Magnesium oxide powder, vulcanizing agent composition for rubber, rubber composition, and method for producing magnesium oxide powder

Abstract

To provide magnesium oxide powder that has a high effect of promoting vulcanization of rubber, has high hydration resistance when blended in a rubber composition, and is excellent in texture.SOLUTION: The magnesium oxide powder has a D50 of 10 μm or less, at which a cumulative value on a volume-based undersize cumulative particle size distribution curve is 50%, and a ratio D90 / D10 of 10 or less, where D90 is a particle diameter at which the cumulative value is 90% relative to a particle diameter D10 at which the cumulative value on the volume-based undersize cumulative particle size distribution curve is 10%, and a citric acid activity of 500 seconds or more and 2500 seconds or less.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a magnesium oxide powder, a rubber vulcanizing agent composition, a rubber composition, and a method for producing the magnesium oxide powder. This application claims priority based on Japanese Patent Application No. 2021-157106, filed on September 27, 2021, the contents of which are incorporated herein by reference. [Background technology]

[0002] One known use of magnesium oxide powder is as an additive for rubber compositions. For example, Patent Document 1 describes the use of magnesium oxide powder as a rubber additive. In the examples of Patent Document 1, reagent-grade magnesium oxide powder is used. Patent Document 2 describes a magnesium oxide powder having a BET specific surface area of ​​0.1 to 1.0 m 2 / g of low activity magnesium oxide powder is used as an additive to lower the brittle temperature and improve the crack resistance while maintaining the air permeation prevention performance of the rubber composition. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 2019-044137 (A) [Patent Document 2] Japanese Patent Application Publication No. 2015-052031 (A) Summary of the Invention [Problem to be solved by the invention]

[0004] The reagent-grade magnesium oxide powder used in Patent Document 1 generally has high activity and is highly effective in accelerating vulcanization. Accelerated vulcanization improves the tensile strength of rubber compositions. Therefore, accelerated vulcanization is desirable. However, highly active magnesium oxide powders tend to be highly reactive with moisture and have low hydration resistance. Because magnesium hydroxide powder has a lower density than magnesium oxide powder, rubber compositions containing reagent-grade magnesium oxide powder may expand and change shape due to hydration of the magnesium oxide powder. On the other hand, the low-activity magnesium oxide powder described in Patent Document 2 has a low effect of accelerating vulcanization, making it difficult to use as a vulcanization accelerator. Chemical stability, such as hydration resistance, is particularly required for rubber compositions used in tires. Furthermore, rubber compositions used in tire surface layers are required to have a low surface roughness and an appearance with excellent texture, such as gloss.

[0005] The present invention has been made in view of the above-mentioned circumstances, and its object is to provide a magnesium oxide powder and a rubber vulcanizing agent composition that are highly effective in accelerating rubber vulcanization, and that, when incorporated into a rubber composition, have high hydration resistance and excellent texture, as well as a method for producing the magnesium oxide powder. Another object of the present invention is to provide a rubber composition that has high tensile strength and hydration resistance, small surface roughness, gloss, and other excellent appearances. [Means for solving the problem]

[0006] In order to solve the above problems, the present inventors conducted extensive research and found that by adjusting the particle size distribution and citric acid activity of magnesium oxide powder within predetermined ranges, it is possible to suppress reactivity with moisture while improving the effect of accelerating rubber vulcanization. They then confirmed that by blending magnesium oxide powder with an adjusted particle size distribution and citric acid activity into a rubber composition, it is possible to obtain a rubber composition with high tensile strength, high hydration resistance, small surface roughness, and high gloss, and thus completed the present invention. Therefore, the present invention has the following features.

[0007] [1] D where the cumulative value according to the volume-based undersieve cumulative particle size distribution curve reaches 50% 50 is 10 μm or less, and the particle diameter D at which the cumulative value according to the volume-based undersieve cumulative particle size distribution curve is 10% 10 The particle diameter D at which the cumulative value is 90% of the 90 Ratio of D 90 / D 10 Magnesium oxide powder having a citric acid activity of 500 seconds or more and 2500 seconds or less. Method for measuring citric acid activity: A mixed solution containing 100 mL of 0.13 mol / L citric acid aqueous solution and 2 mL of 1% phenolphthalein solution is adjusted to a temperature of 30°C ± 0.5°C, and then 2 g of magnesium oxide powder is added to the mixed solution. 10 seconds after adding the magnesium oxide powder, the mixed solution is stirred, and the time from adding the magnesium oxide powder until the color of the mixed solution changes to pink is measured as the citric acid activity. [2] D above 50 The magnesium oxide powder according to [1] above, wherein the particle size is in the range of 3 μm or more and 10 μm or less. [3] D above 90 The magnesium oxide powder according to the above [1] or [2], wherein the particle size is 50 μm or less. [4] BET specific surface area is 2.0m 2 The magnesium oxide powder according to any one of [1] to [3] above, wherein the magnesium oxide powder has a viscosity of 1000 MPa or less. [5] The magnesium oxide powder according to any one of [1] to [4] above, which has a magnesium oxide content of 90% by mass or more. [6] The magnesium oxide powder according to any one of [1] to [5] above, which is used as a vulcanization accelerator aid for rubber.

[0008] [7] A rubber vulcanizing agent composition comprising a vulcanizing agent, a rubber vulcanization accelerator, and the magnesium oxide powder described in [6] above.

[0009] [8] A rubber composition comprising a rubber component, a rubber vulcanizing agent, a rubber vulcanization accelerator, and the magnesium oxide powder described in [6] above.

[0010] [9] A step of calcining a magnesium compound at a temperature of 1200 ° C or more and 2500 ° C or less to produce magnesium oxide, and performing either one or both of pulverization and classification on the magnesium oxide to obtain a magnesium oxide having a cumulative particle size distribution curve of 90% by volume. 90 and obtaining magnesium oxide powder having a BET specific surface area of ​​2.0 m 2 / g or less, and the citric acid activity measured by the above method is in the range of 500 seconds or more and 2500 seconds or less. [Effects of the Invention]

[0011] According to the present invention, it is possible to provide a magnesium oxide powder and a rubber vulcanizing agent composition that have high tensile strength because they accelerate the vulcanization of rubber, have high hydration resistance when blended into a rubber composition, and have an excellent texture, as well as a method for producing the magnesium oxide powder.The present invention also makes it possible to provide a rubber composition that has high tensile strength and hydration resistance, small surface roughness, and an excellent appearance, such as gloss. DETAILED DESCRIPTION OF THE INVENTION

[0012] Hereinafter, embodiments of the magnesium oxide powder of the present invention, a method for producing the same, a rubber vulcanizing agent composition using the magnesium oxide powder, and a rubber composition will be described.

[0013] The magnesium oxide powder of this embodiment has a D where the cumulative value of the undersieve cumulative particle size distribution curve on a volume basis is 50%. 50 The magnesium oxide powder has a particle diameter D at which the cumulative value of the undersieve cumulative particle size distribution curve based on volume is 10%. 10 The particle diameter D at which the cumulative value is 90% of the 90 Ratio of D 90 / D 10 is set to 10 or less. D 90 / D 10 D is an index of the width of the particle size distribution of magnesium oxide powder. 90 / D 10The smaller the value, the narrower and sharper the particle size distribution. 50 is fine, less than 10 μm, and D 90 / D 10 By adding magnesium oxide powder with a narrow particle size distribution, i.e., D = 10 or less, to a rubber composition, it is possible to obtain a rubber composition with a small surface roughness Rz and high gloss. 50 It is preferable that the thickness is in the range of 3 μm or more and 10 μm or less. 50 The thickness may be 3.5 μm or more and 9 μm or less, or 4 μm or more and 8 μm or less. 90 / D 10 is preferably in the range of 2 or more and 8 or less. 90 / D 10 may be 2 or more and 7 or less, or 2 or more and 6 or less. D 90 is preferably 50 μm or less, and more preferably in the range of 11 μm to 30 μm. 90 is D 50 It is preferably 3.0 times or less, and more preferably in the range of 1.3 times or more and 2.5 times or less. 10 is preferably 0.5 μm or more, and more preferably in the range of 2 μm to 9 μm. 10 is D 50 It is preferably 1 / 5.0 times or more, and more preferably in the range of 1 / 4.0 times or more and 1 / 1.3 times or less. The volume-based cumulative undersieve particle size distribution curve can be measured by laser diffraction / scattering method.

[0014] The magnesium oxide powder of this embodiment has a citric acid activity (CAA) measured by the following method within the range of 500 seconds or more and 2500 seconds or less. The CAA (the time from when the magnesium oxide powder is added until the color of the mixed solution changes to pink) shortens as the magnesium oxide powder becomes more reactive with citric acid, and lengthens as the magnesium oxide powder becomes less reactive with citric acid. That is, the CAA shortens as the activity of the magnesium oxide powder increases, and lengthens as the activity of the magnesium oxide powder decreases. If the CAA is too short, the reactivity of the magnesium oxide powder to moisture may become too high, resulting in reduced hydration resistance. On the other hand, if the CAA is too long, the effect of promoting vulcanization of the magnesium oxide powder may be reduced. For this reason, in this embodiment, the CAA of the magnesium oxide powder is set within the range of 500 seconds or more and 2500 seconds or less. The CAA of the magnesium oxide powder may also be within the range of 1000 seconds or more and 2500 seconds or less. The method for measuring CAA will be described later.

[0015] The magnesium oxide powder of this embodiment has a BET specific surface area of ​​2.0 m 2 / g or less. The BET specific surface area is an index of the activity of the magnesium oxide powder. 2 When the BET specific surface area of ​​the magnesium oxide powder is 0.3 m / g or less, the hydration reaction of the magnesium oxide powder is unlikely to occur, and the hydration resistance is improved. 2 / g or more, and 0.5m 2 It is particularly preferable that the saturation coefficient is 1 / g or more. The BET specific surface area can be measured by the BET single-point method.

[0016] The magnesium oxide powder of this embodiment may have a magnesium oxide content of 90% by mass or more. A high magnesium oxide content of 90% by mass or more improves the effect of accelerating rubber vulcanization. The magnesium oxide content is preferably 95% by mass or more, and particularly preferably 97% by mass or more. Although not particularly limited, the magnesium oxide content in the magnesium oxide powder of this embodiment may be 99.9 mass % or less.

[0017] The magnesium oxide powder of the present embodiment having the above-described structure is D 50 is 10 μm or less, and D 90 / D 10 Since the citric acid activity measured by the above method is within the range of 500 seconds to 2500 seconds, the vulcanization of rubber is accelerated, the tensile strength is high, and when compounded in a rubber composition, the hydration resistance is high. Furthermore, when compounded in a rubber composition, the surface roughness Rz of the rubber composition is small, the gloss is high, and an appearance with excellent texture is exhibited.

[0018] In the magnesium oxide powder of this embodiment, D 50 When D is in the range of 3 μm or more and 10 μm or less, the magnesium oxide particles become finer, and the effect of accelerating the vulcanization of rubber is further improved. 90 When the particle size is 50 μm or less, the number of coarse magnesium oxide particles is reduced, and therefore when compounded in a rubber composition, the surface roughness Rz of the rubber composition becomes smaller, the gloss level becomes higher, and the texture becomes excellent.

[0019] In the magnesium oxide powder of this embodiment, the BET specific surface area is 2.0 m 2 When the magnesium oxide content is 90% by mass or more, the hydration reaction is less likely to occur, and the hydration resistance is improved when the compound is blended into a rubber composition. Furthermore, when the magnesium oxide content is 90% by mass or more, the effect of accelerating the vulcanization of rubber is further improved.

[0020] Next, a method for producing the magnesium oxide powder of this embodiment will be described. The method for producing magnesium oxide powder of this embodiment includes (a) a firing step and (b) a particle size adjusting step.

[0021] (a) Firing process In the firing step, the magnesium compound is fired at a temperature of 1200°C or higher and 2500°C or lower to produce magnesium oxide. The magnesium compound is a compound that generates magnesium oxide upon firing, and examples of the magnesium compound that can be used include magnesium hydroxide and magnesium carbonate. The firing temperature is preferably in the range of 1500°C or higher and 2000°C or lower, and particularly preferably in the range of 1600°C or higher and 2000°C or lower. As the calcination device, various devices used for producing magnesium oxide, such as an electric furnace or a rotary kiln, can be used. The magnesium oxide obtained in the firing step may be in the form of granules or powder, and the citric acid activity (CAA) of the magnesium oxide obtained in the firing step may be greater than 2500 seconds.

[0022] (b) Particle size adjustment process In the particle size adjustment process, the magnesium oxide obtained in the above-mentioned firing process is subjected to either pulverization or classification, or both, to obtain a particle size distribution curve of 90% under sieve. 90 Magnesium oxide powder having a particle size of 50 μm or less is obtained. As a pulverization method, a pulverization method using a pulverizer such as a hammer pulverizer, an impact pulverizer, a roll pulverizer, a stone pulverizer, a vibration pulverizer, or a jet stream pulverizer can be used. The crushing device may be a single crushing device or a combination of two or more crushing devices. The classification method is not particularly limited, and classification methods using classification devices such as a vibrating sieve, an air classifier, or a cyclone classifier can be used. The classification device may be used alone or in combination with two or more types of classification devices.

[0023] When the magnesium oxide obtained in the calcination step has a low citric acid activity, it is preferable to perform pulverization in the particle size adjustment step, more preferably to perform both pulverization and classification, and particularly preferably to perform classification after pulverization. By pulverizing the magnesium oxide and exposing the newly crushed surfaces of the magnesium oxide, the citric acid activity of the resulting magnesium oxide powder increases, and the effect of promoting rubber vulcanization is enhanced.

[0024] In this way, the BET specific surface area was increased to 2.0 m 2 / g or less, a magnesium oxide powder having a citric acid activity measured by the above method in the range of 500 seconds or more and 2500 seconds or less can be obtained.

[0025] According to the method for producing magnesium oxide powder of the present embodiment configured as described above, the magnesium compound is calcined at a temperature of 1200°C or higher and 2500°C or lower in the calcination step (a), so that the resulting magnesium oxide powder is less likely to undergo a hydration reaction, and therefore, when compounded with a rubber composition, the hydration resistance is improved. Furthermore, in the (b) particle size adjustment step, the magnesium oxide is subjected to either pulverization or classification, or both, so that D90, at which the cumulative value according to a volume-based undersize cumulative particle size distribution curve reaches 90%, is 50 μm or less. Therefore, the obtained magnesium oxide powder is fine, has a high effect of accelerating the vulcanization of rubber, and has an excellent texture when blended into a rubber composition.

[0026] The magnesium oxide powder of the present embodiment can be used as a vulcanization accelerator for rubber. By using the magnesium oxide powder of the present embodiment, a rubber composition in which the vulcanization of rubber is advanced can be obtained. Examples of methods for producing a rubber composition containing magnesium oxide powder include: (1) preparing a compound containing rubber and magnesium oxide, and mixing this compound with a vulcanizing agent and a vulcanization accelerator to vulcanize it; (2) preparing a rubber vulcanizer composition containing a vulcanizing agent, a rubber vulcanization accelerator, and magnesium oxide powder, and mixing rubber with the rubber vulcanizer composition to vulcanize it; and (3) preparing a compound containing rubber, a vulcanizing agent, and a vulcanization accelerator, and mixing this compound with magnesium oxide to vulcanize it.

[0027] Examples of rubber that can be used include butadiene rubber, styrene-butadiene rubber, chloroprene rubber, acrylic rubber, nitrile rubber, isoprene rubber, urethane rubber, ethylene propylene rubber, chlorosulfonated polyethylene, epichlorohydrin rubber, silicone rubber, fluororubber, and polyisobutylene rubber. These rubbers may be used alone or in combination of two or more.

[0028] Examples of vulcanizing agents that can be used include organic peroxides, phenolic resins, sulfur compounds, hydrosilicone compounds, amino resins, quinones or their derivatives, amine compounds, azo compounds, epoxy compounds, and isocyanate compounds, all of which are commonly used in rubber vulcanization. Examples of vulcanization accelerators that can be used include sulfenamide vulcanization accelerators, thiuram vulcanization accelerators, thiazole vulcanization accelerators, thiourea vulcanization accelerators, guanidine vulcanization accelerators, and dithiocarbamate vulcanization accelerators, and can be used alone or in appropriate mixtures. Examples of vulcanization accelerator assistants that can be used include fatty acids such as acetyl acid, propionic acid, butanoic acid, stearic acid, acrylic acid, and maleic acid, and magnesium oxide powder.

[0029] The rubber vulcanizing agent composition of this embodiment contains a vulcanizing agent, a rubber vulcanization accelerator, and the above-mentioned magnesium oxide powder. When a sulfur compound is used as the vulcanizing agent, the rubber vulcanizing agent composition may contain 5% by mass to 20% by mass of the sulfur compound, 5% by mass to 60% by mass of the vulcanization accelerator, and 5% by mass to 60% by mass of the magnesium oxide powder. Furthermore, the rubber vulcanizing agent composition may further contain 5% by mass to 20% by mass of the above-mentioned fatty acid other than the magnesium oxide powder.

[0030] The rubber vulcanizing agent composition of the present embodiment, configured as described above, contains the magnesium oxide powder, and therefore is highly effective in accelerating rubber vulcanization, and when blended into a rubber composition, it has high hydration resistance and excellent texture. Therefore, by using the rubber vulcanizing agent composition of the present embodiment, vulcanization proceeds sufficiently, and a rubber composition can be obtained that has high tensile strength and hydration resistance, small surface roughness, gloss, and other excellent appearances.

[0031] The rubber composition of this embodiment includes a rubber component, a rubber vulcanizing agent, a rubber vulcanization accelerator, and the magnesium oxide powder according to claim 6. When a sulfur compound is used, the content of the vulcanizing agent relative to 100 parts by mass of the rubber component may be in the range of 0.1 parts by mass to 10 parts by mass, the content of the vulcanization accelerator may be in the range of 0.1 parts by mass to 20 parts by mass, and the content of the magnesium oxide powder may be in the range of 0.1 parts by mass to 20 parts by mass.

[0032] The rubber composition may further contain various additives used in rubber compositions, such as fillers, dispersants for fillers, rubber softeners, and antioxidants. Examples of fillers that can be used include silica, talc, calcium carbonate, magnesium carbonate, barium sulfate, calcium sulfate, calcium sulfite, calcium phosphate, calcium hydroxide, magnesium hydroxide, aluminum hydroxide, aluminum oxide, titanium oxide, iron oxide, zinc oxide, diatomaceous earth, dolomite, mica, calcium silicate, bentonite, and carbon black. Examples of filler dispersants that can be used include coupling agents such as silane coupling agents. Examples of rubber softeners that can be used include naphthenic oils and roughening oils.

[0033] The rubber composition of the present embodiment configured as described above contains the above-mentioned magnesium oxide powder and has sufficiently accelerated vulcanization, and therefore has high tensile strength and hydration resistance, small surface roughness, and excellent appearance such as gloss.

[0034] Although the embodiment of the present invention has been described above, the present invention is not limited to this and can be modified as appropriate within the scope of the technical idea of ​​the invention. [Example]

[0035] The present invention will be specifically described below based on examples, but the object of the present invention is not limited to these examples. First, the evaluation items and measurement methods for the magnesium oxide powder and rubber composition produced in this example are shown below.

[0036] [Evaluation of magnesium oxide powder] (Measurement of particle size distribution) 30 mL of ethanol (Kishida Chemical Co., Ltd., first-grade ethanol) and 1.5 g of magnesium oxide powder were placed in a 50 mL beaker and dispersed for 2 minutes using an ultrasonic homogenizer (Nippon Seiki Seisakusho Co., Ltd., US-150T model, 150 W) to obtain a magnesium oxide dispersion. The obtained magnesium oxide dispersion was then placed in a laser diffraction / scattering analyzer (Microtrac Bell Corporation, particle size distribution analyzer, MT3300EX model) to measure the particle size (D 10 , D 50 , D 90 ) is measured.

[0037] (Measurement of citric acid activity (CAA)) In a 200 mL beaker, 100 mL of 0.13 mol / L citric acid aqueous solution and 2 mL of 1% phenolphthalein solution are mixed to obtain a mixed solution. The temperature of the resulting mixed solution is adjusted to 30°C ± 0.5°C. 2.00 g of magnesium oxide powder is added to the mixed solution whose temperature has been adjusted. 10 seconds after adding the magnesium oxide powder, stirring of the mixed solution is started with a stirrer (rotation speed: 400 rpm). The time from adding the magnesium oxide powder to the mixed solution until the color of the mixed solution changes to pink is measured. This measured time is the citric acid activity.

[0038] (BET specific surface area measurement) Three grams of magnesium oxide powder is placed in a measurement cell and degassed for 10 minutes at 200° C. The degassed measurement cell is attached to a fully automatic specific surface area measuring device (Macsorb HM model-1200, manufactured by Mountec Co., Ltd.), and the specific surface area is measured by the BET single-point method using a nitrogen-helium mixed gas (30% by volume of nitrogen, 70% by volume of helium).

[0039] (Measurement of magnesium oxide (MgO) content) Measure the contents of CaO, SiO2, B2O3, Fe2O3, and Al2O3 in accordance with JIS R2212-4:2006 (Methods for chemical analysis of refractory products - Part 4: Magnesia and dolomite refractories). The MgO content is calculated by subtracting the total content of CaO, SiO2, B2O3, Fe2O3, and Al2O3 from 100% by mass.

[0040] [Evaluation of Rubber Composition] (Measurement of tensile strength (M100)) Measurements are made in accordance with JIS K 6251:2017 (Vulcanized rubber and thermoplastic rubber - Determination of tensile properties). The rubber composition is punched into a dumbbell-shaped No. 3 test piece. Two benchmark lines are drawn at 20 mm intervals in the center of the obtained dumbbell-shaped No. 3 test piece, and the width and thickness are measured with a vernier caliper at three points, on the benchmark lines and in the center between the benchmark lines, and the median values ​​are taken as the width and thickness of the dumbbell-shaped No. 3 test piece. The dumbbell-shaped No. 3 test piece is attached to a universal testing machine (Shimadzu Corporation, AGS-5kNX model), and the stress and elongation of the test piece are measured at a test temperature of 23°C and a tensile speed of 500 mm / min. The stress (N) at 100% elongation of the dumbbell-shaped No. 3 test piece and the cross-sectional area (mm ) calculated from the width and thickness of the test piece are used. 2 ) The tensile strength (M100) (Mpa) was calculated according to the following formula. Formula: Tensile strength (M100) (Mpa) = stress at 100% elongation (N) / cross-sectional area (mm 2 )

[0041] (Measurement of weight gain rate) The weight (W1) of the dumbbell-shaped No. 3 test piece obtained in the same manner as in the measurement of the tensile strength (M100) above was measured. The weight-measured dumbbell-shaped No. 3 test piece was placed in a thermo-hygrostat (IW222 model, manufactured by Yamato Scientific Co., Ltd.) set at a temperature of 85°C and a humidity of 85%, and left to stand for 48 hours. After standing, the dumbbell-shaped No. 3 test piece was removed from the thermo-hygrostat and allowed to cool to room temperature in a desiccator, after which the weight (W2) of the dumbbell-shaped No. 3 test piece was measured. The weight gain rate (mass%) was calculated from the measured weights W1 and W2 using the following formula. Formula Weight increase rate (mass%) = (W2 - W1) / W1 x 100

[0042] (Measurement of surface roughness Rz (maximum height roughness)) Two benchmark lines are drawn 20 mm apart in the center of a dumbbell-shaped No. 3 test piece obtained in the same manner as in the measurement of tensile strength (M100) above. Next, using a laser microscope VK-9700 (manufactured by Keyence Corporation), the roughness of the dumbbell-shaped No. 3 test piece at the center between the benchmark lines (20 mm) is measured. The sum of the highest part (maximum peak height: Rp) and the deepest part (maximum valley depth: Rv) over the reference length of the roughness curve is taken as the surface roughness (Rp + Rv = Rz) (μm).

[0043] (Gloss measurement) For the dumbbell-shaped No. 3 test pieces obtained in the same manner as in the measurement of the tensile strength (M100) above, the gloss is measured at measurement angles of 20° and 60° using a handy gloss meter (Horiba, Ltd., Gloss Checker IG-331 type) in accordance with JIS Z 8741:1997 (Specular gloss - measurement method).

[0044] [Example 1] (1) Manufacturing of magnesium oxide powder Magnesium hydroxide was prepared by reacting milk of lime (calcium hydroxide) with magnesium salt in seawater. This magnesium hydroxide was fired in a rotary kiln at 1700°C. The resulting magnesium oxide sintered body (magnesia clinker) was pulverized using a multiple impact pulverizer (Super Mill, manufactured by Earth Technica Corporation), and the fine powder collected in a cyclone was further subjected to a centrifugal air classifier with a classification point of 20 μm (Micron Separator MS-3, manufactured by Hosokawa Micron Corporation) to remove coarse particles, thereby obtaining magnesium oxide powder A-1 according to Example 1.

[0045] (2) Production of rubber compositions The compounding ingredients shown in Table 1 below were prepared in the amounts (parts by mass) shown in Table 1 below. BR, SBR, silica, naphthenic oil, silane coupling agent, stearic acid, antioxidant, and magnesium oxide powder A-1 obtained in the above (1) Magnesium Oxide Powder Production were each charged in the amounts shown in Table 1 into an internal mixer (Brabender, Labo Station) and mixed at 90°C for 5 minutes to obtain a primary compound. The resulting primary compound was charged into a twin-screw roll mixer (Kansai Roll Co., Ltd., 6-inch test roll) and further mixed at 65°C for 1 minute. Next, vulcanization accelerator A, vulcanization accelerator B, and sulfur were added to the primary compound in the amounts shown in Table 1, and the mixture was mixed in the twin-screw roll mixer for 3 minutes to obtain a secondary compound. The resulting secondary compound was aged for 20 hours. The aged secondary compound was charged into a predetermined mold and press-vulcanized at 160°C for 20 minutes to obtain rubber composition A-1.

[0046] [Table 1]

[0047] [Comparative Example 1] (1) In the production of magnesium oxide powder, magnesium oxide powder B-1 was produced in the same manner as in Example 1, except that the classification point of the centrifugal air classifier was set to 150 μm. (2) In the production of rubber composition, a rubber composition was produced in the same manner as in Example 1, except that magnesium oxide powder B-1 was used instead of magnesium oxide powder A-1.

[0048] Comparative Example 2 (1) In the production of magnesium oxide powder, magnesium hydroxide produced in the same manner as in Example 1 was calcined in a rotary kiln at 1000°C to produce magnesium oxide powder B-2. (2) In the production of rubber composition, rubber composition B-2 was produced in the same manner as in Example 1, except that magnesium oxide powder B-2 was used instead of magnesium oxide powder A-1.

[0049] [evaluation] The measurement results of the particle size distribution, CAA, BET specific surface area, and MgO content of the magnesium oxide powders obtained in Example 1 and Comparative Examples 1 and 2 are shown in Table 2 below. The measurement results of the tensile strength (M100), weight gain due to moisture, surface roughness Rz, and gloss of the rubber compositions obtained in Example 1 and Comparative Examples 1 and 2 are shown in Table 3 below.

[0050] [Table 2]

[0051] [Table 3]

[0052] The results in Tables 2 and 3 confirm that the rubber composition containing the magnesium oxide powder of Example 1, whose particle size distribution and citric acid activity are within the ranges of the present invention, has high tensile strength and hydration resistance, low surface roughness Rz, and improved gloss. In contrast, the particle size distribution D 90 / D 10 It was confirmed that the rubber composition containing the magnesium oxide powder of Comparative Example 1, which has a citric acid activity lower than the range of the present invention and a citric acid activity lower than the range of the present invention, had a lower tensile strength, a higher surface roughness Rz, and a lower gloss. The lower tensile strength is due to the lower citric acid activity compared to the magnesium oxide powder of Example 1, which reduced the effect of promoting vulcanization. The higher surface roughness Rz and lower gloss are also due to the D of the particle size distribution. 90 / D 10 This is because the particle size distribution width is wide. 90 / D 10 However, it was confirmed that the rubber composition containing the magnesium oxide powder of Comparative Example 2, which had a citric acid activity higher than the range of the present invention, had reduced hydration resistance. This was because, similar to the citric acid activity, the reactivity with water became too high. [Industrial Applicability]

[0053] The present invention provides a magnesium oxide powder and rubber vulcanizing agent composition that have high tensile strength because they accelerate the vulcanization of rubber, high hydration resistance when blended into a rubber composition, and excellent texture, as well as a method for producing the magnesium oxide powder. It also provides a rubber composition that has high tensile strength and hydration resistance, small surface roughness, and excellent appearance, such as gloss.

Claims

1. D where the cumulative value according to the volume-based undersieve cumulative particle size distribution curve is 50% ５０ is 10 μm or less, and the particle diameter D at which the cumulative value according to the volume-based undersize cumulative particle size distribution curve becomes 10% １０ The particle diameter D at which the cumulative value is 90% ９０ Ratio D ９０ / D １０ is 10 or less, and the particle diameter D ９０ is 11 μm or more and 30 μm or less, and the particle diameter D １０ and a citric acid activity measured by the following method is in the range of 500 seconds to 2500 seconds. Method for measuring citric acid activity: A mixed solution containing 100 mL of a 0.13 mol / L aqueous citric acid solution and 2 mL of a 1% phenolphthalein solution was adjusted to a temperature of 30°C ± 0.5°C, and then 2 g of magnesium oxide powder was added to the mixed solution. 10 seconds after the addition of the magnesium oxide powder, the mixed solution was stirred, and the time from the addition of the magnesium oxide powder until the color of the mixed solution changed to pink was measured as the citric acid activity.

2. The above D ５０ 2. The magnesium oxide powder according to claim 1, wherein the particle size is in the range of 3 μm or more and 10 μm or less.

3. The above D ９０ / D １０ 2. The magnesium oxide powder according to claim 1, wherein the value of σ is 2 or more and 8 or less.

4. The above D ９０ / D １０ 2. The magnesium oxide powder according to claim 1, wherein the value of σ is 2 or more and 7 or less.

5. The above D ９０ / D １０ 2. The magnesium oxide powder according to claim 1, wherein the value of σ is 2 or more and 6 or less.

6. BET specific surface area is 2.0m ２ 2. The magnesium oxide powder according to claim 1, wherein the molecular weight of the magnesium oxide powder is 1 / g or less.

7. 2. The magnesium oxide powder according to claim 1, wherein the magnesium oxide content is 90% by mass or more.

8. 2. The magnesium oxide powder according to claim 1, which is used as a vulcanization accelerator aid for rubber.

9. 2. The magnesium oxide powder according to claim 1, which is obtained by crushing and classifying magnesia clinker, and which has a citric acid activity in the range of 1000 seconds to 2500 seconds.

10. A rubber vulcanizing agent composition comprising a vulcanizing agent, a rubber vulcanization accelerator, and the magnesium oxide powder according to claim 8.

11. A rubber composition comprising a rubber component, a rubber vulcanizing agent, a rubber vulcanization accelerator, and the magnesium oxide powder according to claim 8.

12. A process of calcining magnesium hydroxide produced by the reaction of calcium hydroxide with magnesium salt in seawater at a temperature of 1200°C or higher and 2500°C or lower to produce magnesium oxide as magnesia clinker; and a process of crushing and classifying the magnesium oxide to obtain a magnesia clinker having a cumulative undersize particle size distribution of 90% by volume. ９０ is 11 μm or more and 30 μm or less, and the particle diameter D at which the cumulative value becomes 10% １０ and obtaining magnesium oxide powder having a BET specific surface area of ​​2.0 m ２ / g or less, and a citric acid activity measured by the following method is in the range of 500 seconds or more and 2500 seconds or less. Method for measuring citric acid activity: A mixed solution containing 100 mL of a 0.13 mol / L aqueous citric acid solution and 2 mL of a 1% phenolphthalein solution was adjusted to a temperature of 30°C ± 0.5°C, and then 2 g of magnesium oxide powder was added to the mixed solution. 10 seconds after the addition of the magnesium oxide powder, the mixed solution was stirred, and the time from the addition of the magnesium oxide powder until the color of the mixed solution changed to pink was measured as the citric acid activity.

Images