JP2011118988A - Method of manufacturing magnetic recording medium - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a kneading method and a diluting method in consideration of a diluting process, in a manufacturing method of a coating type magnetic recording medium for reducing a magnetic cluster size as a cohesion parameter of magnetic powder as much as possible and forming a smooth magnetic surface. <P>SOLUTION: In a method of manufacturing a magnetic recording medium, an energy ratio Ek/Ed between kneading energy (Ek) and diluting energy (Ed) to be added to per unit volume of a kneaded material is 0.5 to 1.1, in a kneading process and a diluting process. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

 The present invention relates to a method for manufacturing a magnetic recording medium, and more particularly to a method for kneading magnetic powder used in a coating type magnetic recording medium.

 The coating-type magnetic recording medium is manufactured as a magnetic tape mainly through a coating process in which a magnetic coating is applied to a continuously running belt-like support (hereinafter referred to as “web”) to form a coating film. In recent years, magnetic tape has been mainly used for computer backup, which uses a magnetoresistive MR head (Magneto Resistive head) as a reproducing head. With the rapid trend toward higher capacity and higher recording density, Accordingly, magnetic tapes having a better C / N (ratio of signal level to noise) are also demanded.

For this reason, in terms of materials, there is an increasing demand for the use of finer magnetic powders than before and the smoothness of the surface properties of the magnetic layer of the magnetic tape to reduce the spacing loss. As a method for smoothing the surface of the magnetic layer, a method of using a multilayer coating method at present and providing a nonmagnetic layer as a lower layer and applying a thin magnetic layer as an upper layer is generally used. As the demand for high recording density and large capacity increases, the upper magnetic layer has recently become increasingly thinner to 0.1 μm or less, even in a multi-layer structure.

On the other hand, the MR head described above can obtain a reproduction output several times that of the induction type magnetic head and does not use an induction coil. It has a feature that C / N can be improved by reducing noise on the recording medium side.

However, the MR head has a problem that thermal asperity noise is generated due to heat generated by contact with the magnetic recording medium or heat absorption due to a dent on the tape. In addition to good sensitivity compared to induction type magnetic heads, the surface of the magnetic layer was previously negligible because the bit length of magnetic recording media for high-density recording is short. Concavities and convexities, slight slight thickness fluctuations, and disturbances at the interface with the lower layer increase noise during recording and reproduction and lower C / N. Therefore, efforts are being made to reduce these as much as possible.

 In response to this, the dispersibility of the finely divided magnetic powder in the magnetic paint can be improved to a high degree, and various fields related to the magnetic paint from the field focusing on the manufacturing process for improving the dispersibility of the magnetic powder can be obtained. Proposals have been made (see Patent Documents 1 to 4).

The good dispersion of the magnetic powder essentially means that the individual particles of the magnetic powder in the magnetic coating are scattered and are present in the magnetic coating in a state closer to the non-aggregated primary particles.
The process most related to the dispersibility in the magnetic paint manufacturing process is a process called kneading in which magnetic powder is kneaded into a medium such as a binder. Therefore, many proposals related to the magnetic coating from the manufacturing process are also related to this process.

  In any of Patent Documents 1 to 4, an attempt is made to easily disperse the magnetic powder by increasing the shearing force applied to the kneaded product by controlling the solid concentration and the treatment temperature of the kneaded product in the kneading step. However, the particle diameters of magnetic powders that are the subject of any literature, including those that are not clearly displayed, are now becoming mainstream, assuming that the major axis length is 80 nm or more from the crystallite size and specific surface area. There is no long axis length of 60 nm or less. In addition, the relationship with the dilution step following the kneading step and the quantitative handling of the shearing force itself are not made.

  In Patent Document 5 (Japanese Patent Application Laid-Open No. 10-21538), the average power of magnetic powder to be processed is determined by quantitatively handling the power per unit weight of magnetic coating material when kneading fine magnetic powder having an average major axis length of 1.0 μm or less. Examining the relationship with the major axis length, the range of power (energy) suitable for uniformly dispersing the magnetic powder is disclosed.

  Further, Patent Document 6 (Japanese Patent Application Laid-Open No. 2005-339649) discloses that the coating temperature in the kneading step and the diluting step is specified with reference to the shearing force in the diluting step.

  However, even if the particle diameter of the magnetic powder that is the subject of Patent Document 5 and Patent Document 6 is the smallest, the major axis length is 80 nm. Currently, there is no disclosure of a magnetic powder that is mainly used in computer backup recording media and has a particle size of 60 nm or less and has a large interparticle cohesive force and is more difficult to disperse.

Further, Patent Document 5 does not mention any dilution process following the kneading process. Moreover, although patent document 6 also mentions the dilution process, the idea of the invention is that the shearing force should be given simply and larger as in the kneading process.

In addition, evaluation of the dispersibility of the magnetic powder is a well-known item up to the average surface roughness (Ra), coercive force distribution (SFD), gloss, and a certain height in a unit area. The number of protrusions. Such items are powerful in the sense that they provide direct information on properties resulting from physical agglomeration caused by insufficient dispersion of the magnetic powder. However, as described above, in the coating film using the current ultrafine particle magnetic powder, the size of the magnetic cluster (magnetic aggregation) that essentially affects the C / N noise level of the electromagnetic conversion characteristics rather than the parameters listed above. It is effective to evaluate the size of the magnetic powder (Patent Document 7) in order to know a measure of the dispersibility of the magnetic powder.

JP-A-5-298690 Japanese Patent Laid-Open No. 9-320051 Japanese Patent Laid-Open No. 7-14158 JP-A-8-96357 Japanese Patent Laid-Open No. 10-21538 JP 2005-339649 A Japanese Patent No. 4001532

  In other words, there are no patents that improve the dispersibility of magnetic powders that have focused on the manufacturing process so far, and have not evaluated the size of the magnetic cluster size involved in the noise level of electromagnetic conversion characteristics. There has been no demonstration of powders with an average particle diameter of 60 nm or less, and those involved only in the kneading process or those that include a dilution process have been dealt with in exactly the same way as the kneading process. . Moreover, none of the two processes clarifies the relationship between the dispersion of the magnetic powder in the magnetic paint and quantitatively defines the energy ratio for the dispersion of both processes.

  Therefore, excessive energy is applied in the kneading process to cause agglomeration of minute magnetic powder, which cannot be loosened in the subsequent dilution process, and there is magnetic aggregation in the dispersed magnetic paint, or conversely kneading. Since the energy in the process is small, there is a problem that even when a large energy is applied in the dilution process, the magnetic aggregation remains and the size of the magnetic cluster after dispersion does not decrease. Further, in the multi-layered magnetic recording medium, these dispersion defects cause the above-described fluctuations in the thickness of the magnetic layer or disturbances in the interface between the upper and lower layers.

These also cause a decrease in magnetic properties and a loss of smoothness on the surface of the magnetic layer, which increases noise during reproduction of high-density recording and causes a decrease in C / N.

  The present invention has been made in view of such circumstances, solves the above problems, and makes the magnetic cluster size as small as possible, which is essentially an agglomeration parameter of the magnetic powder, and a smooth magnetic surface. It is an object of the present invention to provide a method for kneading magnetic powder used in a coating type magnetic recording medium capable of obtaining the above.

In order to achieve the above object, the present invention uses a kneader to knead a ferromagnetic powder and / or nonmagnetic powder and a binder in an organic solvent, a resin solution and / or Alternatively, in a method for producing a magnetic recording medium obtained by applying a magnetic coating produced through a dilution step of adding an organic solvent to a non-magnetic support, the unit of the kneaded product in the kneading step and the diluting step Magnetic powder for magnetic recording medium, characterized in that the energy ratio Ek / Ed of kneading energy Ek (MJ / m ³ ) and dilution energy Ed (MJ / m ³ ) applied per volume is 0.5 to 1.1 A kneading method is provided.

As described above, in order to disperse the fine magnetic powder, the kneading step in which a large shearing force (energy) is applied is emphasized, but the subsequent dilution step before dispersion is also an important step. The functions of these steps are as follows. First, the function of the kneading step with respect to the composition containing the magnetic powder to be dispersed (hereinafter referred to as kneaded product) is as follows: The function of the dilution process is (2) the shearing force is applied to the surface of the loosened magnetic powder, and the binder resin as the binder is spread and covered as much as possible to cover the surface. This is considered to be a function of adjusting the magnetic powder in the state where the binder is adsorbed in the next dispersion step so as to maintain an appropriate fluidity so that the magnetic powder can be stably and uniformly dispersed. Both of the processes for providing these functions are generally performed in a kneading apparatus.

  In order to satisfactorily disperse the magnetic powder, it is important to maximize the functions of (1) and (2). The kneading and crushing function of (1) is effective due to the physical force of essentially loosening the magnetic powder formed by agglomerating several so-called primary particles, and the function in the dilution step of (2) is It is necessary to consider the time factor because it includes the chemical phenomenon of adsorption.

Based on this idea, the amount of energy converted per unit volume of the kneaded material is calculated from the amount of work calculated from the power given to the equipment by the shear force applied to the kneaded material in both steps. By clarifying the relationship between the amount and the smoothness of the coating film obtained from the magnetic paint after dispersing the kneaded material and the magnetic cluster size as a measure of the degree of aggregation of the magnetic powder, a magnetic paint having good dispersibility is obtained. The present invention provides a kneading method for magnetic powder.

In the kneading method of the present invention, the kneading energy Ek per unit volume of the kneaded product is preferably 1000 MJ / m ³ or more.

In the kneading method of the present invention, the kneader is preferably one of a batch kneader, a pressure kneader, a continuous kneader, a biaxial continuous kneader, and a biaxial continuous extruder.

In the kneading method of the present invention, it is preferable to adjust the solid content concentration of the kneaded product in the kneading step to 75 wt% or more because the aggregation of the magnetic powder can be well broken.

According to the present invention, it is possible to obtain a magnetic coating material having greatly improved smoothness of the magnetic coating film surface and a small magnetic cluster size.

It is process drawing to which the manufacturing method of the magnetic coating concerning this invention is applied. It is a cross-sectional schematic diagram of the apparatus used for the kneading | mixing process and dilution process concerning this invention. It is a figure which shows the change of the electric current value of the motive power of the kneader containing the kneading | mixing process and dilution process concerning this invention. It is a figure which shows the relationship between the energy ratio of the kneading | mixing energy Ek (MJ / m < ³ >) added per unit volume of a kneaded material, and the dilution energy Ed (MJ / m < ³ >), and the characteristic of a magnetic sheet.

Hereinafter, the method for producing a magnetic paint according to the present invention will be described in more detail. As described above, the present invention uses a kneader to knead a ferromagnetic powder and / or nonmagnetic powder and a binder in an organic solvent, and further add a binder resin and / or Or a method of manufacturing a magnetic recording medium by manufacturing a magnetic coating material through a dilution process in which an organic solvent (hereinafter abbreviated as a resin solution or the like) is added for dilution, and coating the magnetic coating material on a nonmagnetic support. In particular, the present invention relates to a method for kneading magnetic powder used in a coating type magnetic recording medium.

FIG. 1 is a process diagram showing an example of a method for producing a coating type magnetic recording medium of the present invention. In this figure, the present invention relates to a kneading step and a dilution step. The apparatus used for these steps is a kneader, and any of an open kneader, a pressure kneader, a continuous kneader, a biaxial continuous kneader, and the like can be used.

The magnetic coating material produced from the kneading step through the dilution step is supplied after the coating step via a dispersing step represented by a sand grinder mill and conveyed at a predetermined speed (non-magnetic support). ) To form a magnetic coating film.

FIG. 2 schematically shows a cross section of the batch kneader used in the present invention.
A lid (not shown) is installed on the upper surface of the kneader 1 so that the internal space volume can be made constant. Raw materials are charged, and blades (stirring blades) 3 are arranged in parallel inside 1 and are driven to rotate in opposite directions to give shearing force to the raw materials. Are kneaded. The raw material is shown as kneaded material 2.

The apparatus and method after the dispersion step can be freely selected because they do not directly affect the essence of the present invention even if a conventionally known apparatus is adopted. Further, each component of the magnetic recording medium is not contrary to the idea of the present invention except that the effect of the present invention is remarkable when the average particle diameter of the magnetic powder used in the magnetic layer is 60 nm or less. Any conventionally known component can be used.

FIG. 3 schematically shows changes over time in the current value of the power necessary to drive the kneader in the kneading step and dilution step performed in the kneader during the manufacturing process of FIG.
As shown at point A in FIG. 3, when the surface treatment process is completed and a resin solution is added to the composition in the kneader and a shearing force is applied, the torque increases and the current value of the kneader power increases. Increasing so-called wet peaks appear. In the present invention, the kneading is started on the basis of the point (B point in FIG. 3) where the torque has been reduced after the occurrence of this peak.

  The solid content concentration of the kneaded product in the kneading step is preferably 75 wt% or more. If the concentration is lower than this, the kneaded material is soft and shearing force is not applied. Although it may be 75 wt% or more, the solid content concentration is preferably within 95 wt%. If it exceeds 95 wt%, the solvent is insufficient, the kneaded material does not become one lump and falls apart, and no shear force is applied.

   The current value of the power of the kneading machine, which is a measure of the torque, starts to decrease when a new resin solution is added to maintain a constant value during kneading and reduce the solid content concentration for dilution (see FIG. 3). C point). The kneading step of the present invention refers to points B to C. In the case of the present invention, the solid content concentration at the end of the dilution step was set to 50 wt%.

After passing through the dilution step, an organic solvent is added so that the composition can be taken out from the kneader for sending to the next step (point D in FIG. 3). Since the solid content concentration at the point D is usually 30 wt% or less, the final time point (solid content concentration is 50 wt%) set in the present invention exists between the points C and D in FIG.

   The present invention calculates the shear force required per unit volume of the composition (kneaded material) from the power applied to the kneading machine in the kneading step and dilution step, and calculates the energy required for each step and the magnetic powder in the paint. The relationship with the dispersibility of the film is clarified from the characteristics of the coating film obtained from the magnetic paint.

   In addition, the energy required per unit volume in each process was calculated by using the power when the kneader was driven without any material as the energy reference zero.

   As described above, the constituent members of the magnetic recording medium have essentially no influence on the present invention, but the main constituent members of the recording medium using the magnetic paint obtained by the manufacturing method of the present invention will be briefly described.

(Magnetic layer)
A nonmagnetic layer (hereinafter also referred to as “lower layer”) and a magnetic layer (hereinafter also referred to as “upper layer”) can be provided on both sides or one side of the support. The upper and lower layers can be provided with an upper magnetic layer either after the lower layer is applied or while the lower layer is in a wet state (Wet on Wet) or after being dried (Wet on Dry).

(Magnetic powder)
Examples of the magnetic powder in the present invention include conventionally known powders such as ferromagnetic metal powder, iron nitride powder, and hexagonal ferrite powder. The size of these magnetic powders is the long axis length for needle-shaped or spindle-shaped ones, the longer diameter for spherical or amorphous shapes, and the maximum length for the diameter for plate-shaped ones. A thickness of 10 to 60 nm is preferable. If it is larger than 60 nm, the effect of the present invention is not so remarkable, and the reason why it is smaller than 10 nm is that it is difficult to produce industrially in the actual situation.

  The magnetic powder described above may be treated in advance with a known dispersant, lubricant, surfactant, antistatic agent, or the like before dispersion.

The coercive force (Hc) of the magnetic powder is preferably 159 to 240 kA / m (2000 to 3000 Oe). The saturation magnetization (σs) in the case of the ferromagnetic metal powder is preferably 80 to 120 A · m 2 / kg (80 to 120 emu / g).

(Nonmagnetic layer)
The lower layer in the present invention comprises at least carbon black and at least two kinds of nonmagnetic powders and a binder.

(Non-magnetic powder)
Nonmagnetic powders other than carbon black can be selected from inorganic compounds such as metal oxides, metal carbonates, metal sulfates, metal nitrides, metal carbides, and metal sulfides.

 Known techniques relating to the magnetic layer can be applied to the binder resin (type and amount) of the lower layer, the type, amount, solvent, dispersion method, and the like of the lubricant / dispersant / additive.

(Binder)
As the binder used in the present invention, conventionally known thermoplastic resins, thermosetting resins, reactive resins, and mixtures thereof are used.

(Carbon black, abrasive)
In the present invention, as carbon black used in the magnetic layer, furnace for rubber, thermal for rubber, black for color, acetylene black, and the like can be used.

In the present invention, as the abrasive, known materials having a Mohs hardness of 6 or more are mainly used alone or in combination. These abrasives preferably have an average particle size of 0.01 to 2 μm and a narrow particle size distribution.

These abrasive | polishing agents can also be added to a lower layer as needed. The particle size and amount of the abrasive added to the magnetic layer and the lower layer should be set to optimum values.

All or part of the additives used in the present invention may be added in any step of magnetic and non-magnetic coating production. For example, when mixed with a magnetic material before the kneading step, the magnetic material and the binder When adding in a kneading step with a solvent, when adding in a dispersion step, when adding after dispersion, or when adding just before coating.

As the organic solvent used in the present invention, known ones can be used. For example, the solvents described in JP-A-6-68453 can be used.

(Layer structure)
In the magnetic recording medium according to the present invention, for example, the thickness of the support may be 4 to 10 μm, and an undercoat layer may be provided between the support and the lower layer to improve adhesion.

In the present invention, a structure in which a lower layer and a magnetic layer are provided on one side of a support and a back layer is provided on the other side can also be employed. The thickness of the back layer can be 0.3 to 0.7 μm. For these undercoat layer and back layer, known structures can be used.

  In the present invention, the thickness of the magnetic layer of the magnetic recording medium is optimized depending on the saturation magnetization amount of the head to be used, the head gap length, and the band of the recording signal, but can be 0.05 to 0.3 μm.

The thickness of the lower layer of the magnetic recording medium in the present invention can be 0.5 to 1.5 μm.

(Support)
The support used in the present invention is preferably nonmagnetic. Nonmagnetic supports such as polyesters such as polyethylene terephthalate and polyethylene naphthalate, polyolefins, cellulose triacetate, polycarbonate, polyamides (including aromatic polyamides such as aliphatic polyamides and aramids), polyimides, polyamideimides, etc. Can be used.

(Production method)
The step of producing the magnetic coating liquid for the magnetic recording medium in the present invention comprises at least a kneading step, and a dilution step of diluting the kneaded product by adding a resin solution or the like. And a mixing step provided as necessary before and after these steps. Each process may be divided into two or more stages.

In the kneading step, it is preferable to use an apparatus having a strong kneading force such as an open kneader, a pressure kneader, a continuous kneader, an extruder, a biaxial continuous kneader, and a biaxial continuous extruder.

When using a kneader, the range is 10 to 500 parts by weight with respect to 100 parts by weight of magnetic powder or nonmagnetic powder and all or part of the binder (however, preferably 30% by weight or more of the total binder) and magnetic powder. It is preferable to perform kneading with.

In order to disperse the magnetic layer solution and the lower layer solution, zirconia beads, titania beads, and steel beads, which are high specific gravity dispersion media, are suitable. The particle diameter and filling rate of these dispersion media are optimized. A well-known thing can be used for a disperser.

When applying a magnetic recording medium having a multilayer structure in the present invention, the following method can be used.

1) First, the lower layer is applied by a gravure coating device, roll coating device, blade coating device, extrusion coating device, etc., which are generally used in the application of magnetic paints, and the support pressure type extrusion while the lower layer is wet. A method of applying the upper layer with a coating apparatus.
2) A method in which the upper and lower layers are applied almost simultaneously with a single application head having two coating liquid passage slits.
3) A method in which the upper and lower layers are applied almost simultaneously using an extrusion coating apparatus with a backup roll.

In order to realize the configuration of the present invention, it is possible to use a sequential multilayer coating in which a magnetic layer is provided on a lower layer after being applied and dried, and the effects of the present invention are not lost.

In the present invention, the orientation device is preferably a known orientation device, such as applying an alternating magnetic field with a rare earth magnet and a solenoid.

Next, the kneading method in the production of the magnetic recording medium of the present invention will be specifically described with reference to examples, but the present invention is not limited thereto. From the gist of the present invention, in order to pay close attention to the dispersibility of the magnetic powder of the magnetic paint, a coating film alone having only the magnetic layer was prepared and evaluated. In addition, the display of “parts” in the examples indicates “parts by weight”.

表１
<Magnetic powder>
Table 1 shows various magnetic powders used in the present invention. All the magnetic powders were surface-treated with 3 wt% phosphoric acid dispersant with respect to the magnetic powder.

Table 1

<Magnetic paint>
A kneading process and a dilution process will be described. The following composition (1) is preliminarily mixed at a high speed with a high-speed stirring mixer, and then charged into a kneader to obtain a kneaded product having a solid concentration of 81% by weight. At this time, the torque applied to the blade of the kneader increases, and a peak of current value (so-called wet peak) as shown in FIG. 3 appears. Eventually, the peak of this current value decreases. The point at which the current value has completely decreased (point B in FIG. 3) is taken as the start of the kneading process. The end point of kneading is the point (point C in FIG. 3) where the resin or solvent is then introduced into the kneader and the solid content concentration falls below 81 wt% and the current value of torque begins to drop. The blades of the kneader were kneaded for 40 minutes (the time from B to C in FIG. 3) at a rotation speed of 25 rpm. This time becomes the kneading process time.
To the kneading machine containing the kneaded product, a resin and a solvent were added, and the kneaded product was gradually added so as not to become lumpy and diluted. A dilution having a final diluted solid content concentration of 50 wt% was obtained. At this time, the blade rotation speed remains at 25 rpm, and the dilution process is performed from the point where the solid content concentration is lower than 81 wt% (C point in FIG. 3) to the time when the solid content concentration is 50 wt% (not shown). The kneader was operated for 90 minutes.
Composition (1)
Magnetic powder (A) 100 parts Vinyl chloride copolymer (MR-104 manufactured by ZEON Corporation) 17.0 parts Polyester polyurethane (UR8300 manufactured by Toyobo) 6.0 parts Tetrahydrofuran 2.2 parts Methyl ethyl ketone 81.9 parts Toluene 42. 3 parts

The diluted amount of the solvent was added to the diluted composition to obtain a premixing composition having a final diluted solid content concentration of 30 wt%. After the premix treatment, it was dispersed in a bead mill using ceramic beads.
Methyl ethyl ketone 115.5 parts Toluene 52.5 parts

A magnetic coating film was prepared on a polyethylene naphthalate support having a thickness of 10 μm using a simple applicator.

   The obtained magnetic sheet was not subjected to the calendering process in the mirror surface process, which is a magnetic layer surface treatment process, in order to evaluate the dispersibility more directly. In this state, the magnetic cluster size, which is an aggregation parameter of the magnetic powder, and the smoothness of the magnetic layer of the magnetic sheet were evaluated.

Calculation of the shear force applied to the kneaded material was performed as follows. About the kneaded material in the kneading machine of a kneading | mixing process and a dilution process, the volume of the whole kneaded material is calculated using each specific gravity from the weight of each composition material.
The volume of the entire kneaded product is constant in the kneading process without changing the solid concentration in the process, but in the dilution process, the resin solution is intermittently added to reach a solid concentration of 50 wt% at the end of the dilution. Therefore, in the present invention, the volume of the entire kneaded product in the dilution step is the average of the volumes of both the kneaded product at the start and end of the dilution step.

The power and work amount of the kneader in each process are calculated by the following formula.
Power = 2π × T × N / 60 [W] T: Torque [N · m], N: Number of revolutions [rpm]
Work amount = power × t × 10 ⁻⁶ [MJ] t: process required operation time [sec]
Here, the torque (T) does not show a constant value that persists throughout the process time. For this reason, the average torque obtained by integrating all torques during the process time and making it unit time (minutes) for convenience is taken as the torque value of the process.

From the obtained work amount, the energy (MJ / m ³ ) given to the kneaded product per unit volume is calculated,
The kneading energy (Ek) applied per unit volume of the kneaded product was used. The dilution energy (Ed) applied per unit volume of the kneaded product was also determined for the dilution step by the same calculation method, and the energy ratio Ek / Ed of the two was determined.

  Evaluation items of the magnetic sheet are the surface roughness Ra and the magnetic cluster size of the magnetic layer.

  The surface roughness was evaluated by the following procedure. Using a general-purpose three-dimensional surface structure analyzer NewView 5000 manufactured by ZYGO, the 10-point average roughness Ra was determined at a scan length of 2 μm by scanning white light interferometry. In the measurement, a 50 × objective lens was used and the measurement was performed at 2 × zoom. Therefore, the magnification is 100 times. The measurement visual field is 70 μm × 52 μm.

  The magnetic cluster size was evaluated by the following procedure. As a magnetic force microscope, a digital instrument company, Nano Scope III was used, and a leakage magnetic field image of the magnetic layer was measured by a frequency detection method. A probe having a cobalt alloy coat (tip radius of curvature: 25 to 40 nm, coercive force: about 400 Oe, magnetic moment: about 1 × 10 −13 emu) is used as a measurement probe, the scanning range is 5 μm square, and the scanning speed is 5 μm / sec. A portion having a magnetization intensity larger than the sum (C + δ) of the center value C and standard deviation δ of the magnetization intensity of the obtained leakage magnetic field image is displayed by binarization processing. The average value of equivalent diameters was measured.

  In Examples and Comparative Examples other than Example 1, the solid content concentration of the kneaded material was changed using the magnetic powders shown in Table 2, and the kneading step and the diluting step were also the operating conditions of the kneader shown in Table 2. A magnetic sheet was prepared for evaluation in the same manner as in Example 1 except that the work was performed.

  Table 3 shows the energy per unit volume of the kneaded product and the ratio in the kneading step and dilution step of each example and each comparative example. The surface smoothness Ra and the magnetic cluster size of the magnetic sheet produced by applying the energy shown in the table are summarized.

図４には、表３の結果から、磁性粉末Ａ1を用いた場合についての混練物の単位体積当りに加える混練エネルギーＥｋ（MJ/ｍ^３）と希釈エネルギーＥｄ（MJ/ｍ^３）のエネルギー比と磁気シートの特性との関係をまとめて示した。
同程度の分散度であっても、磁性塗膜のRａや磁気クラスターサイズは、磁性粉末の平均粒子径によって大きく影響を受ける。よって、図４には同一磁性粉末A1を用いた磁気シートの結果をまとめている。

FIG. 4 shows the energy ratio between the kneading energy Ek (MJ / m ³ ) and the dilution energy Ed (MJ / m ³ ) applied per unit volume of the kneaded material when the magnetic powder A1 is used. The relationship between the magnetic sheet and the characteristics of the magnetic sheet is summarized.
Even with the same degree of dispersion, the Ra and magnetic cluster size of the magnetic coating film are greatly affected by the average particle size of the magnetic powder. Therefore, FIG. 4 summarizes the results of the magnetic sheet using the same magnetic powder A1.

  As described above, the characteristics of the magnetic sheet are the surface smoothness Ra and the magnetic cluster size, which are a measure of the dispersibility of the magnetic powder of the magnetic paint. The preferred values for the surface smoothness Ra and the magnetic cluster size before the mirror finishing treatment are good when Ra is 6.0 or less and the magnetic cluster size is 52 nm or less from the accumulation of data of actual prototype tapes from our past. These values were used as threshold values because it was confirmed that a good C / N was secured.

  From FIG. 4, in order to obtain an excellent C / N, that is, to obtain surface smoothness Ra and magnetic cluster size indicating good dispersion of the magnetic powder, the kneaded product of the kneading step and the dilution step in the production of the magnetic coating material It can be seen that it is necessary to control the energy ratio per unit volume within a certain range.

  That is, for the first time in the present invention, both Ra and the magnetic cluster size, which are the characteristics of the magnetic coating film, which provide good C / N, satisfy both the above-mentioned threshold values. It was proved that the energy ratio Ek / Ed of kneading energy (Ek) and dilution energy (Ed) applied per hit is in the range of 0.5 to 1.1.

  From comparison between Example 17 and Comparative Example 8, when a magnetic powder having a particle size larger than 60 nm was used, kneading energy (Ek) and dilution energy (Ed) applied per unit volume of the kneaded material in the kneader were used. The effect of the preferable range on the energy ratio is not so remarkable. From the results when A1 or A3 is used as the magnetic powder, it can be seen that the present invention is particularly useful when the fine particle magnetic powder having an average particle diameter of 60 nm or less is used.

  From the results of Examples 18 to 20 and Comparative Examples 9 to 11, even if the magnetic powder is an iron nitride magnetic powder or a hexagonal magnetic powder, the kneading energy (Ek) and dilution applied per unit volume of the kneaded product It can be seen that a preferable surface smoothness Ra and magnetic cluster size can be obtained when the energy ratio Ek / Ed of the energy (Ed) is in the range clearly defined in the present invention.

  DESCRIPTION OF SYMBOLS 1 ... Kneading machine, 2 ... Kneaded material, 3 ... Blade

Claims (3)

Magnetic material produced through a kneading step using a kneader to knead ferromagnetic powder and / or non-magnetic powder and a binder in an organic solvent, and a dilution step in which the organic solvent is added to the kneaded product to dilute. In a method for producing a magnetic recording medium obtained by applying a paint on a nonmagnetic support,
The magnetism is characterized in that the kneading step and the diluting step are performed at an energy ratio Ek / Ed of kneading energy (Ek) and dilution energy (Ed) applied per unit volume of the kneaded product of 0.5 to 1.1. A method for manufacturing a recording medium. The method of manufacturing a magnetic recording medium according to claim 1, wherein the kneading energy Ek is 1000 MJ / m ³ or more. 3. The method of manufacturing a magnetic recording medium according to claim 1, wherein, in the kneading step, the solid content concentration of the kneaded material is adjusted to 75 wt% or more.

Images