JP3864339B2 - Magnetic tape - Google Patents

Description

  The present invention relates to a coating-type magnetic tape that is suitably used, for example, for computer data backup.

  Magnetic tapes have various uses such as audio tapes, video tapes, computer tapes, etc. Especially in the field of data backup tapes, several tens of GB to 100 GB per volume as the hard disk capacity to be backed up increases. The one with the recording capacity is commercialized. In the future, a large-capacity backup tape exceeding 100 GB has been proposed, and its high recording density is indispensable.

  In order to cope with such a high recording density, it is necessary to use a ferromagnetic powder with fine particles and improved magnetic properties, to further improve the filling and dispersibility of the ferromagnetic powder when manufacturing a magnetic tape. In addition, as the recording becomes shorter, it is necessary to reduce the thickness of the magnetic layer in order to reduce the demagnetization due to the demagnetizing field at the time of recording / reproducing.

In order to improve the magnetic properties of the ferromagnetic powder, it is desirable that the magnetic layer has a larger residual magnetization for higher output. For this reason, it is becoming mainstream to use ferromagnetic iron-based metal powder as magnetic powder instead of conventional oxide magnetic powder and cobalt-containing iron oxide magnetic powder. For example, the coercive force (Hc) is 119 kA / m ( Ferromagnetic iron-based metal powders of 1500 Oe) or more have also been proposed (Patent Documents 1 to 3). Further, an essentially spherical or elliptical rare earth-iron-boron magnetic powder (Patent Document 4), rare earth-iron magnetic powder (Patent Document) having an axial ratio (major axis length / minor axis length) of 1 to 2. A rare earth-iron magnetic powder as in 5) has also been proposed. As the rare earth element used in these magnetic powders, at least one element selected from yttrium, ytterbium, cesium, praseodymium, samarium, lanthanum, europium, neodymium, terbium and the like is used. Among them, neodymium ( Nd), samarium (Sm), terbium (Tb), and yttrium (Y) are preferably used. Furthermore, an essentially spherical (spherical or cubic) iron nitride magnetic powder (Patent Document 6) having a BET specific surface area of 10 m ² / g or more and having a Fe ₁₆ N ₂ phase as a main phase is also known. These essentially spherical or elliptical metallic magnetic powders have a coercive force of 80 kA / m or more.

  In order to improve the magnetic properties of the magnetic layer using the ferromagnetic powder, it is effective to improve the dispersibility of the ferromagnetic powder. As a means of improving the dispersibility of the ferromagnetic powder, a binder having a polar functional group such as a sulfonic acid group, a phosphoric acid group or an alkali metal salt thereof is used, or a low molecular weight dispersant is used in combination with the binder. In addition, there have been proposed means for continuously performing the kneading and dispersing step of the magnetic paint, or adding a lubricant after the dispersion (for example, see Patent Documents 7 to 10).

  In order to improve the magnetic properties, it is necessary to increase the filling of magnetic powder. In general, the magnetic powder filling amount per unit area in the magnetic layer is represented by the Mrt value (the product of the residual magnetic flux density of the magnetic layer and the thickness of the magnetic layer; hereinafter, also simply referred to as “Mrt”). The larger the value, the higher the playback output. However, when Mrt is increased, the PW50 value of the isolated waveform (the PW50 value referred to here is the length of the peak half-value width of the reproduced waveform of the signal recorded on the magnetic recording medium by one pulse of recording current). This is a value expressed in units, hereinafter also simply referred to as “PW50”). An increase in PW50 means a decrease in signal resolution, and therefore recording and reproduction cannot be performed at a high recording density. For this reason, Mrt cannot be increased unnecessarily.

  On the other hand, with the development of a reproduction system using a recent MR head (a magnetic head for reproduction using a magnetoresistive element), the recording wavelength has been further shortened. The latest equipment employs the shortest recording wavelength of 500 nm or less. When the MR head is employed, the MR element is saturated when an appropriate Mrt value for each MR element (magnetoresistance effect element) is exceeded, and the output hardly increases but exists as a background. The increase in noise becomes larger. For this reason, the value of C / N detected at the time of actual signal reproduction becomes small, and good electromagnetic conversion characteristics cannot be obtained (for example, Patent Documents 11 and 12). Therefore, if the residual magnetic flux density is not changed, the magnetic layer thickness is required to be about 1/3 or less of the shortest recording wavelength. In the above magnetic recording / reproducing system, the magnetic layer thickness is set to 100 nm. The following extremely thin layers are required.

  By the way, in addition to the high performance of the magnetic layer as described above, a lower nonmagnetic layer is provided between the magnetic layer and the nonmagnetic support in order to obtain a magnetic recording medium structure suitable for short wavelength recording. Has been proposed to be a thin layer of 600 nm or less (Patent Document 13). In these magnetic recording media, the Mrt value is optimized by reducing the thickness of the upper magnetic layer, self-demagnetization loss and reproduction loss are reduced, and the decrease in runnability and durability due to the magnetic layer is improved. The purpose is to do. It has also been proposed to provide an intermediate layer between the upper magnetic layer and the lower nonmagnetic layer (Patent Document 14).

  Furthermore, in a system using an MR head, the larger the number of ferromagnetic powders contained in the magnetization reversal volume, the lower the noise and the higher C / N, which is advantageous for high recording density. It becomes. For this reason, it is necessary to use an ultrafine particle powder whose major axis length is 100 nm or less.

Japanese Patent Laid-Open No. 5-234064 JP-A-6-25702 JP-A-6-139553 JP 2001-181754 A JP 2002-56518 A JP 2000-277311 A JP-A-2-101624 JP-A-3-216812 JP-A-3-17827 Japanese Patent Laid-Open No. 8-235656 Japanese Patent No. 3046579 JP-A-10-134306 Japanese Patent Laid-Open No. 5-234063 JP-A-5-290353

  However, there is a limit to reducing the thickness of the magnetic layer by providing the lower nonmagnetic layer as described above. Actually, it is difficult to produce a magnetic layer of 100 nm or less from the viewpoint of film thickness control and productivity. Even if the magnetic layer can be produced with a film thickness close to 100 nm, the magnetic layer at the interface between the upper magnetic layer and the lower nonmagnetic layer is formed. It is very difficult to control the interface fluctuation to 50 nm or less. If the fluctuation of the upper and lower layer interface is large, the modulation noise (carrier frequency vicinity noise) becomes high, and good electromagnetic conversion characteristics cannot be obtained.

  Further, as described above, in Patent Document 14, it is proposed to provide an intermediate layer between the upper magnetic layer and the lower nonmagnetic layer. This is because the upper magnetic layer is oriented obliquely or vertically. There is a problem that, when the thickness is 100 to 800 nm, the fluctuation of the interface between the upper magnetic layer and the intermediate layer becomes extremely large. Further, when the intermediate layer is thick, the lubricant cannot be supplied from the lower nonmagnetic layer to the upper magnetic layer, so that the durability of the magnetic tape is lowered.

  The present invention addresses the above-described problems, and an object of the present invention is to provide a magnetic tape having a small PW50 value and a high C / N and excellent in electromagnetic conversion.

  As a result of intensive studies to achieve the above-mentioned object, the present inventors have found that a middle-layer bonded layer having an average dry thickness of 10 nm or more and less than 50 nm substantially containing only a binder (binder resin) immediately below the upper magnetic layer. It has been found that the thickness of the upper magnetic layer can be controlled to 100 nm or less, and the fluctuation of the interface between them can be suppressed to be small, and as a result, good electromagnetic conversion characteristics can be obtained.

  The present invention has been completed based on the above findings. That is, the present invention has a lower nonmagnetic layer containing nonmagnetic powder and a binder and an upper magnetic layer containing ferromagnetic powder and a binder on a tape-like nonmagnetic support. The coating type magnetic tape is configured as follows.

  That is, an intermediate layer having an average dry thickness of 10 nm or more and less than 50 nm, which substantially contains only the binder, is provided immediately below the upper magnetic layer. Further, an upper magnetic layer is provided while the intermediate layer is in a wet state, and the average dry thickness d of the upper magnetic layer is set to 5 nm to 100 nm. Further, the squareness ratio in the longitudinal direction of the upper magnetic layer is set to 0.8 or more. In addition, the middle layer bonding layer is configured to include substantially only the binder, and the term “substantially” as used herein means a low molecular weight organic substance having a molecular weight of 1000 or less that enhances the dispersibility of the paint, and electrical conductivity. This means that the total amount of inorganic substances for adjustment is 3% by weight or less in terms of dry weight.

  As the binder constituting the intermediate layer, an organic solvent-soluble or water-soluble organic polymer can be used. The intermediate layer is provided while the lower nonmagnetic layer is in a wet state or after the lower nonmagnetic layer is applied and dried (claims 4 and 5).

  When PVt is the maximum value of the interface fluctuation measured along the width direction at the interface between the upper magnetic layer and the middle coupling layer, the ratio of this value PVt to the average dry thickness d of the upper magnetic layer (PVt / A configuration in which d) is (PVt / d) <0.5 (Claim 2) is preferable because the interface between the upper magnetic layer and the intermediate coupling layer becomes smooth. In addition, when PVm is the maximum value of the interface variation measured along the longitudinal direction at the interface between the upper magnetic layer and the middle bonded layer, the ratio between this value PVm and the average dry thickness d of the upper magnetic layer ( A configuration in which (PVm / d) is such that (PVm / d) <0.5 (Claim 2) is preferable because the interface between the upper magnetic layer and the middle bonding layer door becomes smooth.

  The center line average roughness (Ra) of the surface of the upper magnetic layer is preferably 0.5 to 5 nm, more preferably 4 nm or less, and most preferably in the range of 1 to 3 nm. The reason why the thickness is 5 nm or less is that if the surface property exceeds this value, the spacing between the magnetic head and the medium increases, and a sufficient reproduction output cannot be obtained. Further, if the surface property is too good and the thickness is less than 0.5 nm, the friction coefficient between the magnetic head and the medium is increased, and the running property and durability are deteriorated.

Further, the residual magnetic flux density (Br) of the upper magnetic layer is preferably 0.1 to 0.5 T, more preferably 0.12 T, still more preferably 0.3 to 0.5 T, and still more preferably 0.32 T or more. Most preferably, it is in the range of 0.35 to 0.4T. The reason why Br is preferably 0.1 T or more is that, if less than this, the leakage magnetic flux from the magnetic recording medium becomes small, and sufficient reproduction output cannot be obtained. In addition, if Br exceeds 0.5T (5000G), it is difficult to manufacture, and if it exceeds 0.5T, the value of Mrt also increases, so that the PW50 value increases.
A material having Br of 0.3T or more is easily obtained when iron-based magnetic powder is used.

  The ferromagnetic powder is preferably a spherical or elliptical iron-based magnetic powder. It is preferable that the shape is essentially spherical or elliptical because the interface between the magnetic layer and the adjacent layer is not disturbed due to the disturbance of the magnetic powder during the magnetic field orientation, and the amount of interface fluctuation is reduced.

  The ferromagnetic powder is preferably a plate-like ferromagnetic powder. A plate-shaped hexagonal magnetic powder is preferable because a high coercive force can be maintained even when the particle size (plate diameter) is reduced, and a magnetic tape having low noise and excellent short wavelength recording characteristics can be obtained.

  According to the present invention, it is possible to control the interfacial fluctuation at the interface between the upper magnetic layer and the intermediate coupling layer immediately below, while reducing the thickness of the upper magnetic layer. With this thin and uniform magnetic layer, a magnetic recording medium having a narrow PW50 value and a high C / N, excellent in electromagnetic conversion characteristics, and suitable as a data backup tape for computers can be obtained. Further, according to the present invention, a magnetic recording medium having a higher recording density and recording capacity can be obtained.

  The present invention is mainly applied to a magnetic tape for digital recording. In the magnetic tape of the present invention, a lower nonmagnetic layer is provided on at least one surface of a nonmagnetic support, and an intermediate coupling layer and an upper magnetic layer are provided in this order on the nonmagnetic support. In particular, when high running reliability is required, the other surface of the nonmagnetic support, that is, the surface on which the surface coating layer composed of the lower nonmagnetic layer, the middle layer coupling layer, and the upper magnetic layer is formed. A back coat layer can be provided on the opposite surface. Hereinafter, the magnetic tape of the present invention will be described more specifically.

<Non-magnetic support>
In the present invention, a tape-like nonmagnetic support is used. The nonmagnetic support to be used preferably has a Young's modulus in the longitudinal direction of 5.9 GPa (600 kg / mm ² ) or more and a Young's modulus in the width direction of 3.9 GPa (400 kg / mm ² ) or more. More preferably, the Young's modulus in the longitudinal direction is 9.8 GPa (1000 kg / mm ² ) or more, and the Young's modulus in the width direction is 7.8 GPa (800 kg / mm ² ) or more. The Young's modulus in the longitudinal direction of the non-magnetic support is preferably 5.9 GPa (600 kg / mm ² ) or more. If the Young's modulus in the longitudinal direction is less than 5.9 GPa (600 kg / mm ² ), the tape running becomes unstable. It is to become. The Young's modulus in the width direction of the non-magnetic support is preferably 3.9 GPa (400 kg / mm ² ) or more. If the Young's modulus in the width direction is less than 3.9 GPa (400 kg / mm ² ), tape edge damage occurs. It is because it becomes easy to do.

  Nonmagnetic supports satisfying such properties include polyethylene terephthalate film, polyethylene naphthalate film, biaxially stretched aromatic polyamide film, aromatic polyimide film and the like. In addition, although the thickness of a nonmagnetic support body changes with uses, a 2-7 micrometers thing is used normally. More preferably, it is 2.5-4.5 micrometers. This is because if the thickness of the nonmagnetic support is less than 2 μm, film formation becomes difficult, and the tape strength decreases, and if it exceeds 7 μm, the total thickness of the tape increases and the recording capacity per tape roll decreases. Further, the surface centerline average roughness (Ra) of the one surface (surface coating layer forming surface) of the nonmagnetic support is preferably 2.5 nm or more and 20 nm or less. If the thickness is 20 nm or less, unevenness on the surface of the lower nonmagnetic layer and the surface of the magnetic layer is reduced even if the lower nonmagnetic layer is thinned. The reason why Ra is preferably 2.5 nm or more is that when it is less than 2.5 nm, it becomes difficult to transport the base film during production, resulting in poor productivity.

<Lower nonmagnetic layer>
The lower nonmagnetic layer contains a nonmagnetic inorganic powder for the purpose of increasing strength. As the inorganic powder, metal oxides, alkaline earth metal salts and the like, particularly metal oxides are preferable. Specifically, for example, iron oxide is preferable, the particle size is more preferably 50 to 400 nm, and the addition amount is preferably 35 to 83% by weight based on the weight of the total inorganic powder. The particle size within this range is preferable because uniform dispersion is difficult when the particle size is less than 50 nm, and unevenness at the interface between the lower nonmagnetic layer and the upper magnetic layer immediately above it increases when the particle size exceeds 400 nm. The addition amount within the above range is preferable because the effect of improving the coating film strength is small if it is less than 35% by weight, and the coating film strength is lowered if it exceeds 83% by weight.

  In addition, it is preferable to add alumina, particularly alumina mainly composed of a corundum phase, to the lower nonmagnetic layer. The addition amount of alumina is more preferably 2 to 30% by weight, further preferably 8 to 20% by weight, and still more preferably 11 to 20% by weight based on the weight of the total nonmagnetic powder. This is because if the amount of alumina added is less than 2% by weight, the paint flow characteristics are insufficient, and if it exceeds 30% by weight, the unevenness at the interface between the lower nonmagnetic layer and the upper magnetic layer immediately above it becomes large. The particle size of the alumina to be added is preferably 100 nm or less, more preferably 10 to 100 nm, more preferably 30 to 90 nm, and even more preferably 50 to 90 nm. Alumina having a thickness of 100 nm or less is preferable because a non-magnetic support having a low smoothness with a surface roughness of the one surface (surface coating layer forming surface) of 20 nm or more is used, and the thickness of the lower non-magnetic layer is 1 This is because when the particle size of alumina exceeds 100 nm when the thickness is as thin as 0.5 μm or less, the smoothing effect on the surface of the lower non-magnetic layer becomes insufficient. In addition, the alumina mainly composed of the corundum phase (α conversion rate: 30% or more) is particularly good when Young of the lower nonmagnetic layer is used in a small amount compared to the case of using θ-, δ-, γ-alumina or the like. This is because the rate increases and the tape strength increases. Further, since the tape strength is also increased, the output variation due to the tape edge undulation (edge weave) is also improved.

  It is not excluded to add 100 to 800 nm α-alumina of less than 3% by weight based on the weight of the total inorganic powder together with the alumina having the above particle diameter.

  Carbon black (CB) can be added to the lower nonmagnetic layer for the purpose of improving conductivity. As the carbon black to be added, acetylene black, furnace black, thermal black or the like can be used. Usually, those having a particle size of 5 to 200 nm are used, but those having a particle size of 10 to 100 nm are preferred. This range is preferable because carbon black has a structure, so that when the particle size is 10 nm or less, it is difficult to disperse the carbon black, and when it is 100 nm or more, the smoothness is poor. The amount of carbon black added varies depending on the particle size of the carbon black, but is preferably 15 to 40% by weight based on the total nonmagnetic powder in the lower nonmagnetic layer. This range is preferable because if the amount is less than 15% by weight, the effect of improving conductivity is poor, and if it exceeds 40% by weight, the effect is saturated. It is more preferable to use 15 to 35% by weight of carbon black having a particle size of 15 to 80 nm, and further preferable to use 20 to 30% by weight of carbon black having a particle size of 20 to 50 nm. By adding carbon black having such a particle size and amount, electric resistance is reduced, and generation of electrostatic noise and tape running unevenness are reduced.

  The thickness of the lower nonmagnetic layer is usually set to 300 to 3000 nm. More preferably, it is 500-2000 nm. This is because the lower non-magnetic layer with a thickness of less than 300 nm is difficult to form by coating and the productivity is poor, and when the thickness exceeds 3000 nm, the total thickness of the tape is increased and the recording capacity per tape roll is reduced. It is. Further, a known undercoat layer may be provided between the nonmagnetic support and the lower nonmagnetic layer in order to improve adhesion. In this case, the thickness of the undercoat layer is preferably 10 to 2000 nm, more preferably 50 to 500 nm.

<Upper magnetic layer>
The upper magnetic layer has a configuration in which ferromagnetic powder is dispersed in a binder, and this layer becomes a magnetic recording layer. As the ferromagnetic powder added to the upper magnetic layer, it is preferable to use a ferromagnetic iron-based metal powder, an essentially spherical or elliptical iron-based magnetic powder, or a plate-shaped ferromagnetic powder. The coercivity of the ferromagnetic iron-based metal powder or the essentially spherical or elliptical iron-based magnetic powder is preferably 135 to 279 kA / m (1700 to 3500 Oe), and the saturation magnetization is 90 to 200 A · m ² / kg. (90 to 200 emu / g) is preferable, and 100 to 180 A · m ² / kg (100 to 180 emu / g) is more preferable. The coercive force of the plate-like ferromagnetic powder is preferably 135 to 279 kA / m (1700 to 3500 Oe), and the saturation magnetization is preferably 30 to 75 A · m ² / kg (30 to 75 emu / g), and 40 to 70 A.・ M ² / kg (40 to 70 emu / g) is more preferable, and 40 to 60 A · m ² / kg (40 to 60 emu / g) is most preferable. Note that the magnetic properties of these ferromagnetic powders and the magnetic properties of the upper magnetic layer (described later) are values measured with an external magnetic field of 1.27 MA / m (16 kOe) using a sample vibration type magnetometer. is there.

The particle diameter of the ferromagnetic powder is usually preferably 10 to 100 nm, more preferably 10 to 60 nm, and most preferably 10 to 30 nm. If the average particle diameter of the ferromagnetic powder is larger than 100 nm, particle noise based on the particle size increases, and it becomes difficult to improve the C / N characteristics. In addition, when the average particle size is less than 10 nm, the coercive force may decrease, and at the same time, the cohesive force of the magnetic powder increases, so that dispersion in the paint becomes difficult. The above average particle diameter is obtained by measuring the maximum size of each particle from a photograph taken with a transmission electron microscope (TEM) (magnification is 100,000 times) and calculating the average value per 100 particles. It is. Further, BET specific surface area of the ferromagnetic powder is preferably 35~85m ² / g, more preferably ^{40~80m 2 / g, 50~70m 2 /} g being most preferred.

  In the present invention, the average dry thickness d of the upper magnetic layer is set to 5 nm or more and 100 nm or less. If the average dry thickness d of the upper magnetic layer is less than 5 nm, it is very difficult to form the magnetic layer, the productivity is very poor, and the leakage magnetic field from the magnetic layer is small, so the head output is small. If the thickness exceeds 100 nm, the head output decreases due to thickness loss, the Mrt value also increases, the PW50 value increases, and high recording density magnetic recording cannot be performed. The average dry thickness d of the upper magnetic layer is more preferably 10 nm to 90 nm, and still more preferably 20 nm to 60 nm.

  The coercive force of the upper magnetic layer is preferably 135 to 279 kA / m (1700 to 3500 Oe) in the head running direction, and the residual magnetic flux density is preferably 0.10 T (1000 G) or more in the tape longitudinal direction, more preferably 0.30 T (3000 G) or more. preferable. In addition, the thing exceeding 0.5T (5000G) is difficult to manufacture, and the practical upper limit is 0.5T (5000G). This is because when the coercive force of the upper magnetic layer is less than 135 kA / m, the output decreases due to the demagnetizing field, and when it exceeds 279 kA / m, writing by the head becomes difficult. The reason why the residual magnetic flux density is preferably 0.10 T (1000 G) or more is that the output is reduced below 0.10 T. Those having a coercive force of 159 to 239 kA / m (2000 to 3000 Oe) and a residual magnetic flux density of 0.12 to 0.5 T (1200 to 5000 G) are more preferable.

When the magnetic recording medium of the present invention is used in a system using an MR head as a reproducing head, the Mrt value, which is the product of the residual magnetic flux density in the longitudinal direction of the upper magnetic layer and the thickness of the magnetic layer, is 72 nTm (5.7 memu / cm). ² ) It is preferable that the squareness ratio is 0.80 or more. The reason why the Mrt value is 72 nTm or less is that when it is 72 nTm or more, the output detected by most MR heads becomes too large and becomes saturated. In order to increase the reproduction output, 2 nTm or more is preferable. Furthermore, the reason why the squareness ratio is preferably 0.80 or more is that if it is less than 0.80, recording demagnetization occurs due to thermal disturbance. The Mrt value is more preferably in the range of ^{2 to} 36 nTm (0.16 to 2.9 memu / cm ² ), and the squareness ratio is more preferably in the range of 0.90 to 0.97.

  A conventionally known abrasive can be added to the upper magnetic layer. These abrasives include α-alumina, β-alumina, silicon carbide, chromium oxide, cerium oxide, α-iron oxide, corundum, artificial diamond, silicon nitride, silicon carbide, titanium carbide, titanium oxide, silicon dioxide, Boron nitride or the like having a Mohs hardness of 6 or more is mainly used alone or in combination. Among these, alumina is particularly preferable because of its high hardness and excellent head cleaning effect with a small addition amount. Although the particle diameter of the abrasive depends on the thickness of the magnetic layer, the average particle diameter is usually preferably 20 to 400 nm, more preferably 30 to 300 nm. The addition amount is preferably 5 to 20 parts by weight with respect to 100 parts by weight of the ferromagnetic powder. More preferably, it is 8 to 18 parts by weight.

  Furthermore, conventionally known carbon black (CB) can be added to the upper magnetic layer in the magnetic recording medium of the present invention for the purpose of improving conductivity and improving surface lubricity. As this carbon black, acetylene black, furnace black, thermal black or the like can be used. Usually, carbon black having a particle size of 5 to 200 nm is used, but a particle size of 10 to 100 nm is preferable, and a particle size of 10 to 50 nm is more preferable. This range is preferable because when the particle size is less than 10 nm, it is difficult to disperse the carbon black, and when it exceeds 100 nm, it is necessary to add a large amount of carbon black. In either case, the surface becomes rough and the output decreases. This is because it causes. The amount of carbon black added is preferably 0.2 to 5 parts by weight, more preferably 0.5 to 4 parts by weight with respect to 100 parts by weight of the ferromagnetic powder.

<Middle bonding layer>
An intermediate coupling layer is provided immediately below the upper magnetic layer, that is, between the upper magnetic layer and the lower nonmagnetic layer. The thickness of the intermediate layer is preferably 10 nm or more and less than 50 nm, and more preferably 20 nm or more and less than 40 nm. The interface between the middle coupling layer and the upper magnetic layer is the ratio (PVt / d) of the maximum thickness fluctuation (PVt, PVm) and the upper magnetic layer thickness d measured in the width direction and the longitudinal direction (PVm / d), (PVm / It is preferable that any one of d) is less than 0.5. The preferable dry thickness of the intermediate layer is within this range because the interface variation between the lower nonmagnetic layer and the intermediate layer is usually larger than this value when the dry thickness is less than 10 nm. This is because it is difficult to achieve sufficient interface fluctuation with the lower nonmagnetic layer. This is because when the dry thickness is 50 nm or more, not only the leveling effect is saturated, but also the upper magnetic layer cannot be stably placed on the middle nonmagnetic layer. Further, according to Japanese Patent No. 2666610, the interface fluctuation average value Δd and the magnetic layer thickness d (corresponding to “t” of “Mrt” described above) can be achieved by optimizing conditions without providing an intermediate layer. ) And the standard deviation σ of the interface, it is disclosed that Δd / d ≦ 0.5 or σ ≦ 0.2 μm can be satisfied, but the interface fluctuation values PVt and PVm in the text are 3 × Δd, 5 Since it corresponds to × σ, by providing the intermediate layer, according to the above-mentioned title, it is possible to achieve the interface fluctuation corresponding to Δd / d ≦ 0.17 and σ ≦ 0.04 μm. For example, a magnetic layer with much smaller interface fluctuations than the technology disclosed in Japanese Patent No. 2666610 can be obtained. In addition, the interface fluctuation value is represented by the difference between the maximum value and the minimum value, which represents a specific interface disturbance. By realizing a recording medium with the value range in the text, sudden electromagnetic conversion characteristics Deterioration can be eliminated.

<Ferromagnetic powder>
The essentially spherical or elliptical iron-based magnetic powder used in the present invention may contain transition metals such as Mn, Zn, Ni, Cu, and Co as an alloy. Among these, Co and Ni are preferable, and Co is particularly preferable because it can improve saturation magnetization most. As a quantity of said transition metal element, it is preferable to set it as 5-50 atomic% with respect to iron, and it is more preferable to set it as 10-30 atomic%. Further, it may contain at least one rare earth element selected from yttrium, cerium, ytterbium, cesium, praseodymium, samarium, lanthanum, europium, neodymium, terbium and the like (sintering inhibitor). Among these, when cerium, neodymium and samarium, terbium, and yttrium are used, the shape is favorably maintained, and a uniform ceramic layer is formed on the surface of the magnetic powder, which is preferable. The amount of rare earth elements is 0.2 to 25 atomic%, preferably 0.3 to 20 atomic%, more preferably 0.5 to 15 atomic%, based on iron.

  Boron may be included in the iron-based magnetic powder. The amount of boron is 0.5 to 30 atomic%, preferably 1 to 25 atomic%, more preferably 2 to 20 atomic%, based on iron in the entire magnetic powder. Both the atomic% are values measured by fluorescent X-ray analysis (reference patent: Japanese Patent Application Laid-Open No. 2001-181754).

  As the iron nitride magnetic powder, a known one can be used, and the shape can be an irregular shape such as a spherical shape or a cubic shape in addition to a needle shape. Regarding the particle diameter and specific surface area, in order to clear the required characteristics as magnetic powder for magnetic recording, it is necessary to make the production conditions of the limited magnetic powder (reference patent: JP-A-2000-277311). .

The coercive force of the iron-based magnetic powder and the iron nitride magnetic powder is preferably 135 to 279 kA / m, and the saturation magnetization is preferably 90 to 200 A · m ² / kg (90 to 200 emu / g), and 100 to 180 A · m ² / kg (100 to 180 emu / g) is more preferable.

The average particle size of the iron-based magnetic powder and the iron nitride magnetic powder is preferably 10 or more and less than 100 nm, more preferably 10 to 60 nm, and most preferably 10 to 30 nm. This range is preferable when the average particle diameter is less than 10 nm, the coercive force is reduced, the surface energy of the particles is increased, and thus dispersion in the paint becomes difficult, or the average particle diameter is 100 nm or more. This is because the particle noise based on the particle size increases. Further, BET specific surface area of the ferromagnetic powder is preferably 35~85m ² / g, more preferably ^{40~80m 2 / g, 50~70m 2 /} g being most preferred.

  Furthermore, the iron-based magnetic powder and the iron nitride magnetic powder may be used after being surface-treated (corrosion resistance) with Al, Si, P, Y, Zr or an oxide thereof.

The essentially spherical or elliptical iron-based magnetic powder used in the present invention is mainly composed of a rare earth element having a high anti-sintering effect, a high coercive force effect, and a stability (corrosion resistance) improving effect on the outer layer portion of the magnetic powder. An iron-based magnetic powder containing a Fe ₁₆ N ₂ phase as a main phase in the core portion is preferable. The nitrogen content is 1.0 to 20 atomic% with respect to iron. Further, as described above, a part of iron (50 atomic% or less) may be substituted with another transition metal element. However, when a large amount of cobalt is added, nitriding reaction (Fe ₁₆ N ₂ phase in the core part) Since the production reaction) takes a long time, it is usually 10 atomic% or less. In addition, aluminum and / or silicon may be contained instead of the rare earth element. In this case, the aluminum and / or silicon content is preferably 1 to 20 atomic% based on Fe, and 2 to 20 Atomic% is more preferable, and 4 to 18 atomic% is more preferable. Also in this case, it is more preferable to contain a rare earth element together with aluminum and / or silicon.

The coercive force of the plate-like ferromagnetic powder is preferably 135 to 279 kA / m, and the saturation magnetization is preferably 30 to 75 A · m ² / kg (30 to 75 emu / g). Further, the particle size (size in the plate surface direction) is preferably 10 to 60 nm, and more preferably 10 to 30 nm. When the particle size is less than 10 nm, the surface energy of the particles increases, so that dispersion in the paint becomes difficult. When the particle size exceeds 50 nm, particle noise based on the particle size increases. The plate ratio (plate diameter / plate thickness) is preferably 2 to 10, more preferably 2 to 5, and still more preferably 2 to 4. A BET specific surface area of 35 to 85 m ² / g is preferably used. Examples of such plate-like ferromagnetic powder include hexagonal-ferrite magnetic powder such as Ba-ferrite magnetic powder.

  Further, as the above-described ferromagnetic iron-based metal powder, conventionally known ones such as iron-based acicular magnetic powder can be used.

  Note that the magnetic properties of these ferromagnetic powders all mean values measured with an external magnetic field of 1.27 MA / m (16 kOe) using a sample vibration type magnetometer.

  In addition, the above average particle diameter was measured by measuring the maximum diameter of each particle (major axis diameter for needle-like powder and plate diameter for plate-like powder) from a photograph taken with a transmission electron microscope (TEM). The average value is obtained.

<lubricant>
The lower nonmagnetic layer contains 0.5 to 5.0% by weight of higher fatty acid with respect to the total powder contained in the upper magnetic layer and lower nonmagnetic layer, and 0.2 to 3.0% by weight of higher fatty acid. The inclusion of an ester of fatty acid is preferable because the coefficient of friction with the head is reduced. The addition of higher fatty acids within this range is preferable when the content is less than 0.5% by weight, and the effect of reducing the friction coefficient is small. When the content exceeds 5.0% by weight, the lower nonmagnetic layer may be plasticized and the toughness may be lost. Because there is. The addition of higher fatty acid esters within this range is preferred because if the amount is less than 0.2% by weight, the effect of reducing the friction coefficient is small, and if it exceeds 3.0% by weight, the amount transferred into the magnetic layer is too large. This is because side effects such as sticking to the head may occur. As the fatty acid, it is preferable to use a fatty acid having 10 or more carbon atoms. The fatty acid having 10 or more carbon atoms may be any of isomers such as linear, branched and cis / trans, but is preferably a linear type having excellent lubricating performance. Examples of such fatty acids include lauric acid, myristic acid, stearic acid, palmitic acid, behenic acid, oleic acid, linoleic acid, and the like. Among these, myristic acid, stearic acid, palmitic acid and the like are preferable. The amount of fatty acid added in the magnetic layer is not particularly limited since the fatty acid is transferred between the lower non-magnetic layer and the magnetic layer, and the amount of fatty acid added to the magnetic layer and the lower non-magnetic layer is the same as above. The amount of If a fatty acid is added to the lower nonmagnetic layer, it is not always necessary to add a fatty acid to the magnetic layer.

  When the upper magnetic layer contains 0.5 to 3.0% by weight of fatty acid amide and 0.2 to 3.0% by weight of higher fatty acid ester relative to the magnetic powder, the friction during tape running This is preferable because the coefficient is small. Fatty acid amides in this range are preferred if the amount is less than 0.5% by weight, the direct contact at the head / magnetic layer interface is likely to occur, and the effect of preventing seizure is small. If the amount exceeds 3.0% by weight, bleed out and dropout occurs. This is because there is a risk that defects such as these may occur. As the fatty acid amide, a fatty acid amide having 10 or more carbon atoms such as palmitic acid and stearic acid can be used. Also, the addition of higher fatty acid esters within the above range is preferable because if the amount is less than 0.2% by weight, the effect of reducing friction is small, and if it exceeds 3.0% by weight, side effects such as sticking to the head may occur. It is. The mutual movement of the lubricant in the magnetic layer and the lubricant in the lower nonmagnetic layer is not excluded.

<Binder etc.>
Examples of the binder used for the lower nonmagnetic layer and the upper magnetic layer include vinyl chloride resin, vinyl chloride-vinyl acetate copolymer, vinyl chloride-vinyl alcohol copolymer, vinyl chloride-vinyl acetate-vinyl alcohol copolymer. A combination of at least one selected from a polymer, vinyl chloride-vinyl acetate-maleic anhydride copolymer, vinyl chloride-hydroxyl group-containing alkyl acrylate copolymer, nitrocellulose, and the like, and a polyurethane resin can be given. . Among them, it is preferable to transform the vinyl chloride-hydroxyl group-containing alkyl acrylate copolymer and the polyurethane resin. Examples of the polyurethane resin include polyester polyurethane, polyether polyurethane, polyether polyester polyurethane, polycarbonate polyurethane, and polyester polycarbonate polyurethane. These binders are 7 to 50 parts by weight, preferably 10 to 35 parts per 100 parts by weight of the ferromagnetic powder in the upper magnetic layer and 100 parts by weight of the total nonmagnetic powder in the lower nonmagnetic layer, respectively. It can be used in the range of parts by weight. In particular, it is most preferable to use a composite of 5 to 30 parts by weight of a vinyl chloride resin and 2 to 20 parts by weight of a polyurethane resin as a binder.

-COOH as a functional _{group, -SO 3 M, -OSO 3 M} , -P = O (OM) 3, -O-P = O (OM) 2 [In these formulas, M represents a hydrogen atom, an alkali metal base or Represents an amine salt], —OH, —NR ₁ R ₂ , —N ⁺ R ₃ R ₄ R ₅ [In these formulas, R ₁ , R ₂ , R ₃ , R ₄ , R ₅ are hydrogen or hydrocarbon groups; A binder composed of a polymer having an epoxy group is used. The reason why such a binder is used is that the dispersibility of magnetic powder and the like is improved as described above. When two or more kinds of resins are used in combination, the polarities of the functional groups are preferably matched, and a combination of —SO ₃ M groups is particularly preferable.

  In addition to these binders, it is desirable to use a thermosetting crosslinking agent that is bonded to a functional group contained in the binder and crosslinked. Examples of the crosslinking agent include tolylene diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate, reaction products of these isocyanates with a plurality of hydroxyl groups such as trimethylolpropane, and condensation products of the above isocyanates. Various polyisocyanates are preferred. These crosslinking agents are usually used in a proportion of 10 to 50 parts by weight with respect to 100 parts by weight of the binder. More preferably, it is 15 to 35 parts by weight.

  Either an organic solvent-soluble resin or a water-soluble resin is used as the binder for forming the middle layer bonding layer. The component of the middle layer bonding layer is substantially composed only of a polymer resin. However, it is possible to add a total of 3% by weight or less of low-molecular organic substances that improve the dispersibility of the paint and inorganic substances for adjusting electric conductivity.

  As the organic solvent-soluble resin, a combination of a vinyl chloride resin and a polyurethane resin can be used in the same manner as that used for the lower nonmagnetic layer and the upper magnetic layer described above. Specifically, as the vinyl chloride resin, vinyl chloride resin, vinyl chloride-vinyl acetate copolymer, vinyl chloride-vinyl alcohol copolymer, vinyl chloride-vinyl acetate-vinyl alcohol copolymer, vinyl chloride-vinyl acetate- Mention may be made of maleic anhydride copolymers, vinyl chloride-hydroxyl group-containing alkyl acrylate copolymers, nitrocellulose and the like. Examples of the polyurethane resin include polyester polyurethane, polyether polyurethane, polyether polyester polyurethane, polycarbonate polyurethane, and polyester polycarbonate polyurethane. In particular, these binders are preferably used in combination of 50 to 95 parts by weight of a vinyl chloride resin and 5 to 50 parts by weight of a polyurethane resin with respect to 100 parts by weight of the total binder.

In the middle-layer bonding layer, -COOH, -SO ₃ M, -OSO ₃ M, -P = O (OM) ₃ , -OP = O (OM) ₂ [in these formulas, M Represents a hydrogen atom, an alkali metal base or an amine salt], —OH, —NR ₁ R ₂ , —N ⁺ R ₃ R ₄ R ₅ [in these formulas, R ₁ , R ₂ , R ₃ , R ₄ , R ₅ represents hydrogen or a hydrocarbon group], and a binder composed of a polymer having an epoxy group is used. The reason why such a binder is used is that the dispersibility between the resins is improved. When two or more kinds of resins are used in combination, the polarities of the functional groups are preferably matched, and a combination of —SO ₃ M groups is particularly preferable.

  In preparing a coating for the middle layer using the above organic solvent-soluble resin, as a solvent, ketones such as acetone, methyl ethyl ketone, methyl isobutyl ketone, diisobutyl ketone, cyclohexanone, isophorone, tetrahydrofuran, methanol, ethanol, propanol, Alcohols such as butanol, isobutyl alcohol, isopropyl alcohol, methylcyclohexanol, esters such as methyl acetate, butyl acetate, isobutyl acetate, isopropyl acetate, ethyl lactate, glycol acetate, glycols such as glycol dimethyl ether, glycol monoethyl ether, dioxane Aromatic hydrocarbons such as ether, benzene, toluene, xylene, cresol, chlorobenzene, methylene chloride, ethylene chlora , Chlorinated hydrocarbons such as carbon tetrachloride, chloroform, ethylene chlorohydrin, dichlorobenzene, and organic solvents such as N, N-dimethylformamide, hexane, etc., can be used alone or mixed in any ratio. .

As described above, a water-soluble resin can also be used as the binder for forming the intermediate layer. Examples of such water-soluble resins include polyvinyl alcohol, polyvinyl pyrrolidone, carboxymethyl cellulose, starch, polyacrylic acid soda, and polymethacrylic acid soda. Water is used as the solvent. Further, as other solvents, ethanol, methyl ethyl ketone, or the like can be used alone or mixed with water. In this case, a water-soluble resin such as an acrylic acid polymer, polyvinyl alcohol, polyvinyl pyrrolidone, fatty acid acrylate, glycerin, and polyethylene glycol is used. Furthermore, an electrical conductivity regulator such as LiNO ₃ or LiCl is used in combination as necessary.

  It is desirable to use a thermosetting crosslinking agent in combination with these organic solvent-soluble and water-soluble binders. Examples of the crosslinking agent include tolylene diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate, reaction products of these isocyanates with a plurality of hydroxyl groups such as trimethylolpropane, and condensation products of the above isocyanates. Various polyisocyanates are preferred. These crosslinking agents are usually used in a proportion of 10 to 50 parts by weight with respect to 100 parts by weight of the binder. More preferably, it is 15 to 35 parts by weight.

<Back coat layer>
A back coat layer can be provided on the other surface of the non-magnetic support (the surface opposite to the surface coating layer forming surface) for the purpose of improving running performance and preventing charging. The thickness of the back coat layer is preferably 200 to 800 nm. The reason why this range is good is that if it is less than 200 nm, the effect of improving running performance is insufficient, and if it exceeds 800 nm, the total thickness of the tape becomes thick and the recording capacity per roll becomes small. The backcoat layer can be formed by a conventionally known coating method such as gravure coating, roll coating, blade coating, or die coating.

  Carbon black (CB) is usually added to the backcoat layer for the purpose of improving running performance and preventing static charge. As carbon black, acetylene black, furnace black, thermal black, etc. can be used. Usually, small particle size carbon black and large particle size carbon black are used. As the small particle size carbon, those having a particle size of 5 to 200 nm are used, and those having a particle size of 10 to 100 nm are more preferable. This range is more preferable because when the particle size is less than 10 nm, it is difficult to disperse the carbon black, and when the particle size exceeds 100 nm, it is necessary to add a large amount of carbon black, and in either case, the surface becomes rough. This is because it causes setback (embossing) to the magnetic layer. When a large particle size carbon having a particle size of 200 to 400 nm is used in combination at a ratio of 5 to 15% by weight of the small particle size carbon black, the surface is not roughened, and the running performance improvement effect is increased. The total addition amount of the small particle size carbon black and the large particle size carbon black is preferably 60 to 98% by weight, more preferably 70 to 95% by weight, based on the weight of the inorganic powder in the backcoat layer. The center line average roughness Ra of the back coat layer surface is preferably 3 to 12 nm, and more preferably 4 to 8 nm.

  Further, it is preferable to add iron oxide to the back coat layer for the purpose of improving the strength. The particle size of the iron oxide to be added is preferably 100 to 600 nm, and more preferably 200 to 500 nm. The amount of iron oxide added is preferably 2 to 40% by weight, more preferably 5 to 30% by weight, based on the weight of the inorganic powder. Further, when 0.5 to 5% by weight of alumina having a particle size of 100 to 600 nm is added based on the weight of the inorganic powder, the strength of the back coat layer is further improved.

<Production method>
By forming an intermediate layer containing substantially only a binder immediately below the upper magnetic layer, the average dry thickness of the upper magnetic layer can be reduced to an arbitrary thickness of 5 to 100 nm and the interface fluctuation can be reduced and the productivity can be improved. Can do. However, in order to obtain this multi-layer structure, it is necessary to use a so-called wet-on-wet simultaneous multi-layer coating method in which the upper magnetic layer is applied in a superimposed manner while the middle layer coupling layer is in a wet state. At that time, the middle layer coupling layer and the upper magnetic layer are coated almost simultaneously by a single die coating head containing two coating liquid passage slits. In order to improve the coating stability, it is preferable that the surface tension of the solvent used for the intermediate bonding layer is higher than the surface tension of the solvent used for the upper magnetic layer. Examples of the solvent having a high surface tension include cyclohexanone and dioxane.

Furthermore, the ratio of the upper and lower layer viscosities at the shear rate of 10 ⁴ s ⁻¹ between the coating solution for the upper magnetic layer and the coating solution for the middle layer at the time of coating is such that the intermediate layer viscosity / upper layer magnetic layer viscosity is 3. It is preferable to adjust so that it may exceed. More preferably, this ratio is in the range of 5-10. By simultaneously applying the coating solution thus adjusted, the maximum value (PVt and / or PVm) of the interfacial fluctuation at the interface measured in the width direction and / or the longitudinal direction between the middle layer coupling layer and the upper magnetic layer is obtained. , Can be controlled to the specific range described above.

  A magnetic recording medium having a three-layer structure in which a lower nonmagnetic layer, an intermediate coupling layer, and an upper magnetic layer are provided on one surface of a nonmagnetic support can be realized by the following method. First, a lower layer is applied by a gravure coating, a roll coating, a blade coating, a die coating apparatus or the like generally used as a magnetic paint coating method, and then dried and calendered. Thereafter, an intermediate layer and an upper magnetic layer are applied by a simultaneous multilayer coating apparatus.

  Moreover, the surface coating layer of the above three-layer structure can be created by simultaneously coating the three layers. This simultaneous coating method includes a so-called sequential coating method in which a middle non-magnetic layer and an upper magnetic layer are simultaneously coated as soon as possible in a wet state after the lower non-magnetic layer is coated, and a coating liquid passing slit. There are two methods in which three layers are applied simultaneously by one die application head incorporating three of them.

  After applying and drying the surface coating layer, calendering between metal rolls can increase the effects of suppressing thickness fluctuations at the interface between the middle layer bonded layer and the upper magnetic layer, and suppressing variations in the upper magnetic layer thickness. it can. Also, a heat-resistant plastic roll such as epoxy, polyimide, polyamide, polyimide amide or the like can be used as a calender roll. The treatment temperature during calendering is preferably 70 ° C or higher, more preferably 80 ° C or higher. The linear pressure during calendering is preferably 147 kN / m (150 kg / cm), more preferably 196 kN / m (200 kg / cm) or more, and the speed is in the range of 20 m / min to 700 m / min. By setting the linear pressure to 294 kN / m (300 kg / cm) or higher at a temperature of 80 ° C. or higher, the effects described above can be further enhanced.

  In the case where no intermediate layer is provided, it has been conventionally possible to smooth the surface of the upper magnetic layer by calendering, but in that case, the interface between the upper magnetic layer and the lower nonmagnetic layer could not be smoothed. As a result, the surface of the upper magnetic layer was smooth, and the interface between the upper magnetic layer and the lower nonmagnetic layer had waviness, which appeared as a variation in the thickness of the upper magnetic layer. Providing the intermediate bonding layer of the present invention originally has the effect of smoothing the surface of the lower nonmagnetic layer during coating, and the intermediate layer substantially does not contain fillers. Due to the high degree of freedom, the undulation effect of the undulation of the lower non-magnetic layer occurs during calendering and the upper magnetic layer is smoothed, and the interface between the upper magnetic layer and the intermediate coupling layer is also smoothed. The thickness of the upper magnetic layer The fluctuation can be extremely small. Further, by providing an intermediate layer as in the magnetic recording medium of the present invention, the saturation magnetic flux density of the upper magnetic layer can be increased and the surface roughness can be reduced even under the same calendar conditions.

  The back coat layer is applied in any of the steps before and after the application of the surface coating layer and the calendar process. Further, after the coating of the surface coating layer and the backcoat layer and the calendar treatment, an aging treatment at 40 to 80 ° C. may be performed in order to accelerate the curing of the surface coating layer and the backcoat layer.

  The Young's modulus of the surface coating layer formed by laminating the lower nonmagnetic layer, the middle coupling layer, and the upper magnetic layer is 50 to 200% of the average Young's modulus in the longitudinal direction and the width direction of the nonmagnetic support. Is preferred. When the Young's modulus of the surface coating layer is controlled within this range, the durability of the tape is large and the tape-head touch is improved. The ratio is more preferably in the range of 70 to 180%, further preferably in the range of 80 to 160%. This range is preferable because if the amount is less than 50%, the durability of the coating film decreases, and if it exceeds 200%, the tape-head touch becomes poor. In the present invention, a control method based on calendar conditions is used as one of the methods for controlling the Young's modulus of the surface coating layer.

  Furthermore, the Young's modulus of the lower nonmagnetic layer is preferably 60 to 99% of the Young's modulus of the upper magnetic layer. The Young's modulus of the intermediate coupling layer is preferably 10 to 60% of the Young's modulus of the upper magnetic layer. The reason why the Young's modulus of the intermediate coupling layer and the lower nonmagnetic layer is preferably lower than that of the magnetic layer is that the intermediate coupling layer and the lower nonmagnetic layer act as a kind of cushion during the calendar process.

In the magnetic recording medium of the present invention, the coefficient of dynamic friction with respect to the stainless steel of the surface coating layer surface and the opposite back coating layer surface is preferably 0.5 or less, more preferably 0.3 or less. Further, the surface specific resistance (surface resistivity in JIS) of the surface coating layer is 10 ⁴ to 10 ¹¹ ohm / sq (10 ⁴ to 10 ¹¹ Ω in JIS display), and the surface electrical resistance of the backcoat layer is 10 ³ to 10 ^9. Ohm / sq (10 ³ to 10 ⁹ Ω in JIS display) is preferable. A magnetic tape cartridge (cassette tape) in which the magnetic recording medium produced in the above manner is incorporated into a tape cartridge (also referred to as a cassette) has a large recording capacity per roll, high reliability, and a data backup tape for a computer or the like. As especially good.

  Examples of the present invention will be described below, but the present invention is not limited thereto. In the following examples and comparative examples, “parts” means “parts by weight” unless otherwise specified.

Example 1:
《Coating component for upper magnetic layer》
(1)
Ferromagnetic iron-based metal powder 100 parts (Co / Fe: 30 at%, Y / (Fe + Co): 8 at%,
Al / (Fe + Co): 5 wt%, σs: 155 A · m ² / kg,
(Hc: 188 kA / m, pH: 9.5, average long axis length: 100 nm)
Vinyl chloride - hydroxypropyl acrylate copolymer 10 parts (containing -SO ₃ Na group: 0.7 × 10 ^-4 eq / g)
Polyester polyurethane resin 4 parts (containing -SO ₃ Na group: 1.0 x 10 ^-4 equivalent / g)
・ Α-alumina (average particle size: 0.2 μm) 15 parts ・ carbon black 2 parts (average particle size: 75 nm, DBP oil absorption: 72 cc / 100 g)
-Methyl acid phosphate 2 parts-Palmitic acid amide 1.5 parts-N-butyl stearate 1.0 parts-Tetrahydrofuran 65 parts-Methyl ethyl ketone 245 parts-Toluene 85 parts (2)
・ Polyisocyanate 4 parts ・ Cyclohexanone 167 parts

《Coating component for middle layer bonding layer》
(1)
Vinyl chloride copolymer 50 parts (containing -SO ₃ Na group: 0.7 × 10 ^-4 eq / g)
Polyester polyurethane resin 25 parts (Tg: 40 ° C., contained -SO ₃ Na group: 1 × 10 ⁻⁴ equivalent / g)
・ Cyclohexanone 125 parts ・ Methyl ethyl ketone 40 parts ・ Toluene 10 parts (2)
・ Polyisocyanate 25 parts ・ Cyclohexanone 10 parts ・ Methyl ethyl ketone 15 parts ・ Toluene 10 parts

《Coating component for lower non-magnetic layer》
(1)
Iron oxide powder (average particle size: 110 × 20 nm) 68 parts Alumina (alpha conversion: 50%, average particle size: 70 nm) 8 parts Carbon black (average particle diameter: 25 nm) 24 parts Stearic acid 2 parts ^-10 parts of vinyl chloride copolymer (containing -SO ₃ Na group: 0.7 × 10 ^-4 equivalent / g)
Polyester polyurethane resin 4.5 parts (Tg: 40 ° C., contained-SO ₃ Na group: 1 × 10 ⁻⁴ equivalent / g)
・ Cyclohexanone 25 parts ・ Methyl ethyl ketone 40 parts ・ Toluene 10 parts (2)
・ Butyl stearate 1 part ・ Cyclohexanone 70 parts ・ Methyl ethyl ketone 50 parts ・ Toluene 20 parts (3)
・ Polyisocyanate 4.5 parts ・ Cyclohexanone 10 parts ・ Methyl ethyl ketone 15 parts ・ Toluene 10 parts

  The upper magnetic layer coating component (1) is kneaded with a kneader and then dispersed in a sand mill using zirconia beads having a bead diameter of 0.5 mm for 45 minutes. The upper magnetic layer coating component (2 ) Was added, and after stirring, filtration, and ultrasonic dispersion, a coating for the upper magnetic layer was obtained. Separately from this, after stirring (1) in the above-mentioned coating composition for the middle layer bonding layer, (2) was added thereto and stirred and filtered to obtain a coating for the middle layer bonding layer. Furthermore, separately from these, after kneading (1) in the above-described coating material for the nonmagnetic layer with a kneader, adding (2), stirring, and then dispersing with a sand mill for 60 minutes, After adding (3), stirring, and filtering, it was set as the coating material for lower layer nonmagnetic layers. This lower layer nonmagnetic layer coating material is coated on a nonmagnetic support made of a polyethylene terephthalate film (thickness 6 μm, longitudinal Young's modulus MD = 5.9 GPa, widthwise Young's modulus TD = 3.9 GPa, manufactured by Toray Industries, Inc.). In addition, it was applied so that the thickness after drying / calendering was 1100 nm, calendered at 90 ° C., and on this lower nonmagnetic layer, the thickness of the upper magnetic layer after further magnetic field orientation treatment, drying and calendering One surface of the non-magnetic support is formed by simultaneously applying the upper magnetic layer coating material and the middle layer coupling layer so that the thickness of the intermediate coupling layer is 10 nm and the thickness of the intermediate coupling layer is 10 nm. A magnetic sheet obtained by laminating a lower nonmagnetic layer, an intermediate coupling layer, and an upper magnetic layer was obtained. In the magnetic field orientation treatment, an NN counter magnet (0.5 T) is installed in front of the dryer, and the NN counter magnet (0.5 T) is installed in the dryer from the front side 75 cm of the finger-drying position of the coating film. Two sets were installed at 50 cm intervals. The coating speed was 100 m / min.

《Paint component for back coat layer》
Carbon black (average particle diameter: 25 nm) 80 parts Carbon black (average particle diameter: 370 nm) 10 parts Iron oxide (average particle diameter: 400 nm) 10 parts Nitrocellulose 45 parts Polyester polyurethane resin 30 parts (containing- SO ₃ Na group: 1.0 × 10 ⁻⁴ equivalent / g)
・ 260 parts of cyclohexanone ・ 260 parts of toluene ・ 525 parts of methyl ethyl ketone

  After the coating component for the backcoat layer was dispersed with a sand mill for a residence time of 45 minutes, 15 parts of polyisocyanate was added to adjust the coating for the backcoat layer and filtered, and the other side of the magnetic sheet prepared above (magnetic layer) The surface on the side opposite to the side where the etc. are formed was applied so that the thickness after drying and calendering was 500 nm and dried. The magnetic sheet thus obtained was calendered with a seven-stage calendar composed of metal rolls at a temperature of 100 ° C. and a linear pressure of 147 N / mm (150 kg / cm). After aging at 72 ° C. for 72 hours, cut to 1/2 inch width, while running at 200 m / min, the surface of the upper magnetic layer was lapped with tape, blade polished, and surface wiping post-treatment, A tape was prepared. At this time, K10000 was used for the wrapping tape, a super hard blade was used for the blade, and Toraysee (trade name) manufactured by Toray was used for wiping the surface, and processing was performed with a running tension of 0.29 N (30 gf). The magnetic tape obtained as described above was incorporated into a single reel type cartridge to produce a magnetic tape cartridge for a computer (hereinafter also simply referred to as a computer tape).

Examples 2-5:
The conditions of the solvent for the middle layer paint, the viscosity difference between the upper layer paint and the middle layer paint (change of conditions of the ultrasonic dispersing machine used for the upper layer paint), the lower layer coating method, and the calendar temperature were changed to the conditions shown in Table 1. Except this, the tapes for computers of Examples 2 to 4 were produced in the same manner as Example 1.

Example 6:
The magnetic powder used for the upper magnetic layer is iron-based acicular ultrafine particle magnetic powder (Co / Fe: 30 at%, Y / (Fe + Co): 8 at%, Al / (Fe + Co): 5 wt%, σs: 155 A · m ² / kg, Hc: 188 kA / m, pH: 9.5, average major axis length: 60 nm) A computer tape was produced in the same manner as in Example 3.

Example 7:
The magnetic powder used for the upper magnetic layer was synthesized in the following manner as a substantially spherical YN—Fe ultrafine spherical magnetic powder (Y / Fe: 5.3 atomic%, N / Fe: 10.8). Except for atomic%, σs: 135.2 Am ^{2 /} kg (135.2 emu / g), Hc: 227 kA / m (2,850 Oe), average axis length: 20 nm, average axis ratio: 1.2) In the same manner as in Example 3, a computer tape was produced.

<Synthesis of ultrafine spherical magnetic powder>
A rare earth-iron nitride magnetic powder was synthesized. As a synthesis example, an example in which yttrium is used as a rare earth will be described below.

  0.419 mol of iron (II) sulfate heptahydrate and 0.974 mol of iron (III) nitrate nonahydrate were dissolved in 1500 g of water. Next, 3.76 moles of sodium hydroxide were dissolved in 1500 g of water. Next, 3.76 moles of sodium hydroxide were dissolved in 1500 g of water. An aqueous solution of sodium hydroxide was added to the aqueous solution of these two types of iron salts and stirred for 20 minutes to generate magnetite particles.

  The magnetite particles were placed in an autoclave and heated at 200 ° C. for 4 hours. Washed with water after hydrothermal treatment. The magnetite particles were spherical or elliptical with a particle size of 25 nm.

  10 g of the magnetite particles were dispersed in 500 cc of water for 30 minutes using an ultrasonic disperser. To this dispersion, 2.5 g of yttrium nitrate was added and dissolved, and stirred for 30 minutes. Separately, 0.8 g of sodium hydroxide was dissolved in 100 cc of water. This aqueous sodium hydroxide solution was added dropwise to the above dispersion over about 30 minutes, and after completion of the addition, the mixture was further stirred for 1 hour. By this treatment, yttrium hydroxide was deposited on the surface of the magnetite particles. This was washed with water, filtered, and dried at 90 ° C. to obtain a powder in which yttrium hydroxide was deposited on the surface of magnetite particles.

  Thus, the powder in which the hydroxide of yttrium was deposited on the surface of the magnetite particles was heated and reduced at 450 ° C. for 2 hours in a hydrogen stream to obtain an yttrium-iron magnetic powder. Next, the temperature was lowered to 150 ° C. over about 1 hour in a state of flowing hydrogen gas. When the temperature reached 150 ° C., the gas was switched to ammonia gas, and nitriding was performed for 30 hours while maintaining the temperature at 150 ° C. Thereafter, the temperature was lowered from 150 ° C. to 90 ° C. with ammonia gas flowing, and at 90 ° C., the ammonia gas was switched to a mixed gas of oxygen and nitrogen, and a stabilization treatment was performed for 2 hours.

  Next, with the mixed gas flowing, the temperature was lowered from 90 ° C. to 40 ° C., held at 40 ° C. for about 10 hours, and then taken out into the air.

The thus obtained yttrium-iron nitride magnetic powder (the YN-Fe ultrafine spherical magnetic powder) was measured for its yttrium and nitrogen content by fluorescent X-rays, respectively. Atomic% and 10.8 atomic%. Further, from the X-ray diffraction pattern to obtain a profile indicating a Fe ₁₆ N ₂ phase. In the X-ray diffraction pattern of this yttrium-iron nitride magnetic powder, a diffraction peak based on Fe ₁₆ N ₂ and a diffraction peak based on α-Fe are observed, and this yttrium-iron nitride magnetic powder is in the Fe ₁₆ N ₂ phase. It was found to be composed of a mixed phase of γ and α-Fe phase.

Furthermore, when the particle shape was observed with a high-resolution analytical transmission electron microscope, it was found that the particles were almost spherical and the average particle size was 20 nm. The specific surface area determined by the BET method was 53.2 m ² / g.

Further, this magnetic powder had a saturation magnetization measured by applying a magnetic field of 1274 kA / m (16 kOe) at 135.2 Am ² / kg (135.2 emu / g) and a coercive force of 226.9 kA / m (2850 Oe). there were. Further, after storing this magnetic powder at 60 ° C. and 90% RH for one week, the saturation magnetization was measured in the same manner as described above. As a result, it was 118.2 Am ^{2 /} kg (118.2 emu / g). The maintenance ratio of magnetization was 87.4%. In the production of the magnetic coating material, the rare earth-iron nitride magnetic powder was prepared by scaling up the production method of this example 100 times.

Example 8:
The magnetic powder used for the upper magnetic layer is an ultrafine particle-like magnetic powder (Ba-ferrite) (average particle diameter (plate diameter) = 30 nm, BET = 67 m ² / g, Hc = 222 kA / m (2790 Oe), σs = 51 A computer tape was produced in the same manner as in Example 3 except that the change was made to 5 A · m ² / kg (51.5 emu / g).

Comparative Examples 1-8:
Upper layer magnetic layer thickness (adjusted by paint discharge rate), middle layer bonded layer thickness (adjusted by paint discharge rate), viscosity difference between upper layer coating and middle layer coating (change in conditions of ultrasonic disperser used for upper layer coating) ), The computer tape of Comparative Examples 1 to 8 in the same manner as in Example 1 except that the conditions of the orientation machine for forming the upper layer, the coating method of the lower layer, and the calendar temperature were changed to the conditions shown in Table 1. Was made.

<Evaluation of characteristics>
The characteristics were evaluated by the following method. The cross section of the magnetic recording medium was processed with a focused ion beam (FIB) in the longitudinal direction or the width direction, observed with a scanning electron microscope (SEM) at a magnification of 50,000, and a photograph thereof was taken. Thereafter, the interface between the upper magnetic layer and the middle bonding layer was visually judged and blackened, and the upper magnetic layer surface was similarly blackened. At this time, the inorganic filler present on the interface was bordered on the assumption that it did not belong to the upper magnetic layer. Thereafter, image analysis was performed with an image analysis apparatus OPTIMAS ^™ manufactured by OPTIMAS, and an average value of the upper magnetic layer thickness was obtained. The average value of the thickness of the intermediate bonding layer was determined in the same manner.

  The maximum values PVt and PVm of the interfacial fluctuation in the width direction and the longitudinal direction at the interface between the upper magnetic layer and the middle coupling layer were measured as follows. In the field of view having a length of 4 μm, the one having the largest difference in the thickness direction is obtained from the top of the crest and the bottom of the trough formed by the interface between the trimmed upper magnetic layer and the middle coupling layer. This difference was measured at five or more locations by changing the field of view, and the average values in the width direction and the longitudinal direction were PVt and PVm, respectively.

  The center line average roughness Ra of the surface of the upper magnetic layer was measured at 10 points with an optical surface roughness meter (manufactured by Zygo, model Maxim · 3D5700) using a Mirau lens and a visual field of 250 × 250 μm, and the average value was calculated. Adopted. Magnetic properties were measured by cutting a magnetic tape into a predetermined size and using an external magnetic field of 1.27 MA / m (16 kOe) using a VSM manufactured by Toei Kogyo.

  A drum tester was used for measuring the electromagnetic conversion characteristics of the tape. The drum tester was equipped with an electromagnetic induction head (track width 25 μm, gap 0.2 μm) and an MR head (track width 8 μm), and recording was performed with the induction head and reproduction was performed with the MR head. Both heads are installed at different locations with respect to the rotating drum, and tracking can be adjusted by operating both heads in the vertical direction. An appropriate amount of the magnetic tape was withdrawn from the state of being wound in the cartridge and discarded, and further 60 cm was cut out, further processed into a width of 4 mm, and wound around the outer periphery of the rotating drum.

  The PW50 writes a rectangular wave with a wavelength of 10000 nm, that is, 10 μm by a function generator, and reads the output of the MR head into a digital oscilloscope. A value obtained by converting the half width of the output solitary wave into a length was defined as PW50, and a relative value with respect to the value of the DDS4 tape used as a reference was obtained.

  For output and noise, a rectangular wave with a wavelength of 200 nm, that is, 0.2 μm was written by a function generator, and the output of the MR head was read into a spectrum analyzer. The carrier value of 0.2 μm was set as the medium output C. Further, when a rectangular wave of 0.2 μm was written, the integrated value of the value obtained by subtracting the output and system noise from the spectral component corresponding to the recording wavelength of 0.2 μm or more was used as the noise value N. Furthermore, the ratio of the two was taken as C / N, and the relative value to the value of the DDS4 tape used as a reference for both C and C / N was obtained.

  The above evaluation results are shown in Table 2.

  As is clear from the results shown in Table 2, it can be seen that the magnetic tape of the example (product of the present invention) has a narrower PW50 than the magnetic tape of the comparative example. The magnetic tape of Example 2 has substantially the same PW50 value as those of Comparative Examples 4 and 5, but the characteristics of C and C / N are superior to those of Comparative Examples 4 and 5. . In particular, in the magnetic tapes of Examples 3 and 4 in which the interface between the upper magnetic layer and the intermediate coupling layer is smooth, the C / N is high. Further, when a spherical powder or a plate-like powder is used as the ferromagnetic powder added to the upper magnetic layer, an ultrafine particle size powder can be obtained.

Claims (9)

A nonmagnetic support; a lower nonmagnetic layer containing nonmagnetic powder and a binder provided on the nonmagnetic support; and an upper magnetic layer containing ferromagnetic powder and a binder. A coating-type magnetic tape,
Immediately below the upper magnetic layer, an intermediate layer containing substantially only a binder is provided,
The upper magnetic layer is provided while the middle layer coupling layer is in a wet state,
The average dry thickness d of the upper magnetic layer is 5 nm or more and 100 nm or less, the average dry thickness of the intermediate coupling layer is 10 nm or more and less than 50 nm, and the squareness ratio in the longitudinal direction of the upper magnetic layer is 0.8 or more. And magnetic tape.   PVt is the maximum value of the interfacial fluctuation at the interface between the upper magnetic layer and the intermediate coupling layer, measured along the width direction, and the maximum interfacial fluctuation at the interface between the upper magnetic layer and the intermediate coupling layer is measured along the longitudinal direction. When the value is PVm, any one of the ratio (PVt / d) and (PVm / d) of PVt, PVm and the average dry thickness d of the upper magnetic layer is less than 0.5. The magnetic tape according to claim 1.   The magnetic tape according to claim 2, wherein the binder used in the intermediate layer is an organic solvent-soluble or water-soluble organic polymer.   The magnetic tape according to claim 3, wherein the intermediate layer is provided while the lower nonmagnetic layer is in a wet state.   The magnetic tape according to claim 3, wherein the intermediate layer is provided after coating and drying the lower nonmagnetic layer.   The magnetic tape according to claim 4 or 5, wherein the center line average roughness (Ra) of the surface of the upper magnetic layer is 5 nm or less.   The magnetic tape according to claim 6, wherein the residual magnetic flux density (Br) of the upper magnetic layer is 0.3 T (3000 G) or more. A nonmagnetic support; a lower nonmagnetic layer containing nonmagnetic powder and a binder provided on the nonmagnetic support; and an upper magnetic layer containing ferromagnetic powder and a binder. A coating-type magnetic tape,
Immediately below the upper magnetic layer, an intermediate layer containing substantially only a binder is provided,
The upper magnetic layer is provided while the middle layer coupling layer is in a wet state,
The average dry thickness d of the upper magnetic layer is 5 nm or more and 100 nm or less, the average dry thickness of the middle bonded layer is 10 nm or more and less than 50 nm, and the squareness ratio in the longitudinal direction of the upper magnetic layer is 0.8 or more,
A magnetic tape, wherein the ferromagnetic powder is essentially a spherical or elliptical iron-based magnetic powder. A nonmagnetic support; a lower nonmagnetic layer containing nonmagnetic powder and a binder provided on the nonmagnetic support; and an upper magnetic layer containing ferromagnetic powder and a binder. A coating-type magnetic tape,
Immediately below the upper magnetic layer, an intermediate layer containing substantially only a binder is provided,
The upper magnetic layer is provided while the middle layer coupling layer is in a wet state,
The average dry thickness d of the upper magnetic layer is 5 nm or more and 100 nm or less, the average dry thickness of the middle bonded layer is 10 nm or more and less than 50 nm, and the squareness ratio in the longitudinal direction of the upper magnetic layer is 0.8 or more,
A magnetic tape, wherein the ferromagnetic powder is a plate-like ferromagnetic powder.