JPH06236077A - Magnetic developer - Google Patents

Abstract

PURPOSE:To provide a binary system magnetic developer capable of simultaneously attaining excellent image density and resolution by an image forming method such as a cleanerless system. CONSTITUTION:A ferrite carrier of small particle diameter having 30mum average diameter and a ferrite carrier of large particle diameter having 70mum average particle diameter are prepared and a magnetic carrier is formed by mixing these ferrites in the weight ratio of 30:70 to 70:30%. The ferrite carrier mainly contains BaO, ZnO, NiO and Fe2O3, which are dry-mixed by a ball-mill. The mixed powder is calcined and the calcined powder is pulverized. A caking agent is added into the pulverized powder to granulate and the granule is graded after sintered in air.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a two-component magnetic developer used for developing an electrostatic latent image in an image forming method such as a cleanerless system, and more particularly to improve image density and resolution. is there.

[0002]

2. Description of the Related Art In the electrophotographic image forming method currently used in copying machines, printers and the like, an electrostatic latent image is generally formed on the surface of a photoconductor which is initially charged by light image exposure.
Toner is attached to the formed electrostatic latent image to develop the latent image, and the toner image formed by the development is transferred onto a sheet, fixed, and finally copied. To develop this electrostatic latent image,
The magnetic brush development method is mainly used. Developers used in this magnetic brush development method include one-component developers and two-component developers. Among these developers, the usual method of transferring a developed toner image on a sheet is composed of a one-component developer composed of a charged magnetic toner or a magnetic carrier and an insulating non-magnetic toner. Two-component developers are widely used.

However, in a one-component developer composed of a charge-type magnetic toner, charge aggregation of magnetic toners easily occurs, and in a two-component developer composed of a magnetic carrier and an insulating non-magnetic toner, an image quality is improved. There is a problem that the halftone reproducibility and the like are poor, and a two-component magnetic developer composed of a magnetic carrier and a magnetic toner is now used.

[0004]

A magnetic carrier conventionally used in a two-component magnetic developer used in an image forming method such as a cleanerless system can obtain a sufficient image density when its average particle size becomes large. However, it has a defect that the image quality is rough. On the contrary, when the average particle diameter of the magnetic carrier becomes small, the image quality becomes fine, but there is a drawback that the image density cannot be sufficiently obtained. Therefore, in the past, the above-mentioned effects on the image quality of the large particle size and the small particle size, which are competing with each other, have been adopted. I was trying to solve it. Therefore, the defects of large particle size and small particle size do not appear on the image quality, but on the other hand, the obtained image quality also falls within a certain range depending on the average particle size of the set magnetic carrier. However, both the image density and the resolution could not be further improved.

Therefore, an object of the present invention is to provide a two-component magnetic developer capable of simultaneously obtaining excellent image density and resolution in an image forming method such as a cleanerless system.

[0006]

In order to achieve the above object, in a two-component magnetic developer comprising a toner and a carrier, a magnetic carrier having a small particle diameter of 10 to 50 μm and an average particle diameter of 10 to 50 μm are used. It is a mixture with a magnetic carrier having a large particle diameter of 50 to 150 μm. Further, the magnetic carrier having a small particle diameter and the magnetic carrier having a large particle diameter are mixed in a ratio of 10:90 to 90: 1.
Mix at a weight ratio of 0. Small particle average particle size 10-50
The reason for setting μm is that when a magnetic carrier having this particle size range is used for actual image formation, the characteristics as a magnetic carrier having a small particle size are sufficiently exhibited in terms of image quality. For the same reason, the average particle size range of the large particle size magnetic carrier is 50 to 15
It was set as 0 μm.

The mixing ratio of carriers having large and small particle sizes is set within the range of 10:90 to 90:10 by weight because one of the large and small particle size carriers has 10% by weight. If it is less than 100% (the other is more than 90% by weight), the defect of the other carrier having a large composition amount greatly affects the image quality. Further, preferably, the mixing ratio of the two types of carriers having an average particle size of large and small is 30:70.
Within the range of the weight ratio of 70:30. As the magnetic carrier, for example, magnetic powder such as ferrite, magnetite, iron powder or the like can be used as it is, or the magnetic powder may be dispersed in resin. Alternatively, a magnetic powder whose surface is coated with a resin may be used.

The magnetic toner may be prepared by a pulverizing method, a spray drying method or the like by mixing a binder resin and magnetic powder, and if necessary, a charge control agent and the like. Further, for example, a fluidizing agent such as silica fine powder, a resistance adjusting agent such as carbon black, a releasing agent such as low molecular weight polyolefin, and the like may be added to the inside and / or the surface of the toner particles. As the magnetic powder, ferrite, iron powder, magnetite or the like can be used, but the content of the magnetic powder in the toner is 1
0 to 70% by weight is preferred. When the content of the magnetic powder is less than 10% by weight, the magnetic force of the toner decreases, and when it is more than 70% by weight, the volume resistivity decreases. A more preferable range is 2
It is in the range of 0 to 60% by weight. On the other hand, the mixing ratio of the magnetic carrier and the magnetic toner is such that the magnetic toner content is 20 to 8
It is preferably in the range of 0% by weight.

[0009]

In the magnetic developer of the present invention, a magnetic carrier having a large particle diameter and a magnetic carrier having a small particle diameter are mixed, so that the defects of the respective image quality can be compensated for each other. That is, in the present invention, since the small particle size magnetic carrier having good resolution characteristics is mixed, the poor resolution, which is a defect in image quality of the large particle size magnetic carrier, can be eliminated. On the other hand, in the present invention, since the large-diameter magnetic carrier having a good image density characteristic is mixed, the low image density which is a defect in the image quality of the small-diameter magnetic carrier can be solved.
As described above, by using the magnetic developer according to the present invention, a sufficient image density characteristic of a carrier having a large particle diameter can be obtained, and at the same time, a fine image quality characteristic of a carrier having a small particle diameter can be obtained. be able to.

[0010]

EXAMPLES The toner used in the magnetic developer according to the present invention is a magnetic toner using magnetite. The magnetic toner is a styrene-n-butyl methacrylate copolymer (weight average molecular weight: 210000, number average molecular weight: 16
000) 54% by weight, magnetite (manufactured by Toda Kogyo Co., E
40% by weight of PT500, 5% by weight of polypropylene (manufactured by Sanyo Kasei Co., Ltd., Viscole 550P), and 1% by weight of a negatively charged Cr-containing azo dye (manufactured by Orient Chemical Co., Ltd., Bontron E81) were mixed and dry mixed. Then, the mixture was heated and mixed, cooled and solidified, and then pulverized and classified. Further, 0.5 wt% of hydrophobic silica (Aerosil R972 manufactured by Nippon Aerosil Co., Ltd.) was added to 100 wt% of the magnetic toner to obtain an average particle diameter of 10 μm, a volume resistivity of 10 ¹⁴ Ω · cm, and a triboelectric charge amount of −. A magnetic toner of 15 μc / g was prepared.

The magnetic carrier used in the present invention is a mixture of two types of carriers having the same composition but different particle sizes. Taking a ferrite carrier as an example, the molar ratio is BaO: 15%, ZnO: 15%, NiO: 10.
%, Fe ₂ O ₃ : 60% and weighed and dry mixed with a ball mill. The obtained mixed powder was calcined at 900 ° C. for 2 hours, and the calcined powder was pulverized with a ball mill to a particle size of 1 μm or less. Further, a binder is added to the pulverized powder, the mixture is granulated by a spray drying method, sintered in air at 1300 ° C. for 2 hours, and further heat-treated at 800 ° C. in a nitrogen atmosphere having an oxygen concentration of 10% for about 3 minutes. Two types of ferrite carriers having different average particle diameters of 30 μm and 70 μm were prepared.
The volume resistivity of these two types of ferrite carriers is 10
It was ⁹ Ω · cm.

The average particle size of the magnetic toner was measured by a particle size analyzer (manufactured by Coulter Electronics Co., Coulter Counter Model TA-II). The triboelectric charge amount of the magnetic toner was measured at a toner concentration of 5 wt% with a blow-off triboelectric charge amount measuring device (TB-200 type, manufactured by Toshiba Chemical Co., Ltd.) as a reference. The volume resistivity of the magnetic toner is obtained by filling a dozen or so mg of a sample in a Teflon (trade name) cylinder having an inner diameter of 3.05 mm,
It was measured under an electric field of DC 4 KV / cm under a load of 0.1 kg. Similarly, the volume resistivity of the magnetic carrier also has an inner diameter of 3.
Fill a dozen or more mg of a sample into a cylinder made of Teflon (trade name) of 05 mm, and load DC of 200 V / c under 0.1 kg.
It was measured in an electric field of m.

The small-diameter ferrite carrier (A) having an average particle diameter of 30 μm and the large-diameter ferrite carrier (B) having an average particle diameter of 70 μm obtained by the above method are mixed at various ratios and mixed. Several magnetic carriers with different ratios were prepared. A two-component developer was prepared by mixing these several types of magnetic carriers and the magnetic toner so that the toner concentration was 30% by weight. In this embodiment, this developer was used for an electrophotographic image forming method using corona discharge.

The image forming condition is that the peripheral speed of the OPC drum (negative charging property) is 60 mm / sec, the developing sleeve is made of SUS304 having a diameter of 20 mm, the built-in magnet (fixed) is 8 poles, the rotation speed of the sleeve is 200 rpm, and the sleeve is The magnetic force above is 7
50G, bias voltage applied to the sleeve is -450
v, the developing gap was 0.35 mm, the doctor gap was 0.25 mm, and after corona transfer, heat roll fixing was performed at a heating roll surface temperature of 190 ° C. and a roll-to-roll linear pressure of 1 Kg / cm. In this embodiment, the method of fixing the magnet in the magnet roll and rotating only the sleeve is used as the developer conveying method, but a method of rotating both the magnet and the sleeve may be used.

An image was actually formed under the above conditions, and the effect of the present invention was confirmed from the obtained image quality. The results are shown in Table 1.

[Table 1]

From Table 1, when two kinds of ferrite carriers A and B are mixed, the ferrite carrier A having a small particle size is 1
It can be seen that the quality of the image density when it is 00% by weight (without B) is improved as the mixing ratio of the ferrite carrier B having a large particle diameter is increased. Similarly, it can be seen that the resolution in the case of only the ferrite carrier B having a large grain size is improved by the increase of the ferrite carrier A having a small grain size. Fog did not occur in any case. Therefore, it can be seen from the results of the image density and the resolution in Table 1 that the optimum range of the mixing ratio of the large and small ferrite carriers is within the range of 30 to 70% by weight.

[0017]

INDUSTRIAL APPLICABILITY The magnetic developer according to the present invention contains a magnetic carrier obtained by mixing two types of carriers having different average particle sizes. Therefore, a magnetic developer obtained by combining this magnetic carrier with a magnetic toner is a two-component developer excellent in both image quality density and resolution, which are advantages of the ferrite carrier having a large particle size or a small particle size. Is. Further, in a conventional two-component developer having an average particle size of one kind of carrier as a component, it is necessary to set the average particle size of the magnetic carrier within a range in which the defects of the large particle size and the small particle size are not noticeable. Therefore, the image density and resolution have to be compromised within a certain level of quality. However, according to the present invention, this point has been solved, and it has become possible to dramatically improve the image density and the resolution that conflict with each other.

Claims (2)

[Claims] 1. A two-component magnetic developer comprising a toner and a carrier, wherein a magnetic carrier having a small particle size having an average particle size of 10 to 50 μm and a magnetic carrier having a large particle size having an average particle size of 50 to 150 μm. A magnetic developer characterized by being a mixture with. 2. The magnetic developer according to claim 1, wherein the magnetic carrier having a small particle size and the magnetic carrier having a large particle size are mixed in a weight ratio of 10:90 to 90:10. .