JPS6350695B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to improvements in magnetic developers, and more particularly to one-component magnetic developers for electrophotography that are excellent in image density, tone, sharpness (notch), resolution, and reproducibility of halftones. Conventionally, when developing an electrostatic latent image, a so-called one-component magnetic developer containing powder of a magnetic material in developer particles has been used as a developer capable of developing a latent image without using a special carrier. widely known. As one type of this one-component magnetic developer, developer particles contain fine powder of magnetic material to impart magnetic attraction properties, and conductive agents such as conductive carbon black are added to the surface of the particles. So-called conductive magnetic developers are also known, which are made to have conductivity by distributing them (for example, as disclosed in U.S. Pat.
3639245 and 3965022). When this conductive magnetic developer is brought into contact with the electrostatic latent image supporting substrate in the form of a so-called magnetic brush and the latent image is developed, it provides an excellent visible image without so-called edge effects or fog. It is also known that quite serious problems arise when an image of this developer is transferred from a substrate onto ordinary transfer paper. That is, as described in JP-A-50-117435, the specific electrical resistance of the transfer paper used is 3x like that of plain paper.
If it is lower than 10 ¹³ Ω-cm, there is a tendency for outline broadening and transfer efficiency to decrease due to scattering of developer particles during transfer. Although this tendency can be improved to some extent by applying high electrical resistance resin, wax, or oil to the toner-receiving surface of the transfer paper, such improvement effect is relatively small under high humidity conditions. In addition, coating with resin or the like increases the cost of the transfer paper, and furthermore, there are disadvantages such as a decrease in texture. As another type of one-component magnetic developer, a one-component non-conductive magnetic developer consisting of a granular mixture of fine powder of magnetic material and an electroscopic binder is already known. For example, US Pat. No. 3,645,770 discloses that a magnetic brush (layer) of the above-mentioned non-conductive magnetic developer is charged by corona discharge to an electric charge of opposite polarity to that of the electrostatic latent image to be developed. An electrostatographic reproduction process is disclosed which comprises contacting a developed developer with an electrostatic latent image supporting substrate to develop the latent image and then transferring the formed developer image to a transfer paper. This electrophotographic copying method has the advantage of being able to form a transferred image on transfer paper made of so-called plain paper, but it is also difficult to uniformly charge the deep part of the magnetic brush of the non-conductive magnetic developer. In general, it is difficult to form images with sufficiently high density, and furthermore, since a corona discharge mechanism must be provided in the developing device, there are disadvantages such as the complexity of the device. Recently, a method for developing an electrostatic latent image using charging of the developer caused by friction between a non-conductive magnetic developer and the surface of the electrostatic latent image supporting substrate (Japanese Unexamined Patent Application Publication No. 1989-1989) has been developed.
62638), and a method of developing using dielectric polarization of a non-conductive magnetic developer (Japanese Patent Application Laid-open No. 1983-
133026) has already been proposed, but in the former method, the developing conditions must be strictly controlled, otherwise fog in non-image areas (between the photoreceptor surface and magnetic toner particles) may occur. This is particularly likely to occur when there is strong mutual contact with the tips of the ears)
This causes the occurrence of magnetic toner particles, adhesion of magnetic toner particles onto the developing sleeve, and blocking, which becomes an important problem especially when continuous copying is performed. Also,
In the latter case, fogging is not a problem, but because a developing charge is obtained by the dielectric polarization effect induced in the magnetic toner against the electrostatic latent image and a visible image is formed, it is disadvantageous for the low-potential latent image area. It becomes a state. Therefore, in the resulting copy, it is difficult to copy the low-density portions of the original, and it is difficult to reproduce halftones in the copy. Furthermore, the copies obtained by both methods lack sharpness, and when a P-type photoreceptor such as selenium is used as a photosensitive plate and a positively charged image is developed, no matter which method is used, It is difficult to form an image with sufficiently high density. Furthermore, U.S. Pat. No. 4,102,305 describes a one-component magnetic developer whose electrical resistance changes depending on the electric field strength, i.e., one component that is substantially conductive in high electric fields, while having high electrical resistance in low electric fields. Using a component-based magnetic developer, development is performed under conditions where a high voltage is applied between the magnetic brush forming sleeve and the photosensitive plate to make the developer particles conductive, while the developer particles are transferred to copy paper. It is disclosed that excellent reproduced images can be formed by performing the transfer in a low electric field or no electric field. This specification also states that the above-mentioned developer whose electrical resistance is highly dependent on the electric field is composed of 50% by weight of magnetite coated with stearate and 50% by weight of styrene-n-butyl methacrylate copolymer.
It is disclosed that it can be obtained by spray granulation of % by weight. Although this method is excellent in terms of the above idea, it has the disadvantage of requiring a special high-voltage device for development, and although the formed image is certainly high in density, the sharpness of the image is still insufficient. is not satisfactory. Furthermore, US Pat. No. 4,121,931 states:
Using an electrically insulating one-component magnetic developer, a magnetic brush-forming sleeve is used as an electrode, and a voltage is applied between this electrode and the photosensitive plate, giving intense turbulent agitation to the developer on the sleeve for development. It is disclosed that the agent particles are uniformly charged. This invention not only requires a high voltage device in the developing device section, but also
There is a problem in that special measures are required to agitate the developer particles on the sleeve. In this way, past research on one-component magnetic developers and development methods using them has focused on the developer composition,
The current situation is that research has focused exclusively on the developer manufacturing method, the charging method of developer particles, etc., and there has been almost no research on the characteristics of the magnetite itself contained in the developer. Generally, when a magnetic brush of a one-component magnetic developer is brought into contact with the surface of a substrate supporting an electrostatic latent image,
Both the electrostatic attraction force (Coulomb force) between the developer particles and the electrostatic latent image and the magnetic attraction force between them and the magnet for forming the magnetic brush act on each developer particle.
Thus, developer particles with a greater Coulomb force will be attracted toward the electrostatic latent image, while developer particles with a greater magnetic attraction will be attracted toward the developer sleeve and will be attracted to the electrostatic latent image on the substrate. Development will be performed depending on the image. Thus, a one-component magnetic developer requires a certain balance between magnetic properties and charging properties during development. Thus, it will be understood that in a one-component magnetic developer, the characteristics of the magnetic material powder used also have an important effect on the characteristics of the image formed. According to the invention, a powder magnetic material is dispersed in a binding resin having a resistance value of at least 1×10 ¹⁵ Ωcm and consisting of a homopolymer or a copolymer of vinyl aromatic monomers or acrylic monomers. In the one-component magnetic developer which is a fine particle composition comprising: (A) a cubic aggregate of magnetite;
The agglomerates (A) are composed of non-friable agglomerates which are aggregates that do not substantially change the particle size distribution even when subjected to ball milling for hours; and (B) magnetite particles of 0.2 to 1.0 μm; is a particle size of 1 to 10 microns measured with an electron microscope, JIS (Japanese Industrial Standards)
Apparent density of 0.5 to 1.5 g/ml, saturation magnetization of 75 to 88 emu/g, measured by the method of K-5101, 3 to 1.5 g/ml.
It has a residual magnetization of 12 emu/g and a coercive force of 40 to 150 oersted, and the weight ratio of agglomerate (A) and magnetite (B) is A/B = 95/5 to 30/70. A one-component magnetic developer is provided. According to the present invention, the non-friable agglomerate particles described above are used as part of the magnetite in a one-component magnetic developer, thereby making it possible to develop a one-component magnetic developer using conventional acicular, cubic or amorphous magnetite. Compared to developers, it can significantly improve image sharpness (cuts) and resolution,
Furthermore, the reproducibility of intermediate tones can also be improved. Furthermore, these non-friable agglomerate particles,
When used as a magnetic material in combination with fine ordinary magnetite particles in a certain ratio, image density can be significantly improved compared to when either one is used alone, and the color tone of the image can be improved. A desirable color tone close to pure black can be achieved. The first component of the magnetic material powder used in the present invention is:
As shown in the electron micrograph of FIG. 1 and the X-ray diffraction image of FIG. 2, it is magnetite consisting of non-crushable agglomerates of cubic particles. In this specification, non-friable agglomerates are those in which fine particles are densely aggregated, as is clear from FIG. , refers to aggregates that do not substantially change the particle size distribution. This non-friable agglomerate has a particle size of 1 to 10 microns, especially 2 to 7 microns, as measured by electron microscopy.
It is in the micron range and has a coarser particle size than normal magnetite particles. The magnetic material particles of the present invention have the above-mentioned dense agglomerated structure and are relatively coarse in particle size. Compared to crystalline or amorphous magnetite particles, it has a characteristic that its volume per unit weight, that is, its bulk, is small. Thus, in the one-component magnetic developer of the present invention, when the weight ratio of magnetite is kept constant, the resin/
It is possible to make the volume ratio of magnetite considerably larger than that of conventional one-component magnetic developers, and this allows the one-component magnetic developer to have more of the charging characteristics unique to resin. will be understood. It has been known that polymer materials with higher dielectric constants are more likely to be positively charged due to triboelectric charging (The Society of Photographic Scientists and
Engineers 2nd Int. Conf. 1974, pp. 95-100).
Furthermore, the present inventors have found that even in a magnetic developer formed by dispersing magnetic material powder in a fixing medium, if the dielectric constant of this magnetic developer is small, it is likely to be negatively charged, and if it is large, it is likely to be triboelectrically charged. It has been discovered. In fact, developer particles containing 55% by weight of magnetite, which is conventionally used in styrenic resin, exhibit a dielectric constant of 3.85 to 4.05, whereas the aforementioned cubic non-crushable agglomerates It was confirmed that when the same amount was mixed, the dielectric constant was 3.79, and the material was more likely to be negatively charged. As described above, the first component of the magnetic material powder used in the present invention has a smaller bulk than ordinary magnetite,
That is, it has a large apparent density, for example, JIS K5101
It has an apparent density of 0.5 to 1.5 g/ml, in particular 0.7 to 1.3 g/ml, as measured by the method. Furthermore, this non-fragile agglomerate of cubic particles has a saturation magnetization of 75 to 88 emu/g, 3 to 88 emu/g.
It has magnetic properties of a remanent magnetization of 12 emu/g and a coercive force of 40 to 150 oersteds. Non-crushable agglomerates of cubic particles used in the present invention include, but are not limited to,
Manufactured by the following method. That is, a weak alkaline aqueous solution such as aqueous ammonia is added to an aqueous solution of iron sulfate () to form a precipitate of iron hydroxide (). This precipitate was subjected to pressure hydrothermal treatment with the pH of the mother liquor set to 3 to 9, and the pseudoglue-like precipitate of iron hydroxide was transformed into fine cubic α
-Change to Fe ₂ O ₃ (Hematite). That is, in this case, by using the above-mentioned weak alkali and bringing the pH of the mother liquor close to the acidic side, fine cubic particles that are easily agglomerated are generated, and furthermore, the hydrothermal treatment is
By aging at a temperature of 230°C for a long time of 50 hours or more, α-iron sesquioxide in the form specified in the present invention can be obtained. By reducing the obtained α-iron sesquioxide under conditions known per se, for example, with hydrogen in a reduction furnace at a temperature of 400°C, triiron tetroxide (Fe ₃ O ₄ ) in the form described above can be obtained. . The reduction treatment is performed so that the atomic ratio of Fe ²⁺ /Fe ³⁺ in triiron tetroxide is generally from 0.9/1.0 to 1.1/1.0 to obtain triiron tetroxide having the above-mentioned microstructure. The X-ray diffraction image of the magnetite used as the first magnetic material component in the present invention is the same as that of ordinary cubic magnetite, and judging from the height of the diffraction peak, there is no particular difference in the degree of crystallinity. It was confirmed that there was no. In the present invention, the above-mentioned non-crushable agglomerate of magnetite (A) and magnetite (B) having a fine internal particle size of normal magnetite, that is, a number average particle size of 0.2 microns or more and less than 1 micron, A:B is used in combination at a weight ratio of 95:5 to 30:70, particularly 90:10 to 40:60. That is, when these two components are used in combination in the above-mentioned ratio as a magnetic material powder, a significant increase in image density can be obtained compared to the case where either component is used alone, and this increase in image density This is achieved without sacrificing the advantages of non-crushable agglomerate magnetite. Magnetite, a non-fragile agglomerate, is particularly suitable for the purpose of improving image sharpness (cuts), resolution, and midtone reproduction, but it produces images with a color bias from pure black to slightly brownish. There is still room for improvement in terms of ease of giving. According to the invention,
By combining this agglomerate magnetite with fine-grained magnetite, the color tone of the image can be made pure black or close to pure black. This fine particle size magnetite has a particle size of
Any form of magnetite can be used, such as acicular, cubic or amorphous forms, so long as it is greater than or equal to 0.2 microns and less than 1 micron, more preferably from 0.3 to 0.8 microns. The above-mentioned form of magnetite is based on the crystal form of the raw material α-iron sesquioxide (hematite). These magnetite fine particles have a large bulk due to their fine particle size, and the apparent density measured according to JIS K 5101 is generally in the range of 0.2 to less than 0.45 g/ml. As the fixing medium in which these magnetites are dispersed, resins, wax-like substances or rubbers which exhibit fixing properties under the application of heat or pressure are used.
These fixing media can be used alone or in combination of two or more types, but these fixing media have a
It is desirable to have a volume resistivity of ×15 ¹⁵ Ω-cm or more. As the fixing medium, various mono- or diethylenically unsaturated monomers are used, especially homopolymers and copolymers of (a) vinyl aromatic monomers, and (b) acrylic monomers. Ru. The vinyl aromatic monomer has the following formula: In the formula, R ₁ is a hydrogen atom, lower (having 4 or less carbon atoms)
is an alkyl group or a halogen atom, R ₂ is a substituent such as a lower alkyl group or a halogen atom, and n is an integer of 2 or less including zero, such as styrene, vinyltoluene, Examples include α-methylstyrene, α-chlorostyrene, vinylxylene, and vinylnaphthalene. Among these, styrene,
Vinyltoluene is preferred. As an acrylic monomer, the following formula In the formula, R ₃ is a hydrogen atom or a lower alkyl group, and R ₄ is a hydroxyl group, an alkoxy group, a hydroxyalkoxy group, an amino group, or an aminoalkoxy group. Methacrylic acid, ethyl acrylate, methyl methacrylate, butyl acrylate, butyl methacrylate, 2-ethylhexyl acrylate, 2-ethylhexyl methacrylate, 3-hydroxypropyl acrylate, 2-hydroxyethyl methacrylate, 3-aminopropyl acrylate, 3-N,N-diethylamino Examples include propyl acrylate and acrylamide. Examples of other monomers used alone or in combination with these monomers (a) or (b) include those of the following formula: In the formula, R ₅ is a hydrogen atom, a lower alkyl group, or a chlorine atom.Conjugated diolefin monomers represented by, for example, butadiene, isoprene, chloroprene, etc.Other examples include maleic anhydride, fumaric acid, crotonic acid, and itacon. Other ethylenically unsaturated carboxylic acids such as acids or their esters, vinyl esters such as vinyl acetate, vinylpyridine, vinylpyrrolidone,
Vinyl ethers, acrylonitrile, vinyl chloride, vinylidene chloride and the like can also be mentioned. The molecular weight of these vinyl polymers is 3000 or
300,000, preferably in the range of 5,000 to 200,000. In the present invention, the above-mentioned combination of magnetite is used per total amount of fixing medium and magnetic material powder.
Preferably, the magnetite is used in an amount of 35 to 75% by weight, especially 40 to 70% by weight, and the magnetite is uniformly and uniformly kneaded in this fixing medium, and then granulated.
A one-component dry type magnetic developer. Prior to kneading and granulating the developer components, auxiliary components of the developer that are known per se may be blended according to a recipe that is known per se. For example, to improve the color tone of the developer, pigments such as carbon black,
Dyes such as acid violet alone or in combination of two or more, 0.5 to 5% by weight of the total
Can be used in amounts of In addition, for the purpose of increasing the volume, fillers such as calcium carbonate and finely powdered silicic acid are added to the total weight.
It can be incorporated in amounts up to 20% by weight. In the method of fixing the developer with a heated roll, an offset preventing agent such as silicone oil, low molecular weight olefin resins, various waxes, etc. can be used in an amount of 2 to 15% by weight based on the total amount. Further, in applications where the developer is fixed with a pressure roll, a pressure fixing agent such as paraffin wax, various animal/vegetable waxes, fatty acid amide, etc. may be used in an amount of 5 to 30% by weight based on the total amount. Furthermore, in order to prevent mutual agglomeration of developer particles and improve its fluidity, a fluidity improver such as polytetrafluoroethylene fine powder or fine powder silica is added in an amount of 0.1 to 1.5% by weight based on the total amount. May be blended. For molding, the above-mentioned kneaded composition is cooled, then pulverized and, if necessary, sieved. Of course, rapid mechanical stirring may be used to round off irregularly shaped particles. The particle size of the developer particles is related to resolution, etc., but is generally desirably in the range of 5 to 35 microns. According to the present invention, a developer made of amorphous particles formed by kneading and pulverization achieves a further increase in transfer efficiency and the formation of sharp images. In the electrostatographic copying method using the developer of the present invention, the formation of the electrostatic latent image can be carried out in any manner known per se, for example after uniformly charging the photoconductive layer on the conductive substrate. , an electrostatic latent image can be formed by imagewise exposure. The surface of the substrate having this electrostatic latent image is brought into contact with the magnetic brush of the one-component magnetic developer described above to form a visible image of the developer. To develop an electrostatic latent image using the developer of the present invention, a developer hopper is filled with the one-component magnetic developer described above. A non-magnetic sleeve is rotatably provided at the lower end opening of the hopper, and a magnet is provided inside the sleeve so as to be rotatable in the opposite direction to the sleeve. In this way, when the sleeve and magnet are rotated, a brush layer of magnetic developer is formed on the sleeve, and after this brush layer is cut to an appropriate length with a cutting board, a selenium drum rotating in the same direction as the sleeve is inserted. With light contact, the electrostatic image on the selenium drum is developed with a magnetic developer. Next, the developer image on the substrate is brought into contact with the transfer paper, and corona charging with the same polarity as the electrostatic latent image described above is performed from the back side of the transfer paper to transfer the developer image onto the transfer paper. In the present invention, the transferred image can be fixed by any method such as heat roller fixing, flash lamp fixing, or pressure roller fixing, depending on the type of developer. The developer of the present invention is particularly useful for developing p-type photosensitive plates having a positive latent image, such as selenium photosensitive plates and organic photoconductor photosensitive plates. Conventional triboelectric-component magnetic developers can generally be used to develop photosensitive plates with negatively charged latent images, but they cannot be used to develop positively charged latent images on p-type photosensitive plates as described above. It only shows extremely unsatisfactory results. In contrast, according to the present invention, excellent effects can be achieved in developing and transferring such positive charge latent images. The invention is illustrated by the following example. Example 1 38.5 parts by weight (hereinafter referred to as parts) of agglomerate magnetite (Fe ₃ O ₄ ) shown in Table 1, coercive force
72Oe, apparent density 0.40g/ml, particle size 0.3μ magnetite (e) 16.5 parts, styrene/butyl methacrylate copolymer (weight average molecular weight 27000) 37 parts, low molecular weight polypropylene (average molecular weight 4000) 8 parts,
0.5 part of zinc stearate is kneaded and melted at 140°C for 35 minutes using a two-roll mill, and after cooling, it is coarsely ground to a size of 10.5 to 2 mm using a cutting mill. Next, the powder is finely pulverized using a jet mill and classified using a zigzag classifier to obtain a magnetic toner having a particle size ranging from 5 to 35 microns. In addition, the lower limit of this particle size range is classified so that it is at least twice the particle size of magnetite, and hydrophobic silica (Nippon Aerosil Co., Ltd., R-
972) at 0.2 wt% of the total amount to prepare a magnetic toner.

[Table] The prepared magnetic toners are a', b', c' and d' (each mixed with magnetite e in Table 1),
Comparative examples include magnetic toners (a″, b″, c″, d″, and
e′) is prepared in the same manner. However, the particle size distribution of the e' toner was 5 to 25 microns. The magnetic properties were measured using a magnetic property measuring instrument manufactured by Toei Kogyo (Model VSMP-1 type, magnetic field 5K Oersted). The following copying test was conducted using these prepared magnetic toners. In a copying machine that uses a selenium drum (outer diameter 150 mm) as a photoconductor, the strength of the magnetic field on the developing sleeve (outer diameter 33 mm) containing a built-in magnet is set to about 900 Gauss through a non-magnetic member. The above magnetic toner was deposited on a developing roller of a so-called double-rotation system in which the sleeves could be rotated independently and individually, with a spacing of 0.3 mm between the cutting board and the sleeve.
The magnetic toner was arranged so that it could be supplied from the hopper to the developing roller, and the distance between the surface of the photoreceptor and the developing roller was 0.5 mm. Under rotating conditions in which the developing sleeve and photoconductor rotate in the same direction, and the magnet rotates in the opposite direction, electricity (+6.7KV),
Exposure, development, transfer (+6.3KV), heater roller fixing, and fur brush cleaning were performed.
High-quality paper with a thickness of 80 μm was used as the transfer paper. The copy test results are shown in Table 2. The image density was determined by measuring solid black areas using a commercially available reflection densitometer (manufactured by Konishiroku Photo Industry). The copy test chart uses a commercially available test pattern from Dataquest, and the gradation and resolution are judged from the copy, the sharpness is judged from the line image in the copy, and the notches are judged from the line image in the copy. The good ones were classified as excellent.

[Table] The magnetic toner of the present invention can be applied as is to developing devices that use conventionally used conductive magnetic toners, and can be used with plain transfer paper as is, resulting in clear copies and conductive magnetic toners. There was no image broadening or toner scattering that sometimes occurs when transferring magnetic toner, and a high-density black image could be obtained, with good halftone reproduction. Example 2 Agglomerate magnetite (apparent density 0.735
g/ml, number average particle size 2.8μ, coercive force 580e, saturation magnetization
87.2emu/g, residual magnetization 5.1emu/g) in combination with the four types of magnetite shown in Table 3, and in the same manner as in Example 1 with the composition ratio shown in Table 4, a magnetic toner (particle size distribution 6 to 20μ) was prepared. Created.

【table】

[Table] In addition to the above composition, 0.6 parts of zinc stearate was added, and styrene/styrene was added to the thermoplastic resin.
Acrylic copolymer (weight average molecular weight 71000),
High-density polyethylene with an average molecular weight of 4000 was used. Using these produced magnetic toners, the following copying test was conducted. In a copying machine that uses a selenium drum as a photoreceptor, the magnetic toner described above is deposited on a developing roller with a built-in magnet via a non-magnetic member with a spacing of 0.3 mm between the cutting plate and the developing roller, and the photosensitive The distance between the body surface and the developing roller is 0.5 mm,
Charging, exposure, development, transfer, and heat fixing were performed under conditions in which the developing roller was moved in the same direction as the photoreceptor and at a speed twice as fast as the photoreceptor. High-quality paper with a thickness of 80 μm was used as the transfer paper. The copy test results are shown in Table 5. Image density was measured at solid black areas.

[Table] It was found that the obtained copy image was clear with a high degree of blackness, and the reproducibility of halftones was also good. However, when the total magnetite content was less than 40% based on the total amount of the fixing resin medium and magnetic material powder, fogging was noticeable, and when it was more than 70%, the image density tended to decrease significantly. Also, using h magnetic toner,
I made 10,000 continuous copies, but the image density was
The value was 1.55 or more, and the condition was stable with no fogging observed. Note that when copies are made using toner made only of agglomerate magnetite, the color tone of the image is not pure black but has a bluish tinge. Furthermore, toners prepared using only the four types of magnetite shown in Table 3 produced copies with low overall image density and poor gradation.

[Brief explanation of the drawing]

FIG. 1 is an electron micrograph of magnetite composed of non-fragile agglomerates of cubic particles used in the present invention, and FIG. 2 is an X-ray diffraction image of the agglomerates of FIG. 1.

Claims (1)

[Claims] 1. Powdered magnetic material in a binder resin having a resistance value of at least 1×10 ¹⁵ Ωcm and consisting of a homopolymer or copolymer of a vinyl aromatic monomer or an acrylic monomer. In a one-component magnetic developer which is a fine particle composition formed by dispersing, the magnetic material is (A) an aggregate of cubes of magnetite, and 5
The agglomerates (A) are composed of non-friable agglomerates which are aggregates that do not substantially change the particle size distribution even when subjected to ball milling for hours; and (B) magnetite particles of 0.2 to 1.0 μm; is a particle size of 1 to 10 microns measured with an electron microscope, JIS (Japanese Industrial Standards)
Apparent density of 0.5 to 1.5 g/ml, saturation magnetization of 75 to 88 emu/g, measured by the method of K-5101, 3 to 1.5 g/ml.
It has a residual magnetization of 12 emu/g and a coercive force of 40 to 150 oersted, and the weight ratio of agglomerate (A) and magnetite (B) is A/B = 95/5 to 30/70. A one-component magnetic developer characterized by the presence of: 2. The magnetic material is present in an amount of 35 to 75% by weight based on the total of the binding resin and the magnetic material, and the developer has a particle size of 5 to 35 μm. Component magnetic developer.