. The barium titanate particles had a number average primary particle diameter of at least 100 nm and no greater than 500 nm. The barium titanate particles had a content of at least 4 parts by mass and no greater than 45 parts by mass relative to 100 parts by mass of the coating resin. The coat layer/core rate was at least 0.10% by mass and no greater than 4.90% by mass. The average of the coverage rates of the carrier cores was at least 80.0% and less than 100.0%. As shown in Tables 6 to 8, all of the evaluation results of fog resistance, the evaluation results of image density, and the evaluation results of inhibition of occurrence of image defects resulting from cleaning failure for the developers (A-1) to (A-22) were determined to be passed. As shown in Tables 6 to 8, the evaluation results of inhibition of decrease in texture and the evaluation results of inhibition of occurrence of carder development for the developers (A-1) to (A-22) were also determined to be passed.

From the above, it was demonstrated that the developer of the present invention, which encompasses the developers (A-1) to (A-22), can contribute to excellent fog resistance, stable formation of images with desired image density, and inhibition of occurrence of image defects resulting from cleaning failure.

INDUSTRIAL APPLICABILITY

The developer according to the present invention can be used for image formation in copiers, printers, and multifunction peripherals, for example.