Magnetic iron oxide particles

Abstract: Magnetic iron oxide particles, suitable for use as magnetic particles for magnetic recording media, comprise magnetic iron oxide core particles containing ferrous iron, a first coating layer of a Zn compound on the surface of the said core particles and a second coating layer of a Si compound on the said first coating layer.

Description: The present invention relates to the magnetic iron oxide particles, to their preparation and to magnetic recording media containing them. With progress in miniaturization and weight reduction of magnetic recording and reproducing devices in recent years,the necessity has arisen keenly for higher quality and performance of recording media such as magnetic tapes and discs. Especially, demands are heard for higher recording density, higher sensitivity,higher output capacity, and higher magnetic and chemical stability of the magnetic media. Ferromagnetic particles comprising metallic iron have been proposed for use in magnetic recording media. EP-A-0041727 describes a process for preparing such ferromagnetic particles from iron oxide particles provided with a first coating layer containing at least one compound chosen from compounds of aluminum, zinc and alkaline earth metals and a second coating layer containing at least one silicon compound. EP-A-0305819 is also concerned with the preparation of ferromagnetic metal particles composed mainly of iron. In this case, however, the particles are prepared from iron oxide particles provided first with an inner coating layer of a silicon compound and second with an outer coating containing a nonferrous transition metal element. For satisfying requirements for magnetic recording media, magnetic iron oxide particles have also been proposed. Such particles for recording media essentially need to be fine in size, to have a high coercive force and also to be stable magnetically and chemically. This fact is referred to in much literature. For instance, "Development of Magnetic Materials and High Dispersion Techniques for Magnetic Powders" (1982) published by Sogo Gijutsu Center Co., Ltd. states on page 310: "Since the main themes of improvement on magnetic tape performance were the realization of higher sensitivity, higher output capacity and lower noise level, the studies have been directed to the attainment of higher coercive force and smaller size of acicular γ-Fe₂O₃ particles". Also, Japanese Patent Publication No. 55-6580 states: "There is seen a tendency of recording signals to transfer to the shorter wavelength region in recent years, such a tendency being particularly conspicuous in video cassette recorders. In other words, demands is rising for higher-density recording, higher-output capacity and, in particular, improvement of frequency characteristics and magnetic stability. The properties of the magnetic material for satisfying these requirements for the magnetic recording media are magnetic stability and a high coercive force (Hc)". Among the known magnetic iron oxide particles, magnetite particles have the advantages that they are high in coercive force and saturated flux density in comparison with maghemite particles, and that when a magnetic recording medium is made by using magnetite particles, such recording medium is hardly electrically charged because of high electroconductivity. As a typical example of magnetic iron oxide particles having high coercive force, there are known so-called Co-modified magnetic iron oxide particles which have been produced by coating on or substituting the surfaces of magnetite or maghemite particles as precursor particles with a Co compound. In preparation of such Co-modified magnetic iron oxide particles, it is commonly practiced to incorporate ferrous along with Co in the particles, when coating on or substituting their surfaces with a Co compound, for further enhancing the coercive force. Thus, the magnetic iron oxide particles which are fine in size, high in coercive force, and also magnetically and chemically stable are the most eagerly required in the art at present. However, known magnetite particles or Co-modified magnetic iron oxide particles, although having high coercive force as mentioned above, have the defect that they are magnetically and chemically unstable due to incorporation of ferrous. That is, when the magnetic iron oxide particles containing ferrous are left in the air, ferrous contained therein is oxidized into ferric, thereby deteriorating magnetic properties, especially reduction of coercive force, with the passage of time. This phenomenon tends to become more marked as the particle size becomes smaller. Recently the problem has also been pointed out that, when magnetic iron oxide particles containing ferrous are used as magnetic tape coating material, the colour tone of the coat is changed from normal black into dark brown due to oxidation of ferrous in the said particles into ferric to cause an increase of light transmittance, resulting in inducing improper running of magnetic tape in recording divices such as a video deck. It has now been found that magnetic iron oxide core particles containing ferrous and having their surfaces coated with a lower layer of a Zn compound and an upper layer of a Si compound are satisfactorily fine in size, high in coercive force and also magnetically and chemically stable. Accordingly, the present invention provides magnetic iron oxide particles comprising magnetic iron oxide core particles containing ferrous, a first coating layer of a Zn compound on the surface of the core particles and a second coating layer of a Si compound on the first coating layer, the magnetic iron oxide core particles containing ferrous having a particle diameter of not more than 0.40 µm and being magnetite (FeO x .Fe₂O₃, O<x≦1) particles or Co-modified magnetite or maghemite particles having their surfaces coated or modified with Co and Fe(II). The invention also provides a process for the preparation of the magnetic iron oxide particles according to the invention which process comprises coating the surface of magnetic iron oxide core particles containing ferrous with a first coating layer of a Zn compound and coating the first coating layer with a second coating layer of a Si compound, the magnetic iron oxide core particles containing ferrous having a particle diameter of not more than 0.40 µm and being magnetite (FeO x .Fe₂O₃, O<x≦1) particles or Co-modified magnetite or maghemite particles having their surfaces coated or modified with Co and Fe(II). In an embodiment of this process, the magnetic iron oxide core particles containing ferrous are mixed with a solution of a Zn compound to cause the Zn compound to be adsorbed on the surface of the core particles and the particles thus coated with the Zn compound are mixed with a solution of a Si compound to cause the Si compound to be adsorbed on the surface of the particles coated with the Zn compound. The invention further provides a magnetic recording medium in which the magnetic particles are magnetic iron oxide particles according to the invention or magnetic iron oxide particles prepared according to the process of the invention. In the accompanying drawing: FIG. 1 is a graph showing change in content of ferrous iron in the Co-modified magnetite particles when the said particles were left under the conditions of 60°C and 90% RH. In the graph, A to F represent the patterns of change in the Co-modified magnetite particles of Example 2, Example 1, Comparative Example 7, Comparative Example 4, Comparative Example 3 and Comparative Example 1, respectively. The term "magnetic iron oxide core particles containing ferrous" used in the present invention means magnetite (FeO x .Fe₂O₃, O < x ≦ 1) particles, and Co-modified magnetite or maghemite particles having their surfaces coated or modified with Co and Fe (II). The Zn compound of the lower first layer on the surface of the magnetic iron oxide particles according to the present invention includes zinc sulfate, zinc nitrate, zinc chloride, zinc acetate, zinc oxides, zinc hydroxides and the like. Preferably the Zn compound of the first coating layer is present in an amount of from 0.1 to 5% by weight, calculated as Zn, based on the magnetic iron oxide core particles containing ferrous. When this amount is less than 0.1% by weight, the desired effect tends not to be obtained. On the other hand, when the amount exceeds 5% by weight, although there can be obtained the magnetically and chemically stable magnetic iron oxide particles, saturation magnetization of the particles tends to lower due to the increase of Zn compound which has no concern with magnetism of the particle. The Si compound of the upper second layer on the surface of the magnetic iron oxide particle according to the present invention includes water glass, colloidal silica, silicon oxide, sodium silicate, potassium silicate and the like. Preferably the Si compound of the second coating layer is present in an amount of from 0.1 to 5% by weight, calculated as SiO₂, based on the magnetic iron oxide core particles containing ferrous. When the amount of Si compound is less than 0.1% by weight, the desired effect tends not to be obtained. When the amount exceeds 5% by weight, although there can be obtained the magnetically and chemically stable magnetic iron oxide particles, saturation magnetization tends to lower because of the increase of Si compound which has no concern with magnetism of the particle. The weight ratio of the first layer to the second layer in the present invention is typically from 0.2:1 to 5:1. When this ratio is less than 0.2:1 or exceeds 5:1, it becomes hard to obtain the desired effect. The Si compound layer (upper coating layer) on the surface of the particles according to the present invention may be a layer which contains a Si compound and a compound of a metal selected from Al, Ca, Zr, Sb, Ti, V, Mg, Ba and Zn. The compounds of metals Al, Ca, Zr, Sb, Ti, V, Mg, Ba and Zn usable in the present invention are the sulfates, nitrates, chlorides oxides, hydroxides, etc., of the said metals. Preferably the Si compound and the compound of a metal selected from Al, Ca, Zr, Sb, Ti, V, Mg, Ba and Zn are present on the surface of the magnetic iron oxide particles in a total amount of from 0.05 to 5% by weight, calculated as SiO₂ plus the said metal, based on the magnetic iron oxide core particles containing ferrous. When the amount is less than 0.05% by weight, the desired effect tends not to be obtained. When the amount exceeds 5% by weight, although there can be obtained the magnetically and chemically stable magnetic iron oxide particles, saturation magnetization of the particle tends to lower due to the increase of Si and specified metal(s) which has no concern with magnetism of the particle. The molar ratio of the Si compound to the said compound of a metal selected from Al, Ca, Zr, Sb, Ti, V, Mg, Ba and Zn is from 1:1 - 10:1. When this ratio is less than 1:1 or exceeds 10:1, it is difficult to obtain the desired magnetically and chemically stable magnetic iron oxide particles containing ferrous. The total amount of Zn compound of the lower layer and Si compound or compounds containing Si and at least one metal selected from Al, Ca, Zr, Sb, Ti, V, Mg, Ba and Zn of the upper layer is preferably not more than 6% by weight, calculated as the sum of the amount of SiO₂ and the amount of metal element(s), based on the weight of the magnetic iron oxide core particles containing ferrous. When the total amount of the lower and upper layers exceeds 6% by weight, although there can be obtained the chemically stable magnetic iron oxide particles, saturation magnetization of the particles tends to lower due to the increase of non-magnetic compounds in the particle surface. As seen from the Examples and Comparative Examples described later, it has been confirmed that the desired effect of the present invention cannot be obtained either in case of coating the particle surface with a single Zn compound layer or Si compound layer, or in case of coating the particle surface with double layers in which the lower layer is an Si compound layer and the upper layer is a Zn compound layer. However, there can be obtained a magnetically and chemically stable magnetic iron oxide particles containing ferrous when the particle surface is coated with double layers in which the lower layer is a Zn compound layer and the upper layer is an Si compound layer. FIG. 1 is a graph showing the change of content of ferrous in the Co-modified magnetite particles when they were left under the conditions of 60°C and 90% RH. In the graph, curves A and B represent the change of ferrous content in the Co-modified magnetite particles obtained in Example 2 and Example 1, respectively; curve E represents the change of ferrous content in the Co-modified magnetite particles obtained in Comparative Example 3, and curve D represents the change of ferrous content in the Co-modified magnetite particles obtained in Comparative Example 4. As seen from the FIG. 1, the Co-modified magnetite particles having a double-layer coating comprising a lower layer composed of a zinc hydroxide and an upper layer composed of silicon oxide are very chemically stable in comparison with the Co-modified magnetite particles having their surfaces coated with a single layer of a zinc hydroxide or silicon oxide. Coating of the particle surfaces with a Zn compound layer in the present invention can be accomplished by, for instance, the following methods: the magnetic iron oxide core particles containing ferrous are mixed and dispersed in a solution of a Zn compound such as zinc sulfate, zinc nitrate, zinc chloride, zinc acetate, zinc oxides or zinc hydroxides so that the Zn compound is adsorbed on the particle surfaces; a solution of a Zn compound such as zinc sulfate, zinc nitrate, zinc chloride, zinc acetate or the like is neutralized with an alkali and the magnetic iron oxide core particles containing ferrous are mixed and dispersed in the resultant solution so that zinc hydroxides is deposited on the particle surfaces. Coating of the particle surfaces with an Si compound layer or a layer of compounds containing Si and at least one metal selected from Al, Ca, Zr, Sb, Ti, V, Mg, Ba and Zn in the present invention can be accomplished by, for instance, the following methods: the magnetic iron oxide core particles containing ferrous and coated with Zn compound layer are mixed and dispersed in a solution of an Si compound such as water glass, colloidal silica, silicon oxide, sodium silicate, potassium silicate etc., or a mixed solution of such an Si compound and compound(s) of one or more of the said metals such as sulfate, nitrate, chloride, oxide or hydroxide of the said metals, so that the Si compound or the compounds containing Si and the specified metal(s) is (are) adsorbed on the particle surfaces; a solution of an Si compound or a mixed solution of Si and specified metal(s) is neutralized with an acid or alkali and the magnetic iron oxide core particles containing ferrous and coated with Zn compound layer are mixed and dispersed in the resultant solution so that silicon oxide, or an oxide or hydroxide containing Si and specified metal(s) is (are) deposited on the particle surfaces. When carrying out the coating treatment with a Zn compound in the present invention, it is preferable to have the particles dispersed well in the solution by previously adding a dispersing agent into an aqueous suspension of the magnetic iron oxide core particles containing ferrous. As the dispersing agent, iron sol, alumina sol, zirconia sol and the like can be effectively used. The magnetic iron oxide particles according to the present invention have the following properties: the particle shape is typically acicular, spindle, cubic, octahedron, spherical or plate-like; the particle diameter is not more than 0.40 µm, preferably 0.05 to 0.35 µm; in the case of acicular particles, typically the major axial diameter is not more than 0.40 µm, preferably 0.05 to 0.35 µm, and the axial ratio (major axial diameter/minor axial diameter) is not less than 5.0, preferably not less than 6.0; the coercive force (Hc) in the case of acicular particles non Co-modified is typically 300-400 Oe and the coercive force (Hc) in the case of acicular particles Co-modified is typically 400-1200 Oe, preferably 600-900 Oe; the coercive force (Hc) in the case of cubic particles non Co-modified is typically 70-110 and the coercive force (Hc) in the case of cubic particle Co-modified is typically 100-900 Oe, preferably 100-600 Oe; the coercive force (Hc) in the case of octahedron particles non Co-modified is typically 110-140 Oe and the coercive force (Hc) in the case of octahedron particles Co-modified is typically 150-950 Oe, preferably 150-650 Oe; the coercive force (Hc) in the case of spherical particles non Co-modified is typically 40-80 Oe and the coercive force (Hc) in the case of spherical particles Co-modified is typically 50-700 Oe, preferably 50-500 Oe; the reduction in coercive force after a lapse of 7 days is typically not more than 2%, preferably not more than 1.5%; and the reduction of Fe²⁺ after a lapse of 7 days is typically less than 20 % by weight, preferably less than 10% by weight. The magnetic iron oxide particles according to the present invention are suited for use as magnetic iron oxide particles for producing magnetic recording media with high recording density, high sensitivity and high output capacity. Further, since the magnetic iron oxide particles according to the present invention are magnetically and chemically stable, and a color tone change thereof from black into dark brown is prevented, they can be used as magnetic iron oxide particles for electrophography. [EXAMPLES] The present invention will be more precisely explained while referring to Examples as follows. In the following Examples and Comparative Examples, the major axial diameter of particles and the axial ratio (major axial diameter/minor axial diameter) were shown by the average of the values determined from electron micrographs. The contents of Zn, Si, Al, Ca and Co were determined by a fluorescent X-ray analysis, and Fe²⁺ was measured according to a chemical analytical method. The magnetic properties were determined by applying an external magnetic field of up to 10 K0e by using a sample vibrating type magnetometer Model VSM-3S-15 manufactured by Toei Kogyo K.K. The initial Fe²⁺ content in the magnetic iron oxide particles containing ferrous was shown by the value determined from the particles obtained after drying in high-purity nitrogen at 60°C for 24 hours. The change of Fe²⁺ content with the passage of time was shown by the values determined in the air under the conditions of 60°C and 90% RH. EXAMPLE 1 500 g of Co-modified acicular magnetite particles having a major axial diameter of 0.2 µm, an axial ratio (major axial diameter/minor axial diameter) of 7 : 1, an Fe²⁺ content of 16.7% by weight, a Co content of 4.24% by weight and a coercive force of 850 Oe were mixed and dispersed in 5 litres of an 8.6 x 10⁻³ mol/l aqueous solution of zinc chloride (equivalent to 0.56% by weight, calculated as Zn, based on the core particle). Then NaOH solution was added to the resultant dispersion to adjust its pH to 7.0, thereby forming a zinc hydroxides layer on the particle surfaces, and then the thus obtained particles were filtered and dried by the usual methods to obtain black particles. The amount of Zn present on the surface of the obtained black particle, as measured by fluorescent X-ray analysis, was 0.55% by weight. To an aqueous suspension containing 100 g of the said black particles was added sodium hydroxide to adjust pH of the suspension to 11.0, followed by addition of 6 g of #3 water glass (equivalent to 1.74% by weight, calculated as SiO₂, based on the core particle). Then the suspension was stirred and mixed up, and sulfuric acid was added to the resultant suspension to adjust pH to 7.0, thereby forming an SiO₂ layer on the surface of the acicular magnetite particle coated with zinc hydroxides. The suspension containing the thus obtained sediment of black particles was filtered, washed with water and dried in N₂ gas at 60°C for 24 hours, all in the usual methods. The amount of SiO₂ present on the thus obtained black particle surface, as measured by fluorescent X-ray analysis, was 1.71% by weight. The results of chemical analysis also showed that the particle had an Fe²⁺ content of 16.4% by weight and a coercive force of 846 Oe. The change with the passage of time (of Fe²⁺ content and coercive force) of the obtained black particles when left under the environmental conditions specified above was as shown by curve B in FIG. 1 and in Table 2, which indicates very excellent chemical and magnetic stability of the particles. EXAMPLES 2 - 4 and COMPARATIVE EXAMPLES 1 - 6 Coated particles were obtained by following the same procedure as Example 1 except for the change of the kind of core particle, the kind and amount of Zn compound, the kind and amount of Si compound and the kind and amount of specified metal used. The change with the passage of time (of Fe²⁺ content and coercive force) of the Co-modified acicular magnetic iron oxide particles which had no coating-treatment and of the coated particles described above was as shown in Table 2. Also, the change of Fe²⁺ content in the particles of Example 2, Comparative Example 1, Comparative Example 3 and Comparative Example 4 when they were left under the specified environmental conditions was as shown in FIG. 1, in which curves A, F, E and D represent the particles of Example 2, Comparative Example 1, Comparative Example 3 and Comparative Example 4, respectively. As seen from Table 2 and FIG. 1, the magnetic iron oxide particles of Examples 2 to 4 are very excellent chemical and magnetic stability. In contrast, the magnetic iron oxide particles of Comparative Examples 1 to 6 were very poor in chemical and magnetic stability. COMPARATIVE EXAMPLE 7 Coated particles were obtained in the same procedure as Example 1 except that the order of coating-treatment for forming the Zn compound layer and the Si compound layer was reversed. The change with the passage of time of Fe²⁺ content and coercive force of these particles when left under the specified environmental conditions is shown in Table 2. Also, the change of content Fe²⁺ content in the particles when left under said conditions is represented by curve C in FIG. 1. As seen from Table 2 and FIG. 1, the magnetic iron oxide particles of Comparative Example 7 were poor in chemical and magnetic stability.

Claims: Magnetic iron oxide particles comprising magnetic iron oxide core particles containing ferrous, a first coating layer of a Zn compound on the surface of the core particles and a second coating layer of a Si compound on the first coating layer, the magnetic iron oxide core particles containing ferrous having a particle diameter of not more than 0.40 µm and being magnetite (FeO x .Fe₂O₃, O<x≦1) particles or Co-modified magnetite or maghemite particles having their surfaces coated or modified with Co and Fe(II). Particles according to claims 1, wherein the Zn compound of the first coating layer present in an amount of from 0.1 to 5% by weight, calculated as Zn, based on the core particles. Particles according to claim 1 or 2, wherein the Si compound of the second coating layer is present in an amount of from 0.1 to 5% by weight, calculated as SiO₂, based on the core particles. Particles according to any one of the preceding claims, wherein the weight ratio of the first coating layer to the second coating layer is from 0.2:1 to 5:1. Particles according to any one of the preceding claims, wherein the first and second coating layers are present in a total amount of not more than 6% by weight based on the core particles. Particles according to any one of the preceding claims, wherein the second coating layer contains a Si compound and a compound of a metal selected from Al, Ca, Zr, Sb, Ti, V, Mg, Ba and Zn. Particles according to claim 6, wherein the Si compound and the compound of a metal selected from Al, Ca, Zr, Sb, Ti, V, Mg, Ba and Zn are present in a total amount of from 0.05 to 5% by weight, calculated as Sio₂ plus the said metal, based on the core particles. Particles according to claim 6 or 7, wherein the molar ratio of the Si compound to the said compound of a metal selected from Al, Ca, Zr, Sb, Ti, V, Mg, Ba and Zn is from 1:1 to 10:1. A process for the preparation of particles as claimed in claim 1, which process comprises coating the surface of magnetic iron oxide core particles containing ferrous with a first coating layer of a Zn compound and coating the first coating layer with a second coating layer of a Si compound, the magnetic iron oxide core particles containing ferrous having a particle diameter of not more than 0.40 µm and being magnetite (FeO x .Fe₂O₃, O<x≦1) particles or Co-modified magnetite or maghemite particles having their surfaces coated or modified with Co and Fe(II). A process according to claim 9, wherein the magnetic iron oxide core particles containing ferrous are mixed with a solution of a Zn compound to cause the Zn compound to be adsorbed on the surface of the core particles and the particles thus coated with the Zn compound are mixed with a solution of a Si compound to cause the Si compound to be adsorbed on the surface of the particles coated with the Zn compound. A magnetic recording medium in which the magnetic particles are magnetic iron oxides particles as claimed in any one of the claims 1 to 8 or which have been prepared by a process as claimed in claim 9 or 10.

Inventors: HAYASHI KAZUYUKI, ITAMOCHI HIROKO, IWASAKI KEISUKE, TANAKA YASUYUKI, HAYASHI, KAZUYUKI, ITAMOCHI, HIROKO, IWASAKI, KEISUKE, TANAKA, YASUYUKI
Applicants: TODA KOGYO CORP, TODA KOGYO CORP.
Application year: 1990
Main classification: G11B5
IPC codes: G11B5, H01F1, C01G49