EP0024692A2 - Process for preparing acicular ferromagnetic iron particles, and their use - Google Patents

Abstract

The invention relates to a method for producing acicular ferromagnetic iron particles by reducing iron (III) oxide provided with a shape-stabilizing surface coating, which is obtained by tempering acicular iron (III) oxide hydroxide at 250 to 390 ° C in a water vapor-containing atmosphere with a water vapor partial pressure of at least 30 mbar is obtained with hydrogen at 275 to 425 ° C and the use of these iron particles for the production of magnetic recording media.

Description

The invention relates to a process for the production of acicular ferromagnetic iron particles by reducing iron (III) oxide provided with a shape-stabilizing surface coating, which is obtained by annealing acicular iron (III) oxide hydroxide, with hydrogen at 275 to 425 ° C. and the use thereof Iron particles for the production of magnetic recording media.

Because of their high saturation magnetization and the high coercive field strength achieved, ferromagnetic metal powders and metal thin layers are of particular interest for the production of magnetic recording media, since in this way the energy product and the information density can be increased considerably and such recording media result in narrower signal widths and better signal amplitudes compared to the current standard .

When using needle-shaped ferromagnetic metal powders as magnetizable materials in the production of magnetic recording media, in contrast to homogeneous metal thin layers, the mechanical properties of such information media can be achieved Beeinflussen influence by a suitable selection of the polymeric organic binder systems within wide limits, but then in addition to the magnetic properties there are other requirements with regard to shape, size and dispersibility of the metal particles.

High coercive field strength and high remanence are prerequisites for materials for magnetic storage layers. For this reason, the corresponding metal particles must show magnetic single-range behavior; moreover, the anisotropy that is present or can additionally be achieved by the magnetic alignment in the strip should be only slightly impaired by external influences, such as temperature or mechanical stress, that is, the small particles should preferably be shape-anisotropic Case should be acicular and generally should be between 10 ² and 10 ⁴ Å in size.

It is known to remove iron particles of the type described by reducing finely divided acicular iron compounds such as e.g. the oxides, with hydrogen or another gaseous reducing agent. In order for the reduction to take place at a practical speed, it must be carried out at temperatures above 300 ° C. However, this brings with it the difficulty that the metal particles formed sinter. As a result, however, the particle shape no longer corresponds to that required for the magnetic properties.

To reduce the reduction temperature, it has already been proposed to catalyze the reduction by applying silver or silver compounds to the surface of finely divided iron oxide (DE-OS 20 14 500). The treatment of iron oxide with tin (II) chloride _{└ has also been} described (DE-OS 19 07 691).

┌The catalytic acceleration of the reduction of preferred _, needle-shaped starting compounds, however, generally results in needles which are much smaller than the starting product, and which also has a low length / thickness ratio. As a result, the end product has a fairly large particle size spectrum. However, it is known that the particle size dependency of coercive force and remanence in magnetic substances is very strong in the order of magnitude of the single-range particles. If there are also the influences which arise from a proportion of superparamagnetic particles which can arise as fragments in the above-mentioned procedure, such magnetic materials are unsuitable for use in the production of magnetic recording media. In such heterogeneous mixtures, the magnetic field strength, which is necessary to remagnetize the particles, is very different, and the distribution of the remanent magnetization as a function of the applied external field also results in a less steep remanence curve.

Attempts could also be made to provide the iron oxides to be reduced with a surface coating layer in order to prevent the sintering of the individual particles due to the required reduction temperature, e.g. described in DE-OSen 24 34 058, 24 34 096, 26 46 348 and 27 14 588, do not fully satisfy.

The object of the invention was therefore to provide a method for producing acicular ferromagnetic iron particles, with which pronounced shape-anisotropic particles with high values for coercive field strength, remanence and relative remanence can be obtained in a simple manner.

It has now been found that acicular ferromagnetic iron _{particles are} acicular, with a dimensionally stable Have ironing surface coating provided with ferric oxide with the required properties by reduction with hydrogen at 275 to 425 ° C if the acicular iron (III) oxide used from acicular iron (III) oxide hydroxide by annealing for 10 minutes to 10 Hours at 250 to 390 ° C in a water vapor atmosphere with a water vapor partial pressure (pH ₂ 0) of at least 30 mbar is obtained.

In a particularly advantageous manner, iron (III) oxide hydroxide in the form of lepidocrocite-FeOOH) or a mixture of goethite (α-FeOOH) and lepidocrocite, but with a minimum lepidocrocite content of 20% by weight, is used as the starting material for this process, and this material Annealed for 10 minutes to 10 hours at 250 to 390 ° C. in an atmosphere containing water vapor with a water vapor partial pressure of 30 to 1013 mbar based on normal conditions.

The iron (III) oxide hydroxides mentioned have a BET specific surface area of 20 to 75 m 2 / g, an average particle length between 0.2 and 1.5 and preferably between 0.3 and 1.2 _/ μm and a length-increase -Thickness ratio of at least 10, advantageously 10 to 40 characterized. They can be prepared from iron (II) salt solutions with alkalis with simultaneous oxidation, such as in DE-AS 10 61 760. For this purpose, iron (III) oxide hydrate nuclei up to an amount of 25-60 mol% are made from an aqueous iron (II) chloride solution using alkalis, such as alkali hydroxide or ammonia, at temperatures between 10 and 36 ° C. with vigorous stirring to produce fine air bubbles. of the iron used, from which the final product is then formed at a temperature between 20 and 70 ° C and at a pH of 4.0 to 5.8 set by adding further amounts of alkali, with intensive air distribution _L. After the _beendet Growth the solids content of iron (III) oxide hydroxide in the aqueous suspension should be between 10 and 70 g / l, preferably 15-65 g / l. After filtering off and washing out the precipitate, the iron (III) oxide hydrates thus obtained are dried at 60 to 200 ° C.

These iron (III) oxide hydroxides required for the process according to the invention are now provided in a known manner with a shape-stabilizing surface coating which participates in maintaining the outer shape during the further processing steps. Suitable for this is e.g. the treatment of ferric oxide hydroxides with an alkaline earth metalation and a carboxylic acid or another organic compound which has at least two groups capable of chelating with the alkaline earth metalation. These processes are described in DE-OSes 24 34 058 and 24 34 096.

Also known and described in DE-OS 26 46 348 is the shape-stabilizing treatment of iron (III) oxide hydroxides on their surface with hydrolysis-resistant oxygen acids of phosphorus, their salts or esters and aliphatic mono- or polybasic carboxylic acids. Possible hydrolysis-resistant substances are phosphoric acid, soluble mono-, di- or triphosphates such as potassium, ammonium dihydrogen phosphate, disodium or dilithium orthophosphate, trisodium phosphate, sodium pyrophosphate and metaphosphates such as sodium metaphosphate. The compounds can be used alone or as a mixture with one another. The esters of phosphoric acid with aliphatic monoalcohols having 1 to 6 carbon atoms, such as, for example, tert-butyl ester of phosphoric acid, can advantageously be used. Carboxylic acids in the process are saturated or unsaturated aliphatic carboxylic acids with up to 6 carbon atoms and up to 3 acid ┌ groups, where one or more hydrogen atoms of the aliphatic chain can be substituted by hydroxyl or amino radicals. Oxidic and oxitricarboxylic acids such as oxalic acid, tartaric acid and citric acid are particularly suitable.

The iron (III) oxide hydroxides which have been given a shape-stabilizing effect in the manner described are then annealed for 10 minutes to 10 hours at temperatures between 250 to 390 ° C. in a water vapor-containing atmosphere with a water vapor partial pressure of at least 30 mbar. The end product is acicular iron (III) oxide provided with the surface coating formed according to the previous equipment.

This tempering can be carried out discontinuously or continuously. Reactors such as muffle furnaces, rotary tube furnaces or swirl furnaces are suitable for batch drainage. For better mixing, air, inert gases or air / inert gas mixtures can be passed over or through the resting or moving iron oxide, with these gases being loaded beforehand with the appropriate amount of water vapor. The gases or gas mixtures are expediently saturated with water vapor at temperatures between 40 ° C. and the boiling point of the water, in particular between 50 ° C. and the boiling point of the water, and introduced into the reactors in this state. The water can of course also be mixed directly in the form of steam or the other gases. The tempering can be carried out particularly advantageously in continuous reactors, for example in a continuous rotary tube furnace, since here, in addition to the water vapor in the gas passed through, water vapor from the dehydration _{reaction of} the iron (III) oxide hydroxide is always _supplied in the same amount _└ . This can therefore be done without or with little ┌gas flows can be worked. After a short setting time, the corresponding required water vapor partial pressure of preferably 70 to 1013 mbar in the reaction space is reached.

In another advantageous embodiment of the process according to the invention, the iron (III) oxide hydroxide of the specified composition is subjected directly to this tempering and only then - as described - provided with a surface finish.

To produce the acicular ferromagnetic iron particles, the iron (III) oxide provided with a shape-stabilizing surface coating is reduced in a manner known per se with hydrogen at 275 to 425, preferably at 300 to 400 ° C. It is recommended that the finely divided iron powder obtained in this way be passed over an air or
Passivating the oxygen-inert gas mixture, since this enables the pyrophoric character of the needle-shaped iron particles with a length between 0.1 to 0.8 µm and a length-to-thickness ratio of 5 to 25: 1 to be controlled.

With the aid of the method according to the invention, it is possible to produce acicular ferromagnetic iron particles which are distinguished by a pronounced shape anisotropy. This is achieved in that the starting products are both largely free of dendrites and treated to maintain the outer shape and, in addition, result in a well-crystallized iron (III) oxide for the subsequent reduction reaction due to the inventive tempering. The resulting iron particles are characterized by markedly improved values for coercive field strength, specific remanence and relative remanence. If the iron particles _L J obtained according to the invention ┌Used in the usual way for the production of magnetogram carriers, the needle-shaped particles can be magnetically oriented particularly easily, and important electroacoustic values, such as depth and height controllability, are also improved.

The process according to the invention is illustrated by the following examples and the achievable technical progress is shown by comparison tests.

The nitrogen surface SN2 determined according to BET was primarily used to characterize the acicular iron (III) oxide hydroxides used, such as lepidocrocite and goethite-lepidocrocite mixture. The appearance and the dimensions (L / D ratio) of the iron oxide hydroxide particles provide information from electron micrographs. The goethite-lepidocrocite ratio was determined by X-ray analysis.

The magnetic values of the iron powder were measured with a vibration magnetometer at a magnetic field of 160 or 800 kA / m. The values of the coercive field strength, H _c , measured in kA / m, were related to a stuffing density of ρ = 1.6 g / cm ³ in the powder measurements. Specific remanence (M _{r / ρ} ) and saturation (M _{m / ρ} ) are given in nTm ³ / g.

In addition to high coercive field strength Hc and high remanence, so-called remanent coercive field strength H _{R is} an important assessment variable. In the case of constant field demagnetization, half of the particles are remagnetized at the field strength H _{R in} terms of volume. It thus represents a characteristic quantity for recording processes, which in particular determines the operating point in magnetic recording. The more inconsistent the retentive ┌ititive field strength of the individual magnetic particles in the recording layer, the wider

Der Zahlenindex beim Buchstaben H besagt, wieviel der Teilchen in Prozenten jeweils ummagnetisiert sind.the distribution of the magnetic fields which can remagnetize a limited volume of the recording layer. This is particularly effective if the border area between oppositely magnetized areas should be as narrow as possible due to high recording densities or short wavelengths. To characterize the distribution of the switching field strengths of the individual particles, a value h ₅ for the total width of the remanence curve and h ₂₅ for the steepness of the remanence curve is determined from the constant field demagnetization curve. The values are determined according to

The number index at the letter H indicates how many of the particles are magnetized in percent.

Comparative experiment 1

According to DE-AS 10 61 760, an iron (III) oxide hydroxide (sample A) with a specific surface area SN2 of 37.6 m ² / g, which consists of a mixture of 9 ₅ % FeOOH and 5% α-FeOOH, produced.

70 parts of sample A were annealed in a tube furnace at 350 ° C. at a pressure of 25 mbar. The test lasted one hour. In order to keep the pressure constant, air is metered in via a vacuum valve, which has previously been dried over silica gel. The resulting iron (III) oxide (sample B) has a surface area of 51.3 m ² / g.

┌Another 70 parts of sample A are annealed in the same tube furnace at 350 ° C. The test lasted one hour. In this case, however, a mixture of 100 Nl / h air and water vapor (P _H2O 845 mbar) is passed over the pigment. The resulting iron (III) oxide (sample C) has a surface area of 34.9 m ² / g.

5 parts each of samples A, B and C are reduced to iron pigments 1 to 3 in a rotary tube at 350 ° C. in a hydrogen stream of 30 Nl / h within 8 hours. The measurement results are shown in Table 1.

example 1

45 parts each of samples B and. C from comparative experiment 1 are suspended in 450 parts by volume of H ₂ O with vigorous stirring. Then 0.35 parts by volume of an 85% phosphoric acid (H ₃ PO ₄ ) and 0.5 parts of H ₂ C ₂ O ₄ .2H ₂ O (oxalic acid) are dissolved in 20 parts by volume of water and added to the dispersion. After stirring for a further 20 minutes, the solid is filtered off and the filter cake is dried in air at 170 ° C. The sample D produced from sample B has a surface area of 42.1 m ² / g, a phosphate content of 1.1% by weight and a carbon content of 0.06% by weight. The corresponding values of sample E produced from sample C are: surface area 36.3 m ² / g, phosphate content 1.2% by weight and carbon content 0.04% by weight. Samples D and E are reduced to iron pigments 4 and 5a as described in comparative experiment 1. Part of the sample 5a is passivated in an air-nitrogen mixture at a temperature below 50 ° C. (sample 5b). The measurement results are shown in Table 1.

┌Comparison 2

50 parts of sample A from comparative experiment 1 are stirred into 400 parts by volume of water. After a dispersion time of 10 minutes, a solution of 4.5 parts by volume of water, 0.35 parts by volume of H ₃ PO ₄ (85% strength) and 0.5 part of H ₂ C ₂ O ₄ .2H ₂ O is added. After the dispersion has ended, this becomes Filtered off water and the filter cake was dried in air at 170 C (sample F). Sample F has a surface area of 37 m ² / g, a phosphate content of 1.4% by weight and a carbon content of 0.14% by weight.

70 parts of the sample F as described in Comparative Experiment 1, at 25 mbar pressure for iron (III) oxide sample G m with a surface area of 53.9 ² / g, and then annealed in the manner described therein reduced (Eisenpi ₃ - ment no. 6). The measurement results are shown in Table 1.

Example 2

┌Vergleichsversuch 3 ┐A further 70 parts of the sample F were prepared as in United ^g leichsver- described seeking 1, oxide sample to the iron (III) H with a surface area of 47.9 m ² / g in water vapor-containing air stream annealed. The reduction of sample H to iron pigment 7 also takes place as described in comparative experiment 1. The measurement results are shown in Table 1.

┌Comparison 3 ┐

An iron (III) oxide hydroxide produced according to DE-AS 10 61 760 consists of 97% τ-FeOOH and 3% α-FeOOH and has a surface area of 32.7 m ² / g (sample J).

, 70 parts of sample J are, as described in comparative experiment 1, annealed in a vacuum to pigment Kl with a surface area of 44.8 m ² / h within one hour and a further 70 parts in the same way within 3 hours to sample K2 with a surface area of 40 , 8 m ² / g. In addition, 70 parts of sample J are annealed to samples L1 and L2 in 1 or 3 hours, as also described in comparative test 1, in an atmosphere containing water vapor. II has a surface area of 33.0 m ² / g and L2, such a 30, ₄ m ² / g ·

According to comparative experiment 1, the samples Kl, K2, L1 and L2 are then reduced to the iron pigments 8 to 11 at 350 ° C. The measurement results are shown in Table 2.

Comparative Experiment 4 and Example 3

und dann wie in Vergleichsversuch 1 beschrieben bei 350°C zu den Eisenpigmenten 12a bis 15a reduziert, sowie Teile der Proben 12a bis 15a wie in Beispiel 1 beschrieben zu den Eisenpigmenten 12b bis 15b passiviert. Die Meßergebnisse sind in den Tabellen 2 und 3 aufgeführt.

45 parts each of the tempering products K1, K2, L1 and L2 are equipped as described in Example 1 for the samples K3, K4, L3 and L4:

and then reduced to the iron pigments 12a to 15a as described in comparative experiment 1 at 350 ° C., and passivated parts of the samples 12a to 15a as described in example 1 to the iron pigments 12b to 15b. The measurement results are shown in Tables 2 and 3.

┌Comparison 5

An X-ray-pure δ-FeOOH with a surface area of 33.4 m ² / g, prepared in a manner known per se, is used as the starting material (sample M).

50 parts of this sample M are, as described in Example 1, equipped with 1% H ₃ PO ₄ and 1% H ₂ C ₂ O ₄ .2H ₂ 0 (data in% by weight, based on -FeOOH), filtered and dried. The resulting product M1 has a phosphate content of 1.4% by weight, a carbon content of 0.06% by weight and a surface area of 36.8 m ² / g.

The reduction is carried out as described in comparative experiment 1. The measurement results on the resulting iron pigment No. 15 are given in Table 4.

Example 4

50 parts of sample M are annealed in a continuous rotary tube at 350 C and an average residence time of 45 minutes in a steam containing nitrogen. In order to set a water vapor partial pressure pH ₂ O = 88 mbar, only a small stream of inert gas of 400 N1 nitrogen / h is passed through the reactor in cocurrent. The resulting iron (III) oxide is finished as described in Example 1, the phosphate content is 1.2% by weight, the Konlenstoff content 0.06% by weight and the surface is 23.4 m ² / g (sample M2). This sample M2 is reduced in the same way as the sample M1 from comparative experiment 5. This produces iron pigment No. 17, the magnetic properties of which are given in Table 4.

Example 5

The iron (III) oxide hydroxide sample N (δ-FeOOH with an α-FeOOH content of 30% and a surface area of 26.1 m ² / g) and sample Ö (γ- FeOOH with an α-FeOOH content of 68% and a surface area of 39.0 m ² / g).

50 parts each of the FeOOH samples N and 0 are suspended in 500 volume parts of water and then 0.70 volume parts of an 85% phosphoric acid (H ₃ PO ₄ ) and one part H ₂ C ₂ O ₄ .2 _H20 (oxalic acid) dissolved in ₃ 0 parts by volume of water; added. After stirring for a further 10 minutes, the solid is filtered off and the filter cake is dried in air at 170 ° C. (samples N1 and 01). The process conditions for the respective tempering and, if applicable; Subsequent surface finishing as well as the surface values and carbon / phosphorus analysis values are listed in Table 5. The magnetic properties of the iron pigments 18a, 21a and 22a obtained from samples N5, 06 and 07 by reduction with hydrogen at 350 ° C. and the passivated iron pigments 18b resulting from passing a nitrogen-air mixture at a temperature below 50 ° C. 21b and 22b are listed in Table 6.

┌Comparison test 6

The process conditions of the iron pigments 17, 19, 20 and 21 derived from samples N and 0 are shown in Tables 5 and 6, as are the corresponding measurement results.

Example 6

50 parts of a ferric oxide hydroxide prepared in the customary manner with a proportion of 6% α-FeOOH and 94% γ-FeOOH and a surface area of 29.4 m ² / g are, as described in Example 4, in a rotary tube at 350 ° C and a p _{H 0} of 88 mbar with an average residence time of 30 minutes and then equipped as described in comparative experiment 2. The resulting sample R1 has a surface area of 32.8 m ² / g, a phosphate content of 1.0% by weight and a carbon content of 0.03% by weight. After reduction with hydrogen during L ┘ ┌8 hours at 335 ° C, the resulting iron pigment 23 shows the measurement results given in Table 7. The material is then passivated by passing an air / nitrogen mixture at temperatures below 50 ° C.

Comparative experiment 7

The sample R given in Example 6 is provided with a surface coating, also reduced (iron pigment 24) and passivated, without tempering, as also described. The measurement results are shown in Table 7.

Examples 7 and 8

Each 800 parts of the passivated iron particles No. 23 and 24 produced according to Example 6 and Comparative Experiment 7 are in a 600-volume steel cylinder mill, which contains 9000 parts of steel balls with a diameter between 4 and 6 mm, with 456 parts of a 13 percent solution of a thermoplastic Polyester urethane from adipic acid, 1,4-butanediol and 4,4'-diisocyanatodiphenylmethane in a solvent mixture of equal parts of tetrahydrofuran and dioxane, 296 parts of a 10 percent solution of a polyvinylformal binder containing 82 percent vinyl formal, 12 percent vinyl acetate and 6 percent vinyl alcohol units , in the solution Oil mixture, 20 parts of butylocarate and a further 492 parts of the solvent mixture mentioned and mixed for 4 days. Then another 456 parts of the specified polyester urethane solution, 296 parts of the polyvinyl formal solution used, 271 parts of the solvent mixture and 2 parts of a commercially available silicone oil are added and dispersed for a further 24 hours and filtered through a cellulose / asbestos gas layer. In a conventional coating apparatus is applied to starch and dried after passing through a directional magnetic field within 2 minutes at 80 to 100 ° C, the magnetic dispersion thus prepared on a polyethylene terephthalate carrier film of 11.5 _/. The magnetic layer is smoothed and compacted by pulling over heated and polished rollers at temperatures from 60 to 80 ° C. The finished magnetic layer is 3.9 _/ um thick.

The magnetic properties of the magnetic layer are listed in Table 8.

Claims (5)

1. A method for producing acicular ferromagnetic iron particles from acicular iron (III) oxide provided with a shape-stabilizing surface coating by reduction with hydrogen at 275 to 425 ° C, characterized in that the acicular iron (III) oxide used from acicular iron (III ) oxide hydroxide is obtained by tempering for 10 minutes to 10 hours at 250 to 390 ° C in a water vapor-containing atmosphere with a water vapor partial pressure of at least 30 mbar. . 2. The method according to claim 1, characterized in that the needle-shaped iron (III) oxide hydroxide from lepidocrocite (γ-FeOOH) or a mixture of goethite (α-FeOOH) and lepidocrocite with a minimum content of 20 percent by weight of lepidocrocite and in in an atmosphere containing water vapor with a water vapor partial pressure between 30 and 1013 mbar at 250 to 390 ° C. for 10 minutes to 10 hours. 3. A process for producing acicular ferromagnetic iron particles from acicular iron (III) oxide provided with a shape-stabilizing surface coating by reduction with hydrogen at 275 to 425 ° C, characterized in that the acicular iron (III) oxide hydroxide used from lepidocrokite (γ-FeOOH ) or a mixture of goethite (α-FeOOH) and lepidocrocite with a minimum content of 20 percent by weight, on the surface of which hydrolysis-resistant oxygen acids of phosphorus, the Salts or esters in an amount of 0.2 to 2% by weight of phosphorus, based on the iron oxide hydroxide, and aliphatic mono- or polybasic carboxylic acids having 1 to 6 carbon atoms in an amount of 0.02 to 1.2% by weight of carbon be applied to the iron oxide hydroxide and that in a water vapor-containing atmosphere with a water vapor partial pressure between 30 and 1013 mbar at 250 to 390 ° C for 10 minutes to 10 hours. 4. A process for producing acicular ferromagnetic iron particles from acicular iron (III) oxide provided with a shape-stabilizing surface coating by reduction with hydrogen at 275 to 425 ° C, characterized in that the acicular iron (III) oxide hydroxide used from lepidocrokite (γ-FeOOH ) or a mixture of goethite (α-FeOOH) and lepidocrocite with a minimum content of 20% by weight lepidocrocite, is annealed in a water vapor-containing atmosphere with a water vapor partial pressure between 30 and 1013 mbar at 250 to 390 ° C for 10 minutes to 10 hours and on the surface of the resulting iron (III) oxide hydrolysis-resistant oxygen acids of phosphorus, their salts or esters in an amount of 0.2 to 2 percent by weight of phosphorus, based on the iron oxide hydroxide, and aliphatic mono- or polybasic carboxylic acids with 1 to 6 carbon atoms in an amount of 0.02 to 1.2 weight percent carbon, based on a on the iron oxide hydroxide. ┌5. Use of the acicular ferromagnetic iron particles produced according to one of claims 1 to 4 for the production of magnetic recording media.