JPH08334932A - Two-component developer - Google Patents

Abstract

PURPOSE: To obtain a two-component developer contg. no harmful substance, using a ferrite carrier moderately imparting electric charges to a negative charge type polyester-contg. toner and a positive charge type styrene-acrylic resin-contg. toner without coating and having satisfactory developing characteristics. CONSTITUTION: This two-component. developer consists of a ferrite carrier of <=20μm grain diameter and a negative charge type toner contg. polyester resin as the principal component, of the bonding resin or a positive charge type toner contg. styrene-acrylic copolymer resin as the principal component of the bonding resin. The ferrite carrier is made of multiple ferrite obtd. by substituting Mn for a part of Me or Fe in cubic single ferrite represented by Me0.5 Fe2.5 O4 (Me is a monovalent. metal) and has a compsn. consisting, e.g., of 4-30mol% Li2 O, >0 to 50mol% MnO and 50-95mol% Fe2 O3 .

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a dry two-component developer used for developing an electrostatic latent image in electrophotography, electrostatic recording or electrostatic printing.

[0002]

2. Description of the Related Art In electrophotography, after a uniform electrostatic charge is applied to a photoreceptor having a photosensitive layer made of a photoconductive material,
An electrostatic latent image is formed on the surface of the photoconductor by imagewise exposure, and charged fine powder, that is, toner is attached (developed) to form a visible image. The visible image is transferred onto a transfer material such as plain paper and then permanently fixed by heating or pressing. As a developing method applied to these electrophotographic methods, there are a method using a one-component developer containing only a toner and a method using a two-component developer containing a toner and a magnetic carrier. In the method using the two-component developer, the magnetic carrier and the toner are mixed in a predetermined ratio,
Both of them are triboelectrically charged, and only the toner charged to a predetermined polarity can be supplied to the electrostatic latent image formed on the surface of the photoconductor by each carrier that connects the magnet roller and the photoconductor in a cross-linking manner. Let it be a visual image.

A two-component developer is generally required to have stability and maintainability of image quality in continuous development. For that purpose, it is necessary to maintain the charge imparting ability of the carrier to the toner and the developer resistance within appropriate ranges, and it is desirable that the initial charge imparting ability and the developer resistance do not change even after long-term use. As the toner, a fine powder obtained by mixing and pulverizing a binder resin with a colorant such as a dye or a pigment, a magnetic powder, a charge control agent, and various functional additives such as wax is pulverized. Iron powder or ferrite powder is often used for the carrier. In order to maintain the charge imparting ability and resistance of the developer in a stable manner, conventionally, the design and selection of the binder resin, the charge control agent, and other additives in the toner have been studied, and the iron powder oxidation treatment method in the carrier. , The constituent material of ferrite, the surface shape of these magnetic powders, the coating material and the treatment method thereof have been variously studied.

[0004]

The iron powder carrier is used after its surface is appropriately treated, but when used for a long time, the surface of the particle physically or chemically changes and toner adheres (toner spent). There is a drawback that the life is short because the charge imparting ability is reduced, or the humidity of the use environment makes the image sharp and the image sharpness deteriorates. This drawback can be improved by coating with a resin, but it causes an increase in cost. Further, when the coating film is peeled off, there is a disadvantage that the charge imparting ability and the developer resistance change.

Ferrite carrier is practically used because it is chemically more stable than iron powder carrier, has less resistance change during use, and has an apparent density of about 2/3 of iron powder. Was advanced. The ferrite carrier is composed of a perfect mixture of suitable metal oxides and iron oxides, and contains Ni, Zn, Mn, Mg, Cu, Li, B.
It is a sintered body of an oxide such as a, V, Cr, or Ca and a trivalent iron oxide. Various compositions of ferrite carrier are known, but Ni-Zn ferrite and Mn-Z are known.
N-type ferrite and Cu-Zn-type ferrite are generally used. However, since Zn, Ba, etc. contained in these are harmful to the human body, there is a problem in practical use such as legal regulation when these ferrite carriers are treated as waste.

Further, with respect to the charge imparting property to the toner, which is another purpose of the ferrite carrier, the ferrite carrier and the iron powder are largely influenced by the level of the work function of the substance. Charging properties are also determined by the composition of the ferrite carrier.
Ni-Zn system and Mn- which have been generally used until now.
When a Zn-based or Cu-Zn-based ferrite carrier is combined with a polyester-based toner having a strong negative charging property, the charge amount becomes too large, so that a sufficient image density cannot be obtained or charge-up occurs. there were. On the contrary, such a carrier having a strong negative charging property has a weak charge imparting ability to the positive charging toner, and a sufficient charge cannot be obtained even in the case of the combination with the styrene-acrylic toner which is relatively easily positively charged. Problems such as image fogging occurred.

Therefore, an object of the present invention is to use a ferrite carrier which does not contain harmful substances and gives a proper charge to negatively chargeable polyester type toners and positively chargeable styrene-acrylic type toners without coating, and to obtain good developing characteristics. A two-component developer having

[0008]

The first invention for achieving the above object is to provide a cubic unitary ferrite of Me _0.5 Fe _2.5 O ₄ (Me is a monovalent metal) or Me or A composite ferrite in which a part of Fe is replaced with Mn, and having a molar ratio of Li ₂ O 2 to 15%, MnO 50% or less (not including 0%), and Fe ₂ O ₃ 50 to 95%. And a negatively-chargeable toner containing a polyester resin as a main component of a binder resin, and a ferrite carrier having an average crystal grain size of 20 μm or less.
A second invention is characterized by comprising the ferrite carrier used in the first invention and a toner using a styrene-acrylic copolymer resin as a main component of a binder resin.

The inventors of the present invention have investigated and examined conventional electrophotographic developers, and as a result, as to adjusting the charge imparting ability of the carrier, which affects the image quality, only by changing the electric resistance of the carrier, it is possible to copy each of them. It was found that the effect may not be exerted at all due to differences in the environment and mechanism of machines and printers, and subsequently various studies were conducted on the relationship between the material composition of the carrier and the charge imparting ability and the relationship between them and the image density. As a result, the present invention has been completed based on the finding of a composition region of a ferrite carrier in which a desired image density can be easily obtained in combination with a change in saturation magnetization value.

In the present invention, each of the Li ₂ O, MnO and Fe ₂ O ₃ components is harmless to the human body and is a basic component for satisfying the characteristics as a ferrite carrier.
The content of Li ₂ O is 2 to 15% (preferably 5 to 13%) and MnO 50% or less (0% is not included) in molar ratio.
Preferably 10 to 30%), Fe ₂ O ₃ 50 to 95%
The range may be (preferably 60 to 90%). To make the crystal structure of the ferrite carrier finer, As, V,
Metal compounds such as Bi, Sb, B, Si and Ca are added in an amount of 0.1 to 0.1
Add 1.5% by weight. Thereby, the sintering density is improved and abnormal crystal grain growth is suppressed, and the average crystal grain size is 20 μm.
The following ferrite carrier is obtained. By mixing this ferrite carrier and a toner using a polyester resin or a styrene-acrylic resin as the main component of the binder resin in a predetermined ratio, a two-component developer having the desired charging characteristics can be obtained. MnO contributes to the improvement of the saturation magnetization value when it forms a solid solution in the composition of Me _0.5 Fe _2.5 O ₄ , and the saturation magnetization value is M
The more nO, the better. When the saturation magnetization value is high, a high magnetic brush is formed on the magnet roll which is the developer transporting means in the magnetic brush method which is a normal developing method. On the other hand, the unit system Li _0.5 F containing no MnO
With e _2.5 O ₄ , the saturation magnetization value itself decreases due to the theory of ferrimagnetism, so that a low magnetic brush is formed on the magnet roll.

In the present invention, the molar ratio of Li ₂ O 2
15%, MnO 50% or less (not including 0%), Fe ₂ O
₃ Add 50 to 95% of the basic composition of V ₂ O ₅ and Bi ₂ O _{3 having} a low melting point, or add 0.1 to 1.5% by weight of a metal salt that can be a metal oxide. May be.
In the present invention, as the additive, for example, As ₂ O ₃ ,
V ₂ O ₅ , Bi ₂ O ₃ , Sb ₂ O ₃ , B ₂ O ₃ , SiO ₂ , Ca
A metal oxide such as O is applied, but these may be a metal compound that can become a metal oxide by heating.
In order to improve the sintering density and suppress abnormal crystal grain growth according to the present invention, the amount of the metal compound added is preferably 0.5% or less in weight, but abnormal crystal grain growth occurs. Even if the addition is 0.5% or more, the weight is 1.
Up to about 5% can be put into practical use without coating the particle surface with a resin or the like. The addition amount of the metal compound is most preferably used within the range of 0.1 to 0.5%.

In the present invention, the carrier particles preferably have an average particle size in the range of 20 to 150 μm, and when the particle size is less than 20 μm, except for special applications such as electrostatic recording. On the other hand, the carrier tends to adhere to the surface of the photoconductor, while when the particle size exceeds 150 μm, the image itself becomes rough, which is not preferable. More preferably, it is in the range of 30 to 100 μm. In the present invention,
The ferrite carrier has a saturation magnetization value of 40 emu /
It is preferably g or more. When the saturation magnetization value is smaller than 40 emu / g, the attracting force to the magnet roll is lowered, so that carrier adhesion is likely to occur and defects such as white spots occur on the image, which is inconvenient. Furthermore, σ ₁₀₀₀
(Value of magnetization measured in a magnetic field of 1000 Oe) is 40 em
It is preferably u / g or more. In the present invention, the ferrite carrier has a volume specific electric resistance value of 10 or less.
It is desirable to be in the range of ³ to 10 ¹³ Ω · cm. This is because when the resistance value is less than 10 ³ Ω · cm, the carrier easily separates from the magnetic brush and causes adhesion to the surface of the photoconductor. On the other hand, when the resistance value exceeds 10 ¹³ Ω · cm, the edge effect is strengthened and solid black development is performed. This is because the concentration becomes non-uniform. The more preferable range of the resistance value is 10 ⁵ to 10 ¹⁰ Ω · cm. In the present invention, when the coercive force of the ferrite carrier exceeds 50 Oe, the particles themselves have the property of a permanent magnet and strongly adhere to a magnet roll or the like to deteriorate the fluidity of the toner that directly contributes to the development. , The image coercive force becomes 50 Oe because a good image cannot be obtained due to image unevenness.
The following is desirable. The ferrite carrier needs wear resistance because it leads to a long life. The fracture strength of the ferrite particles is preferably 5000 g / cm ² or more.

The ferrite carrier of the present invention can be manufactured, for example, by the following method. At first,
A predetermined metal oxide, iron oxide (Fe ₂ O ₃ ) and a predetermined amount of a metal compound as an additive are weighed and mixed. Next, the obtained mixture is calcined in the range of 800 to 1000 ° C. for several hours, and then pulverized to a particle size of several μm or less. The crushed powder obtained is
If necessary, a binder is added, and then spray-dried in a heating atmosphere for granulation. The obtained spherical particles are 1100 to 13
The ferrite carrier is obtained by sintering at a temperature of 00 ° C. and then crushing and classifying.

The developer of the present invention is prepared by mixing the above carrier with a negatively chargeable toner or a positively chargeable toner.
It is desirable to mix the toner so that the toner concentration is in the range of 2 to 20% by weight. The toner used as the developer of the present invention comprises a binder resin and a colorant as essential components, a charge control agent,
It contains a magnetic material, a release agent, a fluidizing agent and the like as optional components.

First, the negatively chargeable toner will be described. The negatively chargeable toner is applied to a regular development method using a positively charged selenium photoreceptor or a reversal development method using a negatively charged zinc oxide or an organic photoreceptor. The copying speed of electrophotographic copying machines and the printing speed of laser beam printers have improved, and the demand for high-speed fixing has increased, and toner is strongly required to have high-speed fixing properties. Further, from the viewpoint of energy saving, a heat roller fixing device which consumes less power is used. To that end, a toner that fixes at a lower temperature is required. As the polyester resin, a resin having a low softening temperature is easily obtained, and for this reason, the polyester resin is used as the binder resin when the toner is a negatively chargeable toner. The polyester resin includes linear polyester resin and non-linear polyester resin, and the non-linear polyester resin having more excellent high-speed fixing property is desirable.

The non-linear polyester resin is formed from one or more polyols and one or more polycarboxylic acids. However, at least one of polyol and polycarboxylic acid has a valence of 3 or more and forms a non-linear polymer skeleton by a polycondensation reaction. Here, "non-linear" means a state in which the polymer skeleton has a branch due to the presence of a trivalent or higher valent polyol and / or a polycarboxylic acid. The polyol includes a divalent diol and a trivalent or higher polyol, and the polycarboxylic acid includes a divalent dicarboxylic acid and a trivalent or higher polycarboxylic acid. Further, aromatic bisphenol A type polyester resin is desirable.

As the colorant, carbon black, aniline blue, chalco oil blue, chrome yellow, ultramarine blue, DuPont oil red, quinoline yellow, methylene blue chloride, phthalocyanine blue, malachite green oxalate, lamp black, rose bengal, etc. Can be mentioned. Usually, 5 to 15% by weight is added to the total amount of toner. Examples of the negative charge control agent include metal-containing azo dyes, copper phthalocyanine dyes, and alkyl derivative complexes of salicylic acid. The addition amount is 1 to 10% by weight based on the total amount of the toner.
The range is appropriate.

When the magnetic toner is used, it contains a magnetic material. As the magnetic body, alloys or compounds containing ferromagnetism elements such as ferrite and magnetite, iron, cobalt, nickel and the like, and various alloys exhibiting ferromagnetism by some treatment such as heat treatment are used. . Specific examples include magnetite, cobalt-containing magnetite, spinel type ferrite, and magnet brambite type ferrite. These magnetic materials are dispersed in the binder resin as fine powder having an average particle size of 0.3 to 1 μm, and the content thereof is 4 with respect to the total amount of toner.
A range of 0 to 60% by weight is suitable.

The release agent is used to prevent the offset phenomenon. The offset phenomenon is a phenomenon in which a part of the toner adheres to the surface of the heat roll in the heat roll fixing method and adheres to the paper again to stain the image. Examples of the release agent include polyolefin, fatty acid metal salt, fatty acid ester, partially saponified fatty acid ester, higher fatty acid, higher alcohol, paraffin wax, polyhydric alcohol ester and the like. Particularly, low molecular weight polypropylene having a weight average molecular weight of 50,000 or less, or polyethylene wax is effective. The content of the release agent is appropriately in the range of 0.5 to 30% by weight, preferably 1 to 10% by weight.

As the fluidizing agent, silica, alumina, titanium oxide, barium titanate, magnesium titanate,
A small amount of fine powder of calcium titanate, strontium titanate, zinc oxide, silica sand, clay, mica, diatomaceous earth, etc. is added to the surface of the toner particles. The amount of addition is suitably in the range of 0.05 to 3% by weight with respect to the total amount of toner.

Next, the positively chargeable toner will be described. The positively chargeable toner is applied to a reversal development method using a positively charged selenium photoreceptor or a regular development method using a negatively charged zinc oxide or an organic photoreceptor. Polyester resin has a strong negative charging property and is not suitable for a positive charging toner.
A styrene-acrylic copolymer resin is often used for the positively chargeable toner.

The styrene-acrylic copolymer resin is a copolymer obtained from a styrene monomer and an acrylic monomer. Specific examples of the styrene-based monomer include, for example, styrene, O-methylstyrene, m-methylstyrene, p-methylstyrene, α
-Methylstyrene, p-ethylstyrene, 2,4-dimethylstyrene, pn-butylstyrene, p-tert-butylstyrene, pn-hexylstyrene, pn-octylstyrene, p-
n-nonylstyrene, pn-decylstyrene, pn-dodecylstyrene, p-methoxystyrene, p-phenylstyrene,
Examples thereof include p-chlorostyrene and 3,4-dichlorostyrene. These monopounds may be used alone,
A plurality of items may be used in combination. Specific examples of the acrylic monomer include acrylic acid, methyl acrylate, ethyl acrylate, n-butyl acrylate, isobutyl acrylate, propyl acrylate, n-octyl acrylate, dodecyl acrylate. , Lauryl acrylate, 2-ethylhexyl acrylate, stearyl acrylate, 2-chloroethyl acrylate, phenyl acrylate, α-chloromethyl acrylate, methacrylic acid, methyl methacrylate, ethyl methacrylate, propyl methacrylate, n-methacrylic acid -Butyl, isobutyl methacrylate, n-octyl methacrylate, dodecyl methacrylate,
Lauryl methacrylate, 2-ethylhexyl methacrylate, stearyl methacrylate, phenyl methacrylate,
Examples thereof include α-methylene aliphatic monocarboxylic acid esters such as dimethylaminoethyl methacrylate and diethylaminoethyl methacrylate; acrylic acid or methacrylic acid derivatives; and others. These single weights may be used alone or in combination of two or more. As a method for producing the styrene-acrylic copolymer, various methods such as a solution polymerization method, a suspension polymerization method and an emulsion polymerization method can be used.

Examples of the colorant include carbon black and the like as described above. Usually, 1 to 20% by weight is added to the total amount of toner. As the positive charge control agent, nigrosine dyes are most widely used, and in addition, quaternary ammonium salts, triphenylmethane dyes, piperazine polymers and the like are also used. The addition amount is appropriately in the range of 1 to 10% by weight based on the total amount of the toner. When the magnetic toner is used, an alloy or compound of the same magnetic material as described above is used. These magnetic materials are dispersed in the binder resin as fine powder having an average particle size of 0.3 to 1 μm, and the content thereof is 40 to 6 with respect to the total amount of the toner.
A range of 0% by weight is suitable. Examples of the release agent include the same low molecular weight polypropylene and polyethylene wax as those described above. The release agent content is 0.5
-30% by weight is suitable, and preferably 1-10.
% By weight. As the fluidizing agent, a minute amount of fine powder such as silica as described above is added to the surface of the toner particles. The amount of addition is suitably in the range of 0.05 to 3% by weight with respect to the total amount of toner.

The toner of the present invention preferably has the following characteristics. In a negatively chargeable toner containing a polyester resin as a main component of a binder resin, if the average particle size is too small, fog or scattering is caused, and if it is too large, a rough image is formed. The volume specific electric resistance is preferably 10 ¹⁴ Ω · cm or more in order to maintain good transferability. When the absolute value of the triboelectrification amount is too small, the background fog increases, and when it is too large, the image density becomes low.
0 to -40 μc / g is desirable. For the same reason, the positively chargeable toner containing the styrene-acrylic copolymer resin as the main component of the binder resin has an average particle diameter of 5 to 15 μm, a volume specific electric resistance of 10 ¹⁴ Ω · cm or more, and friction. Charge amount +5
It is preferably +30 μc / g.

The toner can be manufactured, for example, as follows. The colorant and the charge control agent are preliminarily mixed in a mixer such as a ball mill or a super mixer, and then a binder resin, a magnetic material and a release agent are added and mixed. Then 2
After melt-kneading with a kneader such as a main roll or a kneader, the mixture is cooled and solidified, then coarsely pulverized by a mechanical pulverizer, then finely pulverized by a wind pulverizer, and then classified by a wind classifier to obtain an average particle size. A toner having a diameter of 5 to 15 μm is obtained.

Next, the method of measuring the physical properties described above will be described.
Saturation magnetization value (σ _s ) is a vibrating sample magnetometer (Toei Kogyo V
It can be measured in a maximum magnetic field of 10 KOe with the SM-3 type).
The volume resistivity (R) was measured by inserting a sample into a cylinder of Teflon (registered trademark) having a diameter of 3.04 mm to a thickness of about 2 mm, applying a load of about 200 g, and applying 200 v / c between both electrodes.
A DC voltage of m (4 kv / cm when measuring the resistance value of the toner) was applied and measured. Tribo-electrostatic charge (Tribo-Electrostati
c Charge; TEC) is a value obtained by measuring the triboelectric charge amount of a combination of a ferrite carrier and a toner using a commercially available triboelectric charge amount measuring device (TB-200 manufactured by Toshiba Chemical Co., Ltd.). For the measurement of the average particle diameter (Dav), a Coulter counter model TA-II type (manufactured by Coulter Counter), which is also commercially available, was used.

[0027]

EXAMPLES The present invention will now be described in more detail based on examples. (Example 1) Raw materials and additives were weighed so as to obtain samples of various compositions shown in Table 1 as Example 1, and mixed by a ball mill. The obtained mixed powder was calcined at a temperature of 900 ° C. for 2 hours, and the calcined sample was crushed by an attritor. The average particle size after pulverization was about 0.7 μm. Next, 0.5 to 1.0% by weight of PVA (polyvinyl alcohol) as a binder was added to the crushed sample, and spray-dried with a spray dryer to granulate. The obtained granulated powder was placed in an alumina container and fired at a temperature of 1200 ° C.,
Further, it was crushed and classified to obtain a ferrite carrier shown in Example 1 of Table 1 having an average particle size of about 40 μm. No coating is applied.

The toner was manufactured as follows. As shown in Example 1 in Table 1, carbon black (colorant; # MA-100 manufactured by Mitsubishi Kasei Co., Ltd.) 10 parts (parts by weight, the same applies hereinafter) and charge control agent (Kayacharge T- manufactured by Nippon Kayaku Co., Ltd.). 2N) 1
Parts are premixed with a ball mill, and 87 parts of bisphenol A type polyester (binder resin) and 2 parts of polypropylene (release agent; TP-32 manufactured by Sanyo Kasei Co., Ltd.) are added and mixed. 150 it
Melted and kneaded in a heated twin-screw ruder, then cooled, and the cooled material is roughly crushed by a mechanical crusher to the extent that it passes through a wire net with an opening diameter of 1 mm, and then finely crushed by a wind crusher / jet mill. did. The powder was classified with a wind force classifier (100MZR manufactured by Alpine Co., Ltd.) so that the volume average diameter was about 10 μm, and a powder having a negative charging property was prepared. Hydrophobic silica (fluidizing agent; Aerosil R97 manufactured by Nippon Aerosil Co., Ltd.)
2) 0.5 part was added to obtain a toner shown in Example 1 of Table 1. The developer of Example 1 was prepared by mixing 97 parts by weight of the ferrite carrier and 3 parts by weight of the toner.

(Example 2) The average particle size of the carrier is 70 μm.
A carrier was manufactured under the same conditions as in Example 1 except that m was used. The toner is the same as in Example 1. 97 parts by weight of the ferrite carrier and 3 parts by weight of the toner were mixed to obtain the developer of Example 2.

(Comparative Example 1) The carrier is Cu-Zn type ferrite which is generally used at present. Raw materials were weighed so as to obtain samples of various compositions shown in the carrier of Comparative Example 1 in Table 1, and the carrier was manufactured under substantially the same conditions as in Example 1. The toner is the same as in Example 1. 97 parts by weight of the ferrite carrier and 3 parts by weight of the toner were mixed to prepare a developer of Comparative Example 1.

(Comparative Example 2) The carrier is a Ba-Ni-Zn type ferrite which is generally used at present. The raw materials were weighed so as to obtain samples of various compositions shown in the carrier of Comparative Example 2 in Table 1, and the carrier was manufactured under the same conditions as in Example 1. The toner is the same as in Example 1. 97 parts by weight of a ferrite carrier and 3 parts by weight of toner were mixed to prepare a developer of Comparative Example 2.

[0032]

[Table 1]

The toners used in Examples 1 and 2 and Comparative Examples 1 and 2 are the same and have a negative charging property. An image was formed using these developers under the following conditions. Photoreceptor: negatively chargeable OPC (diameter 30mm), process speed: 60mm / sec, surface potential: -650v, magnet roll (fixed); 4 poles, sleeve: SUS304 (diameter 20m)
m), developing magnetic pole; 800 G (other 700 G), sleeve rotation speed; 150 rpm, developing gap; 0.4 mm, doctor gap; 0.3 mm, bias voltage (applied to sleeve);
-500V (DC), Toner image is transferred onto plain paper by corona transfer, then heat roll fixing (180 ℃, 1kg / cm), environmental condition; 20
℃, 60% RH.

[0034]

[Table 2]

Table 2 shows the evaluation results of the obtained images. From Table 2, in Examples 1 and 2, the initial image density value (Image De
nsity; ID) and the background fog value hardly change even after copying 50,000 sheets, whereas in Comparative Examples 1 and 2, ID
It can be seen that both the value and the ground fog value have deteriorated.

Next, an example using a positively chargeable toner will be described. Example 3 A carrier was manufactured under the same conditions as in Example 1 except that the average particle size of the carrier was 30 μm. A styrene-n-butyl methacrylate copolymer (M _w = 210,000, M _n = 14,000) was used as a binder resin to obtain a positively chargeable toner, and the raw materials shown in the toner of Example 3 in Table 2 were used. Example 1
A positively chargeable toner was obtained in the same manner as in. However, the charge control agent is Bontron N03 manufactured by Orient Chemical Company. 97 parts by weight of the ferrite carrier and 3 parts by weight of the toner were mixed to obtain the developer of Example 3.

(Example 4) The average particle size of the carrier is 50 μm.
A carrier was manufactured under the same conditions as in Example 1 except that m was used. The toner is the same as in Example 3. 97 parts by weight of the ferrite carrier and 3 parts by weight of the toner were mixed to obtain the developer of Example 4.

(Comparative Example 3) The carrier is Cu-Zn type ferrite which is generally used at present. Raw materials were weighed so as to obtain samples of various compositions shown in the carrier of Comparative Example 3 in Table 3, and the carrier was manufactured under the same conditions as in Example 1. The toner is the same as in Example 3. 97 parts by weight of a ferrite carrier and 3 parts by weight of toner were mixed to prepare a developer of Comparative Example 3.

(Comparative Example 4) The carrier is a Ba-Ni-Zn type ferrite which is generally used at present. The raw materials were weighed so as to obtain samples of various compositions shown in the carrier of Comparative Example 4 in Table 3, and the carrier was manufactured under the same conditions as in Example 1. The toner is the same as in Example 3. 97 parts by weight of the ferrite carrier and 3 parts by weight of the toner were mixed to prepare a developer of Comparative Example 4.

[0040]

[Table 3]

The toners used in Examples 3 and 4 and Comparative Examples 3 and 4 are the same and have a positive charging property. An image was formed using these developers under the following conditions. Photoconductor: Positively chargeable OPC (diameter 30 mm), process speed: 60 mm / sec, surface potential: +550 v, magnet roll (fixed); 4 poles, sleeve: SUS304 (diameter 20 m)
m), developing magnetic pole; 800 G (other 700 G), sleeve rotation speed; 150 rpm, developing gap; 0.4 mm, doctor gap; 0.3 mm, bias voltage (applied to sleeve);
+ 400V (DC), toner image on plain paper after corona transfer, heat roll fixing (180 ℃, 1kg / cm), environmental condition; 20
℃, 60% RH. Table 4 shows the evaluation results of the obtained images. From Table 4, in Examples 3 and 4, the initial ID value and the background fog value hardly changed even after copying 50,000 sheets, whereas in Comparative Examples 3 and 4, both the ID value and the background fog value deteriorated. You can see that

[0042]

[Table 4] In Tables 2 and 4, the ID value should be 1.2 or more, and "background fog" is the difference in optical density between the non-image area (white background area) of plain paper after printing and before printing. If this value is larger than 0.05, the fog in the image becomes noticeable.

Next, the charge imparting ability of the carrier will be described. FIG. 1 shows changes with time in the charge amount of toner when the developers of Example 1 and Comparative Examples 1 and 2 described above are put into a developing device and stirred. It can be seen from FIG. 1 that only the toner of Example 1 shows a substantially constant value without a decrease in the charge amount.

[0044]

As described above, since the two-component developer of the present invention is used by mixing it with a specific toner without coating a ferrite carrier containing no harmful substance, the image quality is stable and maintainable. An excellent image can be obtained.

[Brief description of drawings]

FIG. 1 is a diagram showing a change in charge amount depending on a stirring time of Example 1 which is a developer of the present invention and Comparative Examples 1 and 2 which are conventional developers.

Claims (4)

[Claims] 1. A cubic unitary ferrite Me represented by Me _0.5 Fe _2.5 O ₄ (Me represents a monovalent metal).
Alternatively, it is a composite ferrite in which a part of Fe is replaced by Mn, and has a molar ratio of Li ₂ O 2 to 15%, MnO 50% or less (not including 0%), and Fe ₂ O ₃ 50 to 95%. A two-component developer comprising a ferrite carrier having a composition and an average crystal grain size of 20 μm or less, and a negatively chargeable toner containing a polyester resin as a main component of a binder resin. 2. The ferrite carrier according to claim 1,
A two-component developer comprising a styrene-acrylic copolymer resin as a main component of a binder resin and a positively chargeable toner. 3. The ferrite carrier is composed of spherical particles, has an average particle diameter of 20 to 150 μm, a volume specific electric resistance of 10 ³ to 10 ¹³ Ω · cm, and a saturation magnetization value of 40 emu / g or more. The average particle size is 5 to 15 μm, the volume resistivity is 10 ¹⁴ Ω · cm or more, and the triboelectric charge is -10 to -4.
The two-component developer according to claim 1, which is 0 μc / g. 4. The ferrite carrier is composed of spherical particles, has an average particle size of 20 to 150 μm, a volume specific electric resistance of 10 ³ to 10 ¹³ Ω · cm, and a saturation magnetization value of 40 emu / g or more. The average particle size is 5 to 15 μm, the volume specific electric resistance is 10 ¹⁴ Ω · cm or more, and the triboelectric charge is +5 to +30.
The two-component type developer according to claim 2, wherein the two-component type developer is μc / g.