EP0479285A1

EP0479285A1 - Electrophotographic toner

Info

Publication number: EP0479285A1
Application number: EP91116862A
Authority: EP
Inventors: Nakano Tetsuya
Original assignee: Mita Industrial Co Ltd
Current assignee: Kyocera Mita Industrial Co Ltd
Priority date: 1990-10-05
Filing date: 1991-10-02
Publication date: 1992-04-08
Anticipated expiration: 2011-10-02
Also published as: DE69125083D1; ES2100916T3; DE69125083T2; EP0479285B1; KR920008541A; KR950001824B1; CA2052571A1

Abstract

The electrophotographic toner in accordance with the present invention comprises a binder resin, a coloring agent, an electric charge controlling agent and a release agent, said electric charge controlling agent being represented by the following general formula (1):

(wherein R¹, R², R³, R⁴, Y and Z⁺ are defined as in the specification), said electric charge controlling agent having a pH value in a range from 3 to 5. With the use of the toner of the present invention, stable images can be reproduced with toner scattering effectively prevented.

Description

Background of the Invention

The present invention relates to an electrophotographic toner, and more particularly to an electrophotographic toner to be used for an image forming apparatus such as an electrostatic copying apparatus, a laser printer or the like.
In the image forming apparatus above-mentioned, an electrostatic latent image formed on the surface of a photoreceptor by exposure to light is let come in contact with an electrophotographic developer by a developping device. Toner in the electrophotographic developer is electrostatically sticked to the electrostatic latent image. This causes the electrostatic latent image to be turned into a toner image. Then, the toner image is transferred to paper from the surface of the photoreceptor and fixed on the paper, thus achieving image forming.
As an electrophotographic toner, there may be generally used toner particles containing a binder resin, a coloring agent such as carbon black or the like, an electric charge controlling agent, a release agent, a flowability imparting agent as necessary and the like. As the electric charge controlling agent, there is generally used an azo-type metal complex salt dye (azo-type chromium dye or the like).
To improve the flowability of the toner particles, silica fine powder, particularly hydrophobic silica fine powder, is generally mixed with and dispersed in the toner particles.
However, such a conventional electrophotographic toner presents the problems that the electric charge characteristics are not stabilized to provoke fog, decrease in image density, toner scattering or a so-called letter dispersion, i.e., spots as formed by the toner scattering around reproduced letters, so that stable images cannot be obtained. In particular, when a black toner containing carbon black as a coloring agent is used, the problems above-mentioned are remarkable.

Summary of the Invention

It is an object of the present invention to provide an electrophotographic toner with which there can be obtained stable images free from fog, decrease in image density, letter dispersion, toner scattering and the like.
To achieve the object above-mentioned, the inventors have studied hard and paid their attention to the pH value of an azo-type metal complex salt dye used as the electric charge controlling agent. The inventors have found the novel fact that the electric charge characteristics and humidity resistance of a toner and dispersibility of the electric charge controlling agent in the resin vary with this pH value to cause a variety of problems such as defective image (insufficient image density, fog and the like), toner scattering and the like.
The electrophotographic toner in accordance with the present invention contains, as the electric charge controlling agent, a compound represented by the following general formula (1) and presenting a pH value in a range from 3 to 5:
[wherein R¹, R², R³ and R⁴ may be the same as or different from one another, and each is a hydrogen atom, a halogen atom or the following group:
(wherein R⁵ and R⁶ may be the same as or different from one another, and each is an alkyl or aryl group), and R¹, R², R³ and R⁴ should not be simultaneously a hydrogen atom; Y is a Cr, Fe, Co, Zn or Ti atom; Z` is a cation selected from the group consisting of an ammonium ion, a hydrogen ion, a potassium ion and a sodium ion.]
For a toner containing carbon black as the coloring agent, it is preferable to use carbon black of which pH value is in a range from 6 to 11, in addition to the use of the electric charge controlling agent having a pH in the range mentioned earlier.
When hydrophobic silica fine powder is mixed with and dispersed in toner particles containing, as the electric charge controlling agent, the compound (1) having a pH value in a range from 3 to 5, the pH value of the hydrophobic silica is preferably in a range from 3.5 to 4.5.
The pH value above-mentioned may be measured in accordance with the method set forth in JIS K 6221. More specifically, 10 g of a sample is added to 100 ml of distilled water. The sample-water mixture is then boiled for 15 minutes and cooled to a room temperature, after which pH value is measured.

Detailed Description of the Present Invention

Since the compound of the general formula (1) presents a pH value in a range from 3 to 5, it can be uniformly dispersed, as the electric charge controlling agent, in the binder resin of the toner. Accordingly, the electrophotographic toner in accordance with the present invention can be stabilized in electric charge characteristics.
If the pH value of the compound (1) is less than 3, the toner is lowered in humidity resistance. If the pH value is greater than 5, the dispersibility of the compound (1) in the binder resin is defective. In both cases above-mentioned, there are caused the problems such as decrease in image density, letter dispersion, toner scattering and the like.
The pH value of the compound of the general formula (1) is greatly influenced by the polar group connected to this compound. When the polar group is an electron attractive group (e.g., a halogen atom), the pH value is liable to decrease. It is therefore required to select the respective groups such that the pH value is located in the range above-mentioned. Table 1 shows the relationship between the combination of the substituting groups and the pH value. It is however noted that the pH value varies with a trace amount of a by-product included in the course of production of the compound (1) or with the presence of unreacted substances, and is therefore not a definite value.
When there is used, as the coloring agent, carbon black of which pH value is less than 6, the humidity resistance of the toner is not sufficient. When there is used, as the coloring agent, carbon black of which pH value is greater than 11, the dispersibility of the compound (1) and the carbon black in the binder resin is lowered. In both cases above-mentioned, there are caused the problems of fog, decrease in image density, letter dispersion, toner scattering and the like.
When hydrophobic silica fine powder is to be mixed with and dispersed in toner particles containing, as the electric charge controlling agent, the compound (1) of which pH value is in a range from 3 to 5, it is preferable to use hydrophobic silica fine powder of which pH value is in a range from 3.5 to 4.5. In this case, the electric charge characteristics are stabilized to produce stable images. More specifically, if the pH values of the compound of the general formula (1) and the hydrophobic silica fine powder are below the ranges above-mentioned, the amount of negative electric charge becomes great, causing the toner to be separated from the carrier with difficulty. This provokes the problem of decrease in image density. If both pH values exceed the ranges above-mentioned, the amount of negative electric charge becomes small, causing the toner to be insufficiently sticked to the carrier. This provokes the problem of toner scattering, fog or the like.
Examples of the halogen atom include a fluorine atom, a chloride atom, a bromine atom and an iodine atom.
Examples of the alkyl group include methyl, ethyl, propyl, isopropyl, butyl, t-butyl, pentyl and hexyl groups, each having 1 to 6 carbon atoms.
Examples of the aryl group include phenyl, tolyl, xylyl, biphenyl, naphthyl, antolyl and phenantolyl groups.
As the electric charge controlling agent, the compound (1) is used in an amount from 0.5 to 8 parts by weight, preferably from 1 to 3 parts by weight, for 100 parts by weight of binder resin. If the blending ratio of the compound (1) is smaller than the range above-mentioned, the electric charge characteristics become unstable. If the blending ratio of the compound (1) is greater than the range above-mentioned, the carrier is sticked to the toner, thereby to provoke toner scattering, fog and the like.
The toner is produced by a method of mixing a binder resin, a coloring agent, the compound (1) as an electric charge controlling agent, a release agent (an off-set preventive agent) and an additive such as a flowability imparting agent or the like to be used as necessary, and pulverizing the mixture into particles having a predetermined particle size. More specifically, the toner is produced by previously mixing and kneading the components above-mentioned uniformly with the use of a dry blender, a Henschel mixer, a ball mill or the like, uniformly melting and kneading the resultant mixture with the use of a kneading device such as a Banbury mixer, a roll, a single- or double-shaft extruding kneader or the like, cooling and grinding the resultant kneaded body, and classifying the resultant ground pieces as necessary. The toner may also be produced by suspension polymerization or the like.
Examples of the binder resin include styrene resins (monopolymers and copolymers containing styrene or a styrene substituent) such as polystyrene, chloro-polystyrene, poly-a-methylstyrene, a styrene-chlorostyrene copolymer, a styrene-propylene copolymer, a styrene-butadiene copolymer, a styrene-vinyl chloride copolymer, a styrene-vinyl acetate copolymer, a styrene-maleic acid copolymer, a styrene-acrylate copolymer (a styrene-methyl acrylate copolymer, a styrene-ethyl acrylate copolymer, a styrene-butyl acrylate copolymer, a styrene-octyl acrylate copolymer, a styrene-phenyl acrylate copolymer or the like), a styrene-methacrylate copolymer (a styrene-methyl methacrylate copolymer, a styrene-ethyl methacrylate copolymer, a styrene-butyl methacrylate copolymer, a styrene-phenyl methacrylate copolymer or the like), a styrene-a-methyl chloroacrylate copolymer, a styrene-acrylonitrile-acrylate copolymer and the like. Examples of the binder resin further include polyvinyl chloride, low-molecular-weight polyethylene, low-molecular-weight polypropylene, an ethylene-ethyl acrylate copolymer, polyvinyl butyral, an ethylene-vinyl acetate copolymer, rosin modified maleic acid resin, phenyl resin, epoxy resin, polyester resin, ionomer resin, polyurethane resin, silicone resin, ketone resin, xylene resin, polyamid resin and the like. The examples above-mentioned may be used alone or in combination of plural types. In the examples above-mentioned, there may be preferably used styrene resin, particularly a styrene-(meth)acrylate copolymer and more particularly a styrene-methyl methacrylate-butylacrylate copolymer. In particular, there may be preferably used a styrene-methyl methacrylate-butylacrylate copolymer containing 75 to 85 % by weight of styrene, 0.5 to 5 % by weight of methylmethacrylate and 10 to 20 % by weight of butylacrylate.
Examples of the coloring agent include: a black coloring agent such as carbon black (furnace black, channel black, thermal, gas black, oil black, acetylene black), lamp black, aniline black or the like; a brown coloring agent as obtained by mixing red, yellow and black coloring agents. Of these, the black coloring agent may be particularly suitably used. The coloring agent may be used in an amount of 1 to 20 parts by weight and preferably 3 to 15 parts by weight for 100 parts by weights of the binder resin.
Examples of the release agent (off-set preventing agent) include aliphatic hydrocarbon, aliphatic metal salts, higher fatty acids, fatty esters, its partially saponified substances, silicone oil, a variety of waxes and the like. Of these, there is preferably used a low-molecular-weight aliphatic hydrocarbon of which weight average molecular weight is from about 1,000 to about 10,000. More specifically, there is suitably used one or a combination of plural types of a low-molecular-weight polypropylene, low-molecular-weight polyethylene, paraffin wax, a low-molecular-weight olefin polymer composed of an olefin unit having 4 or more carbon atoms and the like. The release agent may be used in an amount of 0.1 to 10 parts by weight and preferably from 1 to 5 parts by weight for 100 parts by weight of the binder resin.
As conventionally done, the toner particles may have sizes in a range from 3 to 35 µm and preferably from 5 to 25 µm, but it is preferable that the distribution of toner particle sizes satisfies the following formula:
[wherein N is the percentage by the number of toner particles of which sizes as measured with a coulter counter exceed 16 am, and C is surface dye density (g/g) of the toner particles]
When the distribution of toner particle sizes is in the range above-mentioned, it is possible, in view of the relationship with the surface dye density, to further eliminate variations in electric charging characteristics of the toner.
To obtain toner particles presenting a distribution of particle sizes which satisfies the formula above-mentioned, the ground toner particles may be classified to remove particles having sizes greater than 16 am, or toner particles may be ground such that the peak of the toner particle-size distribution is shifted to a smaller-size zone to reduce the content of particles having sizes greater than 16 am.
According to the present invention, as the hydrophobic silica fine powder to be mixed with and dispersed in the toner particles, there may be used silica fine powder of which surface is treated with, for example, a (poly)alkyl group, a (poly)alkylsilil group, a (poly)alkylsilane or silicone oil. Preferably, there may be used silica fine powder of which surface has been treated with a compound having a polymethylsilil group such that the powder becomes hydrophobic. Such powder is higher in hydrophobic nature than conventional silica fine powder treated with a compound having a low-molecular-weight alkyl group.
As a commercially available product of such silica fine powder, there may be mentioned "Cabosil TS720" manufactured by Cabot Co., Ltd. This product is hydrophobic fumed silica fine powder, which is obtained by treating high-purity fumed silica fine powder (99.8% Si0₂) with an organic silicone compound, and on the surface of which a polymethylsilil group is present to increase the hydrophobic nature of the surface of the silica fine powder.
The particle sizes of the silica fine powder so treated as to be hydrophobic are suitably in a range from 0.01 to 0.04 urn.
The pH value of the hydrophobic silica fine powder varies with a variety of factors which are not always clarified. However, it is known that the surface functional group is influenced by reaction by-products.
The hydrophobic silica fine powder may be added in an amount of 0.01 to 5 % by weight and preferably from 0.05 to 1 % by weight for the total amount of toner. If the amount of the hydrophobic silica fine powder is greater than the range above-mentioned, the amount of electric charge is excessive. If this amount is smaller than the range above-mentioned, the effect of improving the toner flowability cannot be expected.

Examples

The following description will discuss in more detail the electrophotographic toner in accordance with the present invention with reference to Examples and Comparative Examples.

Examples 1 to 9 and Comparative Examples 1 to 8

The components above-mentioned were mixed. The mixture was molten and kneaded with a double-shaft kneader, and then cooled, ground and classified to prepare toner particles having the average particle size of 10 µm. Table 2 shows the substituting groups contained in the compounds (1) used, as the electric charge controlling agent, in Examples 1 to 9 and Comparative Examples 1 to 8. Table 3 shows the pH values of carbon black and the compounds (1) used. Each pH value was measured in the manner that 10 g of a sample was added to 100 ml of distilled water and the sample-water mixture was then boiled for 15 minutes on a hot plate and cooled to a room temperature, after which pH value was measured with a glass electrode pH meter. The pH value of the hydrophobic silica fine powder used was 4.1.
The moisture contents of the resultant electrophotographic toners thus obtained were measured under the condition of ambient temperature/ambient humidity (temperature : 20 C, humidity : 65%, hereinafter referred to as N/N) and under the condition of high temperature/high humidity (temperature : 35 °C, humidity : 85%, hereinafter referred to as H/H), respectively, according to the Karl Fischer method. Table 3 shows the results.
The following evaluation tests were conducted on the electrophotographic toners obtained in Example 1 to 9 and Comparative Examples 1 to 8. Table 3 shows the test results.

(1) Test of Dispersibility

The circumference of each of the electrophotographic toners was covered with and solidified by epoxy resin. Each of the toners as cut with a microtome was observed with a transmission-type electro microscope. The toner dispersibility was evaluated according to the following criteria:

@ : Extremely finely dispersed
O : Substantially finely dispersed
Δ : Some large particles observed
X : Many large particles observed

(2) Test of Image Density

Each of the electrophotographic toners was mixed with a carrier to prepare a developer having a toner density of 3%. With an electrophotographic copying apparatus (DC-7085 manufactured by Mita Industrial Co., Ltd.) using (i) each developer above-mentioned as a start developer and (ii) the same toner as that contained in the start developer as a resupply toner, a solid-black document was continuously copied for 150,000 pieces under the condition of ambient temperature/ambient humidity (N/N), i.e., temperature of 20°C and humidity of 65%, except that intermediate 8000 copied pieces from 16001st piece to 24000th piece were taken at temperature of 35 ° C and humidity of 85% (H/H). Every thousandth copied pieces were extracted, as samples, from 150,000 copied pieces for each of the developers and were measured as to the density values thereof with a reflection densitometer (TC-6D manufactured by Tokyo Denshoku Co., Ltd.). The averages were calculated for these samples for all the developers.

(3) Test of Fog Density

With the use of the reflection densitometer above-mentioned, the density of the blank spaces of each sample obtained in Test of Image Density was measured to measure fog density. The averages were calculated for the samples for all the developers.

(4) Test of Letter Dispersion

All the samples obtained in Test of Image Density were visually checked for a so-called letter dispersion of toner spots.

(5) Test of Toner Scattering

For each developer, there were checked (i) the blank spaces of the reproduced image of the 150,000th piece, and (ii) the inside of the copying apparatus after 150,000 copies had been taken. The toner dispersibility was evaluated according to the following criteria:
It is understood from Table 3 that each of the electrophotographic toners of Examples 1 to 9 containing, as the electric charge controlling agent, the compound (1) presenting a pH value in a range from 3 to 5, is superior in dispersibility to the electrophotographic toners of Comparative Examples 1 to 8 containing a compound presenting a pH value which deviates from the range above-mentioned. Further, each of the toners of Examples 1 to 9 presents less variations of moisture content under both conditions of ambient temperature/ambient humidity (N/N) and high temperature/high humidity (H/H), and is therefore excellent in humidity resistance.
It is also understood that the reproduced images obtained with the use of the electrophotographic toners of Examples 1 to 9 are superior in any of image density, fog density, letter dispersion and toner scattering to the reproduced images obtained with the use of the electrophotographic toners of Comparative Examples 1 to 8.
It is also understood that variations between reproduced images obtained under the ambient temperature/ambient humidity (N/N) condition and reproduced images obtained under the high temperature/high humidity (H/H) condition both with the use of each of the electrophotographic toners of Examples 1 to 9, are less than variations between reproduced images obtained under the ambient temperature/ambient humidity (N/N) condition and reproduced images obtained under the high temperature/high humidity (H/H) condition both with the use of each of the electrophotographic toners of Comparative Examples 1 to 8. Thus, stable reproduced images can be obtained with the toners of Examples 1 to 9.
It is also understood that, out of the electrophotographic toners of Examples 1 to 9, the toners of Examples 1 to 4 and 7 using carbon black presenting a pH value in a range from 6 to 11 are particularly excellent.

Examples 10

The components above-mentioned were molten and kneaded with a double-shaft kneader, and then prepared as toner particles having the average toner particle size of 10 µm with a jetmil. Table 4 shows the groups contained in the chromium-containing azo dye used as the electric charge controlling agent. The pH value of the carbon black was 8.5.
Silica fine powder so treated as to be hydrophobic (particle size of 0.02 _/1.m, pH of 3.7, Cabosil TS-720 manufactured by Cabot Co., Ltd.) was mixed with and dispersed in the toner particles thus prepared in an amount of 0.5 % by weight for the total amount of the toner particles, thus preparing a toner.

Examples 11 to 13 and Comparative Examples 9 to 16

Toners were prepared in the same manner as in Example 10 except that there were used (i) metal- containing azo dyes which respectively contained groups shown in Table 4 and of which pH values are shown in Table 6 and (ii) hydrophobic silica fine powders of which pH values are shown in Table 6.

Evaluation Tests

Ferrite carrier having the average particle size of 80 µm was blended with each of the toners of Examples 10 to 13 and Comparative Examples 9 to 16. Each mixture was uniformly mixed and agitated to prepare a two-component developer presenting toner density of 4%. With the use of an electrophotographic copying apparatus (DC-3255 manufactured by Mita Industrial Co., Ltd.) using each of the developers thus prepared, an original document was copied totally 80,000 pieces under different operating conditions under which a predetermined number of copied pieces were respectively taken. All the copied pieces were checked for image density, fog density, amount of electric charge and toner scattering for each of the operating conditions. More specifically, the copying operation was carried out with the operating condition changed in the order shown in Table 5 for a predetermined number of pieces, and the reproduced images were checked for the items above-mentioned. It is however noted that the measured values of the images reproduced under the N/N condition were those obtained after 80,000 pieces were copied.
The respective tests were conducted in the following manners.

(1) Measurement of Image Density (I.D.)

Each image density was measured with the use of a reflection densitometer (TC-6D manufactured by Tokyo Denshoku Co., Ltd.)

(2) Measurement of Fog Density (F.D.)

With the use of the reflection densitometer above-mentioned, the density of blank portions of each reproduced image was measured and defined as fog density.

(3) Amount of Electric Charge

The amount of electric charge was measured with a blow-off electric charge measuring instrument manufactured by Toshiba Chemical Co., Ltd.

(4) Toner Scattering

The inside of the copying apparatus and the surface of each reproduced image were visually checked for toner scattering, and evaluated according to the following criteria:
The test results are shown in Table 6.
It is apparent from Table 6 that, by adjusting the pH values of the hydrophobic silica fine powder and the electric charge controlling agent within respective predetermined ranges, the electric charge characteristics can be stabilized to remarkably improve the image density and toner scattering.

Claims

1. An electrophotographic toner comprising a binder resin, a coloring agent, an electric charge controlling agent and a release agent, said electric charge controlling agent being represented by the following general formula (1);

[wherein R¹, R², R³ and R⁴ may be the same as or different from one another, and each is a hydrogen atom, a halogen atom or the following group:

(wherein R⁵ and R⁶ may be the same as or different from one another, and each is an alkyl or aryl group), R¹, R², R³ and R⁴ should not be simultaneously a hydrogen atom; Y is a Cr, Fe, Co, Zn or Ti atom; Z is a cation selected from the group consisting of an ammonium ion, a hydrogen ion, a potassium ion and a sodium ion.], said electric charge controlling agent being a compound of which pH value is in a range from 3 to 5.

2. An electrophotographic toner according to Claim 1, wherein the compound of the general formula (1) is contained in an amount of 0.5 to 8 parts by weight for 100 parts by weight of binder resin.

3. An electrophotographic toner according to Claim 1, wherein the distribution of toner particle sizes is in a range represented by the following formula:

[wherein N is the percentage by the number of toner particles of which sizes as measured with a coalter counter exceed 16 µm, and C is surface dye density (g/g) of the toner particles].

4. An electrophotographic toner according to Claim 1, wherein the coloring agent is carbon black of which pH value is in a range from 6 to 11.

5. An electrophotographic toner according to Claim 1, wherein hydrophobic silica having a pH value in a range from 3.5 to 4.5 is mixed with and dispersed in said toner.

6. An electrophotographic toner according to Claim 5, wherein the hydrophobic silica is fine powder of 0.01 to 0.04 µm.

7. An electrophotographic toner according to Claim 5, wherein the hydrophobic silica is contained in an amount of 0.01 to 5 % by weight for the amount of all toner particles.