DE69016689T2 - Imaging process and apparatus. - Google Patents

Abstract

An image forming method, including the steps of: developing an electrostatic image formed on an electrostatic image-bearing member with a developer to form thereon thereon a developed image, the developer comprising 100 wt. parts of a toner and 0.05 to 3 wt. parts of fine powder treated with a silicone oil or silicone varnish; and transferring the developed image on the electrostatic image-bearing member to a transfer material while causing a transfer means to contact the electrostatic image-bearing member by the medium of the transfer material under a line pressure of 3g/cm or higher.

Description

FIELD OF THE INVENTION AND RELATED ART

The present invention relates to an image forming method and an image forming apparatus in which a transfer device is caused to contact an image holding member for holding an electrostatic latent image via a transfer material (or transfer image receiving material), and a magnetic toner image formed on the image holding member for holding an electrostatic latent image is transferred to the transfer material.

As an image forming apparatus in which a toner image formed on an image holding member for holding a latent image is electrostatically transferred to a sheet-like transfer material such as paper, there have been proposed devices in which an image holding member for holding a latent image in the form of a rotary cylinder, an endless belt, etc. is used, a transfer device provided with a bias voltage is caused to contact with such an image holding member for holding a latent image under pressure, and a transfer material is passed between these members, whereby the toner image formed on the image holding member for holding a latent image is transferred to the transfer material, as disclosed, for example, in Japanese Laid-Open Patent Application (JP-A, KOKAI) No. 46664/1984 or EP-A 0 297 911 is disclosed.

In such an apparatus, if the contact pressure between a transfer roller and the image holding member for holding a latent image is appropriately adjusted, the area in which the transfer material contacts the image holding member for holding a latent image can be expanded as compared with a transfer device utilizing corona discharge which has been widely used heretofore. Since the transfer material is positively held under pressure in the transfer position, the above-mentioned Furthermore, the above-mentioned apparatus is less likely to cause a transfer deviation due to a tracking error caused by a transfer material conveying means or due to a curl or a loop present in the transfer material. As a result, the above-mentioned apparatus can easily meet the demand for shortening the conveying path of the transfer material and miniaturization of the image holding member for holding a latent image together with miniaturization of an image forming apparatus.

On the other hand, in the device for performing the transfer operation which can cause a transfer device to contact an image holding member for holding a latent image via a transfer material, it is necessary to apply a certain pressure to the transfer device because a transfer current is supplied to the transfer material in the contact position. When such a contact pressure is applied to the transfer material, the pressure is also applied to the toner image formed on the image holding member for holding a latent image, and therefore the toner particles constituting the toner image tend to accumulate.

Furthermore, in the case where the surface portion of the latent image holding member comprises a resin, the above-mentioned toner aggregates are likely to adhere firmly to the latent image holding member, and the transfer of the toner to the transfer material may be hindered. In an extreme case, toner particles corresponding to a portion showing firm adhesion are not transferred at all, so that the resulting toner image is likely to be missing.

Such a phenomenon is particularly noticeable in a line image area with a width of 0.1 to 2 mm. Since a so-called "edge phenomenon (or edge effect)" can occur in the line image area, a larger amount of toner particles adhere thereto, so that the accumulation of toner particles due to printing and image defects due to the transfer process are likely to occur. When such a phenomenon occurs, the resulting toner image becomes a copied image with toner particles adhering only to its outline portion. Such a phenomenon is called a "partial white image (e.g., hollow character or letter)". Figures 1B and 1D show examples of the partial white image.

The partial white images are particularly likely to occur in the case of thick paper of more than 100 g/cm², an OHP (overhead projector) transparency with high smoothness, or the operation of copying the second side in double-sided copying, etc. In the case of the thick paper or OHP transparency, it is considered that the effect of the electric field used for transfer is weakened and the pressure becomes strong because the transfer material is thick, whereby the partial white images are likely to occur. In the case of the second copying operation in the double-sided copying, it is assumed that a release agent for preventing the offset phenomenon from a copying device adheres to a transfer material when the transfer material is allowed to pass through the fixing device during the copying of the first side, and that the release agent prevents firm adhesion between the toner particles and the transfer material during the process of transferring the second side, whereby partial white images are liable to occur.

As described above, the application of a transfer device using a contact part has many advantages such as miniaturization and low power consumption, but the requirements for the transfer materials become more stringent.

Since image forming devices such as electrophotographic copying machines have been widely used recently, their applications have also been expanded in various ways and higher image quality has been demanded. For example, when copying original images such as general documents and books, even very small letters are required to be reproduced extremely finely and accurately without thickening or distortion or interruption. In ordinary image forming apparatus such as flat paper copying machines, when the latent image formed on a photosensitive member thereof comprises images of thin lines having a width of 100 micrometers or less, however, the ability to reproduce thin lines is generally poor and the clarity of line images is still insufficient.

Specifically, in recent image forming apparatuses such as electrophotographic printers using digital image signals, the resulting latent image is formed by an aggregation of dots having a constant potential, and the solid areas, halftone areas and highlight areas of the image can be expressed by changing the densities of the dots. However, in a state where the dots are not exactly covered by toner particles and the toner particles protrude from the dots, there is a problem that a gradation curve of a toner image corresponding to the dot density ratio of the black area to the white area in the digital latent image cannot be obtained. Furthermore, in the case where it is intended to increase the resolution by reducing the dot size in order to improve the image quality, the ability to reproduce the latent image comprising very small dots becomes worse, so that an image without sharpness, having a low resolution and a poor gradation curve, tends to occur.

On the other hand, in an image forming apparatus such as an electrophotographic copying machine, there is sometimes a phenomenon that good image quality is obtained in an initial stage, but the image quality deteriorates as the copying or printing process is successively carried out. For such a phenomenon, it can be considered that only toner particles which can contribute more to the developing process are consumed in advance while the copying or printing process is successively carried out, and that toner particles which show poor developing performance accumulate and remain in the developing device of the image forming apparatus.

To date, some developers have been proposed to improve image quality. For example, in Japanese Patent Application Laid-Open (JP-A, KOKAI) No. 3244/1976 (corresponding to U.S. Patent Nos. 3942979, 3969251 and 4112024), a non-magnetic toner in which the particle size distribution is adjusted to improve image quality has been proposed. This toner comprises relatively coarse particles and mainly contains toner particles having a particle size of 8 to 12 micrometers. However, according to our study, with such a particle size, it is difficult to achieve uniform and dense coverage of a latent image with the toner particles. Furthermore, the above-mentioned toner has a feature that it contains 30% (by number) or less of particles having a size of 5 micrometers or less and 5% (by number) or less of particles having a size of 20 micrometers or more, and thus has a broad particle size distribution, which tends to reduce uniformity in the resulting image. In order to form a clear image by using such relatively coarse toner particles having a broad particle size distribution, it is necessary that the spaces between the toner particles be filled by thickly superposing the toner particles so as to increase the apparent image density. As a result, there is a problem that the toner consumption increases for obtaining a prescribed image density.

In Japanese Patent Application Laid-Open No. 72054/1979 (corresponding to US Patent No. 4284701), a non-magnetic toner has been proposed which has a sharper particle size distribution than the above-mentioned toner. In this toner, medium weight particles have a relatively large particle size of 8.5 to 11.0 micrometers, and the toner could be even better as a high resolution toner.

In Japanese Patent Application Laid-Open No. 129437/1983 (corresponding to British Patent Publication No. 2114310), a non-magnetic toner has been proposed in which the average particle size is 6 to 10 micrometers and the mode value of the particle size is 5 to 8 micrometers. However, this toner contains particles having a size of 5 micrometers or less only in a small amount of 15% (by number) or less and tends to produce an image without sharpness.

Further, in U.S. Patent No. 4,299,900, a jump development method has been proposed in which a developer containing 10 to 50 mass% of magnetic toner particles having a size of 20 to 35 micrometers is used. In this method, the particle size distribution of the toner is improved to triboelectrically charge the magnetic toner, form a uniform and thin toner layer on a developing sleeve (developer carrying member), and improve the environmental resistance of the toner. However, further improvements are currently required in the developing and transfer steps.

Furthermore, document EP-A 0 270 063 discloses the use of a developer comprising toner and silicone oil treated powder in a ratio of 0.1 to 3 parts powder to 100 parts toner. However, this type of developer is used in corona type transfer processes.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an image forming method and an image forming apparatus in which the above-mentioned problems encountered in the known art have been solved.

It is another object of the present invention to provide an image forming method and an image forming apparatus which employ an electrostatic pressure transfer method such as a contact transfer method and have a transfer step capable of providing high quality images which faithfully reproduce a latent image regardless of the conditions of the transfer materials.

It is another object of the present invention to provide an image forming method and an image forming apparatus in which the above-mentioned partial white images are avoided or suppressed.

It is another object of the present invention to provide an image forming method and an image forming apparatus capable of providing high quality images without partial white images even in the case of using a transfer material such as thick paper.

It is a further object of the present invention to provide an image forming method capable of exhibiting consistently good operating characteristics even under changes in environmental conditions such as under conditions of high temperature and high humidity and low temperature and low humidity.

According to the present invention, there is provided an image forming method in which

an electrostatic image formed on an image holding member for holding an electrostatic image is developed with a developer to form a developed image thereon, the developer comprising 100 parts by weight of a toner and 0.05 to 3 parts by weight of fine powder treated with 1 to 35 parts by weight of a silicone oil or silicone varnish per 100 parts by weight of fine powder; and

the developed image on the image holding member for holding an electrostatic image is transferred to a transfer material while causing a transfer means to contact the image holding member for holding an electrostatic image via the transfer material under a line pressure of 3 g/cm or higher.

The method of the present invention can be used in conjunction with an image forming apparatus which

an image holding member for holding an electrostatic image;

a device for developing the electrostatic image comprising a toner carrying member, on the toner carrying member there is carried a developer comprising 100 parts by weight of a toner and 0.05 to 3 parts by weight of fine powder treated with a silicone oil or silicone varnish; and

a transfer means for transferring a developed image developed with the developer from the image holding member for holding an electrostatic image to a transfer material while causing the transfer means to contact the image holding member for holding an electrostatic image via the transfer material under a line pressure of 3 g/cm or higher,

includes.

The method of the present invention can also be used in connection with a facsimile machine comprising an image generating device and a receiving device for receiving image data from a remote data station; wherein the image generating device

an image holding member for holding an electrostatic image;

a device for developing the electrostatic image, comprising a toner carrier member, wherein on the toner carrier member a developer comprising 100 parts by weight of a toner and 0.05 to 3 parts by weight of fine powder treated with a silicone oil or silicone varnish; and

a transfer means for transferring a developed image developed with the developer from the image holding member for holding an electrostatic image to a transfer material while causing the transfer means to contact the image holding member for holding an electrostatic image via the transfer material under a line pressure of 3 g/cm or higher,

includes.

These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Figures 1A and 1C are schematic views for explaining toner images showing a good transfer state, and Figures 1B and 1D are schematic views for explaining toner images showing a poor transfer state;

Figures 2 and 3 are schematic partial sectional views each illustrating an apparatus used for a transfer step;

Figures 4 and 5 are a front sectional view and a perspective sectional view, respectively, of an apparatus embodiment for performing multiple subdivision classification;

Figure 6 is a graph showing a particle size range in terms of the ratio "number percentage (N)/volume percentage (V)" and the indicates the numerical percentage of magnetic toner particles having a particle size of 5 micrometers or less;

Figure 7 is a schematic sectional view showing an embodiment of the image forming method and the image forming apparatus according to the present invention; and

Figure 8 is a block diagram showing a facsimile machine in which the image forming apparatus according to the present invention is applied as a printer.

DETAILED DESCRIPTION OF THE INVENTION

We have found that satisfactory results can be achieved by carrying out a transfer step in which a developer obtained by mixing a toner and a fine powder such as silica which has been subjected to a surface treatment with a silicone oil or silicone varnish is used in conjunction with a transfer device in which a transfer material and an image holding member for holding a latent image are caused to contact a transfer member under a line pressure of 3 g/cm or higher.

The contact pressure used in the present invention may preferably be 3 g/cm or more in terms of line pressure. The line pressure can be calculated according to the following formula:

Line pressure (g/cm) = [total pressure (g)]/[length of the contact area (cm)]

The above-mentioned contact area is an area in which a transfer material contacts with the transfer member constituting a transfer device, and its length is measured in a direction perpendicular to the moving direction of the transfer material.

If the above-mentioned contact pressure is less than 3 g/cm, deviation in the conveyance of the transfer material or transfer failure may undesirably occur. In the present invention, the contact pressure may preferably be 20 g/cm or more, and more preferably 25 to 80 g/cm.

Within the scope of the present invention, the transfer device can be a transfer roller as shown in Figure 2 or a transfer belt as shown in Figure 3.

Figure 2 is a schematic side sectional view showing an important part of a typical embodiment of the image forming apparatus according to the present invention. The apparatus shown in Figure 2 comprises a cylindrical image holding member 1 for holding a latent image (hereinafter referred to as "photosensitive member") which extends along a direction perpendicular to the drawing plane and rotates in the direction of arrow A, and an electroconductive transfer roller 2 arranged in contact with the photosensitive member 1.

In the apparatus shown in Figs. 2 and 3, along the peripheral surface of the photosensitive member 1 serving as an image holding member for holding a latent image, there are arranged members not shown to be used for image formation. Specific examples thereof may include a primary charging means for uniformly charging the surface of the photosensitive member 1; an exposure section for supplying a light image comprising laser light modulated according to a predetermined image or reflection light obtained from an original image to the charged surface of the photosensitive member 1 to reduce the potential of the exposed area, thereby forming an electrostatic latent image on the photosensitive member 1; a developing device; the above-mentioned transfer device 2; and a cleaning device for removing residual toner remaining after the transfer operation. left on the surface of the photosensitive member. The above-mentioned components may be arranged in this order along the moving direction of the photosensitive member 1.

The transfer roller 2 comprises a metal core 2a and an electrically conductive elastic (or elastomeric) layer 2b disposed thereon. The electrically conductive elastic layer 2b may comprise an elastic (elastomeric) material such as polyurethane type resin and ethylene-propylene-diene ternary copolymer (EPDM) having a volume resistivity of 106 to 1010 ohm cm and an electrically conductive material such as carbon dispersed therein. A bias voltage may be applied to the metal core 2a by means of a constant voltage source 8. Regarding the bias voltage conditions, a current of 0.1 to 50 µA and a voltage (absolute value) of 100 to 5000 V (particularly 500 to 4000 V) may preferably be applied. In order to apply pressure to the transfer roller 2, pressure may generally be applied to bearings (not shown) supporting both ends of the metal core 2a.

Figure 3 shows the use of a transfer belt 9 within the scope of the present invention. The transfer belt 9 can be supported and driven by an electrically conductive roller 10.

The present invention is particularly preferably applied to an image forming apparatus having an image holding member for holding an electrostatic image whose surface portion comprises an organic compound such as a resin.

When the surface layer of the image holding member for holding an electrostatic image comprises an organic compound, a binder resin contained in a toner easily adheres to such a surface layer. Particularly, in the case where the binder resin and the surface layer comprise materials of the same or similar species, Chemical bonds easily form at the contact points between the toner particles and the photosensitive member, thereby raising the problem that the transferability of the toner is reduced.

Specific examples of the surface material for forming the image holding member for holding an electrostatic image may include silicone resins, vinylidene chloride type resins, ethylene-vinylidene chloride type resins, styrene-acrylonitrile type resins, styrene-methyl methacrylate type resins, styrene type resins, polyethylene terephthalate resins and polycarbonate resins. However, the resin usable in the present invention is not limited to these specific examples, but other copolymers of monomers constituting the above-mentioned resin, copolymers of such a monomer and another monomer, or polymer blends of the above-mentioned polymers may be used.

The present invention is particularly effective in the case of an image forming apparatus comprising, as the photosensitive member 1, a photosensitive drum having a diameter of 50 mm or less (and preferably 40 mm or less).

In the case of a photosensitive drum having a small diameter, since its curvature is larger even under the same line pressure, the pressure is easily concentrated at the contact point. Since the same phenomenon can occur in the case of a belt type photosensitive member, the present invention is also effective in an image forming apparatus comprising a photosensitive member in the form of a belt having a radius of curvature of 25 mm or less at the transfer point.

The developer to be used in the present invention contains fine powder treated with a silicone oil or a silicone varnish. The fine powder to be used in the present invention may preferably have a particle size of 0.001 to 2 micrometers, and more preferably 0.005 to 0.2 micrometers.

The fine powder used in the present invention may preferably comprise an inorganic compound. Preferred examples thereof may include metal oxides containing a Group III or IV metal such as silica (or silicon dioxide), alumina and titania.

In the present invention, it is preferable to use dry process silica fine powder prepared by vapor phase oxidation of a silicon halide. In the above-mentioned preparation step, it is also possible to obtain a composite fine powder of silica and another metal oxide by using another metal halide compound such as aluminum chloride and titanium chloride together with the silicon halide compound. Such a one is also included in the silica fine powder to be used in the present invention.

The silicone oil used for treating the fine powder used in the present invention may preferably be one represented by the following formula:

wherein R represents an alkyl group having 1 to 3 carbon atoms; R' represents a silicone oil modifying group such as alkyl, halogen-modified alkyl, phenyl and modified phenyl (i.e., phenyl having a substituent); and R" represents an alkyl or alkoxy group having 1 to 3 carbon atoms.

Specific examples of such a silicone oil may include dimethyl silicone oil, alkyl-modified silicone oil, α-methylstyrene-modified silicone oil, chlorophenyl silicone oil, fluorine-modified silicone oil, etc. The silicone oil used in the present invention, but is not limited to the specific examples mentioned above.

The above-mentioned silicone oil may preferably be one having a viscosity of 50 to 1,000 centistokes at 25°C. If the viscosity is less than 50 centistokes, the silicone oil may be partially evaporated to deteriorate the charging performance of silicon dioxide. If the viscosity exceeds 1,000 centistokes, handling of the silicone oil in the treatment process is difficult.

Known methods can be used to carry out the silicone oil treatment. For example, a method in which fine powder and a silicone oil are mixed by means of a mixer; a method in which a silicone oil is sprayed onto fine powder by means of a sprayer; and a method in which a silicone oil is dissolved in a solvent and fine powder is mixed into the resulting solution can be used. However, the treatment method applicable to the present invention is not limited to these specific examples.

The silicone varnish to be used in the present invention for treating fine powder may be a known material. Specific examples thereof may include commercially available silicone varnishes such as KR-251 and KP-112 (each manufactured by Shinetsu Silicone K.K.). However, the silicone varnish usable in the present invention is not limited to these specific examples.

To carry out the silicone varnish treatment, known methods can be used in the same way as in the case of silicone oil.

In the present invention, an amino-modified silicone oil represented by the following structural formula (I) can also be used:

TEXT MISSING

Particular examples of the saturated nitrogen-containing heterocycle may include those represented by the following formulas:

Although the present invention is not limited to the above-mentioned specific examples, a heterocycle having a five- or six-membered ring structure can be preferably used.

In the present invention, the heterocycle may be a derivative thereof such that a functional group such as hydrocarbon group, halogen group, amino group, vinyl group, mercapto group, methacrylic group, glycidoxy group and ureido group is introduced therein.

The amino-modified silicone oil used in the present invention may preferably have a nitrogen atom equivalent of 10,000 or less, and more preferably 300 to 2,000. The nitrogen atom equivalent used herein is an equivalent mass (g/equivalent) per nitrogen atom, i.e., a value obtained by dividing the molecular mass by the number of nitrogen atoms contained in one molecule. These silicone oils may be used singly or as a mixture of two or more species thereof.

The silicone varnish used to provide an amino-modified silicone varnish for treating fine powder in the present invention may include methyl silicone varnish, phenylmethyl silicone varnish, etc. Of these, methyl silicone varnish is particularly preferred.

The methyl silicone varnish may comprise a polymer containing the following T³¹ unit, D³¹ unit and M³¹ unit, and may a three-dimensional polymer comprising a larger amount of the T³¹ unit. [T³¹ unit] [D³¹ unit] [M³¹ unit]

Specific examples of the methylsilicone varnish or phenylsilicone varnish may include those comprising a chemical structure represented by the following formula:

wherein R³¹ represents a methyl or phenyl group.

In the silicone varnish mentioned above, the T³¹ unit is particularly effective in imparting good heat curability to form a three-dimensional network structure.

When fine powder is subjected to surface treatment with the silicone varnish comprising such a T³¹ unit, the fine particles constituting the fine powder can have a hard and tough film on their surfaces, whereby the fine particles exhibit excellent impact resistance, moisture resistance and releasability. The above-mentioned T³¹ unit may be contained in the silicone varnish preferably in an amount of 10 to 90 mol%, and more preferably 30 to 80 mol%.

If the content of the T31 unit is too low, the silicone varnish film may be softened due to a low molecular weight component contained therein, so that its adhesiveness increases, which may sometimes lower the moisture resistance, durability or stability of triboelectric chargeability. Further, in some cases, the cleaning performance of the toner is deteriorated to cause scattering of toner, which may cause image unevenness, fog, etc., and further lower the durability of a developing device.

On the other hand, if the content of the T31 unit is too high, the coating layer to be formed on inorganic fine particles may become uneven and, in some cases, the stability of triboelectric chargeability and the durability may be deteriorated.

The silicone varnish may have a hydroxyl group at the end of the molecular chain or in the side chain, and may be crosslinked or cured by dehydration condensation based on such a hydroxyl group. Specific examples of the curing accelerator for accelerating the above-mentioned curing reaction may include fatty acid salts such as those containing zinc, lead, cobalt and tin; amines such as triethanolamine and butylamine, etc. Of these, an amine is particularly preferred.

In order to convert the above-mentioned silicone varnish into an amino-modified silicone varnish, the methyl or phenyl group contained in the above-mentioned T³¹, D³¹ or M³¹ unit can be partially replaced by an amino group-containing group.

Specific examples of the amino group-containing group may include those represented by the following formula, but the amino group-containing group applicable to the present invention is not limited to these specific examples.

-CH₂CH₂-NH₂

-CH₂(CH₂)₂-NH₂

-CH₂(CH₂)₂-NH-(CH₂)₃-NH₂

To carry out the treatment with the amino-modified silicone varnish, known methods can be used in the same way as in the case of the silicone oil.

In the present invention, it is preferred that 1 to 35 parts by mass (and particularly 2 to 30 parts by mass) of the amino-modified silicone oil or amino-modified silicone varnish (based on its solid content) be used for the treatment per 100 parts by mass of the fine powder.

It is preferable that 0.05 to 3 parts by mass (preferably 0.1 to 3 parts by mass, and particularly 0.6 to 3 parts by mass) of the fine powder treated with the silicone oil or silicone varnish is used per 100 parts by mass of the toner.

When the material of the fine powder comprises silica, the silica can preferably exhibit its effect when added in an amount of 0.1 to 1.6 parts by mass, and in particular, it can exhibit excellent stability, when it is added in an amount of 0.3 to 1.6 parts by mass per 100 parts by mass of toner. If the addition amount is less than 0.1 parts by mass, the effect of the addition is small. If the addition amount exceeds 1.6 parts by mass, a problem is likely to occur during the development and fixing processes.

In the present invention, it is more preferable that the fine powder is first treated with a silane coupling agent and then treated with a silicone oil or a silicone varnish.

When the fine powder is treated only with a silicone oil, in general, agglomeration of the fine powder easily occurs in the treatment because the surface of the fine powder is coated with a larger amount of the silicone oil, and the fluidity of a developer may sometimes be reduced when such a fine powder is applied to the developer. It is therefore preferred that sufficient attention be paid to the step of using the silicone oil. In order to remove agglomeration of the fine powder while maintaining its good moisture resistance, it is preferred that the fine powder is treated with a silane coupling agent and then treated with a silicone oil to achieve a sufficient effect of the treatment with the silicone oil.

The silane coupling agent used in the present invention may preferably be one represented by the following general formula:

RmSiYn,

wherein R represents an alkoxy group or a chlorine atom; m represents an integer of 1 to 3; Y represents a hydrocarbon group comprising an alkyl, vinyl, glycidoxy or methacrylic group; and n represents an integer of 3 to 1.

Typical examples of such a silane coupling agent may include dimethyldichlorosilane, trimethylchlorosilane, allylmethyldichlorosilane, hexamethyldisilazane, allylphenyldichlorosilane, benzyldimethylchlorosilane, Vinyltriethoxysilane, γ-methacryloxypropyltrimethoysilane, vinyltriacetoxysilane, divinylchlorosilane, dimethylvinylchlorosilane, etc.

The treatment of the fine powder with the silane coupling agent can be carried out by a dry method in which fine powder is converted into a cloud state by stirring, etc., and a vaporized silane coupling agent is reacted with the resulting cloud; or by a wet method in which fine powder is dispersed in a solvent and a silane coupling agent is dropped into the resulting dispersion to be reacted therewith.

The silane coupling agent may be used for the treatment preferably in an amount of 1 to 50 parts by mass, and more preferably 5 to 40 parts by mass per 100 parts by mass of the fine powder.

In the present invention, the amount of the solid content of the silicone oil or silicone varnish to be used for the treatment is 1 to 35 parts by mass, and preferably 2 to 30 parts by mass per 100 parts by mass of the fine powder. Such a treatment amount is used for the following reasons.

If the amount of the silicone oil used for the treatment is too small, the result of the treatment may be substantially the same as in the case of treatment with a silane coupling agent alone, and the moisture resistance is not sufficiently improved, so that the resulting fine powder may absorb moisture and it is difficult to obtain high-quality copied images under the condition of high humidity. If the amount of the silicone oil used for the treatment is too large, the above-mentioned accumulations of fine powder are likely to occur, and in an extreme case, free silicone oil may also occur. As a result, even in the case of applying such a silica to a developer, there may be a problem that it does not sufficiently improve the fluidity of the developer.

The mechanism of improvement by the fine powder treated with the silicone oil or silicone varnish with respect to the partially white images or hollow characters is not necessarily clear. However, to the best of our knowledge, it is considered that the separability of magnetic toner particles from an image holding member for holding a latent image is improved based on a low surface energy of the treating agent.

In the present invention, the toner contained in the developer may preferably have an average particle size (volume average) of 5 to 13 micrometers.

In an embodiment of the present invention, in which the toner contained in the developer comprises an insulating magnetic toner and a developed image having excellent image quality is desired, an insulating magnetic toner having an average particle size (volume average) of 6 to 8 micrometers may preferably be used.

It is preferable that the above-mentioned insulating magnetic toner contains 17 to 60% (by number) of magnetic toner particles having a particle size of 5 micrometers or less, 5 to 50% (by number) of magnetic toner particles having a particle size of 6.35 to 10.08 micrometers, and 2.0% or less by volume of magnetic toner particles having a particle size of 12.70 micrometers or more, and the magnetic toner has an average particle size (volume average) of 6 to 8 micrometers, and the magnetic toner particles having a particle size of 5 micrometers or less have a particle size distribution satisfying the following formula:

N/V = -0.05 N + k,

where N is the percentage (in number) of magnetic toner particles having a particle size of 5 micrometers or less, V is the volume percentage of magnetic toner particles having a particle size of 5 micrometers or less, k denotes a positive number from 4.6 to 6.7, and N denotes a positive number from 17 to 60.

The insulating magnetic toner having the above-mentioned particle size distribution can accurately reproduce thin lines present in a latent image formed on a photosensitive member and is excellent in reproduction of latent dot images such as halftone dot images and digital images, so that it provides images with excellent gradation and resolution characteristics. Furthermore, such a toner can maintain high image quality even in the case of successive copying or printing and can effect good development even in the case of high density images while consuming a smaller amount thereof as compared with the conventional magnetic toner. As a result, the above-mentioned magnetic toner has excellent economy and also has the advantage of miniaturizing the main body of a copying machine or printer.

The reason for the above-mentioned effects of the magnetic toner according to the present invention is not necessarily clear, but the reason can be considered as follows.

The magnetic toner according to the present invention can be characterized, first, by containing 17 to 60% (by number) of magnetic toner particles having a size of 5 micrometers or less. Conventionally, it has been considered that the content of magnetic toner particles having a size of 5 micrometers or less must necessarily be reduced because it is difficult to adjust the amount of charge, because they impair the fluidity of the magnetic toner, and because they cause scattering of toner to contaminate the apparatus.

However, according to our investigation, it has been found that the magnetic toner particles with a size of 5 micrometers or less an essential component in producing a high quality image.

For example, we conducted the following experiments.

Thus, a latent image was formed on a photosensitive member whereby the surface potential on the photosensitive member varied from a high development potential contrast, where the latent image would be easily developed with a large number of toner particles, to a low development potential contrast, where the latent image would be developed with only a small number of toner particles.

Such a latent image was developed with a magnetic toner whose particle size distribution was in the range of 0.5 to 30 micrometers. Then, the toner particles adhering to the photosensitive member were collected and their particle size distribution was measured. As a result, it was found that many magnetic toner particles having a particle size of 8 micrometers or less, and particularly 5 micrometers or less, were present. Based on this result, it was found that when magnetic toner particles having a size of 5 micrometers or less were adjusted to be uniformly supplied for developing a latent image formed on a photosensitive member, an image having truly excellent reproducibility could be obtained and the toner particles adhered precisely to the latent image without protruding therefrom.

The magnetic toner used in the present invention preferably contains 5 to 50% (by number) of magnetic toner particles having a size of 6.35 to 10.08 micrometers. This second feature is related to the above-mentioned necessity of the presence of the toner particles having a size of 5 micrometers or less.

As described above, the toner particles having a particle size of 5 micrometers or less have the ability to cover a latent image accurately and reproduce it accurately. On the other hand, in the latent image itself, the field strength at its peripheral edge portion is higher than the field strength at its central portion. Consequently, toner particles sometimes cover the inner portion of the latent image in a smaller amount than that at its peripheral portion, whereby the image density in the inner portion appears to be lower. In particular, the magnetic toner particles having a size of 5 micrometers or less show such a tendency to a large extent. However, we have found that not only the above-mentioned problem can be solved, but also the resulting image can be made clearer if 5 to 50% (by number) of toner particles having a size of 6.35 to 10.08 micrometers are contained in a toner.

To the best of our knowledge, the reason for such a phenomenon can be considered to be that the toner particles having a size of 6.35 to 10.08 micrometers have a charge amount appropriately adjusted with respect to those having a size of 5 micrometers or less, and these toner particles are supplied to the inner region of the latent image having a lower field strength than the peripheral region, thereby compensating for the reduction in the coverage of the toner particles in the inner region compared with the coverage in the peripheral region and producing a uniform developed image. As a result, a sharp image having a high image density and excellent resolution and gradation characteristics can be obtained.

The third feature of the magnetic toner preferably used in the present invention may be that toner particles having a particle size of 5 micrometers or less contained therein satisfy the following relationship between their percentage by number (N) and their percentage by volume (V):

N/V = -0.05 N + k,

where 4.6 ≤ k ≤ 6.7 and 17 ≤ N ≤ 60.

The range satisfying such a relationship is shown in Fig. 6. The magnetic toner according to this embodiment of the present invention, which has the particle size distribution satisfying such a range in addition to the above-mentioned features, can achieve excellent developing performance.

We have found, according to our study on the state of particle size distribution with respect to toner particles having a size of 5 micrometers or less, that there is a proper state of the presence of fine powder in magnetic toner particles. Specifically, in the case of a certain value of N, it is understood that a large value of N/V shows that the particles having a size of 5 micrometers or less are contained in a considerable proportion, and a small value of N/V shows that the frequency of the presence of particles having a size near 5 micrometers is high and that of particles having a smaller particle size is low. When the value of N/V is in the range of 1.6 to 5.85, N is in the range of 17 to 60, and the relationship represented by the above-mentioned formula is satisfied, good thin line reproducing ability and high resolution are obtained.

In the magnetic toner preferably used in the present invention, magnetic toner particles having a particle size of 12.70 micrometers or more may be contained in an amount of 2.0% by volume or less. The amount of these particles may preferably be as small as possible.

As described above, the magnetic toner can solve the problems encountered in the prior art from a viewpoint completely different from that of the prior art and can meet the recently strongly demanded high image quality.

Another embodiment of the present invention will be described in more detail below.

In this embodiment of the present invention, the magnetic toner particles having a particle size of 5 micrometers or less are contained in an amount of 17 to 60%, preferably 25 to 60%, and more preferably 30 to 60%, based on the total number of particles. If the amount of these magnetic toner particles is less than 17% (in number), the proportion of the toner particles effective for improving the image quality is insufficient. Above all, as the toner particles are consumed in successive copying or printing, the component of the effective magnetic toner particles decreases and the balance in the particle size distribution of the magnetic toner exhibited by the present invention deteriorates, so that the image quality gradually decreases. On the other hand, if the above-mentioned amount exceeds 60% (in terms of number), the magnetic toner particles are likely to aggregate to produce toner aggregates whose size is larger than the original particle size. As a result, coarse images are obtained, the resolution is lowered, and the density difference between the peripheral areas and the inner areas increases, so that an image whose inner area has a somewhat low density is likely to occur.

In the magnetic toner according to the present invention, it is preferred that the amount by number of particles in the size range of 6.35 to 10.08 micrometers is 5 to 50%, and preferably 8 to 40%. If the above-mentioned amount is larger than 50% (by number), not only the image quality deteriorates, but also excessive development (i.e., excessive coverage of toner particles) occurs, resulting in an increase in toner consumption. On the other hand, if the above-mentioned amount is less than 5%, it is difficult to obtain a high image density.

In the present invention, it is preferable that the number percentage (N%) and the volume percentage (V%) of magnetic toner particles having a particle size of 5 micrometers or less satisfy the relationship of N/V = -0.05 N + k, where k is a positive number, satisfy the relationship of 4.6 ≤ k ≤ 6.7. The number k may preferably satisfy the relationship of 4.6 ≤ k ≤ 6.2, and particularly the relationship of 4.6 ≤ k ≤ 5.7. As described above, furthermore, the number N may preferably satisfy the relationship of 17 ≤ N ≤ 60, particularly the relationship of 25 ≤ N ≤ 60, and particularly the relationship of 30 ≤ N ≤ 60.

If k < 4.6, the proportion of magnetic toner particles having a size of 5 micrometers or less is insufficient, and the resulting image density, resolution and sharpness may decrease. If fine toner particles, which have been conventionally considered unusable, are present in a magnetic toner in a proper amount, they achieve a densest toner packing in development (i.e., in a latent image formed on a photosensitive drum) and contribute to the formation of a uniform image free from coarsening. In particular, these particles fill in thin line areas and contour areas of an image, thereby visibly improving its sharpness. If k < 4.6 in the above formula, the proportion of such a component in the particle size distribution becomes insufficient, and the above-mentioned properties may become poor.

Furthermore, in view of the manufacturing process, a large amount of fine powder must be removed by classification in order to satisfy the condition of k < 4.6. Such a process is disadvantageous in terms of yield and toner cost.

On the other hand, an excess of fine powder is present when k > 6.7, so that the resulting image density slightly decreases in successive copying. The reason for such a phenomenon can be assumed to be that an excess of fine magnetic toner particles with an excess amount of charge triboelectrically adheres to a developing cylinder and prevents normal toner particles from being carried on the developing cylinder and charged.

In the magnetic toner used in this embodiment, the amount of magnetic toner particles having a particle size of 12.7 micrometers or more may preferably be 2.0% by volume or less, more preferably 1.0% by volume or less, and most preferably 0.5% by volume or less.

If the above-mentioned amount is greater than 2.0% by volume, these particles may impair the ability to reproduce thin lines.

In the present invention, the average particle size (volume average) of the toner may preferably be 6 to 8 micrometers. This value is closely related to the above-mentioned characteristics of the magnetic toner according to this embodiment. If the average particle size (volume average) is smaller than 6 micrometers, in the case of an image such as a graphic image in which the ratio of the image area to the entire area is high, problems that the amount of toner particles transferred to a transfer paper is insufficient and the image density is low are likely to occur. The reason for such a phenomenon can be considered to be the same as in the above-mentioned case in which the inner portion of a latent image provides a lower image density than that in its peripheral portion. If the average particle size (number average) exceeds 8 micrometers, the resulting resolution is not good and a phenomenon easily occurs that the image quality is reduced in successive printing even if it is good in its initial stage.

The particle size distribution of a toner can be measured using a Coulter counter in the present invention, but it can be measured in various ways.

A Coulter counter (Model TA-II, available from Coulter Electronics Inc.) is used as the measuring device, to which an interface (available from Nikkaki K.K.) for providing a number-based distribution and a volume-based distribution and a workstation computer (available from Canon K.K.) are connected.

For measurement, a 1% aqueous NaCl solution is prepared as an electrolytic solution using reagent-grade sodium chloride. Into 100 to 150 ml of the electrolytic solution, 0.1 to 5 ml of a surfactant, preferably an alkylbenzenesulfonic acid salt, is added as a dispersant, and 2 to 20 mg of a sample is added thereto. The resulting dispersion of the sample in the electrolytic liquid is subjected to a dispersion treatment with an ultrasonic disperser for about 1 to 3 minutes, and then subjected to measurement of particle size distribution in the range of 2 to 40 micrometers using the above-mentioned Coulter counter (Model TA-II) with an aperture of 100 micrometers to obtain a volume distribution and a number distribution. From the results of the volumetric distribution and the numbertric distribution, the above-mentioned respective parameters characterizing the magnetic toner are obtained.

In the present invention, the true density of the magnetic toner may preferably be 1.45 to 1.8 g/cm³, and more preferably 1.55 to 1.75 g/cm³. When the true density is in such a range, the magnetic toner having a specific particle size distribution as described above functions very effectively in a reversal development system in the presence of a magnetic field in terms of high image quality and stability in successive use.

If the true density of the magnetic toner particles is less than 1.45, the mass of the particle itself may be too small and there is a tendency for reversal fog and distortion of thin lines, scattering of toner and deterioration of resolution in reversal development because an excess of toner particles adheres to the latent image. On the other hand, if the true density of the magnetic toner is more than 1.8, an image in which the image density is low, thin lines are broken and sharpness is lacking occurs. Furthermore, tufts of the toner particles are easily elongated or turned into a branched form because the magnetic field strength becomes relatively high in such a case. As a result, when a latent image is developed, the image quality is deteriorated so that a coarse image is easily caused.

In the present invention, the true density of the magnetic toner can be measured in the following manner, which can easily provide an accurate value in the measurement of fine powder, but the true density can be measured in various ways.

A stainless steel cylinder with an inner diameter of 10 mm and a length of about 5 cm and a disc (A) with an outer diameter of about 10 mm and a height of about 5 mm and a piston (B) with an outer diameter of about 10 mm and a length of about 8 cm, which can be tightly inserted into the cylinder, are provided.

In the measurement, first the disc (A) is placed at the bottom of the cylinder, and about 1 g of a sample to be measured is filled into the cylinder, and the piston (B) is gently pushed into the cylinder. Then a force of 400 kg/cm2 is applied to the piston by means of a hydraulic press, and the sample is pressed for 5 minutes. The mass (W g) of the sample thus pressed is measured, and its diameter (D cm) and height (H cm) are measured by means of a micrometer. Based on such a measurement, the true density can be calculated according to the following formula:

True density (g/cm³) = W/[π × (D/2)² × L]

To achieve better development performance, the magnetic toner used in the present invention may will preferably have the following magnetic properties: a remanence r of 1 to 5 emE/g and in particular 2 to 4.5 emE/g; a saturation magnetization s of 15 to 50 emE/g and in particular 20 to 40 emE/g and a coercive force Hc of 20 to 100 Oe, preferably 40 to 100 Oe and in particular 40 to 70 Oe. These magnetic properties can be measured under a measuring magnetic field of 1 kOe.

The magnetic toner having a particle size as used in the present invention generally tends to have a larger amount of charge and to aggregate as compared with the conventionally known toner having a volume average particle size of 9 micrometers or more. Therefore, as the particle size becomes smaller, it is necessary to add a fluidity improving agent thereto in accordance with the increase in the specific surface area. When hydrophobic silica subjected to surface treatment with a silicone oil or silicone varnish according to the present invention is used in an image forming method in which a transfer charger arranged in contact with an image holding member for holding an electrostatic image under a contact pressure of 3 g/cm or higher is used, the fluidity can be improved and further partial white images (e.g., hollow characters or hollow letters) can be avoided.

In the present invention, it is also preferred that hydrophobic silicon dioxide is used as a fluidity-improving agent in combination with the above-mentioned fine powder which has been surface-treated with the silicone oil or silicone varnish.

Specific examples of the binder for use in forming the magnetic toner according to the present invention may include homopolymers of styrene and its derivatives such as polystyrene, poly-p-chlorostyrene and polyvinyltoluene; styrene copolymers such as styrene-p-chlorostyrene copolymer, styrene-propylene copolymer, styrene-vinyltoluene copolymer, styrene-vinylnaphthalene copolymer, styrene-methyl acrylate copolymer, styrene-ethyl acrylate copolymer, styrene-butyl acrylate copolymer, styrene-octyl acrylate copolymer, styrene-methyl methacrylate copolymer, styrene-ethyl methacrylate copolymer, styrene-butyl methacrylate copolymer, styrene-methyl-α-chloromethacrylate copolymer, styrene-acrylonitrile copolymer, styrene-vinyl methyl ether copolymer, styrene-vinyl ethyl ether copolymer, styrene-vinyl methyl ketone copolymer, styrene-butadiene copolymer, styrene-isoprene copolymer and styrene-acrylonitrile-indene copolymer; Polyvinyl chloride, polyvinyl acetate, polyethylene, polypropylene, silicone resin, polyester resin, epoxy resin, polyvinyl butyral, rosin, modified rosin, terpene resin, phenolic resins, xylene resin, aliphatic or alicyclic hydrocarbon resins, aromatic petroleum resin, chlorinated paraffin, paraffin wax, etc. These binder resins can be used either individually or as a mixture.

Of these, the binder used in the present invention may preferably comprise a styrene-acrylic resin type copolymer (including styrene-acrylic ester copolymer and styrene-methacrylic ester copolymer). Particularly preferred examples may include styrene-n-butyl acrylate (St-nBA) copolymer, styrene-n-butyl methacrylate (St-nBMA) copolymer and styrene-n-butyl acrylate-2-ethylhexyl methacrylate (St-nBA-2EHMA) copolymer in view of the fixing performance and the offset resistance of the resulting toner in hot roll fixing.

The magnetic toner according to the present invention may also contain a known colorant. Specific examples thereof may include carbon black, copper phthalocyanine, iron black, etc.

The magnetic toner according to the present invention may contain a magnetic material. The magnetic material to be mixed into the toner may be powder of a material which is magnetizable when placed in a magnetic field, including a metal such as Fe, Ni and Co or an alloy or compound of these metals such as magnetite, γ-Fe₂O₃ and ferrite.

The magnetic fine powder may preferably have a specific surface area determined by the BET method of 2 to 20 m²/g, and more preferably 2.5 to 12 m²/g, and may preferably have a Mohs hardness of 5 to 7. The magnetic powder may be used in a proportion of 70 to 120 parts by mass per 100 parts by mass of the binder resin.

The toner according to the present invention may also contain a charge control agent if desired. Specific examples thereof may include negative charge control agents such as metal salts of monoazo dyes and metal complex salts of salicylic acid, alkylsalicylic acid, dialkylsalicylic acid and naphthoic acid.

In the present invention, the hydrophobicity (or wettability) of the silica fine powder can be measured in the following manner, but other methods may be used with respect to the following method.

A sample in an amount of 0.1 g is placed in a 200 ml separatory funnel fitted with a stopper, and 100 ml of ion-exchanged water is added thereto. The mixture is shaken for 10 min by a shaker mixer (Turbula Shaker Mixer model T2C) at 90 rpm. The separatory funnel is then left to stand for 10 min to separate a silica powder layer and an aqueous layer, and 20 to 30 ml of the contents are drained from the bottom. A portion of the water is taken up in a 10 mm cell, and the transmittance of the water thus drained is measured by a colorimeter (wavelength: 500 nm) in comparison with ion-exchanged water not containing silica fine powder as Blank sample measured. The permeability of the water sample is called hydrophobicity (wettability) of the silicon dioxide.

The hydrophobic silica used in the present invention should preferably have a hydrophobicity of 90% or higher, and particularly 93% or higher. If the hydrophobicity is less than 90%, high-quality images cannot be obtained under a high-humidity condition due to moisture absorption by the silica fine powder.

The magnetic toner according to the present invention can be generally produced in the following manner.

(1) The binder resin and a magnetic material are mixed by uniformly dispersing using a mixing device such as a Henschel mixer together with an optionally added dye or pigment as a colorant.

(2) The mixture obtained by mixing as above is subjected to a melt-kneading process using a melt-kneading device such as a kneader, an extruder and a roll mill.

(3) The kneaded product is roughly crushed by a crushing device such as a cutting mill and a hammer mill and then finely pulverized by a micropulverizing mill such as a jet mill.

(4) The finely pulverized product is classified by means of a classifier such as a zigzag classifier and an elbow jet classifier to obtain a magnetic toner according to the present invention.

As another method for producing the magnetic toner according to the present invention, a polymerization method or an encapsulation method can be used. The outlines of these methods are summarized below.

[Polymerization process]

(1) A monomer mixture comprising a polymerizable monomer (and optionally a polymerization initiator and a colorant) can be dispersed in an aqueous dispersant to form particles.

(2) The particles of the monomer mixture are classified to obtain a suitable particle size range.

(3) The particles of the monomer mixture which, after classification, are within a prescribed particle size range, are subjected to polymerization.

(4) After removing a dispersant by an appropriate treatment, the polymerized product is filtered, washed with water and dried to obtain a toner.

[Encapsulation process]

(1) A binder resin (and optionally a colorant and a magnetic material) is subjected to a melt-kneading process to form a toner core material in a molten state.

(2) The toner core material is vigorously stirred in water to form fine particles of the core material.

(3) The fine core particles are dispersed in a solution of a coating material, and a poor solvent is added thereto with stirring to coat the surfaces of the core particles with the coating material and effect encapsulation.

(4) The capsules obtained above are recovered by filtration and drying to obtain a toner.

The developer to be used in the present invention can be obtained by adding a fine powder such as hydrophobic silica treated with a silicone oil or silicone varnish to the thus-obtained toner and mixing the fine powder with the toner.

A preferred embodiment of the image forming method according to the present invention will be described with reference to Figure 7.

Referring to Figure 7, the surface of a photosensitive member (photosensitive drum) 1 is negatively charged by a primary charger 702, and then an exposure light 705 comprising a laser beam is applied to the surface of the photosensitive member according to an image scanning method so that a digital latent image is formed thereon. The latent image is developed with a magnetic one-component developer 710 at a developing position where a developing sleeve 704 of a developing device 709 is arranged opposite to the surface of the photosensitive member to form a toner image. The developing device 709 comprises a magnetic doctor blade 711 and the developing sleeve 704 having a magnet 714 therein, and contains the one-component developer 710. In the developing position, a bias voltage is applied between the electrically conductive substrate (not shown) of the photosensitive drum 1 and the developing sleeve 704 by a bias voltage applying device 712 as shown in Figure 7, which includes an AC bias voltage, a pulse bias voltage and/or a DC bias voltage.

As shown in Figure 7, when a transfer paper P is transported to a transfer position where a transfer device 2 is opposed to the photosensitive drum 1, the back surface of the image receiving paper P (ie, the surface thereof which is opposite to the surface which is opposite to the photosensitive drum 1) by means of a roller type transfer device 2 and a voltage applying device 8, whereby the developed image (ie, toner image) formed on the surface of the photosensitive drum is electrostatically transferred to the image receiving paper P. Then, the image receiving paper P is separated from the photosensitive drum 1 and transported to a fixing device 707 operating under the application of heat and pressure, whereby the toner image is fixed to the image receiving paper P.

The residual one-component developer remaining on the photosensitive drum 1 downstream of the transfer position is removed by a cleaning device 708 having a cleaning blade. The photosensitive drum 1 after cleaning is discharged by erasing exposure 706 and is again subjected to the above-mentioned process which includes, as an initial step, the step of charging by means of the primary charging device 702.

Referring again to Fig. 7, the photosensitive drum 1 serving as an image holding member for holding an electrostatic image comprises a photosensitive layer and the electrically conductive substrate (not shown) and moves in the direction of an arrow shown in Fig. 7. On the other hand, the developing sleeve 704 made of a non-magnetic cylinder serving as a developer carrying member rotates in the developing position so as to move in the same direction as the photosensitive drum 1. The multi-pole permanent magnet (magnet roller) not shown is arranged in the non-magnetic cylinder 704 so as not to rotate.

The insulating magnetic single-component developer 710, which is arranged in the developing device 709, is applied to the developing sleeve 704, and the friction between the surface of the developing sleeve 704 and the toner particles imparts a negative triboelectric charge to the toner particles contained therein.

A magnetic blade 711 made of iron is arranged near the surface of the developing sleeve (preferably at a gap of 50 to 500 micrometers) and opposite to one of the poles of the multi-pole permanent magnet contained in the developing sleeve 704. In this way, the thickness of the toner layer arranged on the developing sleeve 704 is adjusted to a uniform and small value (preferably at a thickness of 30 to 300 micrometers) to form a developer layer whose thickness is smaller than the above-mentioned gap between the photosensitive drum 1 and the developing sleeve 704 in the developing position, so that the developer layer formed on the developing sleeve 704 does not contact the image holding member 1. The rotational speed of the developing sleeve 704 can be adjusted so that the speed of its surface is substantially the same as (or close to) the speed of the surface of the photosensitive drum 1.

The magnetic blade 711 may also comprise a permanent magnet instead of iron to form a magnetic counterpole. An AC bias or a pulse bias may be applied between the developing sleeve 704 and the photosensitive drum 1 by the bias applying means 712. The AC bias may preferably have a frequency of 200 to 4000 Hz and a Vpp (peak-to-peak voltage) of 500 to 3000 V. In the developing position, the toner particles are transferred to an electrostatic image formed on the photosensitive drum 1 under the action of an electrostatic force attributable to the surface holding an electrostatic image and under the action of the AC bias or pulse bias.

In the above-mentioned embodiment, instead of the magnetic blade 711, an elastic blade comprising an elastic or elastomeric material such as silicone rubber may be used so that the developer is applied to the developer carrying member 704 is applied while the thickness of the developer layer is adjusted under pressure.

In the case where the present invention is applied to a printer for a facsimile machine, the imagewise exposure corresponds to the exposure for printing received data. Figure 8 shows such an embodiment using a block diagram.

Referring to Fig. 8, a controller 511 controls an image reader (or image reading unit) 510 and a printer 519. The entire controller 511 is controlled by a CPU (central processing unit) 517. Data read from the image reader 510 is transmitted to another terminal such as a facsimile machine through a transmission circuit 513. On the other hand, data received from another terminal such as a facsimile machine is transmitted to the printer 519 through a reception circuit 512. An image memory 516 stores prescribed image data. A printer control unit 518 controls the printer 519. In Fig. 8, reference numeral 514 denotes a telephone system.

More specifically, an image received from a line (or circuit) 515 (i.e., image data received from a remote terminal connected through the line) is demodulated by the receiving circuit 512, decoded by the CPU 517, and sequentially stored in the image memory 516. When image data corresponding to at least one page has been stored in the image memory 516, image-wise recording is performed with respect to the corresponding page. The CPU 517 reads out image data corresponding to one page from the image memory 516 and transmits the decoded data corresponding to one page to the printer control unit 518. When the printer control unit 518 receives the image data corresponding to one page from the CPU 517, the printer control unit 518 controls the printer 519 so that recording of image data corresponding to the page is performed. During recording by the printer 519, the ZVE 517 receives other image data corresponding to the next page.

In this way, the reception and recording of an image can be carried out in the manner mentioned above by means of the device shown in Figure 8.

The present invention will be explained in more detail with reference to examples, by which the present invention is not limited at all. In the following formulations, parts are parts by weight.

example 1

Styrene-butyl acrylate-divinylbenzene copolymer 100 parts by weight

[Copolymerization mass ratio = 84/15.5/0.5,

Average molecular weight (weight average) = 25 × 10&sup4;]

Magnetite 100 parts by mass

(average particle size = 0.2 micrometers)

Ethylene-polypropylene copolymer with low molecular weight 3 parts by mass

[Average molecular weight (weight average) = 10,000)

Chromium complex of a monoazo dye 0.5 parts by mass

(Spiron Blak TRH, manufactured by Hodogaya Kagaku)

The above components were well mixed by a mixer and subjected to melt-kneading by means of a two-axis extruder heated to 130 ºC. Incidentally, a magnetic toner which easily caused fog could be obtained if the set temperature at that time was too high.

The above-mentioned kneaded product was cooled and then coarsely crushed by a cutting mill and then finely pulverized by a micropulverizer using an air jet. The finely pulverized product was classified by a fixed wall air classifier to obtain a classified powder product. Ultrafine powder and coarse powder were separated from the classified powder by a multi-section classifier using a Coanda effect (Elbow Jet Classifier, available from Nittetsu Kogyo KK), thereby obtaining an insulating magnetic toner (A) having a volume average particle size of 6.5 micrometers. When the magnetic toner (A) thus obtained was mixed with an iron powder carrier and then its triboelectric charge was measured, it was found to have a negative charge.

The number distribution and volume distribution of the magnetic toner thus obtained were measured in the same manner as described above using a Coulter counter (Model TA-II) with an aperture of 100 micrometers. The results thus obtained are shown in Table 1 below.

To 100 parts of the negatively chargeable insulating magnetic toner, 1.3 parts of dry process silica fine powder (specific surface area determined by BET method = 200 m2/g, wettability with water = 97%) treated with hexamethyldisilazane and silicone oil was added and mixed by a Henschel mixer to obtain a one-component type negatively chargeable magnetic developer.

The resulting developer was introduced into a modification of a commercially available copying machine (trade name: FC-5, manufactured by Canon K.K.) having a negatively chargeable laminate type photosensitive member (drum) (diameter = 30 mm) with an organic photoconductor (OPC) in which a transfer material is separated from the photosensitive member due to the curvature thereof. The copying machine used here was modified to effect reversal development and a transfer device having a transfer roller as shown in Figure 2 was incorporated therein.

The transfer roller used here had a surface area made of rubber with a rubber hardness of 27 degrees according to JIS-A (JIS K 6301-1975) and comprised an electrically conductive elastic layer made of EPDM and electrically conductive carbon black dispersed therein, which had a volume resistivity of 10⁸ ohm cm. Furthermore, with respect to the transfer conditions used here, a transfer current of 1 µA, a transfer voltage of +2000 V and a contact pressure of 50 g/cm were used.

In the above-mentioned image formation, the photosensitive drum was subjected to primary charging at -700 V, the gap between the photosensitive drum and a developing drum (in which a magnet was incorporated) was set so that the developer layer formed on the developing drum did not contact the photosensitive drum, and an AC bias (frequency = 1800 Hz, Vpp value = 1600 V) and a DC bias (VDC = -500 V) were applied to the developing drum. The resulting developed image was fixed to a transfer material by a fixing device having a heating and pressure roller.

The thus obtained fixed toner images were evaluated in the following manner.

(1) Image density

1000 sheets of ordinary flat paper (75 g/m²) for a copying machine were passed through the above-mentioned copying machine, and the image density at the time of copying 1000 sheets was evaluated.

(Excellent): The image density was 1.35 or more.

Δ (good): The image density was 1.0 to 1.34.

× (not good): The image density was below 1.0.

(Transmission state)

A sharper transfer condition was applied by passing thick paper (120 g/m²) through the copier, and the resulting transfer error (or transfer dropout) was evaluated.

: The resulting image was good, as shown in Figure 1A.

Δ: The resulting image was acceptable for practical application.

×: The resulting image was not good, as shown in Figure 1B.

(3) Paper transport condition

1000 sheets of thin paper (50 g/m²) were passed through the copier and the occurrence of transport errors such as skewed movement was evaluated.

: The number of occurrences of transport errors per pass of 1000 sheets was 1 or less.

Δ: The number of occurrences of transport errors per pass of 1000 sheets was 2 to 4.

×: The number of occurrences of transport errors per pass of 1000 sheets was 5 or more.

(4) Image quality

Scattering of toner particles, coarsening, etc. in the resulting image were evaluated with the naked eye.

: Good.

Δ: Acceptable for practical use.

×: Not acceptable for practical use.

(5) Ability to reproduce thin lines

The ability to reproduce a latent image in the form of transverse lines with a width of 50 micrometers was evaluated.

: Good.

Δ: Acceptable for practical use.

' : Not acceptable for practical use.

The multi-section classifier and the classification step applied in this case are explained below with reference to Figures 4 and 5.

Referring to Figures 4 and 5, the multi-partitioned sifter 101 has side walls 122, 123 and 124 and a bottom wall 125. The side wall 123 and the bottom wall 125 are provided with knife-edge-shaped classifying wedges 117 and 118, respectively, dividing the classifying chamber into three sections. In a lower portion of the side wall 122 there is a raw material supply nozzle 116 leading into the classifying chamber. Along the lower contact line of the nozzle 116 there is a Coanda block 126 arranged so as to form a long elliptical arc formed by bending the contact line downwards. The classifying chamber has an upper wall 127 provided with a knife-edge-shaped gas inlet wedge 119 extending downwards. Above the classifying chamber there are gas inlet pipes 114 and 115 leading into the classifying chamber. In the inlet pipes 114 and 115 there are a first gas introduction control device 120 and a second gas introduction control device 121, which comprise, for example, a throttle; and pressure gauges 128 and 129 for measuring the static pressure are also arranged in communication with the pipes 114 and 115. At the bottom of the classifying chamber there are arranged outlet pipes 111, 112 and 113 with outlets corresponding to the respective classifying sections and leading into the chamber.

Powder to be classified, which serves as a raw material, is introduced into the classifying zone through the feed nozzle 116 under reduced pressure. The powder, which serves as a raw material, thus fed is dropped along curved lines 130 due to the Coanda effect given by the Coanda block 126 and the action of the air streams having a high speed, so that the powder serving as a raw material is classified into coarse powder 111, black fine powder (magnetic toner) 112 having a prescribed average particle size (volume average) and particle size distribution, and ultrafine powder 113.

Example 2

Image formation was carried out in the same manner as in Example 1, except that 0.6 parts of aluminum oxide (specific surface area determined by the BET method) was used as an additive to be mixed with the insulating magnetic toner (A). = 100 m²/g) treated with silicone varnish and 1.0 part of hydrophobic silica fine powder obtained by treating dry process silica fine powder having a specific surface area of 300 m²/g determined by the BET method with hexamethyldisilazane.

The results are shown in Table 2 below.

Example 3

Image formation was carried out in the same manner as in Example 1 except that a transfer condition of 5 g/cm was applied.

The results are shown in Table 2 below.

Example 4

Image formation was carried out in the same manner as in Example 1 except that 80 parts by mass of magnetite, a magnetic toner (B) having a particle size distribution shown in Table 1 and 0.8 part of hydrophobic dry process silica fine powder treated with hexamethyldisilazane and dimethylsilicone oil were used.

The results are shown in Table 2 below.

Comparison example 1

Image formation was carried out in the same manner as in Example 2 except that silicone varnish-treated alumina was not used.

The results are shown in Table 2 below.

Comparison example 2

Image formation was carried out in the same manner as in Example 1 except that a transfer condition of 2 g/cm was applied.

The results are shown in Table 2 below.

Comparison example 3

Image formation was carried out in the same manner as in Example 1 except that 60 parts by mass of magnetite, a magnetic toner (C) having a particle size distribution shown in Table 1 and 3.5 parts of dry-process hydrophobic silica treated with hexamethyldisilazane and dimethylsilicone oil were used.

The results are shown in Table 2 below.

Comparison example 4

Image formation was carried out in the same manner as in Example 1 except that 140 parts by mass of magnetite, a magnetic toner (D) having a particle size distribution shown in Table 1 and 4.4 parts of fine powder were used.

The results are shown in Table 2 below. The developer of this example showed quite poor fixing behavior. Table 1 Average particle size (volume average) (µm) % (by number) of particles ≤ 5 µm % (by volume) of particles ≥ 12.7 µm % (by number) of particles from 6.35 to 10.08 µm [% (by number)]/[% (by volume)] of particles ≤ 5 µm True density Magnetic toner Table 2 Image density Transfer condition Paper transport condition Image quality Thin line reproduction ability Example Comparative example

Example 5

Styrene-butyl acrylate copolymer 100 parts by weight

(Copolymerization mass ratio = 8:2)

Magnetic material 60 parts by mass

(Magnetite)

Release agent 3 parts by mass

(polypropylene wax)

Chromium complex of a monoazo dye 1 part by mass

(charge control agent)

The above components were subjected to melt-kneading by means of a two-axis extruder heated to 160 ºC, and the kneaded product was cooled and then roughly crushed by means of a hammer mill (mechanical pulverizer) so that the resulting product passed through sieve meshes having an opening diameter of 2 mm, and then finely pulverized by means of a jet mill (wind power pulverizer) so that a particle size of about 10 micrometers was obtained.

The finely pulverized product was classified using a DS classifier (air classifier) in such a way that the classified product had a had a volume average particle size of 11.5 micrometers (measured by a Coulter counter), thereby obtaining a negatively chargeable, insulating magnetic toner.

When the insulating magnetic toner thus obtained was mixed with an iron powder carrier and then its triboelectric charge was measured by the blow-off method, it showed a value of -13 uC/g.

Separately, 100 parts of silica fine powder (Aerosil #200, manufactured by Nihon Aerosil K.K.) having a specific surface area of 200 m2/g was treated with 20 parts of hexamethyldisilazane (HMDS) and then treated with a solution obtained by diluting 10 parts of dimethylsilicone oil (trade name: KF-96, 100 cSt, manufactured by Shinetsu Kagaku) with a solvent. The resulting mixture was dried and subjected to heat treatment at about 250 °C, thereby obtaining silica fine powder treated with hexamethyldisilazane and then treated with dimethylsilicone oil.

To 100 parts of the above-mentioned insulating magnetic toner, 0.8 parts by mass of the treated silica powder was added and mixed by a Henschel mixer to obtain a one-component type magnetic developer.

The resulting developer was introduced into a modification of a commercially available copying machine (trade name: FC-5, manufactured by Canon K.K.) having a negatively chargeable laminate type photosensitive member (drum) (diameter = 30 mm) with an organic photoconductor (OPC) and a surface layer of polycarbonate. The copying machine used here was modified to effect reversal development and a transfer device having a transfer roller as shown in Figure 2 was incorporated therein.

The transfer roller used here had a surface area made of rubber with a rubber hardness of 27 degrees. Furthermore, with respect to the transfer conditions used here, a transfer current of 1 µA and a contact pressure of 50 g/cm were used.

In the above-mentioned image formation, the photosensitive drum was subjected to primary charging at -700 V, the gap between the photosensitive drum and a developing drum (incorporating a magnet) was set to about 300 micrometers so that the developer layer formed on the developing drum did not contact the photosensitive drum, and an AC bias (frequency = 1800 Hz, Vpp (peak-to-peak voltage) = 1600 V) and a DC bias (VDC = -500 V) were applied to the developing drum. The resulting developed image was fixed to a transfer material by a fixing device having a heating and pressure roller.

The thus obtained fixed toner images were evaluated in the following manner. The results are shown in Table 3 below.

(1) Image density

1000 sheets of ordinary flat paper (75 g/m²) for a copier were passed through the copier, and the image density at the time of copying 1000 sheets was evaluated.

(Excellent): The image density was 1.35 or more.

Δ (good): The image density was 1.0 to 1.34.

× (not good): The image density was below 1.0.

(Transmission state)

A sharper transfer condition was applied by passing thick paper (120 g/m²) or an OHP (overhead projector) transparency through the copier, and the resulting transmission error (or transmission failure) was evaluated.

: The resulting image was good, as shown in Figure 1A or 1C.

Δ: The resulting image was acceptable for practical application.

×: The resulting image was not good, as shown in Figure 1B or 1D.

(3) Paper transport condition

1000 sheets of thin paper (50 g/m²) were passed through the copier and the occurrence of transport errors such as skewed movement was evaluated.

: The number of occurrences of transport errors per pass of 1000 sheets was 1 or less.

Δ: The number of occurrences of transport errors per pass of 1000 sheets was 2 to 4.

×: The number of occurrences of transport errors per pass of 1000 sheets was 5 or more.

(4) Image quality

Scattering of toner particles, coarsening, etc. in the resulting image were evaluated with the naked eye.

: Good.

Δ: Acceptable for practical use.

×: Not acceptable for practical use.

Example 6

100 parts of silica fine powder (Aerosil #200, manufactured by Nihon Aerosil K.K.) was treated with a solution obtained by diluting 20 parts of dimethyl silicone oil (trade name: KK-96) with a solvent. The resulting mixture was dried and subjected to heat treatment at about 280 ºC to obtain treated silica.

Image formation was carried out in the same manner as in Example 5 except that the above-mentioned treated silica was used.

The results are shown in Table 3 below.

Example 7

Image formation was carried out in the same manner as in Example 6 except that silica fine powder having a specific surface area of 130 m²/g was used and 100 parts of the fine powder was treated with 27 parts of silicone oil.

The results are shown in Table 3 below.

Example 8

Image formation was carried out in the same manner as in Example 5 except that a transfer condition of 5 g/cm was applied.

The results are shown in Table 3 below.

Example 9

Image formation was carried out in the same manner as in Example 6 except that α-alumina (average particle size = 0.020 micrometers, specific surface area determined by the BET method = 100 m2/g) was used as the base material for the fine powder.

The results are shown in Table 3 below.

Example 10

Image formation was carried out in the same manner as in Example 5 except that the added amount of the treated fine powder was 2 parts by mass.

The results are shown in Table 3 below.

Example 11

Image formation was carried out in the same manner as in Example 6, except that silica fine powder having a specific Surface area of 300 m²/g and an average particle size of 0.008 micrometers and 4 parts silicone oil are used.

The results are shown in Table 3 below.

Example 12

Image formation was carried out in the same manner as in Example 5 except that a transfer condition of 20 g/cm was applied.

The results are shown in Table 3 below.

Example 13

Image formation was carried out in the same manner as in Example 5 except that the added amount of the treated fine powder was 0.2 parts by mass.

The results are shown in Table 3 below.

Comparison example 5

Image formation was carried out in the same manner as in Example 5 except that untreated silica fine powder was used instead of the treated silica fine powder.

The results are shown in Table 3 below.

Comparison example 6

Image formation was carried out in the same manner as in Example 5 except that a transfer condition of 2 g/cm was applied.

The results are shown in Table 3 below.

Comparison example 7

Image formation was carried out in the same manner as in Example 5 except that the added amount of the treated fine powder was 4 parts by mass.

The results are shown in Table 3 below. Table 3 Transfer condition Image density Paper transport condition Image quality Thick paper OHP film Example Comparative example

Example 14

Styrene-butyl acrylate copolymer 100 parts by weight

(Copolymerization mass ratio = 8:2)

Magnetic material 60 parts by mass

(Magnetite)

Release agent 3 parts by mass

(polypropylene wax)

Nigrosine dye 1 part by mass

(charge control agent)

The above components were subjected to melt-kneading by means of a two-axis extruder heated to 160 ºC, and the kneaded product was cooled and then roughly crushed by means of a hammer mill (mechanical pulverizer) so that the resulting product passed through sieve meshes having an opening diameter of 2 mm, and then finely pulverized by means of a jet mill (wind power pulverizer) so that a particle size of about 10 micrometers was obtained.

The finely pulverized product was classified by a DS classifier (air classifier) so that the classified product had a volume average particle size of 12.0 micrometers (measured by a Coulter counter), thereby obtaining a positively chargeable insulating magnetic toner.

When the insulating magnetic toner thus obtained was mixed with an iron powder carrier and then measured for its triboelectric charge by the blow-off method, it showed a triboelectric charge amount of +11 uC/g.

Separately, 100 parts of silica fine powder (average particle size = 0.016 micrometers) having a specific surface area of 130 m2/g was treated with 20 parts of an amino-modified silicone oil having an amine value of 700 to obtain treated fine powder.

To 100 parts of the insulating magnetic toner, 0.8 parts by mass of the treated silica fine powder was added and mixed by a Henschel mixer to obtain a one-component type developer.

The resulting developer was incorporated into a modification of a commercially available copier (trade name: FC-5, manufactured by Canon KK) with a negatively chargeable photosensitive member (drum) of the laminate type (diameter = 30 mm) with an organic photoconductor (OPC). The copier used here was modified in such a way that a transfer device with a transfer roller as shown in Figure 2 was installed therein.

The transfer roller used here had a surface area made of rubber with a rubber hardness of 27 degrees. Furthermore, with respect to the transfer conditions used here, a transfer current of 1 µA, a transfer voltage of -2000 V and a contact pressure of 50 g/cm were used.

In the above-mentioned image formation, the photosensitive drum was subjected to primary charging at -700 V, the gap between the photosensitive drum and a developing drum (in which a magnet was incorporated) was set so that the developer layer formed on the developing drum did not contact the photosensitive drum, and an AC bias (frequency = 1800 Hz, Vpp value = 1600 V) and a DC bias (VDC = -300 V) were applied to the developing drum by a normal developing process. The resulting developed image was fixed to a transfer material by a fixing device having a heating and pressure roller.

The thus obtained fixed toner images were evaluated in the same manner as described above. The results are shown in Table 4 below.

Example 15

Image formation was carried out in the same manner as in Example 14 except that 100 parts of the fine powder was treated with 35 parts of the amino-modified silicone oil.

The results are shown in Table 4 below.

Example 16

Image formation was carried out in the same manner as in Example 14 except that a transfer condition of 5 g/cm was applied.

The results are shown in Table 4 below.

Example 17

Image formation was carried out in the same manner as in Example 14 except that α-alumina fine powder (average particle size = 0.020 micrometers, BET specific surface area = 100 m2/g) was used as the base material for the fine powder.

The results are shown in Table 4 below.

Example 18

Image formation was carried out in the same manner as in Example 14 except that the added amount of the treated fine powder was 2 parts by mass.

The results are shown in Table 4 below.

Example 19

Image formation was carried out in the same manner as in Example 14 except that silica fine powder (average particle size = 0.008 micrometers) having a specific surface area of 300 m2/g was used and 100 parts of the fine powder was treated with 5 parts of the amino-modified silicone oil.

The results are shown in Table 4 below.

Example 20

Image formation was carried out in the same manner as in Example 14 except that a transfer condition of 20 g/cm was applied.

The results are shown in Table 4 below.

Example 21

Image formation was carried out in the same manner as in Example 14 except that the added amount of the treated fine powder was 0.2 parts by mass.

The results are shown in Table 4 below.

Comparison example 8

Image formation was carried out in the same manner as in Example 14 except that silica fine powder treated with aminopropyltriethoxysilane was used.

The results are shown in Table 4 below.

Comparison example 9

Image formation was carried out in the same manner as in Example 14 except that a transfer condition of 2 g/cm was applied.

The results are shown in Table 4 below.

Comparison example 10

Image formation was carried out in the same manner as in Example 14 except that the added amount of the treated fine powder was 4 parts by mass.

The results are shown in Table 4 below. Table 4 Image density Transfer condition Paper transport condition Image quality Example Comparison example

An image forming method includes the steps of: developing an electrostatic image formed on an image holding member for holding an electrostatic image with a developer to form a developed image thereon, the developer comprising 100 parts by weight of a toner and 0.05 to 3 parts by weight of fine powder treated with 1 to 35 parts by weight of a silicone oil or silicone varnish per 100 parts by weight of fine powder, and transferring the developed image on the image holding member for holding an electrostatic image to a transfer material while causing a transfer means to contact the image holding member for holding an electrostatic image via the transfer material under a line pressure of 3 g/cm or higher.

Claims (40)

1. Image forming process in which an electrostatic image formed on an image holding member (1) for holding an electrostatic image is developed with a developer to form a developed image thereon, and the developed image located on the image holding member (1) for holding an electrostatic image is transferred to a transfer material while causing a transfer device (2, 9, 10) to contact the image holding member (1) for holding an electrostatic image via the transfer material under a line pressure of 3 g/cm or higher, characterized in that said developer comprises 100 parts by weight of a toner and 0.05 to 3 parts by weight of fine powder which has been treated with 1 to 35 parts by weight of a silicone oil or silicone varnish per 100 parts by weight of fine powder. 2. A method according to claim 1, wherein the developer comprises an insulating magnetic toner and silica fine powder treated with a silicone oil or silicone varnish. 3. A method according to claim 1, wherein the developer is carried on a developing sleeve and is triboelectrically charged by its contact with the developing sleeve. 4. Method according to claim 1, wherein the transfer device comprises a transfer roller (2) or a transfer belt (9). 5. Method according to claim 4, wherein the transfer device comprises a transfer roller (2) which comprises a metal core (2a) and an electrically conductive, elastic layer (2b) arranged thereon. 6. The method according to claim 5, wherein the electrically conductive, elastic layer (2b) of the transfer roller (2) has a specific volume resistivity of 10⁶ to 10⁸ Ω cm. 7. A method according to claim 1, wherein the developed image is electrostatically transferred to the transfer material while causing the transfer means (2, 9, 10) to contact with the image holding member (1) for holding an electrostatic image under a line pressure of 20 g/cm or higher. 8. A method according to claim 1, wherein the developed image is electrostatically transferred to the transfer material by the transfer device (2, 9, 10) to which a bias voltage having a transfer current of 0.1 to 50 uA and a transfer voltage of 500 to 4000 V (absolute value) is applied. 9. The method according to claim 1, wherein 100 parts by weight of the fine powder have been treated with 2 to 30 parts by weight of the silicone oil. 10. The method of claim 1, wherein the silicone oil comprises one having a partial structure wherein R denotes an alkyl group having 1 to 3 carbon atoms and m denotes an integer. 11. The method of claim 1, wherein the silicone oil comprises an amino-modified silicone oil. 12. The method according to claim 1, wherein the fine powder treated with silicone oil or silicone varnish comprises one obtained by treating an inorganic oxide having a particle size of 0.001 to 2 micrometers with the silicone oil or silicone varnish. 13. A process according to claim 12, wherein the silicone oil has a viscosity of 50 to 1000 centistokes at 25ºC. 14. The method according to claim 1, wherein the toner comprises an insulating magnetic toner and the fine powder comprises hydrophobic silica fine powder treated with the silicone oil or silicone varnish. 15. The method according to claim 14, wherein the hydrophobic silica fine powder has been treated with a silane coupling agent and the silicone oil. 16. The method according to claim 14, wherein the hydrophobic silica fine powder is used in an amount of 0.1 to 1.6 parts by mass per 100 parts by mass of the insulating magnetic toner. 17. The method according to claim 2, wherein the insulating magnetic toner has a remanence r of 1 to 5 emE/g, a saturation magnetization s of 15 to 50 emE/g and a coercive force of 20 to 100 Oe. 18. The method of claim 1, wherein the toner comprises an insulating magnetic toner, and the insulating magnetic toner contains 17 to 60% (by number) of magnetic toner particles having a particle size of 5 micrometers or less, 5 to 50% (by number) of magnetic toner particles having a particle size of 6.35 to 10.08 micrometers, and 2.0% or less by volume of magnetic toner particles having a particle size of 12.7 micrometers or more; the magnetic toner has a mean particle size (volume average) of 6 to 8 micrometers, and the magnetic toner particles having a particle size of 5 micrometers or less have a particle size distribution satisfying the following formula: N/V = -0.05 N + k, where N is the percentage (in number) of magnetic toner particles with a particle size of 5 micrometers or less, V denotes the volume percentage of magnetic toner particles having a particle size of 5 micrometers or less, k denotes a positive number from 4.6 to 6.7, and N denotes a positive number from 17 to 60. 19. The method of claim 1, wherein the toner has a volume average particle size of 5 to 13 micrometers. 20. The method according to claim 1 or 19, wherein the toner is a magnetic toner containing 17 to 60% (by number) of magnetic toner particles having a particle size of 5 micrometers or less. 21. The method of claim 1, 19 or 20, wherein the toner is a magnetic toner containing 5 to 50% (by number) of magnetic toner particles having a size of 6.35 to 10.08 micrometers. 22. Use of a developer (710) comprising 100 parts by weight of a toner and 0.05 to 3 parts by weight of fine powder which has been treated with 1 to 35 parts by weight of a silicone oil or silicone varnish per 100 parts by weight of fine powder, in an image forming apparatus which an image holding part (1) for holding an electrostatic image; a device (704, 709) for developing the electrostatic image, which comprises a toner carrier member, said developer (710) being carried on the toner carrier member; and a transfer device (2, 9, 10) for transferring a developed image developed with the developer (710) from the image holding part (1) for holding an electrostatic image to a transfer material (P) while causing the transfer device (2, 9, 10) to contact the image holding part (1) via the transfer material (P) for Holding an electrostatic image under a line pressure of 3 g/cm or higher, includes. 23. Use according to claim 22, wherein the developer comprises an insulating magnetic toner and silica fine powder treated with a silicone oil or silicone varnish. 24. Use according to claim 22, wherein the transfer device comprises a transfer roller (2) or a transfer belt (9). 25. Use according to claim 24, wherein the transfer device comprises a transfer roller (2) which comprises a metal core (2a) and an electrically conductive, elastic layer (2b) arranged thereon. 26. Use according to claim 25, wherein the electrically conductive elastic layer (2b) of the transfer roller (2) has a specific volume resistivity of 10⁶ to 10⁸ Ω cm. 27. Use according to claim 22, wherein the transfer means (2, 9, 10) is caused to contact the image holding member (1) for holding an electrostatic image under a line pressure of 20 g/cm or higher. 28. Use according to claim 22, wherein the image holding member (1) for holding an electrostatic image comprises a photosensitive drum with an organic photoconductor (OPC). 29. Use according to claim 28, wherein the image holding member (1) for holding an electrostatic image comprises a laminate type organic photoconductor (OPC) drum having a diameter of 50 mm or less. 30. Use according to claim 22, wherein 100 parts by weight of the fine powder have been treated with 2 to 30 parts by weight of the silicone oil. 31. Use according to claim 22, wherein the silicone oil comprises one having a partial structure wherein R denotes an alkyl group having 1 to 3 carbon atoms and m denotes an integer. 32. Use according to claim 22, wherein the silicone oil comprises an amino-modified silicone oil. 33. Use according to claim 22, wherein the fine powder treated with silicone oil or silicone varnish comprises one obtained by treating an inorganic oxide having a particle size of 0.001 to 2 micrometers with the silicone oil or silicone varnish. 34. Use according to claim 33, wherein the silicone oil has a viscosity of 50 to 1000 centistokes at 25ºC. 35. Use according to claim 22, wherein the toner comprises an insulating magnetic toner and the fine powder comprises hydrophobic silica fine powder treated with the silicone oil or silicone varnish. 36. Use according to claim 35, wherein the hydrophobic silica fine powder has been treated with a silane coupling agent and the silicone oil. 37. The use according to claim 35, wherein the hydrophobic silica fine powder is used in an amount of 0.1 to 1.6 parts by mass per 100 parts by mass of the insulating magnetic toner. 38. Use according to claim 35, wherein the insulating magnetic toner has a remanence r of 1 to 5 emE/g, a saturation magnetization s of 15 to 50 emE/g and a coercive force of 20 to 100 Oe. 39. Use according to claim 22, wherein the toner comprises an insulating magnetic toner, and the insulating magnetic toner contains 17 to 60% (by number) of magnetic toner particles having a particle size of 5 micrometers or less, 5 to 50% (by number) of magnetic toner particles having a particle size of 6.35 to 10.08 micrometers, and 2.0% or less by volume of magnetic toner particles having a particle size of 12.7 micrometers or more; wherein the magnetic toner has a mean particle size (volume average) of 6 to 8 micrometers, and the magnetic toner particles having a particle size of 5 micrometers or less have a particle size distribution satisfying the following formula: N/V = -0.05 N + k, where N denotes the percentage (by number) of magnetic toner particles having a particle size of 5 micrometers or less, V denotes the percentage by volume of magnetic toner particles having a particle size of 5 micrometers or less, k denotes a positive number from 4.6 to 6.7, and N denotes a positive number from 17 to 60. 40. Use of a developer (710) comprising 100 parts by weight of a toner and 0.05 to 3 parts by weight of fine powder which has been treated with 1 to 35 parts by weight of a silicone oil or silicone varnish per 100 parts by weight of fine powder, in a facsimile machine comprising an image forming device and a receiving device for receiving image data from a remote data station; said image forming device an image holding part (1) for holding an electrostatic image; a device (704, 709) for developing the electrostatic image, which comprises a toner carrier member, said developer (710) being carried on the toner carrier member; and a transfer means for transferring a developed image developed with the developer from the image holding member for holding an electrostatic image to a transfer material while causing the transfer means to contact the image holding member for holding an electrostatic image via the transfer material under a line pressure of 3 g/cm or higher, includes.