JP2728929B2 - Negatively chargeable magnetic developer - Google Patents

Description

Description: FIELD OF THE INVENTION The present invention contains an insulating magnetic toner used as a developer for developing an electrostatic image in electrophotography, electrostatic recording, electrostatic printing, and the like. It relates to a negatively chargeable magnetic developer.

More specifically, in a direct or indirect electrophotographic developing method, a negatively chargeable magnetic toner containing a negatively chargeable magnetic toner that uniformly and strongly negatively charges and visualizes a negative electrostatic image by reversal development to give a high quality image. Related to the developer.

[Background Art] In recent years, as image forming apparatuses such as electrophotographic copying machines have become widespread, the applications thereof have been diversified, and the requirements for image quality have become strict. 2. Description of the Related Art In copying images such as ordinary documents and books, it is required to reproduce very finely and faithfully without crushing or breaking even fine characters. In particular, when the latent image on the photoreceptor of the image forming apparatus is a line image of 100 μm or less, thin line reproducibility is generally poor, and the line image is not yet sufficiently sharp. Recently, in an image forming apparatus such as an electrophotographic printer using a digital image signal, a latent image is formed by collecting dots of a constant potential, and a solid portion,
The halftone portion and the line portion are represented by changing the dot density. However, if the toner particles do not adhere exactly to the dots and the toner particles protrude from the dots, the gradation of the toner image corresponding to the dot density ratio of the black portion and the white portion of the digital latent image cannot be obtained. There is a problem. Furthermore, when the resolution is improved by reducing the dot size in order to improve the image quality, the reproducibility of a latent image formed from minute dots becomes more difficult, and the resolution and gradation are poor. Image tends to be lacking.

Although the image quality is good in the early stage, the image quality may deteriorate while copying or printing out. It is considered that this development is caused by the fact that only toner particles which are easily developed are consumed first while copying or printing is continued, and the toner particles having poor developability accumulate and remain in the developing machine.

Some developers have been proposed for the purpose of improving the image quality. JP-A-51-3244 proposes a non-magnetic toner intended to improve image quality by regulating the particle size distribution. In the toner, 8 to
The toner mainly has a particle diameter of 12 μm, and is relatively coarse. With this particle diameter, it is difficult for the present inventors to closely “glue” the latent image, and the particle diameter of 5 μm or less is 30 μm.
From the characteristic that the particle size distribution is not more than 5% by number when the particle size is 20% or more, the particle size distribution is also broad, which tends to lower the uniformity. In order to form a clear image using such a coarse toner particle and a toner having a broad particle size distribution, it is necessary to fill the gap between the toner particles by thickly overlapping the toner particles. There is also a problem that it is necessary to increase the image density, and the amount of toner consumption required to obtain a predetermined image density increases.

Further, Japanese Patent Application Laid-Open No. 54-72054 proposes a non-magnetic toner having a sharper distribution than the former,
The size of particles having an intermediate weight is as coarse as 8.5 to 11.0 μm, and there is still room for improvement as a high-resolution toner.

In Japanese Patent Application Laid-Open No. 58-1229437, the average particle size is 6 to 10 μm.
Non-magnetic toners having the largest number of particles of 5 to 8 μm have been proposed, but the number of particles of 5 μm or less is as small as 15% by number or less, and an image lacking in sharpness tends to be formed.

According to the study of the present inventors, it has been found that toner particles having a size of 5 μm or less clearly reproduce the outline of the latent image and have a main function of dense toner adhesion to the entire latent image. In particular, in the electrostatic latent image on the photoreceptor, due to the concentration of lines of electric force, the contoured edge portion has a higher electric field intensity than the inside, and the sharpness of the image quality is determined by the quality of the toner particles collected in this portion. According to the study of the present inventors, it has been found that the amount of particles of 5 μm or less is effective in solving the problem of sharpness of image quality.

Also, in U.S. Pat.No. 4,299,900, 20-35 μm
A jumping development method using a developer having 10 to 50% by weight of a magnetic toner has been proposed. In other words, the toner particle size suitable for frictionally charging the magnetic toner, uniformly and thinly applying the toner layer on the sleeve, and further improving the environmental resistance of the developer has been devised. However, considering stricter requirements such as reproducibility of fine lines and resolution, it is not sufficient, and further improvements are required.

Conventionally, in an electrophotographic apparatus, a normal development method of developing an unexposed portion is generally used. this is,
Since the reflected light from the document is projected onto the photoreceptor after being optically processed, the non-exposed portion (reflected portion of the document) without reflected light is developed.

Recently, electrophotographic systems have been used for printers and the like used for computer output, in addition to obtaining copied images. In the case of a printer application, a light-emitting body (such as a semiconductor laser) is turned on and off (ON-OFF) according to an image signal, and the light is projected on a photoconductor. At this time, the printing rate (the ratio of the printing area per page) is usually 3
The method of exposing the character portion (reversal development) is superior in terms of the life of the luminous body.

Reversal development is used for devices that output a positive image and a negative image from the same document (for example, a microfilm output device). In addition, regular development is performed to develop two or more colors in the same device. And devices that combine reversal development.

However, reversal development has the following problems.
The transfer electric field in normal development (hereinafter, a normal image) has the same polarity as the primary charging, and even if the transfer electric field is applied to the photoreceptor after passing through an image carrier (hereinafter, a paper, etc.), the influence thereof is the same as the erase exposure ( It is erased by 6) in FIG.

On the other hand, the transfer electric field in the reversal development has the opposite polarity to the primary charge, and even after passing through paper etc., when the transfer electric field is applied, the charge of the opposite polarity occurs on the photoreceptor. Appears. This is a development called "paper mark".

As a countermeasure against paper marks, there is a method of reducing the transfer current after the paper or the like has passed as shown in Japanese Patent Application Laid-Open No. 60-256173. However, this method requires various components (such as microswitches). This is necessary, and the apparatus becomes complicated and the apparatus cost increases. In addition, a method of reducing the transfer electric field so that the opposite polarity charge does not occur on the photoconductor can be considered. However, in this method, since the transfer efficiency is lowered, image quality is deteriorated due to poor transfer. As another complication of the reversal developing method, since the photoconductor and the paper are charged to opposite polarities, when charged by a strong electric field, the photoconductor and the paper are electrostatically attracted, Even after the completion of the transfer process, the paper does not separate, and paper and the like enter into the next process (cleaning process and the like), causing paper jams and the like. This is a phenomenon called “wrapping”. As a countermeasure against winding, there is a means for preventing adhesion between the photoreceptor and paper or the like as disclosed in JP-A-56-60470. However, this method is not always effective in reversal development. That is, it is considered that this is because adhesion at the time of separation in the transfer step of reversal development is stronger than that in the normal image system. As another method, there is an apparatus provided with a separation charging or belt separation as an auxiliary means for separation as seen in US Pat. No. 3,357,400. This has an effect on wrapping development, but has no effect on the paper mark phenomenon. This is because the separation charging is smaller than the transfer charging and does not affect the potential on the photoconductor.

As another means, there is a means for lowering the transfer electric field to lower the electrostatic attraction force. However, this method is liable to cause a deterioration in image quality due to poor transfer as described above. Also, when the transfer electric field is lowered, the transfer efficiency is lowered, and postcards and OH
Can't respond to various needs such as P film. Also, when the transfer electric field is lowered, the outline portion of the image, line drawing portion,
In a portion where the developer is likely to concentrate (developed portion), “transfer missing” which is a part of the transfer failure occurs. This is thought to be due to the fact that the developer has a larger amount of developer in the developing section than in the normal section, the developer is liable to agglomerate, and the response to the transfer electric field is lowered. Therefore, it is possible to obtain a high-quality image which is faithful to the latent image. Has a problem that it becomes difficult.

In addition, in order to form a visible image of good image quality in a method using a dry toner, it is necessary that the toner has high fluidity and uniform chargeability. Is contained in the toner. However, since the silica fine powder is hydrophilic as it is, the toner containing the silica fine powder causes agglomeration due to the moisture in the air to reduce the fluidity. In extreme cases, the toner is charged by the moisture absorption of the silica fine powder. Performance will be reduced.

Accordingly, use of silica fine powder subjected to hydrophobic treatment is disclosed in JP-A-46-5792, JP-A-48-47345, and JP-A-48-47346.
And JP-A-55-120041 and JP-A-59-34539. Specifically, for example, silica fine powder obtained by reacting silica fine powder with an organosilicon compound such as dimethyldichlorosilane, hexamethyldisilazane, silicone oil, etc., and replacing the silanol groups on the surface of the silica fine powder with an organic group to make the silica fine powder hydrophobic. Is used.

Among these, silicone oil treatment is preferred as the hydrophobic treatment which shows sufficient hydrophobicity and exhibits excellent transferability when contained in the toner. However, since silicone oil is a polymer substance, it is hydrophobicized. During processing, the silica fine powder is agglomerated, and some of the fine particles are dispersed in the toner and remain tens of μ in size, and have the same negative charge as the remaining toner. It had the disadvantage of worsening.

[Problem to be Solved by the Invention] An object of the present invention is to provide an image forming method and a toner which solve the above-mentioned problems.

The object of the present invention will be described below in more detail.

SUMMARY OF THE INVENTION An object of the present invention is to provide a magnetic toner having a high image density, excellent fine line reproducibility, and excellent gradation.

It is a further object of the present invention to provide a magnetic toner whose performance does not change due to environmental changes.

Another object of the present invention is to provide a magnetic toner capable of obtaining a high image density with a small consumption.

Another object of the present invention is to provide a negatively chargeable toner capable of obtaining a high-quality image in an image forming method requiring transfer by a low transfer electric field such as a reversal developing method.

It is another object of the present invention to provide an image forming method in which phenomena such as "paper mark", "wrapping", and "transfer missing" are suppressed or the phenomena are suppressed.

Another object of the present invention is to provide a toner and an image forming method which can provide a high quality image without fogging even when a thick transfer paper is used.

Another object of the present invention is to provide a negatively chargeable toner and an image forming method suitable for developing a digital latent image used in a digital copying machine, a laser beam printer, and the like.

It is also an object of the present invention to
An object of the present invention is to provide a negatively chargeable toner and an image forming method which do not cause a hollowing phenomenon even under a low transfer electric field and have good durability.

Another object of the present invention is to provide a toner having good fluidity and free of white spots due to silica aggregates on a solid black image.

[Summary of the Invention] More specifically, the present invention relates to a magnetic toner having at least a binder resin and a magnetic powder, in which 17 to 60% by number of magnetic toner particles having a particle diameter of 5 µm or less, and 6.35 to 10.08 µm.
5 to 50% by number of magnetic toner particles having a particle size of 12.7 μm.
Volume% or less, and the volume average diameter of the magnetic toner is 6 to
8 μm, and the magnetic toner particles having a particle size of 5 μm or less [Wherein N represents the number% of magnetic toner particles having a particle size of 5 μm or less, V represents the volume% of magnetic toner particles having a particle size of 5 μm or less, and k represents a positive number of 4.6 to 6.7. . Here, N indicates a positive number of 17 to 60. ] 100 parts by weight of a negatively chargeable insulating magnetic toner (black fine powder) having a particle size distribution satisfying the following formula: 0.6 to 0.6 parts by weight of the negative silica fine powder hydrophobized with the treatment agent represented by the composition formula (I).
The present invention relates to a negatively chargeable magnetic toner characterized by containing 1.6 parts by weight.

[R ₁ is an alkyl group or an alkoxy group, R ₂ is a C 1 to C 3
Alkyl group, R ₃ is a long chain alkyl group, a halogen-modified alkyl, phenyl group, sill corn oil modified group such as a modified phenyl group, m, n, m ', n' is n> m a positive integer including 0 , n ′
> M ', n + m + n' + m '<30] In the toner of the present invention, when n' + m '+ n + m is 30 or more, the viscosity of the processing agent becomes high, and silica lumps are formed during processing, and white spots occur in the image area.

Further, according to the present invention, the toner of the present invention is used under conditions where the ratio ( _Vtr / _Vpr ) of the primary charging electric field _Vpr to the transfer charging electric field _Vtr is negative and when the absolute value of ( _Vtr / _Vpr ) is 0.5. This is effective in an image forming method for electrostatically transferring a toner image under the condition of 1.6.

The magnetic toner and the magnetic developer of the present invention having the above-mentioned particle size distribution have improved performance of faithfully reproducing even a fine line of a latent image formed on a photoreceptor, and have a dot latent like dot and digital. Excellent image reproduction, excellent gradation and resolution.

Furthermore, even if the printout is continued, high image quality is maintained, and even in the case of a high-density image, good development can be performed with less toner consumption than the conventional magnetic toner, and economy and This also has an advantage in reducing the size of the printer body.

In particular, the magnetic developer of the present invention develops a digital electrostatic latent image of minute spots formed on a negatively charged organic photoconductor by a reversal development method, and transfers the toner image by plain paper or OHP by electrostatic transfer. It is preferably used in an image forming method of transferring and fixing to a transfer material such as a plastic sheet for use.

The reason why such effects are obtained in the magnetic toner of the present invention is not necessarily clear, but is presumed as follows.

That is, one feature of the magnetic toner of the present invention is that 17 to 60% by number of magnetic toner particles having a particle diameter of 5 μm or less. Conventionally, 5 μm
The following magnetic toner particles are difficult to control the charge amount, impair the fluidity of the magnetic toner, and actively decrease as a component that scatters the toner and stains the machine, as well as a component that causes image fogging. Was thought to be necessary.

However, studies by the present inventors have revealed that magnetic toner particles having a size of 5 μm or less are essential components for forming high-quality images.

For example, using a magnetic toner having a particle size distribution ranging from 0.5 μm to 30 μm, the surface potential on the photoreceptor is changed, and from a large development potential contrast in which a large number of toner particles are easily developed, to a halftone, and further, a very small amount. A latent image with a changed surface potential on the photoreceptor was developed to a small development potential contrast where only toner particles were developed, and the developed toner particles on the photoreceptor were collected. The toner particle size distribution was measured. It has been found that there are many magnetic toner particles, especially magnetic toner particles of 5 μm or less. That is, when magnetic toner particles having a particle size of 5 μm or less, which are most suitable for development, are smoothly supplied to the development of the latent image on the photoreceptor, the toner is faithful to the latent image, and does not protrude from the latent image, and is truly reproducible. And excellent images can be obtained.

In the magnetic toner of the present invention, 6.35 to 10.08
One feature is that particles in the range of μm are 5 to 50% by number. This is related to the need for the presence of magnetic toner particles having a particle size of 5 μm or less, as described above. The magnetic toner particles having a particle size of 5 μm or less strictly cover the latent image and have an ability to faithfully reproduce the latent image. However, in the latent image itself, the electric field strength of the edge portion around the latent image is higher than that of the central portion, so that the toner particles may be thinner inside the latent image than in the edge portion, and the image density may appear to be light. In particular, 5μ
The tendency is strong for magnetic toner particles of m or less. However, the present inventors have found that this problem can be solved and made clearer by containing 5 to 50% by number of toner particles in the range of 6.35 to 10.08 μm.
That is, it is considered that the toner particles having a particle size in the range of 6.35 to 10.08 μm have a moderately controlled charge amount with respect to the magnetic toner particles having a particle size of 5 μm or less.
The toner is supplied to the inner side of the latent image where the electric field intensity is smaller than the edge portion, and a small amount of the toner particles on the edge portion is compensated for, so that a uniform developed image is formed. A sharp image with excellent gradation is provided.

Further, for particles having a particle size of 5 μm or less, N / V = −0.05 N + k between the number% (N) and the volume% (V).
(However, one of the characteristics is that the magnetic toner of the present invention satisfies the relationship of 4.6 ≦ k ≦ 6.7; 17 ≦ N ≦ 60).
FIG. 5 shows this range, and together with other features, the magnetic toner of the present invention having a particle size distribution satisfying this range can achieve excellent developability.

The present inventors have studied the state of the particle size distribution of 5 μm or less and found that there is a state of existence of the fine powder most suitable for achieving the object as shown by the above formula. In other words, for a given value of N, a large N / V means that 5 μm
It indicates that the following particles are widely contained, and N / V
Is small, it is understood that the abundance of particles around 5 μm is high, and that particles having a particle size smaller than 5 μm are small, and the value of N / V is in the range of 1.6 to 5.85; and N is
When the ratio is in the range of 17 to 60 and the above relational expression is further satisfied, good fine line reproducibility and high resolution are achieved.

Further, for magnetic toner particles having a particle diameter of 12.7 μm or more, the content is preferably set to 2.0% by volume or less, and as small as possible.

By a completely different idea from the conventional viewpoint, the magnetic developer of the present invention solves the conventional problems and can withstand recent severe demands for high image quality.

 The configuration of the present invention will be described in more detail.

Magnetic toner particles having a particle size of 5 μm or less have a total particle number of 17 to
The number is preferably 60% by number, preferably 25% to 60% by number, and more preferably 30% to 60% by number. When the number of magnetic toner particles having a particle diameter of 5 μm or less is less than 17% by number, the amount of magnetic toner particles effective for high image quality is small. In particular, as the toner is used by continuing printout, the effective magnetic toner particle component is reduced. Decrease, the balance of the particle size distribution of the magnetic toner as shown in the present invention deteriorates,
Image quality gradually decreases. If it exceeds 60% by number, the aggregation state of the magnetic toner particles tends to occur,
Since the toner mass is larger than the original particle size, the image quality becomes rough, the resolution is reduced, or the density difference between the edge portion and the inside of the latent image is increased, so that the image tends to be slightly hollow.

Further, particles in the range of 6.32 to 10.08 μm are 5 to 50% by number.
And preferably 8 to 40% by number. 50
If the number is more than the number%, the image quality is deteriorated, and the development is performed more than necessary, that is, the toner is excessively applied, which leads to an increase in toner consumption. On the other hand, if it is less than 5% by number, it becomes difficult to obtain a high image density. The number% (N%) and the volume% (V%) of the magnetic toner particles having a particle size of 5 μm or less.
Has a relationship of N / V = −0.05N + k, and 4.6 ≦ k ≦
Indicates a positive number in the range of 6.7. Preferably, 4.6 ≦ k ≦ 6.2, and more preferably, 4.6 ≦ k ≦ 5.7. As indicated above, 17 ≦ N ≦ 60, preferably 25 ≦ N ≦ 60, more preferably 30 ≦ N ≦ 60.

When k <4.6, the number of magnetic toner particles having a particle diameter smaller than 5.0 μm is small, and the image density, resolution, and sharpness are poor. The appropriate presence of fine magnetic toner particles, which was conventionally considered unnecessary, contributes to the fine packing of the toner in development and contributes to the formation of a uniform image without roughness. In particular, by uniformly filling the fine lines and the contours of the image, the sharpness is further enhanced visually.
That is, when k <4.6, these characteristics are inferior due to the lack of the particle size distribution component.

From another aspect, in terms of production, it is necessary to cut a large amount of fine powder by classification or the like to satisfy the condition of k <4.6, which is disadvantageous in terms of absorption rate and toner cost.
Also, at k> 6.7, the presence of more fine powder than necessary
As printout is repeated, the image density tends to decrease. Such development occurs when excessive fine powdered magnetic toner particles having an unnecessary charge are charged and adhered to the developing sleeve, thereby hindering normal magnetic toner from being carried on the developing sleeve and applying charge. Conceivable.

The content of the magnetic toner particles having a particle size of 12.7 μm or more is preferably 2.0% by volume or less, more preferably 1.0% by volume or less, and further preferably 0.5% by volume or less. If it is more than 2.0% by volume, it will hinder the reproduction of fine lines. Further, the volume average diameter of the magnetic toner is 6 to 8 μm, and this value cannot be considered separately from the above-mentioned respective constituent elements. If the volume average particle diameter is less than 6 μm, there is a problem that the amount of toner applied on the transfer paper is small and the image density is low in digital latent images having a high image area ratio, such as graphic images. This is considered to be due to the same reason as described above for lowering the density inside the edge portion of the latent image. When the volume average particle size exceeds 8 μm, the resolution of fine spots of 100 μm or less is not good, and there are many spots on non-image areas. In addition, at the beginning of printout, image quality tends to deteriorate if use is continued at best. .

The particle size distribution of the toner can be measured by various methods, but in the present invention, the measurement was performed using a Coulter counter.

That is, Coulter Count-TA
-Type II (manufactured by Coulter), connected to an interface (manufactured by Nikkaki) and a CX-1 personal computer (manufactured by Canon) that output the number distribution and volume distribution, and using primary-grade sodium chloride as the electrolyte. Prepare 1% NaCl aqueous solution. As a measuring method, 0.1 to 5 ml of a surfactant, preferably an alkylbenzene sulfonate, is added as a dispersing agent to 100 to 150 ml of the electric field aqueous solution, and 2 to 20 mg of a measurement sample is further added. The electrolyte in which the sample was suspended was treated with an ultrasonic
Perform a dispersion treatment for minutes, using the Coulter Counter TAII, using a 100μ aperture as an aperture,
The particle size distribution of the particles of 2 to 40 μ was measured on the basis of the number, and the values according to the present invention were determined therefrom.

The true density of the magnetic toner of the present invention is 1.45 to 1.80 g / cm ³
It is. Within this range, the magnetic toner having the specific particle size distribution of the present invention can exhibit effects in terms of high image quality and durability stability in the reversal development system in the presence of a magnetic field. If the true density of the magnetic toner is less than 1.45, the magnetic toner particles themselves are too light in weight, and thin lines are liable to be crushed, jumped, and degraded in resolving power due to fog and excessive toner particles in reversal development. Also, if the true density of the magnetic toner is larger than 1.8, the image density becomes lighter,
The resulting image lacks sharpness, such as breaks in thin lines, and the magnetic force is relatively large, so that the toner ears are likely to be long or branched, and in this case, the image quality when a digital latent image is developed is reduced. The image is likely to be disturbed and coarse.

The true density of the magnetic toner can be measured by any of several methods. In the present application, the following method was adopted as an accurate and simple method when measuring fine powder.

A stainless steel cylinder with an inner diameter of 10 mm and a length of about 5 cm, a disk (A) with an outer diameter of about 10 mm and a height of 5 mm that can be closely inserted into it, and a piston (B) with an outer diameter of 10 mm and a length of about 8 cm are prepared. I do. The disk (A) is placed at the bottom of the cylinder, then about 1 g of the measurement sample is placed, and the piston (B) is gently pushed. A 400 kg / cm ² force is applied to this with a hydraulic press,
Take out what was compressed for 5 minutes. The weight of the compressed sample is weighed (wg), the diameter (Dcm) and the height (Lcm) of the compressed sample are measured with a micrometer, and the true density is calculated by the following equation.

In order to obtain better developing characteristics, the magnetic toner of the present invention has a residual magnetization σ _{r of 1} to 5 emu / g, preferably 2 to 4.5 emu / g.
an emu / g, a saturation magnetization sigma _s is 20~40emu / g, both coercivity H _c is 40 to 100 Esutetsudo _(e) (measurement magnetic field
It is preferable to satisfy the magnetic properties of 1K is _e).

Further, in the negatively chargeable black fine powder having a particle size distribution according to the present invention, it is necessary to add silica fine powder from the viewpoint of improving fluidity and transferability and preventing sleeve contamination.

The present inventors can obtain satisfactory results in the transfer step in reversal development by incorporating silica fine powder hydrophobized with the above-described processing agent into a negatively chargeable-component toner, and at the same time, use silica silica in the image area. It has been found that high quality images can be obtained without white spots caused by the image.

In the present invention, as the fine silica powder constituting one component of the developer, fine silica powder produced by a dry method and a wet method can be used.

The dry method referred to here is a method for producing silica fine powder generated by vapor phase oxidation of a silicon halide. For example, in a method utilizing the thermal decomposition oxidation reaction of silicon tetrachloride gas in an oxyhydrogen flame, the basic reaction formula is as follows.

SiCl ₄ + 2H ₂ + O ₂ → SiO ₂ + 4HCl Also, in this production process, for example, by using another metal halide such as aluminum chloride or titanium chloride together with a silicon halide, a fine composite powder of silica and another metal oxide is obtained. It is also possible to obtain and include them.

Commercially available fine silica powder produced by vapor phase oxidation of a silicon halide used in the present invention includes, for example, those commercially available under the following trade names.

AEROSIL 130 (Nippon Aerosil) 200 300 380 OX50 TT600 MOX80 MOX170 COK84 Ca-O-SiL M-5 (CABOT Co.) MS-7 MS-75 HS-5 EH-5 Wacker HDK N20 V15 (WACKER-CHEMIE GMBH) N20E T30 T40 DC Fine Silica (Dow Corning Co.) Fransol (Fransil) Leoloseal (Tokuyama Soda) On the other hand, a method for producing the silica fine powder used in the present invention by a wet method is conventionally known. Various methods can be applied. For example, the decomposition of sodium silicate by acid, as shown by the general reaction formula (hereinafter, the reaction formula is abbreviated), Na ₂ O.XSiO ₂ + HCl + H ₂ O → SiO ₂ .nH ₂ O + NaCl In addition, ammonia salts of sodium silicate or alkali Decomposition with salts, after producing an alkaline earth metal silicate from sodium silicate, a method of decomposing with an acid to produce silicic acid, a method of transforming sodium silicate solution into silicic acid with ion exchange resin, natural silicic acid or There is a method using a silicate.

Silica such as aluminum silicate, sodium silicate, potassium silicate, magnesium silicate, zinc silicate, etc. can be applied to the silica fine powder here, in addition to anhydrous silicon dioxide (silica).

Commercially available silica fine powder synthesized by a wet method includes, for example, those commercially available under the following trade names.

Among the above silica fine powders, the specific surface area by nitrogen adsorption measured by the BET method is 30 cm ² / g or more (particularly 100 to 400 m ² / g)
Those within the range give good results. The toner having a particle size distribution of the present invention has a large BET specific surface area since electrostatic aggregation tends to be strong.

Conventionally, there has been known an example in which fine silica powder generated by vapor phase oxidation of a silicon halide is added to a developer.

As the silica hydrophobizing agent contained in the toner of the present invention, those having the above-mentioned formula (I) are used. In particular, l + l 'is preferably 4-20.

The treating agent has a high hydrophobizing property of dimethyl silicone oil, which is conventionally known as a hydrophobizing treating agent, a high lubricating property which gives a good effect on the transfer characteristics of the toner, and a silanol group on the silica surface of hexamethyldisilazane. High reactivity to Further, the viscosity of the treating agent at 25 ° C. is preferably 70 cs or less (preferably 50 cs or less).

The silica lumps are developed together with the toner during solid black development to form white dots. In the toner of the present invention having a particle size distribution, the white spots are conspicuous because the consumption amount is small and the amount of applied toner when a solid black image is formed is remarkably small.

The amount of hydrophobic silica added to the toner of the present invention is preferably 0.6 to 1.6 parts by weight based on 100 parts by weight of the toner.

When the amount is less than 0.6 parts by weight, the effect of addition is small, and a problem concerning development transfer occurs. If it is 1.6 parts by weight or more, fog increases, which is not preferable. Particularly preferably, it is 0.7 to 1.4 parts by weight.

The triboelectric charge of the hydrophobic silica contained in the developer of the present invention is preferably from -100 to -300 µc / g.

The hydrophobic silica contained in the toner of the present invention is characterized in that it behaves together with the toner. This is completely different from the method disclosed in JP-A-56-60470 in which particles are positively arranged on a non-image to reduce the attraction force between a transfer material and a photosensitive member.

According to the method disclosed in JP-A-56-60470, the winding phenomenon can be improved without lowering the transfer electric field, but there is no effect on the paper mark phenomenon, and the transfer rate is improved under a low transfer electric field. It has no effect.

Corona discharge charger as the transfer step used in the present invention,
An electrostatic transfer method using an electric field generated by a contact roller charger or the like can be used. The transfer conditions are measured as follows. Referring to FIG. 1 of the accompanying drawings, the cleaning device 8, the developing device 9, the transfer charger 3 and the like are removed from the image forming apparatus, and the photosensitive member (photosensitive drum) 1 serving as an electrostatic image holding member is primary It is charged by the charger 2. Leak light,
After charging the photoreceptor 1 for one week with substantially complete shading, the surface potential of the photoreceptor 1 is measured with a surface voltmeter. The value of the surface potential at this time is defined as V _pr [V]. Next, the surface of the photoreceptor is wiped with a cloth impregnated with alcohol or the like, and the surface of the photoreceptor is neutralized. Then, the primary charger 2 is removed, and the transfer charger 3 is attached. After that, the surface potential of the photoconductor is measured. The value of the surface potential at this time is V _tr
[V]. In the present invention, the value of V _tr / V _pr is negative, preferably the absolute value of V _tr / V _pr Is preferably 0.5 to 1.6. When the absolute value is less than 0.5, the transfer electric field is too low, and image deterioration tends to occur during transfer. On the other hand, when the absolute value exceeds 1.6, the transfer electric field is too strong, and the photoconductor is positively charged. This easily causes a paper mark phenomenon and a winding phenomenon.

The absolute value Is more preferably from 0.9 to 1.4.

INDUSTRIAL APPLICABILITY The present invention is effective for an image forming method (apparatus) using an organic photoreceptor (hereinafter referred to as an OPC photoreceptor), and the OPC photoreceptor is composed of a multilayer OPC having at least a charge generation layer and a charge transport layer. This is particularly effective for an image forming method of a reversal development system using In the case of the OPC photoreceptor, when the photosensitive layer is charged to the opposite polarity, the movement of the electric charge is slow.In particular, in the laminated type OPC, this tendency is remarkable, and the paper mark is easily generated, so the present invention is particularly effective. I do.

The value of V _pr used in the present invention, -300 1,000
[V] is preferable, and especially -500 to -900 [V] is preferable. If the voltage is less than −300 [V], it is difficult to secure a potential difference during development, and the image tends to be unclear.
When the voltage exceeds 000 [V], dielectric breakdown of the photosensitive layer due to an electric field occurs, and image quality deterioration such as black spots easily occurs. From the viewpoint of durability and the like, −500 to −900 [V] is particularly preferable.

The image forming method according to the present invention is an image forming method in which the transfer material is separated from the photoreceptor by the elastic force of the transfer material (paper or the like), the curvature of the photoreceptor, a static elimination brush or the like without using a mechanical separating unit. This is particularly effective for (apparatus). The present invention is particularly effective because the state of separation in an apparatus without a mechanical separation mechanism depends on the transfer conditions and winding is likely to occur.

In the present invention, the diameter of the photosensitive member 1 ("φ" in FIG. 1) is 50 mm.
It is particularly effective for the following image forming method (apparatus) using a photoconductor. In an apparatus using a photosensitive drum of φ50 mm or less, it is intended to reduce the size, and it is necessary to reduce the number of parts. Usually, the separation step is constituted by separation by only the elastic force of paper and a neutralization brush 7 and the like. (See FIG. 2). At this time, in the neutralization step, only the paper or the like is neutralized, and usually does not act on the surface potential of the photoconductor.

The image forming process will be described with reference to FIG. The surface of the photoreceptor is negatively charged by the primary charger 2, a latent image is formed by image scanning by exposure 5 with a light source or a laser beam, and a developing sleeve 4 including a magnetic blade 11 and a developing sleeve 4 containing a magnet is provided. The latent image is reversal-developed with the one-component magnetic developer 13 in the container 9. In the developing section, a bias is applied between the photosensitive drum 1 and the developing sleeve 4 by bias applying means. When the transfer paper P is conveyed and reaches the transfer section, the transfer charger 3 charges the transfer paper P with a positive polarity from the rear surface (the surface opposite to the photosensitive drum side), thereby loading the negatively charged toner image on the surface of the photosensitive drum. Is electrostatically transferred onto the transfer paper P. Immediately after passing through the transfer charger 3, the transfer paper P is separated from the photosensitive drum 1 by curvature separation while the charge on the back surface of the transfer paper is discharged by the discharging brush 10. The transfer paper P separated from the photosensitive drum 1 by the curvature separation is
The toner image on the transfer paper P is fixed by the heating and pressing roller fixing device 7.

The one-component developer remaining on the photosensitive drum after the transfer process is removed by a cleaning device 8 having a cleaning blade. After the cleaning, the photosensitive drum 1 is neutralized by the erase exposure 6, and the process starting from the charging process by the primary charger 2 is repeated again. Next, the negatively charged toner used in the present invention will be described.

In the present invention, as the binder resin of the toner, for example,
Styrenes such as polystyrene, poly-p-chlorostyrene, polyvinyltoluene and homopolymers of substituted styrenes;
Styrene-p-chlorostyrene copolymer, styrene-propylene copolymer, styrene-vinyltoluene copolymer, styrene-vinylnaphthalene copolymer, styrene-
Methyl acrylate copolymer, styrene-methyl acrylate copolymer, styrene-ethyl acrylate copolymer, styrene-octyl acrylate copolymer, styrene-methyl methacrylate copolymer, styrene-butyl methacrylate Copolymer, styrene-α-chloromethyl methacrylate copolymer, styrene-acrylonitrile copolymer, styrene-vinyl methyl ether copolymer, styrene-vinyl ethyl ether copolymer, styrene-vinyl methyl ketone copolymer Styrene copolymers such as styrene-butadiene copolymer, styrene-isoprene copolymer, styrene-acrylonitrile-indene copolymer; polyvinyl chloride, polyvinyl acetate, polyethylene,
Polypropylene, silicone resin, polyester, epoxy resin, polyvinyl brillal, rosin, modified rosin, terpene resin, phenol resin, xylene resin, aliphatic or alicyclic hydrocarbon resin, aromatic petroleum resin, chlorinated paraffin, paraffin Tux or the like is used alone or as a mixture.

Here, as the cross-linking agent, a compound having two or more polymerizable double bonds is mainly used, for example, an aromatic divinyl compound such as divinylbenzene, divinylnaphthalene and the like; for example, ethylene glycol diacrylate, ethylene glycol dimethacrylate Carboxylic acid esters having two double bonds, such as 1,3-butanediol methacrylate; divinyl compounds such as divinylaniline, divinylether, divinylsulfide, divinylsulfone; and having three or more vinyl groups Compound alone or as a mixture.

In the present invention, among these resins, a styrene-acrylic copolymer is preferably used.
N-butyl acrylate (St-nBA) copolymer, styrene-n-butyl methacrylate (St-nBMA) copolymer, styrene-n-butyl acrylate-2-ethylhexyl methacrylate (St-nBA- 2EHMA) copolymers and the like or those containing a small amount of divinylbenzene are preferably used.

Further, as a coloring material that can be added to the toner according to the present invention, conventionally known carbon black, copper phthalocyanine, iron black and the like can be used.

As the magnetic fine particles contained in the magnetic toner according to the present invention, a substance that is magnetized when placed in a magnetic field is used,
Powders of comagnetic metals such as iron, cobalt and nickel or alloys and compounds such as magnetite, γ-Fe ₂ O ₃ and ferrite can be used.

These magnetic particles preferably have a BET specific surface area of ^{2 to 20} cm ² / g, particularly 2.5 to 12 m ² / g, and a Mohs hardness of 5 to 7 by a nitrogen adsorption method. The content of the magnetic powder is preferably 70 to 120% by weight based on 100 parts by weight of the binder resin.

Further, the toner of the present invention preferably contains a charge control agent as necessary, and a negative charge control agent such as a metal complex salt of a monoazo dye; a metal complex salt of salicylic acid, alkylsalicylic acid, dialkylsalicylic acid or naphthoic acid is used. .

Further, the magnetic toner according to the present invention has a volume resistivity of 10
It is preferably at least ¹⁰ Ω · cm, particularly preferably at least 10 ¹² Ω · cm, in view of triboelectric charge and electrostatic transferability. The volume resistivity referred to herein, by molding the toner in pressure of 100 kg / cm ^2, this 100
An electric field of V / cm is applied, and is defined as a value converted from a current value one minute after the application.

The triboelectric charge amount of the negatively chargeable magnetic developer used in the present invention must be in the range of −20 μc / g to −35 μc / g. −20
When the value is less than μc / g, the image density tends to be low, and the effect particularly under high humidity is remarkable. On the other hand, if it exceeds -35 µc / g, the toner charge is too high, and the line image or the like becomes thin, resulting in a poor image particularly under low humidity.

In the present invention, the amount of triboelectric charge was measured at 23.5 ° C. and 60% in an environment of 2/9 in the case of iron powder carrier (Nippon Iron Powder EFV200 / 300) having a particle size of 200/300 mesh and silica in an environment of 60%.
8. In the case of toner, mix at a ratio of 10/90 and try to shake thoroughly for about 20 seconds. 0.5-1.5 g of this mixture was precisely weighed and sucked by a pressure of 25 cmH ₂ O on a metal 400 mesh screen connected to an electrometer, and the amount of the test substance sucked at that time and the charge amount were used per unit weight. By the method of determining the charge amount of

The toner according to the present invention is generally manufactured as follows.

(A) A binder resin, a magnetic substance, a coloring agent as a colorant, and the like are dispersed uniformly by a mixer such as a hexiel mixer.

(B) The dispersion obtained above is melt-kneaded with a kneader, extruder, roll mill or the like.

(C) The kneaded material is roughly pulverized by a cutter mill, a hammer mill or the like, and then finely pulverized by a jet mill or the like.

(D) The finely pulverized product is classified using a zigzag classifier or the like to have a uniform particle size distribution and classified to obtain a toner.

Preferably, a multi-divider as shown in FIGS. 3 and 4 is used.

In addition, as a method for producing the toner, a polymerization method, a capsule method, or the like can be used.

The method of treating the treating agent according to the present invention employs a known technique, for example, silica fine powder and the treating agent may be directly mixed using a mixer such as a Herciel mixer, or silica as a base. A method of injecting the treatment agent may be used. Alternatively, the treating agent may be prepared by dissolving or dispersing the treating agent in an appropriate solvent, then mixing with a base silica fine powder, and removing the solvent.

Further, it is preferable that the heat treatment is performed at 100 ° C. to 400 ° C. as the final step of the treatment.

The water wettability of the silica contained in the toner of the present invention is 80%
(Preferably 90%). If the water wettability is less than 80%, high quality images cannot be obtained due to moisture adsorption of the silica fine powder under high humidity.

The degree of water wetting is controlled by the amount of the treatment agent and the treatment conditions.

The water wettability of the silica in the present invention is measured as follows. A 0.1 g sample was collected in a 200 ml separating funnel,
Add 100 ml of ion-exchanged water using a measuring cylinder. Shake it at 90 rpm for 10 minutes using a T2C Tarbrush Acre Mixer. Leave the separating funnel for 10 minutes and then
After extracting ~ 30 ml, the solution was collected in a 10 mm cell, ion exchanged water was used as a blank, and the turbidity of the aqueous layer was measured with a colorimeter (wavelength 50
0 nm) The transmittance% relative to the blank is defined as the degree of water wetting.

The magnetic developer of the present invention having at least a hydrophobic silica fine powder and an insulating magnetic toner is obtained by a nitrogen gas adsorption method.
It has a BET specific surface area 1.8~3.5m ² / g, has a triboelectric charging characteristic of -20~-35μc / g, has an apparent density 0.4~0.52g / cm ^3,
Having a true specific gravity 1.45~1.8g / cm ^3.

If the triboelectric charge is less than −20 μc / g, a sufficient charge for development cannot be obtained on the developer carrying member, and the image density is low from the beginning. Also, when the value is larger than -35 μc / g, the image is repeated, so that the charge amount of the developer in the vicinity of the carrier on the developer carrier becomes large, which inhibits the proper charging of the developer on the carrier. The charge-up phenomenon occurs and the image density gradually decreases. This development is likely to occur when developing a digital latent image, which is a development of a dot latent image, and is remarkable in a low potential contrast reversal development system using an OPC photosensitive member.

When the BET specific surface area of the developer according to the nitrogen gas adsorption method of the present invention is less than 1.8 m ² / g, on the developer carrying member, it takes time to obtain a sufficient charge amount for development, and the initial concentration is thin,
It becomes an image with a lot of fog. On the other hand, if it is larger than 3.5 m ² / g, the reflection power with the sleeve is increased, and the development rate is reduced, and as a result, the image density is reduced.

In the measurement of the BET specific surface area in the present invention, QVANTACHROM
It was determined by the BET one-point method using a specific surface area meter Autosorb 1 manufactured by Company E.

The true specific gravity of the developer of the present invention is 1.45~1.8g / cm ^3, prone to fog in method of developing by applying an AC bias in the magnetic field is less than 1.45, and the line width is the resolution deteriorates thick .

If it is larger than 1.8, line blurring tends to occur, and the image density also decreases.

Hereinafter, a specific example of the present invention will be described. All parts in the text are parts by weight.

Example 1 Styrene / n-butyl acrylate / divinylbenzene copolymer (85 / 14.5 / 0.5) 100 parts Magnetite 100 parts Low molecular weight polypropylene 3 parts Monoazo dye chromium complex 1 part The above mixture was heated to 130 ° C. in a biaxial rudder. The mixture was melted and kneaded, and then cooled. The cooled product was roughly pulverized into a pass having a diameter of 2 mm using a hammer mill (mechanical pulverizer), and then 6 μm using a jet mill (wind pulverizer).
m. After classifying the finely pulverized product with a DS classifier (wind classifier), ultra-fine and coarse powders are simultaneously strictly removed by a multi-segmentation classifier using the Coanda effect (Nippon Mining Co., Ltd. Elbodiet classifier). Volume average particle size 6.
A black fine powder of 5 μm (a negatively charged magnetic toner made of insulating material) was obtained. The particle size distribution characteristics are shown in Table 1A.

The multi-segmentation classifier used in the present embodiment and the classification process by the classifier will be described with reference to FIGS. 1 and FIG. 2, the side wall has a shape indicated by 122 and 124, and the lower wall
And the side wall 123 and the lower wall 125 are provided with knife edge type classification edges 117 and 118, respectively. The classification edges 117 and 118 divide the classification zone into three. A material supply nozzle 116 is provided below the side wall 122 and opens into the classification chamber, and is bent downward with respect to the direction of extension of the tangent at the bottom of the nozzle to form a long elliptical arc.
6 is provided. The classifying chamber upper wall 127 is provided with a knife edge type inlet edge 119 in the lower direction of the classifying chamber, and further provided with air inlet pipes 114 and 115 opening to the classifying chamber at the upper part of the classifying chamber.
The inlet pipes 114 and 115 are provided with first and second gas introduction adjusting means 120 and 121 such as dampers and static pressure gauges 128 and 129, respectively. Discharge pipes 111, 112, and 113 having discharge ports that open into the room are provided on the lower surface of the classifying chamber so as to correspond to the respective dividing areas.
The classified powder is introduced under reduced pressure from the supply nozzle 116 into the classification area,
Due to the Coanda effect of the Coanda block 126 due to the Coanda effect and the action of the high-speed air flowing in at that time, it moves along a curved line 130, and the coarse powder 111, a black fine powder having a predetermined volume average particle size and particle size distribution (Toner) 112
And ultrafine powder 113.

The insulating magnetic toner had a triboelectric charge of −14 μc / g with respect to the iron powder carrier.

100 parts of the toner was treated with a treating agent (II) (viscosity at 25 ° C of 50 c
s. ) Treated with dry silica (BET specific surface area 200 m ² / g) in hydrophobic silica A (water wettability 93%, triboelectric charge −170 μc / g) 1.
Two parts were added and mixed with a Hensiel mixer to obtain a developer.

The surface treatment of silica in the present invention was performed as follows.

(1) Silica fine powder (BET specific surface area 200m ² / g dry silica)
Stir 100 parts in a mixing tank.

(2) The above treatment agent is diluted 4 times with xylene and sprayed on silica in a mixing tank. (80 parts of diluent) (3) The temperature in the tank is raised to 300 ° C. and maintained for 2 hours while stirring is continued.

(4) Cool and remove.

The obtained developer is used as a commercially available copier FC-5 (manufactured by Canon Inc .; OPC laminated negatively charged photoreceptor, curvature separated type using a drum diameter of φ30, static elimination needle to which -1.0 KV is biased).
Was modified for reversal development (see FIG. 1), V _pr was -700 V, Under the transfer condition (V _tr = + 700 V) where is 1.0, a gap is set in a non-contact manner between the photosensitive drum and the developer layer on the developing drum (including magnets), and AC bais (f = 1,800 Hz, V _pp = 1,600 V) ) And a DC bias (V _DC = −500 V) were applied to the developing drum to perform image formation. The toner-fixed image formed and fixed by the heating and pressing roller was evaluated as follows. The results are shown in Table 1.

(A) Image density: ordinary copier plain paper (75 g / m ² ) 1,00
The evaluation was performed by maintaining the image density when passing 0 sheets.

○ (good): 1.35 or more, △ (acceptable): 1.0 to 1.34, × (impossible): 1.0 or less (b) Transfer condition: The transfer conditions were severe, and 120 g / m ² thick paper was printed out on both sides, and the The evaluation was made based on the state of missing transfer on the back side (second side) and the back side (second side). (Note that the transfer state usually tends to be worse on the second side during double-sided printout, and a difference that does not appear in the first side evaluation tends to appear.) ○: good, Δ: practical, ×: impractical (c ) Wound condition: 1,000 sheets of 50 g / m ² thin paper were passed, and the occurrence of paper jam was evaluated.

: 1: Within 1 time / 1,000 sheets, △: 2 to 4 times / 1,000 sheets, ×: 5 times or more / 1,000 sheets (d) Paper mark: All solid images were output and evaluated by their uniformity.

：: Density difference within 0.05 △: 0.06 to 0.15 ×: 0.16 or more (e) Image quality of 50 μm fine lines: 50 μm horizontal fine lines are printed every 150 μm, the outline of fine lines is rough, and toner scattering between fine lines is 10 Observed and evaluated with a magnifying loupe.

◎: very good (the contour is less rugged, and there is almost no scattering between fine lines) ○: good (the contour is slightly rugged, but there is almost no scattering between the thin lines) △: practical (the contour is slightly more) ×: Possible or bad for practical use (contour is rough, and scattering is very large) (f) Image white spot: A4 solid black image is output and visually observed. It was evaluated by the number of white spots.

O: 0 to 3 Δ: 4 to 10 ×: 11 or more Example 2 The same as Example 1 except that the transfer conditions were changed so that the ratio of V _tr / V _pr became −0.5. I did the drawing. The results are shown in Table 3.

Example 3 An image was formed in the same manner as in Example 1 except that the transfer conditions were changed so that the ratio of V _tr / V _pr became −1.6. The results are shown in Table 3.

Examples 4 and 5 An image was formed in the same manner as in Example 1 except that silica B and C shown in Table 2 were used instead of silica A. The results are shown in Table 3.

Example 6 Image formation was performed in the same manner as in Example 1 except that the amount of magnetite was 85 parts, the particle size distribution of the magnetic toner was B in Table 1, and the addition amount of hydrophobic silica A was 0.8 part. I did it. The results are shown in Table 3.

Comparative Example 1 Untreated dry silica (BET200m ² /
Except for adding g), image formation was performed in the same manner as in Example 1, and evaluation was performed. The results are shown in Table 3.

Comparative Examples 2 and 3 An image was formed in the same manner as in Example 1 except that silicas D and E shown in Table 2 were used instead of silica A. The results are shown in Table 3.

Comparative Examples 4 and 5 An image was formed and evaluated in the same manner as in Example 1 except that the transfer conditions were such that the ratio of V _tr / V _pr was −2.0 and −0.3, respectively. The results are shown in Table 3.

Comparative Example 6 Image formation was performed in the same manner as in Example 1 except that the amount of magnetite was 60 parts, the particle size distribution of the magnetic toner was C in Table 1, and the addition amount of hydrophobic silica A was 0.4 part. It was evaluated. The results are shown in Table 3.

Table 4 shows the physical properties of the magnetic developer used in each example.

[Effect of the Invention] When an image is formed by the method of the present invention using the toner of the present invention as described above, a high-quality image can be obtained in any environment.

[Brief description of the drawings]

In the accompanying drawings, FIG. 1 is a view schematically showing an image forming apparatus used in an embodiment of the present invention, and FIG. 2 is an enlarged view of a transfer portion in which an AC bias and a DC bias are applied to a discharging brush. FIG. FIG. 3 is an explanatory view of a classification process using a multi-division classification device, FIG. 4 is a schematic cross-sectional perspective view of the multi-division classification device, and FIG. FIG. 4 is a diagram showing a range of a content ratio of particles having the particles. DESCRIPTION OF SYMBOLS 1 ... Photosensitive drum 2 ... Primary charger 3 ... Transfer charger 4 ... Developing sleeve 5 ... Exposure 6 ... Erase exposure 7 ... Heat and pressure roller fixing device 8 ... Blade cleaning device 9 ... Developing device 10 ... Static elimination brush 11 ... Blade 12 ... Bias applying means

Claims (1)

(57) [Claims] 1. A condition absolute value conditions the ratio of the primary charging electric field V _pr and the transfer charging field _{V tr (V tr / V pr} ) is negative and (V _tr / V _pr) is 0.5 to 1.6 In a negatively chargeable magnetic developer used in an image forming method having a step of electrostatically transferring a toner image below, a particle size distribution of a negatively chargeable insulating magnetic toner having at least a binder resin and a magnetic powder is 5 μm or less. Magnetic toner particles having a particle size of 17 to 60% by number, 6.35 to 10.0
5 to 50% by number of magnetic toner particles having a particle size of 8 μm, 1
2.0% by volume of magnetic toner particles having a particle size of 2.7μm or more
Hereinafter, the magnetic toner particles having a volume average diameter of 6 to 8 μm and a particle size of 5 μm or less are represented by the following formula: N / V = −0.05N + k [wherein N represents the number% of magnetic toner particles having a particle size of 5 μm or less. V represents the volume% of the magnetic toner particles having a particle size of 5 μm or less, and k represents a positive number of 4.6 to 6.7. Here, N indicates a positive number of 17 to 60. The following formula (I) is used for 100 parts by weight of the negatively chargeable magnetic toner satisfying [R ₁ is an alkyl group or an alkoxy group, R ₂ is a C 1 to C 3
Alkyl group, R ₃ is a long chain alkyl group, a halogen-modified alkyl, phenyl group, silicone oil modified group such as a modified phenyl group, m, n, m ', n' is n> m at a positive integer including 0 , n ′
> M ', n + m + n' + m '<30] The silica fine powder hydrophobized with a treating agent represented by the formula:
A negatively-chargeable magnetic developer comprising 1.6 parts by weight.