Ferro-Magnetic Acicular Particles for Recording Medium and Process for Preparing the Same

The present invention relates to ferro-magnetic, acicular particles for a recording medium (e.g. recording tape, recording sheet, recording drum, recording disc), particularly of high density, and their preparation.

The term "high density" herein used is intended to means a recording medium having a higher number of signals in a unit length.

With the intensification of magnetic recording density, demand was made for a recording medium of high coercive force. For this purpose, it has been proposed to use a uniform composition of magnetic particles consisting of iron oxide with a small amount of any other appropriate metal oxide such as cobalt oxide. The preparation of such magnetic particles is usually effected by crystallizing them out from an aqueous solution of an iron salt and any other metal salt, but the thus prepared particles are not uniform in size and their use for a recording medium produces noise and print through. In order to overcome such defect, it has been proposed to use acicular particles constituted with crystal nuclei of non-magnetic iron oxide and a coating of magnetic metal oxide provided on their surface. On the formation of the coating, however, the magentic metal oxide is apt to make nuclei by itself and the particles of uniform size are difficultly obtained. Further, the coating adheres unevenly on the surface of the crystal nuclei so that, when blended with a binder, the coating is apt to be eliminated so as to make inferior the magnetic characteristics of the acicular particles.

As the result of the extensive study, it has been found that magnetic, acicular particles of iron oxide having a coating of metal oxides, the metal component mainly consisting of iron and coblat, crystallized on their surface show a high coercive force and, when used for a recording medium, affords a prominent electro-magnetic characteristics. It has also been found that the incorporation of zinc into the said metal component can provide the resulting particles with a higher saturated magnetic flux density. The present invention is based on these findings.

For preparation of the ferro-magnetic, acicular particles of the present invention, magnetic, acicular particles of iron oxide (e.g. .gamma.-Fe.sub.2 O.sub.3, Fe.sub.3 O.sub.4) as crystal nuclei are dispersed in an aqueous alkaline medium containing hydroxides of iron and cobalt with or without zinc hydroxide and an oxidative gas (e.g. air) is introduced therein whereby oxidation proceeds to produce oxides of iron and cobalt with or without zinc oxide, which are epitaxially crystallized on the surface of the crystal nuclei.

In preparation of the metal hydroxides, there may be used the corresponding metal salts such as ferrous sulfate, ferrous nitrate, ferrous chloride, cobalt sulfate, cobalt chloride, cobalt nitrate, zinc sulfate and zinc chloride.

The epitaxial growth of the metal oxides on the surface of the crystal nuclei may be considerably influenced by the concentration of alkali (e.g. sodium hydroxide, potassium hydroxide) in the aqueous medium and the temperature for oxidation. The presence of not less than about 0.5 mol equivalent of alkali, preferably about 1.0 to 2.0 mol equivalent, to 1 mol of the metal ion in the aqueous medium is usually required. Such concentration is advantageous in preventing the formation of the new crystal nuclei of the metal oxides themselves. When the concentration is less than the said lower limit, the growth of the crystals of the metal oxides takes a much longer time. In general, a higher oxidation temperature results in a more uniform growth of the crystals of the metal oxides and a more favorable magnetic property. However, a too high temperature is not preferred, because new crystal nuclei of the metal oxides are apt to be formed. Thus, the oxidation temperature may be usually room temperature or higher, preferably from about 60.degree. to 100.degree. C.

By the said procedure, the crystalline structure of the metal oxides as the coating can be the same as that of the crystal nuclei so that the coating adheres firmly thereon. Further, the metal oxides as the coating are per se ferro-magnetic so that any heat treatment such as reduction and oxidation can be omitted. Thus, the largeness of particles due to sintering does not occur, and ferro-magnetic, acicular particles of uniform size and even quality can be obtained. Furthermore, a desired coercive force and a saturated magnetic flux density can be readily obtained by varying appropriately the incorporation amount of cobalt and/ or zinc to that of iron.

FIG. 1 is a graph showing the relationship between the coercive force of the ferro-magnetic, acicular particles and the amount of alkali employed;

FIG. 2 is a graph showing the relationship of the content of cobalt in the ferro-magnetic, acicular particles and the coercive force obtained; and

FIG. 3 is a graph showing the relationship between the contents of cobalt and zinc in the ferro-magnetic, acicular particles and the saturated magnetic flux density when the atomic ratio of cobalt and zinc is 9:1, 8:2 and 7:3.

Practical and presently preferred embodiments of the invention are illustratively shown in the following Examples.

An aqueous solution of ferrous sulfate (0.2 mol/ liter; 2 liters) and an aqueous solution of cobalt sulfate (0.2 mol/liter; 1 liter) are combined together to make a metal ion solution. To the metal ion solution (1 liter), an aqueous solution of sodium hydroxide (1.5 mol/liter; 1 liter) is added, whereby hydroxides of iron and cobalt are co-precipitated. Magnetic, acicular particles of .gamma.-Fe.sub.2 O.sub.3 (150 g) having 380 oersteds in coercive force and 4000 gauss in saturated magnetic flux density are added thereto. Into the resultant solution maintained at about 80.degree. C., air is introduced at a rate of 3 liters per minute for 3 hours while stirring so that oxidation proceeds whereby oxides or iron and cobalt are crystallized on the acicular particles of .gamma.-Fe.sub.2 O.sub.3. The resultant particles are washed with water and dried to give ferro-magnetic, acicular particles of cobalt-containing iron oxide having about 470 oersteds in coercive force and about 4300 gauss in saturated magnetic flux density.

The above obtained ferro-magnetic, acicular particles (70 parts by weight) are admixed with with vinyl chloridevinyl acetate copolymer (22 parts by weight), dioctyl phthalate (7 parts by weight), toluene (100 parts by weight) and methyl isobutyl ketone (100 parts by weight) by the aid of a ball mill, and the resultant mixture is applied on the surface of a polyester film of 36 .mu. thick to make a coating of 14 .mu. thick after dryness. The thus prepared magnetic recording tape is about 420 oersteds in coercive force, about 1100 gauss in remanence magnetic flux density and about 0.7 in saturated remanence retention.

In the same manner as above, a variety of ferromagnetic, acicular particles of cobalt-containing iron oxide are prepared using a metal ion solution containing iron and cobalt ions in a total concentration of 0.2 mol per liter.

The relationship between the coercive force of the ferro-magnetic, acicular particles and the amount of alkali used when the oxidation temperature is 20.degree. C., 60.degree. C. and 80.degree. C. is shown in FIG. 1 of the accompanying drawings wherein the curves 1, 2 and 3 represent those at the oxidation temperatures of 20.degree. C., 60.degree. C. and 80.degree. C., respectively. From the said Figure, it is understood that a larger coercive force can be obtained with a higher oxidation temperature and also with a larger amount of alkali. When the amount of alkali is less than 0.5 mol equivalent, new crystal nuclei FeO(OH) are formed and the coercive force is lowered.

The relationship between the content of cobalt in the ferro-magnetic, acicular particles and the coercive force is shown in FIG. 2 of the accompanying drawings, from which it is understood that the coercive force is increased proportionally with the increase of the content of cobalt.

The coercive force and the saturated magnetic flux density of the ferro-magnetic, acicular particles according to the invention can be appropriately controlled by varying the composition of the coating on their surface. Such variation in the composition may be attained by changing the mixing ratio of iron and cobalt ions in the said metal ion solution.

To a metal ion solution containing iron and cobalt ions in a total concentration of 0.2 mol per liter (1 liter), an aqueous solution of sodium hydroxide (1.5 mol/liter; 1 liter) is added, whereby iron and cobalt hydroxides are coprecipitated. Magnetic, acicular particles of .gamma.-Fe.sub.2 O.sub.3 having 380 oersteds in coercive force and 4000 gauss in saturated magnetic flux density are added thereto. Into the resultant solution maintained at about 80.degree. C., air is introduced at a rate of 3 liters per minute for 3 hours while stirring so that oxidation proceeds whereby oxides or iron and cobalt are crystallized on the acicular particles of .gamma.Fe.sub.2 O.sub.3. The coercive force (Hc) and the saturated magnetic flux density (Bs) of the resultant ferro-magnetic, acicular particles and the composition of the coating formed on their surface with the molar ratio of iron and cobalt ions in the metal ion solution employed are shown in the following Table:

Ferrous sulfate (0.267 mol), cobalt sulfate (0.12 mol) and zinc sulfate (0.013 mol) are dissolved in water (2 liters) to make a metal ion solution. To the metal ion solution (1 liter), an aqueous solution of sodium hydroxide (1.2 mol/liter; 1 liter) is added, whereby hydroxides of iron, cobalt and zinc are co-precipitated. Magnetic, acicular particles of .gamma.-Fe.sub.2 O.sub.3 (150 g) as in Example 1 are added thereto. Into the resultant mixture maintained at about 60.degree. C., air is introduced at a rate of 3 liters per minute for 3 hours while stirring so that oxidation proceeds whereby oxides of iron, cobalt and zinc are crystallized on the acicular particles of .gamma.-Fe.sub.2 O.sub.3. The resultant particles are washed with water and dried to give ferro-magnetic, acicular particles of cobalt and zinc-containing iron oxide having 600 oersteds in coercive force and about 4420 gauss in saturated magnetic flux density.

The above obtained ferro-magnetic, acicular particles are blended with vinyl chloride-vinyl acetate copolymer, dioctyl phthalate, toluene and methyl isobutyl ketone as in Example 1, and the resultant mixture is applied on the surface of a polyester film of 36 .mu. thick to make a coating of 14 .mu. thick after dryness. The thus prepared magnetic recording tape is about 540 oersteds in coercive force, about 1250 gauss in remanence magnetic flux density and about 0.72 in saturated remanence retention.

The relationship between the contents of cobalt and zinc in the ferro-magnetic, acicular particles and the saturated magnetic flux density when the atomic ratio of cobalt and zinc is 9:1, 8:2 and 7:3 is shown in FIG. 3 of the accompanying drawings wherein the curves 4, 5 and 6 represent those at the Co/Zn atomic ratio of 9/1, 8/2 and 7/3, respectively. From the said Figure, it is understood that a higher saturated magnetic flux density can be obtained with a larger content of cobalt and zinc and also with a smaller Co/Zn atomic ratio. It is also understood that the increase of the content of cobalt and zinc results in a larger coercive force.

As understood from the above descriptions, the coating on the ferro-magnetic, acicular particles of the invention is so firmly bonded that it is scarcely eliminated from the nuclear crystal nuclei even when blended with a binder or any other material under a drastic condition. Thus, the ferro-magnetic, acicular particles hardly cause any material noise or print through. Moreover, it is greatly advantageous that desired magnetic characteristics, particularly high coercive force, can be readily produced by selecting an appropriate content of cobalt with or without zinc.