DETAILED DESCRIPTION OF THE INVENTION

[0090] For the purpose of uniformizing and stabilizing the chargeability of an image-bearing member for a long period in the image forming system of the present invention, it is important to satisfy a specific percentage of iron-containing isolated particles in the magnetic toner, and a specific relationship of material and charged potential of the image-bearing member.

[0091] The iron-containing isolated particles (comprising iron or an iron compound) in the magnetic toner used in the present invention are originated from magnetic iron oxide particles used as magnetic powder in the magnetic toner and play an important role for uniformizing and stabilizing the chargeability of the image-bearing member together with electroconductive fine powder externally added to the magnetic toner particles. Isolated magnetic iron oxide particles exhibit a low resistivity and a weak chargeability and also a property of abrading a member contacting the particles because of a high hardness thereof. On the other hand, the image-bearing member has a photoconductor layer formed of a silicon-based non-single crystal material, so that it exhibits a high surface free energy and has a tendency of showing a strong interaction with inorganic (fine) particles. The magnetic iron oxide particles thus attached to the image-bearing member surface function to enhance the injection charging performance in the charging step, and abrade the image-bearing member at the contact nip between the charging member, thereby refreshing the image-bearing member surface to retain the charging uniformity for a long period.

[0092] If the image-bearing member is charged to a surface potential of 250 to 600 volts and the isolated magnetic iron oxide particles are contained in a percentage of 0.05 to 3.00% by number of the toner particles), the magnetic iron oxide particles and electroconductive fine powder are supplied at an appropriate rate from the magnetic toner to the surface of the photoconductor layer comprising a silicon(Si)-based non-single crystal material and removed at an appropriate rate from the photoconductor layer surface, so that the amounts of the magnetic iron oxide particles and electroconductive fine powder on the photoconductor layer surface are stabilized to further stabilize the injection charging performance in the charging step and suppress the abrasion irregularity on the image-bearing member surface leading to non-uniform chargeability.

[0093] Herein, the “non-single crystal material” constituting a surface or photoconductor layer of the image-bearing is principally in an amorphous state but can contain a minor proportion of microcrystalline or polycrystalline material unlike a single-crystal material as is understood from representative processes for production of such a photoconductor or surface layer described hereinafter. The term “silicon-based” means that the material comprises silicon as a principal element.

[0094] To describe more fully the composition of the magnetic toner used in the present invention, it is important that the magnetic toner includes magnetic toner particles comprising at least a binder resin and a magnetic iron oxide, and inorganic fine powder and electroconductive fine powder present at the surface of the magnetic toner particles; has an average circularity of 0.950 to 0.995; and contains 0.05 to 3.00% of isolated iron-containing particles.

[0095] If the magnetic toner has an average circularity of at least 0.950, the surface unevenness of the magnetic toner particles is alleviated to some extent so that inorganic fine powder and electroconductive fine powder as other components of the magnetic toner of the present invention can be uniformly attached to the magnetic toner particle surfaces, thus providing a level of flowability suitable for use in an electrophotographic process. Below 0.950, a sufficient flowability is liable to be failed in some cases.

[0096] In the image forming system of the present invention, in the case where the developing step (or means) is also used as a step (or means) for recovering residual toner on the image-bearing member, the electroconductive fine powder behaves separately from the toner particles and is supplied to the charging step to promote the charging of the image-bearing member. In this instance, if the toner has an average circularity below 0.950, the effective supply of the electroconductive fine powder from the toner to the charging step is liable to be hindered.

[0097] A higher circularity of toner tends to improve the image forming performances, and an average circularity of 0.970 or higher is preferred.

[0098] A toner comprising toner particles having an average circularity of 0.970 or higher exhibits a very excellent transferability. This is presumably because in such a magnetic toner having a high circularity, the magnetic toner particles are cause to have a small contact area with the photosensitive member, thus resulting in a small force of attachment force attributable to image force and van der Waals force onto the photosensitive member. As a result of a high transferability. The amount of transfer residual toner is reduced, and the amount of the magnetic toner present at the pressure nip between the charging member and the photosensitive member is reduced to prevent the occurrence of toner attachment onto the photosensitive member, thus remarkably reducing image defects.

[0099] Further, magnetic toner particles having an average circularity of at least 0.970 are almost free from surface edges to reduce the friction at the pressure nip between the charging member and the photosensitive member, to suppress the abrasion of the photosensitive member surface. These effects are particularly pronounced in an image forming method including a contact transfer step liable to cause a hollow transfer image dropout. It is particularly preferred that the magnetic toner has a mode circularity of at least 0.990 meaning that particles having a circularity of at least 0.990 are predominant since the effect can be insufficient in some cases if predominant particles have a low circularity even if the average circularity is high.

[0100] If the magnetic toner satisfies preferable features having an average circularity of at least 0.970 and a mode circularity of 0.990, toner ears formed on the toner-carrying member become fine and dense to provide a uniform charge, so that fog is remarkably reduced.

[0101] The average circularity and mode circularity are used as quantitative measures for evaluating particle shapes and based on values measured by using a flow-type particle image analyzer (“FPIA-1000”, mfd. by Toa Iyou Denshi K.K.). A circularity (Ci) of each individual particle (having a circle equivalent diameter (DCE) of at least 3.0 μm) is determined according to an equation (1) below, and the circularity values (Ci) are totaled and divided by the number of total particles (m) to determine an average circularity (Ca) as shown in an equation (2) below:

Circularity Ci=L ₀ /L, (1)

[0102] wherein L denotes a circumferential length of a particle projection image, and L ₀ denotes a circumferential length of a circle having an area identical to that of the particle projection image. 1 $\begin{array}{cc}\mathrm{Average}\mathrm{circularity}\left(\mathrm{Ca}\right)=\sum _{i=1}^m\mathrm{Ci}/m& \left(2\right)\end{array}$

[0103] Further, the mode circularity (Cmod) is determined by allotting the measured circularity values of individual toner particles to 61 classes in the circularity range of 0.40-1.00, i.e., from 0.400-0.410, 0.410-0.420, . . . , 0.990-1.000 (for each range, the upper limit is not included) and 1.000, and taking the circularity of a class giving a highest frequency as a mode circularity (Cmod).

[0104] Incidentally, for actual calculation of an average circularity (Ca), the measured circularity values (Ci) of the individual particles were divided into 61 classes in the circularity range of 0.40-1.00, and a central value of circularity of each class was multiplied with the frequency of particles of the class to provide a product, which was then summed up to provide an average circularity. It has been confirmed that the thus-calculated average circularity (Ca) is substantially identical to an average circularity value obtained (according to Equation (2) above) as an arithmetic mean of circularity values directly measured for individual particles without the above-mentioned classification adopted for the convenience of data processing, e.g., for shortening the calculation time.

[0105] More specifically, the above-mentioned FPIA measurement is performed in the following manner. Into 10 ml of water containing ca. 0.1 mg of surfactant, ca. 5 mg of magnetic toner sample is dispersed and subjected to 5 min. of dispersion by application of ultrasonic wave (20 kHz, 50 W), to form a sample dispersion liquid containing 5,000−20,000 particles/μl. The sample dispersion liquid is subjected to the FPIA analysis for measurement of the average circularity (Ca) and mode circularity (Cm) with respect to particles having D _CE ≧3.0 μm.

[0106] The average circularity (Ca) used herein is a measure of roundness, a circularity of 1.00 means that the magnetic toner particles have a shape of a perfect sphere, and a lower circularity represents a complex particle shape of the magnetic toner.

[0107] Herein, only particles having a circle-equivalent diameter (D _CE =L/π) of at least 3 μm are taken for the circularity measurement because particles smaller than 3 μm include a substantial amount of external additives and the inclusion of such particles can distort the circularity characteristic of magnetic toner particles.

[0108] A magnetic toner having an average circularity (Ca) of at least 0.950, preferably at least 0.970 and a mode circularity (Cmod) of at least 0.990 exhibits a remarkably improved transferability even at a small particle size, which has provided a difficulty in providing an improved transferability, and also exhibits a remarkably improved developing performance for a low-potential latent image. It is particularly effective for development of digital minute spot latent images. This means that the magnetic toner exhibits a good matching with a non-single crystal (or roughly amorphous) silicon photosensitive member used in the image forming system of the present invention.

[0109] If the average circularity (Ca) is below 0.950, the magnetic toner not only exhibits a lower transferability but also can exhibit a lower developing performance. On the other hand, if the average circularity exceeds 0.995, the toner surface deterioration becomes noticeable, thus posing a problem in durability.

[0110] Next, the percentage of isolated iron-containing particles will be described. The isolated iron-containing particles are particles of iron or iron compound (typically magnetic iron oxide particles) isolated from magnetic toner particles. The isolation percentage can also be determined by observation through, e.g., a scanning electron microscope but may conveniently be determined by plasma-induced particle luminescence spectra. In the latter measurement method, the percentage of isolated iron-containing particles (Fe.iso (%)) is determined based on the frequency of atomic luminescence (abbreviated as “AL”) of Fe separate or simultaneous with C (carbon) atomic luminescence and calculated according to the following formula:

Fe.iso (%)=100×{number of AL of Fe alone}/{(number of AL of Fe simultaneous with AL of C)+(number of AL of Fe alone)}

[0111] In this instance, AL of Fe is regarded as simultaneous if it occurs within 2.6 m.sec from AL of C, and regarded as separate if it occurs thereafter.

[0112] In the case of a magnetic toner particle containing magnetic iron oxide particles, the simultaneous luminescences of carbon atom and iron atom means a luminescence from a toner particle containing magnetic iron oxide dispersed therein, and the luminescence of only iron atom means a luminescence from an isolated iron-containing particle.

[0113] In the plasma-induced luminescence measurement method, fine particles like toner particles are introduced into plasma, particle by particle, to determine an element and a particle size of a luminescent particle from its luminescence spectrum. For example, in the case where a magnetic toner particle is introduced into plasma, each toner particle causes one luminescence of carbon (constituting the binder resin) and one luminescence of iron (constituting the magnetic iron oxide) which can be respectively observed. As one toner particle causes one luminescence, the number of toner particles can be determined based on the number of observed luminescences (C with Fe). The measurement may be performed by using, e.g., a particle analyzer (“PT1000”, made by Yokogawa Denki K.K.) according to a principle described in Japan Hardcopy '97 Paper Collection, pp. 65-68.

[0114] More specifically, for the measurement, a sample toner left standing overnight in an environment of 23° C. and 60%RH is subjected to measurement together with 0.1% O ₂ -containing helium gas in the above environment. For spectrum separation, Channel 1 detector is used for carbon atom (at wavelength of 247.86 nm, with a recommended value of K factor) and Channel 2 detector is used for iron atom (at wavelength of 239.56 nm, with K factor of 3.3764). Sampling is performed at a rate of one scan for covering 1000-1400 times of luminescence of carbon atom, and the sampling is repeated until the luminescences of carbon atom reaches at least 10,000 times. By integrating the luminescences, a particle size distribution curve is drawn with the number of luminescences taken on the ordinate and with the cube root of voltage representing a particle size on the abscissa, while effecting the sampling so that the particle size distribution curve exhibits a single peak and no valley. Based on the measured data while taking noise cut level during the measurement at 1.50 volts, Fe.iso (%) is calculated according to the above formula.

[0115] Incidentally, an azo-iron compound as a charge control agent may be contained in a toner in some cases, but the azo iron compound is an organometallic compound, so that it cannot result in a luminescence of only iron atom.

[0116] As a result of our study, there is found a close correlation between the percentage of isolated iron-containing particles (Fe.iso (%)) and the rate of exposure of magnetic iron oxide particles at the toner particle surfaces. More specifically, if Fe.iso (%) is at most 3.00%, the exposure at the toner particle surfaces of magnetic iron oxide particles is suppressed to provide a high chargeability. This is attributable to the uniformity of particle size distribution of the magnetic iron oxide particles and uniformity of surface treatment of the magnetic iron oxide particles. For example, in case where the surface treatment of the magnetic iron oxide particles is ununiform, magnetic iron oxide fine particles having a high hydrophillicity due to insufficient surface treatment are exposed to the toner particle surface, and a portion or all of them can be isolated from the toner particles.

[0117] Accordingly, a magnetic toner containing a lower percentage of isolated iron-containing particles tends to show a higher chargeability. On the other hand, if Fe.iso (%) is higher than 3.00%, the charge-leakage points are increased, thus being liable to result in a magnetic toner having an insufficient chargeability. This tendency becomes particularly remarkable in a high temperature/high humidity environment. A magnetic toner having a low chargeability is not desirable because it causes increased fog, causes a lower transferability and is liable to cause charging failure. Further, a magnetic toner satisfying both a high average circularity and a low percentage of isolated iron-containing particles can acquire a high chargeability and also a very high transferability as a result of synergy with the toner particle shape.

[0118] On the other hand, an Fe-iso (%) of below 0.05% means that substantially no magnetic iron oxide particles are isolated from the magnetic toner particles. Such a magnetic toner having a low Fe.iso (%) has a high chargeability but is liable to cause an excessive charge resulting in images having a low image density and accompanied with roughening, in image formation on a large number of sheets, particularly in a low temperature/low humidity environment. This is presumably because of the following mechanism.

[0119] A magnetic toner carried on a toner-carrying member is not wholly transferred for development onto the photosensitive member, but some magnetic toner remains on the toner-carrying member even immediately after the development. This tendency is particularly noticeable in the jumping developing mode using a magnetic toner. Further, magnetic toner particles having a high circularity form uniformly thin ears in the developing regions, and toner particles present at the tips of ears are used for development and toner particles present close to the toner-carrying member are not readily consumed for the development.

[0120] As a result, the magnetic toner particles close to the toner carrying member are liable to be excessively charged due to repetitive tribo-electrification with the charging members, and the transfer for development thereof becomes further difficult. In such a state, the charge uniformity of the magnetic toner is impaired, to result in rough images.

[0121] Now, if a magnetic toner having Fe.iso (%) >0.05% is used, the excessive charge of the magnetic toner is suppressed due to the isolated magnetic iron oxide particles and magnetic iron oxide particles present at the toner particle surfaces, and the charge uniformity of the magnetic toner is promoted to suppress the roughening of images.

[0122] As a result, even for a magnetic toner having a high circularity and a high chargeability, the excessive charge (charge-up phenomenon) in a long-term use can be alleviated if the exposed magnetic iron oxide particles are present, so that Fe.iso (%) of at least 0.05% is important.

[0123] For the above reason, Fe.iso (%) of 0.05% -3.00% is necessary. Fe.iso (%) is preferably 0.05-2.00%, more preferably 0.05−1.50%, further preferably 0.05−0.80%.

[0124] The magnetic toner used in the present invention may preferably comprise magnetic toner particles produced through the polymerization process. The magnetic toner particles can be produced through the pulverization process, but the magnetic toner particles produced through the pulverization process are generally indefinitely shaped and have to be mechanically or thermally treated in order to have an average circularity of at least 0.950 as an essential requirement, or a preferable circularity of at least 0.970 (and also a preferred mode circularity of at least 0.990).

[0125] Thus, in the present invention, the magnetic toner particles may preferably be produced through the polymerization process, examples of which may include: direct polymerization, suspension polymerization, emulsion polymerization, emulsion-association polymerization and seed polymerization. Among these, the suspension polymerization process is particularly preferred in order to easily provide a good balance of particle size and particle shape.

[0126] In the suspension polymerization process for producing a magnetic toner according to the present invention, a monomeric mixture is formed by uniformly dissolving or dispersing a monomer and magnetic powder (fine particles) (and, optionally, other additives, such as wax, a colorant, a crosslinking agent and charge control agent), followed by dispersing the monomeric mixture in an aqueous medium containing a dispersion stabilizer by means of an appropriate stirrer, and subjecting the dispersed monomeric mixture to suspension polymerization in the presence of a polymerization initiator to obtain toner particles of a desirable particle size.

[0127] The magnetic polymerization toner polymerized through the suspension polymerization process is caused to comprise individual toner particles having a uniformly spherical shape, so that it is easy to obtain a toner having a circularity of at least 0.970 as a preferred physical requirement of the present invention, and further such a magnetic toner has a relatively uniform chargeability distribution, thus exhibiting a high transferability.

[0128] However, by using a monomeric mixture containing ordinary magnetic powder at the time of suspension polymerization, it is difficult to suppress the exposure of the magnetic powder to the resultant toner particle surface, the resultant toner particles are liable to have remarkably lower flowability and chargeability, and also it is difficult to obtain a magnetic toner having a desirable circularity because of strong interaction between the magnetic powder and water. This is (1) because magnetic powder particles are generally hydrophillic, thus being liable to be localized at the toner particle surfaces, and (2) because at the time of suspension of the monomeric mixture in an aqueous medium or at the time of stirring the suspension liquid during the polymerization, the magnetic powder is moved at random within the suspended liquid droplets and the suspended liquid droplet surfaces comprising the monomer are pulled by the randomly moving magnetic powder, thereby distorting the liquid droplets from spheres. In order to solve such problems, it is important to modify the surface property of the magnetic iron oxide powder.

[0129] As for magnetic powder used in the magnetic toner of the present invention, it is extremely preferred that the magnetic iron oxide particles are surface-treated for hydrophobization by dispersing magnetic iron oxide particles in an aqueous medium into primary particles thereof, and while maintaining the primary particle dispersion state, hydrolyzing a coupling agent in the aqueous medium to surface-coat the magnetic iron oxide particles. According to this hydrophobization method in an aqueous medium, the magnetic iron oxide particles are less liable to coalesce with each other than in a dry surface-treatment in a gaseous system, and the magnetic iron oxide particles can be surface-treated while maintaining the primary particle dispersion state due to electrical repulsion between hydrophobized magnetic iron oxide particles.

[0130] The method of surface-treatment of magnetic iron oxide particles with a coupling agent while hydrolyzing the coupling agent in an aqueous medium does not require gas-generating coupling agents, such as chlorosilanes or silazanes, and allows the use of a high-viscosity coupling agent which has been difficult to use because of frequent coalescence of magnetic iron oxide particles in a conventional gaseous phase treatment, thus exhibiting a remarkable hydrophobization effect.

[0131] As a coupling agent usable for surface-treating the magnetic iron oxide particles used in the present invention, a silane coupling agent or a titanate coupling agent may be used. A silicone coupling agent is preferred, and examples thereof may be represented by the following formula (I):

R _m SiY _n (I),

[0132] wherein R denotes an alkoxy group, Y denotes a hydrocarbon group, such as alkyl, vinyl, glycidoxy or methacryl, and m and n are respectively integers of 1-3 satisfying m+n=4.

[0133] Examples of the silane coupling agents represented by the formula (I) may include: vinyltrimethoxysilane, vinyltriethoxysilane, gamma-methacryloxypropyltrimethoxysilane, vinyltriacetoxysilane, methyltrimethoxysilane, methyltriethoxysilane, isobutyltrimethoxysilane, dimethyldimethoxysilane, dimethyldiethoxysilane, trimethylmethoxysilane, hydroxypropyltrimethoxysilane, phenyltrimethoxysilane, n-hexadecyltrimethoxysilane, and n-octadecyltrimethoxysilane.

[0134] It is particularly preferred to use an alkyltrialkoxysilane coupling agent represented by the following formula (II) to treat the magnetic powder for hydrophobization in an aqueous medium:

C _p H _2p+1 —Si—(OC _q H _2q+1 ) ₃ (II),

[0135] wherein p is an integer of 2-20 and q is an integer of 1-3.

[0136] In the above formula (II), if p is smaller than 2, the hydrophobization treatment may become easier, but it is difficult to impart a sufficient hydrophobicity, thus making it difficult to suppress the exposure of the magnetic powder to the toner particle surfaces. On the other hand, if p is larger than 20, the hydrophobization effect is sufficient, but the coalescence of the magnetic powder particles becomes frequent, so that it becomes difficult to sufficiently disperse the treated magnetic powder particles in the toner, thus being liable to result in a toner exhibiting lower fog-prevention effect and transferability.

[0137] If q is larger than 3, the reactivity of the silane coupling agent is lowered, so that it becomes difficult to effect sufficient hydrophobization.

[0138] It is particularly preferred to use an alkyltrialkoxysilane coupling agent represented by the formula (II) wherein p is an integer of 3-15, and q is an integer of 1 or 2.

[0139] The coupling agent may preferably be used in 0.05-20 wt. parts, more preferably 0.1-10 wt. parts, per 100 wt. parts of the magnetic powder.

[0140] Herein, the term “aqueous medium” means a medium principally comprising water. More specifically, the aqueous medium includes water alone, and water containing a small amount of surfactant, a pH adjusting agent or/and an organic solvent.

[0141] As the surfactant, it is preferred to use a nonionic surfactant, such as polyvinyl alcohol. The surfactant may preferably be added in 0.1-5 wt. parts per 100 wt. parts of water. The pH adjusting agent may include an inorganic acid, such as hydrochloric acid. The organic solvent may include methanol which may preferably be added in a proportion of 0-500 wt. % of water.

[0142] For the surface-treatment of magnetic iron oxide particles with a coupling agent in an aqueous medium, appropriate amounts of magnetic iron oxide particles and coupling agent may be stirred in an aqueous medium. It is preferred to effect the stirring by means of a mixer having stirring blades, e.g., a high-shearing force mixer (such as an attritor or a TK homomixer) so as to disperse the magnetic iron oxide particles into primary particles in the aqueous medium under sufficient stirring.

[0143] The thus-surface treated magnetic iron oxide is free from particle agglomerates and individual particles are uniformly surface-hydrophobized. Accordingly, the magnetic powder is uniformly dispersed in polymerization toner particles to provide almost spherical polymerization toner particles free from surface-exposure of the magnetic iron oxide. As a result, by using such magnetic iron oxide particles, it becomes possible to provide a magnetic toner having Ca≧0.970, Cmod≧0.990 and Fe.iso (%)≦1.50%.

[0144] If such a magnetic toner is used in the image forming method of the present invention, the abrasion of and toner melt-attachment onto the photosensitive member are further effectively suppressed, and it becomes possible to form high-quality images stably even in a low humidity environment.

[0145] Further, while the magnetic toner has a uniformly high chargeability due to presence of little or no surface-exposed magnetic iron oxide, the magnetic toner can provide good image during image formation on a large number of sheets in a low temperature/low humidity environment due to the presence of electroconductive fine powder at the magnetic toner particle surfaces.

[0146] It is preferred that the magnetic toner used in the present invention contains a wax as described below in a proportion of 0.1-20 wt. % thereof.

[0147] In the image forming method, a magnetic toner image transferred onto a transfer(-receiving) material, such as paper, is thereafter fixed onto the transfer material by application of energy, such as heat and/or pressure, to provide a semipermanently fixed image. In this instance, a heat-pressure fixing scheme, such as a hot roller-fixing scheme, is frequently adopted.

[0148] By using a weight-average particle size of at most 10 μm, it is possible to obtain a very high-definition image, but such a small-particle size magnetic toner particles are liable to be buried between fibers of paper as a typical transfer medium and fail to receive sufficient heat, thus being liable to cause low-temperature offset. However, by including an appropriate amount of a wax as a release agent, the magnetic toner used in the present invention can satisfy both a high resolution and anti-offset characteristic as well as prevention of abrasion of the photosensitive member.

[0149] Examples of waxes usable in the magnetic toner used in the present invention may include: petroleum waxes and derivatives thereof, such as paraffin wax, microcrystalline wax and petrolactum; montan wax and derivatives thereof; hydrocarbon wax by Fischer-Tiropsch process and derivative thereof; polyolefin waxes as represented by polyethylene wax and derivatives thereof; and natural waxes, such as carnauba wax and candelilla wax and derivatives thereof. The derivatives may include oxides, block copolymers with vinyl monomers, and graft-modified products. Further examples may include: higher aliphatic alcohols, fatty acids, such as stearic acid and palmitic acid, and compounds of these, acid amide wax, ester wax, ketones, hardened castor oil and derivatives thereof, vegetable waxes and animal waxes.

[0150] Among such waxes, it is preferred to use a wax showing a maximum heat-absorption peak in a temperature range of 40-110° C., more preferably 45-90° C., in the course of temperature increase on a DSC cure measured by using a differential scanning calorimeter. The inclusion of a wax having a maximum heat-absorption peak in the above-mentioned temperature range, contributes to improvements in low-temperature fixability and effective releasability. If the maximum heat-absorption peak temperature (Tabs.max) is below 40° C., the wax is liable to exhibit only a weak self-cohesion, thus lowering the anti-high-temperature offset characteristic. On the other hand, if Tabs.max exceeds 110° C., the fixation temperature is raised so that low-temperature offset is liable to occur. Further, in the case of production of magnetic toner particles by particle formation and polymerization in an aqueous medium, the wax is liable to precipitate during the particle formation.

[0151] The maximum heat-absorption peak temperature (Tabs.max) of a wax may be measured by using a differential scanning calorimeter (DSC) (e.g., “DSC-7”, available from Perkin-Elmer Corp.) according to ASTM D3418-8. Temperature correction of the detector may be effected based on melting points of indium and zinc, and calorie correction may be effected based on heat of fusion of indium. For the measurement, a sample is placed on an aluminum pan and subjected to heat at an increasing rate of 10° C./min in parallel with a blank aluminum pan as a control.

[0152] The magnetic toner used in the present invention may preferably contain such a wax in a proportion of 0.1-20 wt. % of the entire magnetic toner. Below 0.1 wt. %, the low-temperature offset-suppression effect is lowered, and above 20 wt. %, the long-term storability is lowered and the dispersibility of the other toner ingredients becomes lowered to result in lower flowability and image forming performances of the resultant magnetic toner.

[0153] The magnetic toner used in the present invention can further contain a charge control agent so as to stabilize the chargeability. Known charge control agents can be used. It is preferred to use a charge control agent providing a quick charging speed and stably providing a constant charge. In the case of polymerization toner production, it is particularly preferred to use a charge control agent showing low polymerization inhibition effect and substantially no solubility in aqueous dispersion medium.

[0154] Specific examples of negative charge control agents may include: metal compounds of aromatic carboxylic acids, such as salicylic acid, alkylsalicylic acids, dialkylsalicylic acids, naphthoic acid, and dicarboxylic acids; metal salts or metal complexes of azo-dyes and azo pigments; polymeric compounds having a sulfonic acid group or carboxylic acid group in side chains; boron compounds, urea compounds, silicon compounds, and calixarenes.

[0155] Positive charge control agents may include: quaternary ammonium salts, polymeric compounds having such quaternary ammonium salts in side chains, quinacridone compounds, nigrosine compounds and imidazole compounds.

[0156] The charge control agent may be included in the toner by internal addition or external addition to the toner particles. The amount of the charge control agent can vary depending on toner production process factors, such as binder resin species, other additives and dispersion methods, but may preferably be 0.1-10 wt. parts, more preferably 0.1-5 wt. parts, per 100 wt. parts of the binder resin.

[0157] In the case of providing a negatively chargeable magnetic toner, it is preferred to add a metal salt or a metal complex of an azo dye or an azo pigment.

[0158] However, it is not essential for the magnetic toner used in the present invention to contain a charge control agent, but the toner need not necessarily contain a charge control agent by positively utilizing the triboelectrification with a toner layer thickness-regulating member and a toner-carrying member.

[0159] Next, description will be made on the magnetic iron oxide and the binder resin contained in the magnetic toner particles.

[0160] The magnetic toner particles contain at least particles of a magnetic iron oxide, such as magnetite, maghemite, or ferrite.

[0161] The magnetic iron oxide particles may preferably have a BET specific surface area (SBET) of 2-30 m/g, more preferably 3-28 m/g, as measured according to nitrogen-adsorption, and a Mohs' hardness of 5-7.

[0162] For providing the magnetic toner used in the present invention, the magnetic iron oxide particles may preferably be used in 10-200 wt. parts, more preferably 20-180 wt. parts, per 100 wt. parts of the binder resin. Below 10 wt. parts, the coloring power of the resultant magnetic toner is liable to be insufficient, and the suppression of fog becomes difficult. Above 200 wt. parts, the resultant toner is held at an excessively large force onto the toner-carrying member, to show a lower developing performance. Moreover, the dispersion of magnetic iron oxide particles to individual toner particles becomes difficult, and the fixability is lowered.

[0163] The magnetic iron oxide particles used for constituting the magnetic toner used in the image forming method of the invention may be produced, e.g., in the following manner, in the case of magnetite-based magnetic iron oxide.

[0164] To a ferrous salt aqueous solution, an alkali, such as sodium hydroxide, in an amount equivalent to the iron in the ferrous salt or larger to prepare an aqueous solution containing ferrous hydroxide. While retaining the pH of the thus-prepared aqueous solution at pH 7, preferably pH 8-10 and warming the aqueous solution at a temperature of 70° C. or higher, air is blown into the aqueous solution to oxidize the ferrous hydroxide, thereby first forming seed crystals functioning as nuclei of magnetic iron oxide particles to be produced.

[0165] Then, to the slurry-form liquid containing the seed crystals, an aqueous solution containing ferrous salt in an amount of ca. 1 equivalent based on the amount of the previously added alkali, is added. While keeping the liquid at pH 6-10, air is blown thereinto to proceed with the reaction of the ferrous hydroxide, thereby growing magnetic iron oxide particles around the seed crystals as nuclei. Along with the progress of the oxidation reaction, the liquid pH is shifted toward an acidic side, but it is preferred not to allow the liquid pH go down to below 6. At a final stage of the oxidation, the liquid pH is adjusted, and the slurry liquid is sufficiently stirred so as to disperse the magnetic iron oxide in primary particles. In this state, a coupling agent for hydrophobization is added to the liquid to be sufficiently mixed under stirring. Thereafter, the slurry is filtered out and dried, and the dried product is lightly disintegrated to provide hydrophobic treated magnetic iron oxide particles.

[0166] Alternatively, the iron oxide particles after the oxidation reaction may be washed, filtered out and then, without being dried, re-dispersed in another aqueous medium. Then, the pH of the re-dispersion liquid is adjusted and subjected to hydrophobization by adding a coupling agent under sufficient stirring. Anyway, it is preferred that untreated iron oxide particles formed in the oxidation reaction system are subjected to hydrophobization in their wet slurry state and without being dried prior to the hydrophobization.

[0167] As the ferrous salt used in the above-mentioned production process, it is generally possible to use ferrous sulfate by-produced in the sulfuric acid process for titanium production or ferrous sulfate by-produced during surface washing of steel sheets. It is also possible to use ferrous chloride. In the above-mentioned process for producing magnetic iron oxide from a ferrous salt aqueous solution, a ferrous salt concentration of 0.5-2 mol/liter is generally used so as to obviate an excessive viscosity increase accompanying the reaction and in view of the solubility of a ferrous salt, particularly of ferrous sulfate. A lower ferrous salt concentration generally tends to provide finer magnetic iron oxide particles. Further, as for the reaction conditions, a higher rate of air supply, and a lower reaction temperature, tend to provide finer product particles.

[0168] By using a magnetic toner containing the thus-produced hydrophobic magnetic iron oxide particles, it becomes possible to realize an image forming method wherein the abrasion of and toner attachment onto the photosensitive member are effectively suppressed to stably provide high-quality images.

[0169] The magnetic iron oxide particles may have octahedral, hexahedral, spherical, acicular or flaky shape, but magnetic iron oxide particles having less anisotropic shapes, such as octahedral, hexahedral or spherical are preferred in order to provide a high image density. Such particle shapes may be confirmed by observation through a scanning electron microscope (SEM).

[0170] It is preferred that the magnetic iron oxide particles have a volume-average particle size of 0.1-0.3 μm and contain at most 40% by number of particles of 0.03-0.1 μm, based on measurement of particles having particle sizes of at least 0.03 μm, also in view of magnetic properties of the magnetic iron oxide particles. It is further preferred that the amount of particles of 0.3 μm or larger is suppressed to at most 10% by number.

[0171] Magnetic iron oxide particles having an average particle size of below 0.1 μm are not generally preferred because they are liable to provide a magnetic toner giving images which are somewhat tinted in red and insufficient in blackness with enhanced reddish tint in halftone images. Further, as the magnetic iron oxide particles are caused to have an increased surface area, the dispersibility thereof is lowered, and an inefficiently larger energy is consumed for the production. Further, the coloring power of the magnetic iron oxide particles can be lowered to result in insufficient image density in some cases.

[0172] On the other hand, if the magnetic iron oxide particles have an average particle size in excess of 0.3 μm, the weight per one particle is increased to increase the probability of exposure thereof to the toner particle surface due to a specific gravity difference with the binder during the production. Further, the wearing of the production apparatus can be promoted and the dispersion stability of a monomer composition containing the magnetic iron oxide particles is liable to become unstable.

[0173] Further, if particles of 0.1 μm or smaller exceed 40% by number of total particles (having particle sizes of 0.03 μm or larger), the magnetic iron oxide particles are liable to have a lower dispersibility because of an increased surface area, liable to form agglomerates in the toner to impair the toner chargeability, and are liable to have a lower coloring power. If the percentage is lowered to at most 30% by number, the difficulties are preferably alleviated.

[0174] Incidentally, magnetic iron oxide particles having particle sizes of below 0.03 μm receive little stress during the toner production so that the probability of exposure thereof to the toner particle surface is low. Further, even if such minute particles are exposed to the toner particle surface, they do not substantially function as leakage sites lowering the chargeability of the toner particles. Accordingly, the particles of 0.03-0.1 μm are noted herein, and the percentage by number thereof is suppressed to below a certain limit.

[0175] On the other hand, if particles of 0.3 μm or larger exceed 10% by number, the magnetic iron oxide particles are caused to have a lower coloring power, thus being liable to result in a lower image density. Further, as the number of magnetic iron oxide particles is decreased at an identical weight percentage, it becomes difficult statistically to have the magnetic iron oxide particles be present up to the proximity of the toner particle surface and distribute equal numbers of magnetic iron oxide particles to respective toner particles. This is undesirable. It is further preferred that the percentage be suppressed to at most 5% by number.

[0176] In the present invention, it is preferred that the magnetic iron oxide production conditions are adjusted so as to satisfy the above-mentioned conditions for the particle size distribution, or the produced magnetic iron oxide particles are used for the toner production after adjusting the particle size distribution as by pulverization and/or classification. The classification may suitably be performed by utilizing sedimentation as by a centrifuge or a thickener, or wet classification using, e.g., a cyclone.

[0177] The volume-average particle size and particle size distribution of iron oxide particles described herein are based on values measured in the following manner.

[0178] Sample particles in a sufficiently dispersed state are photographed at a magnification of 3×10 ⁴ through a transmission electron microscope (TEM), and 100 particles each having a particle size of at least 0.03 μm selected at random in visual fields of the taken photographs are subjected to measurement of projection areas. The particle size (projection area-equivalent circle diameter (DCE)) of each particle is determined as a diameter of a circle having an area equal to the measured projection area of the particle. Based on the measured particle sizes of the 100 particles, a volume-average particle size (Dv=(Σ(nD _CE ³ )/Σn) ^{{fraction (1/30)}} ), percentage by number of particles of 0.03 μm-0.1 μm and percentage by number of particles of 0.3 μm or larger are determined.

[0179] The volume-average particle size and particle size distribution of magnetic iron oxide particles dispersed within toner particles may be measured in the following manner.

[0180] Sample toner particles are sufficiently dispersed in a cold-setting epoxy resin, which is then hardened for 2 days at 40° C. The hardened product is sliced into thin flakes by a microtome. The thin flakes are observed through a TEM and photographic at magnification of 1×10 ⁴ -4×10 ⁴ . One hundred iron oxide particles of at least 0.03 μm in particle size selected at random in visual fields of the taken photographs are subjected to measurement of projection areas. From the projection areas of the 100 iron oxide particles, a volume-average particle size (projection area-equivalent circular diameter), percentage by number of particles of 0.03 μm-0.1 μm and percentage by number of particles of 0.3 μm or larger are determined similarly as the above.

[0181] The magnetic iron oxide particles may preferably have magnetic properties including a saturation magnetization of 10-200 Am ² /kg as measured at a magnetic field of 795.8 kA/m, a residual magnetization of 1-100 Am ² /kg, and a coercive force of 1-30 kA/m.

[0182] It is particularly preferred that the magnetic toner used in the present invention has a magnetization of 10-50 Am/kg at a magnetic field of 79.6 kA/m (1000 oersted).

[0183] The magnetization at a magnetic field of 79.6 mA/m is taken as a property of a magnetic toner in a magnetic field realized in an actual image forming apparatus, while the saturation magnetization is used as a parameter representing magnetic properties of magnetic iron oxide. The magnetic field acting on magnetic toners is most commercially available image fo