Magnetic toner particles containing iron as the main component and process for producing the same

Abstract: Magnetic particles are disclosed which contain iron as the main component, have an average particle diameter of 0.1 to 3.0µm and a liquid absorption of not more than 18 ml. The particles are useful in magnetic toners, for example in copying machines.

Description: BACKGROUND OF THE INVENTION The present invention relates to magnetic particles containing iron as the main component, which have a good affinity for a vinyl aromatic resin, an acrylic resin and a copolymer of monomers thereof generally used for a magnetic toner, and an excellent mixing property with these resins for a magnetic toner, and process for producing the same. A developing method using composite particles obtained by dispersing magnetic particles such as magnetite particles into a resin as a developer without using a carrier, which are generally called a one-component type magnetic toner, is conventionally known and used as one of a method of developing an electrostatic latent image. With the recent development of copying machines having a higher performance such as a capability of continuously copying a higher-quality image at a higher speed, and magnetic toner as a developer is strongly required to improve the properties and for this purpose, magnetic particles which have an excellent mixing property with a magnetic toner resins are strongly in demand. This fact is described in Japanese Patent Application Laid-Open (KOKAI) No. 55-65406 (1980) as "Generally, magnetic particles for a magnetic toner of such as one-component type are required to have the following properties. ... VII) To have an excellent mixing property with a resin. Ordinarily, the particle diameter of a toner is not more than several ten µm, and the microscopic degree of mixing in the toner is an important property of the toner. ..." Magnetite particles having an isotropic shape such as an octahedron and a sphere are mainly used as magnetic particles for a magnetic toner, which are added to a vinyl aromatic resin such as styrene resin and vinyl toluene resin, an acrylic resin such as acrylic acid resin and a methacrylic acid resin, and a copolymer of the monomers thereof, which are used as resins for a magnetic toner. JP-A-60-52859 discloses a magnetic toner containing a binder, a magnetic powder, a titanium type coupling agent and a surfactant. JP-A-62-70862 discloses a magnetic toner incorporating magnetic powders which contain 50-100 wt% metallic iron particles, treated on the surface with a silane and/or a titanium coupler. JP-A-55-28019 discloses a magnetic powder covered with a titanium-base coupling agent. US-A-4,450,221 discloses a magnetic toner comprising nuclear particles comprising lyophilic magnetic particles and a resin having a low softening point. The above four documents disclose neither particles with the properties of those of the present invention, nor the treatment of similar particles with the process of the present invention. JP-A-63-13935 discloses a method for producing a magnetic iron oxide fine powder for magnetic recording, comprising compaction grinding powdery magnetic iron oxide in the presence of a phosphate ester compound. "Powder, Theory and Application": Maruzen K.K. (25.12.1962) discloses methods for classification of crushers. JP-A-61-214227 discloses a process for producing a magnetic tape using a magnetic coating mix which contains uniaxially anisotropic cobalt-containing iron oxide magnetic particles. GB-A-2209842 discloses a magnetic toner which comprises spherical magnetic particles having a specified tap density and linseed oil absorption. JP-A-54-84731 discloses a process for producing a magnetic toner, comprising rendering the surfaces of fine magnetic particles oleophilic, mixing the fine magnetic particles with a binder resin forming material, forming an aqueous dispersion of the mixture, and carrying out suspension polymerization. JP-A-1-251602 discloses a method of treating a magnetic iron oxide powder with a dispersant using a grinder/mixer edge runner. TP-A-57-124404 discloses a process for producing a magnetic iron oxide powder comprising a wet reaction between an iron salt solution and an alkali. Magnetic particles which have an excellent mixing property with a resin are in strong demand at present, but known magnetic particles have poor affinity for resins. No magnetic particles which have an excellent mixing property with a resin have been provided yet. The affinity of particles for a resin in the present invention means the degree of the affinity of the surfaces of magnetic particle for a resin. As an index of the dispersibility of magnetic particles in a resin, the gloss of the surfaces of a resin molding containing the magnetic particles is generally measured, and as the value is higher, the dispersibility is regarded as better. When the present inventor measured the gloss of a resin molding made of known magnetic particles at an incident angle of 60°, most of them were not less than 90%, but when the incident angle was lowered to 20° so as to sense minute projections and dents on the surfaces of the resin molding, the gloss of all the resin molding were reduced to not more than 90%. The present inventor presumed that this phenomenon was caused because the known magnetic particles were present in the resin in the form of agglomerates due to the poor affinity of each particle for the resin, and that if each particle has a superior affinity for the resin, the magnetic particles have an excellent mixing property with the resin so that they are uniformly dispersed in the resin, resulting in a smooth surface of the resin molding, thereby obtaining not less than 90% of gloss even at an incident angle of 20°. That is, it is considered that the gloss measured at an incident angle of 20° is an index of the affinity for a resin and the magnetic particles in a resin molding the surface of which has a gloss of not less than 90% are particles having a good affinity for a resin. As a result of studies undertaken by the present inventors so as to obtain magnetic particles having a good affinity for a resin, it has been found that by kneading, smearing and spatula-stroking magnetic particles containing iron as the main component which have an average particle diameter of 0.1 to 3.0 µm by means of a wheel-type mill or an attrition mill so as to release from an agglomeration thereof, the obtained magnetic particles containing iron as the main component have an average particle diameter of 0.1 to 3.0 µm and a liquid absorption of not more than 18 m l , and a gloss of the surface of a resin molding containing such magnetic particles is not less than 90% when measured at an incident angle of 20°: The present invention has been achieved on the basis of this finding. SUMMARY OF THE INVENTION In a first aspect of the present invention, there are provided magnetic particles suitable for a magnetic toner, which particles contain iron as the main component which have either: (A) an isotropic shape, an axial ratio (major axial diameter/minor axial diameter) of not more than 1.5, an average particle diameter of 0.1 to 1.0 µm and a liquid absorption of not more than 10 ml, and are coated with a titanate or silane coupling agent or a surfactant selected from fatty ester nonionic surfactants and natural fat and oil derivatives; or (B) an acicular or spindle shape, an average major axial diameter of 0.1 to 3.0 µm, an axial ratio of not more than 10 and a liquid absorption of not more than 18 ml, which particles are coated with a titanate or silane coupling agent or a surfactant selected from fatty ester nonionic surfactants and natural fat and oil derivatives; where the liquid absorption is measured in the method wherein (1) a styrene-acrylic resin and xylene are mixed at the resin content: (resin)/(resin + xylene) x 100 of 20 wt%, in a polyester container provided with a cover by using a paint conditioner, thereby obtaining a resin solution; (2) 10 g of magnetic particles containing iron as the main component which are weighed out by an electronic balance are charged in a 100 ml polyester container, to which 50 ml of said resin solution prepared are added dropwise by using a burette and the resultant mixture is stirred with a glass rod; (3) the point at which the first droplet naturally drops from the end of said glass rod as a result of the obtained paste in said polyester container becoming uniform and the fluidity thereof being increased is regarded as the end point; and (4) the amount of resin solution used until said end point is measured is said liquid absorption. In a second aspect of the present invention, there is provided a process for producing magnetic particles as defined in the first aspect suitable for a magnetic toner, said process comprising the step of kneading, smearing and spatula-stroking magnetic particles containing iron as the main component and having an average particle diameter of 0.1 to 1.0 µm when isotropic and of 0.1 to 3.0 µm when acicular or spindle shape, with a titanate or silane coupling agent or a surfactant selected from fatty ester nonionic surfactants and natural fat and oil derivatives, by a wheel-type mill or an attrition mill so as to release particles from an agglomeration thereof. In a third aspect of the present invention, there is provided a magnetic toner comprising magnetic particles as defined in the first aspect and a vinyl aromatic resin, an acrylic resin or a copolymer of monomers thereof. DETAILED DESCRIPTION OF THE INVENTION The magnetic particles of the present invention are classified as follows. (A) Magnetic particles containing iron as the main component have an isotropic shape, an axial ratio (major axial diameter/minor axial diameter) of not more than 1.5, an average particle diameter of 0.1 to 1.0 µm, preferably 0.1 to 0.5 µm, and a liquid absorption of not more than 10 ml as measured in the above-defined method, and are coated with an organic compound having a hydrophobic group which is a titanate or silane coupling agent or a surfactant selected from fatty ester nonionic surfactants and natural fat and oil derivatives. (B) Magnetic particles containing iron as the main component have an acicular or spindle shape, an average major axial diameter of 0.1 to 3.0 µm, preferably 0.1 to 1.0 µm, an axial ratio (major axial diameter/minor axial diameter) of not more than 10, preferably not more than 8, more preferably 1.5 to 7.0, and a liquid absorption of not more than 18 ml, preferably not more than 16 ml, as measured in the above-defined method, and are coated with an organic compound having a hydrophobic group which is a titanate or silane coupling agent or a surfactant selected from fatty ester nonionic surfactants and natural fat and oil derivatives. The reason why the liquid absorption of the magnetic particles (A) and (B) according to the present invention is small is considered to be that when a wheel-type mill or an attrition mill is used, the kneading operation presses the organic compound having a hydrophobic group existent between the magnetic particles to the surfaces of the magnetic particles and spreads the organic compound through the gaps between the particles so as to bring the organic compound into close-contact with the particle surfaces, the smearing operation changes the positions of the particle groups so as to separate the agglomerated particles in a discrete state while spreading the organic compound having a hydrophobic group, the spatula-stroking operation uniformly spreads the organic compound having a hydrophobic group which is existent on the surfaces of the particles with a spatula, and the repetition of these three operations releases from the agglomeration of the magnetic particles without re-agglomeration and makes the surface of each particle highly hydrophobic. As shown in a later-described comparative example, since a Henschel mixer which is a blade-type mill and is ordinarily used for the surface treatment of magnetic particles only has a stirring operation, use of a Henschel mixer does not produce the above-mentioned advantages of the present invention. The magnetic particles (A) according to the present invention are obtained by kneading magnetic particles containing iron as the main component and having an isotropic shape and an average particle diameter of 0.1 to 1.0 µm with an organic compound having a hydrophobic group, kneading, smearing and spatula-stroking the magnetic particles and the organic compound by a wheel-type mill or an attrition mill so as to coat the surfaces of the magnetic particles containing iron as the main component with the organic compound having a hydrophobic group. The magnetic particles (B) according to the present invention are obtained by kneading magnetic particles containing iron as the main ingredient and having an acicular or spindle shape, and average major axial diameter of 0.1 to 3.0 µm and an axial ratio (major axial diameter/minor axial diameter) of not more than 10 with the organic compound having a hydrophobic group, kneading, smearing and spatula-stroking the magnetic particles and the organic compound by a wheel-type mill or an attrition mill so as to coat the surfaces of the magnetic particles containing iron as the main component with the organic compound having a hydrophobic group. As magnetic particles containing iron as the main component, magnetite particles, maghemite particles, magnetite and maghemite particles containing elements other than Fe such as zinc and manganese, and spinel-type ferrite particles containing at least one selected from the group consisting of zinc, manganese and nickel may be exemplified. If magnetic particles containing iron as the main component have an average particle diameter of less than 0.1 µm, the magnetic agglomeration of the magnetic particles becomes so large as to make the dispersion of the magnetic particles in a resin difficult. On the other hand, if the average particle diameter exceeds 3.0 µm, the distribution of the magnetic particles in a resin becomes nonuniform, so that these magnetic particles are unfavorable as magnetic particles of a magnetic toner. In case of the magnetic particles (A) according to the present invention, from the view point of the dispersibility and uniform distribution of the magnetic particles in a resin, the average particle diameter of 0.1 to 0.5 µm is preferable. "Particles having an isotropic shape" means particles in which the ratio of the major axial diameter and the minor axial diameter is not more than 1.5, preferably 1.0 to 1.3, and include not only spherical, hexahedral and octahedral particles but also particles having no definite shapes. In case of the magnetic particles (B) according to the present invention, from the view point of the dispersibility and uniform distribution of the magnetic particles in a resin, an average major axial diameter of 0.1 to 1.0 µm and an axial ratio (major axial diameter/minor axial diameter) of not more than 8, more preferable 1.5 to 7.0 are preferred. "Particles having an acicular or spindle shape" include not only acicular and spindle particles but also rise-type and spheroidal particles. As an organic compound having a hydrophobic group in the present invention, a titanate or silane coupling agent or a surfactant is used. As the titanate coupling agent having a hydrophobic group, isopropyl triisostearoyl titanate, isopropyl tridodecylbenzenesulfonyl titanate, isopropyl tris(dioctylpyrophosphate) titanate, bis(dioctylpyrophosphate) oxyacetate titanate, bis(dioctylpyrophosphate) ethylene titanate and the like are usable. As the silane coupling agent having a hydrophobic group, 3-methacryloxypropyl trimethoxysilane, 3-chloropropyl trimethoxysilane and the like are usable. As the surfactant, fatty ester nonionic surfactants and natural fat and oil derivatives such as alkyl amine and the like are usable. The amount of organic compound having a hydrophobic group added is 0.1 to 10.0 parts by weight, preferably 0.1 to 5 parts by weight, more preferably 0.3 to 5 parts by weight based on 100 parts by weight of magnetic particles. If it is less than 0.1 part by weight, the magnetic particles may be made unsufficiently hydrophobic. If it exceeds 10.0 parts by weight, since the components which do not contribute to the magnetic properties increase, the saturation magnetization of the magnetic particles is reduced, so that the magnetic particles become unfavorable as magnetic particles for a magnetic toner. In the present invention, a wheel-type mill or an attrition mill is used. As the wheel-type mill, a Simpson mix muller, multiple mill, Stotz mill, back-flow mill and Eirich mill. However, a wet pan mill, melanger and whirl mix, which have only the kneading and spatula-stroking operations but do not have a smearing operation, are not applicable. Magnetic toner according to the present invention comprises the magnetic particles and a vinyl aromatic resin, and acrylic resin, or a copolymer of monomers thereof. As the vinyl aromatic resin, styrene resin and vinyl toluene resin may be exemplified. As the acrylic resin, acrylic resin and metharylic resin may be exemplified. As the copolymer, styrene-acrylic resin may be exemplified. In the magnetic toner, the content of the magnetic particles according to the present invention is 20 to 50 wt%. Since the magnetic particles containing iron as the main component according to the present invention have an average particle diameter of 0.1 to 3.0 µm, a saturation magnetization of not less than 70 emu/g, and a liquid absorption of not more than 18 m l , they have a good affinity for a resin, in particular, a vinyl aromatic resin, an acrylic resin and a copolymer of mononers thereof which are generally used for a magnetic toner, and an excellent mixing property with these resins for a magnetic toner. Thus, the magnetic particles according to the present invention are suitable as magnetic particles for a magnetic toner. [Examples] The present invention will now be explained with reference to the following examples and comparative example. It is to be understood, however, that the present invention is not restricted by these examples. The shapes of the particles in the examples and comparative examples were observed by a transmission electron microscope and a scanning electron microscope. The magnetic characteristics of the magnetic particles were measured by using an vibrating sample magnetometer VSM-3S-15 (produced by Toei Kogyo K.K.) applying an external magnetic field of 10 KOe. The gloss of the surface of a resin molding was expressed by the values measured at incident angles of 20° and 60° by using a aigital glossmeter UGV-50 (produced by Suga Shikenki K.K.). Example 1 10 Kg of spherical magnetite particles having an average particle diameter of 0.23 µm, a saturation magnetization of 84.3 emu/g and a coercive force of 52 Oe and 100 g of a silane coupling agent KBM-6000 (produced by Shin-etsu Chemical Industry Co., Ltd) were charged in a Simpson mix muller (produced by Matsumoto Chuzo Co., Ltd.). By one-hour operation of the muller, the surfaces of the spherical magnetite particles were coated with the silane coupling agent. The thus-obtained spherical magnetite particles coated with the silane coupling agent had a liquid absorption of 6.8 m l , and a saturation magnetization and a coercive force thereof were approximately equal to the respective value before treatment. 15 g of the spherical magnetite particles coated with the silane coupling agent and 35 g of a styrene-acrylic resin Hymer TB-1000 (produced by Sanyo Kasei Co., Ltd.) which had been dried at a temperature of 60°C were mulled at a surface temperature of 130°C for 5 minutes by a hot roll. The mulled product obtained was pressed into a sheet by a hot press to produce a sheet-like resin molding. The gloss of the sheet-like resin molding was 96.4% at an incident angle of 60° and 92.0% at an incident angle of 20°. Examples 2 to 5, Comparative Example 1 to 2 Treated magnetic particles were obtained in the same way as in Example 1 except for varying the kinds of magnetic particles which were treated, the kinds and the amount of organic compound having a hydrophobic group and the kinds and the operation time of the machine. The main producing conditions and the properties of the treated magnetic particles are shown in Table 1. Reference Examples 1 to 3 The liquid absorptions and the gloss of the typical articles commercially available as magnetic particles for a magnetic toner were measured in accordance with the present invention. The results are shown in Table 2. All of these magnetic particles have a large liquid adsorption and a small affinity for a resin. Reference Example 4 10 Kg of acicular magnetite particles having an average major axial diameter of 0.25 µm, an axial ratio (major axial diameter/minor axial diameter) of 2.2, a saturation magnetization of 85.0 emu/g and a coercive force of 236 Oe were charged into Simpson mix muller (produced by Matsumoto Chuzo Co., Ltd.). By 0.5 hour operation of the muller, the acicular magnetite particles were treated. The thus-obtained acicular magnetite particles had a liquid absorption of 14.0 m l , and a saturation magnetization and a coercive force thereof were approximately equal to the respective value before treatment. 15 g of the obtained acicular magnetite particles and 35 g of a styrene-acrylic resin Hymer TB-1000 (produced by Sanyo Kasei Co., Ltd.) which had been dried at a temperature of 60°C were mulled at a surface temperature of 130°C for 5 minutes by a hot roll. The mulled product obtained was pressed into a sheet by a hot press to produce a sheet-like resin molding. The gloss of the sheet-like resin molding was 99.2% at an incident angle 60° and 90.8% at an incident angle 20°. Reference Examples 5 to 7 and Comparative Examples 3 to 4 Treated magnetic particles were obtained in the same way as in Reference Example 4 except for varying the kinds of magnetic particles which were treated, the kinds and the operation time of the machine. The main producing conditions and the properties of the treated magnetic particles are shown in Table 3. Example 6 10 Kg of acicular magnetite particles having an average major axial diameter of 0.5 µm, an axial ratio (major axial diameter/minor axial diameter) of 7, a saturation magnetization of 84.2 emu/g and a coercive force of 349 Oe and 30 g of titanate coupling agent Plenact TTS (produced by Ajinomoto Co., Ltd.) were charged into a Shimpson mix muller (produced by Matsumoto Chuzo Co., Ltd.). By one-hour operation of the muller, the surfaces of the acicular magnetite particles were coated with the titanate coupling agent. The thus-obtained acicular magnetite particles coated with the titanate coupling agent had a liquid absorption of 17.5 m l , and a saturation magnetization and a coercive force thereof were approximately equal to the respective value before treatment. 15 g of the acicular magnetite particles coated with the titanate coupling agent and 35 g of a styrene-acrylic resin Hymer TB-1000 (produced by Sanyo Kasei Co., Ltd.) which had been dried at a temperature of 60°C were mulled at a surface temperature of 130°C for 5 minutes by a hot roll. The mulled product obtained was pressed into a sheet by a hot-press to produce a sheet-like resin molding. The gloss of the sheet-like resin molding was 101.5% at an incident angle of 60° and 91.1% at an incident angle of 20°. Example 7 to 12, Comparative Examples 5 to 7 Treated magnetite particles were obtained in the same way as in Example 6 except for varying the kinds of magnetic particles which were treated, the kind and the amount of organic compound having a hydrophobic group and the kind and operation time of the machine. The main producing conditions and the properties of the treated magnetic particles are shown in Table 4. Reference Examples 8 to 9 The liquid absorptions and gloss of the typical articles commercially available as magnetic particles for a magnetic toner were measured in accordance with the present invention, the results are shown in Table 5. All of these magnetic particles have a large liquid absorption and a small affinity for a resin.

Claims: Magnetic particles suitable for a magnetic toner, which particles contain iron as the main component which have either: (A) an isotropic shape, an axial ratio (major axial diameter/minor axial diameter) of not more than 1.5, an average particle diameter of 0.1 to 1.0 µm and a liquid absorption of not more than 10 ml, and are coated with a titanate or silane coupling agent or a surfactant selected from fatty ester nonionic surfactants and natural fat and oil derivatives; or (B) an acicular or spindle shape, an average major axial diameter of 0.1 to 3.0 µm, an axial ratio of not more than 10 and a liquid absorption of not more than 18 ml, which particles are coated with a titanate or silane coupling agent or a surfactant selected from fatty ester nonionic surfactants and natural fat and oil derivatives; where the liquid absorption is measured in the method wherein (1) a styrene-acrylic resin and xylene are mixed at the resin content: (resin)/(resin + xylene) x 100 of 20 wt%, in a polyester container provided with a cover by using a paint conditioner, thereby obtaining a resin solution; (2) 10 g of magnetic particles containing iron as the main component which are weighed out by an electronic balance are charged in a 100 ml polyester container, to which 50 ml of said resin solution prepared are added dropwise by using a burette and the resultant mixture is stirred with a glass rod; (3) the point at which the first droplet naturally drops from the end of said glass rod as a result of the obtained paste in said polyester container becoming uniform and the fluidity thereof being increased is regarded as the end point; and (4) the amount of resin solution used until said end point is measured is said liquid absorption. Magnetic particles according to claim 1 which have an isotropic shape and a particle diameter of from 0.1 to 0.5 µm. Magnetic particles according to claim 1 which have an acicular or spindle shape which have a liquid absorption of not more than 14 ml. Magnetic particles according to claim 1 which have an acicular or spindle shape and have an axial ratio of not more than 8. Magnetic particles according to claim 1 or 4 which have an acicular or spindle shape which have a liquid absorption of not more than 16 ml. A process for producing magnetic particles as defined in claim 1 suitable for a magnetic toner, said process comprising the step of kneading, smearing and spatula-stroking magnetic particles containing iron as the main component and having an average particle diameter of 0.1 to 1.0 µm when isotropic and of 0.1 to 3.0 µm when acicular or spindle shape, with a titanate or silane coupling agent or a surfactant selected from fatty ester nonionic surfactants and natural fat and oil derivatives, by a wheel-type mill or an attrition mill so as to release particles from an agglomeration thereof. A magnetic toner comprising magnetic particles as claimed in any one of claims 1 to 5 and a vinyl aromatic resin, an acrylic resin or a copolymer of monomers thereof. A resin moulding comprising magnetic particles as claimed in any one of claims 1 to 5 which has a gloss value of 90% at an incident angle of 20°. A resin moulding according to claim 8 in sheet form.

Inventors: FUJII YASUHIKO, FUJIOKA KAZUO, KURITA EIICHI, MISAWA HIROMITSU, OKANO YOUZI, FUJII, YASUHIKO, FUJIOKA, KAZUO, KURITA, EIICHI, MISAWA, HIROMITSU, OKANO, YOUZI
Applicants: TODA KOGYO CORP, TODA KOGYO CORPORATION
Application year: 1991
Main classification: G03G9
IPC codes: H01F1, G03G9