ACICULAR MAGNETIC IRON OXIDE PARTICLES, PROCESS FOR PREPARING THE SAME, AND MAGNETIC RECORDING MEDIA USING SUCH PARTICLES

Abstract: Particles for use in magnetic recording media comprise acicular magnetic iron oxide particles coated with an inner layer of a Co compound and an outer layer of a spinel type ferrite layer containing Mn-Zn, Ni-Zn or Mn-Ni-­ Zn.

Description: The present invention relates to acicular magnetic iron oxide particles, their preparation and to magnetic recording media containing them. Recently, with the advancement of long time recording capacities and lightweight and compact designing of magnetic recording and reproducing devices, the necessity of higher performance of magnetic recording media such as magnetic tapes, magnetic discs, etc., has grown increasingly. Especially, improvement of recording density characteristics is a paramount necessity. Generally, magnetic recording media are produced by coating a substrate with a resin composition containing magnetic particles. For improving the recording density characteristics of magnetic recording media, it is necessary to enhance the coercive force of magnetic recording media. For attaining this, it is required that the particles of magnetic material used have as high coercive force as possible. This fact is pointed out in, for instance, "Technological Study Reports of The Institute of Electronics and Communication Engineers of Japan" MR77-36, published by The Institute of Electronics and Communication Engineers of Japan in 1978, which states on page 37: "For increasing the recording density of a magnetic tape, it is necessary to elevate the coercive force of magnetic powder used for the tape.". Presently, so-called Co doped type acicular magnetic iron oxide particles and so-called Co coated type magnetic iron oxide particles are known as magnetic iron oxide particle having high coercive force. These magnetic iron oxide particles have a tendency to be elevated in coercive force as the Co content increases. The Co doped type magnetic iron oxide particles can be produced by first preparing Co-containing acicular goethite particles by adding Co salt to the starting material acicular goethite particles in their preparation reaction and then reducing them to form Co-containing acicular magnetite particles, or, if necessary, further oxidizing the thus obtained magnetite particles to form Co-containing maghemite particles. The Co coated type magnetic iron oxide particles can be obtained by using as precursor particles the acicular magnetite particles obtained by reducing the starting acicular goethite particles or acicular maghemite particles obtained by further oxidizing the magnetite particles, and coating the surfaces of the precursor particles with a Co compound. Since the magnetic recording media are used repeatedly for a long time, it is also strongly required that they are thermally stable, resistant to change with time and also have excellent erasing characteristic. For satisfying these requirements for magnetic recording media, it is necessary that the magnetic properties of the magnetic iron oxide particles used have thermal stability, are resistant to change with time and have excellent erasing characteristic. The magnetic iron oxide particles and magnetic recording media having high coercive force, excellent thermal stability, excellent erasing characteristic and being resistant to change with time are most strongly demanded at present. The afore-mentioned Co doped type magnetic iron oxide particles and the magnetic recording media produced by using the particles have high coercive force, but on the other hand, they have the problem that the coercive force distribution is too wide due to diffusion of Co ions in crystals, resulting in poor thermal stability, poor resistance to change with time and bad erasing characteristic. This phenomenon is mentioned in "Technological Study Reports of The Institute of Electronics and Communication Engineers of Japan" which states: "Co solid-solution type (doped type) magnetic iron oxide powder is liable to change in coercive force thermally and with time, so that it has the serious defect that when a tape is made therefrom, such tape shows poor in print-through and erasing characteristic. Such defect is considered attributable to the fact that Co ions move in crystals even at room temperature.". The Co coated type magnetic iron oxide particles and the magnetic recording media produced by using such particles have high coercive force and are also more stable thermally, resistant to change with time, and improved erasing characteristic than the Co doped type magnetic iron oxide particles. Such a act is noted from "Technological Study Reports of The Insutitute of Electronics and Communication Engineers of Japan" which states: "... the Co epitaxial (Co coated type) magnetic iron oxide powder, because of its double-coated layer structure, is free of these defects and stable thermally and resistant to change with time, and the tape produced by using such magnetic powder has excellent print-through and erasing characteristic.". However, recent technology demands improvements in erasing characteristic, and the Co coated type magnetic iron oxide particles and magnetic recording media produced by using such particles hardly seem to be satisfactory in erasing characteristic, due to a wide spread of coercive force distribution. This fact is referred to in much literature. For instance, Japanese Patent Application Laid-Open (KOKAI) No. 61-17426 (1986) states in its specification: "In the case of magnetic powder using the γ-Fe₂O₃ particles, it was found that the coercive force distribution widens as the γ-Fe₂O₃ particles are reduced in size, and there is a tendency that the coercive force distribution is further widened when the particles are coated with cobalt compound layer. When the cobalt coated type γ-Fe₂O₃ particles are reduced in size for attaining high density recording, although a desired coercive force (Hc) is obtained, the produced magnetic powder has an unfavorable coercive force distribution and is poor in erasing characteristic.". The coercive force distribution of Co coated type magnetic iron oxide particles tends to widen as the amount of Co increases, resulting in worse erasing characteristic of the particles. Thus, there is an inverse correlation between the amount of Co and improvement of coercive force distribution. Under these circumstances, there has been in industry a strong request for the establishment of a process capable of producing Co coated type magnetic iron oxide particles further improved in erasing characteristic while maintaining high coercive force and th magnetic recording media coated with a resin composition containing such particles. The present invention provides acicular magnetic iron oxide particles coated with an inner layer consisting of a Co compound and an outer ferrite layer; characterised in that the outer layer is a spinel type ferrite layer containing Mn-Zn, Ni-Zn or Mn-Ni-Zn and is present in an amount of from 1.0 to 10 % by weight of the acicular magnetic iron oxide particles coated with said Co compound, based on the amount of Mn-Zn, Ni-Zn or Mn-Ni-Zn in the outer layer, and wherein the amount of Mn, Ni or Mn-Ni in the outer layer is from 0.1 to 80 atom % based on the total amount of Mn, Ni and Zn in the outer layer, the acicular magnetic iron oxide particles having an average major axis diameter of 0.5 µm or below and an aspect ratio (major axis:minor axis) of 6:1 or greater and showing a coercive force of 4.77 x 10⁴ Am⁻¹ (600 Oe) or above and an erasing characteristic of 72.0 dB or above. The present invention also provides a process for producing acicular magnetic iron oxide particles, which process comprises; (i) mixing a first water dispersion of acicular magnetic iron oxide particles as core particles with at least an aqueous solution of a Co salt and an aqueous alkaline solution to prepare thereby a second dispersion having a pH of 11 or above, (ii) heat-treating said second dispersion at a temperature of from 50 to 100°C to form thereby a layer of a Co compound on the surfaces of said acicular magnetic iron oxide particles, (iii) adding a Fe (II) salt and (a) a Mn salt and a Zn salt, (b) a Ni salt and a Zn salt or (c) a Mn salt, a Ni salt and a Zn salt to the heat-treated second dispersion; and (iv) heat-treating the dispersion from step (iii) at a temperature of from 50 to 100°C to form thereby a spinel type ferrite layer containing Mn-Zn, Ni-Zn or Mn-Ni-Zn on said Co compound layer. The present invention also provides a magnetic recording medium comprising a substrate coated with a resin containing from 60 to 90 % of magnetic iron oxide particles of the present invention or particles which have been prepared by the process of the present invention. A marked improvement of erasing characteristic can be realized while maintaining high coercive force by use of acicular magnetic iron oxide particles of the present invention. The magnetic recording medium of the present invention produced by coating a substrate with a resin composition containing Co coated type acicular magnetic iron oxide particles having especially a Ni-containing as a spinel type ferrite layer, has an improved erasing characteristic and also enhanced coercive force. Further, according to the present invention, the magnetic recording media produced by coating a substrate with a resin composition containing the Co coated type acicular magnetic iron oxide particles of the present invention which have undergone a heat-treatment at a temperature of 100-200°C after coating treatment with a spinel type ferrite layer can be further enhanced in coercive force while maintaining the excellent erasing characteristic. The magnetic iron oxide particles coated with a Co compound layer followed by further coating with spinel type ferrite are disclosed in JP-A-60-165703 (KOKAI) (1985). In the magnetic iron oxide particles disclosed therein, the surfaces of the magnetic iron oxide particles coated with a Co compound are further coated with Zn ferrite, but such coating is intended to improve saturation magnetization and has no relation to the improvement of erasing characteristic as intended in the present invention. Thus, the magnetic iron oxide particles disclosed in JP-A-60-165703 (KOKAI) are quite different in action and effect from those of the present invention. In a preferred embodiment the core acicular magnetic iron oxide particles of the present invention are acicular maghemite particles, acicular magnetite particles or either of said particles containing at least one of Co, Hi, Si, Al, Zn and P. Coating of the acicular magnetic iron oxide particles with a Co compound can be effected by mixing a water dispersion of acicular magnetic iron oxide particles with at least an aqueous solution of Co salt and an aqueous alkaline solution, and heat-treating the thus obtained mixed solution having a pH of 11 or above, preferably 12 or above at a temperature within the range of 50 to 100°C, preferably 80 to 100 °C. The aqueous solution of Co salt may contain, if necessary, an aqueous solution of Fe (II) salt. As the aqueous solution of Co salt, there can be used an aqueous solution of cobalt sulfate, cobalt chloride, cobalt nitrate or the like. The atmosphere for the coating treatment may be a non-oxidizing atmosphere under a stream of an inert gas such as N₂ or an oxidizing atmosphere under a stream of an oxygen-containing gas such as air. The spinel type ferrite layer in the present invention can be formed by adding Fe (II) salt and (a) Mn salt and Zn salt, (b) Hi salt and Zn salt or (c) Mn salt, Ni salt and Zn salt to a mixed solution of pH 11 or above containing the acicular magnetic iron oxide particles coated with a Co compound layer, and then heat-treating the thus obtained mixed solution at a temperature within the range of 50 to 100°C, preferably 80 to 100 °C. Ferrous sulfate, ferrous chloride and the like can be used as Fe (II) salt. Manganese sulfate, manganese chloride, manganese nitrate and the like can be used as Mn salt. Nickel sulfate, nickel chloride and the like can be used as Ni salt. The atmosphere for the heat-treatment may be a non-oxidizing atmosphere under a stream of an inert gas such as N₂ or an oxidizing atmosphere under a stream of an oxygen-containing gas such as air. The spinel type ferrite outer layer in the present invention is present in an amount of from 1.0 to 10.0% by weight, preferably 3.0 to 9.0% by weight, of the acicular magnetic iron oxide particles coated with a Co compound based on the amount of Mn-Zn, Ni-Zn or Mn-Ni-Zn in the outer layer. In the case of less than 1.0% by weight, the object of the present invention can not be attained satisfactorily. The desired improvement of erasing characteristic can be achieved in the case of over 10.0% by weight, but it is meaningless in the excess addition of the elements. The amount of Mn, Ni or Mn-Ni in the outer layer is from 0.1-80 atom %, preferably 10 - 75 atom %, based on the total amount of Mn, Ni and Zn in the outer layers. When the amount is less than 0.1 atom % or greater than 80 atom %, no satisfactory improvement of erasing characteristic can be achieved. The Co coated type magnetic iron oxide particles used in the present invention may be, if necessary, heated at a temperature of 100 to 200°C, preferably 120 to 160°C, after coating with a spinel type ferrite layer. When the heating temperature is below 100°C, a long heat treatment is required, which is disadvantageous having regard to industrial utility and economy. When the heating temperature is higher than 200°C, cobalt ions may be caused to diffuse in crystals, making the particles produced thermally unstable and poorly resistant to change with time. The atmosphere for this heat-treatment may be either a non-oxidising atmosphere under a stream of an inert gas such as N₂ or an oxidising atmosphere under a stream of an oxygen-containing gas such as air. The magnetic recording media according to the present invention can be obtained by coating a substrate with a resin composition containing the Co coated type acicular magnetic iron oxide particles of the present invention in a known way wherein the resin contains from 60 to 90 % by weight, preferably 65 to 85 % by weight of the acicular magnetic iron oxide particles. The resin composition may contain the ordinarily used additives such as dispersants, lubricants, abrasives, antistatic agents and hardners. As to the substrate material in the present invention, there can be used films of synthetic resins such as polyethylene terephthalate, polyethylene, polypropylene, polycarbonate, polyethylene naphthalate, polyamide, polyamide-imide, polyimide or polysulfone; foils and plates of metals such as aluminum and stainless steel; and various types of paper, which are commonly used in the production of magnetic recording media. As to the resin in the present invention, vinyl chloride-vinyl acetate copolymer, vinyl chloride-vinyl acetate-maleic acid copolymer, polyvinyl butyral, a cellulose derivative such as nitrocellulose, polyurethane elastomer, polyester resin, synthetic rubber such as acrylonitrile-butadiene copolymer and polybutadiene, epoxy resin, polyamide resin, electron radiation cured acrylic urethane resin or a mixture of two or more thereof, which are commonly used in the production of magnetic recording media, may be exemplified. The acicular magnetic iron oxide particles of the present invention have high coercive force and excellent erasing characteristic, so that they are suited for use as magnetic iron oxide particles for high-density recording. The acicular magnetic iron oxide particles of the present invention have an average major axis diameter of 0.5 µm or below, preferably of from 0.1-0.4 µm, an aspect ratio (major axis : minor axis) of 6 : 1 or greater, preferably 7 : 1, and show a coercive force of 4.77 x 10⁴ Am⁻¹ (600 Oe) or above, preferably 5.17 x 10⁴ Am⁻¹ (650 Oe) or above and an erasing characteristic value of 72.0 dB or above, preferably 73.0 dB or above. Further, the magnetic recording media according to the present invention have high coercive force and excellent erasing characteristic owing to the high coercive force and excellent erasing characteristic of the acicular magnetic iron oxide particles of the present invention, so that they are best suited for use as magnetic recording media for high-density recording which are most keenly required in the industries at present. The magnetic recording media according to the present invention have a coercive force of 4.77 x 10⁴ Am⁻¹ (600 Oe) or above, preferably 5.17 x 10⁴ Am⁻¹ (650 Oe) or above and an erasing characteristic value of 71.0 dB or above, preferably 72.5 dB or above. The present invention will hereinafter be described more particularly with reference to the examples thereof and comparative examples. The erasing characteristic of the particles and recording media in the following examples and comparative examples were measured according to the method shown on pages 152-153 of "A Collection of Résumés of Lectures at the Spring Conference, 1986" of Journal of the Japan Society of Powder and Powder Metallurgy. That is, a DC magnetic field of 79.58 x 10⁴ Am⁻¹ (10 kOe) was applied to the sample and the remanent magnetization (Mr) was measured. Then the sample was set in an erasing device and, after applying an erasing magnetic field varying from 11.94 x 10⁴ Am⁻¹ (1,500) Oe to zero, the remanent magnetization (Me) was measured. The erasing characteristic was shown by the value of 20 log Me/Mr (dB), Coating of Co coated type acicular magnetic iron oxide particles with spinel type ferrite layer (Examples 1-4 and Comparative Examples 1-4) Example 1 Used as precursor were the acicular γ-Fe₂O₃ particles [average major axis diameter = 0.3 µm; aspect ratio (major axis minor axis) = 8 : 1; coercive force = 2.94 x 10⁴ Am⁻¹ (370 Oe)]. A dispersion obtained by dispersing 100 g of the precursor particles in 2 litres of water was mixed with 500 ml of an aqueous solution having dissolved therein 0.2 mol of ferrous ions and 0.1 mol of cobalt ions by using ferrous sulfate and cobalt sulfate, and then 500 ml of a 6N NaOH solution was added thereto to prepare a mixed solution of pH 14. This mixed solution was heated to 95°C and stirred for 300 minutes while shutting off air, thereby forming a sedimentation of dark brown particles. A part of the reaction solution was withdrawn and filtered, washed with water and dried by the conventional methods to obtain dark brown particles. The results of fluorescent X-ray analysis and X-ray diffraction of the obtained particles showed that they were the acicular γ-Fe₂O₃ particles coated with a Co compound on their surfaces (the amount of Co corresponding to 2.6 % by weight based on the Co-coated γ-Fe₂O₃ particles). These acicular γ-Fe₂O₃ particles coated with Co compound had an average major axis diameter of 0.3 µm, an aspect ratio (major axis : minor axis) of 8 : 1, a coercive force of 5.59 x 10⁴ Am⁻¹(702 Oe) and an erasing characteristic value of 69.5 dB. To the mother solution containing the dark brown sedimented particles was added 500 ml of an aqueous solution having dissolved therein 0.1 mol of Mn and 0.1 mol of Zn by using manganese sulfate and zinc sulfate, and the mixed solution was maintained at a pH of 12 and a temperature of 95°C under stirring for 180 minutes while preventing entrance of air, thereby forming the dark brown sedimented particles. The reaction solution containing the dark brown sedimented particles was filtered, washed with water and dried by the conventional methods. The results of fluorescent X-ray analysis and X-ray diffraction of the obtained dark brown particles showed that they were the acicular γ-Fe₂O₃ particles having formed on the Co compound layer an additional spinel type ferrite layer containing Mn-Zn [spinel type ferrite containing Mn-Zn{Mn/(Mn + Zn) = 48 atom %} is equivalent to 5.7 % by weight of total amount of Mn and Zn based on Co-coated acicular γ-Fe₂O₃]. The obtained acicular γ-Fe₂O₃ particles coated with a double coated layer consisting of a Co compound layer as an inner layer and a spinel type ferrite layer containing Mn-Zn as an outer layer had an average major axis diameter of 0.3 µm and an aspect ratio (major axis : minor axis) of 7 : 1, and showed a coercive force of 5.55 x 10⁴ Am⁻¹ (698 Oe) and erasing characteristic value of 74.1 dB. When these particles were further subjected to a 60-minute heat-treatment at 150°C under an N₂ gas stream, the resulting particles showed a coercive force of 5.96 x 10⁴ Am⁻¹ (749 Oe) and an erasing characteristic value of 74.5 dB. By way of comparison, the Co compound-coated acicular γ-Fe₂O₃ particles were subjected to a 60-minute heat treatment at 150°C under an N₂ gas stream. The obtained particles showed a coercive force of 5.90 x 10⁴ Am⁻¹ (741 Oe) and an erasing characteristic value of 70.2 dB. Example 2 Used as precursor were the acicular γ-Fe₂O₃ particles [average major axis diameter = 0.3 µm; aspect ratio (major axis : minor axis) = 8 : 1; coercive force = 2.94 x 10⁴ Am⁻¹ (370 Oe)]. A dispersion prepared by dispersing 100 g of the precursor particles in 2 litres of water was mixed with 500 ml of an aqueous solution having dissolved therein 0.2 mol of ferrous ions and 0.1 mol of cobalt ions by using ferrous sulfate and cobalt sulfate. Then 500 ml of a 6N NaOH solution was further added thereto to obtain a mixed solution of pH 14. This mixed solution was heated to 95°C and maintained at this temperature under stirring for 300 minutes while shutting off air, thereby forming dark brown sedimented particles. A part of the mother solution was drawn out and filtered, washed with water and dried to obtain dark brown particles. The results of fluorescent X-ray analysis and X-ray diffraction of these particles showed that they were the acicular γ-Fe₂O₃ particles coated with a layer of Co compound (Co amount corresponds to 2.6 % by weight based on Co-coated γ-Fe₂O₃ particles). These Co compound-coated acicular γ-Fe₂O₃ particles had an average major axis diameter of 0.3 µm and an aspect ratio (major axis : minor axis) of 8 : 1 and showed a coercive force of 5.59 x 10⁴ Am⁻¹ (702 Oe) and an erasing characteristic value of 69.5 dB. To the mother solution containing dark brown sedimented particles was added 500 ml of an aqueous solution having dissolved therein 0.1 mol of Ni and 0.1 mol of Zn by using nickel sulfate and zinc sulfate, and the mixed solution was maintained at a pH of 12 and a temperature of 95°C under stirring for 180 minutes while shutting off air, thereby forming dark brown sedimented particles. The reaction solution containing the dark brown sedimented particles was filtered, washed with water and dried by the conventional methods. The results of fluorescent X-ray analysis and X-ray diffraction of the obtained dark brown particles showed that they were the acicular γ-Fe₂O₃ particles having formed on the Co compound layer an additional spinel type ferrite layer [spinel type ferrite containing Ni-Zn {Ni/(Ni + Zn) = 64 atoms %} corresponds to 6.0 % by weight in terms of total amount of Ni and Zn based on Co-coated acicular γ-Fe₂O₃]. The obtained acicular γ-Fe₂O₃ particles coated with a double coated layer consisting of a Co compound layer as an inner layer and a spinel type ferrite layer containing Ni-Zn as an outer layer had an average major axis diameter of 0.3 µm and an aspect ratio (major axis : minor axis) of 7 : 1, and showed a coercive force of 6.25 x 10⁴ Am⁻¹ (786 Oe) and an erasing characteristic value of 7.73 dB. When these particles were further subjected to a 60-minute heat treatment at 150°C under an N₂ gas stream, the resulting particles showed a coercive force of 6.35 x 10⁴ Am⁻¹ (798 Oe) and an erasing characteristic value of 74.3 dB. For the purpose of comparison, the Co compound-coated acicular γ-Fe₂O₃ particles were subjected to a 60-minute heat treatment at 150°C under an N₂ gas stream. The obtained particles showed a coercive force of 5.90 x 10⁴ Am⁻¹ (741 Oe) and an erasing characteristic value of 70.2 dB. Example 3 Used as precursor were the acicular magnetite particles [average major axis diameter = 0.4 µm; aspect ratio (major axis : minor axis) = 10 : 1; coercive force = 3.06 x 10⁴ Am⁻¹ (385 Oe)] containing 4.2 % by weight of Fe²⁺ ions. A dispersion obtained by dispersing 100 g of the precursor particles in 2 litres of water was mixed with 500 ml of an aqueous solution having dissolved therein 0.2 mol of cobalt ion by using cobalt sulfate. Then 500 ml of a 6N NaOH solution was further added thereto to prepare a mixed solution of pH 14. This mixed solution was heated to 100°C and allowed to stand under stirring for 240 minutes while shutting off air, thereby forming dark brown sedimented particles. A part of the reaction solution was withdrawn and filtered, washed with water and dried according to conventional methods to obtain dark brown particles. The results of fluorescent X-ray analysis and X-ray diffraction of the obtained particles showed that they were the acicular γ-Fe₂O₃ particles coated with Co compound (Co amount corresponds to 5.1 % by weight based on Co-coated γ-Fe₂O₃ particles). These Co compound-coated acicular γ-Fe₂O₃ particles had an average major axis diameter of 0.4 µm and an aspect ratio (major axis : minor axis) of 10 : 1, and showed a coercive force of 5.68 x 10⁴ Am⁻¹ (714 Oe) and an erasing characteristic value of 70.3 dB. To the mother solution containing dark brown sedimented particles was added 500 ml of an aqueous solution having dissolved therein 0.2 mol of Mn and 0.1 mol of Zn by using manganese sulfate and zinc sulfate, and the mixed solution was allowed to stand at a pH of 11 and a temperature of 80°C under stirring for 300 minutes while shutting off air, whereupon dark brown particles were formed as sediment. The reaction solution containing the dark brown sedimented particles was filtered, washed with water and dried by conventional methods. The results of fluorescent X-ray analysis and X-ray diffraction of the obtained dark brown particles showed that they were the acicular γ-Fe₂O₃ particles having formed on the Co compound layer an additional spinel type ferrite layer containing Mn-Zn [spinel type ferrite containing Mn-Zn {Mn/(Mn + Zn) = 68 atom %} corresponds to 7.2 % by weight of total amount of Mn and Zn based on Co-coated acicular γ-Fe₂O₃]. The obtained acicular γ-Fe₂O₃ particles coated with a double coated layer consisting of a Co compound layer as an inner layer and a spinel type ferrite layer containing Mn-Zn as an outer layer had an average major axis diameter of 0.4 µm and an aspect ratio (major axis : minor axis) of 10 : 1, and showed a coercive force of 5.62 x 10⁴ Am⁻¹ (706 Oe) and an erasing characteristic value of 75.0 dB. When these particles were further subjected to a 60-minute heat treatment at 150°C under an N₂ gas stream, the resulting particles showed a coercive force of 5.75 x 10⁴ Am⁻¹ (722 Oe) and an erasing characteristic value of 75.6 dB. By way of comparison, the Co compound-coated acicular γ-Fe₂O₃ particles were subjected to a 60-minute heat treatment at 150°C under an N₂ gas stream. The obtained particles showed a coercive force of 5.81 x 10⁴ Am⁻¹ (730 Oe) and an erasing characteristic value of 70.7 dB. Example 4 Used as precursor were the acicular magnetite particles [average major axis diameter of 0.4 µm; aspect ratio (major axis : minor axis) = 10 : 1; coercive force = 3.06 x 10⁴ Am⁻¹ (385 Oe)] containing 4.2% by weight of Fe²⁺ ions. A dispersion obtained by dispersing 100 g of the precursor particles in 2 litres of water was mixed with 500 ml of an aqueous solution having dissolved therein 0.2 mol of cobalt ions by using cobalt sulfate, followed by further addition of 500 ml of a 6N NaOH solution to prepare a mixed solution of pH 14. This mixed solution was heated to 100°C and allowed to stand under stirring for 240 minutes while shutting off air, thereby forming a sedimentation of dark brown particles. A part of the reaction solution was withdrawn and filtered, washed with water and dried by conventional methods to obtain dark brown particles. The results of fluorescent X-ray analysis and X-ray diffraction of these particles showed that they were the acicular γ-Fe₂O₃ particles coated with Co compound (Co amount corresponds to 5.1% by weight based on Co-coated γ-Fe₂O₃ particles). These Co compound-coated acicular γ-Fe₂O₃ particles had an average major axis diameter of 0.4 µm and an aspect ratio (major axis : minor axis) of 10 : 1, and showed a coercive force of 5.68 x 10⁴ Am⁻¹ (714 Oe) and an erasing characteristic value of 70.3 dB. To the mother solution containing dark brown sedimented particles was added 500 ml of an aqueous solution having dissolved therein 0.2 mol of Ni and 0.1 mol of Zn by using nickel sulfate and zinc sulfate, and the mixed solution was maintained at a pH of 12 and a temperature of 80°C under stirring for 300 minutes while shutting off air, thereby forming a sedimentation of dark brown particles. The reaction solution containing the dark brown sedimented particles was filtered, washed with water and dried by conventional methods. The results of fluorescent X-ray analysis and X-ray diffraction of the obtained dark brown particles showed that they were the acicular γ-Fe₂O₃ particles having formed on the Co compound layer a spinel type ferrite layer containing Ni-Zn [spinel type ferrite containing Ni-Zn {Ni/(Ni + Zn) = 71 atom %} corresponds to 8.6 % by weight of total amount of Ni and Zn based on Co-coated acicular γ-Fe₂O₃]. The obtained acicular γ-Fe₂O₃ particles coated with a double coated layer consisting of a Co compound layer as an inner layer and a spinel type ferrite layer containing Ni-Zn as an outer layer had an average major axis diameter of 0.4 µm and an aspect ratio (major axis : minor axis) of 10 : 1, and showed a coercive force of 5.98 x 10⁴ Am⁻¹ (751 Oe) and an erasing characteristic value of 74.4 dB. When these particles were further subjected to a 60-minute heat treatment at 150°C under an N₂ gas stream, the resulting particles showed a coercive force of 6.15 x 10⁴ Am⁻¹ (773 Oe) and an erasing characteristic value of 74.2 dB. By way of comparison, the Co compound-coated acicular γ-Fe₂O₃ particles were subjected to a 60-minute heat treatment at 150°C under an N₂ gas stream. The obtained particles showed a coercive force of 5.81 x 10⁴ Am⁻¹ (730 Oe) and an erasing characteristic value of 70.7 dB. Comparative Example 1 By following the same procedure as Example 1 except that zinc sulfate alone was added instead of adding manganese sulfate and zinc sulfate, there were produced the acicular γ-Fe₂O₃ particles having a Zn-containing spinel type ferrite layer formed on the Co compound layer. The obtained acicular γ-Fe₂O₃ particles coated with a double coated layer consisting of a Co compound layer as an inner layer and a Zn-containing spinel type ferrite layer as an outer layer showed a coercive force of 5.61 x 10⁴ Am⁻¹ (705 Oe) and an erasing characteristic value of 69.0 dB. Comparative Example 2 By following the same procedure as Example 1 except that manganese sulfate alone was added instead of adding manganese sulfate and zinc sulfate, there were produced the acicular γ-Fe₂O₃ particles having an Mn-containing spinel type ferrite layer formed on the Co compound layer. The obtained acicular γ-Fe₂O₃ particles coated with a double coated layer consisting of a Co compound layer as a lower layer and a spinel type ferrite layer as an upper layer showed a coercive force of 5.49 x 10⁴ Am⁻¹ (690 Oe) and an erasing characteristic value of 70.8 dB. Comparative Example 3 By following the same procedure as Example 2 except that nickel sulfate alone was added instead of adding nickel sulfate and zinc sulfate, there were produced the acicular γ-Fe₂O₃ particles having an Ni-containing spinel type ferrite layer formed on the Co compound layer. The obtained acicular γ-Fe₂O₃ particles coated with a double coated layer consisting of a Co compound layer as an inner layer and an Ni-containing spinel type ferrite layer as an outer layer showed a coercive force of 5.63 x 10⁴ Am⁻¹ (708 Oe) and an erasing characteristic value of 69.7 dB. Comparative Example 4 By using the same method as in Example 1 except for the addition of 500 ml of an aqueous solution having dissolved therein 0.1 mol of Mn and 0.1 mol of Zn by using manganese sulfate and zinc sulfate in addition to cobalt sulfate, there were produced the dark brown sedimented particles. The reaction solution containing the dark brown sedimented particles was filtered, washed with water and dried according to conventional methods. The results of fluorescent X-ray analysis and X-ray diffraction of the obtained dark brown particles showed that they were the acicular γ-Fe₂O₃ particles coated with the Co, Mn and Zn compounds. These particles showed a coercive force of 5.60 x 10⁴ Am⁻¹ (704 Oe) and an erasing characteristic value of 69.3 dB. Production of magnetic recording media (Examples 5-12 and Comparative Examples 5-12) Example 5 100 parts by weight of fine particles coated with a double coated layer consisting of a Co compound layer as an inner layer and an Mn-Zn-containing spinel type ferrite layer as an outer layer, obtained in Example 1 but not subjected to the heat treatment, 14 parts by weight of VAGH (VAGH is a trade mark for a vinyl chloride-vinyl acetate-vinyl alcohol copolymer, made by U.C.C. Inc., U.S.A.), 1 part by weight of myristic acid, 30 parts by weight of toluene, 30 parts by weight of methyl ethyl ketone, 1 part by weight of Al₂O₃ powder and 2 parts by weight of carbon black were kneaded for 90 minutes by a kneader. The kneaded material was diluted by adding 45 parts by weight of toluene and 45 parts by weight of methyl ethyl ketone and then mixed and dispersed by a sand grinder for 3 hours. To the resulting dispersion was added 140 parts by weight of a methyl ethyl ketone solution containing 14 parts by weight in solids of a polyurethane resin (NIPPORAN 2304 (trade mark) made by Nippon Polyurethane Kogyo KK), followed by mixing for 30 minutes and filtration. The resulting material was mixed with 3 parts by weight of COLONATE L (COLONATE L is a trade mark for a trifunctional low-molecular weight isocyanate compound, produced by Nippon Polyurethane Kogyo K.K.) to prepare a magnetic coating material. This magnetic coating material was coated on a 12 µm thick polyester base film and dried to form a 4 µm thick magnetic layer. The coated film was calendered and then cut to a width of 3.81 mm to make a magnetic tape. This magnetic tape had the following magnetic properties : coercive force = 5.41 x 10⁴ Am⁻¹ (680 Oe), erasing characteristic value = 74.4 dB. Examples 6-12 & Comparative Examples 5-12 Magnetic tapes were made according to Example 5 by using the acicular magnetic iron oxide particles coated with a double coated layer obtained in Examples 1-4, the acicular magnetic iron oxide particles coated with a Co compound produced by way of comparison in Examples 1 and 3 and the particles obtained in Comparative Examples 1-4. The magnetic properties of these magnetic tapes are shown in Table 1.

Claims: Acicular magnetic iron oxide particles coated with an inner layer consisting of a Co compound and an outer ferrite layer; characterised in that the outer layer is a spinel type ferrite layer containing Mn-Zn, Ni-Zn or Mn-Ni-Zn and is present in an amount of from 1.0 to 10% by weight of the acicular magnetic iron oxide particles coated with said Co compound, based on the amount of Mn-Zn, Ni-Zn or Mn-Ni-Zn in the outer layer, and wherein the amount of Mn, Ni or Mn-Ni in the outer layer is from 0.1 to 80 atom % based on the total amount of Mn, Ni and Zn in the outer layer, the acicular magnetic iron oxide particles having an average major axis diameter of 0.5 µm or below and an aspect ratio (major axis:minor axis) of 6:1 or greater and showing a coercive force of 4.77 x 10⁴ Am⁻¹ (600 Oe) or above and an erasing characteristic of 72.0 dB or above. Particles according to claim 1, wherein the core acicular magnetic iron oxide particles are acicular maghemite particles, acicular magnetite particles or either of said particles containing at least one of Co, Ni, Si, Al, Zn and P. Particles according to claim 1 or 2, wherein the outer layer is present in an amount from 3.0 to 9.0% by weight of the acicular magnetic iron oxide particles coated with a said Co compound, based on the amount of Mn-Zn, Ni-Zn or Mn-Ni-Zn in the outer layer. Particles according to any one of the preceding claims, wherein the amount of Mn, Ni or Mn-Ni in the outer layer is from 10 to 75 atom % based on the total amount of Mn, Ni and Zn in the layer. Particles according to any one of the preceding claims, which have an average major axis diameter of from 0.1 to 0.4 µm and an aspect ratio (major axis:minor axis) of at least 7:1 and which show a coercive force of 5.17 x 10⁴ Am⁻¹ (650 Oe) or above and an erasing characteristic value of 73.0 dB or above. A process for producing acicular magnetic iron oxide particles, which process comprises; (i) mixing a first water dispersion of acicular magnetic iron oxide particles as core particles with at least an aqueous solution of a Co salt and an aqueous alkaline solution to prepare thereby a second dispersion have a pH of 11 or above, (ii) heat-treating said second dispersion at a temperature of from 50 to 100°C to form thereby a layer of a Co compound on the surfaces of said acicular magnetic iron oxide particles, (iii) adding a Fe (II) salt and (a) a Mn salt and a Zn salt, (b) a Ni salt and a Zn salt or (c) a Mn salt, a Ni salt and a Zn salt to the heat-treated second dispersion; and (iv) heat-treating the dispersion from step (iii) at a temperature of from 50 to 100°C to form thereby a spinel type ferrite layer containing Mn-Zn, Ni-Zn or Mn-Ni-Zn on said Co compound layer. A magnetic recording medium comprising a substrate coated with a resin containing acicular magnetic iron oxide particles; characterised in that the acicular magnetic iron oxide particles are particles as claimed in any one of claims 1 to 5 or which have been produced by a process as claimed in claim 6, wherein the resin contains from 60 to 90% by weight of the acicular magnetic iron oxide particles. A magnetic recording medium according to claim 7, wherein the resin contains from 65 to 85% by weight of the core acicular magnetic iron oxide particles. A magnetic recording medium according to claim 7 or 8, wherein the substrate is a synthetic resin film of polyethylene terephthalate, polyethylene, polypropylene, polycarbonate, polyethylene naphthalate, polyamide, polyamide-imide, polyimide or polysulfone, a metal foil or plate made of aluminium or stainless steel, or a paper. A magnetic recording medium according to any one of claims 7 to 9 wherein the resin is a vinyl chloride-vinyl acetate copolymer, vinyl chloride-vinyl acetate-maleic acid copolymer, polyvinyl-butyral, cellulose derivative, polyurethane elastomer, polyester resin, acrylonitrilebutadiene copolymer, polybutadiene, epoxy resin, polyamide resin, electron radiation cured acrylic urethane resin or a mixture of two or more thereof.

Inventors: NAKAMURA, TATSUYA
Applicants: TODA KOGYO CORP.
Application year: 1987
Main classification: G11B5
IPC codes: G11B5