KR100282008B1 - Magnetic recording media - Google Patents

Abstract

A nonmagnetic substrate and at least one magnetic layer applied thereon, including needle-shaped chromium dioxide uniformly distributed in the polymeric binder and additives, having a coercive force of at least 60KA / m, and having an average particle diameter of 300 nm Less than, and the HD factor is A magnetic recording medium satisfies the recognition. Where KT is the coercive force of the magnetic layer in units of [KA / m] measured at an external field of 380KA / m, L is the average particle diameter, ΔL is the particle size distribution range, and is determined from the numerical distribution of the particle diameter, Is measured in [nm] and is greater than 1.85. The preparation process of this chromium dioxide.

Description

Magnetic recording media

The present invention comprises a non-cyclic chromium dioxide and additives consisting of a nonmagnetic substrate and at least one magnetic layer applied thereto and uniformly distributed in the polymeric binder and having a coercive force of at least 60 kA / m, small particles and narrow sizes A magnetic recording medium having a particle distribution of and a method for producing the chromium dioxide.

The use of acyclic chromium dioxide and its preparation for magnetic recording media have been described in many publications. Compared with other magnetic oxide recording media, magnetic recording media containing chromium dioxide have high coercivity, residual magnetism and saturation magnetization, and in particular, excellent magnetic properties of certain shapes and small sizes of acyclic chromium dioxide particles.

Continued development of analog and digital audio and video signal processing has led to an increased need for suitable magnetic recording media to store the signals. An essential feature of such magnetic recording media is that the storage density must be increased. In the case of a magnetic recording medium including magnetic fine particles such as chromium dioxide in the matrix in the bonding of the magnetic layers, this means that the layer thickness decreases, the coercive force of the magnetic particles needs to be increased, and the magnetic particle size needs to be reduced. However, taking into account the smaller thickness of the magnetic layer, the magnetic moment of entry must be increased to the same extent in order to maintain the residual flux of the magnetic layer unit which is clear at the recording level.

Therefore, continuous development of chromium dioxide materials has been attempted to obtain higher coercivity, higher saturation magnetization and at the same time narrower particle distribution. Chromium dioxide is generally obtained by two processes. In addition to the synthesis of chromium dioxide by the same proportion of chromium (III) oxide and chromium (VI) oxide under hydrothermal conditions, as described in EP 27 640, by temperature decomposition of hydrated chromium (III) oxide Formulations are also well known. In addition, German Patent Publication No. 22 10 059 describes a process in which Cr ₂ (CrO ₄ ) ₃ nH ₂ O decomposes at 250 to 500 ° C. and 30 to 1000 bar when n is 1 to 8. The resulting material is too low for commonly used recording media and has an additional coercive force when n is a high value. When the value of n is greater than 8, residual induction and saturation magnetization decrease as a result of CrOOH formed simultaneously. Improvement of the magnetic properties is obtained using a process according to German Patent Publication No. 23 32 854, wherein the described substrates carrying out the exothermic decomposition under the reaction conditions described above are mixed with chromium chromium (III). According to German Patent Publication No. 29 19 572, the deformation of chromium dioxide consisting of lanthanum, yttrium, barium or strontium leads to an increase in magnetic properties. Furthermore, modified chromium dioxide can be prepared from chromium (III) chromium having a higher water component (hydration (n) of 8 to 12) (DE-B 25 20 030 and DE-B 26 48 305).

A common feature of the process for producing chromium dioxide from chromium chromium (III) is the use of very small amounts of water to prevent loss of saturation magnetization by CrOOH form. Moreover, the production of chromium (III) chromium used in such processes has many time-consuming processes and the resulting uneconomical problems. Moreover, chromium (III) chromate is obtained in powder form using filters and encapsulation devices that require low hydration and expensive safety devices. Higher degrees of hydration result in viscose fasts that are difficult to control (ie, not flowing). Further developments in the production of chromium dioxide from chromium (III) chromium are described in EP 0 304 851 and US Pat. No. 50 30 371. Thus, n Cr ₂ (CrO ₄ ) ₃ nH ₂ O, which is 13, is actually produced for a short time and has a potential water content of a moderator, glycerol, methanol or glycol, under conditions of 200 to 500 ° C. at 50 to 700 bar given to chromium dioxide. Inexpensive devices are actually prepared by reducing the aqueous CrO ₃ suspension with organic compounds. CrO ₂ modified with iron and terulium or antimony, a material obtained according to US Patent Publication No. 5 030 371, has a higher specific saturation magnetization and greater than 60 kA / m due to the consideration of process engineering issues formed during the addition of glycerol. It is very difficult to obtain a relatively large amount having coercive force and very high adhesion of chromium chromium (III) having certain characteristics. It is described in EP 239 089 to obviate the disadvantage of exhibiting high tackiness by using organic moderators which partially react with CrO ₃ during the preparation of the reaction mixture. A portion of the exothermic decrease of CrO ₃ then takes place during the reaction in the reactor. Considerable portion of the target thermal energy is supplied to the chemical system itself (heating) and a constant heat distribution of the reaction mixture is possible. However, this has the disadvantage that the particle size distribution of the resultant material is adversely affected during nucleation since the temperature cannot be controlled. Another disadvantage is that the resulting magnetic powder has a coercive force of less than 53 kA / m.

The chromium dioxide material obtainable in accordance with the prior art is suitable for the production of high quality magnetic recording media which are currently suitable for conventional use, but additionally have disadvantages in the method of recording and storing at high density. Magnetic recording media meeting these requirements have not only a constant high coercivity of the magnetic layer but also a high coercivity of a layer larger than 60 kA / m and are combined in a layer combined with the small means particle length required for the storage area per desired high information density. It has a narrow particle size distribution of acyclic magnetic materials.

It is an object of the present invention to provide a magnetic recording medium which obtains chromium dioxide as a magnetization material of a recording layer and satisfies the above-mentioned conditions suitable for use for high density storage of information.

SUMMARY OF THE INVENTION An object of the present invention described above is a magnetic recording medium comprising a non-magnetic substrate and at least one magnetic layer applied thereto and comprising acyclic chromium dioxide and additives uniformly distributed in a polymer binder and having a coercive force of at least 60 kA / m. In the case of chromium dioxide, the median particle length is less than 300 nm and the Where KT is the coercive force of the magnetic layer of [kA / m], measured at an external system of 380kA / m, L is the intermediate particle length, and ΔL is determined as a distribution of particle lengths as a range of particle size distribution. L and ΔL measured in [nm] are obtained by a magnetic recording medium characterized by being 1.85 or more, preferably 1.85 to 4.0.

In addition, the present invention is suitable for novel magnetic recording media, modified with iron and tellurium and / or antimony, and based on chromium dioxide as the main component, 0.05 in iron dioxide and tellurium oxide and / or antimony compounds. 1.54 to 2.32 parts by weight of CrO ₃ used per 1 part of water, by reacting the water-soluble CrO ₃ suspension to which 10 to 10% by weight was added and taking into account the amount of water formed during the oxidation of the organic reducing agent at 250 to 400 ° C. at least 70 bar. By reacting an organic reducing agent to form a Cr ((VI): Cr (III) ratio of 1 to 1: 1, coercivity greater than 60 kA / m, saturation magnetization greater than 85 nTm3 / g, and greater than 300 nm A process for preparing chromium dioxide having a small particle size distribution of small average particle diameters, wherein the organic reducing agents used are glycerol and octanol and the amount of glycerol is 25 to 70% based on the total reducing equivalents.

The reducing equivalent is defined as the amount of reducing agent required to set the Cr (III) / Cr (VI) ratio by proportional reduction of CrO ₃ . Reducing agents glycerol and octanol are essentially oxidized to CO ₂ and water during CrO ₂ synthesis.

In an embodiment of the process of the invention, the preparation of chromium dioxide occurring in the reactor is carried out in such a way that the temperature is maintained at about 130 ° C. for one to two hours during the heating step. In addition, it can be seen that the object of the present invention is particularly effectively achieved if the heating of the reactor from 200 ° C to 300 ° C is carried out in a short period of time, preferably about 60 minutes or less.

In carrying out the process of the invention, aqueous aqueous glycerol solution and octanol are scored in a mixture of CrO ₃ and water. Exothermic reactions occur where CrO ₃ is completely oxidized and octanol is partially oxidized. Thus, the amount of octanol that is not reacted in the reactor, or the partially oxidized product, and thus the heat during the reaction, is minimized in a controlled manner and conditions are carried out for the free flowing reaction mixture in an easy-to-handle form. For water-soluble CrO ₃ suspensions containing dissolved CrO ₃ , the total number of parts by weight of CrO ₃ used per part of water depends on the amount of water formed during oxidation of glycerol and complete oxidation of octanol and the amount of water in aqueous glycerol solution. This results in a total weight ratio of 1.54 to 3.32 parts of CrO ₃ per part of water. It is necessary to control the very violent reaction of glycerol and CrO ₃ , which dilute glycerol with water. Glycerol is usually diluted with water in a weight ratio of 1: 4.5 to 1: 1.5 in the case of concentrated reaction mixtures. The amount of glycerol and octanol is such that 33.0 to 50.0%, preferably 40%, of the total amount of CrO ₃ is reduced from chromium (VI) to chromium (III).

Prior to commencement of the octanol and glycerol initiation, the aqueous CrO ₃ suspension is fully excited with the dopant and heated to 50-70 ° C. in a suitable apparatus. The excitation agent used is, for example, a solubilizer. Iron and tellurium and / or antimony and / or their compounds are used as modifiers. The modifier is carried out in an amount of 0.05 to 10.0% by weight based on the amount of CrO ₂ and is added to the aqueous CrO ₃ suspension before the organic reducing agent is added. The aqueous aqueous glycerol solution and octanol are then spotted in the mixture at 50-70 ° C. with simultaneous cooling, so that the temperature of the mixture is maintained in the initial temperature range. In a simple and rapid manner, in order to carry out the preparation of the reaction mixture without involving the foaming process, the addition of the organic reducing agent starts with glycerol. Complete reaction of glycerol causes the viscosity of the reaction mixture to increase continuously. The faster the bubble formation time, the smaller the amount of water used during the preparation of CrO ₂ . This is because the foaming step prevents the exhaustion of the oxidized product of CO ₂ and glycerol at high viscosity.

In the examples of the above process, a reducing equivalent of 25% is provided by glycerol when approximately 40% of the CrO ₃ used is reduced, and the weight ratio of 2.32 parts of CrO ₃ per part of water represents the amount of water formed upon complete reaction. The amount of glycerol increases continuously as the weight ratio changes with water and is 70% in the weight ratio of 1.54 parts of CrO ₃ to 1 part of water. In all cases, subsequent addition of octanol is not a problem since the viscosity increases slightly due to incomplete reaction with CrO ₃ . At 1.98 parts by weight of CrO ₃ relative to 1 part of water, and at higher weight ratios, octanol and glycerol may be added simultaneously. The free-flowing reaction mixture obtained by this novel process is introduced into a steel reaction can and converted to chromium dioxide at high pressure reactors at temperatures up to 70 bar and at temperatures between 250 ° C. and 400 ° C. During the reactor heating state, the pre-determined temperature is kept constant at 130 ° C. for 1 to 2 hours to reduce the supply of thermal energy, thereby completely preventing the heat that continues to be generated in a small range at this temperature. In addition, it is necessary to go very quickly through the temperature range from 200 ° C. to 300 ° C., ie preferably in a short period of at most 1 hour. In order to realize this required temperature in the reaction mixture in a uniform manner, it is preferred to use a ring-shaped reaction can (by US-A 4 045 544) of layer thickness smaller than 10 cm, preferably 5 cm. Thereafter, cooling is preferably carried out at 220 to 250 ° C., preferably in a known manner, at a maximum of two hours after reaching the maximum temperature. At the same time, the pressure becomes low. The resulting chromium dioxide is mechanically removed from the reaction vessel and milled. Usually, heat treatment is followed at a temperature of 150-400 ° C. under stabilization by oxidation and alkali and / or reducing surface treatment of chromium dioxide.

The prepared chromium dioxide according to the invention has at least 60 kA / m coercive force and a special saturation magnetization of 85 nTm3 / g or more, measured at 380 kA / m magnetic field. Another advantage is that the properties have very narrow particle size distributions with an average deviation of less than 35% of the average particle length and an average particle length of less than 300 nm.

Therefore, this chromium dioxide material is particularly preferred for use in novel magnetic recording media for recording information of particularly high density. The novel chromium dioxide is processed into a magnetic recording medium by a known method. In order to form a magnetic layer, from 2 to 10 parts by weight of the chromium dioxide is converted into dispersion, such as a suitable dispersant and lubricant and other conventional additives in a total amount of at least 10% by weight of the binder and the chromium dioxide. The dispersion thus obtained is filtered and applied using a conventional coating apparatus. As a knife coater, for example, it is filtered and applied in one or more thin layers to a nonmagnetic gas or in a thin layer to a magnetic recording medium already supplied to another magnetic layer. Before the liquid coating mixture is dried at 50 to 90 ° C., the magnetic orientation of the chromium dioxide particles can be carried out. The coated membrane web passes between the heated and polished rollers under pressure for a special surface treatment of the magnetic layer. The thickness of the magnetic layer is usually 1.5 to 12 mu m.

Binders used in the magnetic layer are known polymer binders such as acrylate copolymers, polyvinyl acetate, polyvinyl foam or polyvinyl butyral, relatively high molecular weight epoxy resins, polyurethanes and mixtures of binders similar to these. Soluble in volatile organic solvents, 6 to 24, especially 8 to 1,2- or 1,3-propylene glycol, 1,4-butanediol, diethylene glycol, neopentylglycol or 1,8-octanediol Chains with 1,4-butanediol when having a relatively small amount of 4 to 4 carbon atoms, preferably toluylene diisocyanate or 4,4'-diisocyanatodiphenylmethane, having 20 carbon atoms of diisocyanate Linear polyesterurethanes free of isocyanates and elastomers that act as extenders have proven desirable. Preferred polyesterurethanes are those obtained from adiric acid, 1,4-butanediol and 4,4'-diisocyanatodiphenyl methane. Preferred polyesterurethanes have a shore strength A of 70 to 100, a tensile strength of 40 to 42 N / mm 2 (according to DIN 53,455), and a brake elongation (according to DIN 53,455) of about 440 to 560%. H. K values according to Pickenser (cellulose-chemistry 13 (1932), page 58, etc.) are 40 to 60 (1% strength in dimethyl formamide) for particularly suitable polymer binders.

The following example illustrates the invention compared to the example of the prior art.

For the resulting chromium dioxide, the specific surface permanent SSA in [m2 / g] in accordance with DIN 66,132 is determined by the Schroder, Dusseldorf using the Schröreline area meter by a one-point difference method according to Howl and Dybrogen. The magnetic properties are determined by a vibrating sample magnetometer with a magnetic field of 380 kA / m, that is, a coercive force of [kA / m] and a saturation magnetization Ms / ζ of [nTm 3 / g]. The average tap density ζ = 1.3 g / cm 3.

The aforementioned impurity percentages of the above examples are in each case based on the amount of chromium dioxide resulting.

The average needle length and standard deviation of chromium dioxide are determined from numerical dispersion of lengths of more than 200 particles in transmission electron micrographs (at 50,000 times expansion).

The results of the measurements are shown in the table.

Example 1

In a 20 L agitation vessel, 3.4 kg of water, 17.7 g of TeO ₂ (0.21 weight percent) and 422 g of Fe ₂ O ₃ (5.0 weight percent) are added to 10 kg of C ₂ O ₃ and the dispersion is carried out with the aid of a solvent. Carried out for minutes. During the dispersion procedure, 226 g of 226 g of glycerol at a concentration of 86.5% in 800 g of water are initially added to the reaction mixture at 50 ° C. and then to 242 g of 1-octanol between 65 ° C. and 70 ° C. (reducing agent) Ratio: 25 percent glycerol / 75 percent octanol). After stirring at 70 ° C. for at least 2 hours, the reaction mixture was cooled to 30 ° C. and led to a 5 cm layer thickness towards the 10 L annular gap can.

The filled reaction cans were heated to 345 ° C. at a high pressure reaction of 360 bar, and the temperature was kept constant at 130 ° C. for 1 hour 30 minutes and in the temperature range from 200 ° C. to 300 ° C. for 1 hour.

Cooling from 345 ° C to 325 ° C is effective within 2 hours and 30 minutes. After that the heating is stopped and the pressure goes down. After cooling to room temperature, the resulting CrO ₂ is mechanically transferred from the reaction can, dried and pulverized. The measurement results are shown in the chart.

[Comparison Experiment 1]

The procedure is described in Example 1 except that CrO ₃ is suspended in 4.3 kg of water and 325 g of octanol is added as reducing agent.

Example 2

In a 400 L reaction vessel, 128 L of water and 14.7 g Fe ₂ O ₃ (5.0 weight percent) and 529 g FeO ₂ (0.18 weight percent) are added to 350 kg of CrO ₃ and dispersion is carried out for 20 minutes with the aid of two solvents. do. During this procedure the suspension is heated to 55 ° C. Initially, 12.76 kg of glycerol at a concentration of 86.5% in 30 kg of water was added to the mixture for 50 minutes while cooling, after which 68.4 kg of 1-octanol had a temperature of 60 to 70 ° C. in the reaction mixture. It is injected into the mixture for 80 minutes at the temperature of. The addition of octanol starts after the glycerol is added for 10 minutes and is carried out simultaneously with the addition of glycerol, so that the total addition time is 90 minutes.

The reaction mixture obtained by the described method is stirred at 55 ° C. for at least 30 minutes, then led to an annular gap can with a layer thickness of 5 cm, heated to 360 ° C. in a high pressure reaction of 360 bar and cooled to 340 ° C. for 3 hours. Thereafter the mixture is cooled to room temperature for 3 hours and the pressure drops after 15 minutes while cooling. The heating curve is a temperature range from 200 ° C. to 300 ° C. for one hour while this temperature remains constant at 130 ° C. for two hours.

The CrO ₂ formed is mechanically moved, crushed and dried from the reaction can.

The measurement results are shown in the chart.

Example 3

The procedure is described in Example 2 except that the reaction mixture is directed towards the annular gap can with 10 cm layer thickness. The measurement results are shown in the chart.

Example 4

In a reaction vessel with a volume of 2 L, 155.5 g of water is added to 500 g of CrO ₃ and dispersion is carried out for 10 minutes with the aid of a solubilizer. Afterwards 21 g of Fe ₂ O ₃ (5.0 wt.%, Based on the theoretical yield of CrO ₂ ) and 0.63 g of TeO ₂ (0.15 wt.%) Are added to the suspension. 15.8 g of glycerol and 9.76 g of 1-octanol are added to the mixture at 60 ° C. while stirring is carried out for at least 20 minutes.

The reaction mixture obtained by the described method is cooled and then heated to 350 ° C. in a high pressure reaction of 320 bar in a metal reaction vessel with a diameter of 5 cm. The heating curve shows the temperature range from 200 ° C to 300 ° C for 1 hour. Immediately after 2 hours at 350 ° C., cooling is carried out and the pressure drops. After cooling to room temperature, the resulting CrO ₂ is mechanically transferred from the reaction can, dried and milled.

The measurement results are shown in the chart.

Example 5

In a reaction vessel having a volume of 2 L, 153.4 g of water is added to 500 g of CrO ₃ and dispersion is carried out for 10 minutes with the aid of a solubilizer. After that, 21 g of Fe ₂ O ₃ (5.0 weight percent, based on the theoretical yield of CrO ₂ ) and 0.67 g of TeO ₂ are added to the suspension. 23.7 g of glycerol and 6.51 g of 1-octanol in 72 ml of water are added to the mixture at 60 ° C. while stirring is carried out for at least 20 minutes.

The reaction mixture obtained by the described method is cooled and then heated to 350 ° C. in a high pressure reaction of 320 bar in a metal reaction vessel with a diameter of 5 cm. The heating curve shows the temperature range from 200 ° C to 300 ° C for 1 hour. After 2 hours at 350 ° C., cooling is performed immediately and the pressure drops. After cooling to room temperature, the resulting CrO ₂ is mechanically transferred from the reaction can, dried and pulverized.

The measurement results are shown in the chart.

Example 6

In a reaction vessel with a volume of 2 L, 153.4 g of water is added to 500 g of CrO ₃ and the dispersion is carried out with the aid of a solubilizer for 10 minutes. After that, 21 g of Fe ₃ O ₃ (5.0 weight percent, based on the theoretical yield of CrO ₂ ) and 0.88 g of TeO ₂ are added to the suspension. 23.7 g of glycerol in 42 ml of water are added to the mixture at 60 ° C. while stirring is carried out for at least 20 minutes.

The reaction mixture obtained by the described method is cooled and heated to 350 ° C. in a high pressure reaction of 320 bar in a metal reaction vessel with a diameter of 5 cm. The heating curve shows the temperature range from 200 ° C to 300 ° C for 1 hour. After 2 hours at 350 ° C. the cooling is carried out immediately and the pressure drops. After cooling to room temperature, the resulting CrO ₂ is mechanically transferred, dried and pulverized.

The measurement results are shown in the chart.

Example 7

In a reaction vessel with a volume of 2 L, 153.4 g of water is added to 500 g of CrO ₃ and mixed for 10 minutes with a dissolver. 21 g of Fe ₂ O ₃ (= 5.0% by weight, based on the theoretical yield of CrO ₂ ) and 0.76 g of TeO ₂ are added to the suspension. 23.7 g of glycerol and 6.51 g of 1-octanol dissolved in 72 ml water are added to the mixture while mixing at 60 ° C. for at least 20 minutes.

The reaction mixture obtained by the above-mentioned method is cooled in a metal reaction vessel 5 cm in diameter in a high pressure reactor and then heated to 350 ° C. at 320 bar. After 2 hours at 350 ° C, it is rapidly cooled and the pressure is reduced. After cooling to room temperature, the by-product CrO ₂ is physically transferred from the reaction vessel, dried and pulverized.

The results of the measurements are shown in the chart.

Example 8

In a reaction vessel with a volume of 2 liters, 154.5 g of water is added to 500 g of CrO ₃ and mixed for 10 minutes with a dissolvers. Then 21 g of Fe ₂ O ₃ (= 5.0% weight, based on the theoretical yield of CrO ₂ ) and 0.55 g of TeO ₂ are added to the suspension.

19.8 g of glycerol and 8.13 g of 1-octanol dissolved in 72 ml water are added to the mixture while mixing at 60 ° C. for 20 minutes.

The reaction mixture obtained by the above-mentioned method is cooled in a metal reaction vessel having a diameter of 5 cm in the high pressure reactor and then heated to 330 ° C. at 225 bar. The heating curve shows that the temperature range varies from 200 ° C to 300 ° C for 1 hour. After 2 hours at 330 ° C, cool rapidly and let the pressure go down. After cooling to room temperature, the by-product CrO ₂ is physically transferred from the reaction vessel, dried and pulverized.

The measurement results are shown in the chart.

[Comparison Experiment 2]

In a reaction vessel with a volume of 4 L, 463.5 g of water is added to 1,000 g of CrO ₃ and mixed for 10 minutes with a dissolver. 30.24 g of Fe ₂ O ₃ and 1.26 g of TeO ₂ are then added to the suspension. Add 32.53 g of 1 octanol to the mixture while mixing 20 minutes at 70 ° C.

The reaction mixture obtained by the above-mentioned method is cooled in a metal reaction vessel having a diameter of 5 cm in the high pressure vessel and then heated again to 350 ° C. at 320 bar. The heating curve shows that the temperature range changes from 200 ° C. to 300 ° C. for 1 hour. After 2 hours at 350 ° C the metal is cooled and the pressure is reduced. After cooling to room temperature, the by-product CrO ₂ is physically removed from the reaction vessel, dried and milled.

The measurement results are shown in the chart.

[Comparison Experiment 3]

251.2 g of water is added to 500 g of CrO ₃ in a 2 L volumetric reaction vessel and mixed for 10 minutes with a dissolver. 21 g of Fe ₂ O ₃ (= 5.0% weight, based on the theoretical yield of CrO ₂ ) and 0.76 g of TeO ₂ are then added to the suspension. 16.3 g of 1 octanol is added to the mixture while mixing 20 minutes at 70 ° C.

The reaction mixture obtained by the above-mentioned method is cooled in a metal reaction vessel having a diameter of 5 cm in the high pressure vessel and then heated to 350 ° C. at 320 bar. The heating curve shows that the temperature range changes from 200 ° C. to 300 ° C. for 1 hour. After 2 hours at 350 ° C it is rapidly cooled and the pressure is reduced. After cooling to room temperature, the by-product CrO ₂ is physically transferred from the reaction vessel, dried and pulverized.

Example B1

In a pulverizer comprising 200 parts by volume of a 1.5 mm diameter steel ball having a capacity of 500 parts by volume, 120 parts of chromium dioxide obtained by Example 1 and treated with sodium sulfite are of the same amount of tetrahydrofran and dioxane 50% in a mixture of tetrahydrofuran and dioxane in an amount equal to 27 parts of a 10% strength solution of thermoplastic polyesterurethane obtained from adipic acid, 1,4-butanediol and 4,4-diphenylmethanediisocyanat in the mixture A volatile mixture of tetrahydrofuran and dioxane in the same amount of 48 parts in the commercial polyvinyl form of the concentration solution mixed 144 parts with 2 parts of zinc stear and stirred for 4 hours. The same amount of two conjugates, 13.5 parts of a stable volatile mixture and 0.1 parts of a commercial silicone oil, were then stirred for a further 30 minutes. This mixture is applied to polyethylene terephthalite which is hung and dried in a known epidermis by a knife epidermis and calendered to a dry layer thickness of 5.5 μm. As soon as the dispersion was poured, the acicular chromium dioxide particles were directed in the direction recorded by the magnetic field. Magnetic properties and coercive forces Hc [kA / m] residual inductance Mr [mT] and the return rate OR, and the rates of residual inductance along the return direction and cross-hairs, and the williams and bean stock Switching field distribution SFD (comp) by comstock (AIP conf. Proc. 5 (1971), 738) is shown in the table.

Furthermore, the average peak-bone height Rz of the magnetic layer is measured according to DIN 4768. The recording characteristics were determined at a wavelength of 0.7 µm by measuring the luminance signal at 4.5 MHz for the reference tape TDK HD-Pro at 0 dB.

The relevant measurements are shown in the chart.

Example B2-B7 and Comparative Experiment BV1-BV3

The process was the same as in Example B1 except for the chromium dioxide material obtained by the example shown in the table used. The measurement results are shown in the chart.

Claims (3)

In a magnetic recording medium composed of a nonmagnetic material and at least one magnetic layer, containing pointed cron dioxide uniformly distributed in a polymer binder and an additive, and having a coercive force of at least 60 kA / m, the average particle length of the chromium dioxide is 300 nm or less, and the acquired HD is Where KT is the coercivity of the magnetic layer measured in [kA / m] at an external field of 380 kA / m, L is the average particle length, and ΔL is the area of particle size dispersion determined from the numerical dispersion of the particle length, L and ΔL are both measured in [nm], and the factor HD is 1.85 or more. 2. The magnetic recording medium of claim 1, wherein the factor HD is 1.85 to 4.0. Modified with iron and tellurium or antimony; The amount of water formed in the oxide of the organic reducing agent at 250 to 400 ° C. at least 70 bar by reacting a water-soluble CrO ₃ suspension added with 0.05 to 10% by weight of iron dioxide and tellurium oxide or antimony compound, mainly based on chromium dioxide. Taking into account the reaction of an organic reducing agent for forming a ratio of Cr (VI): Cr (III) of 1.54 to 2.32 parts by weight of CrO ₃ used per part of water, 4: 2 to 1: 1, which is larger than 60 kA / m. Coercive force, saturation magnetization greater than 85 nTm3 / g, and small particle size distribution with an average particle diameter smaller than 300 nm; The organic reducing agent used is glycerol and octanol and the amount of glycerol is 25% to 70% based on the total reducing equivalents.