JPH0991683A - Magnetic recording medium and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a coating type magnetic recording medium excellent in the surface smoothness and hardness of the magnetic layer. SOLUTION: This magnetic recording medium is produced through a process for coating the top of a nonmagnetic substrate with a magnetic coating material contg. magnetic powder of Co-contg. γ-iron oxide and a resin binder. The coating material gives a dry coating film whose coercive force is lower than that of the magnetic powder by 1-6%.

Description

Detailed Description of the Invention

[0001]

[0001] The present invention relates to a magnetic recording medium. In particular, the present invention relates to a magnetic recording medium excellent in surface smoothness and hardness of a magnetic layer.

[0002]

2. Description of the Related Art A coating type magnetic recording medium manufactured through a process of coating a magnetic coating material on a flexible non-magnetic support such as polyethylene terephthalate is widely used as a floppy disk or a magnetic tape. . There is a strong demand for higher performance of these magnetic recording media, and research is being conducted to improve recording density and electromagnetic conversion characteristics.

[0003]

One of the keys to improving the performance of the coating type magnetic recording medium is the magnetic paint. The magnetic paint is prepared by kneading a mixture of a magnetic powder, a binder resin and a solvent, and a dispersant, an antistatic agent, an abrasive, a lubricant and the like. Fine magnetic powders must be used to manufacture high-performance magnetic recording media, but since such magnetic powders are extremely bulky and entrain a large amount of air, air is completely eliminated by kneading. Is extremely difficult. That is, during kneading, the air around the magnetic powder is successively replaced with a solvent or a binder resin, but this replacement becomes more difficult as the magnetic powder becomes finer and bulkier. The magnetic recording medium manufactured by using the magnetic coating which is not sufficiently replaced has voids inside the magnetic layer, so that the hardness of the magnetic layer is inferior. Further, since such magnetic paint does not disperse the magnetic powder well, the performance as a recording medium tends to be poor.

It is considered that in order to sufficiently remove the air around the magnetic powder, a strong shearing force should be applied to carry out kneading for a long time. However, the magnetic recording medium produced by using the magnetic coating material prepared by such kneading tends to be inferior in terms of surface gloss and smoothness of the magnetic layer although the hardness of the magnetic layer is high. Accordingly, the present invention provides a method for producing a magnetic coating material that provides a high-performance magnetic layer and a magnetic recording medium produced using this magnetic coating material.

[0005]

According to the findings of the present inventors, a magnetic coating material containing a cobalt-containing-γ-iron oxide magnetic powder and a binder resin is applied onto a non-magnetic support to form a magnetic coating. When a recording medium is manufactured, a magnetic coating material having a coercive force lower than that of the magnetic powder used for preparing the magnetic coating material by 1 to 6% is used as the magnetic coating material. A magnetic recording medium having excellent surface properties such as hardness and smoothness can be obtained.

Further, such a magnetic coating material has a kneading device equipped with a mechanism for conveying contents while kneading the contents, and at least a cobalt-containing-γ-iron oxide magnetic powder, a dispersant having a phosphate ester group, A magnetic coating material composed of a binder resin and a solvent is supplied, the solvent concentration in the mixture is 15 to 35% by weight, the shear rate is 1.0 × 10 ^{1 to} 1.0 × 10 ³ / sec, and 50 to 500. It can be easily prepared by going through the step of kneading for a second.

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION To explain the present invention in more detail, the magnetic paint used in the production of the magnetic recording medium in the present invention is prepared by using a known cobalt-containing-γ-iron oxide magnetic powder. It was done. Cobalt-containing-γ
Any iron oxide magnetic powder can be used. The specific surface area of the magnetic powder (in the present specification, the specific surface area means the specific surface area measured by the BET method) is usually 20 to 70 m ² / g. For high-density recording, it is preferable to use fine magnetic powder, but the finer the magnetic powder, the more difficult it is to prepare a magnetic coating material. Therefore, a magnetic powder having a specific surface area of 45 to 60 m ² / g is generally used. Is preferred. The average major axis length of the magnetic powder is usually 0.01 to 0.5 μm, but it is generally preferable to use the one having a length of 0.01 to 0.3 μm. Also, axial ratio (ratio of major axis length to minor axis length)
Is preferably 12 or less.

The magnetic powder preferably accounts for 50 to 90% by weight of the magnetic layer. When the ratio of the magnetic powder becomes small, it becomes unsuitable for high density recording. On the contrary, if the ratio becomes too large, the mechanical strength of the magnetic layer will decrease. The preferable ratio of the magnetic powder in the magnetic layer is 65 to 75% by weight. As the binder resin, which is a main component constituting the magnetic layer together with the magnetic powder, it is preferable to use one having excellent adhesion to the non-magnetic support, affinity with the magnetic powder, and abrasion resistance. Commonly used binder resins include polyurethane resins, polyester resins, cellulose acetate butyrate, cellulose diacetate, cellulose derivatives such as nitrocellulose, vinyl chloride-vinyl acetate copolymers, vinyl chloride-vinylidene chloride copolymer copolymers. Examples thereof include a polymer, a vinyl chloride resin such as a vinyl chloride-acrylic copolymer, a synthetic rubber such as a styrene-butadiene copolymer, an epoxy resin, and a phenoxy resin. These may be used alone or in combination of two or more, and for example, the combined use of vinyl chloride resin and polyurethane resin is one of the preferable embodiments. When a binder resin having a polar group such as a sulfonic acid ester group, a phosphoric acid ester group, an amino group, a quaternary ammonium group, or a carboxylic acid group is used, since it has high affinity with the magnetic powder, Generally, the preparation of magnetic paint becomes easy. Usually, the binder resin has a glass transition point of −100 to 150 ° C. and a number average molecular weight of 1000 to
The one of 150,000 is used.

The magnetic paint contains a low molecular weight polyisocyanate compound having a plurality of isocyanate groups such as a trimethylolpropane adduct of tolylene diisocyanate as a cross-linking agent, and has a three-dimensional network structure in the magnetic layer. May be formed to improve its mechanical strength. The cross-linking agent is preferably contained in the binder resin in an amount of 10 to 50% by weight. The binder resin including the crosslinking agent is 2 to 45% by weight of the magnetic layer, especially 5 to
It preferably accounts for 25% by weight.

The magnetic paint preferably contains a dispersant in addition to the magnetic powder and the binder resin, and may further contain a lubricant, an abrasive, an antistatic agent and the like. As the dispersant, lecithin or other one having a phosphoric acid ester group is preferable, and usually, a phosphoric acid monoester represented by the following general formula or a mixture of phosphoric acid monoester and phosphoric acid diester is used.

[0011]

[Image Omitted] R-OP (= O) (OM) ₂ ... (1) (RO) ₂ P (= O) (OM) ... (2) (wherein R is an organic group. And M represents a cation containing a hydrogen ion) Preferably, a phosphoric acid ester having a polyoxyalkylene chain, particularly RO in the above formula is

[0012]

Embedded image (In the formula, R ′ represents an alkyl group or an alkylphenyl group), a polyoxyethylene alkyl (or alkylphenyl) phosphate ester is used.

In addition to the above-mentioned dispersant having a phosphoric acid ester group, another dispersant may be used in combination. Examples of such a dispersant include capric acid, lauric acid, myristic acid, oleic acid, linoleic acid and the like having 12 carbon atoms.
The fatty acids of -18, their alkali metal salts, alkaline earth metal salts and the like can be mentioned. The dispersant is 0.1-1 of the magnetic layer.
It is preferably contained so as to occupy 0% by weight, particularly 1 to 5% by weight.

As the lubricant, various kinds of lubricants such as aliphatic type, fluorine type, silicone type and hydrocarbon type can be used. Examples of aliphatic lubricants include fatty acids, fatty acid metal salts, fatty acid esters, fatty acid amides, and aliphatic alcohols. Examples of the fatty acid include oleic acid, lauric acid, myristic acid, palmitic acid, stearic acid and behenic acid. Examples of the fatty acid metal salt include magnesium salts, aluminum salts, sodium salts and calcium salts of these fatty acids.
Examples of the fatty acid ester include butyl ester, octyl ester and glyceride of the above fatty acids, and examples of the fatty acid amide include amides of the above acids, linoleic acid amide and caproic acid amide. Examples of the aliphatic alcohol include lauryl alcohol, myristyl alcohol, palmityl alcohol, stearyl alcohol, oleyl alcohol and the like. Examples of the fluorine-based lubricant include perfluoroalkyl polyether and perfluoroalkyl carboxylic acid. Examples of the silicone lubricant include silicone oil and modified silicone oil. Also,
Solid lubricants such as molybdenum disulfide and tungsten disulfide and phosphoric acid esters can also be used. Examples of hydrocarbon lubricants include paraffin and squalane.

The content of the lubricant in the magnetic layer is 0.1 to 1
It is preferably 0% by weight, especially 1 to 7% by weight. In the case of a medium having a multilayer structure, the content of the lubricant may be changed in each layer. As the abrasive, α-alumina, β-alumina, γ-alumina, α-
Iron oxide, silicon nitride, boron nitride, titanium oxide, silicon dioxide, tin oxide, zinc oxide, calcium carbonate, calcium sulfate, barium sulfate, molybdenum disulfide, tungsten oxide, silicon carbide, chromium oxide, etc., alone or in combination. Used in combination. Of these, it is preferable to use one having a relatively high hardness. Also,
The particle size (number average particle size) of the abrasive is preferably 0.5 μm or less. The amount of the abrasive used is such that the content in the magnetic layer is 1 to
Preferably, it is in the range of 10% by weight.

As the antistatic agent, carbon black,
Conductive metals and their compounds and oxides, natural surfactants such as saponins, nonionic surfactants such as alkylene oxides and glycerin, higher alkylamines, fourth
Cationic surfactants such as secondary ammonium salts, pyridinium salts and other heterocyclic salts, anionic surfactants containing carboxylic acid group, sulfonic acid group, phosphoric acid group, sulfuric acid ester group, acidic group such as phosphoric acid ester group, amino acids, Amphoteric surfactants such as aminosulfonic acids, sulfuric acid or phosphoric acid esters of aminoalcohol, etc. are used. Some of these surfactants can be mixed and used.

Among these antistatic agents, carbon black includes acetylene black, furnace black, thermal black and the like, and any of them can be used. A specific example is BLACK manufactured by Cabot.
PEARLS 2000, 1000, 900, 800,
VULCAN XC-72, Columbian Carbon Co., Ltd. RAVEN 8800, 8000, 7000, Mitsubishi Chemical Co. # 3750B, # 3750, # 3250B, # 32.
50, # 950, # 850B, # 650B, # 45, #
40, # 5, MA-77, MA-7 and the like.
These carbon blacks can be used alone or in combination. The carbon black surface may be treated with a dispersant or the like, or a part thereof may be graphitized before use.

As the conductive metal oxide, tin oxide, indium-tin oxide or the like can be used. The content of the antistatic agent in the magnetic layer is usually 0.1 to 0.1%.
It is used so as to be 10% by weight. As is apparent from the above description, a certain additive may have several effects at the same time.

Examples of the solvent used for preparing the magnetic paint include ketones such as methyl ethyl ketone, methyl isobutyl ketone and cyclohexanone, alcohols such as methanol, ethanol, propanol and isopropanol, esters such as methyl acetate, ethyl acetate and butyl acetate. , Ethers such as diethyl ether and tetrahydrofuran, aromatic hydrocarbons such as benzene, toluene and xylene, and aliphatic hydrocarbons such as hexane.

When a magnetic paint is prepared by kneading the above-mentioned magnetic powder, binder resin, dispersant and other additives with a solvent, the coercive force of the dry coating film of the magnetic paint (in the present specification, drying of the magnetic paint is used). The coercive force of a coating film means the coercive force of a magnetic coating material applied to a non-magnetic support and dried in the absence of a magnetic field in exactly the same manner as in the production of a magnetic recording medium using this). However, the coercive force of the magnetic powder of the raw material is 1 to 6
It is necessary to carry out kneading so that the percentage becomes low. That is, magnetic powder is very fine and bulky, but when preparing magnetic paint, it is necessary to replace the air around the magnetic powder or the air between particles with a binder resin or solvent and coat the magnetic powder with these. Is. Therefore, in the kneading process, a strong shearing force is applied to the magnetic powder to replace air between particles or around the particles with a binder resin or a solvent. At the same time, this causes the magnetic powder to be compressed, the distance between the particles is reduced, and the coercive force is reduced. If the shearing force is too small, the exclusion of air will not be sufficient, and the decrease in coercive force will naturally be small. On the contrary, if the shearing force is too large, the replacement with the binder resin or the solvent is sufficiently performed, but since the magnetic powders are strongly compressed, they stick to each other strongly and their surroundings are hard covered with the binder resin etc. It is considered that the dispersion of the magnetic powder in the subsequent step becomes difficult. Further, since the cobalt-containing-γ-iron oxide magnetic powder used in the present invention is needle-shaped, the coercive force may be lowered due to the needle-shaped magnetic powder being broken or deformed when a strong shearing force acts.
Therefore, the degree of decrease in the coercive force of the magnetic coating material reflects whether or not the magnetic coating material was well prepared, and it is considered that it is not preferable that the decrease in the coercive force is too large or too small.

However, since the magnetic powder in the magnetic coating material is oriented by the magnetic field, it is impossible to measure the coercive force of the magnetic coating material itself. The present inventors applied the magnetic coating material to a non-magnetic support in exactly the same manner as when manufacturing a magnetic recording medium using the magnetic coating material as a measure for coercive force of the magnetic coating material, and Paying attention to the coercive force of the dry coating film dried without application, in the case of cobalt-containing-γ-iron oxide magnetic powder, the decrease of the coercive force of the dry coating film of the magnetic coating with respect to the raw material magnetic powder was 1 It has been found that it is most preferable as a magnetic paint when it is in the range of -6%, especially 2-5%.

To prepare such a magnetic paint, a dispersant having a phosphoric acid ester group is used, and the raw material of the magnetic paint is first kneaded with the contents such as an extruder or a similar extrusion continuous kneader. It is preferable that the mixture is kneaded by a kneading device equipped with a mechanism for transporting it, and then an additional solvent or the like is added thereto to obtain a final magnetic coating composition, and further kneading / dispersion is performed. In this kneading process, the solvent concentration of the mixture is set to 15 to 35% by weight, and the shear rate is 1.0 × 10 ^{1 to} 1.0 × 10 ³ / sec, especially 2.0 × 10 ^{2 to} 5 for 50 to 500 seconds. It is preferable to knead at 0.0 × 10 ² / sec. The shear rate is defined as V (cm / S), which is the peripheral speed of the rotating shaft of the kneading device, and d (cm) is the gap between the rotating shaft and the device wall (= the gap through which the contents pass during kneading).
Is a value calculated by the following formula.

[0023]

## EQU1 ## Shear rate (S ^-1 ) = V / d

When the solvent concentration is less than 15%, the magnetic powder is excessively compressed and it becomes difficult to disperse the magnetic powder in the subsequent step. On the other hand, if the solvent concentration exceeds 35%, the fluidity of the mixture becomes too high, and the kneading effect may be drastically reduced. The preferred solvent concentration is 20 to 30% by weight. Kneading time is 5
If it is less than 0 seconds, the kneading is insufficient, and conversely, if kneading is performed for a long time exceeding 500 seconds, it may be difficult to disperse the magnetic powder in the subsequent step. The preferred kneading time is 60-30
0 seconds.

In the kneading step, the total amount of the magnetic powder and the dispersant is kneaded with a part of the binder resin and the solvent, and then the other raw materials and the additional solvent and the binder resin are added to the obtained mixture. It is preferable to carry out kneading / dispersion treatment. In a preferred embodiment of the present invention, a solvent is added to the internally kneaded mixture from the middle of the kneading device for kneading, and the mixture is diluted to a solvent concentration of 25 to 60% by weight and further diluted. Knead. By doing so, dispersion in the subsequent step is facilitated and the surface properties of the magnetic layer are improved. In this method, the addition of the solvent may be carried out in multiple stages,
Further, other components such as a binder resin may be supplied together with the solvent. Moreover, when using this method, the kneading is 50 to 1
50 seconds is sufficient.

The mixture discharged from the kneading device equipped with a mechanism for conveying while kneading is added with a solvent and the like so as to obtain a desired magnetic coating composition, and then subjected to a dispersion treatment to finish into a magnetic coating. As a dispersing device, a ball mill,
A conventional one such as a sand grinder or a roll mill can be used. According to the present invention, a high performance magnetic recording medium can be obtained by applying the magnetic coating material thus prepared to a flexible non-magnetic support by a conventional method. After coating, it is preferable that a magnetic field treatment (alignment treatment or non-orientation treatment) is carried out by a conventional method, followed by drying, followed by calendering treatment.

Examples of the material for forming the non-magnetic support include polyethylene terephthalate, polyethylene-2,
Examples thereof include polyesters such as 6-naphthalate, polyolefins such as polypropylene, cellulose derivatives such as cellulose triacetate and cellulose diacetate, aramid and polycarbonate. Also,
Although the form of the non-magnetic support is arbitrary, it is mainly in the form of tape, film, sheet, card, disk, drum or the like. The thickness is usually 3 to 100 μm, preferably 4 to 75 μm. The non-magnetic support may have a single structure or a multi-layer structure. Further, in order to improve the adhesiveness between the non-magnetic support and the magnetic layer, for example, corona discharge treatment or surface treatment with a surface modifier such as amine aqueous solution, trichloroacetic acid or phenols may be performed. .

[0028]

EXAMPLES The present invention will be described more specifically with reference to the following examples, but the present invention is not limited to the following examples. In the following examples and comparative examples, the raw materials used for the preparation of the magnetic paint are as follows.

Cobalt-containing-γ-iron oxide magnetic powder: coercive force 780 Oe, specific surface area 48 m ² / g, average major axis length 0.
2μm, axial ratio 7 Lecithin (dispersant) Polyurethane resin (binder resin): Toyobo Co., Ltd. UR
8300, glass transition point 23 ° C, number average molecular weight about 300
00 Vinyl chloride-vinyl acetate-vinyl alcohol copolymer (binder resin): UCC product VAGH carbon black (antistatic agent): Cabot product
VULCAN XC-72 α-Alumina (abrasive): Average particle size 0.3 μm Curing agent: Nippon Polyurethane Company product Coronate L Butyl stearate (lubricant) Methyl ethyl ketone (solvent) Cyclohexanone (solvent)

The compositions of the magnetic paints finally prepared in Examples and Comparative Examples are as follows. Cobalt-containing-γ-iron oxide magnetic powder 100.00 parts by weight Polyurethane resin 12.14 〃 Vinyl chloride-vinyl acetate-vinyl alcohol copolymer 12.14 〃 α-alumina 2.86 〃 Carbon black 2.86 〃 Lecithin 1.43 〃 Butyl stearate 4.29 〃 Hardener 7.14 〃 Methyl ethyl ketone 266.70 〃 Cyclohexanone 66.70 〃

Example 1 A raw material was continuously supplied to a kneader (T2-KRC kneader manufactured by Kurimoto Iron Works Co., Ltd.) of the type in which the contents were kneaded and conveyed, and the shearing rate was at the maximum. 4.1 × 10 ² /
The kneading was performed for 200 seconds so that the time became 2 seconds. Cobalt-containing-γ-iron oxide magnetic powder 100 parts by weight / min Lecithin 1.43 〃 Polyurethane resin 3.0 〃 Methyl ethyl ketone 44.76 〃 The resulting kneaded product (solvent concentration 30.0% by weight) was mixed with the final composition. Thus, additional polyurethane resin, methyl ethyl ketone, and other raw materials were added, and dispersion treatment was performed with a sand grinder to prepare a magnetic paint.

Comparative Example 1 The feed rate of methyl ethyl ketone to the kneader was set to 69.6 parts by weight / minute (therefore, the solvent concentration of the kneaded product was 40% by weight).
Except for the above), a magnetic coating material was prepared in the same manner as in Example 1.

Examples 2 to 4 and Comparative Examples 2 to 4 The raw materials were fed to the T2-KRC kneader at the feeding rate shown in Table 1 to carry out kneading under the conditions shown in Table 1, and the kneaded product inside the kneader The raw materials were fed at the feeding rate shown in Table 2 to continue kneading. The remaining raw materials were added to the obtained kneaded product so that the final composition was obtained, and dispersion treatment was carried out with a sand grinder in the same manner as in Example 1 to prepare a magnetic coating material.

[0034]

[Table 1]

[0035]

[Table 2]

Production of Magnetic Recording Medium and Measurement of Surface State The magnetic coating material obtained in each of the above Examples and Comparative Examples was applied to a polyethylene terephthalate film having a thickness of 10 μm using an extrusion type die, and 90 ° C. Dried in. Next, this was calendered at a temperature of 85 ° C. and a linear pressure of 300 kg / cm to manufacture a magnetic recording medium. For the magnetic recording medium thus obtained, the surface gloss of the magnetic layer (Gloss of 60 ° / 20 °) and the surface roughness (R
a) and hardness were measured. The results are shown in Table 3.

The physical properties in the examples were measured as follows. Coercive force of magnetic powder: Toei Kogyo Co., Ltd. magnetometer (VSM) is used to measure magnetic powder closely packed in a designated plastic container. Coercive force of dried coating of magnetic paint: Toei Kogyo Co., Ltd. A dry coating is sandwiched between two sample holders using a magnetometer (VSM) Surface roughness of magnetic layer: Measured using Taylstep manufactured by Taylor Hobson Hardness: Micro Vickers micro hardness manufactured by Shimadzu Gloss: Measured with a gloss meter manufactured by Suga Test Instruments Co., Ltd.

[0038]

[Table 3]

As is clear from Table 3, the hardness of the surface of the magnetic layer is low when the decrease rate of the coercive force of the dried coating film of the magnetic coating does not reach 1%. On the contrary, when the coercive force reduction rate exceeds 6%, the surface roughness is large and the surface gloss is also reduced.

Claims (6)

[Claims] 1. A magnetic recording medium manufactured through a step of applying a magnetic paint containing cobalt-containing .gamma.-iron oxide magnetic powder and a binder resin onto a non-magnetic support, which is dried as a magnetic paint. A magnetic recording medium having a coercive force of the coating film lower than that of the magnetic powder used by 1 to 6%. 2. The specific surface area of the magnetic powder is 45 to 60 m ² / g.
The magnetic recording medium according to claim 1, wherein 3. A magnetic coating material is added to a kneading device equipped with a mechanism for conveying contents while kneading the contents.
-Iron oxide magnetic powder, a dispersant having a phosphate group,
Supplying magnetic coating material consisting of binder resin and solvent,
It is a magnetic paint produced through a process in which the solvent concentration in the mixture is 15 to 35% by weight and the shear rate is 1.0 × 10 ^{1 to} 1.0 × 10 ³ / sec for 50 to 500 seconds. The magnetic recording medium according to claim 1 or 2. 4. The magnetic coating material is added to a kneading device equipped with a mechanism for conveying contents while kneading the contents, and at least cobalt-containing-γ
-Iron oxide magnetic powder, a dispersant having a phosphate group,
Supplying magnetic coating material consisting of binder resin and solvent,
After kneading the mixture at a solvent concentration of 15 to 35% by weight and a shear rate of 1.0 × 10 ^{1 to} 1.0 × 10 ³ / sec for 50 to 500 seconds, an additional solvent is added to the mixture in the kneading device. The solvent concentration of the mixture is 25-
2. A magnetic paint produced by a process of diluting to 60% by weight and further kneading.
Or the magnetic recording medium according to 2. 5. A method for producing a magnetic recording medium, which comprises applying a magnetic coating material onto a non-magnetic support, wherein a kneading device equipped with a mechanism for kneading and transporting the content as the magnetic coating material contains at least cobalt-containing-γ. -A magnetic coating material consisting of iron oxide magnetic powder, a dispersant having a phosphoric acid ester group, a binder resin and a solvent is supplied so that the solvent concentration in the mixture is 15 to 35.
% By weight, shear rate 1.0 × 10 ^{1 to} 1.0 × 10 ³ /
A method comprising using a magnetic paint produced through a process of kneading for 50 to 500 seconds per second. 6. A method for producing a magnetic recording medium, which comprises applying a magnetic coating material onto a non-magnetic support, wherein a kneading device equipped with a mechanism for kneading and transporting the content as the magnetic coating material contains at least cobalt-containing-γ -A magnetic coating material consisting of iron oxide magnetic powder, a dispersant having a phosphoric acid ester group, a binder resin and a solvent is supplied so that the solvent concentration in the mixture is 15 to 35.
% By weight, shear rate 1.0 × 10 ^{1 to} 1.0 × 10 ³ /
After kneading for 50 to 500 seconds per second, a magnetic coating material containing an additional solvent is supplied to the mixture in the mixing device so that the solvent concentration in the mixture rises to 25 to 60% by weight,
A method comprising using a magnetic paint produced through a kneading process.