Plate-like maghemite particles and process of producing the same

Abstract: Poreless, non-sintered plate-like maghemite particles, useful as materials for electromagnetic wave absorption and shields, have an average particle diameter of more than 0.5 µm and not more than 2.0 µm, and a BET specific surface area of not more than 6.0 m²/g.

Description: The present invention relates to plate-like maghemite particles useful as materials for manufacturing electromagnetic wave absorbers and shields, their preparation and a paint containing the particles. The use of maghemite particles as the manufacturing materials for electromagnetic wave absorbers and shields is known. For example, JP-A-54-110496 (1979) discloses: "the electromagnetic wave absorption properties of magnetite (Fe₃O₄) were examined in comparison to those of conventional composite ferrites and in the microwave region of 5-10 GHz, the material can be made thinner, and has better workability and superior electromagnetic wave absorption to composite ferrites.---". Electromagnetic wave absorption and shielding are carried out by applying a paint, obtained by preparing a dispersion of maghemite particles in a vehicle, to buildings, ships and aircraft which reflect such waves, or to instruments which generate such waves. It is required that the maghemite particles used as materials for electromagnetic wave absorption and shield materials are plate-like particles, have excellent dispersibility in the vehicle, and have a large particle size, specifically a particle size of not less than 0.5µm. Concerning the shape of the particles, it is disclosed, for example in JP-A-61-77616 (1986) that: "plate-like ferrite particles have excellent properties as electromagnetic wave absorption and shielding materials in the low frequency region, and these properties are not achieved by ordinary ferrite particles"; in JP-A- 55-104926 (1980) that: "--- a very remarkable parallel orientation of individual particles is produced in the covering material. Accordingly, a very high packing density can thus be achieved, and as a result, for example, anti-corrosion is enhanced, screening of electromagnetic interference becomes more effective, and electrical conductivity is increased"; and "there are other uses for hexagonal thin-plate (plate-like) iron oxide particles having a magnetite or maghemite structure. ---a very remarkable parallel orientation of individual particles is produced. A very high packing density can be achieved.---". It is, therefore, evident that plate-like particles are necessary. With regard to the particle size, it is disclosed in JP-A- 61-77626 (1986) that: "--- the average major diameter of the particles is not less than 0.5 µm ---". Conventional processes for producing plate-like maghemite particles are, for example: (i) a process comprising the steps of autoclaving an alkaline suspension containing ferric hydroxide or goethite to form plate-like hematite particles in aqueous solution, reducing the thus obtained plate-like hematite particles by heating in a reducing gas, and then oxidizing the obtained particle with heating; and (ii) a process comprising the steps of rapidly oxidizing an alkaline suspension containing ferrous hydroxide with a strong oxidizing agent, or reacting a ferric salt with an alkali in an aqueous medium in the presence of specific additives to produce ferric hydroxide, autoclaving the thus obtained ferric hydroxide, thereby obtaining plate-like goethite particles in aqueous solution, dehydrating the said plate-like goethite particles by heating, reducing the obtained particle by heating in a reducing gas, and then oxidizing the obtained particles with heating. Process (i) is disclosed in JP-A- 51-28700 (1976) and JP-A- 55-104923 (1975). Process (ii) is disclosed in JP-A- 61-266311 (1986) and JP-A- 55-104923 (1980). Plate-like maghemite particles having excellent dispersibility and large size, in particular having a particle size of more than 0.5 µm, are strongly demanded at the present. However, in the case of the known processes mentioned above, it is necessary to reduce the plate-like particles formed in aqueous solution by heating in a reducing gas. In this reducing process, rapid particle growth takes place, whereby sintering of the particles and between the particles arises. As a result, it becomes difficult to disperse the particles in the vehicle or the resin, and accordingly the packing density lowers and particle orientation is impaired. It is well-known that when plate-like magnetite particles are oxidized with heating to form plate-like maghemite particles, particle growth does not take place, and therefore sintering of the particles and between the particles does not occur. In process (ii), water in the goethite crystals is removed when the plate-like goethite particles are heated, and many pores are therefore produced on the surfaces and inside the plate-like magnetite particles obtained. When the plate-like maghemite particles are obtained by oxidizing the plate-like magnetite particles with heating, many pores remain on the surfaces and inside the particles. If such porous plate-like maghemite particles are dispersed in a vehicle or a resin, other fine particles are attracted to the portions where there are superficial magnetic poles. As a result, many particles agglomerate together, and agglomerated lumps of a fairly large size are formed. Dispersion of the particles in a vehicle is then rendered difficult, and the particle orientation is impaired. As is clear from the above discussion, in order to obtain poreless, non-sintered maghemite particles, plate-like magnetite particles should be formed directly in aqueous solution. In order to overcome the above disadvantages, the present inventors have already proposed a process of forming directly poreless, non-sintered plate-like magnetite particles in aqueous solution (EP-A-0279626). Using this process, however, plate-like magnetite particles of a large particle size, specifically of a particle size of more than 0.5µm, could not be obtained. The coercive force and residual magnetization of maghemite particles is correlated with particle size. As the size increases, the coercive force and residual magnetization decrease, so particles of a large size are preferable for electromagnetic wave absorbers and shielding materials. A process is therefore required for forming plate-like maghemite particles having a particle size of more than 0.5 µm in aqueous solution. The present invention provides poreless, non-sintered plate-like maghemite particles having: (i) an average particle diameter of more than 0.5µm and not more than 2.0µm; (ii) a BET specific surface area of not more than 6.0m²/g; (iii) a coercive force of not more than 6.37 kA/m (80 Oe); (iv) an aspect ratio (ratio of plate surface diameter:thickness) of from 3:1 to 35:1; and (v) a squareness ratio (ratio of σ r :σ s of from 0.070:1 to 0.125:1. The present invention also provides a process for producing poreless, non-sintered plate-like maghemite particles as defined above, comprising (i) autoclaving an alkaline suspension containing ferric oxide hydroxide particles and ferrous hydroxide in the presence of sulfate ion at a temperature of from 200 to 300°C to form plate-like magnetite particles, the ferric oxide hydroxide particles being present in an amount of from 0.05 to 1.0 mol/l, the molar ratio of the ferric oxide hydroxide particles to ferrous hydroxide being from 2:1 to 1:1.5, the concentration of sulfate ion in the suspension being from 0.025 to 1.0 mol/l and the concentration of free OH ion in the alkaline suspension being from 0.1 to 10 mol/l, and (ii) oxidizing the obtained plate-like magnetite particles under heating in air. The present invention further provides a paint comprising a vehicle and particles as defined above. In the accompanying drawings: Fig.1 and Figs. 3 to 5 are electron micrographs of the plate-like magnetite particles obtained in Synthesis Examples 1 to 4 respectively. Fig. 2 is the X-ray diffraction pattern of the plate-like magnetite particles obtained in Synthesis Example 1. Fig. 6 is an electron micrograph of the isotropic magnetite particles obtained in Comparative Example 1. Fig. 7 is an electron micrograph of the plate-like maghemite particles obtained in Example 1. The maghemite particles of the present invention are large particles having an average particle diameter of more than 0.5 µm and not more than 2.0 µm, preferably from 0.7 to 2.0 µm. As the particles are formed directly in aqueous solution, they are poreless, and as the particles do not undergo a reduction with heating, they are also non-sintered. As the plate-like maghemite particles of the present invention are large and poreless, their BET specific surface area is not more than 6.0 m²/g, preferably from 1.5 to 6.0 m²/g, more preferably from 2.5 to 5.5 m²/g. These plate-like particles are poreless and non-sintered. They can easily be made into paints. Their dispersibility and orientation are excellent, and a high packing density in the vehicle or resin can be achieved. The coercive force (Hc) of the plate-like maghemite particles of the present invention is not more than 80 Oe, preferably from 50 to 75 Oe. As a result, since a high magnetic permeability (µ) is easily obtained, the plate-like maghemite particles are very useful as powders for electromagnetic wave absorber and shield materials. In the present invention, as the ferric oxide hydroxide particles α, β or γ-FeOOH may be used. In the present invention, the ferrous hydroxide may be produced by reacting a ferrous salt with an alkali. As the ferrous salt, ferrous sulfate or ferrous chloride may be used, and as the alkali, sodium hydroxide may be used. The sulfate ion of the present invention may be produced by the reaction of a ferrous salt with an alkali, or it may be supplied by adding a sulfate such as sodium sulfate or metal sulfate such as manganese sulfate, zinc sulfate or nickel sulfate. The concentration of ferric oxide hydroxide particles in the alkaline suspension is from 0.05 to 1.0mol/ℓ, preferably from 0.1 to 0.5 mol/ℓ and the mole ratio of the ferric oxide hydroxide particles to the ferrous hydroxide is from 2:1 to 2:1.5, preferably from 2:1 to 2:1.2 The concentration of sulfate ion in the alkaline suspension is from 0.025 to 1.0 mol/ℓ, preferably from 0.05 to 0.5 mol/ℓ/. If the concentration of sulfate ion in the alkaline suspension is less than 0.025 mol/ℓ, it tends to become difficult to form plate-like magnetite particles. The concentration of free OH ion in the alkaline suspension is from 0.1 to 10 mol/ℓ, preferably from 1.0 to 7 mol/ℓ. With increasing concentration of free OH ion, the plate-like magnetite particles obtained tend to become thinner and the plate ratio increases. If the concentration of free OH ion is less than 0.1 mol/ℓ, the reaction slows down and it tends to become difficult to form plate-like magnetite particles. The autoclaving process used in the present invention may be carried out in a high temperature, high pressure vessel such as an autoclave. The autoclaving temperature used in the present invention is from 200 to 300°C. If the temperature is less than 200°C the particle size distribution of the plate-like magnetite particles widens. If the temperature exceeds 300°C, plate-like magnetic particles can still be formed, but from the viewpoint of safety of the equipment, the upper limit of temperature is 300°C. The autoclaving time in the present invention may be the time required to form plate-like magnetite particles directly in aqueous solution, there being no special restriction on this time. The oxidation of the plate-like magnetite particles in the present invention under heating in air may be carried out by a usual method. The temperature is preferably from 200 to 400°C. The plate-like maghemite particles of the present invention have an average particle diameter of more than 0.5 µm and not more than 2.0 µm, preferably from 0.7 to 2.0 µm; a BET specific surface area of not more than 6.0 m²/g, preferably from 1.5 to 6.0 m²/g, more preferably from 2.5 to 5.5 m²/g; a coercive force (Hc) of not more than 6.37 kA/m (80 Oe), preferably from 3.98 to 5.97 kA/m (50 to 75 Oe); an aspect ratio (ratio of plate surface diameter: thickness) of from 3:1 to 35:1, preferably from 5:1 to 30:1; and a squareness ratio (ratio of σ r :σ s ) of from 0.070:1 to 0.125:1, preferably from 0.075:1 to 0.125:1. In addition, as the plate-like particles are poreless and non-sintered, they have excellent dispersibility in a vehicle or a resin, and since plate-like particles are large particles which especially exceed 0.5 µm in particle size, they are suitable for use as powders for electromagnetic wave absorber and shield materials. Further, the plate-like maghemite particles of the present invention are of a brown to black colour, and as the plate-like particles have excellent dispersibility in a vehicle or resin, they may be used as colouring pigments. In addition to the above properties, as these plate-like particles are magnetic, they are also expected to be useful as magnetic colour toners. Furthermore, since the plate-like maghemite particles of the present invention have excellent mechanical properties such as rigidity and apparent density, they are expected to be useful as a vehicle damping material. Examples The present invention is now further described in the following Examples and Comparative Examples. Average particle diameters and aspect ratios (ratio of plate surface diameter: thickness) given in the Examples and Comparative Examples are averages of values measured from electron micrographs, and the specific surface areas were measured by the BET method. Measurement of magnetization was carried out by a vibrating sample magnetometer VSMP-1 (Toei Kogyo Co.) under a field of 796 kA/m (10 KOe). {Manufacture of Plate-Like Magnetite Particles} Synthesis Examples 1 to 4 Comparative Example 1 Reference Example 1 Synthesis Example 1 0.093 mol of a-FeOOH, 0.046 mol of FeSO₄ and 0.7 mol of NaOH were mixed together, and the total volume made up to 0.7 ℓ. After introducing the resultant mixture into an autoclave, it was heated to 220°C, and maintained at this temperature with mechanical stirring for 2 hours to form a black precipitate. After cooling to room temperature, the obtained black precipitate was filtered off, washed with water, dried and disagregated by the usual methods. The obtained black particles were shown by transmission electron microscopy to have an average particle diameter of 1.0 µm. As is seen from the scanning electron micrograph in Fig. 1 (x 10,000), the plate-like particles had an aspect ratio (ratio of plate surface diameter: thickness) of 7:1, and there were no pores on the surface or inside the particles. The obtained particles were individually distributed (non-sintered), their BET specific surface area was 2.8 m²/g, their squareness ratio (σ r :σ s ) was 0.078:1, and the particles had excellent orientation. The coercive force (Hc) of the particles was 4.62 kA/m (58 Oe). Fig. 2 shows the X-ray diffraction pattern of the particles. In Fig. 2, the peaks A corresponds to magnetite, and it was thus be concluded that the particles consisted of magnetite. Synthesis Example 2 0.093 mol of α-FeOOH, 0.046 mol of FeSO₄ and 4.2 mol of NaOH were mixed together, and the total volume made up to 0.7 ℓ. After introducing the resultant mixture into an autoclave, it was heated to 250°C, and maintained at this temperature with mechanical stirring for 2 hours to form a black precipitate. After cooling to room temperature, the obtained black precipitate was filtered off, washed with water, dried and disagregated by the usual methods. The obtained black particles were shown by transmission electron microscopy to have an average particle diameter of 1.5 µm. As is seen from the scanning electron micrograph in Fig. 3 (x 10,000), the plate-like particles had an aspect ratio (ratio of plate surface diameter: thickness) of 21:1, and there were no pores on the surface or inside the particles. The obtained particles were individually distributed (non-sintered), their BET specific surface area was 5.5 m²/g, their squareness ratio (σ r :σ s ) was 0.087:1, and the particles had excellent orientation. The coercive force (Hc) of the particles was 5.09 kA/m (64 Oe). The X-ray diffraction pattern of the particles showed only peaks corresponding to magnetite, and it was thus be concluded that the particles consisted of magnetite. Synthesis Example 3 0.093 mol of α-FeOOH, 0.046 mol of FeSO₄ and 0.35 mol of NaOH were mixed together, and the total volume made up to 0.7 ℓ. After introducing the resultant mixture into an autoclave, it was heated to 280°C, and maintained at this temperature with mechanical stirring for 2 hours to form a black precipitate. After cooling to room temperature, the obtained black precipitate was filtered off, washed with water, dried and disagregated by the usual methods. The obtained black particles were shown by transmission electron microscopy to have an average particle diameter of 0.7 µm. As is seen from the scanning electron micrograph in Fig. 4 (x 10,000), the plate-like particles had an aspect ratio (ratio of plate surface diameter: thickness) of 6:1, and there were no pores on the surface or inside the particles. The obtained particles were individually distributed (non-sintered), their BET specific surface area was 3.5 m²/g, their squareness ratio (σ r :σ s ) was 0.092:1, and the obtained particles had excellent orientation. The coercive force (Hc) of the particles was 4.54 kA/m (57 Oe). The X-ray diffraction pattern of the particles showed only peaks corresponding to magnetite, and it was thus be concluded that the particles consisted of magnetite. Synthesis Example 4 0.093 mol of α-FeOOH, 0.046 mol of FeCl₂ and 5.6 mol of NaOH were mixed together, 0.1 mol of Na₂SO₄ was added, and the total volume made up to 0.7ℓ. After introducing the resultant mixture into an autoclave, it was heated to 220°C, and maintained at this temperature with mechanical stirring for 2 hours to form a black precipitate. After cooling to room temperature, the obtained black precipitate was filtered off, washed with water, dried and disagregated by the usual methods. The obtained black particles were shown by transmission electron microscopy to have an average particle diameter of 2.0 µm. As is seen from the scanning electron micrograph in Fig. 5 (x 15,000), the plate-like particles had an aspect ratio (ratio of plate surface diameter: thickness) of 25:1, and there were no pores on the surface or inside the particles. The obtained particles were individually distributed (non-sintered), their BET specific surface area was 4.8 m²/g, their squareness ratio (σ r :σ s ) was 0.082, and the obtained particles had excellent orientation. The coercive force (Hc) of the particles was 4.29 kA/m (53 Oe). The X-ray diffraction pattern of the particles showed only peaks corresponding to magnetite, and it was thus be concluded that the particles consisted of magnetite. Comparative Example 1 A black precipitate was formed as in Example 1 except that FeC1₂ was used instead of FeSO₄. After cooling to room temperature, the obtained black precipitate was filtered off, washed with water, dried and disagregated by the usual methods. The obtained black particles were shown by transmission electron microscopy to have an average particle diameter of 0.25 µm. As is seen from the scanning electron micrograph in Fig. 6, (x 30,000), the obtained particles exhibited an isotropic form. Reference Example 1 0.83 ℓ of a 1.35 mol/l aqueous solution of ferrous sulfate was added to a reaction vessel kept under a flow of N₂ gas, in which had been placed 3.67ℓ of an aqueous solution of Na₂CO₃ of 0.62 mol/ℓ to which had been added 1.65 g of trisodium citrate dihydrate, such that the resulting solution contained 0.5 mol % based on Fe (correspoding to CO₃/Fe = 2.0 equivalents), and FeCO₃ was formed at a temperature of 60°C. The iron concentration was then 0.25 mol/ℓ (calculated as Fe). The resultant aqueous solution containing FeCO₃ was subjected to an aging treatment at 85°C for 30 min. while blowing in N₂ gas at a rate of 15 ℓ/min, and particles were then formed by blowing in air at 85°C for 2.0 hours at a rate of 18 ℓ/min. When the oxidation reaction was completed, part of the reaction solution was removed, and was made acid with hydrochloric acid. The presence or absence of Fe²⁺ was determined by the presence or absence of a blue coloration on adding a solution of red prussiate of potash. The particles obtained were filtered off, washed with water, dried and disagregated by the usual methods. The particles obtained were shown by transmission electron microscopy to have an average particle diameter of 0.39 µm. From a scanning electron micrograph, the plate-like particles had an aspect ratio (ratio of plate surface diameter: thickness) of 8:1, and there were no pores on the surface or inside the particles. The obtained particles had a BET specific surface area of 8.5 m²/g. Their coercive force Hc was 8.76 kA/m (110 Oe), saturation magnetization σ s was 87.8 emu/g, and squareness ratio (σ r :σ s ) was 0.168:1. The X-ray diffraction pattern of the obtained particles showed only peaks corresponding to magnetite, and it was thus be concluded that the particle consisted of magnetite. {Manufacture of Plate-Like Maghemite Particles} Examples 1-4: Example 1 70 g of the plate-like magnetite particle obtained in Synthesis Example 1 was heated in air at 250°C for 30 min so as to obtain maghemite particles. The maghemite particles obtained were shown by transmission electron microscopy to have an average particle diameter of 1.0 µm. As is seen from the scanning electron micrograph in Fig. 7 (x 12,000), the plate-like particles had an aspect ratio (ratio of plate surface diameter: thickness) of 7:1, and there were no pores on the surface or inside the particles. The BET specific surface area of these plate-like maghemite particles was 2.7 m²/g. The coercive force Hc was 4.78 kA/m (60 Oe), saturation magnetization as was 80.0 emu/g, and squareness ratio (σ r :σ s ) was 0.105:1. Examples 2 to 4 Plate-like maghemite particles were prepared by the same method as in Example 1 except that various plate-like magnetite particles were used. Table 1 shows the principal manufacturing conditions used and the properties of the particles obtained. The X-ray diffraction patterns of the particles obtained in Examples 2 to 4 all showed only peaks correspoding to maghemite. Further, the scanning electron micrographs showed that all the particles consisted of plate-like particles, and that there were no pores on the surface or inside the particles.

Claims: Poreless, non-sintered plate-like maghemite particles having: (i) an average particle diameter of more than 0.5µm and not more than 2.0µm; (ii) a BET specific surface area of not more than 6.0m²/g; (iii) a coercive force of not more than 6.37 kA/m (80 Oe); (iv) an aspect ratio (ratio of plate surface diameter:thickness) of from 3:1 to 35:1; and (v) a squareness ratio (ratio of σ r :σ s of from 0.070:1 to 0.125:1. Particles according to claim 1, which have a coercive force of from 3.98 to 5.97 kA/m (50 to 75 Oe). Particles according to claim 1 or 2, which have an aspect ratio (ratio of plate surface diameter:thickness) of from 5:1 to 30:1. Particles according to any one of the preceding claims, which have a squareness ratio (ratio of σ r :σ s ) of from 0.075:1 to 0.125:1. A process for producing poreless, non-sintered plate-like maghemite particles as defined in any one of the above claims, comprising (i) autoclaving an alkaline suspension containing ferric oxide hydroxide particles and ferrous hydroxide in the presence of sulfate ion at a temperature of from 200 to 300°C to form plate-like magnetite particles, the ferric oxide hydroxide particles being present in an amount of from 0.05 to 1.0 mol/l, the molar ratio of the ferric oxide hydroxide particles to ferrous hydroxide being from 2:1 to 1:1.5, the concentration of sulfate ion in the suspension being from 0.025 to 1.0 mol/l and the concentration of free OH ion in the alkaline suspension being from 0.1 to 10 mol/l, and (ii) oxidizing the obtained plate-like magnetite particles under heating in air. A process according to claim 5, wherein the concentration of sulfate ion is from 0.05 to 0.5 mol/l. A process according to claim 5 to 6, wherein the concentration of ferric oxide hydroxide particles is from 0.1 to 0.5 mol/l, and the ratio (molar ratio) of the ferric oxide hydroxide particles to ferrous hydroxide is from 2:1 to 2:1.2. A process according to any one of claims 5 to 7, wherein the concentration of free OH ion in the alkaline suspension is from 1 to 7 mol/l. A process according to any one of claims 5 to 8, wherein the oxidation is carried out at a temperature of 200-400°C. A paint comprising a vehicle and particles as defined in any one of claims 1 to 4.

Inventors: SADAMURA HIDEAKI, TAKEDOI ATSUSHI, SADAMURA, HIDEAKI, TAKEDOI, ATSUSHI
Applicants: TODA KOGYO CORP, TODA KOGYO CORP.
Application year: 1990
Main classification: C01G49
IPC codes: H01F1, C01G49, C09D5