Toner for amorphous silicon photoreceptor and developer including the same

Description

[0001] The present invention relates to a toner and a developer for use in an image forming apparatus equipped with an amorphous silicon photoreceptor. More particularly, it relates to a toner and a developer which can ensure an image development without uncontrolled toner attraction, fogging, toner scattering and blurring even under high humidity.

[0002] With the recent widening demand for high speed copying and resistance to high humidity environment, an amorphous silicon photoreceptor is noted as an electrophotograhic photoreceptor, which has high surface hardness and high durability, and is useful for high speed copying. Unfortunately, the amorphous silicon photoreceptor has a relatively low surface potential, which makes it difficult to obtain a high quality copy image. In addition, the amorphous silicon photoreceptor has the problem of "blurring". Specifically, when the surface of the amorphous photoreceptor is subject to oxidization by ozone generated, for example, in the process of coloring particles being charged, an oxidation layer occurs on the surface of the photoreceptor. The oxidation layer is likely to adsorb moisture from the air, and consequently disturb an electrostatic latent image formed on the photoreceptor. As a result, a developed toner image has a so-called "blurring". Blurring decrease the image quality. In the case that the blurred image includes characters, there is a likelihood that characters cannot be recognized.

[0003] To prevent blurring, a variety of improvements in terms of image forming system and photoreceptor materials have been made in the past without great success. On the other hand, improvements in developers have been attempted though not as many as the above. JP-A-278661/1996 proposes an image forming method in which a specific resin-coated carrier and toner are used for an amorphous silicon photoreceptor. With this method, however, blurring cannot be prevented satisfactorily and, under high humidity environment, the chargeability of toner is reduced to cause fog and toner scattering in some cases.

[0004] It is an object of the present invention to provide a toner and a developer which have overcome the problems residing in the prior art.

[0005] It is another object of the present invention to provide a toner and a developer which can prevent the occurrence of blurring in an image forming apparatus using an amorphous silicon photoreceptor, and ensure a high quality image free from fog even under high humidity.

[0006] According to an aspect of the present invention, a toner comprises coloring particles each including a main particle, particles made of a hydrophobic silica and attached on a surface of the main particle, and magnetic particles attached on a surface of the main particle. The main particle includes coloring elements and a binder resin containing a polyester resin having an acid value of not less than 20 for binding coloring elements. Primary particles of the hydrophobic silica particles have a mean particle size of from 7 to 20 nm. Primary particles of the magnetic particles have a mean particle size of from 100 to 1000 nm.

[0007] According to another aspect of the present invention, a developer comprises carrying particles and the above-mentioned coloring particles.

[0008] The inventive toner and developer effectively prevents blurring, which is liable to occur in a photoreceptor including amorphous silicon, and assures formation of a high quality image free from fog under high humidity.

[0009] A variety of considerations have been made with regard to the solution to the above stated problems inherent in using amorphous silicon materials for a photoreceptor, and the toner and developer as set forth were invented when it was found that the problems are solvable by employing a toner in which a specific polyester resin is used as a binder resin of the coloring particles, and a specific silica and magnetic powder are used for treating the surface of each coloring particle.

[0010] It is a primary feature of the present invention to use a polyester resin having an acid value of not less than 20, as a binder resin of coloring particles. Since a polyester resin has an excellent fixability at a low temperature, it is suitable for a toner used in high speed image forming apparatus. In addition, using a polyester resin having the acid value of not less than 20 as a binder resin of a toner ensures that the toner is negatively charged and thereby high quality image is obtained, even when the toner is used in combination with an amorphous silicon photoreceptor, which has a low surface potential and the potential tends to decay rapidly. However, since the binder resin having such a high acid value, under high humidity, the carboxyl group in the polyester resin is likely to be reacted with the moisture in the air, resulting in a decreased chargability of the toner having the resin. A low chargeability of toner causes too high image density, as well as fog and toner scattering. In the present invention, the problems attributed to high humidity are solved by adding to the surface of main particles, which include a coloring agent and a binder resin, a hydrophobic silica whose primary particles have a mean particle size of from 7 to 20 nm. This is because of the fact that the surface of the hydrophobic silica is generally negatively charged, adding the hydrophobic silica to the surface of the toner can increase chargeability of the toner. Thus, the surface treatment for the toner with the hydrophobic silica enables to suppress the chargeability of the toner from decreasing under high humidity.

[0011] It is another primary feature of the present invention to add, to the surface of main particles, magnetic powder whose primary particles have a mean particle size of from 100 to 1000 nm. Since the magnetic powder can act as an abrasive, an oxidation layer on the surface of an amorphous silicon photoreceptor is abraded i.e., removed by the magnetic powder added to the main particles. This enables to maintain the photoreceptor surface be always fresh and to effectively prevent blurring even under high humidity. It is also possible to suppress toner scattering by adding magnetic powder with high specific gravity to the toner surface. The components included in the toner of the present invention will be described as below.

Polyester Resin

[0012] Polyester resin used in the present invention is usually obtained by condensation polymerization of polyhydric carboxylic acids and polyhydric alcohols.

[0013] Examples of the polybasic carboxylic acids used in the polyester resin are aromatic polyhydric carboxylic acids such as phthalic acid, isophthalic acid, terephthalic acid, 1,2,4-benzene tricarboxylic acid, 2,5,7-naphthalene tricarboxylic acid, 1,2,4-naphthalene tricarboxylic acid and pyromellitic acid; fatty dicarboxylic acids such as maleic acid, fumaric acid, succinic acid, adipic acid, sebacic acid, malonic acid, azelaic acid, mesaconic acid, citraconic acid and glutaconic acid; alicyclic dicarboxylic acids such as cyclohexane dicarboxylic acid and cyclohexene dicarboxylic acid; anhydrides of these carboxylic acids; and lower alkyl esters. These can be used solely or in a combination of two or more kinds.

[0014] Since degree of crosslinking depends upon the total amount of components of the alcohol having more than three hydroxyl groups and the carboxylic acid having more than three carboxyl groups, a desired degree of crosslinking is obtainable by adjusting the amounts of such components. It is usually preferable that the components are present in the amount of not more than 15 mol percent.

[0015] Examples of the polyhydric alcohols used in the polyester resin are alkylene glycols such as ethylene glycol, 1,2-propylene glycol, 1,3-propylene glycol, 1,4-butanediol, 1,4-butenediol, neopentyl glycol, 1,5-pentane glycol and 1,6-hexane glycol; alkylene ether glycols such as diethylene glycol, triethylene glycol, dipropylene glycol, polyethylene glycol, polypropylene glycol and polytetramethylene glycol; fatty polyhydric alcohols such as 1,4-cyclohexane dimethanol and hydrogenerated bisphenol A; bisphenols such as bisphenol A, bisphenol F and bisphenol S; and alkylene oxides of bisphenols. These may be used solely or in a combination of two or more kinds.

[0016] The polyester resin used in the present invention is resins with an acid value of not less than 20. Such resins is made from alcohol monomers and carboxylic acid monomers whose amounts are in a specified proportion so that the rate of carboxylic groups contained carboxylic acid monomers to hydroxyl groups contained alcohol monomers is more than one. Specifically, the rate is preferably 1.1 to 1.5. The acid value of the polyester resin is preferably not less than 35, and its upper limit is preferably 70.

[0017] In order to obtain a polyester resin having a desired molecular weight and a desired acid value, monobasic carboxylic acid and monohydric alcohol may be used as required in the present invention. Examples of the monobasic carboxylic acid are benzoic acid, paraoxy benzoic acid, toluene carboxylic acid, salicylic acid, acetic acid, propionic acid and stearic acid. Examples of the monohydric alcohol are benzil alcohol, toluene-4-methanol and cyclohexane methanol.

[0018] The polyester resin used in the present invention is prepared by using the above materials in the usual way. For instance, alcohol compositions and acid compositions are placed in a reactor in predetermined amounts and, while an inert gas, e.g., nitrogen, is blown into the reactor, they are allowed to react at temperatures between 150 and 190 °C in the presence of a catalyst. Low molecular compounds by-produced in the course of the reaction is successively removed outside of the system. Thereafter, the reaction is accelerated by raising the reaction temperature to between 210 and 250 °C, to obtain the desired polyester resin. The reaction can be conducted at atmospheric pressure, under reduced pressure, or high pressure. After a conversion of 50 to 90 % is reached, however, the reaction is preferably conducted under reduced pressure of not more than 200 mmHg.

[0019] Examples of the catalyst are metals such as tin, titanium, antimony, manganese, nickel, zinc, lead, iron, magnesium, calcium and germanium; metals thereof; and compounds containing these metals.

[0020] The glass transition temperature of the above polyester resin is preferably from 45 to 90 °C. If a toner contains the resin whose glass transition temperature is below 45 °C, the toner is likely to solidify in a toner cartridge or developing machines. On the other hand, if a toner contains the resin whose glass transition temperature is above 90 °C, the fixability of the toner to a transfer material is likely to be insufficient.

[0021] In addition to the polyester resin used in the present invention, if necessary, other resins may be jointly used as a binder resin, to such an extent that the effect of the present invention is not inhibited.

Preparation of Toner

[0022] The toner according to the present invention can be prepared by a number of methods which are well known in the art, such as pulverization classification method, melt granulating method, spray granulating method, and polymerization method. In the pulverization classification method, for example, the above binder resin is premixed together with toner compositions which comprises a coloring agent, a charge control agent and a mold releasing agent, in a mixer such as Henschel's mixer, and the mixture is kneaded with a kneading machine, e.g., a biaxial extruder. The obtained kneaded composition is then cooled, pulverized and, if necessary, classified, to prepare main particles.

[0023] As the coloring agent incorporated in the polyester resin, there are black pigments such as acetylene black, lamp black and aniline black; yellow pigments such as chrome yellow, zinc chromate, cadmium yellow, yellow iron oxide, mineral fast yellow, nickel titanium yellow, naples yellow, naphthol yellow S. Hansa yellow G, Hansa yellow 10G, benzidine yellow G, benzidine yellow GR, quinoline yellow lake, permanent yellow NCG and tartrazine lake, orange color pigments such as chrome orange, molybdenum orange, permanent orange GTR, pyrazolone orange, Balkan orange, indanthrene brilliant orange RK, benzidine orange G, indanthrene brilliant orange GK; red pigments such as iron oxide red, cadmium red, red lead, mercury sulfide cadmium, permanent red 4R, lithol red, pyrazolone red, watching red calcium salt, lake red D, brilliant carmin 6B, eosin lake, rhodamine lake B, alizarin lake and brilliant carmin 3B; purple pigments such as manganese purple, fast violet B and methyl violet lake; blue pigments such as iron blue, cobalt blue, alkali blue lake, Victoria blue lake, phthalocyanine blue, metal-free phthalocyanine blue, partial chloride of phthalocyanine blue, fast sky blue and indanthrene blue BC; green pigments such as chrome green, chromium oxide, pigment green B, malachite green lake and Final Yellow Green G; and white pigments such as zinc white, titanium oxide, antimony white, zinc sulfide, baryta powder, barium carbonate, clay, silica, white carbon, talc and alumina white. The above pigment is preferably present in an amount of from 2 to 20 parts by weight, more preferably from 5 to 15 parts by weight, to 100 parts by weight of a binder resin.

[0024] As a mold releasing agent incorporated in the binder resin, a variety of waxes and low molecular weight olefin resins can be employed. Olefin resin has a number average molecular weight (Mn) of from 1000 to 10000, particularly from 2000 to 6000. Examples of the olefin resin are polypropylene, polyethylene and propylene-ethylene copolymer. Especially preferred is polypropylene.

[0025] As a charge control agent, any charge control agents normally used can be used. Examples of charge control agent of positively charged type are nigrosine dye, fatty acid modified nigrosine dye, carboxyl group containing fatty acid modified nigrosine dye, quaternary ammonium salt, amine compounds and organic metallic compounds. Examples of charge control agent of negatively charged type are metallic complex dye and salicylic acid derivatives.

[0026] It may be preferable that the main particle has a median size from 5 to 15 µm, particularly from 7 to 12 µm, in terms of measurement of a Coulter counter.

[0027] To the surface of the main particles, a hydrophobic silica and magnetic powder are added, to prepare coloring particles. The added silica prevents aggregation of coloring particles and increases chargeability of coloring particles by the negatively-charged surface of the silica. The added magnetic powder decreases the resistance of coloring particles, which results in an increased image density, and decreases the magnetic power of the coloring particles, which results in a decreased amount of coloring particles scattered in an image forming apparatus.

[0028] It is necessary that the primary particles of the hydrophobic silica has a mean particle size of from 7 to 20 nm. Below 7 nm, the coloring particles having such hydrophobic silica are likely to aggregate, i.e., a dispersibility of the toner is decreased. Above 20 nm, the coloring particles are likely to have a decreased chargeability under high humidity. More suitable mean particle size of the primary particle of the hydrophobic silica is from 7 to 16 nm. In the present invention, the mean particle size of the primary particle of the hydrophobic silicon was measured with microscopy.

[0029] In preparing the hydrophobic silica, fine powder of silicon dioxide in which the silicon atom on the surface is silanol group is allowed to react with a compound, so that a hydrophobic group is bonded to the silicon atom on the surface of the silicon dioxide particles, via an oxygen atom. Examples of the above compound are octyltrichlor silane, decyltrichlor silane, nonyltrichlor silane, 4-isopropylphenyl- trichlor silane, 4-tert-buthylphenyl trichlor silane, dimethylchlor silane, dipentyldichlor silane, dihexyldichlor silane, dioctyldichlor silane, dinonyldichlor silane, deciledichlor silane, didodecyldichlor silane, 4-tert-buthylphenyloctyldichlor silane, dioctyldichlor silane, didecenyldichlor silane, dinonenyldichlor silane, di-2-ethylhexyldichlor silane, di-3,3-dimethyl pentyldichlor silane, trimethylchlor silane, trihexylchlor silane, trioctylchlor silane, tridecylchlor silane, dioctylchlor silane, octyldimethylchlor silane and 4-isopropylphenyl diethylchlor silane.

[0030] Of these hydrophobic silicas, preferred is one having on its surface polydimethyl siloxane group. The presence of polydimethyl siloxane group enables to provide the coloring particles an improved chargeability under high humidity.

[0031] The hydrophobic silica is preferably present in an amount of from 0.1 to 2 wt% per main particle. If an amount of hydrophobic silica added to main particles is below 0.1 wt%, the obtained coloring particles are likely to have a decreased chargeability On the other hand, if an amount of hydrophobic silica added to main particles is above 2 wt%, a larger part of the silica is likely to separate from the surface of the coloring particles to adhere to a photoreceptor surface, resulting in a so-called photoreceptor filming.

[0032] The primary particles of the magnetic powder used in the present invention preferably have a mean particle diameter of from 100 to 1000 nm. If the diameter of the magnetic particles is below 100 nm, the magnetic particles have a decreased abrasive property in the aspect of removing an oxidation layer on the surface of an amorphous silicon photoreceptor drum, resulting in a blurring. If the diameter of the magnetic particles is above 1000 nm, the magnetic particles are likely to adhere to the surface of the photoreceptor drum, resulting in black spots. In addition, magnetic particles having such large diameter hinder contact of coloring particles with carrying particles or the other coloring particles to decrease the charging of coloring particles. Furthermore, magnetic particles having such large diameter increase an amount of toner present in a developer. These decreased chargeability and the excessive amount of the toner cause toner scattering or uncontrolled toner attraction under high temperature and high humidity. More suitable mean particle size is from 300 to 800 nm. In the present invention, the mean particle size of the primary particle of the magnetic powder was measured with microscopy.

[0033] The term "black spots" means the following phenomenon: coloring particles are fused to the surface of a photoreceptor; other coloring particles easily adhere on the fused coloring particles; and such other particles are transferred on a paper sheet as black spots. The term "toner scattering" means coloring particles with poor chargeability scattered within an image forming apparatus by the centrifugal force exerted on a rotating magnetic brush. The presence of the scattered coloring particles on a path through which a transfer paper sheet is conveyed is likely to cause contamination of the rear face of the transfer paper. The term "uncontrolled toner attraction" means the following phenomenon: when a toner image is developed by poor charged coloring particles, coloring particles near to a charging brush are liable to be attracted to the charging brush.

[0034] The magnetic powder is preferably present in an amount of from 0.1 to 10 wt% per main particle. Below 0.1 wt%, it is likely to cause a decreased image density, toner scattering and blurring. Above 10 wt%, such larger amount of the magnetic powder hinder contact of coloring particles with carrying particles or the other coloring particles to decrease the charging of coloring particles, and also it increases the amount of toner present in a developer. These decreased chargeability and the excessive amount of the toner cause an uncontrolled toner attraction, toner scattering, a decreased image quality, and a decreased fixability.

[0035] Examples of the above magnetic powder are triiron tetroxide (Fe₃O₄), diiron trioxide (γ-Fe₂O₃), iron oxide zinc (ZnFe₃O₄), iron oxide yttrium (Y₃Fe₅O₁₂), iron oxide cadmium (CdFe₂O₄), iron oxide gadolinium (Gd₃Fe₅O₁₂), iron oxide copper (CuFe₂O₄), iron oxide lead (PbFe₁₂O₁₉), iron oxide nickel (NiFe₂O₄), iron oxide neodymium (NdFeO₃), iron oxide barium (BaFe₁₂O₁₉), iron oxide magnesium (MgFe₂O₄), iron oxide manganese (MnFe₂O₄), iron oxide lanthan (LaFeO₃), iron powder (Fe), cobalt powder (Co), and nickel powder (Ni). Particularly suitable magnetic powder is fine particles of triiron tetroxide (magnetite). Suitable magnetite is of regular octahedron.

[0036] The surface of magnetic powder is preferably coated with, for example, a long chain aliphatic compound, in terms of dispersibility and the adhesive properties to the surface of coloring particles. Examples of the long chain aliphatic compound are stearic acid, oleic acid, palmitic acid, hexanoic acid, linoleic acid, ricinoleic acid, fatty dicarboxylic acid having 10 to 22 carbon atoms, and hydroxy containing compounds of these acids, and salts which are composed of a combination of these acids and zinc, magnesium, calcium, cadmium, lead, iron, nickel, cobalt, copper, or aluminum.

[0037] In order that the magnetic powder adhered to the surface of main particles can effectively act to prevent the toner scattering, the strength of magnetization is preferably from 30 to 80 emu/g, when the external magnetic field is 1000 Oe.

[0038] Further, other modifiers can be added to the coloring particles of the present invention, if necessary. There are, for example, aluminum oxide, zinc oxide, titanium oxide, magnesium oxide, calcium carbonate and polymethyl methacrylate. They can be used solely or in a combination of two or more kinds.

[0039] In the case of adding the hydrophobic silica and magnetic powder to the surface of main particles, it is preferable to premix them closely, and add the mixture to main particles, and then mix all the components so sufficiently as to disperse the silica and magnetic powder in the main particles uniformly.

[0040] As an apparatus used in such an addition process, any well-known mixing apparatuses can be employed, such as Henschel's mixer, V-shape mixer, Tumbler's mixer, and Hybritizer.

[0041] The toner of the present invention can be used as it is, as a one-component developer. In this case, if necessary, magnetic powder is incorporated into the main particles to prepare a magnetic toner.

[0042] Also, the toner may be mixed with a carrier so as to be used as a two-component developer. In this case, a toner is preferably present in an amount of from 2 to 20 wt% to the total weight of the developer. Below 2 wt%, the obtained image by using such a developer has too low image density. Above 20 wt%, toner scattering and fog are liable to occur due to the excessive amount of the toner.

[0043] As examples of carrier for the two-component developer, there may be iron powder carrier, ferrite carrier, magnetite carrier. Also, it may be appreciated to use these carriers coated with a suitable resin. Developer containing a resin-coated carrier can give excellent high quality and a prolonged life to a developed image. No special limitations are imposed upon the form of carrier, and it may be in the form of flat, sponge, coin, ball, sphere, etc.

[0044] A toner according to the present invention is applicable to any fixing methods such as heat roll fixing with/without oil, flash fixing and oven fixing, and it is applicable to both cleaning methods of fur brush method and blade method.

[0045] A toner according to the present invention is more advantageous when it is used in combination with an amorphous silicon photoreceptor. However, applications of the toner is not limited thereto. As a photoreceptor suitable for the toner of the present invention, there are, for example, one which has a photosensitive layer containing amorphous silicon, amorphous silicon germanium or amorphous silicon tin; these amorphous ones incorporated by carbon, nitrogen and oxygen atoms; and these amorphous ones doped with an element of the group III a or Va in periodic law table.

[0046] When the toner of the present invention is used in image forming apparatus, it is suitably used for high speed type ones which can copy not less than 50 paper sheets of longitudinal A4 size, per minute.

[0047] The following examples and comparative examples are being supplied to further define the present invention, it being noted that these examples are intended to illustrate and not limit the scope of the present invention. Parts and percentage (%) are by weight unless otherwise indicated.

[0048] First of all, Material Resins A to C were prepared as shown in Table 1. More specifically, Material Resin A was prepared as follows. Dibutyl tin oxide as a polymerization catalyst was added into a mixture of 9.6 mol % of ethylene glycol and 17.8 mol % of neopentyl glycol as fatty alcohol, 19.7 mol % of polyoxypropylene-(2,2)-2,2-bis(4-hydroxyphenyl) propane as an aromatic alcohol, 46.4 mol % of terephthalic acid as a carboxylic acid, and 6.5 mol % of absolute 1,2,4-benzene tricarboxylic acid. This mixture was then placed in a four-mouth flask. To the flask, a stirrer, a condenser, a thermometer, and a gas conduit pipe were attached and then placed in a mantle heater.

[0049] From the gas conduit pipe, nitrogen gas was conducted into the flask to hold it in an inert gas atmosphere. In this state, the flask was heated with stirring to 210 °C for dehydration condensation reaction. When a predetermined Tm was reached, the resulting resin was taken out of the flask and cooled to room temperature to terminate the reaction, thereby obtaining the desired polyester resin A. Its glass transition temperature (Tg) was 68 °C, its softening point (TM) was 142 °C, and its acid value was 50.4. At this time, the reaction product was taken out appropriately as a preliminary experiment, to check the relationship between the softening point (Tm) and the reaction time. Thereby, the softening point of the resulting resin is predictable from the reaction time. The glass transition point and softening point were measured on a "flow tester" manufactured by Shimadzu Corp.

[0050] Material Resins B and C were prepared in the same manner as Material Resin A. Specific prescription and the characteristic features of the resulting resin are given in Table 1.

Table 1

Fatty series alcohol	Resin A	Resin B	Resin C
Ethylene glycol	9.6	14.8	16.2
Neopentyl glycol	17.8	18.9	19.5

Aromatic series alcohol
Polyoxypropylene-(2,2)-2,2-bis(4-hydroxyphenyl) propane	19.7	20.4	21.1

Carboxylic acid
Terephthalic acid	46.4	39.4	36.7
Absolute 1,2,4-benzene Tricarboxylic acid	6.5	6.5	6.5

Resulting Resin's Physical Properties
Glass transition point	68	69	67
Softening point	142	145	149
Acid value	50.4	21.6	15.1

Example 1

[0051] 100 parts of Polyester resin A as a binder resin, 10 parts of carbon black "Morgal L" (manufactured by Cabot Corp.) as a coloring agent, 5 parts of "Biscall 550P" (manufactured by Sanyo Kasei Industries Ltd.) as a mold releasing agent, were all placed in a Henschel's mixer and then mixed. This mixture was subjected to melt kneading by a biaxial extruder and then cooled by a drum flaker. Subsequently, this was roughly pulverized by a hammer mill, finely pulverized by a jet mill, and classified by a pneumatic classifier, to obtain main particles having a mean particle size of 9.0 micron.

[0052] As a surface treatment agent, 0.5 wt% of hydrophobic silica "TG308F" having on its surface polydimethyl siloxane (manufactured by Cabot Corp.) and 0.5 wt% of magnetite whose primary particle has a mean particle size of 700 nm, were added to main particles and mixed with high speed stirring.

[0053] Then, 5 parts of the toner and 95 parts of ferrite carrier (being coated with acrylic resin) were mixed by a ball mill, to prepare a developer. This developer was put into an image forming apparatus "Anesis6050" (manufactured by Mita Industrial Co., Ltd.) equipped with an amorphous silicon photoreceptor, to measure the following characteristic values. The results are given in Table 2.

(i) Image Density:

[0054] The density of a black solid area of the initially copied image was measured on a reflection density meter (Model #TC-6D, Tokyo Denshoku Co., Ltd.). The obtained value was employed as image density.

(ii) Fog Density

[0055] A fog density was obtained by measuring the density of a blank portion of the paper sheet bearing a copy image by a reflection density meter (Model #TC-6D, Tokyo Denshoku Co., Ltd.). The fog density is usually required to be below 0.005.

(iii) Black Spots:

[0056] A copied image was observed visually to evaluate whether black spots are present or not. In Table 2, mark "○" represents the absence of black spots; and mark "X" represents the presence of black spots.

(iv) Uncontrolled toner Attraction:

[0057] A copied image was observed visually to evaluate whether an uncontrolled toner attraction occurs or not. In Table 2, mark "○" represents the absence of an uncontrolled toner attraction; and mark "X" represents the presence of an uncontrolled toner attraction.

(v) Blurring:

[0058] A copied image formed under high humidity (temperature: 35°C, humidity: 85%) was observed visually to evaluate whether a blurring occurs or not. In Table 2, mark "○" represents the absence of blurring; and mark "X" represents the presence of blurring.

(vi) Toner Scattering:

[0059] The presence or absence of contamination on the rear face of copied transfer paper sheets was examined by visual observation. After a continuous copying of 30000 paper sheets, an amount of toner scattered inside of the image forming apparatus was also examined visually. In Table 2, mark "○" represents that little or no toner scattering was observed, and the rear face of the paper was free from contamination; and mark "X" represents that much toner scattering was observed, and the rear face of the paper was contaminated.

(vii) Chargeability:

[0060] A 0.2 g of a sample developer was placed in a Faraday cage and nitrogen was sprayed for 30 seconds at a pressure of 1 kg/cm², to measure chargeability of the toner contained the sample developer by using a Blow-Off Charge Measuring Device (manufactured by Toshiba Chemical corp.). The measurement was performed under an ordinary condition (temperature: 20°C, humidity: 60%) and under high humidity condition (temperature: 35°C, humidity: 85%).

Example 2

[0061] Toner was prepared in the same manner as in Example 1, except for the use of Polyester resin B as a binder resin. The same evaluations were made and the results are given in Table 2.

Example 3

[0062] Toner was prepared in the same manner as in Example 1, except for the use of hydrophobic silica "R812S" having on its surface polydimethyl siloxane (manufactured by Nippon Aerosil Co., Ltd.), as a finishing agent. The same evaluations were made and the results are given in Table 2.

Example 4

[0063] Toner was prepared in the same manner as in Example 1, except for the use of hydrophobic silica "R972" having on its surface polydimethyl siloxane (Nippon Aerosil Co. Ltd.), as a surface treatment agent. The same evaluations were made and the results are given in Table 2.

Example 5

[0064] Toner was prepared in the same manner as in Example 1, except for the use of magnetite whose primary particle has a mean particle size of 100 nm. The same evaluations were made and the results are given in Table 2.

Example 6

[0065] Toner was prepared in the same manner as in Example 1, except for the use of magnetite whose primary particle has a mean particle size of 1000 nm. The same evaluations were made and the results are given in Table 2.

Comparative Example 1

[0066] Toner was prepared in the same manner as in Example 1, except for the use of Polyester resin C as a binder resin. The same evaluations were made and the results are given in Table 2.

Comparative Example 2

[0067] Toner was prepared in the same manner as in Example 1, except for the use of magnetite whose primary particle has a mean particle size of 60 nm. The same evaluations were made and the results are given in Table 2.

Comparative Example 3

[0068] Toner was prepared in the same manner as in Example 1, except for the use of magnetite whose primary particle has a mean particle size of 1300 nm. The same evaluations were made and the results are given in Table 2.

Table 2

	Ex.1	Ex.2	Ex.3	Ex.4	Ex.5	Ex.6	Comp. Ex.1	Comp. Ex.2	Comp. Ex.3
Resin used	Resin A	Resin B	Resin A	←	←	←	Resin C	Resin A	←
Carbon black "Morgal L"	10 parts	←	←	←	←	←	←	←	←
Wax "550P"	5 parts	←	←	←	←	←	←	←	←
Silica Kind	TG308F	←	R812S	R972	TG308F	←	←	←	←
Particle size (nm)	14	←	7	16	14	←	←	←	←
Magnetic powder Particle size (nm)	700	700	700	700	100	1000	700	60	1,300
Image density	1.45	1.41	1.40	1.38	1.37	1.47	1.27	1.27	1.52
Fog density	0.001	0.001	0.003	0.001	0.004	0.004	0.007	0.009	0.005
Black spots	"○"	"○"	"○"	"○"	"○"	"○"	"○"	"○"	X
Uncontrolled toner Attraction	"○"	"○"	"○"	"○"	"○"	"○"	"○"	"○"	X
Blurring	"○"	"○"	"○"	"○"	"○"	"○"	"○"	X	"○"
Toner scattering	"○"	"○"	"○"	"○"	"○"	"○"	X	"○"	X

Chargeability (µC/g)
20°C/60%	-25.0	-24.4	-20.5	-21.7	-24.4	-22.5	-29.1	-24.7	-16.3
35°C/85%	-22.8	-22.0	-20.8	-20.0	-23.0	-19.8	-27.2	-23.8	-13.1

[0069] Examples 1 to 6 gave good results in all the evaluation items of image density, fog density, black spots, uncontrolled toner attraction, blurring, toner scattering, and chargeability of toner.

[0070] In Comparative Example 1 using Polyester resin C with an acid value of 15.1, the chargeability of toner was too high in both conditions, thus failing to satisfy the required image properties such as image density of above 1.30 and fog density of below 0.005. It also did not meet the toner scattering criteria.

[0071] In Comparative Example 2 in which the primary particle of magnetite had a mean particle size of 60 nm that is below 100 nm, the copies did not satisfy the required image density and fog density. There was also observed a blurring due to the presence of an oxidation layer on the surface of the amorphous silicon photoreceptor, since the surface of the photoreceptor was not abraded sufficiently by such small magnetite particles.

[0072] In Comparative Example 3 in which the primary particle of magnetite had a mean particle size of 1300 nm that exceeds 1000 nm, good results were given in image density, fog density and blurring, hit particle size of the magnetite was too large and thus the magnetite was liable to separate from the surface of the main particles. This caused black spots, as well as uncontrolled toner attraction and toner scattering.

Claims

1. A toner comprising coloring particles, each coloring particle including:

a main particle having:

coloring elements; and

a binder resin containing a polyester resin having an acid value of not less than 20 for binding coloring elements;

particles made of a hydrophobic silica and attached on a surface of the main particle, primary particles of the hydrophobic silica particles having a mean particle size of from 7 to 20 nm; and

magnetic particles attached on a surface of the main particle, primary particles of the magnetic particles having a mean particle size of from 100 to 1000 nm.

2. A toner according to claim 1, wherein the polyester resin has a glass transition temperature of from 45 to 90 °C.

3. A toner according to claim 1 or 2, wherein each hydrophobic silica particle has a polydimethyl siloxane group on a surface thereof.

4. A toner according to claim 1, 2 or 3, wherein the hydrophobic silica particles are present in an amount of from 0.1 to 2 wt% to the main particle.

5. A toner according to one or more of claims 1 to 4, wherein the magnetization strength of the magnetic particles is from 30 to 80 emu/g when the strength of the external magnetic field is 1000 Oe.

6. A toner according to one or more of claims 1 to 5, wherein the magnetic particles are present in an amount of from 0.1 to 10 wt% to the main particle.

7. A toner according to one or more of claims 1 to 6, wherein each magnetic particle comprises a triiron tetroxide.

8. A toner according to one or more of claims 1 to 7, wherein each coloring particle further comprises a mold releasing agent including an olefin resin having a number-average molecular weight of from 1000 to 10000.

9. A toner according to anyone of claims 1 to 8, wherein the toner is for use in an amorphous silicon photoreceptor.

10. A developer comprising:

carrying particles; and

coloring particles, each coloring particle including:

a main particle having:

coloring elements; and

a binder resin containing a polyester resin having an acid value of not less than 20 for binding coloring elements;

particles made of a hydrophobic silica and attached on a surface of the main particle, primary particles of the hydrophobic silica particles having a mean particle size of from 7 to 20 nm; and

magnetic particles attached on a surface of the main particle, primary particles of the magnetic particles having a mean particle size of from 100 to 1000 nm.

11. A developer according to claim 10, wherein each carrying particle is coated by a resin.

12. A developer according to claim 10 or 11, wherein the polyester resin has a glass transition temperature of from 45 to 90 °C.

13. A developer according to claim 10, 11 or 12, wherein each hydrophobic silica particle has a polydimethyl siloxane group on a surface thereof.

14. A developer according to one or more of claims 10 to 13, wherein the hydrophobic silica particles are present in an amount of from 0.1 to 2 wt% to the main particle.

15. A developer according to one or more of of claims 10 to 14, wherein the magnetization strength of the magnetic particles is from 30 to 80 emu/g when the strength of the external magnetic field is 1000 Oe.

16. A developer according to one or more of claims 10 to 15, wherein the magnetic particles are present in an amount of from 0.1 to 10 wt% to the main particle.

17. A developer according to one or more of claims 10 to 16, wherein each magnetic particle comprises a triiron tetroxide.

18. A developer according to one or more of claims 10 to 17, wherein each coloring particle further comprises a mold releasing agent including an olefin resin having a number-average molecular weight of from 1000 to 10000.

19. A developer according to anyone of claims 10 to 18, wherein the developer is for use in an amorphous silicon photoreceptor.