EP0024692B1 - Process for preparing acicular ferromagnetic iron particles, and their use - Google Patents

Description

The invention relates to a process for the production of acicular ferromagnetic iron particles by reduction of 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 force 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 influenced within a wide range by a suitable selection of the polymeric organic binder systems, but the magnetic properties are then excluded to make further demands regarding the shape, size and dispersibility of the metal particles.

High coercive field strength and high remanence are prerequisites for materials for magnetic storage layers. Therefore, the corresponding metal particles must show magnetic single-range behavior. In addition, the anisotropy that is present or that can additionally be achieved by the magnetic alignment in the strip should be caused by external influences, such as, B. temperature or mechanical stress, to be little affected, ie the small particles should be shape-anisotropic, in the preferred case needle-shaped, and they should generally be between 10 ^-2 and 1 µm in size.

It is known to iron particles of the type described by reducing finely divided acicular iron compounds, such as. B. the oxides with hydrogen or another gaseous reducing agent. In order for the reduction to take place at a speed that is suitable for practice, 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-A-2 014 500). The treatment of iron oxide with tin (II) chloride has also been described (DE-A-1 907 691).

However, the catalytic acceleration of the reduction of preferably needle-shaped starting compounds generally results in needles which are much smaller than the starting product with an additionally small 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. In addition to the influences caused by 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. With 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. in DE-A-2 434 058, 2 434 096, 2 646 348 and 2 714 588, do not fully satisfy.

Another way of producing pronounced finely dispersed metal particles is proposed in CH-A-528 320. Thereafter, a finely divided inert carrier material is provided with oxidic metal deposits and this is then reduced to metal with hydrogen, if necessary to enlarge the particles in the presence of water vapor. A disadvantage of this method is that because of the non-magnetic carrier material, only a little magnetic material is available per unit volume.

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 can be produced from acicular iron (III) oxide with a shape-stabilizing surface coating by reduction with hydrogen at 275 to 425 ° C with the required properties if the acicular iron (III) oxide used is out Lepidocrokit (y-FeOOH) or a mixture of goethite (a-FeOOH) and lepidocrokit with a minimum content of lepidocrokit of 20 percent by weight, and consists of a water vapor containing atmosphere with a water vapor partial pressure between 30 to 1013 mbar at 250 to 390 ° C for 10 minutes to 10 hours.

The iron (III) oxide hydroxides mentioned have a BET specific surface area of 20 to 75 m ² / g, an average particle length of between 0.2 and 1.5 and preferably between 0.3 and 1.2 pm and a length of -Thickness ratio of at least 10, suitably 10 to 40 characterized. They can be prepared from iron (II) salt solutions with alkalis with simultaneous oxidation, as described, for example, in DE-B-1 061760. For this purpose, iron (III) oxide hydrate nuclei up to an amount of 25-60 mol are formed 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 precipitates, from which the final product is then formed at a temperature between 20 and 70 ° C and at a pH value of 4.0 to 5.8 set by adding further amounts of alkali with intensive air distribution through growth. After growth has ended, 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 helps to maintain the outer shape during the further reworking 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-A-2434058 and 2 434 096.

Also known and described in DE-A-2 646 348 is the shape-stabilizing treatment of the ferric 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. Advantageously, the esters of phosphoric acid with aliphatic monoalcohols with 1 to 6 carbon atoms, such as. B. tert-butyl ester of phosphoric acid. Carboxylic acids in the process are saturated or unsaturated aliphatic carboxylic acids with up to 6 carbon atoms and up to 3 acid groups, it being possible for one or more hydrogen atoms in the aliphatic chain to 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 are designed to stabilize the shape 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 between 30 and 1013 mbar according to the inventive method. The end product is an 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 vortex 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, 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 introduced directly in the form of steam or added to the other gases. Annealing can be carried out particularly advantageously in continuous reactors, e.g. in a continuous rotary kiln, 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. It is therefore also possible to work with little or no gas flows. 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 advisable to passivate the finely divided iron powders obtained in this way by passing an air or oxygen-inert gas mixture over them, since this changes the pyrophoric character of the needle-shaped iron particles with a length between 0.1 to 0.8 µm and a length-to-length ratio. Thickness ratio of 5 to 25: 1 can be mastered.

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 dendrite-free and treated to maintain the outer form 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 obtained according to the invention are used in the usual way for the production of magnetogram carriers, the needle-shaped particles can be oriented magnetically particularly easily, and important electroacoustic values such as depth and height modulation are 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 S _N2 determined according to BET was primarily used to characterize the acicular iron (III) oxide hydroxides such as lepidocrocite and goethite-lepidocrocite mixture used. Electron microscope images provide information about the appearance and dimensions (LID ratio) of the iron oxide hydroxide particles. 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 _e , measured in kA / m, were based on a stuffing density of p = 1.6 g / cm ³ in the powder measurements. Specific remanence (M _{r / P} ) and saturation (M _{m / P} ) 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 variable characteristic of recording processes, which in particular determines the working point in magnetic recording. The more non-uniform the remanent coercive field strength of the individual magnetic particles in the recording layer, the wider 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. For the characterization of the distribution of the switching field strengths of the individual particles, a value h _s 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-B-1 061 760, an iron (III) oxide hydroxide (sample A) with a specific surface area S _N2 of 37.6 m ² / g, which consists of a mixture of 95% y-FeOOH and 5% a -FeOOH is made.

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 ^z / 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 NI / h air and water vapor ( _PH20 845 mbar) is passed over the pigment. The resulting iron (III) oxide (sample C) has a surface area of 34.9 m ² / g.

In each case 5 parts 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 NI / h within 8 hours. The measurement results are shown in Table 1.

example 1

45 parts each of samples B and C from comparative test 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 part 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. 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.

Comparative experiment 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 0 is added. After the dispersion has ended the water is filtered off and the filter cake is 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 sample F are, as described in comparative experiment 1, annealed at 25 mbar pressure to give iron (III) oxide sample G with a surface area of 53.9 m ² / g and then reduced in the manner described there (iron pigment No. 6) . The measurement results are shown in Table 1.

Example 2

Another 70 parts of sample F were, as described in comparative experiment 1, annealed to iron (III) oxide sample H with a surface area of 47.9 m 2 / g in a water vapor-containing air stream. 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.

Comparative experiment 3

An iron (III) oxide hydroxide prepared according to DE-B-1 061 760 consists of 97% y-FeOOH and 3% a-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 K1 with a surface area of 44.8 m ² / h within one hour and a further 70 parts in the same way to sample K2 with a surface area of 40 within 3 hours. 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. L1 has a surface area of 33.0 m ² / g and L2 a surface area of 30.4 _{m2 /} g.

According to comparative experiment 1, samples K1, K2, L1 and L2 are then reduced to 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 parts of the samples 12a to 15a passivated to the iron pigments 12b to 15b as described in Example 1. The measurement results are shown in Tables 2 and 3.

Comparative experiment 5

An X-ray-pure γ-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 .2 H ₂ O (data in% by weight, based on γ-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. 16 are given in Table 4.

Example 4

50 parts of sample M are annealed in a continuous rotary tube at 350 ° C. and a mean residence time of 45 minutes in a steam stream containing nitrogen. In order to set a water vapor partial pressure pH ₂ 0 = 88 mbar, only a small stream of inert gas of 400 NI 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 carbon content is 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 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 starting materials used are the iron (III) oxide hydroxides sample N (y-FeOOH with an a-FeOOH content of 30% and a surface area of 26.1 m ² / g) and sample O (γ -FeOOH with an a-FeOOH content of 68% and a surface area of 39.0 m ² / g).

50 parts each of the FeOOH samples N and O are suspended in 500 parts by volume of water and then 0.70 parts by volume of an 85% phosphoric acid (H ₃ PO ₄ ) and one part H ₂ C ₂ O ₄ .2 H ₂ 0 (oxalic acid) added dissolved in 30 parts by volume of water. 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 any subsequent surface treatment, as well as the surface values and carbon / phosphorus analysis values are listed in Table 5. The magnetic properties of the iron pigments 18a, 21 and 22a obtained from the 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.

Comparative experiment 6

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

Example 6

50 parts of a ferric oxide hydroxide prepared in a conventional manner with a proportion of 6% a-FeOOH and 94% y-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 pH ₂ 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 for 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 without tempering, as also described, reduced (iron pigment 24) and passivated. The measurement results are shown in Table 7.

Examples 7 and 8

In each case, 800 parts of the passivated iron particles No. 23 and 24 produced according to Example 6 and Comparative Experiment 7 are extracted with 456 parts of a 13 percent solution of a thermoplastic polyester urethane in a steel cylinder mill comprising 600 parts by volume, which contains 9000 parts of steel balls with a diameter between 4 and 6 mm Adipic acid, 1,4-butanediol and 4,4'-diisocyanatodiphenylmethane in a mixed solvent of equal parts 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 solvent mixture mentioned , 20 parts of butyl octoate and a further 492 parts of the solvent mixture mentioned and dispersed 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 fiber layer. The magnetic dispersion thus produced is applied to a 11.5 μm thick polyethylene terephthalate carrier film on a conventional coating machine and, after passing through a magnetic field, is dried at 80 to 100 ° C. within 2 minutes. 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 µm thick.

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

Claims (4)

1. A process for the manufacture of acicular ferromagnetic iron particles from acicular iron(III)oxide, provided with a shape-stabilizing surface coating, by reduction with hydrogen at a temperature of from 275 to 425°C, wherein the acicular iron(III)oxide hydroxide used consists of lepidocrocite (y-FeOOH) or a mixture of goethite (a-FeOOH) and at least 20% by weight of lepidocrocite, and is heated at a temperature of from 250 to 390°C for a period of from 10 minutes to 10 hours in an atmosphere containing water vapor at a partial pressure of from 30 to 1013 mbar.

2. A process as claimed in claim 1, wherein, prior to heating, there are applied to the surface of the acicular iron(III)oxide hydroxide such an amount of a hydrolysis-resistant phosphorus oxyacid or a salt or ester thereof that from 0.2 to 2% by weight, based on the iron oxide hydroxide, of phosphorus is present, and such an amount of an aliphatic monobasic or polybasic carboxylic acid of 1 to 6 carbon atoms that from 0.02 to 1.2% by weight, based on the iron oxide hydroxide, of carbon is present.

3. A process as claimed in claim 1, wherein there are applied to the surface of the iron(III)-oxide hydroxide, formed in the heating operation, such an amount of a hydrolysis-resistant phosphorus oxyacid or a salt or ester thereof that from 0.2 to 2% by weight, based on the iron oxide hydroxide, of phosphorus is present, and such an amount of an aliphatic monobasic or polybasic carboxylic acid of 1 to 6 carbon atoms that from 0.02 to 1.2% by weight, based on the iron oxide hydroxide, of carbon is present.

4. The use of the acicular ferromagnetic iron particles produced as claimed in any of claims 1 to 3 for the production of magnetic recording media.