EP0000749B1 - Method for the preparation of needle-shaped ferrimagnetic iron oxides and their application - Google Patents

Description

The invention relates to a method for producing acicular ferrimagnetic iron oxides by dewatering acicular iron (III) oxide hydroxides to acicular iron (III) oxides, then reducing these iron (III) oxides at temperatures between 300 and 650 ° C by means of organic compounds which are decomposable in this temperature range Oxidation of the resulting products with oxygen-containing gases at 200 to 500 ° C and their use for the production of magnetic recording media.

Acicular ferrimagnetic iron oxides, such as magnetite and gamma-iron (III) oxide, have long been widely used as a magnetizable material in the production of magnetic recording media. A large number of processes have already become known for producing the gamma-iron (III) oxide used in particular. For example, GR-PS 675 260 already describes a process for the production of gamma-iron (III) oxide, in which acicular iron oxide hydroxide (goethite) is dewatered to α-iron (III) oxide, in a reducing atmosphere at more than 300 ° C is converted to magnetite and oxidized in air at temperatures below 450 ° C to acicular gamma iron (III) oxide. In the course of efforts to improve the crystalline, mechanical and also magnetic properties of such materials, this process was varied several times in its individual stages, and was also modified by changing the starting materials.

Almost simultaneously with the work on the method according to the British patent specification, suitable magnetic iron oxides were obtained in the manner disclosed in DE-PS 801 352, namely by treating non-magnetic iron oxides with the salts of short-chain carboxylic acids and subsequent heating. The magnetite obtained afterwards can also be converted into gamma-iron (III) oxide by oxidation at 200 to 400 ° C.

It is then known from US Pat. No. 2,900,236 that all organic compounds which are decomposable at temperatures below 540 ° C. with little formation of tar and ash are suitable for the reduction of the non-magnetic iron oxides to magnetite. For this purpose, the iron oxide is brought into contact with the gaseous, solid or liquid organic substance and heated to a temperature of 540 to 650 ° C. While US Pat. No. 2,900,236 specifies all of the corresponding organic substances as being usable with particular mention of wax, starch and oil, DE-AS 12 03 656 describes salts of soluble soaps precipitated on iron oxide, and DE-OS 20 64 804 both higher hydrocarbons, higher alcohols and amines, higher fatty acids and their salts as well as oils, fats and waxes, in DD-PS 91 017 also long-chain carboxylic acids or their salts, in DE-AS 17 71 327 aliphatic monocarboxylic acids with 8 to 24 carbon atoms optionally mixed with morpholine 'and in JP-OS 80 499/1975 organic compounds, such as Ethanol, containing inert gases as a reducing agent for the extraction of magnetite from non-magnetic iron oxides. In the previously known processes mentioned, heating is carried out partly in the absence of air, the reaction remaining at the magnetite stage or in the presence of air, as a result of which the magnetite is immediately oxidized to gamma-iron (III) oxide.

The starting materials for this conversion of the iron oxides by means of organic substances were predominantly the corresponding a-modifications, such as a-FeOOH or α-Fe ₂ O ₃ , but 8-FeOOH (DE-AS 1203656) and y-Fe00H (DE- OS 22 12 435) used successfully.

The stated diverse efforts to improve the magnetic iron oxides suitable for the production of magnetic recording media make it clear that in this way both the increasing technical demands on the information media and the disadvantages of other magnetic materials which can also be used are counteracted.

The invention was therefore based on the object of improving the known acicular ferrimagnetic iron oxides and of eliminating the disadvantages to date. In particular, it was an object of the invention to provide acicular gamma-iron (III) oxide which is distinguished by high values for the coercive field strength and the remanence, by good crystallinity and by mechanical and magnetic stability.

It has now been found that acicular ferrimagnetic iron oxides are formed by dewatering acicular iron (III) oxide hydroxides to acicular iron (III) oxides, then reducing these iron (III) oxides at temperatures between 300 and 650 ° C by means of organic compounds which are decomposed in this temperature range and Allow oxidation of the reduction product with oxygen-containing gases to be obtained at 200 to 500 ° C with the properties required by the task if the acicular iron (III) oxide hydroxide used consists of a mixture of goethite and lepidocrocite with a lepidocrocite content of at least 60% and up to 98% and is dewatered at temperatures from 200 to 700 ° C.

It is particularly advantageous if, in the process according to the invention, the acicular iron (III) oxyhydroxides of the mixture have a length to thickness ratio of at least 10 and are dewatered at 300 to 650 ° C.

The iron (III) oxyhydroxides composed of goethite and lepidocrocite preferably consist of 70 to 98% of lepidocrocite.

The batches used for the process according to the invention can be prepared under suitable reaction conditions from iron (II) salt solutions with alkalis with simultaneous oxidation. It has proven to be particularly expedient to use iron (III) oxide hydrate seeds from an aqueous iron (II) chloride solution using alkalis, such as alkali hydroxide or ammonia, at temperatures between 10 and 32 ° C. and vigorous stirring to produce fine air bubbles To form amount of 25 to 60 mole percent of the iron used, from which then at a temperature between 25 and 70 ° C and at a pH adjusted by adding further amounts of alkali from 4.0 to 5.8 with intensive air distribution through growth The end product is created.

After growth has ended, the solids content of iron (III) oxide hydroxide in the aqueous suspension should be between 10 and 50 g / l, preferably 15 to 45 g / l. After filtering off and washing out the precipitate, the iron (III) oxide hydrate mixture thus obtained is dried at 60 to 200 ° C.

Stable crystal needles of the goethite-lepikrokite mixture with at least 60% and up to 98% lepidocrocite, which have almost no dendritic branches, can be obtained by the procedure described by way of example. These acicular goethite-lepicrokite mixtures have an average particle size of 0.2 to 1.5, usually 0.3 to 1.2 µm. The length-to-thickness ratio is generally at least 10, mostly 12 to 40. The surface area measured according to BET is between 18 and 70 m ² / g.

If a goethite-lepidocrocite mixture characterized in this way is dewatered according to the invention at temperatures between 200 and 700.degree. C., preferably between 300 and 650.degree. C. and in particular between 450 and 600.degree Improve values for coercive field strength and remanence of the end products available from them. The dewatering can be carried out both in air and in an inert gas atmosphere.

The product present after the dewatering step is converted in a manner known per se with largely ash- and tar-free decomposable organic substances at temperatures between 300 and 650 ° C. to acicular ferrimagnetic iron oxides.

For this purpose, the dewatered goethite-lepidocrocite mixture is mechanically mixed with the solid or liquid organic substance or coated in a suitable solution or suspension of the substance and then heated to temperatures of 300 to 650 ° C. under inert gas. In the same way, the method can be carried out by means of gaseous organic substances which are added to the inert gas. Organic compounds which can be used in the process according to the invention are all compounds which are stated to be suitable according to the prior art, provided they can be added at temperatures between 300 and 650 ° C. Expediently, higher fatty acids, their derivatives, glycerin, inert gas / alcohol vapor mixtures and also methane are used for this purpose. Depending on the organic substance used and the selected reaction temperature, the conversion of the dewatered goethite-lepidocrocite mixture to the acicular magnetite is complete after about 1 to 120 minutes.

The acicular magnetite obtained after this reduction reaction is usually oxidized to gamma-iron (III) oxide, advantageously by passing air or adding oxygen. Temperatures from 200 to 500 ° C.

However, if the conversion reaction specified is not carried out under an inert gas, but rather in the presence of oxygen, If carried out in air, gamma-iron (III) oxide can be obtained directly if the oxidation takes place at temperatures below about 500 ° C.

The acicular ferrimagnetic iron oxides produced according to the invention, especially the y-iron (III) oxide, show unexpectedly advantageous properties when used as magnetic pigments for the production of magnetogram carriers. To produce magnetic layers, the gamma-iron (III) oxide is dispersed in polymeric binders. Compounds known for this purpose are suitable as binders, such as homopolymers and copolymers of polyvinyl derivatives, polyurethanes, polyesters and the like. The binders are used in solutions in suitable organic solvents, which may optionally contain further additives. The magnetic layers are applied to rigid or flexible supports such as plates, foils and cards.

The acicular ferrimagnetic iron oxides produced according to the invention, in particular the gamma-iron (III) oxide obtainable in this way, differ significantly from the known gamma-iron (III) oxides by their more uniform needle shape, since agglomeration is avoided by sintering together their improved crystallinity and the higher coercive field strength and remanence, which can surprisingly be achieved in the process according to the invention. These improvements in the magnetic material are also very noticeable in the magnetic tapes made therefrom.

The subject matter of the invention is illustrated by the following examples.

The magnetic powder values are determined by measuring an oxide sample brought to a stuffing weight _t of D = 1.2 g / cm ³ using a conventional vibration magnetometer at a measuring field strength of 100 kA / m. The coercive field strength (H _e ) is expressed in [kA / m], the specific remanence (M, / p) and the specific magnetization (M _{m /} p) are given in [nT cm ³ / g].

example 1

In a 26 liter reaction vessel, 7.31 mol FeCl ₂ (926.6 g) are dissolved with deionized water to 9 liters and a temperature of 26 ° C. is set while simultaneously passing 500 liters nitrogen / h. With stirring at 500 rpm, 7.38 mol of NaOH (295.2 g) dissolved in 4.5 l of demineralized water are then added in 30 to 40 minutes. After the NaOH egg has ended, stirring is continued for 10 minutes and then 150 l of air are introduced per hour instead of nitrogen until the pH has dropped to about 3.4. An orange-colored germ suspension was obtained in this way after 2 h 30 minutes.

The germ suspension is heated to 40 ° C. with further stirring at 500 rpm and passing through 150 l of air / h. After this temperature has been reached, the amount of air is increased to 400 l / h and the pH is kept at pH = 4 by the addition of aqueous sodium hydroxide solution until the end of the reaction. Duration of growth: 2 hours 40 minutes.

The suspension is washed on a suction filter with water until the filtrate is chloride-free and then dried at 130 ° C. in a drying cabinet.

The resulting iron oxide hydroxide is a mixture of 94% lepidocrocite and 6% goethite, has a length-to-thickness ratio of 31 and a BET specific surface area of 34.7 _m ² / g.

This goethite-lepidocrocite mixture is heated in air for one hour at 490 ° C. The resulting product is now mixed with 2% by weight of stearic acid, divided into two identical samples 1 A and 1 B and converted to magnetite under nitrogen under the conditions specified in Table 1 and oxidized to gamma-iron (III) oxide in air. The magnetic properties are also shown in Table 1.

Comparative experiment 1

The procedure is as described in Example 1, but the goethite-lepidocrocite mixture is not dewatered before the conversion and oxidation. Reaction conditions and measurement results are given in Table 1.

Example 2

On the basis of the procedure according to Example 1, iron (III) oxide hydroxide mixtures with a lepidocrocite fraction of 72% with a BET specific surface area of 33.2 m ² / g are produced. These goethite-lepidocrocite mixtures are each dewatered in air at the temperatures given in Table 2 for one hour, then reduced to magnetite at 300 ° C. while passing over a nitrogen / methanol vapor mixture and then in air at 400 ° C. to the gamma Oxidized iron (III) oxide. The nitrogen-methanol mixture is formed when nitrogen is passed through in an amount of 50 l per hour through a container heated to 105 ° C., to which methanol is metered. The resulting magnetic values of the tests are also shown in Table 2.

Example 3

As described in Example 1, 4 goethite-lepidocrocite mixtures were prepared. Sample D in a 26 l reaction vessel, samples E ― G in a 700 l stirred kettle (Table 3).

The conversion conditions and the magnetic powder values are shown in Tables 4 and 5 below.

Table 5 lists samples which were dewatered in 30 minutes at the lower dewatering temperature of 4 ° C. and then converted to γ-Fe ₂ O ₃ .

Example 4

49.5 liters of a 30.1% FeCl ₂ solution and 180 liters of drinking water are placed in a 700 liter kettle. The pH of the template was 3.6; the temperature door was 16 ° C.

120 l of an aqueous solution of NaOH, prepared from 97.2 l of drinking water and 22.8 l of a 25% sodium hydroxide solution, were introduced with stirring over the course of 30 minutes. This corresponds to a 56% precipitation based on Fe (II). After the alkali was added, a pH of 7.2 and a temperature of 17 ° C. were measured.

After the NaOH entry has ended, stirring is continued for 5 minutes and then 5 m ^{3 of} air / h are initiated for the oxidation. The temperature was simultaneously raised to 24 ° C. within 40 minutes. The nucleation was complete after 4 hours and 5 minutes. The pH had dropped to 3.4.

With further passage of 5 m ^{3 of} air / h, the germ suspension was heated to 33 ° C. in 20 minutes. The amount of air was then increased to 8 m ³ / h and the pH was adjusted to 5.5 by adding aqueous sodium hydroxide solution. and regulated to this value until the end of the reaction. Beginning with the increase in the air volume to 8 m ³ / h, the mixture was simultaneously heated to 38 to 39 ° C. in 30 minutes and this temperature was maintained until the end of the growth reaction.

The oxidation was complete after 1 hour and 40 minutes. After filtration through a filter press, the product is washed with water until the filtrate is chloride-free and then dried at 110 ° C. in a drying cabinet.

The resulting iron (III) oxide hydrate is a mixture of 92% lepidocrocite and 8% goethite, has a length-to-thickness ratio of 16 and a BET specific surface area of 23.3 _{m2 /} g _.

200 g of this goethite-lepidocrocite mixture are heated to 500 ° C. in air for 30 minutes for dewatering. The resulting product is mixed with 2% stearic acid and then kept in a drying cabinet at 110 to 120 ° C for 30 minutes. The subsequent reduction to Fe ₃ 0 ₄ was carried out at 550 ° C in 30 minutes, the oxidation to γ-Fe ₂ O ₃ with air after cooling the Fe ₃ 0 ₄ to 280 ° C in 90 minutes (sample H).

A comparative sample J was produced without the dewatering from the same starting product under otherwise identical conditions (Table 6).

Magnetic pigment dispersions and then magnetic tapes are produced from the two gamma-iron (III) oxide samples H and J.

To produce the two magnetic dispersions, pot mills are filled with 8,000 parts of steel balls with a diameter of 5 mm and then with 700 parts of the respective magnetic material, 420 parts of a mixture of equal parts of tetrahydrofuran and dioxane, 8.75 parts of lecithin, 8.75 parts of one neutral polyamino salt and 210 parts of a 20% solution of a Provide copolymers of 80% vinyl chloride, 10% dimethyl maleinate and 10% diethyl maleinate (K value = 58) in a mixture of equal parts of tetrahydrofuran and dioxane. The mixture is predispersed for 40 hours. 1,090 parts of a 10% strength solution of a thermoplastic polyester urethane consisting of adipic acid, 1,4-butanediol and 4,4'-diisocyanatodiphenylmethane in a mixture of equal parts of tetrahydrofuran and dioxane, 0.7 part of polydimethylsiloxane are then added. After a further 5 hours of dispersion, the magnetic dispersion obtained is filtered under pressure through a filter with a pore size of 5 μm. A 6 µm thick polyethylene terephthalate film is coated with the magnetic dispersion using a straightedge caster and dried at temperatures between 60 and 100 ° C after passing through a magnetic field. After drying, the film has a magnetic layer 5 µm thick. The magnetic layer is compacted by passing between heated rollers (80 ° C under a line pressure of approx. 3 kg / cm). The coated film is cut into strips 6.25 wide.

The electroacoustic measurement is carried out according to DIN 45 512, part 11.

The measurement results are shown in Table 7.

Claims (3)

1. A process for the manufacture of acicular ferrimagnetic iron oxide by dehydrating acicular iron(III) oxide hydroxide to give acicular iron(III) oxide, then reducing this iron(lll) oxide at from 300 to 650⁰C by means of an organic compound which decomposes in this temperature range, and oxidizing the reduced product with an oxygen-containing gas at from 200 to 500°C, characterized in that the acicular iron(III) oxide hydroxide employed consists of a mixture of goethite and lepidocrocite comprising at least 60% and up to 98% of lepidocrocite and is dehydrated at from 250 to 700°C.

2. A process as claimed in claim 1, characterized in that the acicular iron(III) oxide hydroxide in the mixture has a length/width ratio of at least 10 and is dehydrated at from 300 to 650°C.

3. Use of the acicular ferrimagnetic iron oxide produced in accordance with claim 1 as magnetic material for the production of magnetic recording media.