JP2000207732A - Magnetic recording medium and manufacture of the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a magnetic recording medium equipped with a thin magnetic layer and a non-magnetic lower layer with the high reliability of electromagnetic converting characteristics, drop-out, and durability, and with high productivity. SOLUTION: A lower non-magnetic layer in which non-magnetic inorganic power and carbon black is dispersed in connecting agent is formed on a non- magnetic supporting body, and a magnetic layer in which ferromagnetic powder is dispersed in the connecting agent is formed on this so that dry thickness can be set so as to be 0.5 μm or less. The top end part of the magnetic layer can be prevented from being projected from the top end part of the lower non-magnetic layer on the longitudinally cut cross-section of the magnetic recording medium.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magnetic recording medium for high density recording, and more particularly to a magnetic recording medium having a coating type thin magnetic layer and a method of manufacturing the same.

[0002]

2. Description of the Related Art In recent years, digital media have been required to have a higher recording density in order to respond to the trend of miniaturization of equipment, the demand for improving the quality of recording / reproducing signals, the demand for longer recording time, and the demand for increasing the recording capacity. Further improvement has always been desired. For this purpose, improvements have been made to the surface properties of the magnetic layer, to improve the dispersibility of the magnetic particles in the magnetic layer, and to enhance the magnetic properties of the magnetic layer. The demand for high-density recording requires that the thickness of the magnetic layer be extremely thin. However, if the thickness of the magnetic layer is reduced to about 2 μm or less, the effect of the non-magnetic support on the surface of the magnetic layer may occur, and Inferior film properties affect the electromagnetic conversion characteristics and dropout. For this reason, if the nonmagnetic thick undercoat layer is provided on the support surface and then the thin magnetic layer is provided as the upper layer, the above problem can be solved and overwriting is advantageous, and thickness loss and self-demagnetization loss are reduced. Thus, a magnetic recording medium having high electromagnetic conversion characteristics can be obtained.

A typical method of applying such a thin multilayer medium is, for example, a non-magnetic layer coated on a non-magnetic support as disclosed in JP-A-63-191315. There are a wet-on-wet coating method in which a coating liquid for a magnetic layer is applied while the coating is in a wet state, and a wet-on-dry coating method in which a lower non-magnetic layer is dried and calendered, and then the upper layer is applied. In general, the wet-on-wet method has advantages in that the coating thickness of the upper coating film is stable and the cost is excellent, but the viscoelastic properties of the lower non-magnetic layer and the coating liquid of the upper layer are advantageous. If they are not similar, the liquid flow may be disturbed at the boundary of the coating surface, or a streak-like defect called a coating streak on the magnetic layer may occur, or the surface of the magnetic layer may be rough, resulting in a magnetic layer with excellent surface properties. There is a problem that can not be obtained. Further, when the paint components of the upper layer and the lower non-magnetic layer are different from each other, there is a problem that noise is generated due to disturbance of the interface between the upper and lower layers due to poor dispersibility of each paint and difference in viscosity.

To solve this problem, Japanese Unexamined Patent Publication No. 4-325915 discloses the use of acicular pigments in the lower non-magnetic layer. In the wet-on-wet coating, when the needle-shaped pigment is contained in the non-magnetic layer, the pigment is flow-oriented in the longitudinal direction, and when the magnetic layer is subsequently applied to the magnetic field, the rotational movement of the magnetic powder is suppressed. Therefore, the disturbance of the interface between the upper magnetic layer and the lower nonmagnetic layer can be suppressed, and the orientation of the ferromagnetic powder can be improved. In JP-A-7-326043, the powder contained in the lower layer is mainly composed of α-Fe ₂ O ₃ having an acicular ratio of 2 to 20, and the lower layer has a saturation magnetic flux density Bm of 0.01 to 29 Gauss. By using powder, the orientation probability in the longitudinal direction is set to 60% or more, and disturbance of the interface in wet-on-wet coating can be suppressed to provide a magnetic recording medium with high smoothness.

However, when wet-on-dry coating is used, although the cause is unknown, even if needle-like pigments are used for the lower non-magnetic layer, the needle-like pigments are not oriented in the longitudinal direction but rather are oriented randomly. I understand. Since the needle-shaped nonmagnetic inorganic powder is not oriented in the longitudinal direction, there is a problem that the life of the cutting blade during cutting is extremely short. Usually, in the case of a medium having a single magnetic layer having a thickness of about 2 μm, since the needle-shaped magnetic powder is oriented in the longitudinal direction with a considerable probability, the cutting blade can maintain a substantially good cutting state up to a total cutting length of 1,000,000 m. Most of the time it was kept. But,
When cutting a multilayer medium using needle-like pigments for the lower non-magnetic layer, the life of the cutting blade is as short as 100,000 m or less, and the cut surface deteriorates sharply, especially at the interface between the upper and lower layers, causing dropout and durability. However, it has been difficult to make a stable product. This is thought to be because, in the case of the wet-on-dry method, the upper layer is applied after the lower non-magnetic layer is once dried, so that the adhesiveness of the upper and lower layers is poor, and the cut surface is greatly changed due to the life of the cutting blade. Can be That is, if the cutting blade becomes poorly sharp, the coating film is torn off and cut off. If the adhesiveness at the interface between the upper and lower layers is poor, the thin magnetic layer is pulled and then cannot be restored, and jumps out of the cross section. It is damaged and lost in the process after cutting,
It causes dropout and clogging.

[0006] The relationship between the cross-sectional shape of the cut magnetic tape and characteristics such as dropout and head contamination has been studied. For example, in JP-A-9-153212,
It has been studied to reduce the abrasion of the back coat layer and reduce the dropout by adopting a configuration in which the end face of the back coat layer does not protrude from the apex of the maximum protrusion of the nonmagnetic support. In addition, Japanese Patent No. 2722128 discloses that a tape winding shape and head contamination are improved by defining the shape of the cross section of the slit end portion to a specific shape.

[0007]

However, in the above examination of the cross-sectional shape, almost no consideration has been given to a multilayer recording medium having a thin magnetic layer and a lower non-magnetic layer. SUMMARY OF THE INVENTION An object of the present invention is to provide a multilayer magnetic recording medium having a thin magnetic layer and a non-magnetic lower layer, a magnetic recording medium having excellent reliability such as electromagnetic conversion characteristics, dropout and durability, and further having excellent productivity, and production thereof. It is to provide a method.

[0008]

The above objects can be achieved by the present invention described in the following (1) to (7). (1) A lower nonmagnetic layer in which a nonmagnetic inorganic powder and carbon black are dispersed in a binder is provided on a nonmagnetic support, and a magnetic layer in which a ferromagnetic powder is dispersed in a binder is dried on the nonmagnetic support. The outermost portion of the magnetic layer is not protruded from the outermost portion of the lower non-magnetic layer on the cut surface of the magnetic recording medium obtained by cutting so as to be 5 μm or less and obtained by cutting in the longitudinal direction. Magnetic recording medium. (2) The thickness of the lower nonmagnetic layer is 0.01 μm to 0.5 μm
(1) The magnetic recording medium according to (1), (3) The non-magnetic inorganic powder of the lower non-magnetic layer has a major axis length of 0.
A needle pigment having a size of 20 μm or less, wherein the binder of the lower nonmagnetic layer is an electron beam (EB) curable resin, and the volume ratio of carbon black is based on the total inorganic powder (nonmagnetic inorganic powder + carbon black) The magnetic recording medium according to (1) or (2), wherein the magnetic recording medium is 40% or more and 80% or less. (4) The carbon black contained in the lower non-magnetic layer has a primary particle size of 25 to 40 nm, a specific surface area by a BET method of 50 to 80 m ² / g, and a DBP oil absorption of 40 to 55 m.
The magnetic recording medium according to any one of (1) to (3), wherein the weight is 1/100 g. (5) The magnetic recording medium according to any one of (1) to (4), wherein the binder of the magnetic layer is made of a thermosetting resin. (6) The magnetic powder contained in the magnetic layer is a ferromagnetic powder containing iron (Fe) as a main component, and contains 10 to 15 at% of Al element with respect to Fe element.
(5) The magnetic recording medium according to (5). (7) A coating for a lower non-magnetic layer in which a non-magnetic inorganic powder and carbon black are dispersed in a binder is applied on a non-magnetic support, dried, calendered, and cured, and then the lower non-magnetic layer is obtained. (1) to (6), wherein a coating material for a magnetic layer in which a ferromagnetic powder is dispersed in a binder is applied.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION In the study of the present inventors, a multilayer magnetic recording medium having a very thin magnetic layer provided on
When used in a rotating head type deck such as a VC, a phenomenon that dropout abnormally increases was observed. Therefore, as a result of repeated studies focusing on the relationship between the upper magnetic layer and the non-magnetic lower layer on the cut surface of the magnetic recording medium, a multilayer magnetic recording medium in which this phenomenon rarely occurs can be obtained.

The basic idea of the present invention is to provide a multilayer structure in which the outermost end of the upper magnetic layer does not protrude from the outermost end of the lower nonmagnetic layer. Here, the outermost portion of the magnetic layer or the non-magnetic layer refers to a magnetic layer or a magnetic layer at a cut surface of a long magnetic recording medium obtained by cutting in the longitudinal direction at the time of coating as shown in FIG. Alternatively, it is a portion that protrudes most in the width direction in the cut surface shape of the nonmagnetic layer. With such a configuration, the contact between the end portion of the extremely thin magnetic layer and the side surface of the winding hub in the magnetic tape cartridge or the width regulating portion of the guide roll during the winding process after cutting is reduced. The drop of the magnetic layer from the end of the magnetic layer can be prevented, and the clogging of the dropout head can be improved.

A magnetic recording medium having a cross-sectional shape after cutting as in the present invention can be specifically obtained by using the following method. The thickness of the magnetic layer is 0.5 μm or less, and the thickness of the lower non-magnetic layer is 0.5 μm or more. The lower non-magnetic layer contains acicular non-magnetic inorganic powder having a major axis length of 0.20 μm or less and carbon black, and the volume ratio of carbon black is 40% of the total inorganic powder (non-magnetic inorganic powder + carbon black). At least 80%. An electron beam (EB) curable resin is used for the lower nonmagnetic layer, and a thermosetting resin is used for the upper magnetic layer.

First, a description will be given. In the present invention, carbon black is not included in the definition of the nonmagnetic inorganic powder.

The thickness of the lower non-magnetic layer of the present invention is from 0.5 to
It is set to 2.0 μm, preferably 0.8 to 1.5 μm. If this value exceeds 2.0 μm, curling and cupping of the coating film will increase, resulting in an inconvenience that envelope defects easily occur. On the other hand, this value is 0.5 μm
If it is less than the lower limit, it becomes difficult to make the outermost end of the lower non-magnetic layer protrude from the outermost end of the magnetic layer. Further, the effect of covering the surface roughness of the non-magnetic support and smoothing the surface of the non-magnetic lower layer is weakened, and furthermore, the surface property of the magnetic layer cannot be improved. The thickness of the magnetic layer is 0.5 μm or less, preferably 0.3 μm or less. This value is 0.5
Above μm, even if the thickness of the nonmagnetic lower layer is 0.5 μm or more, it becomes difficult to make the outermost end of the lower nonmagnetic layer protrude from the outermost end of the magnetic layer. The lower limit of the magnetic layer can be determined depending on the electromagnetic conversion characteristics required for the magnetic recording medium. However, it is generally difficult to apply the magnetic layer uniformly below 0.01 μm.

The following is a description. In order to obtain the cross-sectional shape of the present invention, it is effective that the lower non-magnetic layer contains needle-like non-magnetic powder and carbon black at a specific ratio. The amount of carbon black added to the lower non-magnetic layer is specified by the volume ratio of the non-magnetic inorganic powder and carbon black in the lower non-magnetic layer. Specifically, the carbon black is added so that the volume ratio of the carbon black is 40% or more and 80% or less based on all the inorganic pigments (nonmagnetic inorganic powder + carbon black) in the lower nonmagnetic layer. By adding in this manner, the coating film has appropriate hardness, appropriate voids and conductivity, can extend the life of the cutting blade, and can easily form the cut surface of the present invention. This value is preferably between 45 and 70%. 40%
In the following, since the ratio of the randomly oriented pigment on the cut surface is large, the hard needle-like pigment is cut, and the life of the cutting blade is significantly shortened, whereby the shape of the cut surface of the medium becomes unstable. On the other hand, if it exceeds 80%, the ratio of carbon black becomes high, the dispersion of the coating material for the lower non-magnetic layer is poor, the surface smoothness of the lower non-magnetic layer is deteriorated, and the electromagnetic conversion characteristics are reduced. By appropriately adding carbon, the coating film has appropriate hardness, appropriate voids and conductivity, can extend the life of the cutting blade, and can produce a medium with a stable cut surface. This volume ratio can be calculated from the added weight and specific gravity of the added carbon black and nonmagnetic inorganic powder. It is also possible to calculate the area ratio of each of the powder particles in the photograph of the cross section of the lower non-magnetic layer of the magnetic recording medium after the preparation, and calculate the area ratio by performing this at a plurality of locations. .

The acicularity of the acicular nonmagnetic inorganic powder contained in the lower nonmagnetic layer is defined as an axial ratio (a ratio of the length of the major axis to the minor axis).
Is in the range of 3 to 20 and does not include granular, spherical, and thin plate shapes. This is because the relationship between the solid content concentration and the viscosity of the lower non-magnetic layer coating, when the needle-shaped inorganic non-magnetic powder of the lower non-magnetic layer is used, is the same as that of the magnetic layer coating containing the needle-shaped ferromagnetic powder. This is because it is similar to the relationship between the solid content concentration and the viscosity, and is easy to handle in the dispersion step and the coating step. In the case of granules or spheres having an axial ratio of 3 or less, the viscosity is too low with respect to the solid concentration, which causes a problem in the dispersion step and the coating step. In the case of a thin plate, stacking between pigments is so strong that high dispersion cannot be achieved in many cases, which is not preferable. The major axis length of these nonmagnetic inorganic powders is 0.01 to 0.20 μm.
m, preferably 0.05 to 0.18 μm. If this value exceeds 0.20 μm, there arises a disadvantage that the surface roughness of the lower non-magnetic layer is deteriorated.
When the thickness is less than 1 μm, the dispersibility of the nonmagnetic inorganic powder in the coating material is deteriorated, and there is a disadvantage that the surface property cannot be improved by reducing the particle size. This major axis length is
It is defined as follows: That is, the maximum length of each powder particle is measured for each non-magnetic inorganic powder, and the maximum length is obtained as an arithmetic average thereof. As a specific measuring method, a sample obtained by encapsulating a finally obtained magnetic recording medium with a thermosetting epoxy resin, and cutting the sample thinly with a diamond cutter (TEM) (manufactured by JEOL Ltd.) : JEM100CX) (observation locations are increased one after another if necessary), and a total of 100 or more particles are measured, and the average value is obtained. Such a nonmagnetic inorganic powder is combined with the following carbon black in the solid content of the nonmagnetic layer in an amount of 40 to 85 wt%, preferably 50 to 85 wt%.
It is contained at a ratio of 83 wt%, more preferably 50 to 80 wt%. If this value exceeds 85% by weight, the dispersibility of the non-magnetic inorganic powder or carbon black becomes worse due to the decrease in the resin content, and the inconvenience that the coating of the non-magnetic layer becomes brittle occurs. This value is 40w
When it is less than t%, the abrasive of the magnetic layer is easily buried in the lower non-magnetic layer side, and the efficiency as the abrasive tends to decrease. There is a disadvantage that it is difficult to contribute to the improvement of surface properties. Specific non-magnetic inorganic powders used include α-iron oxide, α-goethite, acicular Ti
O ₂ , Cr ₂ O ₃ , CrOOH and the like are listed, and among them, α-iron oxide is particularly preferable.

The carbon black has a primary particle size of 25.
４０40 nm, specific surface area by BET method is 50-100 m
² / g, preferably 50 to 80 m ² / g, DBP oil absorption 1
0 to 50 ml / 100 g, preferably 40 to 55 ml /
100 g. Specific examples include those manufactured by Columbian Carbon: Leven 760, Leven 1060
B, manufactured by Mitsubishi Chemical Corporation: # 42, CF9B, # 45 and the like. By using the carbon black defined by the above characteristic values, it is possible to balance the life of the cutting blade, the surface smoothness of the lower non-magnetic layer, the applicability, the conductivity, and the amount of lubricant retained. These primary particle size, specific surface area, DBP
Characteristic values such as oil absorption are closely related to each other, and it is often difficult to change them independently.However, the characteristic value tends to be such that the primary particle size is larger than this range. If the BET value or the DBP oil absorption is small, the surface property of the lower non-magnetic layer is inferior. When the primary particle size is small, the dispersibility is poor, and when the BET value or DBP oil absorption is large, the viscosity of the coating material is poor and the coating property is likely to be deteriorated.

The total amount of the above pigment and carbon black is 100 to 1 with respect to the binder 100 in a weight ratio.
000 is preferred.

The following is a description. In the present invention, it is preferable to use an electron beam (EB) curable resin for the lower nonmagnetic layer and to use a thermosetting resin for the upper magnetic layer. If an electron beam-curable resin is used for the non-magnetic lower layer, the curing reaction by electron beam irradiation will result in a coating film with a high degree of crosslinking, and will be damaged by the winding process after cutting and the contact with the side of the hub in the magnetic tape cartridge. And the effect of protecting the thin magnetic layer is further enhanced.

As the electron beam-curable resin contained in the lower non-magnetic layer of the present invention, known resins can be used. Among them, an electron beam-curable vinyl chloride resin having a sulfur-containing polar group and a phosphorus-containing polar group are used. And an electron beam-curable urethane resin.

As such a vinyl chloride resin, known resins such as a vinyl chloride-vinyl acetate-vinyl alcohol copolymer and a vinyl chloride-vinyl alcohol copolymer can be used. A copolymer with the contained monomer is preferred. As the sulfur (S) -containing polar group contained in the vinyl chloride resin, a sulfuric acid group and / or a sulfo group are particularly preferable. The sulfate group and a sulfo _group, -SO 4 Y, in -SO ₃ Y, Y
May be either H or an alkali metal, but Y = K
In, -SO ₄ K, particularly preferably -SO ₃ K.
These sulfate groups and sulfo groups may be either one or both. In addition, these S
It is preferable that the polar group is contained in the molecule as 0.01 to 10% by weight, particularly 0.1 to 5% by weight as S atom. Vinyl chloride resin has a vinyl chloride content of 60 to 10
0% by weight, especially 60 to 95% by weight is preferred. The average degree of polymerization of the vinyl chloride resin is preferably about 100 to 900. Further, this vinyl chloride resin is
Acrylic group CH ₂ア ク リ ル CH—COO—
Or it contains a methacryl group CH ₂ ＣＨCHCH ₃ COO—. These (meth) acryl groups have an average of 1 in the molecule.
It is preferred that there are ~ 20, preferably 2-10. Further, the (meth) acryl group is preferably bonded to the resin skeleton through one urethane bond. Such an electron beam-curable vinyl chloride resin is commercially available from Toyobo Co., Ltd. as TB-0246.

The electron beam-curable urethane resin used in combination with the above-mentioned electron beam-curable vinyl chloride resin contains a phosphorus (P) -containing polar group. As the phosphorus-containing polar group, one or more of a phosphonic acid group = PO ₃ Y, a phosphinic acid group = PO ₂ Y, and a phosphinous acid group = POY (Y is H or an alkali metal) is preferable. Particularly, Na is preferable, and among these polar groups, only = PO ₃ Na is contained,
= PO ₃ Na is preferably contained as a main component. These P-containing polar groups have a P atom of 0.01 to
It is preferable to contain 10% by weight, particularly 0.02 to 3% by weight. Even if these exist in the main chain of the skeleton resin,
It may be present during branching. In addition, it contains at least one acryl group or methacryl group in the molecule similarly to the above-mentioned vinyl chloride resin. It is preferable that the (meth) acryl group be bonded via a urethane bond to bond to the resin skeleton. Such an electron beam-curable urethane resin may be prepared by a known method by mixing an acrylic double bond-containing compound with a specific phosphorus compound and / or a raw material containing a raw material resin reacted with a specific phosphorus compound in a solvent, or It is obtained by reacting in the absence of a solvent. These production methods are described in JP-A-62-43830, JP-A-61-77134, JP-A-62-40615, and JP-A-62-195720.
And JP-A-8-185624. Such an electron beam-curable urethane resin is commercially available from Toyobo Co., Ltd. as TB-0242.

It is preferable that the S-containing polar group-containing vinyl chloride resin and the P-containing polar group-containing urethane resin are mixed in a weight ratio of 7: 3 to 3: 7. By using such a resin as a binder in the non-magnetic lower layer, the dispersibility of the carbon black and the non-magnetic inorganic powder is improved, and the object of the present invention can be easily achieved. In addition, in addition to these resins, various known resins may be contained in a range of 20% by weight or less of the whole resin.

An electron beam is used for curing the electron beam-curable resin, and the irradiation amount is preferably 1 to 10 Mrad, and 3 to 7 Mrad.
Mrad is more desirable, and the irradiation energy (acceleration voltage) is preferably 100 kV or more. The lower non-magnetic layer is preferably irradiated with an electron beam after calendering the non-magnetic layer, so that the surface property of the lower non-magnetic layer is good and most desirable.
Irradiation may be performed before calendering the nonmagnetic lower layer.
Irradiation may be performed separately before and after the application of the upper magnetic layer as long as the curability of the lower layer which does not cause any problem during the application of the upper layer is obtained.

As the binder contained in the magnetic layer, conventionally known thermoplastic resins, thermosetting or reactive resins, electron beam curable resins and the like are used, and the combination thereof depends on the characteristics of the medium and the process conditions. Is used as appropriate. When using an electron beam curable resin for the lower non-magnetic layer, it is better to use a highly versatile thermoplastic resin, thermosetting or reactive resin,
A thermosetting resin is preferable because it is easy to design the physical properties of the coating film by combining the upper and lower layers.

Of these, a combination of a vinyl chloride copolymer and a polyurethane resin is most preferably used.
The vinyl chloride resin preferably contains a sulfate group and / or a sulfo group and an epoxy group as polar groups. Polyurethane resins are particularly effective in that they have good abrasion resistance and good adhesion to a support, and may have a polar group, a hydroxyl group, or the like in a side chain thereof. Particularly, sulfur (S) or phosphorus (P) ) Are preferred. As the crosslinking agent for curing the binder resin, various known polyisocyanates can be used. Specifically, Coronate L and H manufactured by Nippon Polyurethane Industry Co., Ltd.
L, 3041, 24A-100, T made by Asahi Kasei Corporation
PI-100, Death Modules L and N manufactured by BF Goodrich, and the like. These resins and crosslinking agents are described in detail in JP-A-8-185624. Also,
To cure the reactive or thermosetting resin, the resin is heated in a heating oven at 50 to 80 ° C. for 6 to 100 hours, or is run at a low speed in an oven at 80 to 120 ° C. These resins may be used alone or as a mixture of two or more of the same type.

Hereinafter, preferred embodiments of the present invention will be described. The magnetic layer of the present invention contains a ferromagnetic powder. Examples of the ferromagnetic powder used include iron oxide magnetic powder, ferromagnetic metal powder, plate-like hexagonal ferrite, and chromium dioxide. Among these, it is preferable to use ferromagnetic metal powder mainly composed of iron (Fe) and plate-like hexagonal ferrite,
In particular, when Fe is the main component and Al is 10
Ferromagnetic powders containing up to 15 at% are preferred. If the Al element is less than 10 at%, the shape of the magnetic powder deteriorates due to sintering during the production of the magnetic powder, and the dispersibility when formed into a paint tends to be poor, resulting in an uneven magnetic coating film. On the other hand, if the content is more than 15 at%, the magnetic energy of the magnetic powder is weakened, and the electromagnetic conversion characteristics deteriorate. The characteristics of the ferromagnetic powder are as follows: coercive force Hc = 1800 to 2500 Oe, saturation magnetization σs = 120 to 150 emu / g, specific surface area by BET method = 30 to 70 m ² / g, major axis length 0.05
0.15 μm and a short axis length of 20 nm or less.

In the case of a ferromagnetic metal powder, water-soluble N
a, K, Ca, Fe, may contain inorganic ions such as Ni, but the amount is preferably 500 ppm or less,
More preferably, it is 100 ppm or less. Such a ferromagnetic powder is usually contained in a weight ratio of binder 100 in the magnetic layer.
, And the content of the ferromagnetic powder in the magnetic layer is 50 to 95 watts of the entire dried magnetic layer.
t%, preferably 55 to 90 wt%. If the content of the ferromagnetic powder is too large, the amount of additives including the resin in the magnetic layer is relatively reduced, so that defects such as reduced durability of the magnetic layer are likely to occur, and if the content is too small, High playback output cannot be obtained.

Hereinafter, other general matters relating to the present invention will be described.

It is preferable that the magnetic layer further contains an abrasive. The average particle size of the abrasive used is 0.05 to
0.40 μm, preferably 0.08 to 0.30 μm,
More preferably, it is 0.08 to 0.20 μm. Since the magnetic layer of the present invention is very thin, if the average particle diameter of the abrasive contained exceeds 0.40 μm, the wear of the magnetic head in contact with the magnetic recording medium tends to increase. If it is less than 0.05, the height of the projections due to polishing on the surface of the magnetic layer is reduced, and the durability tends to decrease. The average particle size of the abrasive in the present invention is defined as follows. That is, the maximum length of each particle is measured for each particle, and the maximum length is obtained as an arithmetic average thereof. As a specific measuring method, a cross-sectional photograph of the magnetic recording medium is taken, 100 or more abrasive particles are arbitrarily sampled from the photograph, the maximum length of these particles is measured, and the maximum length of the particles is obtained according to the above calculation method. . Such an abrasive is preferably used in an amount of 1 to 10 parts by weight with respect to 100 parts by weight of the ferromagnetic powder, but can be determined by the balance of wear, seizure, and clogging depending on the type of head of the recording / reproducing apparatus used. preferable. Examples of usable abrasives include inorganic powders such as metals, metal oxides, metal carbonates, metal sulfates, metal nitrides, metal carbides, and metal sulfides. It is also possible to use a part of the magnetic inorganic powder. More specifically, α-alumina, β-alumina, γ-alumina, θ-alumina, δ
-Alumina, Cr ₂ O ₃ , α-iron oxide, SnO ₂ , silicon carbide, barium sulfate and the like can be used alone or in combination. These abrasives may be contained in the lower non-magnetic layer as needed.

The coating material for the magnetic layer and the coating material for the lower non-magnetic layer usually contain a lubricant. As the lubricant to be used, various known lubricants can be used without limitation, and it is particularly preferable to use a fatty acid and / or a fatty acid ester. Specifically, a monobasic fatty acid having 12 to 24 carbon atoms (which may contain an unsaturated bond or may be branched), a monobasic fatty acid having 10 to 24 carbon atoms (which may contain an unsaturated bond, Monovalent fatty acid and a monovalent, divalent, trivalent, tetravalent, pentavalent monovalent fatty acid having 2 to 22 carbon atoms (which may contain an unsaturated bond or may be branched). Mono-fatty acid ester, di-fatty acid ester, tri-fatty acid ester, or a mixture thereof, or a mixture of two or more of any of cyclic or polysaccharide-reducing alcohols such as polyvalent, hexahydric alcohol, sorbitan, and sorbitol Good.

The content of these fatty acids and / or fatty acid esters may be changed as appropriate according to the required characteristics of the magnetic recording medium. It is also possible to adopt a method in which the above-mentioned lubricant is also contained in the magnetic layer and the magnetic layer is gradually transferred to the upper magnetic layer. Further, when a back coat layer is applied, a large amount of a lubricant is contained on the back coat layer side to improve the surface lubricity by transfer to the magnetic layer surface.

In the coating material for the magnetic layer, in addition to the fatty acid and fatty acid ester, a lubricating effect, an antistatic effect, a dispersing effect,
An additive for exhibiting a plasticizing effect or the like is contained. The amount of these additives is 10 wt% in total with respect to the ferromagnetic powder.
Hereinafter, it may be used particularly in the range of 0.01 to 5 wt%.

Further, carbon black may be contained in the coating material for the magnetic layer. Furnace carbon black, thermal carbon black, acetylene black and the like can be used as carbon black. The particle size and the like of these carbon blacks may be appropriately selected depending on the electrical resistance and friction characteristics required of the medium and the balance of the output or the surface roughness at the shortest recording wavelength.
These are 20 to 4 when considering the electromagnetic conversion characteristics with priority.
0 nm is preferable.
It is preferable to use the largest possible particle size in the range of 50 nm which is acceptable in the electromagnetic conversion characteristics. The amount of addition is 0.05 to 2 parts by weight based on 100 parts of the ferromagnetic powder.
The value may be selected experimentally in consideration of the required characteristics of the medium and the dispersibility and flow characteristics when a coating material is used. These carbon blacks are described, for example, in "Carbon Black Handbook" (edited by Carbon Black Association).

Next, a method and steps for producing the magnetic recording medium of the present invention will be described.

The lower non-magnetic layer in the present invention is formed by mixing and dispersing a coating material containing the above-described materials and a solvent by a known method, coating the mixture on a non-magnetic support, and drying. You. At this time, the lower non-magnetic layer is firmly cured by electron beam irradiation using an electron beam (EB) curable resin as a binder and a needle-shaped non-magnetic inorganic powder and carbon black. Even when the magnetic layer paint is applied, the interface between the upper and lower layers is not disturbed, and as a result, the interface between the upper and lower layers can be made very regular. In addition, by adopting a so-called wet-on-dry method in which the magnetic layer is coated after the lower non-magnetic layer is dried and hardened in this manner, the abrasive in the magnetic layer can sink into the lower non-magnetic layer. Therefore, the effect as an abrasive is effectively exhibited, and as a result, the durability of the magnetic layer is reliably ensured as initially set. However, depending on the thickness and required characteristics of the magnetic layer, the amount of electron beam irradiation may be weakened to control the degree of hardening of the lower layer, and the durability of the magnetic layer may be adjusted by adjusting the amount of abrasive sunk into the lower layer. Is also possible.

Further, it is more preferable to smooth the non-magnetic layer by performing so-called calendering after drying the lower non-magnetic layer and before forming the magnetic layer. By performing this calendering process, the surface smoothness of the non-magnetic layer is improved, and thus the surface smoothness of the magnetic layer applied on the non-magnetic layer is improved, and at the same time, the burial of the abrasive is further suppressed. And the electromagnetic conversion characteristics and durability are significantly improved.

The calendering may be performed by using a conventionally known metal roll or resin roll pair, which compresses the applied non-magnetic support and smoothes the applied surface.
Further, it may be carried out in the application line of the non-magnetic layer, or may be carried out after the non-magnetic layer is once applied and wound. Here, the calendering conditions depend on the material of the non-magnetic support, but are usually at a processing temperature of 80 to 130.
° C, more preferably 90-120 ° C, and a linear pressure of 100-
500 kg / cm, more preferably 200-400 kg
/ Cm range. The hardening treatment of the non-magnetic layer is performed by electron beam hardening as described above. The calendering is preferably performed before the hardening treatment of the non-magnetic layer in consideration of ease of processing.

As the nonmagnetic support used in the present invention, for example, various known plastic films such as polyesters such as polyethylene terephthalate and polyethylene 2,6-naphthalate, and polyamides and polyimides can be used without limitation.

The thickness of the non-magnetic support is not particularly limited, but is preferably 2 to 14 μm, preferably 3 to 10 μm in order to meet the demand for downsizing of the magnetic recording medium. Good.

As a method of applying the magnetic paint, a conventionally known method can be used. Examples thereof include an extrusion nozzle coating method, a reverse roll coating method, a gravure roll coating method, a knife coater coating method, a doctor blade coating method, a kiss coat coating method, a color coat coating method, and a slide bead coating method. Above all, the gravure roll coating method is excellent in productivity, and the reverse roll coating method is preferable because the range of paints that can be used for coating is wide. In addition, the extrusion nozzle coating method is excellent in controllability of the coating film thickness. Such a coating method is also used when the lower non-magnetic layer and the back coat layer described later are coated. The magnetic paint applied on the lower non-magnetic layer by such a method is subjected to a magnetic field orientation treatment, a drying treatment, a smoothing treatment and the like. For smoothing the magnetic layer, calendering is performed in the same manner as for the lower non-magnetic layer. As calendering rolls, heat-resistant plastic rolls such as epoxy, polyester, nylon, polyimide, polyamide, and polyimideamide (carbon, metal, etc.) Or other inorganic compounds may be kneaded) and a combination of metal rolls or metal rolls, and the treatment temperature is preferably 70 ° C. or higher, more preferably 80 ° C. or higher, The line pressure is preferably 2
00 kg / cm or more, more preferably 300 kg / c
m or more, the speed ranges from 20 m / min to 700 m / min.

Further, the magnetic recording medium of the present invention may be provided with various known backcoat layers on the back side (the side on which the magnetic layer is not formed) of the nonmagnetic support.

By the above procedure, a non-magnetic lower layer, a magnetic layer and, if necessary, a back coat layer are provided and then wound up to obtain a coated material.

Thereafter, the magnetic recording medium is cut into a desired form, for example. As the cutting method, use the conventional method of cutting by passing the coated raw material between the upper blade and the lower blade while engaging and rotating the upper and lower blades as shown in Fig. 2 together. Can be.

Hereinafter, the present invention will be described in more detail by showing specific examples of the present invention. (Example 1) As shown below, a magnetic paint for forming a magnetic layer and a paint for forming a non-magnetic layer were prepared. The mixing ratios of the following materials were all parts by weight. Magnetic paint for forming magnetic layer Ferromagnetic powder Iron-based ferromagnetic metal magnetic powder (HB167, manufactured by Dowa Mining Co., Ltd.) 79.5 Fe containing 12.6 at% of Al Hc = 2375 Oe, BET value = 51 m ² / G, σs = 143 emu / g, crystallite size = 165 °, major axis length = 0.10 μm, PH = 9.4 binder resin vinyl chloride copolymer (manufactured by Zeon Corporation, MR-110) 6 2.0 PVC / 2HEMA / AGE / OSO ₃ K at molecular terminal = 84.5 / 4.5 / 7.4 / 0.36 Degree of polymerization 300 Polyurethane resin (TS9121 manufactured by Toyobo Co., Ltd.) 3.0 Aromatic system Polyester polyol Urethane SO ₃ Na = 0.07 mmol / g content Average molecular weight = 30,000 Polyurethane resin (TS7400, manufactured by Toyobo Co., Ltd.) 1.0 Aliphatic polyester resin Riolurethane SO ₃ Na, F group-containing average molecular weight = 23,000 Dispersant (Toho Chemical Co., Ltd., Phosphanol RE610) 2.3 Abrasive α-Al ₂ O ₃ (Sumitomo Chemical Co., Ltd., HIT70) 4.0 Average particle size 0.14 μm Lubricant (NAA180, manufactured by NOF CORPORATION) 0.9 (NIKKOLBS, manufactured by Nikko Chemicals Co., Ltd.) 0.8 Solvent Methyl ethyl ketone 141.7 Toluene 141.7 Cyclohexanone 283. 3 A part or all of this composition was sufficiently kneaded with a kneader and mixed, and then dispersed by a sand grind mill.
2.5 parts by weight of a hardener (Coronate L, manufactured by Nippon Polyurethane Industry Co., Ltd.) was added and mixed with the magnetic paint thus obtained to prepare a paint for forming a magnetic layer. Next, a coating composition for forming a nonmagnetic layer was prepared using the following coating composition for forming a nonmagnetic layer. Paint composition for forming non-magnetic layer Non-magnetic inorganic powder α-iron oxide (DPN250BW, manufactured by Toda Kogyo Co., Ltd.) 65.2 Average major axis length = 0.15 μm, axial ratio = 6 BET value = 50 m ² / g, specific gravity = 5.1 carbon black (R760, manufactured by Columbian Co., Ltd.) 16.3 Primary particle size = 30 nm, BET value = 70 m ² / g DBP lubrication amount = 48 ml / g, specific gravity = 1.85 Binder resin Vinyl chloride resin (TB0246, manufactured by Toyobo Co., Ltd.) 7.4 Vinyl chloride-epoxy-containing monomer copolymer Average polymerization degree = 310, epoxy content = 3 wt% Potassium persulfate S content = 0.6 wt% 2 -Use of isocyanate ethyl methacrylate (MOI) Polyurethane resin (Toyobo Co., Ltd., TB0242) 7.4 Hydroxy-containing acrylic compound-Phosphonic acid-containing phosphation Product-Hydroxy-containing polyester polyol Average molecular weight = 23,000, P content = 0.2 wt% Acrylic content = 8 mol / 1 mol Dispersant (Toho Chemical Co., Ltd., Phosphanol RE610) 2.5 Lubrication (Nippon Oil & Fats Co., Ltd., NAA180) 0.5 (Nikko Chemicals Co., Ltd., NIKKOLBS) 0.6 Solvent Methyl ethyl ketone 81.2 Toluene 81.2 Cyclohexanone 40.6 Part or all of this composition is kneaded After thoroughly kneading and mixing at, after performing dispersion with a sand grind mill,
A paint for forming a non-magnetic layer was prepared. Coating composition for forming back coat layer Carbon black 80.0 (Sc Ultra: manufactured by Colombian Carbon Co., Ltd.) Average particle size 21 nm, BET 220 m ² / g Soluble ion: Na 17 ppm, K 9 ppm Carbon black 1.0 (MT-CI: (Colombian Carbon Co., Ltd.) Average particle size 350 nm, BET 8 m ² / g Soluble ion Na46 ppm, K27 ppm Nonmagnetic inorganic powder α-iron oxide (100ED, manufactured by Toda Kogyo Co., Ltd.) 1.0 Average particle size 0.1 μm Binder resin Vinyl chloride copolymer (MPR-TA, manufactured by Nissin Chemical Industry Co., Ltd.) 40.0 Vinyl chloride-vinyl acetate-vinyl alcohol copolymer, average degree of polymerization 420 Vinyl chloride copolymer 25.0 (Nissin Chemical Industry MPR-ANO (L) manufactured by Co., Ltd.) PVC-vinyl acetate-vinyl alcohol copolymer Nitrogen atom 390 pp m, average degree of polymerization 340 Polyester polyurethane (TS9555: manufactured by Toyobo Co., Ltd.) 35.0 SO ₃ Na-containing, number average molecular weight 40000 Solvent Methyl ethyl ketone 700.0 Toluene 400.0 Cyclohexanone 300.0 Curing agent (Coronate L: Japan 2.5 Polyurethane Co., Ltd. 2.5 The above-mentioned materials other than the curing agent were mixed and dispersed to prepare a coating for the back coat layer. Various magnetic recording samples were prepared using the above-mentioned paint in the following manner. First, the thickness 5.2
The coating material for forming a nonmagnetic layer was applied on a nonmagnetic support made of polyethylene naphthalate (PEN) having a thickness of μm using a reverse coater and dried. Then the temperature 100
The film was calendered at a temperature of 300 ° C. and a linear pressure of 300 kg / cm, and irradiated with an electron beam at 3 Mrad. The calendered thickness of the nonmagnetic layer was 1.4 μm. The above magnetic paint was applied on such a non-magnetic layer with a nozzle coater, subjected to an orientation treatment with a magnetic field of 7000 gauss, and dried. Thereafter, the temperature is 100 ° C and the linear pressure is 300kg / c.
calendering under the condition of m and winding into a roll,
A heat curing treatment was performed at 60 ° C. for 24 hours. The thickness of the obtained magnetic layer was 0.15 μm. Further, a curing agent is added to the coating for the back coat layer, applied on the non-magnetic support surface opposite to the magnetic layer forming surface with a gravure cylinder, and dried,
A backcoat layer having a thickness of 0.5 μm was obtained. The coated material thus obtained was cut into a 1/4 inch width to prepare a tape sample. Example 2 A sample was prepared in the same manner as in Example 1 except that the volume ratio between the pigment and the carbon black was changed to 52:48. Example 3 A sample was prepared in the same manner as in Example 1 except that the volume ratio of the pigment to the carbon black was changed to 46:54. Example 4 A sample was prepared in the same manner as in Example 1 except that the volume ratio of the pigment to the carbon black was changed to 27:73. Comparative Example 1 A sample was prepared in the same manner as in Example 1 except that the volume ratio between the pigment and the carbon black was changed to 100: 0. Comparative Example 2 A sample was prepared in the same manner as in Example 1 except that the volume ratio of the pigment to the carbon black was changed to 77:23. Comparative Example 3 A sample was prepared in the same manner as in Example 1 except that the volume ratio of the pigment to the carbon black was changed to 19:81. Comparative Example 4 In Comparative Example 1, the binder of the lower non-magnetic layer was changed as follows.

Vinyl chloride resin (manufactured by ZEON Corporation, MR110) 6.7 PVC / 2HEMA / AGE / OSO3K at molecular terminals = 84.5 / 4.5 / 7.4 / 0.36 Average degree of polymerization = 300 polyurethane Resin (Toyobo Co., Ltd., TS7400) 6.4 Aliphatic polyester polyol urethane SO3Na, F group-containing coronate L (Nippon Polyurethane Co., Ltd.) 3.3 After calendering this lower nonmagnetic layer with a reverse coater Is applied to a thickness of 1.4 μm, and while the lower non-magnetic layer is in a wet state, the same magnetic layer as in Example 1 is calendered to a thickness of 0.15 by a nozzle coater.
It was coated to a thickness of μm, and orientation, drying and calendering were performed. Comparative Example 5 A sample was prepared in the same manner as in Comparative Example 4 except that the volume ratio of the pigment to the carbon black was changed to 52:48. (Comparative Example 6) In Example 2 described above, the pigment was changed to granular α-
A sample was prepared in the same manner except that iron oxide (Nanotight 30: particle size: 30 nm) was changed. (Comparative Example 7) A sample was prepared in the same manner as in Comparative Example 6 except that the volume ratio between the pigment and the carbon black was changed to 100: 0. Comparative Example 8 A sample was prepared in the same manner as in Example 2 except that the pigment was changed to α-iron oxide having a long axis of 0.3 μm. Comparative Example 9 A sample was prepared in the same manner as in Comparative Example 1 except that the pigment was changed to α-iron oxide having a long axis of 0.3 μm. (Comparative Example 10) In the above-mentioned Example 2, the lower non-magnetic layer carbon black R760 was replaced by Mitsubishi Chemical 3170B.
(Primary particle size 25 nm, BET specific surface area 180 m ² / g,
A sample was prepared in the same manner except that the DBP oil absorption was changed to 114 ml / 100 g). (Comparative Example 11) Same as in Example 2 except that carbon black R760 of the lower nonmagnetic layer was changed to Mitsubishi Chemical CF9B (primary particle size: 40 nm, BET specific surface area: 60 m ² / g, DBP oil absorption: 64 ml / 100 g). A sample was prepared. Comparative Example 12 A sample was produced in the same manner as in Example 2 except that the amount of Al in the ferromagnetic metal powder was changed to 4.0 at%. Comparative Example 13 A sample was prepared in the same manner as in Example 2 except that the amount of Al in the ferromagnetic metal powder was changed to 9.5 at%. Comparative Example 14 A sample was produced in the same manner as in Example 2 except that the amount of Al in the ferromagnetic metal powder was changed to 20.0 at%. 20.96MHz in the electromagnetic conversion characteristics and? Output Matsushita DVC camera NV-DJ1
(１ ／ Tb) and 10.48 MHz (１ ／ Tb)
Was recorded, and the output when this signal was reproduced was measured. The tape position was set in the MP mode. 0 dB at that time is an output at each frequency of the TDKDVC-Ref tape. Running durability 0 ℃, 20 ℃ 60%, manufactured by Matsushita in 40 ° C. 80% of 3 environmental D
Four VC cameras NV-DJ1 were installed, and a sample tape T-60 equivalent was reproduced for 100 passes, and the change in RF output was examined. Those corresponding to the following items were regarded as NG, and the numbers were shown in the numerator. Sudden output drop (clogging) of 3 dB or more and 1 minute or more Output deterioration of 3 dB or more One sample 3 of a sudden output drop of 2 dB or more and within 1 minute
A magnetic tape with a magnetic resistance of 6.35 mm width is placed on a 6.35 mm-wide electrode so that the magnetic surface is placed on the electrode, and a DC voltage of 10 V is applied under a tension of 5 N / mm ^2. The applied electric resistance was measured. In Table 1, when this value is 1 × 10 ⁹ , for example, it is shown as E + 9. An initial dropout Matsushita DVC camera NV-DJ1 records a color bar signal on each sample tape, and at the time of reproduction, a dropout counter VH03AZ (manufactured by Shivasoku) has a width of 3 μs.
Dropouts having a depth of 10 dB were measured for 30 minutes, and the number of dropouts per minute was determined. After storage, re-recording and re-dropout The sample for which the above-mentioned initial drop-out was measured was stored in an in-cassette state in an environment of 60 ° C and 90% for 2 days, and then left at room temperature for 1 day.
A color bar signal was recorded by a Matsushita DVC camera NV-DJ1, and measured for 30 minutes at the time of reproduction, to determine the number of dropouts per minute. Observation of cut surface shape A magnetic recording medium is sampled every 10,000 m of cut length, fixed on silicon rubber with the magnetic surface facing up, and observed with an optical microscope from a direction perpendicular to the magnetic surface. At 400 × magnification, the focus is first adjusted to the end face of the magnetic layer. Next, the focus is gradually shifted farther away, and it is checked whether or not the lower non-magnetic layer protrudes from the end face of the magnetic layer. Observation at a cut length of 180,000 m indicates that the magnetic layers at both ends in the cut width direction did not protrude, the case where both the edges protruded, the case where only one edge protruded, and the case where both edges protruded.
In addition, FIG. 3 shows the cutting length of all the samples until the cut surface shape becomes “X” as “the maximum usage of the cutting blade” together with all the evaluation results described above.

[0046]

The effects of the present invention are clear from the above results. That is, according to the present invention, in a wet-on-dry method, an electron beam curable resin (E) is used as a needle-like pigment having a major axis length of 0.15 μm or less and a binder in a lower nonmagnetic layer.
When B) is used, by specifying the volume ratio of carbon black to all inorganic pigments, the life of the cutting blade is ensured, and by preventing the end face of the magnetic layer from protruding in the edge shape after cutting, a high level is achieved. It has electromagnetic conversion characteristics,
In addition, a magnetic recording medium having high reliability such as dropout and durability can be provided.

[Brief description of the drawings]

FIG. 1 is a diagram for explaining the shape of a cut surface of a magnetic recording medium according to the present invention.

FIG. 2 is a diagram illustrating a method for cutting a magnetic recording medium according to the present invention.

FIG. 3 is a diagram for explaining an example and a comparative example of the magnetic recording medium of the present invention.

Claims (7)

[Claims] An underlayer nonmagnetic layer in which a nonmagnetic inorganic powder and carbon black are dispersed in a binder is provided on a nonmagnetic support, and a magnetic layer in which a ferromagnetic powder is dispersed in a binder is dried on the nonmagnetic support. The outermost end of the magnetic layer is not protruded from the outermost end of the lower non-magnetic layer on the cut surface of the magnetic recording medium obtained by being provided so as to be 0.01 μm to 0.5 μm and being cut in the longitudinal direction. A magnetic recording medium characterized by the above-mentioned. 2. The magnetic recording medium according to claim 1, wherein the thickness of the lower non-magnetic layer is 0.5 μm or more. 3. The non-magnetic inorganic powder of the lower non-magnetic layer is an acicular pigment having a major axis length of 0.20 μm or less, the binder is an electron beam (EB) curable resin, and the volume of carbon black is 3. The magnetic recording medium according to claim 1, wherein the ratio is 40% or more and 80% or less with respect to all inorganic powders (non-magnetic inorganic powder + carbon black). 4. The carbon black contained in the lower nonmagnetic layer has a primary particle size of 25 to 40 nm, a specific surface area by BET method of 50 to 80 m ² / g, and a DBP oil absorption of 40 to 55.
The magnetic recording medium according to any one of claims 1 to 3, wherein the volume is ml / 100g. 5. The magnetic recording medium according to claim 1, wherein the binder of the magnetic layer is made of a thermosetting resin. 6. The magnetic powder contained in the magnetic layer is iron (Fe).
Is a ferromagnetic powder mainly composed of
The magnetic recording medium according to any one of claims 1 to 5, wherein the element contains 10 to 15 at% of the element l. 7. A lower non-magnetic layer obtained by applying a coating for a lower non-magnetic layer in which a non-magnetic inorganic powder and carbon black are dispersed in a binder onto a non-magnetic support, drying, calendering and curing. 7. The method for producing a magnetic recording medium according to claim 1, wherein a coating material for a magnetic layer in which a ferromagnetic powder is dispersed in a binder is applied thereon.