Magnesium oxide powder and method for producing the same

Abstract

A magnesium oxide powder, wherein 0.2 g of the magnesium oxide powder is put into 50 mL of a borate buffer (pH = 9.18), dispersed by an ultrasonic homogenizer for 3 minutes, then put into a measuring device within 1 minute after the dispersion treatment, and a Zeta potential measurement is performed under conditions where the temperature is set to 25°C and the equilibration time is 2 minutes, and the Zeta potential thus measured is -50 mV or more and -5 mV or less. It is preferable that the circularity of the above-described magnesium oxide powder be 0.6 or more and 0.8 or less. It is also preferable that the particle diameter Dv50 of the above-described magnesium oxide powder, which is obtained by a laser diffraction scattering method, be 1 μm or more and 200 μm or less. 50 1 μm or more and 200 μm or less.

Description

Technical Field

[0001] This invention relates to magnesium oxide powder and its manufacturing method. Background Technology

[0002] Magnesium oxide readily reacts with water to form magnesium hydroxide. In particular, pulverized magnesium oxide reacts more readily with water due to its high surface activity. Therefore, water resistance is an issue with magnesium oxide, and improvements in water resistance are desired.

[0003] To improve the water resistance of magnesium oxide, Patent Document 1 proposes to treat the surface of magnesium oxide with fatty acids and various coupling agents.

[0004] Patent document 2 proposes to remove the water-sensitive grain boundary phase present on the surface by grinding the surface of magnesium oxide.

[0005] Existing technical documents

[0006] Patent documents

[0007] Patent Document 1: Japanese Patent Application Publication No. 2015-160781

[0008] Patent Document 2: U.S. Patent Application Publication No. 2020 / 246864 Summary of the Invention

[0009] The techniques described in Patent Documents 1 and 2 can improve the water resistance of magnesium oxide, but there is a problem of increased manufacturing costs when using surface treatment agents. In addition, if the surface of magnesium oxide is polished, the yield will decrease due to the removal of the surface, and for magnesium oxide, which originally has an underdeveloped grain boundary phase with low water resistance on the surface, surface polishing is not effective.

[0010] Therefore, the objective of this invention is to provide a method for producing high water resistance magnesium oxide that can be obtained through simple operation.

[0011] The present invention provides a magnesium oxide powder wherein the zeta potential, as measured by the following method, is above -50mV and below -5mV.

[0012] <Determination Method>

[0013] 0.2 g of the magnesium oxide powder was added to 50 mL of borate buffer (pH = 9.18) and dispersed using an ultrasonic homogenizer for 3 minutes. Then, within 1 minute of dispersion, the powder was added to the measuring apparatus, and the zeta potential was measured at a set temperature of 25°C and an equilibration time of 2 minutes.

[0014] Furthermore, as a preferred method for manufacturing the aforementioned magnesium oxide powder, the present invention provides a method for manufacturing magnesium oxide powder, which includes:

[0015] The first firing process involves firing magnesium compounds to obtain magnesium oxide sintered bodies.

[0016] The pulverization process involves pulverizing the sintered magnesium oxide obtained in the first firing process to obtain pulverized magnesium oxide; and

[0017] The second firing process involves firing the pulverized magnesium oxide obtained in the crushing process to obtain magnesium oxide powder.

[0018] in,

[0019] The firing temperature in the second firing process is set to be lower than the firing temperature in the first firing process. Detailed Implementation

[0020] Hereinafter, the present invention will be described based on its preferred embodiments. First, the method for manufacturing the magnesium oxide powder of the present invention will be described. The method for manufacturing the magnesium oxide powder of the present invention generally consists of the following steps (1) to (3).

[0021] (1) The process of sintering magnesium compounds to obtain magnesium oxide sintered bodies (first sintering process).

[0022] (2) The process of pulverizing the magnesium oxide obtained in the first firing process to obtain magnesium oxide powder (pulverization process).

[0023] (3) The process of firing the magnesium oxide pulverized product obtained in the pulverization process to obtain magnesium oxide powder (second firing process).

[0024] The following sections will describe these procedures in turn.

[0025] (1) First firing process

[0026] In this process, a magnesium compound is calcined to pyrolyze it, thereby obtaining a sintered magnesium oxide body. Magnesium hydroxide, as well as magnesium salts such as magnesium carbonate (magnesite), basic magnesium carbonate, magnesium chloride, magnesium nitrate, and magnesium sulfate can be used as the magnesium compound, with magnesium hydroxide being preferred.

[0027] The preferred purity of the sintered magnesium oxide body is the same as that of the magnesium oxide powder described later. The purity of the sintered magnesium oxide body is determined by the same method as that used for the magnesium oxide powder.

[0028] There are no particular restrictions on the firing method for magnesium compounds; any type of firing furnace can be used, such as rotary kiln, vertical kiln, tunnel kiln, pusher furnace, box furnace, or airflow firing furnace.

[0029] Firing can be carried out while the magnesium compound is stationary, or it can be carried out while the magnesium compound is stirred, flowing, or floating. Alternatively, it can be divided into a process in which the magnesium compound is fired in the first firing step to obtain magnesium oxide, and a process in which the magnesium oxide is granulated and then fired at a higher temperature to obtain a sintered magnesium oxide body, with each process carried out in a different firing furnace.

[0030] In order to obtain a magnesium oxide sintered body from the magnesium compound in the first firing process, the firing temperature of the magnesium compound is preferably 1400°C or higher, more preferably 1600°C or higher, and even more preferably 1800°C or higher.

[0031] In addition, from the perspective of reducing manufacturing costs, the firing temperature of the magnesium compound is preferably below 2600°C, more preferably below 2400°C, and even more preferably below 2200°C.

[0032] The above firing temperature refers to the temperature of the magnesium compound in the first firing process.

[0033] It should be noted that the sintered magnesium oxide obtained through the first firing process can also be fused magnesium oxide that has been electrofused in an electric arc furnace to adjust its properties before being fed into the pulverizing process, thereby further increasing the crystal size.

[0034] The firing time for magnesium compounds varies depending on the type of firing furnace; for example, it can be set to be more than 1 second and less than 24 hours. There are no particular restrictions on the firing atmosphere; any of the following can be used: an inactive atmosphere, an oxidizing atmosphere, or a reducing atmosphere.

[0035] (2) Crushing process

[0036] After obtaining the sintered magnesium oxide body, it is then pulverized to obtain magnesium oxide powder with the desired particle size. The pulverizing device can be appropriately selected according to the properties of the sintered magnesium oxide body being pulverized and the desired properties of the resulting magnesium oxide powder. For example, magnesium oxide powder can be obtained by using pulverizing devices such as roller crushers, jaw crushers, rolling ball mills, vibrating ball mills, roller mills, hammer mills, pin mills, jet mills, or a combination of two or more. Furthermore, grading can be performed during or after the pulverizing process, or a pulverizer equipped with a grading mechanism can be used.

[0037] From the perspective of improving the yield of magnesium oxide powder, the magnesium oxide powder obtained in the pulverizing process preferably has more than 90% by mass passing through a sieve with a mesh size of 200μm.

[0038] There are no particular restrictions on the temperature during pulverization; for example, it can be carried out at room temperature.

[0039] (3) Second firing process

[0040] The second firing step is a process of firing pulverized magnesium oxide to obtain magnesium oxide powder. The inventors' research revealed that the pulverized magnesium oxide obtained through the pulverization process has high surface activity and low water resistance. Furthermore, it was found that firing the pulverized magnesium oxide in this step can reduce the surface activity of the resulting magnesium oxide powder and improve its water resistance.

[0041] The second firing step is preferably carried out in a manner that prevents excessive grain growth or sintering of the magnesium oxide particles. By carrying out the second firing step in this way, the increase in the particle size of the magnesium oxide powder caused by excessive grain growth or sintering can be suppressed. As a result, magnesium oxide powder with the desired particle size can be obtained in a higher yield. From this point of view, the firing temperature T2 in the second firing step is set to be lower than the firing temperature T1 in the first firing step.

[0042] Regarding the firing temperature T2 in the second firing process, in order to prevent the magnesium oxide particles, whose particle size has been adjusted to avoid pulverization, from becoming larger due to thermal adhesion, the firing temperature is lower than the firing temperature T1 in the first firing process. Specifically, it is preferable to fire at a temperature below 1200°C, more preferably at a temperature below 1100°C, and even more preferably at a temperature below 1000°C.

[0043] In addition, from the perspective of fully improving the water resistance of magnesium oxide powder, the firing temperature T2 in the second firing process is preferably above 500°C, more preferably above 700°C, and even more preferably above 800°C.

[0044] The firing temperature T2 mentioned above is the temperature of the magnesium oxide pulverized product in the second firing process.

[0045] There are no particular restrictions on the firing method for magnesia pulverizers; the same firing furnace used in the first firing step can be used. The firing furnace used in the second firing step can be the same as or different from the firing furnace used in the first firing step.

[0046] The firing time of the magnesium oxide pulverized product is preferably 1 second to 100 seconds when the firing furnace is an airflow firing furnace, and preferably 0.1 hours to 10 hours when the firing furnace is not an airflow firing furnace, more preferably 0.5 hours to 5 hours, and even more preferably 1 hour to 2 hours.

[0047] There are no particular restrictions on the firing atmosphere; any of the following can be used: inactive atmosphere, oxidizing atmosphere, and reducing atmosphere.

[0048] Before firing the magnesia pulverized product, it can be mixed with water and / or a binder and granulated (granulation process), and the resulting mixture (granules) is then fed into the second firing process. The granulation process improves the operability of the magnesia pulverized product, such as reducing dust generation, and adjusts the particle size of the resulting magnesia powder. Granulation of magnesia pulverized product can be carried out using equipment such as extrusion granulators, rotary granulators, stirred granulators, fluidized bed granulators, briquetting mills, rolling mills, and spray dryers.

[0049] As a binder, organic solvents such as lower alcohols and water-soluble organic compounds such as polyvinyl alcohol, poly-N-vinyl-2-pyrrolidone, sodium polyacrylate, and polyethylene glycol can be used.

[0050] The total amount of water and / or binder mixed with 100 parts by weight of magnesium oxide powder can be, for example, more than 0.1 parts by weight and less than 10.0 parts by weight, especially more than 0.5 parts by weight and less than 5.0 parts by weight.

[0051] The mixing of magnesium oxide powder with water and / or binder can be carried out using any mixer, such as a screw mixer, conical spiral mixer, V-type mixer, drum mixer, twin-shaft kneader, etc.

[0052] The magnesium oxide powder obtained in the second calcination process does not have the desired particle size (e.g., D as described later). 90 and D 50 In the case of magnesium oxide powder, it can also be graded as needed to produce magnesium oxide powder with the desired particle size (e.g., D). 90 and D 50 (This refers to magnesium oxide powder within the numerical range described later).

[0053] Grading can be performed using vibrating screens, air classifiers, and cyclone classifiers individually, or by combining two or more types.

[0054] As described above, in this manufacturing method, the firing temperature T2 in the second firing step is appropriately controlled, thus suppressing the sintering of magnesium oxide particles together in this step. Therefore, the Dg of the magnesium oxide powder obtained in the second firing step can be suppressed. 90 The increase.

[0055] Specifically, after grading, it is preferable to obtain particles with a particle size of 150 μm or less with a high yield of 70% or more, more preferably 75% or more, further preferably 80% or more, and especially preferably 85% or more.

[0056] As mentioned above, if magnesium oxide is pulverized, the surface activity of the resulting magnesium oxide powder may increase, while its water resistance may decrease. Therefore, from the perspective of preventing a decrease in water resistance, the magnesium oxide powder obtained through the second firing process is preferably not further pulverized. In other words, the magnesium oxide powder obtained through the second firing process is preferably used after being graded as described above, or after being lightly crushed, or used directly.

[0057] Next, the magnesium oxide powder of the present invention will be described. The zeta potential of the magnesium oxide powder of the present invention, measured by the method described later, is -50 mV to -5 mV. The zeta potential represents the state of the surface of the particles constituting magnesium oxide. The inventors' research results show that when the zeta potential of the magnesium oxide powder is -50 mV to -5 mV, the water resistance of the surface of the particles constituting magnesium oxide is improved, and as a result, the water resistance of the magnesium oxide powder as a whole is also improved. Based on this aspect, the zeta potential of the magnesium oxide powder is preferably -48 mV to -7 mV, and more preferably -47 mV to -10 mV.

[0058] The zeta potential of magnesium oxide powder was determined by the following method. Specifically, 0.2 g of magnesium oxide powder was added to 50 mL of borate buffer (pH = 9.18) and dispersed for 3 minutes using an ultrasonic homogenizer (US150T, rated output power 150W, manufactured by Nippon Seiki Co., Ltd.). Then, within 1 minute after dispersion, the powder was added to the measuring apparatus, and the zeta potential was measured at a set temperature of 25°C and an equilibration time of 2 minutes. A Zetasizer Nano ZS ZEN3600 manufactured by Malvern was used for the zeta potential measurement.

[0059] In order to keep the zeta potential within the above-mentioned range, for example, in the above-described manufacturing method, it is preferable to perform a second firing step and not to perform a pulverizing step after the second firing step.

[0060] The magnesium oxide powder of the present invention may contain unavoidable impurities or components such as boron, iron, calcium, aluminum, and silicon added to adjust the properties of magnesium oxide. The purity of the magnesium oxide powder is preferably 88% by mass or more, more preferably 90% by mass or more, and even more preferably 92% by mass or more. Furthermore, from the viewpoint of improving the water resistance of the manufactured magnesium oxide powder, the purity of the magnesium oxide powder is preferably 99% by mass or less, more preferably 98.5% by mass or less, and even more preferably 97% by mass or less.

[0061] To determine the purity of magnesium oxide powder, according to JIS R2212-4, the contents of CaO, SiO2, Fe2O3, Al2O3, and B2O3 are quantified by ICP emission spectroscopy analysis. The contents of these five components (CaO, SiO2, Fe2O3, Al2O3, and B2O3) are then subtracted from the total content using the differential method, thus determining the purity.

[0062] The purity of the magnesium oxide powder is approximately the same as that of the magnesium oxide sintered body used as its raw material. Therefore, the preferred purity of the magnesium oxide powder can be the same as that of the aforementioned preferred purity of the magnesium oxide sintered body.

[0063] The sphericity of the magnesium oxide powder of the present invention is preferably 0.6 to 0.8. Magnesium oxide powder having this sphericity is obtained, for example, by performing a pulverizing process after calcining a magnesium compound (after the first calcination step in the above manufacturing method). If a pulverizing process is performed, the particle size becomes smaller, and the sphericity decreases due to the influence of the fracture surface generated by pulverization on the particle shape. According to the above manufacturing method, after obtaining a magnesium oxide pulverized product with a sphericity of 0.60 to 0.80 through the pulverizing process, by feeding this magnesium oxide pulverized product into the above-described second calcination step, magnesium oxide powder with improved water resistance can be obtained. That is, according to the above manufacturing method, magnesium oxide powder with a sphericity of 0.60 to 0.80, high water resistance, and a desired particle size can be efficiently manufactured.

[0064] Furthermore, if the sphericity of the magnesium oxide powder is set to 0.8 or less, the contact area between the particles of the magnesium oxide powder increases, forming more thermal conductivity pathways. As a result, the thermal conductivity of the magnesium oxide powder can be improved.

[0065] From the perspective of making the above advantages more significant, the roundness of magnesium oxide is more preferably 0.63 or more and 0.79 or less, and even more preferably 0.65 or more and 0.79 or less.

[0066] The roundness is calculated based on a projected image of the magnesium oxide powder according to the present invention. Specifically, SEM images are taken with the magnesium oxide powder dispersed and fixed on a carbon ribbon or similar surface. For particles whose individual shapes can be determined, image analysis software (Mac-view ver.4: manufactured by MOUNTECH Co., Ltd.) is used to calculate the roundness coefficient from the particle shape, which is then taken as the roundness of that particle. The roundness of the powder is determined by measuring more than 100 magnesium oxide particles and their arithmetic mean.

[0067] The aspect ratio of the magnesium oxide powder of the present invention is preferably 1.30 to 1.60 or less. Magnesium oxide powder having this aspect ratio is obtained, for example, by performing a pulverization process after calcining a magnesium compound (after the first calcination step in the above-described manufacturing method). The aspect ratio is calculated based on a projected image of the magnesium oxide powder of the present invention. Specifically, SEM images are taken with the magnesium oxide powder dispersed and fixed on a carbon ribbon or the like. For particles whose individual particle shapes can be determined, image analysis software (Mac-view ver.4: manufactured by MOUNTECH Co., Ltd.) is used to calculate the aspect ratio of the particle based on the short side and long side calculated from the particle shape, with the long side / short side ratio being the aspect ratio of the particle. Measurements are taken on 100 or more magnesium oxide particles, and their arithmetic mean is used as the aspect ratio of the magnesium oxide powder.

[0068] From the perspective of smoothness and aesthetic appearance after compounding with resins, the magnesium oxide powder of the present invention has a particle size D that is 90% of the cumulative frequency based on volume, obtained by laser diffraction scattering. 90 Preferably, the micrometer size is 200 μm or less, more preferably 180 μm or less, and even more preferably 150 μm or less. Additionally, the D0.05 of the magnesium oxide powder... 90 Preferably, it is 5μm or more, and more preferably 10μm or more.

[0069] The particle size D of magnesium oxide powder at 50% cumulative frequency based on volume was obtained using laser diffraction scattering. 50 The median particle size is preferably 1 μm or more and 200 μm or less, more preferably 2 μm or more and 150 μm or less, more preferably 3 μm or more and 150 μm or less, and even more preferably 5 μm or more and 100 μm or less.

[0070] As for the water resistance of the magnesium oxide powder of the present invention, the mass increase rate after placing the magnesium oxide powder at 85°C and 85% relative humidity for 48 hours is preferably 2.0% or less, more preferably 1.8% or less, and even more preferably 1.6% or less.

[0071] The magnesium oxide powder of the present invention is suitable for use as a thermally conductive filler. This thermally conductive filler can, for example, be mixed with various resins to form resin compositions. These resin compositions are suitable for use in various articles, particularly those requiring high thermal conductivity and water resistance. Examples of such articles include, for instance, lamp holders and various electrical components in the automotive industry. In the field of electronic equipment, examples include heat sinks, chip pads, printed circuit boards, semiconductor packaging components, cooling fan components, pickup components, connectors, switches, bearings, housings, thermal interface materials (sheets, greases), and gap fillers.

[0072] In addition, the magnesium oxide powder produced by the above method can also be used as a raw material for various ceramics.

[0073] The magnesium oxide powder and its manufacturing method of the present invention have been described above based on preferred embodiments, but the scope of the present invention is not limited to these embodiments.

[0074] For example, the manufacturing method of the present invention may involve one or more additional firings in the second firing step. In this case, additional firing may be performed after the magnesium oxide powder obtained in the second firing step has been pulverized, as needed. Regardless of the number of firings, it is preferable to use magnesium oxide powder without pulverization after the last firing.

[0075] Example

[0076] The present invention will now be described in more detail through examples. However, the scope of the present invention is not limited to these examples. Unless otherwise stated, "%" means "mass %".

[0077] [Example 1]

[0078] Magnesium hydroxide was produced by a seawater method using seawater and lime slurry as raw materials. The Si content of the magnesium hydroxide was adjusted to achieve the desired MgO purity in the sintered magnesium oxide body (described later). Next, the magnesium hydroxide with adjusted Si content was introduced into a rotary kiln and calcined at 1800°C for 8 hours under an oxidizing atmosphere to obtain the sintered magnesium oxide body (first calcination step). The MgO purity of the obtained sintered magnesium oxide body, as determined by ICP emission spectroscopy, was 95.80% by mass. The composition of the sintered magnesium oxide body determined by ICP emission spectroscopy is shown in Table 1.

[0079] The magnesium oxide sintered body obtained by pulverizing it with a ball mill is used to obtain magnesium oxide powder (pulverization process).

[0080] The above-mentioned magnesium oxide pulverized product is fed into a rotary kiln and fired at 888°C for 2 hours in an oxidizing atmosphere (second firing process).

[0081] The magnesium oxide powder obtained through the second calcination process was classified using a vibrating screen with a mesh size of 150 μm (classification process) to obtain magnesium oxide powder of Example 1 with particles larger than 150 μm removed.

[0082] [Examples 2 and 3]

[0083] The firing temperature in the second firing process was changed to the temperature recorded in Table 2, and the magnesium oxide powders of Examples 2 and 3 were obtained in the same manner as in Example 1.

[0084] [Examples 4 and 5]

[0085] In the second firing process, magnesium oxide pulverized material was introduced into the box-type electric furnace instead of the rotary kiln, and the firing temperature in the second firing process was changed to the temperature recorded in Table 2. Otherwise, the magnesium oxide powders of Examples 4 and 5 were obtained in the same manner as in Example 1.

[0086] [Example 6]

[0087] Magnesium hydroxide was produced by a seawater method using seawater and lime slurry as raw materials. The Si content of the magnesium hydroxide was adjusted to achieve the desired MgO purity in the sintered magnesium oxide body (described later). Next, the magnesium hydroxide with adjusted Si content was introduced into a rotary kiln and calcined at 1800°C for 8 hours under an oxidizing atmosphere to obtain the sintered magnesium oxide body (first calcination step). The MgO purity of the obtained sintered magnesium oxide body, as determined by ICP emission spectroscopy, was 97.47% by mass. The composition of the sintered magnesium oxide body determined by ICP emission spectroscopy is shown in Table 1.

[0088] The magnesium oxide sintered body obtained by pulverizing it with a ball mill is used to obtain magnesium oxide powder (pulverization process).

[0089] Add 5 wt% water to the pulverized magnesium oxide, granulate it using a twin-shaft kneader (granulation process), and then introduce it into a rotary kiln for 2 hours at 924°C in an oxidizing atmosphere (second calcination process).

[0090] The calcined magnesium oxide pulverized product was classified using a vibrating screen with a mesh size of 150 μm (classification process) to obtain magnesium oxide powder of Example 6, in which particles larger than 150 μm were removed.

[0091] [Examples 7-10]

[0092] The composition of magnesium hydroxide was adjusted in accordance with the manner in which the calcined magnesium oxide had the composition described in Table 1, and the calcination temperature in the second calcination process was changed to the temperature described in Table 2. Otherwise, magnesium oxide powders of Examples 7 to 10 were obtained in the same manner as in Example 1.

[0093] [Comparative Example 1]

[0094] Without performing the second firing process and the grading process, the magnesium oxide powder of Comparative Example 1 was obtained in the same manner as in Example 1.

[0095] [Comparative Example 2]

[0096] Magnesium oxide pulverizer of Comparative Example 2 was obtained in the same manner as in Example 6, except that the granulation process, the second firing process, and the grading process were not performed.

[0097] [Comparative Example 3]

[0098] Without performing the second firing process, the magnesium oxide pulverized product of Comparative Example 3 was obtained in the same manner as in Example 9.

[0099] [evaluate]

[0100] For the magnesium oxide powder and pulverized products obtained in the examples and comparative examples, the median particle size (D) was determined using the following method. 50 D 90 The yield of the grading process, the mass gain after the water resistance test, and the Zeta potential were measured. Additionally, the average sphericity was determined using the methods described above. The various physical properties of the measured magnesium oxide powder are shown in Table 2.

[0101] Median particle size (D) 50 ) and D 90 ]

[0102] Particle size distribution was determined using a laser diffraction scattering apparatus (MICROTRAC MT3300EXII, manufactured by Microtrac BELCorp.). Deionized water was used as the solvent. Magnesium oxide powder was added through the sample inlet until the amount of sample added was deemed appropriate by the apparatus. The process was maintained in a cyclic state until the peak shape of the detected particle size distribution stabilized. After peak shape stabilization, the particle size (D) at the 50% cumulative frequency on a volume basis was determined. 50 The median particle size and the particle size D with a cumulative frequency of 90% based on volume are: 90 .

[0103] <Device Conditions>

[0104] Light source: Semiconductor laser 780nm 3mW Class 1

[0105] Laser refractive index: 1.74 (MgO) - 1.333 (water)

[0106] Number of measurements: Avg / 3

[0107] Measurement time: 30 seconds

[0108] [Yield of grading process]

[0109] The yield of the grading process is calculated based on the following formula. The results are shown in Table 2.

[0110] Yield (%) = (Mass of magnesium powder after classification) / (Mass of magnesium powder before classification) × 100

[0111] [Gross increase in mass after water resistance test]

[0112] The weighing bottle was dried at 105°C for 1 hour, then naturally cooled to room temperature in a desiccator containing quicklime-based drying material, and its mass was measured. This process was repeated until a constant mass was reached, and the mass of the container was measured. Approximately 10 g of magnesium oxide powder was weighed into the same weighing bottle, dried at 105°C for 1 hour, and similarly naturally cooled to room temperature in a desiccator, and its mass was measured. This process was repeated until a constant mass was reached, and the mass before humidification was measured. Next, with the cap of the weighing bottle containing magnesium oxide powder open, it was placed in a constant temperature and humidity bath at 85°C and 85% relative humidity for 48 hours, then dried at 105°C for 1 hour to remove adhering moisture, and naturally cooled to room temperature in a desiccator containing quicklime-based drying material, and its mass after humidification was measured. Based on the following formula, the mass increase rate after the water resistance test was calculated from these values. The results are shown in Table 2.

[0113] Mass increase rate (%) = (Mass after humidification - Mass before humidification) / (Mass before humidification - Container mass) × 100

[0114] [Zeta potential]

[0115] Zeta potentials were measured using a Zetasizer Nano ZS ZEN3600 manufactured by Malvern using the method described above.

[0116] [Table 1]

[0117] MgO (%) CaO (%) <![CDATA[SiO2(%)]]> <![CDATA[Fe2O3(%)]]> <![CDATA[Al2O3(%)]]> <![CDATA[B2O3(%)]]> Example 1 95.80 0.98 2.66 0.08 0.13 0.35 Example 2 95.80 0.98 2.66 0.08 0.13 0.35 Example 3 95.80 0.98 2.66 0.08 0.13 0.35 Example 4 95.80 0.98 2.66 0.08 0.13 0.35 Example 5 97.68 0.84 0.68 0.07 0.07 0.66 Example 6 97.47 0.88 0.80 0.06 0.09 0.70 Example 7 97.47 0.88 0.80 0.06 0.09 0.70 Example 8 97.51 0.84 0.78 0.05 0.09 0.73 Example 9 97.51 0.84 0.78 0.05 0.09 0.73 Example 10 97.51 0.84 0.78 0.05 0.09 0.73 Comparative Example 1 95.80 0.98 2.66 0.08 0.13 0.35 Comparative Example 2 97.47 0.88 0.80 0.06 0.09 0.70 Comparative Example 3 97.51 0.84 0.78 0.05 0.09 0.73

[0118]

[0119] As shown in Table 2, the magnesium oxide powders of the embodiments that underwent the second firing process showed a smaller increase in mass and superior water resistance compared to the magnesium oxide powders of the comparative examples that did not undergo the second firing process. Furthermore, the zeta potential of the magnesium oxide powders of each embodiment was between -50 mV and -5 mV.

[0120] If we look at the yield of the grading process (i.e., the proportion of magnesium oxide particles that can pass through a sieve with a mesh size of 150 μm), we can see that by setting the firing temperature of the second firing process to a lower level, the sintering of magnesium oxide powder can be suppressed.

[0121] Industrial applicability

[0122] According to the present invention, a magnesium oxide powder with high water resistance is provided. Furthermore, according to the present invention, the magnesium oxide powder with high water resistance can be manufactured through a simple operation involving at least two firing processes. The magnesium oxide powder of the present invention thus obtained is particularly suitable for use as a filler in resin compositions or as a ceramic raw material.

Claims

1. A magnesium oxide sintered body powder, wherein a Zeta potential is -50 mV or more and -5 mV or less as measured by the following measurement method, The measurement method is as follows: 0.2 g of the magnesium oxide sintered body powder is put into 50 mL of a borate buffer solution having a pH of 9.18, and dispersed for 3 minutes using an ultrasonic homogenizer. Then, the Zeta potential is measured within 1 minute after the dispersion treatment by putting it into a measurement device under conditions of a set temperature of 25°C and an equilibration time of 2 minutes.

2. The magnesium oxide sintered body powder according to claim 1, wherein The circularity is 0.6 or more and 0.8 or less.

3. The magnesium oxide sintered body powder according to claim 1 or 2, wherein, The particle diameter D50 of the cumulative frequency of 50% by volume basis obtained by the laser diffraction scattering method 50 is 1 μm or more and 200 μm or less.

4. The magnesium oxide sintered body powder according to claim 1 or 2, wherein, The particle size D90 of the volume basis cumulative frequency of 90% was obtained by laser diffraction scattering method 90 is 200 μm or less.

5. The magnesium oxide sintered body powder according to claim 1 or 2, wherein The purity of the magnesium oxide sintered body powder is 88 mass% or more and 99 mass% or less, which is obtained by subtracting the contents of the five components from the whole by a difference method after the contents of CaO, SiO2, Fe2O3, Al2O3, and B2O3 are quantified by ICP emission spectroscopic analysis according to JIS R2212-4.

6. The magnesium oxide sintered body powder according to claim 1 or 2, wherein The mass increase rate after being left in an environment of 85°C and 85% RH for 48 hours is 2.0 mass% or less.

7. A method for producing a magnesium oxide powder, the method comprising: a first firing step of firing a magnesium compound at 1400°C or higher to obtain a magnesium oxide sintered body; a pulverization step of pulverizing the magnesium oxide sintered body obtained in the first firing step to obtain a magnesium oxide pulverized product; and a second firing step of firing the magnesium oxide pulverized product obtained in the pulverization step at 800°C or higher to obtain a magnesium oxide powder, wherein the firing temperature in the second firing step is set to be lower than the firing temperature in the first firing step.

8. The manufacturing method according to claim 7, wherein the firing temperature in the second firing step is set to be 1200°C or lower under the condition of being lower than the firing temperature in the first firing step.

9. The production method according to claim 7 or 8, wherein the magnesium oxide pulverized product obtained in the pulverization step is mixed with water and / or a binder and then granulated, and the granulated product thus obtained is supplied to the second firing step.

10. The production method according to claim 7 or 8, wherein The magnesium oxide powder obtained in the second firing step was classified to produce a magnesium oxide powder having a particle diameter D90 of 90% of the cumulative frequency on a volume basis obtained using a laser diffraction scattering method 90 of 200 μm or less.

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