JPS6411577B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing cobalt-containing acicular magnetic iron oxide useful as a magnetic material for magnetic recording media. Cobalt-containing magnetic iron oxide has high coercive force,
In addition, due to its excellent fidelity in the high frequency range, it is widely used in the field of magnetic recording media such as audio and video, but in recent years there has been a significant demand for improved recording density of these magnetic recording media. There is also a growing demand for improved performance such as lower noise. One direction is that the grain size of cobalt-containing magnetic iron oxide is becoming finer, but as the grain size becomes finer, the heat resistance of the grains decreases, and they become more likely to sinter during the heat treatment process, resulting in less homogeneous grain surfaces. In addition, the use of various heat-resistant agents for the purpose of shape retention reduces surface reactivity, and the need for a large amount of cobalt compounds, which in turn reduces other magnetic properties. many. On the other hand, the conventional method for manufacturing magnetic iron oxide is to dehydrate acicular hydrated ferric oxide in air at 450 to 750°C, and then reduce it at 300 to 450°C in a reducing atmosphere such as hydrogen.
A known method is to convert Fe ₃ O ₄ into γ-Fe ₂ O ₃ by further oxidizing it at 150 to 450°C in an oxidizing atmosphere, but in this reduction process, if it is reduced in a strongly reducing atmosphere such as hydrogen, However, many methods have been proposed for reduction in the presence of organic matter, since this causes drawbacks such as particle collapse and sintering between particles. For example, a method in which non-magnetic iron oxide is treated with a salt of a short-chain carboxylic acid (German Patent No. 801352), a method in which non-magnetic alpha iron oxide is treated with wax, starch, kerosene, light oil, etc. (US Patent No. 2900236); A method in which nonmagnetic alpha iron oxide is coated with a hydrophobic fatty acid monocarboxylic acid having 8 to 24 carbon atoms and heated in air to perform reduction and oxidation in one step (US Patent No. 3498748), acicular nonmagnetic oxidation A method of treating iron () or iron oxide () iron hydroxide with a superheated steam mixture consisting of an organic compound and an inert gas (Japanese Unexamined Patent Publications No. 55-104924, No. 57-
118035). However, in either method, the main purpose is to prevent particle collapse, interparticle sintering, etc., and to obtain γ-
This is an attempt to obtain Fe ₂ O ₃ and does not take into consideration γ-Fe ₂ O ₃ suitable for coating with cobalt compounds. Regarding improving the performance of cobalt-containing acicular magnetic iron oxide, various methods are known for incorporating cobalt into magnetic iron oxide, but usually a cobalt compound is added to the particle surface of magnetic iron oxide. In order to obtain acicular magnetic γ-Fe ₂ O ₃ , i.e., a precursor, which has surface properties that allow it to react homogeneously with cobalt compounds, it is necessary to After repeated studies focused on the reduction method, acicular iron oxyhydroxide was immersed in an aqueous dispersion of a specific fatty acid to coat it with the fatty acid, and then, before reduction, it was placed in a non-oxidizing atmosphere. If sufficient dehydration is carried out at a temperature of , at the same time specific fatty acids are thermally decomposed, then reduction is carried out at a specific temperature and time in a non-oxidizing atmosphere, and then oxidized, a surface that reacts homogeneously with cobalt compounds is created. The present invention was completed based on the finding that a precursor suitable for coating with acicular magnetic γ-Fe ₂ O ₃ , that is, a cobalt compound, can be obtained. That is, in the present invention, acicular iron oxyhydroxide is immersed in an aqueous dispersion of a fatty acid having 8 to 24 carbon atoms to coat the particle surface with the fatty acid, and then immersed in a non-oxidizing atmosphere at 250° C.
It is dehydrated at a temperature of ~300°C, then reduced with a decomposition product of the fatty acid at a temperature of 370~500°C in a non-oxidizing atmosphere for over 1 hour, and further oxidized to form acicular γ-Fe ₂ O. ₃ and coating the obtained acicular γ-Fe ₂ O ₃ with at least a cobalt compound. The acicular iron oxyhydroxide used in the method of the present invention includes acicular α, β,
Although any γ-FeOOH may be used, it is preferable to apply the method of the present invention to one that has not been dried during the manufacturing process, for example, after the reaction is completed, the reaction solution is filtered and washed with water. In addition, the acicularity, that is, the axial ratio (L/W), is preferably about 4 to 15, and the particle size is small, that is, the BET specific surface area when γ-Fe ₂ O ₃ is 30 m. ² /g or more is desirable. In the method of the present invention, first, a fatty acid having 8 to 24 carbon atoms is coated on acicular iron oxyhydroxide. This deposition method involves uniformly depositing the above-mentioned fatty acids on the surface of the particles of acicular iron oxyhydroxide, by immersing the acicular iron oxyhydroxide in an aqueous dispersion of the above-mentioned fatty acids. It is carried out at In this immersion deposition, the pH of the aqueous dispersion is usually 8.
Below, it is preferable to adjust it to 4 to 7 in order to avoid coagulation and agglomeration between particles. The fatty acids having 8 to 24 carbon atoms used here are:
Caprylic acid, pelargonic acid, capric acid, undecanoic acid, lauric acid, tridecanoic acid, myristic acid, pentadecanoic acid, palmitic acid, margaric acid, stearic acid, oleic acid, linoleic acid, linoleic acid, nonadecylic acid, arachidic acid, arachidonic acid , saturated or unsaturated fatty acids such as lignoceric acid, or their ammonium salts or amine salts. Among them, unsaturated fatty acids such as oleic acid or linoleic acid, or their ammonium salts or amine salts are desirable. If an alkali metal salt or alkaline earth metal salt of a fatty acid having 8 to 24 carbon atoms is used, the alkali metal or alkaline earth metal will remain in the particles and will be difficult to remove. This is undesirable as it promotes sintering. The amount to be used is usually 1 to 20% by weight, preferably 2 to 10% by weight based on the acicular iron oxyhydroxide. If this amount is less than the above range, the effect of the method of the present invention will not be achieved. On the other hand, if the amount is too high, the coating state of the organic matter on the particle surface may change or the amount of remaining organic matter may increase, reducing the effect. Next, acicular iron oxyhydroxide coated with a specific fatty acid on its surface is heated at 250 to 300% in a non-oxidizing atmosphere.
Dehydrate at a temperature of °C. In this step, dehydration is carried out with care so that substantially no reduction occurs, and even if some reduction occurs, it is preferable to suppress the Fe/total Fe ratio to about 5 to 10% or less. If the dehydration temperature is too low than the above range, it will take a long time to complete the dehydration reaction, while if it is too high, a reduction reaction will occur, making it impossible to obtain the desired effect. The dehydration time may be until the dehydration reaction is completed, depending on the shape of the acicular iron oxyhydroxide used,
Although it cannot be absolutely determined depending on the specific surface area value, the type of fatty acid to be deposited, etc., it is usually 10 minutes to 24 hours, preferably 30 minutes to 6 hours. The atmosphere for this step is preferably a non-oxidizing atmosphere, for example in the presence of an inert gas. Next, the dehydrated product obtained in the previous step is reduced in a non-oxidizing atmosphere at a temperature of 370 to 500° C. for one hour or more. A decomposed product of a specific fatty acid is present on the surface of the dehydrated product after the dehydration reaction in the first step is completed, and a reduction reaction is performed using this decomposed product. If the reduction reaction is performed without fully completing the dehydration reaction in the previous stage, or if the temperature is suddenly raised to a temperature range where the reduction reaction occurs due to insufficient dehydration reaction, the resulting γ-
Perhaps because the homogeneity of the Fe ₂ O ₃ particle surface is impaired, the cobalt-containing acicular magnetic iron oxide produced using such a precursor has a broader coercive force distribution and a switching field distribution (SFD). make it worse The temperature of this reduction reaction is 370-500°C, preferably 400-460°C; if this temperature is too low than the above range, the reduction reaction will be slow and take a long time; on the other hand, if it is too high, the particles will This is undesirable because it causes collapse and interparticle sintering. The reduction time is 1 hour or more, preferably 2 to 2 hours.
It is 3 hours. The atmosphere for this step is preferably a non-oxidizing atmosphere, for example in the presence of an inert gas. Furthermore, the reduced product obtained in the previous step is oxidized to obtain γ-Fe ₂ O ₃ . This oxidation step may be carried out by a normal method. For example, the oxygen-containing gas used in the oxidation reaction may include air, oxygen gas, etc.
The temperature of the oxidation reaction is 150-450°C. Acicular γ-Fe ₂ O ₃ obtained through the above steps
is a precursor suitable for coating with a cobalt compound. Figure 1 shows the relationship between the BET specific surface area value and σs (saturation magnetization) of γ-Fe ₂ O ₃ in the example described below. It has been found that the precursor that lies between and around the point D is desirable, and that around the inflection point C is the most desirable precursor. The maximum value of σs of γ-Fe ₂ O ₃ derived from the acicular iron oxyhydroxide used is 80 to 95%, and the σs from around point L to around point D is 80% to 95%. It was found that σs is about 90-95% of the maximum value. Furthermore, the BET specific surface area value near the bending point C is
When set to 100, the ratio of BET specific surface area values is 90 ~
It was also found that 120 is desirable. here,
In Figure 1, when moving from K to A, the coercive force is difficult to increase and the switching field distribution (SFD) becomes large. It is difficult to remove,
It has been found that defects such as difficulty in increasing the coercive force of cobalt-containing acicular magnetic iron oxide after coating with cobalt arise. Therefore, in the method of the present invention, various conditions may be selected keeping the above matters in mind. Next, the acicular γ-
At least a cobalt compound is deposited on the Fe ₂ O ₃ .
This deposition can be done in various ways. For example, (1) the γ-Fe ₂ O ₃ is dispersed in at least an aqueous cobalt compound solution and an alkali aqueous solution is added thereto; (2) the γ-Fe ₂ O ₃ is dispersed in at least a cobalt compound and an aqueous alkali solution; A method of dispersing in a mixed liquid, (3) dispersing the γ-Fe ₂ O ₃ in water,
A method of adding at least a cobalt compound and an aqueous alkali solution to this, (4) a method of dispersing the γ-Fe ₂ O in an _aqueous alkali solution, and adding at least a cobalt compound thereto, (5) a method of dispersing the γ-Fe 2 O in an aqueous alkali solution, and There are methods such as dispersing at least ₂ O ₃ in an aqueous cobalt compound solution and dropping this dispersion into an aqueous alkaline solution.
Further, in this case, ferrous iron and other metal compounds can be deposited simultaneously with the cobalt compound, or they can be deposited sequentially as appropriate. In any case, the alkali is added to the metal compound such as cobalt in an amount equal to or more than the equivalent amount to neutralize it, and the reaction products are deposited on the surface of the γ-Fe ₂ O ₃ particles. be done. The atmosphere and temperature during the deposition process are not particularly limited, but for example, the deposition process is carried out at a temperature below the boiling point in a non-oxidizing atmosphere in which the metal such as cobalt is not substantially oxidized. This processing time is usually 0.1
~10 hours. Regarding the above-mentioned adhesion treatment, the alkalis used include sodium hydroxide, potassium hydroxide,
These include sodium carbonate and potassium carbonate. Examples of the cobalt compound include inorganic or organic salts of cobalt, such as cobalt sulfate, cobalt chloride, and cobalt acetate. In addition, when a metal compound other than cobalt is deposited in combination with a cobalt compound, for example, ferrous sulfate, ferrous chloride, ferrous manganese sulfate, ferrous manganese chloride, nickel chloride, etc. , zinc chloride, etc. can be used in combination. Usually, when a _cobalt compound is used alone, the amount of these compounds added is determined by _the Co
For example, when depositing a combination of a cobalt compound and a ferrous compound, the former should be 0.5 to 10% as Co and the latter should be 1 to 20% as Fe. Appropriate. The deposition slurry containing magnetic iron oxide that has been subjected to the deposition treatment described above may be further subjected to heat treatment as necessary. For example, this adhering slurry may be subjected to a wet heat treatment at 100 to 250°C in an autoclave, or this slurry may be filtered and washed with water to form a wet cake, which is then redispersed in water and heated at 100 to 250°C in an autoclave. Wet heat treatment at 250°C, or heat treatment of the wet cake at 60~250°C in the presence of water vapor, or drying the wet cake after drying at 100~250°C.
By performing a dry heat treatment at 300°C, it is possible to obtain more preferable magnetic properties. The cobalt-containing acicular magnetic iron oxide obtained by the method of the present invention has excellent dispersibility, and magnetic tapes manufactured using this material have good squareness ratio and orientation, as well as good coercive force distribution. It has switching magnetic field distribution, erasing characteristics, etc. The details of the method of the present invention can be further understood from the following illustrative examples. Explanation example: After neutralizing a portion of an aqueous solution of ferrous sulfate with an alkali, air is introduced into the solution to obtain a slurry containing acicular α-FeOOH, which is filtered and washed with water to form needles. α-FeOOH was obtained. This acicular α-FeOOH [axial ratio (L/W); 10~
15, Average major axis length: 0.2μ, BET specific surface area (measured with Micromeritics #2200): 70m ² /g]
was added to water to make a 100 g slurry, and the pH of this slurry was adjusted to about 5 with aqueous ammonia. Equivalent to 5% of this slurry (based on α-FeOOH weight)
of oleic acid was added little by little with stirring, and after the addition was completed, the mixture was filtered, washed with water, and dried to obtain acicular α-FeOOH coated with oleic acid. The α-FeOOH obtained above was divided into five equal parts, each was subjected to the heat treatment shown in Table 1 below in an _N2 gas atmosphere, and the obtained reduced product was further oxidized in air at 250°C. Acicular γ-Fe ₂ O ₃ was obtained. The coercive force (Hc), BET specific surface area (SG), and saturation magnetization (σs) of each acicular γ-Fe ₂ O ₃ were measured by conventional methods, and the results shown in Table 1 were obtained.

[Table] Next, Examples and Comparative Examples of the method of the present invention are shown. Examples 1 to 2 and Examples 1 to 3 Samples (A) to (E) of the above-mentioned illustrative examples were subjected to the following cobalt adhesion treatment and heat treatment. Disperse 100 g of the sample in 1 part water to make a slurry, and heat it at 45°C while blowing _N2 gas into the liquid.
70 ml of a 0.85 mol/aqueous cobalt sulfate solution and 125 ml of a 1 mol/aqueous ferrous sulfate solution were added, and then 175 ml of a 10 mol/aqueous NaOH solution was added with stirring, followed by further stirring at 45°C for 5 hours. Next, this slurry was filtered and washed with water, and the resulting wet cake was placed in an autolave together with water in a separate container, and after purging with N ₂ gas, it was sealed.
Heat treatment was performed at 130° C. for 6 hours in the presence of steam.
After the treatment, it was dried in the air at 60°C for 15 hours to obtain the desired cobalt-containing acicular γ-Fe ₂ O ₃ (a) to (e). Table 2 shows the results of measuring the coercive force (Hc) and saturation magnetization (σs) of the above samples (A) to (E) by a conventional method. Further, for Samples (A) to (E), compositions were adjusted according to the following composition ratios and kneaded in a ball mill to produce magnetic paints. (1) Cobalt-containing acicular γ-Fe ₂ O ₃ 25 parts by weight (2) Polyurethane resin 5.4 (3) Dispersant 1.0 (4) Mixed solvent (toluene/MEK = 1/1)
68.2 Next, each magnetic coating material was applied to a polyester film by a conventional method, oriented, dried, and calendered to produce a magnetic tape having a magnetic coating film of about 5 μm. The coercive force (Hc), squareness ratio (Br/Bm), orientation (OR), reversal magnetic field distribution (SFD), and erasure characteristic (Er) of each tape were measured using the usual methods, and the results are shown in Table 2. I got it.

[Table] Example 3 In the above explanation example, 5% equivalent (α-FeOOH
(weight basis) oleic acid equivalent to 8% (α-
_Acicular γ- _Fe2O3
(F) was obtained, and this product was treated in the same manner as in Example 1 to obtain the desired cobalt-containing acicular γ-Fe ₂ O ₃ (F). In addition, Fe/total Fe of dehydrated product is 4%
It was hot. Comparative Example 4 In Example 3, the dehydration reaction conditions are
Except for replacing 280℃, 2 hours with 230℃, 3 hours.
In the same manner as in Example 3, comparative acicular γ
−Fe ₂ O ₃ (G) and comparative cobalt-containing acicular γ−
Fe ₂ O ₃ (g) was obtained. Note that Fe/total Fe in the dehydrated product was 0%. Comparative Example 5 In Example 3, the dehydration reaction conditions are
Except for replacing 280℃, 2 hours with 330℃, 2 hours.
In the same manner as in Example 3, comparative acicular γ
Cobalt-containing acicular γ-Fe ₂ O ₃ (H) of −Fe ₂ O ₃ (H) was obtained. In addition, Fe/total Fe of dehydrated product is 13%
It was hot. Comparative Example 6 In Example 3, the reduction reaction conditions are
Except for replacing 2 hours at 470℃ with 4 hours at 340℃.
In the same manner as in Example 3, comparative acicular γ
-Fe ₂ O ₃ (I) and cobalt-containing acicular γ-Fe ₂ O ₃ (I) were obtained. Note that the dehydrated Fe/total Fe is 2
% Comparative Example 7 In Example 3, the reduction reaction conditions were
Except for replacing 2 hours at 470℃ with 1 hour at 530℃.
In the same manner as in Example 3, comparative acicular γ
−Fe ₂ O ₃ (J) and comparative cobalt-containing acicular γ−
Fe ₂ O ₃ (nu) was obtained. Note that Fe/total Fe in the dehydrated product was 2%. Example 4 In Example 3, 8% equivalent (α-
FeOOH weight basis) oleic acid equivalent to 7% (α
The desired acicular γ-Fe ₂ O ₃ (K) and the desired cobalt-containing acicular γ-
Fe ₂ O ₃ (L) was obtained. Note that Fe/total Fe in the dehydrated product was 2%. The coercive force (Hc), BET specific surface area (SG), and saturation magnetization (σs) of the acicular γ-Fe ₂ O ₃ samples (F) to (K) were measured, and the results are shown in Table 3. The above cobalt-containing acicular γ-Fe ₂ O ₃ sample (f) ~
Table 3 shows the results of measuring the coercive force (Hc) and saturation magnetization (σs) of (L) using the usual method. Furthermore, tapes were made for samples (F) to (R) in the same manner as described above, and the coercive force (Hc) of each tape was determined by the usual method.
The squareness ratio (Br/Bm), orientation (OR), switching field distribution (SFD) and erasure characteristic (Er) were measured, and the third
Obtained the results in the table.

【table】 [Brief explanation of the drawing]

Figure 1 shows the BET specific surface area values (m ² /g) of the acicular γ-Fe ₂ O ₃ powder obtained in each example and comparative example.
This figure shows the relationship between σ and saturation magnetization (σs).

Claims (1)

[Claims] 1. Acicular iron oxyhydroxide is immersed in an aqueous dispersion of a fatty acid having 8 to 24 carbon atoms to coat the particle surface with the fatty acid, and then dehydrated at a temperature of 250 to 300°C in a non-oxidizing atmosphere, Then, the fatty acid is reduced by a decomposition product of the fatty acid at a temperature of 370 to 500°C for 1 hour or more in a non-oxidizing atmosphere,
Furthermore, a cobalt-containing acicular magnetic material characterized by oxidizing this material to obtain acicular γ-Fe ₂ O ₃ and depositing at least a cobalt compound on the obtained acicular γ-Fe ₂ O ₃ Method for producing iron oxide.