JP4532341B2 - Magnetic recording medium - Google Patents

Description

  The present invention relates to a magnetic recording medium excellent in high recording density characteristics and excellent in durability.

  Magnetic recording media have various uses such as audio tapes, video tapes, computer tapes, magnetic disks, and magnetic cards. Especially in the field of data backup tapes, as the capacity of hard disks to be backed up increases, 1 Magnetic tapes having a recording capacity of several hundred GB or more per roll have been commercialized. In the future, a large-capacity backup tape exceeding 1 TB has been proposed, and its high recording density is indispensable.

  In producing a magnetic recording medium corresponding to such a high recording density, magnetic powder (hereinafter also referred to as magnetic particles) is made fine (hereinafter also referred to as fine powder) and the amount of the powder in the coating film is increased. Advanced techniques relating to density filling, coating smoothing, and magnetic layer thinning are used.

  With regard to the improvement of the magnetic powder, in order to cope with the short wavelength recording, the magnetic properties are improved along with the formation of fine particles, the needle-like metal magnetic powder having an average particle diameter of 100 nm or less, the plate of 50 nm or less A magnetic recording medium using a hexagonal ferrite magnetic powder having a spherical shape and a spherical or elliptical rare earth-iron nitride magnetic powder of 50 nm or less has been proposed. Also, in order to prevent a decrease in output due to demagnetization during short wavelength recording, a higher coercive force has been achieved year by year.

  On the other hand, with respect to improvements in the manufacturing technology of magnetic recording media, the recording wavelength has become shorter with the recent increase in recording density, so the uppermost magnetic layer (when only one magnetic layer is provided) If the thickness of the magnetic layer is too large, the influence of self-demagnetization loss during recording / reproduction and the thickness loss due to the thickness of the magnetic layer, which has not been a significant problem in the past, is increased, and the output is increased. The problem of decreasing has grown. Therefore, it is necessary to reduce the thickness of the uppermost magnetic layer.

  When the thickness of the top magnetic layer is reduced, the effect of the weak magnetic flux leakage from the magnetic layer and the surface roughness of the nonmagnetic support is greatly reflected on the surface of the magnetic layer, and the surface properties of the magnetic layer are likely to deteriorate. There is a problem. In addition, when only a single magnetic layer is thinned, a method of reducing the solid content concentration of the magnetic coating or reducing the coating amount can be considered, but depending on these methods, defects during coating or magnetic powder There is a problem that the filling property is not improved and the strength of the coating film is weakened.

  For this reason, when the uppermost magnetic layer is thinned by improving the medium manufacturing technique, an undercoat layer (hereinafter also referred to as the lower layer) is provided between the nonmagnetic support and the magnetic layer, and the undercoat layer is wet. A so-called simultaneous multilayer coating method has been proposed in which the uppermost magnetic layer is coated while it is in a state.

  In order to make the leakage flux from the magnetic layer extremely weak, in a system using these magnetic recording media, a high-sensitivity MR (magneto-resistance effect) type head is used as a reproducing head. . Since the MR head does not have an induction coil, the device noise is small, and an excellent C / N can be obtained by reducing the noise of the magnetic recording medium. However, this MR head is more prone to noise (thermal noise) due to the collision between the MR element and minute irregularities on the surface of the magnetic layer, which was not a problem with the magnetic induction type. Need to control.

  In addition, since such a high recording density magnetic recording medium has a surface of the magnetic layer that is controlled to be extremely smooth, in order to have sufficient runnability and durability when used in a recording / reproducing apparatus, Usually, a back coat layer (hereinafter sometimes simply referred to as a back layer) is provided on the surface of the nonmagnetic support opposite to the magnetic recording layer. The backcoat layer is designed to achieve the object by having conductivity and appropriate surface roughness. In particular, the design of the surface roughness is important. In the above-described high recording density magnetic recording medium, if the surface of the backcoat layer is too rough, the tape-shaped magnetic recording medium is wound around a hub. Since it is stored and used, the magnetic layer and the back layer are in intimate contact with each other, and the convex portion on the surface of the backcoat layer causes the magnetic layer surface to be excessively recessed, resulting in a rough surface of the magnetic layer. In some cases, the short wavelength recording characteristics of the magnetic layer may be deteriorated or an error may occur during recording / reproduction. In particular, in the recording / reproducing apparatus using the above-described MR head, the influence of this phenomenon appears greatly, so the design of the surface roughness of the backcoat layer is important.

The surface roughness of the backcoat layer is usually controlled by the particle size and the amount of nonmagnetic powder particles contained in the backcoat layer. For example, Patent Document 1 discloses that fine carbon black and coarse carbon black are included, and discloses that the back coat layer has a two-layer structure, the coarse carbon black is included in the lower layer, and the fine carbon black is included in the upper layer. Patent Document 2, and Patent Document 3 that discloses that the back layer has a two-layer structure, fine carbon black is included in the lower layer, and coarse carbon black is included in the upper layer.

Japanese Patent Laid-Open No. 2-7223 JP-A-11-250445 JP 2001-67652 A

  As mentioned above, the convex portion of the backcoat layer is pressed against the magnetic layer thinned to correspond to the short wavelength recording, and the stable running property and durability are ensured without being excessively dented. It has become important.

  However, in the magnetic recording media disclosed in Patent Document 1 to Patent Document 3, the roughness control of the backcoat layer surface is still insufficient, and the high recording density characteristics and durability of the high recording density magnetic recording medium are not achieved. It was difficult to achieve enough.

  The present invention has been made in order to achieve the above-described problems. For example, the magnetic recording is excellent in high recording density characteristics capable of dealing with a recording capacity of 1 TB or more per tape roll and highly reliable in terms of durability. The purpose is to provide a medium.

  As a result of diligent studies, the inventors of the present invention have a recording layer on one surface on a nonmagnetic support and a plurality of different average particle diameters on the other surface of the nonmagnetic support in order to achieve the above object. A magnetic recording medium having a backcoat layer composed of two or more layers containing various types of carbon black, and found that the above object can be achieved by the following constitution, and has led to the present invention.

  That is, the content of carbon black having the largest average particle size contained in the outermost layer of the backcoat layer is greater than the content of carbon black having the largest average particle size contained in the inner layer adjacent thereto, and The average particle size of carbon black having the maximum average particle size is 150 nm or more.

  Since the backcoat layer is composed of two or more layers, and the particle diameter and the amount of carbon black of each layer are controlled within a preferable range, a magnetic recording medium having both high recording density characteristics and durability can be obtained. It is done.

  A backcoat layer can be provided on the surface of the nonmagnetic support constituting the magnetic recording medium of the present invention opposite to the surface on which the magnetic layer is formed for the purpose of improving running performance. Since the magnetic signal of the magnetic layer may be disturbed if the backcoat layer is magnetic, the backcoat layer is usually nonmagnetic. The thickness of the back coat layer is preferably 0.3 to 1.5 μm, more preferably 0.4 to 1.0 μm. This range is good because if the thickness is less than 0.3 μm, the effect of improving the running performance is insufficient, and if it exceeds 1.5 μm, the total thickness of the tape becomes thick and the recording capacity per roll becomes small. Moreover, 2-12 nm is preferable and, as for centerline average surface roughness Ra, 3-7 nm is more preferable.

  As the carbon black to be included in the back coat layer, acetylene black, furnace black, thermal black and the like can be used.

  The back coat layer of the magnetic recording medium of the present invention is composed of two or more layers. Each layer of the backcoat layer preferably contains a plurality of types of carbon blacks having different average particle sizes and a binder resin. As a combination of a plurality of types of carbon black, a fine particle carbon black having a particle size of less than 100 nm, more preferably 70 nm or less, most preferably 30 nm or less and a coarse particle carbon black having a particle size of 150 nm or more, more preferably 200 nm or more (carbon Black A) is preferably included at least. The fine particle carbon black usually has a particle diameter of 10 nm or more. Coarse particle carbon black usually has a particle size of 500 nm or less. If the particle size of the coarse particle carbon black is too large, C / N may decrease or errors may increase. Therefore, the particle size of the coarse particle carbon black is preferably 400 nm or less, more preferably 360 nm or less, and 300 nm or less. Is most preferred. By including carbon black having a particle diameter of less than 100 nm, preferable conductivity is imparted. By including carbon black having a particle diameter of 150 nm or more, preferable surface roughness is imparted. By stacking two such layers, if necessary, three or more layers, a more preferable surface roughness shape can be formed than a single layer. The content of carbon black having the largest average particle size contained in the outermost layer is preferably larger than the content of carbon black having the largest average particle size contained in the inner layer adjacent thereto. By controlling the particle size and content of carbon black with the largest average particle size contained in the outermost layer, the surface roughness shape of the back layer surface can be mainly controlled, but it is contained in the inner layer adjacent to the outermost layer The surface roughness shape is preferably controlled auxiliary by the content of carbon black having the largest average particle diameter. When the content of carbon black with the largest average particle size contained in the inner layer adjacent to the outermost layer is larger than the content of carbon black with the largest average particle size contained in the outermost layer, the average particle in the outermost layer It becomes difficult to control the surface roughness of the back layer with the carbon black having the largest diameter, and as a result, excellent running stability and durability cannot be obtained. Further, when the inner layer adjacent to the outermost layer does not contain carbon black having a particle diameter of 150 nm or more, the durability may be lowered or the C / N may be lowered.

  In the above-described embodiment, the coarse particle carbon black contained in the outermost layer and the coarse particle carbon black contained in the inner layer adjacent to the outermost layer are the same, but the inner layer adjacent to the outermost layer is shown. The coarse particle carbon black contained in may be different from the coarse particle carbon black contained in the outermost layer as long as the particle diameter is in the range described above. However, in view of the productivity of the coating material for each back layer, it is advantageous in terms of efficiency and cost to use the same coarse particle carbon black.

  Moreover, you may contain carbon black other than these as needed. The ratio of fine particle carbon black / coarse carbon black in each layer of the back coat layer is preferably 99/1 to 85/15 in terms of weight ratio. Moreover, 60-100 weight% is preferable and, as for the ratio of carbon black, 70-95 weight% is more preferable on the basis of the weight of all the powder particles of each layer of a back layer.

  The thickness of each layer of the backcoat layer is preferably 0.1 to 1.0 μm. In particular, the thickness of the outermost layer is preferably 0.2 to 0.6 μm. The surface roughness shape of the backcoat layer can be controlled by controlling the content of the outermost elementary particle carbon black and the thickness of the outermost layer.

  A nonmagnetic plate-like powder having an average particle diameter of 10 to 100 nm can be added to the backcoat layer for the purpose of improving strength, temperature / humidity dimensional stability, and the like.

  The component of the nonmagnetic plate-like powder is not limited to aluminum oxide, and rare earth elements such as cerium, and oxides or composite oxides of elements such as zirconium, silicon, titanium, manganese, and iron are used. For the purpose of improving conductivity, a plate-like carbonaceous powder having an average particle size of 10 to 100 nm, a plate-like ITO powder having an average particle size of 10 to 100 nm, or the like may be added.

  Moreover, you may add the granular iron oxide powder and granular alumina whose average particle diameter is 0.05-0.5 micrometer as needed. The addition amount is preferably 0.5 to 40% by weight, more preferably 5 to 30% by weight, based on the weight of all powder particles in the backcoat layer. Further, if necessary, an organic powder insoluble in an organic solvent such as benzoguanamine, crosslinked polystyrene, polyethylene, silicon resin, or fluororesin may be added.

  In the back coat layer, the same resin as that used for the magnetic layer and the undercoat layer can be used as a binder (binder resin). Among these, in order to reduce the coefficient of friction and improve the running performance, it is desirable to use a cellulose resin and a polyurethane resin in combination.

  The amount of the binder resin used is usually 40 to 200 parts by weight, preferably 50 to 150 parts by weight, and more preferably 60 to 130 parts by weight with respect to 100 parts by weight of the total amount of carbon black and the powder particles.

  If the binder resin is too small, the strength of the backcoat layer becomes insufficient, and if it is excessive, the friction coefficient tends to be high. The binder resin is most preferably 30 to 70 parts by weight of cellulose resin and 20 to 50 parts by weight of polyurethane resin.

  Moreover, it is preferable to use a crosslinking agent such as a polyisocyanate compound in order to cure the binder resin. The usage-amount of a crosslinking agent is 10-50 weight part normally with respect to 100 weight part of binder resin, Preferably it is 10-35 weight part, More preferably, it is 10-30 weight part. If the amount of the crosslinking agent used is too small, the coating strength of the backcoat layer tends to be weak, and if too much, the dynamic friction coefficient with respect to a guide roller (the material is, for example, SUS304) provided in the tape drive running system is large. Become. In addition, the electron beam curable resin similar to the magnetic layer and the undercoat layer can also be used as a crosslinking agent.

  Arbitrary methods can be used as a method of providing a plurality of backcoat layers. For example, in a coating machine having a die head having a plurality of paint discharge ports, a plurality of backcoat layers may be formed simultaneously, or each layer may be formed sequentially. In addition, it is optional to perform a smoothing process on the backcoat layer using a calendar device. Depending on the use and purpose of the magnetic recording medium, a plurality of back coat layers can be provided by a conventionally known method and procedure.

  Hereinafter, other components of the present invention will be described in detail.

<Magnetic layer>
In order to improve the self-demagnetization loss and the reproduction waveform resolution during short wavelength recording, it is preferable to reduce the thickness of the uppermost magnetic layer. The thickness of the magnetic layer is preferably 10 nm or more and less than 200 nm, and more preferably 150 nm or less. If the thickness of the magnetic layer is less than 10 nm, the output obtained may be too small, or it may be difficult to apply a uniform magnetic layer. If the thickness exceeds 200 nm, self-demagnetization and thickness loss become too large and short wavelength recording is possible. This is because the reproduction characteristics deteriorate.

  In a recording / reproducing apparatus that uses an MR head as a reproducing head, it is a big problem to reduce the noise of the magnetic recording medium. For this purpose, it is necessary to increase the number of magnetic powder particles contained in the unit volume of the coating film of the magnetic layer as much as possible. For this purpose, it is preferable to increase the number of magnetic powder particles contained in a unit volume by reducing the particle diameter of the magnetic powder to reduce the particle volume and increasing the packing property of the magnetic powder particles.

  The coercive force of the magnetic layer is preferably 160 to 400 kA / m, more preferably 200 to 350 kA / m, and further preferably 220 to 320 kA / m. This range is preferable because if the recording wavelength is shorter than 160 kA / m, the output is reduced due to demagnetization, and if it exceeds 400 kA / m, recording by the magnetic head may be difficult.

  The binder (binder resin) used for the magnetic layer includes vinyl chloride resin, vinyl chloride-vinyl acetate copolymer resin, vinyl chloride-vinyl alcohol copolymer resin, vinyl chloride-vinyl acetate-vinyl alcohol copolymer resin, A combination of a polyurethane resin and at least one selected from vinyl chloride-vinyl acetate-maleic anhydride copolymer resins, vinyl chloride-hydroxyl group-containing alkyl acrylate copolymer resins, and cellulose resins such as nitrocellulose. and so on. Among these, it is preferable to use a vinyl chloride-hydroxyl group-containing alkyl acrylate copolymer resin and a polyurethane resin in combination.

  Examples of the polyurethane resin include polyester polyurethane resin, polyether polyurethane resin, polyether polyester polyurethane resin, polycarbonate polyurethane resin, polyester polycarbonate polyurethane resin, and the like.

The binder resin has —COOH, —SO ₃ M, —OSO ₃ M, —P═O (OM) ₃ , —O—P═O (OM) ₂ [in these formulas, M is hydrogen. Represents an 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 a hydrogen or hydrocarbon group], and a polymer such as a urethane resin having an epoxy group or the like is particularly preferable.

The reason why such a binder resin is preferable is that dispersibility of magnetic powder and the like is improved. When two or more kinds of resins are used in combination, the polarities of the functional groups are preferably matched, and among them, a combination of —SO ₃ M groups is preferable.

  In the magnetic layer, the binder (binder resin) is usually used in a ratio of 7 to 50 parts by weight, preferably 10 to 35 parts by weight, with respect to 100 parts by weight of the magnetic powder. In particular, it is most preferable to use a composite of 5-30 parts by weight of vinyl chloride resin and 2-20 parts by weight of polyurethane resin as the binder resin.

  Moreover, it is preferable to use together with said binder resin the thermosetting crosslinking agent which couple | bonds with the functional group contained in binder resin, etc. as a binder resin structural component, and bridge | crosslinks.

  Various crosslinking agents include tolylene diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate, reaction products of these isocyanates with those having a plurality of hydroxyl groups such as trimethylolpropane, and condensation products of the above isocyanates. The polyisocyanate is preferably used.

  These crosslinking agents are generally used in a proportion of 1 to 30 parts by weight, preferably 5 to 20 parts by weight, per 100 parts by weight of the binder resin. When a magnetic layer is applied wet-on-wet on the undercoat layer, a certain amount of polyisocyanate is diffused and supplied from the undercoat paint, so that the magnetic layer is crosslinked to some extent without using polyisocyanate.

  Instead of the above-described thermosetting binder resin, a radiation curable resin may be used. As the radiation curable resin, those obtained by acrylic modification of the above thermosetting resin to give a radiation sensitive double bond, acrylic monomers, and acrylic oligomers are used.

  The average particle size of the magnetic powder contained in the magnetic layer is preferably in the range of 5 nm to 30 nm, and more preferably 10 nm or more. Moreover, 25 nm or less is more preferable, and 20 nm or less is further more preferable. This range is preferable because if the average particle size is less than 5 nm, the surface energy of the particles may increase and dispersion may become difficult, and if the average particle size exceeds 30 nm, noise tends to increase. .

  As such magnetic powder, ferromagnetic iron-based metal magnetic powder, iron nitride-based magnetic powder, hexagonal Ba-ferrite magnetic powder and the like are preferably used.

  The ferromagnetic iron-based metal magnetic powder 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, at least one rare earth element selected from yttrium, cerium, ytterbium, cesium, praseodymium, samarium, lanthanum, europium, neodymium, terbium and the like may be included. Among these, when cerium, neodymium and samarium, terbium, and yttrium are used, a high coercive force can be obtained, which is preferable. The amount of the rare earth element is 0.2 to 20 atomic%, preferably 0.3 to 15 atomic%, and more preferably 0.5 to 10 atomic% with respect to iron.

  The ferromagnetic iron-based metallic magnetic powder is usually needle-shaped, spindle-shaped, or rice-grained, but preferably has an average particle diameter (major axis diameter) of 10 nm to 30 nm. This is because if the average particle diameter is less than 10 nm, the coercive force is out of the preferred range, the specific surface area is increased, the dispersion becomes unstable, and the preferred recording / reproducing characteristics may not be obtained. When the average particle diameter exceeds 30 nm, the particle noise based on the particle size increases. The axial ratio (major axis diameter / minor axis diameter) of the ferromagnetic iron-based metal magnetic powder is preferably 1 or more and less than 3. This range is preferred because the degree of filling of the coating film does not decrease when the magnetic field is oriented.

  Known iron nitride magnetic powders can be used. As the shape, a polyhedral shape such as a spherical shape, a substantially spherical shape, an elliptical shape, a substantially elliptical shape, a cubic shape, or a substantially polyhedral shape can be used. The average particle size is preferably 30 nm or less, and more preferably 20 nm or less. The axial ratio of the magnetic powder is preferably 1 or more and less than 2, more preferably 1.5 or less, and even more preferably less than 1.5.

In particular, the outer layer portion is mainly composed of at least one element such as rare earth elements, aluminum, silicon, zirconium, titanium, etc. whose oxide is not reduced by hydrogen reduction at 600 ° C. or less (particularly, rare earth elements, aluminum, silicon). Containing iron or iron-based transition metal nitrides (particularly, Fe ₁₆ N ₂ or a part of Fe substituted with a transition metal) in the core part The powder is preferably used because it easily obtains a coercive force of 200 kA / m or more.

The coercive force of the ferromagnetic iron-based metal magnetic powder or the iron nitride-based magnetic powder is preferably 160 to 400 kA / m, and more preferably 200 to 350 kA / m. Saturation magnetization is preferably a ^{60~200A · m 2 / kg (60~200emu} / g), more preferably from ^{80~180A · m 2 / kg (80~180emu} / g).

Further, the BET specific surface area of these ferromagnetic powders is preferably 35 m ² / g or more, more preferably 40 m ² / g or more, and most preferably 50 m ² / g or more. Usually, it is good that it is 100 m ² / g or less.

  In addition, about the manufacturing method etc. of an iron nitride type magnetic powder, the method as described in the reference patent WO03 / 079333A1 can be used.

The coercive force of the hexagonal Ba-ferrite magnetic powder is preferably 160 to 400 kA / m, and more preferably 200 to 350 kA / m. The saturation magnetization is preferably 40 to 60 A · m ² / kg (40 to 60 emu / g).

  The particle size (size in the plate surface direction) of the hexagonal Ba-ferrite magnetic powder is preferably 10 nm or more and less than 30 nm. Moreover, 25 nm or less is more preferable, and 20 nm or less is further more preferable. When the particle size is less than 10 nm, the surface energy of the particles increases, so dispersion in the paint may be difficult. When the particle size exceeds 30 nm, particle noise based on the size of the particles tends to increase. Because.

The plate ratio (plate diameter / plate thickness) of the hexagonal Ba-ferrite magnetic powder is preferably from 1 to less than 10, more preferably from 1 to 7, and even more preferably from 1 to 5. The hexagonal Ba-ferrite magnetic powder preferably has a BET specific surface area of 1 to 100 m ² / g.

  Further, in order to improve the dispersibility of the magnetic powder, the above-described ferromagnetic iron-based metal magnetic powder, iron nitride magnetic powder or hexagonal Ba-ferrite magnetic powder is converted into an Al, Si, P, Y, Zr compound or an oxidation thereof. It may be used after being surface-treated with an object.

  The magnetic powder contained in the magnetic layer is more preferably an iron nitride-based magnetic powder or hexagonal Ba-ferrite magnetic powder that can obtain a particularly large coercive force among the above-mentioned preferable magnetic powders. The iron nitride magnetic powder is most preferable.

  Note that the magnetic characteristics of these magnetic powders all mean values measured with an external magnetic field 1273.3 kA / m (16 kOe) using a sample vibration magnetometer.

  The average particle diameter of these magnetic powders is the maximum diameter of each particle (major axis diameter for needle-like powder, plate diameter for plate-like powder) from a photograph taken at 200,000 times with a transmission electron microscope (TEM). Is actually obtained and obtained from the number average value of 50 pieces.

  If necessary, a conventionally known carbon black may be added to the magnetic layer for the purpose of improving conductivity and surface lubricity. As carbon black, acetylene black, furnace black, thermal black, etc. can be used.

  Carbon black preferably has an average particle size of 10 to 100 nm. This range is preferable because when the average particle diameter is less than 10 nm, the dispersion of carbon black tends to be difficult. When the average particle diameter exceeds 100 nm, the amount of carbon black particles protruding from the surface of the magnetic layer increases. This is because there is a possibility that the output becomes rough and causes a decrease in output. If necessary, two or more types of carbon black having different average particle diameters may be used.

  Moreover, you may add nonmagnetic powders other than carbon black to a magnetic layer as needed. The nonmagnetic powder other than carbon black contained in the magnetic layer preferably has an average particle size of less than 150 nm. The non-magnetic powder is preferably α-alumina, β-alumina, silicon carbide, chromium oxide, cerium oxide, α-iron oxide, corundum, artificial diamond, silicon nitride, silicon carbide, titanium carbide, titanium oxide, and dioxide. Those having a Mohs hardness of 6 or more, such as silicon and boron nitride, are used alone or in combination.

  In addition, as long as the effect of improving durability is obtained, metal salt powder such as calcium carbonate and barium sulfate, organic powder insoluble in organic solvents such as benzoguanamine, crosslinked polystyrene, polyethylene, silicon resin, polytetrafluoroethylene, etc. Conventionally known non-magnetic powders can be widely used.

  In the magnetic recording medium of the present invention, the addition amount of the nonmagnetic powder may be appropriately selected according to the required specifications of the durability of the magnetic recording medium and the head polishing amount. It is preferably 20% by weight.

<Non-magnetic support>
Although the thickness of a magnetic support body changes with uses, a 1.5-12.0 micrometer thing is used normally. More preferably, it is 2.0-7.0 micrometers, Most preferably, it is 2.0-6.0 micrometers. Nonmagnetic supports with a thickness in this range are used when film thickness is less than 1.5 μm, and the strength of the tape is reduced. When the thickness exceeds 12.0 μm, the total thickness of the tape is increased. This is because the recording capacity per unit becomes small.

Longitudinal Young's modulus of the non-magnetic support is preferably 5.8GPa (590kg / mm ²⁾ or more, more preferably at least ^{7.1GPa (720kg / mm 2),} 7.8GPa (800kg / mm 2) or higher Most preferred. The longitudinal Young's modulus of the non-magnetic support is preferably 5.8 GPa (590 kg / mm ² ) or more. If the Young's modulus in the longitudinal direction is less than 5.8 GPa (590 kg / mm ² ), the tape running becomes unstable. Because.

In the helical scan type, the Young's modulus (MD) in the longitudinal direction / Young's modulus (TD) in the width direction is preferably in the range of 0.60 to 0.80, and more preferably in the range of 0.65 to 0.75. . The above range is good when it is less than 0.60 or exceeds 0.80, but the mechanism is currently unknown, but the output variation (flatness) between the entrance side and the exit side of the track of the magnetic head This is because of the increase.

  This variation is minimized when the Young's modulus in the longitudinal direction / Young's modulus in the width direction is around 0.70. Further, in the linear recording type, the Young's modulus in the longitudinal direction / Young's modulus in the width direction is preferably 0.70 to 2.50, although the reason is not clear.

The temperature expansion coefficient in the width direction of the nonmagnetic support is preferably −10 × 10 ⁻⁶ to 10 × 10 ⁻⁶ , and the humidity expansion coefficient is preferably 0 to 10 × 10 ⁻⁶ . The reason why this range is preferred is that if it is outside this range, off-track occurs due to changes in temperature and humidity, and the error rate may increase.

  Examples of the non-magnetic support that satisfies all the above-mentioned properties include biaxially stretched polyethylene terephthalate film, polyethylene naphthalate film, aromatic polyamide film, and aromatic polyimide film.

<Undercoat layer>
In the present invention, since the thickness of the magnetic layer is as thin as less than 100 nm, it is preferable to provide an undercoat layer in order to increase the smoothness of the magnetic layer and improve the short wavelength recording characteristics.

  The thickness of the undercoat layer is preferably 0.2 μm or more and less than 1.0 μm, and more preferably 0.8 μm or less. This range is preferably less than 0.2 μm, the effect of reducing the thickness unevenness of the magnetic layer, the effect of improving the durability is reduced, and when it is 1.0 μm or more, the total thickness of the magnetic tape becomes too thick, This is because the recording capacity per tape roll is reduced.

  Nonmagnetic powders used for the undercoat layer include titanium oxide, iron oxide, and aluminum oxide. Iron oxide alone or a mixed system of iron oxide and aluminum oxide is preferably used. The particle shape of the non-magnetic powder may be spherical, plate-like, needle-like, or spindle-like, but in the case of needle-like or spindle-like, the major axis length is usually 50 to 200 nm and the minor axis length is 5 to 100 nm. Is preferred. In the case of a granular shape, a particle size of 5 to 200 nm, more preferably 5 to 100 nm is used.

  In many cases, such non-magnetic powder is mainly supplemented with carbon black having a particle diameter of 0.01 to 0.1 μm and acicular or granular aluminum oxide having the above-mentioned particle diameter as necessary. In order to apply the undercoat layer smoothly and with little thickness unevenness, it is particularly preferable to use nonmagnetic powder and carbon black having a sharp particle size distribution.

  In order to reduce the temperature / humidity expansion coefficient of the magnetic recording medium and improve the smoothness of the upper magnetic layer, it is preferable to add a nonmagnetic plate-like powder having an average particle diameter of 10 to 100 nm to the undercoat layer. As a component of the nonmagnetic plate-like powder, a rare earth element such as cerium, an oxide or a complex oxide of an element such as zirconium, silicon, titanium, manganese, or iron is used.

  For the purpose of improving the conductivity, a plate-like carbonaceous powder such as graphite having an average particle size of 10 to 100 nm or a plate-like ITO (indium, tin composite oxide) powder having an average particle size of 10 to 100 nm may be added. . By adding the nonmagnetic plate powder, the film thickness uniformity, surface smoothness, rigidity, and dimensional stability are improved. In addition, as a binder (binder resin) used for an undercoat layer, the thing similar to the below-mentioned magnetic layer can be used.

  An undercoat layer containing magnetic powder is not excluded.

<Lubricant>
The magnetic layer and the undercoat layer preferably contain 0.5 to 3.0% by weight of fatty acid amide with respect to the total powder contained in the magnetic layer and the undercoat layer. It is preferable to contain -5.0 weight% of higher fatty acid, and it is further preferable to contain 0.2-3.0 weight% of higher fatty acid ester.

  It is preferable to add the fatty acid amide within the above range. If the amount is less than 0.5% by weight, 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, bleeding out occurs. This is because defects such as dropout may occur. Also, the addition of higher fatty acids within the above range is preferred because if the amount is less than 0.5% by weight, the effect of reducing the friction coefficient is small, and if it exceeds 5.0% by weight, the primer layer may be plasticized and the toughness may be lost. Because there is. Furthermore, the addition of a higher fatty acid ester within the above range is preferable 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 to the magnetic layer is too large. This is because side effects such as sticking of the head may occur.

  The higher fatty acid is preferably a fatty acid having 10 or more carbon atoms, and the higher fatty acid ester is preferably an ester of the above higher fatty acid. 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.

  In addition, the mutual movement of the lubricant in the magnetic layer and the lubricant in the undercoat layer is not excluded, and when the lubricant is contained in the undercoat layer, the magnetic layer may not contain the lubricant. On the contrary, when the effect is manifested only by being included in the magnetic layer, it may not be included in the undercoat layer.

  In addition, in order to include a lubricant in the undercoat layer and the magnetic layer, in addition to the method of adding the lubricant to the coating material for forming each layer, the lubricant or lubricant is included after the formation of the undercoat layer and the magnetic layer. Various methods such as a method of dipping or spraying the solution can be employed.

  When the thickness of the undercoat layer is 0.5 μm or less, the lubricant component supplied from the undercoat layer may be insufficient. In that case, it is preferable to include a lubricant in the backcoat layer and supply the lubricant from the backcoat layer to the surface on the magnetic layer side. The type of lubricant used is the same as that used for the magnetic layer and the undercoat layer. As the addition amount, 0.5 to 3.0% by weight of fatty acid amide, 0.2 to 3.0% by weight of higher fatty acid ester, 0.5 to 5% with respect to the total nonmagnetic powder in the backcoat layer. It is preferable to contain 0.0% by weight of higher fatty acid.

<Dispersant>
The nonmagnetic powder, carbon black, and magnetic powder contained in the undercoat layer and magnetic layer can be surface-treated with an appropriate dispersant in order to improve dispersibility with the binder (binder resin). Moreover, you may add an appropriate dispersing agent in the coating material for forming the undercoat layer and magnetic layer containing said each powder. As the dispersant, a phosphoric acid dispersant, a carboxylic acid dispersant, an amine dispersant, a chelate agent, various silane coupling agents, and the like are preferably used. These dispersants are preferably blended after the kneading pretreatment step, the kneading step and the initial dispersion step. Examples of phosphate dispersants include alkyl phosphates such as monomethyl phosphate, dimethyl phosphate, monoethyl phosphate, and diethyl phosphate, and aromatic phosphates such as phenylphosphonic acid and monooctylphenylphosphonic acid. As commercial products, “GARFAC RS410” manufactured by Toho Chemical Co., Ltd., “JP-502”, “JP-504”, “JP-508” manufactured by Johoku Chemical Industry Co., Ltd., and the like can be used. Moreover, as a carboxylic acid type | system | group dispersing agent, a C12-C18 fatty acid, specifically, caprylic acid, capric acid, lauric acid, myristic acid, palmitic acid, stearic acid, behenic acid, oleic acid, elaidic acid Linoleic acid, linolenic acid, stearic acid and the like are used. Also, a metal soap composed of an alkali metal or an alkaline earth metal of the fatty acid, an amide of the fatty acid, an ester of the fatty acid or a compound containing fluorine in the fatty acid, a polyalkylene oxide alkyl phosphate ester, lecithin, a trialkyl polyolefinoxy Quaternary ammonium salt (alkyl is 1 to 5 carbon atoms, olefin is ethylene, propylene, etc.), sulfate, sulfonate, phosphate, copper phthalocyanine, benzoic acid, phthalic acid, tetracarboxyl naphthalene, dicarboxyl naphthalene And fatty acids having 12 to 22 carbon atoms. Examples of the amine dispersant include aliphatic amines having 8 to 22 carbon atoms, aromatic amines, alkanolamines, and alkoxyalkylamines. Further, examples of chelating agents include 1,10-phenanthroline, EDTA, dimethylglyoxime, acetylacetone, glycine, dithiazone, nitrilotriacetic acid and the like. These may be used alone or in combination.

  The dispersant is usually added in the range of 0.5 to 20 parts by weight with respect to 100 parts by weight of the binder in any layer.

<Organic solvent>
Examples of organic solvents used in magnetic paints, undercoat paints, and backcoat paints include ketone solvents such as methyl ethyl ketone, cyclohexanone, and methyl isobutyl ketone, ether solvents such as tetrahydrofuran and dioxane, and acetates such as ethyl acetate and butyl acetate. System solvents and the like. These organic solvents are used alone or in combination, and further mixed with toluene.

  EXAMPLES The present invention will be described in detail below by examples, but the present invention is not limited to these. In addition, the part of an Example and a comparative example shows a weight part. Moreover, the average particle diameter of an Example and a comparative example shows a number average particle diameter.

Example 1
<Undercoat paint component>
(1)
・ Nonmagnetic plate-like iron oxide powder (average particle size: 50 nm) 76 parts ・ Carbon black (average particle size: 25 nm) 24 parts ・ Stearic acid 2.0 parts ・ Vinyl chloride-hydroxypropyl acrylate copolymer 8.8 parts (Contained -SO ₃ Na group: 0.7 × 10 ^-4 equivalent / g)
Polyester polyurethane resin 4.4 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)
・ Stearic acid 1 part ・ Butyl stearate 1 part ・ Cyclohexanone 70 parts ・ Methyl ethyl ketone 50 parts ・ Toluene 20 parts (3)
・ Polyisocyanate 1.4 parts ・ Cyclohexanone 10 parts ・ Methyl ethyl ketone 15 parts ・ Toluene 10 parts 《Magnetic paint component》
(1) Kneading process / magnetic powder (YN-Fe) 100 parts (Y / Fe: 5.5 at%, N / Fe: 11.9 at% σs: 103 A · m ² / kg (103 emu / g), Hc : 211.0 kA / m (2650 Oe), average particle diameter: 17 nm, axial ratio: 1.2)
-13 parts of vinyl chloride-hydroxypropyl acrylate copolymer (containing -SO ₃ Na group: 0.7 × 10 ^-4 equivalent / g)
Polyester polyurethane resin (PU) 4.5 parts (containing -SO ₃ Na group: 1.0 × 10 ⁻⁴ equivalent / g)
・ Particulate alumina powder (average particle size: 80 nm) 10 parts ・ Methyl acid phosphate (MAP) 2 parts ・ Carbon black 3 parts (average particle size: 75 nm, oil absorption 72 ml / 100 g)
-Tetrahydrofuran (THF) 20 parts-Methyl ethyl ketone / cyclohexanone (MEK / A) 9 parts (2) Dilution step-Palmitic acid amide (PA) 1.5 parts-N-butyl stearate (SB) 1 part-Methyl ethyl ketone / cyclohexanone ( MEK / A) 350 parts (3) Compounding process: Polyisocyanate 1.5 parts Methyl ethyl ketone / cyclohexanone (MEK / A) 29 parts After kneading (1) with a batch kneader in the above base coat component, (2) After stirring, the mixture was dispersed in a sand mill with a residence time of 60 minutes, and after adding (3), stirring and filtering, an undercoat paint (undercoat layer paint) was obtained.

  Separately, (1) kneading step components are mixed at high speed in advance in the above magnetic coating components, the mixed powder is kneaded with a continuous biaxial kneader, and (2) dilution step components are added and continuously. Dilution was performed in at least two stages using a formula biaxial kneader, and dispersion was performed with a sand mill with a residence time of 45 minutes. To this, (3) ingredients of the blending step were added, stirred and filtered to obtain a magnetic paint.

  On the nonmagnetic support (base film) made of a polyester film (thickness 6.1 μm, MD = 7.2 GPa, MD / TD = 1.5, trade name: Lumirror, manufactured by Toray Industries, Inc.) , Dried and coated so that the thickness after calendering is 1.5 μm, and the magnetic coating is further coated on the undercoat layer with a magnetic layer having a thickness of 0. 0 after magnetic field orientation treatment, drying and calendering treatment. The coating was applied wet-on-wet with an extrusion coater so as to have a thickness of 15 μm, and after magnetic field orientation treatment, it was dried using a dryer and far infrared rays to obtain a magnetic sheet.

<< Back layer A paint component >>
Fine particle carbon black (average particle size: 25 nm) 86 parts Coarse particle carbon black (average particle size: 270 nm) 4 parts Nonmagnetic iron oxide powder (average particle size: 100 nm) 10 parts Nitrocellulose 45 parts Polyurethane resin (-SO ₃ Na group-containing) 30 parts ・ Cyclohexanone 260 parts ・ Toluene 260 parts ・ Methyl ethyl ketone 525 parts The coating component for the back layer A was dispersed in a sand mill for a residence time of 45 minutes, and then 15 parts of polyisocyanate was added to form a back coat. After the layer coating material was adjusted and filtered, it was applied to the opposite surface of the magnetic layer of the magnetic sheet prepared above so that the thickness after drying and calendering was 0.35 μm and dried.

  The magnetic sheet thus obtained was mirror-finished under a condition of a temperature of 100 ° C. and a linear pressure of 196 kN / m with a seven-stage calendar made of a metal roll, and a magnetic sheet with a back layer A was obtained.

<< Coating component for back layer B >>
・ Particulate carbon black (average particle diameter: 25 nm) 82 parts ・ Rough particle carbon black (average particle diameter: 270 nm) 8 parts ・ Nonmagnetic iron oxide powder (average particle diameter: 100 nm) 10 parts ・ Nitrocellulose 45 parts ・ Polyurethane resin (-SO ₃ Na group-containing) 30 parts ・ Cyclohexanone 260 parts ・ Toluene 260 parts ・ Methyl ethyl ketone 525 parts The coating component for the back layer B was dispersed with a sand mill for a residence time of 45 minutes, and then 15 parts of polyisocyanate was added to form a back coat. The layer coating material was adjusted and filtered, and then applied to the back surface of the magnetic sheet with the back layer A produced above so that the thickness after drying was 0.45 μm and dried.

  The magnetic sheet with the back layers A and B prepared above was aged at 70 ° C. for 72 hours in a state of being wound around the core to obtain a magnetic sheet with the back layer.

  The magnetic sheet with the back layer was cut into a 1/2 inch width by a slit machine. A slit machine (an apparatus for cutting a magnetic tape original into a magnetic tape having a predetermined width) was used in which various constituent elements were improved as follows. A tension cut roller was provided in the web path from the unwinding raw fabric to the slit blade group, this tension cut roller was a suction type, and the suction part was a mesh suction in which a porous metal was embedded. A direct drive directly connected to a motor without a mechanism for transmitting power to the blade drive unit was used.

  While running the cut tape at 200 m / min, the surface of the magnetic layer was subjected to post-processing such as lapping tape polishing, blade polishing, and surface wiping to produce a magnetic tape. At this time, K10000 was used for the wrapping tape, a carbide blade was used for the blade, and Toraysee (trade name) manufactured by Toray was used for wiping the surface. The magnetic tape obtained as described above was written with a servo signal on the backcoat layer with a servo writer for S-DLT, and then 4 pieces of cotton having a short fiber diameter of 4 μm were twisted on the backcoat layer. The tape was run while contacting a velvet planted with 2.5 mm long fibers to remove the burning residue at the time of servo signal writing, and it was incorporated into a cartridge to produce a computer tape.

(Example 2)
In the back layer A, the compounding amount of coarse particle carbon black (average particle size: 270 nm) was changed from 4 parts to 6 parts, and the compounding amount of fine particle carbon black (average particle size: 25 nm) was changed from 86 parts to 84 parts. Produced the computer tape of Example 2 in the same manner as Example 1.

(Example 3)
In the back layer B, the blending amount of coarse particle carbon black (average particle size: 270 nm) was changed from 8 parts to 6 parts, and the blending amount of fine particle carbon black (average particle size: 25 nm) was changed from 82 parts to 84 parts, In the back layer A, the amount of coarse carbon black (average particle size: 270 nm) was changed from 4 parts to 2 parts, and the amount of fine particle carbon black (average particle size: 25 nm) was changed from 86 parts to 88 parts. A computer tape of Example 3 was produced in the same manner as Example 1 except for the above.

Example 4
In the back layer B, the compounding amount of coarse particle carbon black (average particle size: 270 nm) was changed from 8 parts to 6 parts, and the compounding amount of fine particle carbon black (average particle size: 25 nm) was changed from 82 parts to 84 parts. Produced the computer tape of Example 4 in the same manner as in Example 1.

(Example 5)
In the same manner as in Example 1, except that the coating component for the back layer was changed as follows, and the final thicknesses of the back layer A and the back layer B were changed to 0.3 μm and 0.4 μm, respectively. The computer tape of Example 5 was produced.

<< Back layer A paint component >>
・ Particulate carbon black (average particle size: 17 nm) 84 parts ・ Coarse particle carbon black (average particle size: 240 nm) 6 parts ・ Nonmagnetic iron oxide powder (average particle size: 100 nm) 10 parts ・ Nitrocellulose 45 parts ・ Polyurethane resin (-SO ₃ Na group-containing) 30 parts / cyclohexanone 260 parts / toluene 260 parts / methyl ethyl ketone 525 parts << coating component for back layer B >>
・ Particulate carbon black (average particle size: 17 nm) 82 parts ・ Coarse particle carbon black (average particle size: 240 nm) 8 parts ・ Nonmagnetic iron oxide powder (average particle size: 100 nm) 10 parts ・ Nitrocellulose 45 parts ・ Polyurethane resin _(-SO 3 Na group-containing) 30 parts cyclohexanone 260 parts 260 parts Methyl ethyl ketone 525 parts toluene (example 6)
The coating component for the back layer was changed as follows, and the final thickness of the back layer A and the back layer B was changed to 0.2 μm and 0.3 μm, respectively, as in Example 1, The computer tape of Example 6 was produced.

<< Back layer A paint component >>
・ Particulate carbon black (average particle size: 17 nm) 84 parts ・ Coarse particle carbon black (average particle size: 180 nm) 6 parts ・ Nonmagnetic iron oxide powder (average particle size: 100 nm) 10 parts ・ Nitrocellulose 45 parts ・ Polyurethane resin (-SO ₃ Na group-containing) 30 parts / cyclohexanone 260 parts / toluene 260 parts / methyl ethyl ketone 525 parts << coating component for back layer B >>
・ Particulate carbon black (average particle size: 17 nm) 82 parts ・ Coarse particle carbon black (average particle size: 180 nm) 8 parts ・ Nonmagnetic iron oxide powder (average particle size: 100 nm) 10 parts ・ Nitrocellulose 45 parts ・ Polyurethane resin _(-SO 3 Na group-containing) 30 parts cyclohexanone 260 parts 260 parts Methyl ethyl ketone 525 parts toluene (Comparative example 1)
In the back layer A, the compounding amount of coarse particle carbon black (average particle size: 270 nm) was changed from 4 parts to 8 parts, and the compounding amount of fine particle carbon black (average particle size: 25 nm) was changed from 86 parts to 82 parts. In the same manner as in Example 1, a computer tape of Comparative Example 1 was produced.

(Comparative Example 2)
In the back layer A, the compounding amount of coarse particle carbon black (average particle size: 270 nm) was changed from 4 parts to 10 parts, and the compounding amount of fine particle carbon black (average particle size: 25 nm) was changed from 86 parts to 80 parts. In the same manner as in Example 1, a computer tape of Comparative Example 2 was produced.

(Comparative Example 3)
In the back layer A, the compounding amount of coarse particle carbon black (average particle size: 270 nm) was changed from 4 parts to 6 parts, and the compounding amount of fine particle carbon black (average particle size: 25 nm) was changed from 86 parts to 84 parts. Produced the computer tape of Comparative Example 3 in the same manner as in Example 3.

(Comparative Example 4)
In the back layer A, the compounding amount of coarse particle carbon black (average particle size: 270 nm) was changed from 4 parts to 8 parts, and the compounding amount of fine particle carbon black (average particle size: 25 nm) was changed from 86 parts to 82 parts. In the same manner as in Example 3, a computer tape of Comparative Example 4 was produced.

(Comparative Example 5)
A computer tape of Comparative Example 5 was produced in the same manner as in Example 1 except that the back layer A was not provided.

(Comparative Example 6)
A computer tape of Comparative Example 6 was produced in the same manner as in Example 5 except that the average particle size of the coarse particle carbon black of the back layer A and the back layer B was changed from 180 nm to 116 nm.

(Comparative Example 7)
A computer tape of Comparative Example 7 was prepared in the same manner as in Example 1 except that the coarse particle carbon black was removed from the composition of the back layer A.

(Comparative Example 8)
A computer tape of Comparative Example 8 was prepared in the same manner as in Example 3 except that the coarse particle carbon black was removed from the composition of the back layer A.

  The evaluation method was performed as follows.

<C / N measurement>
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 drawn out from the state of being wound around the cartridge and discarded, and a further 60 cm was cut out and wound around the outer periphery of the rotating drum.
The output and noise were controlled by inputting a rectangular wave to a recording current / current generator by a function generator, a signal having a wavelength of 0.2 μm was written, the output of the MR head was amplified by a preamplifier, and then read into a spectrum analyzer. The carrier value of 0.2 μm was defined as the medium output C. Further, when a rectangular wave of 0.2 μm was written, an integrated 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 between the two was taken as C / N, and the relative value to the value of the tape of Comparative Example 5 was determined.

<Durability test>
Using the DLT7000 drive manufactured by Quantum, the error information output by the drive before running is read via the RS-232C interface by recording and playing back at 25 ° C and 55% RH, and the error rate is calculated. All the tracks were run continuously for 300 hours, the error rate was obtained in the same manner, and the number of errors per 1 MB of the recording capacity was evaluated.

<Back layer thickness and carbon black content>
The sample magnetic tape is filled with resin, and a cross section in the thickness direction is cut out in the longitudinal direction of the tape with a focused ion beam processing apparatus, and the cross section is photographed with a scanning electron microscope (SEM) at a magnification of 50,000 times and 10 fields of view. Photographing is performed to frame the back layer B surface, the back layer B-back layer A interface, and the back layer B-nonmagnetic support interface. Next, arbitrary five places where nonmagnetic powder is not applied to the interface are selected per field of view of the photograph, and the distance between the bordered lines is measured as the thickness of each layer of the back layer. The same measurement was performed for 10 fields of view of the photograph, and these were averaged to obtain the thickness of each layer of the back layer. Further, in this example, the amount of carbon black contained in each layer of the back was shown based on the blending composition amount, but in the case of obtaining from the magnetic tape, each back from the photograph when the thickness of the back layer was obtained. The number of carbon black particles contained in the layer may be obtained to determine the amount of carbon black.

  Tables 1 and 2 show the evaluation results of each computer tape. As is apparent from the table, the computer tapes of Examples 1 to 5 according to the present invention have better C / N than the computer tapes of Comparative Examples 1 to 8 that do not satisfy Claim 1, The error rate after running is small.

  Next, the critical significance of various numerical values of the present invention will be clarified with reference to FIGS. First, FIG. 1 and FIG. 2 show the relationship between the amount of coarse particles CB contained in the A layer and C / N, and the error rate after running when the back layer B contains 8 wt% and 6 wt% of coarse particle carbon black, respectively. Show. Experiments were performed with Examples 1 and 3 as the basic composition and the amount of carbon black contained in the back layer A varied in the range of 0 to 10 wt% and 0 to 8 wt%. As is apparent from the figure, when the amount of coarse particle carbon black contained in the back layer A increases, C / N decreases and the error rate after traveling increases. It can be seen that when the amount of the coarse particle carbon black contained in the back layer A becomes equal to or greater than the amount of the coarse particle carbon black contained in the back layer B, the error rate after running increases rapidly. Further, it can be seen that when the coarse particle carbon black is not included in the back layer A, the C / N and the error rate are poor.

  FIG. 3 shows the relationship between the particle size of coarse carbon black, C / N, and error rate after running. The experiment was conducted with Example 2 as the basic composition and the particle size of the coarse particle carbon black contained in the back layer B and the back layer A being changed in the range of 270 nm to 116 nm. As is apparent from the figure, when the particle diameter of coarse carbon black increases, C / N decreases and the error rate after running also decreases. In particular, the error rate after running is preferable when the particle size of the coarse-grained carbon black is 150 nm or more, since the error rate decreases rapidly.

It is a figure which shows the relationship between back layer A coarse particle CB amount, C / N, and an error rate. (Back layer B coarse particles CB amount 8wt%) It is a figure which shows the relationship between back layer A coarse particle CB amount, C / N, and an error rate. (Back layer B coarse particles CB amount 6 wt%) It is a figure which shows the relationship between the coarse particle CB diameter, C / N, and ER after driving | running | working.

Claims (1)

Having a recording layer on one side of the non-magnetic support;
Each layer includes coarse carbon black having an average particle size of 150 nm to 500 nm, fine particle carbon black having an average particle size of 10 nm to less than 100 nm, and a binder on the other surface of the nonmagnetic support.
A magnetic recording medium having a backcoat layer comprising two layers having a thickness of 0.1 to 1.0 μm ,
More than the content of the coarse particles of carbon black content of the coarse particles of carbon black contained in the outermost layer of the back coat layer is included in the inner layer adjacent thereto, and,
The weight ratio of the fine particle carbon black to the coarse particle carbon black is 99/1 to 85/15,
The magnetic recording medium according to claim 1, wherein the total amount of the fine particle carbon black and the coarse particle carbon black is 60 to 100 parts by weight based on the weight of all powder particles of the backcoat layer .

Images