JP4934873B2 - Strontium ferrite magnetic powder - Google Patents

Description

  The present invention relates to a ferrite magnetic powder for a bonded magnet fixed with a rubber-based resin binder and a method for producing the same.

  In rubber-based bonded magnets that use a rubber-based resin as a binder, not only the characteristics of the magnetic powder used, but also the filling rate of the magnetic powder into the rubber-based binder greatly affects the magnetic characteristics. The filling rate of the magnetic powder in the rubber binder is affected by various factors such as the particle size and particle size distribution of the magnetic powder, the shape and surface form of the particle, and the type of the rubber binder. It is important that the magnetic powder is a magnetic powder that does not change the original properties of the rubber-based binder and is well-matched with the rubber-based binder. In this specification, the property of the magnetic powder that does not deteriorate the original properties of the rubber-based binder is referred to as “non-reactivity of the magnetic powder”, and the compatibility with the rubber-based binder is referred to as “affinity of the magnetic powder”. .

  If the non-reactivity and affinity of the magnetic powder are not good, the viscosity increases when the kneading with the rubber binder or the molding of the kneaded material (compound) and the fluidity is lowered. Will join. When mechanical stress is applied to the magnetic particles, distortion occurs, reducing the coercive force.

  The non-reactivity and affinity of the ferrite magnetic powder can be evaluated, for example, by measuring the viscosity and shear stress of the kneaded product. It can be said that the smaller the viscosity and shear stress, the better the non-reactivity and affinity (compatibility) with the resin.

  In Patent Document 1, after pulverizing a sintered product having a ferrite composition, ferrite magnetic powder having a pH of 9 or more obtained by annealing is suspended in water, and carbon dioxide gas is blown into this suspension to obtain a carbon content. Teaches that a ferrite magnetic powder having a pH of 0.010 to 0.040% by weight and a pH of less than 6 to 9 can be obtained, which is described as being excellent in non-reactivity and affinity with a resinous binder. Yes. Furthermore, in Patent Document 1, in the neutralization treatment in which mineral acid is added instead of carbon dioxide blowing, the dried product is agglomerated and requires a very strong crushing treatment. It teaches that it is difficult to eliminate the occurrence (see paragraph [0013]).

Patent Document 2 discloses a ferrite magnetic powder having a carbon content of 0.015 to 0.080 wt% and a pH of less than 7 to 10 by contacting the annealed ferrite magnetic powder with a CO ₂ source while stirring. This material is described as being excellent in non-reactivity and affinity with resinous binders.

JP 2001-160506 A Japanese Patent No. 3294507 JP 2002-353019 A JP-A-4-93002 JP-A 63-293806

  Rubber-based bond magnets have the feature that they can be flexibly adapted for mounting on equipment due to their flexibility. For example, a sheet-like rubber-based bond magnet can be used along a curved surface even if it has a curved surface. Can be installed. Such an elastic (tough) property of the bonded magnet may be deteriorated due to the property of the magnetic powder. There is no reported example of such deterioration characteristics of molded products of rubber-based bonded magnets as a factor from the magnetic powder side.

  In rubber-based bonded magnets, when the molded product after being molded into a predetermined shape is deteriorated in toughness or shape, it generally causes deterioration in magnetic properties. For example, when a sheet-like molded product is mounted while being bent and deformed along a curved surface by making use of the characteristics of a rubber-based bond magnet, the function of the bond magnet itself may deteriorate if cracks occur. So far, no measures have been taken from the magnetic powder side to extend the life after forming bonded magnets.

  Accordingly, it is an object of the present invention to clarify the cause from the magnetic powder side that causes deterioration of magnetic properties after forming a bonded magnet, and to obtain a ferrite-based magnetic powder for a rubber-based bonded magnet from which the cause has been removed.

  It has been found that the above-mentioned problems can be achieved by a ferrite magnetic powder comprising an alkaline earth metal as a constituent component and having a chlorine content of 0.05% by weight or less and a powder pH of less than 6. It was. In order to obtain such a magnetic powder, a sintered product of a ferrite composition containing an alkaline earth metal as a constituent is pulverized and then annealed to remove crystal distortion, and the annealed powder is dispersed in water. It is preferable to neutralize with a mineral acid, add a dispersant, separate the solid and liquid, and dry under reduced pressure. It was found that when the dispersant was added and dried under reduced pressure, the effect of preventing agglomeration was enhanced as compared to the case without decompression.

  According to the present invention, a ferrite magnetic powder excellent in non-reactivity and affinity for a rubber binder is obtained, and a rubber-based bonded magnet using this magnetic powder has excellent toughness of a molded product, and is not subject to bending deformation. Since it does not crack even when it is received, it has an excellent effect that it does not impair the deformability that is characteristic of rubber-based bonded magnets.

FIG. 1 is a graph showing changes in viscosity and shear stress when a shear rate is changed in comparison with a control example and a comparative example for a compound obtained by kneading a ferrite magnetic powder according to the present invention with a rubber binder. It is.

  Ferrite magnetic powder has various component compositions and particle forms, but in the case of the dry method, generally the raw material blending → granulation → firing → grinding → water washing / dehydration → drying → The process consists of crushing → annealing → product. “Annealing” in the final step is for removing crystal distortion generated during pulverization after firing (and also during pulverization after drying). This is because crystal distortion generated during pulverization or crushing reduces the magnetic properties, particularly the coercive force. After this annealing process, the ferrite magnetic powder has a pH of about 10 to 12, and exhibits strong alkali. This increase in pH value is particularly noticeable in the case of ferrite magnetic powders containing alkaline earth metals.

  If the ferrite magnetic powder exhibits such a strong alkali, the rubber binder is denatured and the viscosity and fluidity of the compound are greatly affected. Further, in the ferrite magnetic powder that has undergone this annealing step, residual components from the additives and impurity components of the raw material are detected. The residual components may contribute to the deterioration of rubber-based bonded magnets after kneading and molding, but it was found that chlorine has a particularly bad effect.

  According to the present invention, the annealed ferrite magnetic powder is first suspended in water, preferably stirred well, and mineral acid is added thereto. By this treatment, the chlorine accompanying the magnetic powder can be reduced to almost harmless level, and the powder pH can be lowered to less than 6 to prevent the quality deterioration of the molded product of the rubber-based bond magnet. I understood. As the mineral acid, sulfuric acid is preferably used.

  It was found that neutralization of the suspension of annealed powder using mineral acid facilitates aggregation during drying after solid-liquid separation, but agglomeration during drying can be avoided by applying appropriate anti-aggregation treatment. In general, anti-agglomeration treatment includes the addition of inorganic substances with low water absorption, addition of anti-sticking agents such as fatty acid amides and fluorinated fatty acids, and surface treatment with silica-based surface treatment agents and surfactants. However, the content of the anti-aggregation treatment according to the present invention is to prevent the occurrence of aggregation by adding a dispersant (surfactant) before the drying step and performing the drying step after solid-liquid separation under reduced pressure. .

  Thus, the annealed ferrite magnetic powder has a chlorine content of 0.05% by weight or less, preferably 0.02% by weight or less and a powder pH of a dried product after being wet-treated in water. This is less than 6, and this has good non-reactivity and affinity with the rubber binder and can improve the bending strength of the molded product. As a result, the characteristics of rubber-based bonded magnets can be maintained over time.

  Here, the pH value of the ferrite magnetic powder means that obtained according to the measuring method of JIS K 5101. Although the component composition of the ferrite magnetic powder targeted by the present invention is not limited, it is particularly useful for a ferrite magnetic powder containing an alkaline earth metal as a constituent component. The rubber-based binder for fixing the ferrite magnetic powder is not particularly limited as long as it is rubber-based, but vulcanizable rubber such as NBR (acrylonitrile butadiene rubber), EPDM (ethylene propylene diene). Monomer rubber) and thermoplastic resin with rubber elasticity such as CPE (chlorinated polyethylene), plasticized PVC (plasticized vinyl chloride resin), EVA (ethylene-vinyl acetate copolymer), etc. can be applied. Sulfonated polyethylene, silicon rubber, etc. can also be used.

[Example 1]
Iron oxide and strontium carbonate were weighed and mixed so as to have a molar ratio of 5.75, granulated with water, dried, and calcined at 1290 ° C. for 4 hours in a furnace. The fired product was coarsely crushed and further wet pulverized with a wet mill to obtain a strontium ferrite magnetic powder having an average particle size of 1.4 μm. This magnetic powder was annealed in a furnace at 980 ° C. for 1 hour. The obtained annealed product contained 0.055% by weight of chlorine, and had a pH of 10.22 according to the measurement method of JIS K 5101.

  12,000 g of this magnetic powder is mixed with water to make a suspension with a pulp concentration of 25% by weight. While stirring this suspension, sulfuric acid is added so that the sulfuric acid concentration with respect to the suspension becomes 0.10%. Stir for another 15 minutes. Thereafter, washing with water was performed by decantation, and Surfinol CT151 (trade name, manufactured by Nissin Co., Ltd.) as a dispersant was added in an amount of 0.25% with respect to the ferrite magnetic powder, and further stirred for 10 minutes. Subsequently, the cake was dehydrated, and the resulting cake was dried under reduced pressure and then pulverized with a high-speed stirring type pulverizer to obtain a strontium ferrite magnetic powder having an average particle size of 1.4 μm. Chemical analysis of the obtained magnetic powder revealed no chlorine exceeding 0.01% by weight. Moreover, when pH was measured, it was pH = 4.9.

  136.1 g of the obtained magnetic powder and 13.5 g of NBR were loaded into a lab plast mill (manufactured by Toyo Seiki Seisakusho), kneaded at a temperature of 80 ° C. for 10 minutes, discharged once, and kneaded again under the same conditions. The obtained kneaded material is rolled into a sheet with a 6-inch roll, cut into sheets, and several samples (for affinity evaluation) with a thickness of 3 mm, a width of 2 mm and a length of 50 mm, and a thickness of 3 mm are obtained. A sample having a width of 20 mm and a length of 50 mm (for evaluating the deterioration of the molded product) was prepared.

The affinity evaluation was performed by measuring a shear rate (SR), a viscosity (Viscosity), and a shear stress (Share stress) with a capillograph (manufactured by Toyo Seiki Seisakusho). The measurement results are shown in FIG. Figure 1 shows the change in viscosity and shear stress when the shear rate is changed, with the shear rate (sec ^-1 ) on the horizontal axis and the viscosity (PaS) and shear stress (Pa) on the vertical axis. Is. For comparison, FIG. 1 shows a magnetic powder at an annealed stage (as described above, a magnetic powder having a chlorine content of 0.055 wt% and a pH of 10.22 (this is referred to as “control example”). The results of a similar affinity evaluation are also shown.

  From the results shown in FIG. 1, the magnetic powder of this example shows lower values of both viscosity and shear stress in the compound kneaded compound with rubber than the control example, and has better affinity with the rubber-based binder. It can be seen that it is.

Degradability evaluation of molded products was conducted by holding the sheet at 100 ° C for 5 days, cooling it to room temperature, conducting a bending strength test by wrapping the sheet around a 22mmφ cylinder, and evaluating the crack condition of the sheet curved surface in the following three stages. As a result, the degradation evaluation of this example was rank A. In contrast, the magnetic powder in the annealed stage of the control example was ranked C.
A: No cracks are observed.
B: Cracks having a length of less than 3 mm are observed.
C: A crack having a length of 3 mm or more and a width of 0.1 mm or more is observed.

[Reference example]
Example 1 was repeated except that the sulfuric acid concentration was changed from 0.10% to 0.05% during the addition of sulfuric acid. When the obtained magnetic powder was chemically analyzed, chlorine exceeding 0.01% by weight was not detected, and the pH of the powder was 6.6.

  The powder of this reference example was subjected to the same affinity evaluation test as in Example 1, and the results are also shown in FIG. Further, when this powder was subjected to the same deterioration evaluation test of the molded product as in Example 1, it was ranked B.

Claims (2)

A ferrite magnetic powder containing strontium as a constituent, a bond having a chlorine content of 0.02% by weight or less , a powder pH of less than 6 and a dispersing agent added thereto, and a rubber resin fixed as a binder Strontium ferrite magnetic powder for magnets. A bonded magnet in which a strontium ferrite magnetic powder having a chlorine content of 0.02% by weight or less , a powder pH of less than 6 and containing strontium added with a dispersant is hardened with a rubber-based resin.

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