JP2006053995A - Magnetic recording medium and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a magnetic tape having high reliability in durability as a magnetic recording medium which can correspond to a high recording density property. <P>SOLUTION: This medium is a magnetic recording medium having a magnetic layer (application type magnetic layer) including magnetic powders and bonding resin at one side plane of a non-magnetic supporter, also, having a back coat layer at the other side plane. a carbon layer of thickness of 2nm to 10nm is provided on the magnetic layer. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a magnetic recording medium having a coating type magnetic layer and a manufacturing method thereof, and more particularly to a coating type magnetic tape and a manufacturing method thereof.

  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 tape corresponding to high recording density, magnetic powder (hereinafter also referred to as magnetic particles) is made fine (hereinafter also referred to as fine powder) and high-density filling into the coating film, Advanced techniques relating to smoothing of the coating film and thinning of the magnetic layer are used.

  With regard to the improvement of magnetic powder, in order to cope with short wavelength recording, improvement of magnetic properties has been achieved along with the formation of fine particles, and acicular metal magnetic powder having an average particle diameter of 100 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.

  Regarding the coercive force, recording on a magnetic tape having a higher coercive force is possible due to technological innovation of the magnetic head. In particular, in the longitudinal recording method, it is preferable to increase the coercive force as much as possible in order to prevent output reduction due to recording and reproduction demagnetization as long as recording and erasing can be performed with a magnetic head. Therefore, it can be said that a practical and effective method for improving the recording density of the magnetic tape is to increase the coercive force of the magnetic tape.

  On the other hand, with the shortening of the recording wavelength, the influence of the self-demagnetization loss at the time of recording / reproduction and the thickness loss due to the thickness of the magnetic layer, which has not been a problem so far, have been increasing. Therefore, it is necessary to make the magnetic layer thin (thinner).

  However, when the magnetic layer is made thinner, the surface roughness of the nonmagnetic support affects the surface of the magnetic layer, which causes a problem that the surface properties of the magnetic layer are liable to deteriorate. In addition, in a coating-type magnetic recording medium, when only a single magnetic layer is thinned, a method of reducing the solid content concentration of the magnetic paint or reducing the coating amount can be considered. However, it is difficult to improve the filling properties of the magnetic powder, and defects such as coating unevenness are likely to occur during coating, and the coating strength is liable to decrease. For this reason, when the magnetic layer is thinned by improving the medium manufacturing technology, a nonmagnetic undercoat layer is provided between the nonmagnetic support and the magnetic layer, and the upper layer is formed while the undercoat layer is in a wet state. A so-called simultaneous multi-layer coating method has been proposed (for example, Patent Document 1).

  As the recording wavelength is shortened and the magnetic layer is made thinner, the leakage flux from the magnetic layer is becoming extremely weak. In systems using these magnetic recording media, high-sensitivity MR (magnetoresistance effect) The one using a mold head as a reproducing head is becoming mainstream. The MR type head (hereinafter simply referred to as “MR head”) does not have an induction coil, so that the equipment noise is small. Therefore, it is possible to obtain an excellent C / N by reducing the noise of the magnetic recording medium. . However, even the minute unevenness on the surface of the magnetic layer, which was not a problem with the magnetic induction type, greatly affects the noise of the magnetic recording medium in the MR head. Therefore, in the conventional magnetic recording medium used in the system equipped with the MR head. As described above, it is necessary to control the roughness of the magnetic layer surface.

Further, if the magnetic layer is made thinner, its durability tends to be lowered, and this needs to be prevented. As a means for ensuring or improving the durability of the magnetic layer in the coating type magnetic recording medium, for example, as described in Patent Document 1 and Patent Document 2, nonmagnetic powder is included in the magnetic layer. Things are known. The magnetic recording medium described in Patent Document 1 includes an abrasive having an average particle size larger than the thickness of the magnetic layer (1.0 μm or less) in a magnetic layer formed by a wet-on-wet method. The magnetic recording medium described in Patent Document 2 contains Al ₂ O ₃ having two or more different crystal systems as an abrasive in the magnetic layer, both of which improve the durability of the magnetic layer. Therefore, a measure is taken to include nonmagnetic powder particles having a relatively high Mohs hardness in the magnetic layer.

  On the other hand, in a magnetic recording medium having a metal thin film type magnetic layer, as described in, for example, Patent Document 3 and Patent Document 4, a carbon layer is formed on the surface of the magnetic layer as a means for improving the durability of the magnetic layer. What is provided is known. The magnetic recording medium described in Patent Document 3 is provided on one or more layers of Fe—C—O based metal magnetic film having Fe, C and O, and on this Fe—C—O based metal magnetic film. The magnetic recording medium described in Patent Document 4 is provided with a diamond-like carbon film and a fluorine-based lubricant film provided on the diamond-like carbon film. A magnetic layer is provided, and a carbon protective film containing nitrogen deposited by the Filtered Cathodic Arc method is provided on the surface of the magnetic layer.

JP-A-5-197946 (page 2-4) JP 11-238226 A (page 2-3) JP-A-9-91662 (page 2-3) JP 2002-32907 (page 2-4)

  However, in the magnetic recording media disclosed in Patent Document 1 and Patent Document 2, since the magnetic layer contains nonmagnetic powder, it is sufficient that the recording wavelength is reduced or the magnetic layer thickness is reduced. It was difficult to obtain a good electromagnetic conversion characteristic. In addition, as the thickness of the magnetic layer gradually decreases, the tendency of the nonmagnetic powder in the magnetic layer to disturb the orientation of the magnetic powder or to reduce the surface smoothness of the magnetic layer increases. It has become an obstacle to increase the recording density.

  On the other hand, in the thing of patent document 3, since it is a metal thin film type magnetic layer, a lubricant cannot be held therein, and a lubricant film is provided on the uppermost layer of the recording layer. The durability of the magnetic film was not sufficient. Similarly, since the magnetic thin film type magnetic layer described in Patent Document 4 cannot hold the lubricant therein, the lubricant layer is provided as the uppermost layer of the recording layer. However, the durability of the magnetic film was not sufficient.

  As described above, the prior art as described above is insufficient to achieve a high recording density of the magnetic recording medium.

  The present invention has been made in view of the circumstances as described above, and an object thereof is to provide a coating type magnetic recording medium having sufficient durability as a magnetic recording medium that can cope with an increase in recording density. .

  In order to achieve the above object, the present invention has a magnetic layer comprising a coating film containing magnetic powder and a binder resin on one surface of a nonmagnetic support, and a backcoat layer on the other surface. A carbon recording layer having a thickness of 2 nm to 10 nm is provided on the magnetic layer. Here, when the thickness of the carbon layer is 2 nm to 10 nm, if it is 10 nm or more, not only the effect of improving the durability is saturated, but also the spacing between the magnetic head and the magnetic layer becomes too large. This is because the short wavelength recording characteristics are greatly deteriorated, and if the thickness is less than 2 nm, the carbon layer is too thin and the effect of improving the durability cannot be expected.

  In such a magnetic recording medium, a magnetic coating containing magnetic powder and a binder resin is applied to one surface of a nonmagnetic support, and the resulting coating layer (magnetic layer) is dried, It can be manufactured by smoothing the surface and forming a carbon layer having a thickness of 2 nm to 10 nm on the surface of the magnetic layer after the smoothing treatment.

The proportion of sp ³ bonds in the carbon layer in the carbon layer is preferably 25 to 85% (the reason will be described later). In the present application, of carbon C—C bonds, a bond by sp ³ hybrid orbital is called sp ³ bond, and a bond by sp ² hybrid orbit is called sp ² bond.

  According to the magnetic recording medium of the present invention, since the carbon layer having a thickness of 2 nm to 10 nm is provided on the magnetic layer, this carbon layer functions as a protective layer for the surface of the magnetic layer, and as a result, the magnetic layer The durability of the layer will be improved. Moreover, by providing such a carbon layer on the magnetic layer, the friction coefficient of the surface on the magnetic layer side is reduced, and the running performance is improved. In addition, since the magnetic recording medium of the present invention is a coating type, unlike a magnetic recording medium having a metal thin film type magnetic layer, a magnetic layer or an undercoat layer disposed between the magnetic layer and a nonmagnetic support. A lubricant can be added to the coating material for forming the. Therefore, the effect of maintaining good running performance by such a lubricant can be secured. Furthermore, since the thickness of the carbon layer is set as thin as 2 to 10 nm within the range where the surface protection effect on the magnetic layer can be expected, the spacing due to the presence of the carbon layer between the non-magnetic support and the magnetic layer. In other words, there is almost no reduction in output due to spacing loss.

  On the other hand, according to the production method of the present invention, the magnetic layer coated and formed on one surface of the nonmagnetic support is dried, and then the surface is smoothed, and the magnetic layer after the smoothing treatment is performed. Since the carbon layer is formed on the surface of the magnetic layer, fluctuations in the interface between the magnetic layer and the carbon layer can be suppressed. Therefore, the thicknesses of the magnetic layer and the carbon layer can be made uniform, and fluctuations in the reproduction output can be avoided or suppressed.

  The present inventors have made various studies in order to improve the excellent short wavelength recording characteristics required for a high density recording medium. Abrasive powder and carbon black are usually added to the magnetic layer to improve the durability and runnability of the magnetic layer. These abrasive powders and carbon blacks have a particle diameter larger than the minor axis diameter of the magnetic powder in order to exert their effects, and it is inevitable that the surface of the magnetic layer becomes rough. Since the presence disturbs the orientation of the magnetic powder in the magnetic layer, the spacing with the magnetic head is increased, and the short wavelength recording characteristic is deteriorated due to the deterioration of the magnetic characteristic. Furthermore, since these powders are non-magnetic powders, the amount of magnetization per unit volume of the magnetic layer is reduced, so that the reproduction output is reduced. Therefore, it is possible to reduce these nonmagnetic powders contained in the magnetic layer as much as possible and improve durability and runnability by other means, for example, a high density that can correspond to a recording capacity of 1 TB or more per tape roll. This was one problem for developing a recording medium.

The present inventors have developed a magnetic recording medium (coating-type magnetic medium) having a magnetic layer containing magnetic powder and a binder resin on one side of a nonmagnetic support and a backcoat layer on the other side. In the recording medium, the above-mentioned problem was solved by providing a carbon layer having a thickness of 2 nm to 10 nm on the magnetic layer. The durability can be further improved by setting the proportion of sp ³ bonds to 25 to 85% of the C—C bonds of carbon in the carbon layer. The C—C bond of the carbon forming the carbon layer is composed of sp ² bond and sp ³ bond. However, when the ratio of sp ³ bond is 25% or more, the strength of the carbon layer is further increased, which is preferable. . In addition, of the carbon C—C bonds, the greater the proportion of sp ³ bonds, the greater the strength of the carbon layer and the lower the coefficient of friction of the surface on the magnetic layer side. However, it is technically difficult to form a film having a content exceeding 85%.

  The thickness of the carbon layer is preferably 2 to 10 nm in order to minimize the spacing between the magnetic head and the magnetic layer caused by providing the carbon layer, which is a nonmagnetic layer, on the magnetic layer. 5 nm is more preferable. The thickness of the carbon layer here is obtained as follows. First, a sample magnetic recording medium is filled with a resin, cut out with a focused ion beam processing apparatus, and a cross-section of the sample is taken with a transmission electron microscope (TEM) at an appropriate magnification of 100,000 to 1,000,000 times. Photographing is performed, and the boundary between the uppermost carbon layer surface and the uppermost carbon layer-magnetic layer interface is trimmed. Next, measure the distance between the bordered lines at any five locations (50 locations in total) per field of view of the photograph as the thickness of the top carbon layer (hereinafter also referred to as carbon layer), and average them. The thickness of the carbon layer.

  The center line average surface roughness Ra of the carbon layer is preferably 5 nm or less, more preferably 0.5 to 4 nm, still more preferably 1 to 4 nm, and most preferably 1 to 3 nm. The PV of the carbon layer is preferably 70 nm or less, more preferably 6 to 50 nm, from the microscopically the difference in height from the lowest “valley bottom” to the highest “mountain peak” present on the carbon layer surface. 8 to 50 nm is more preferable, and 8 to 40 nm is most preferable. More preferably, Ra is in the range of 0.5 to 4 nm and PV is in the range of 6 to 50 nm. Below these lower limits, when the magnetic recording medium is a magnetic tape, the running of the magnetic tape becomes unstable. If the upper limit is exceeded, the resolution of short wavelength recording deteriorates or the output decreases due to spacing loss, and the error rate increases.

  The carbon layer provided on the magnetic layer is preferably a continuous layer, but when the layer thickness is thin, the carbon layer is not formed on the entire surface of the magnetic layer, and the carbon layer is “sea / island” on the magnetic layer. It may exist in the form. As long as the effect of improving the durability of the magnetic layer is recognized, such a shape may be used.

  The present invention is premised on being applied to a magnetic recording medium having a coating type magnetic layer (magnetic layer coating film) that can easily retain a lubricant as compared with a metal thin film type magnetic layer. Such a coating-type magnetic recording medium includes not only a magnetic tape but also a flexible disk (FD). The main object of the present invention is a coating-type magnetic tape (however, it is applied from the application object of the present invention). Does not exclude flexible disks having a magnetic layer of the mold). Accordingly, the case where the present invention is applied to a magnetic tape having a coating type magnetic layer will be described in more detail below.

<Carbon layer>
The carbon layer is formed on the magnetic layer by using a conventionally known sputtering method, plasma CVD (Chemical) used in the manufacture of semiconductor devices, metal thin film magnetic recording media, and the like.
Vapor Deposition), ion plating, FCVA (Filtered Cathodic Vacuum Arc), and the like can be used. In order to impart high durability to carbon, it is also possible to add hydrogen or nitrogen to the carbon layer. Especially, it is more preferable to form by the FCVA method which is a relatively new technique.

Formation of the carbon layer by the FCVA method is performed as follows. That is, arc discharge is performed between the cathode target and the anode, and target constituent atoms and electrons are ejected from the cathode spot on the target surface formed by the arc. The ejected atoms are ionized by collision with electrons near the cathode spot. In addition to atoms and electrons, macroparticles (decomposed coarse particles of cathode constituent materials) are emitted from the cathode spot. These emitted ions, electrons, neutral atoms, and macro particles are accelerated by the influence of the electric field and plasma and proceed to the filter section. Here, the neutral atoms and macro particles are trapped by the filter, and only the ions and electrons are captured. Reaches the carbon layer adherend. As a result, a carbon thin film is formed from the ions and electrons that have reached the surface of the carbon layer adherend. Compared with other methods, the FCVA method is characterized by being capable of being processed at a low temperature and increasing the content of sp ³ bonds in the C—C bond. Magnetic layer, subbing layer, nonmagnetic support A thin and strong carbon layer can be formed while minimizing the effect on the body.

  A film forming apparatus (FCVA apparatus) used in the FCVA method has a conveyance unit for moving a predetermined web (a magnetic sheet having at least a magnetic layer on a nonmagnetic support), a film forming chamber, a filter unit, and a discharge chamber. . The discharge chamber consists of a cathode target, an anode, a cathode coil, and a striker. Pure graphite is used for the cathode coil, and arc discharge is started by striking the surface of the cathode target with a striker. The cathode coil is for promoting ionization. The carbon particle group generated in the discharge chamber forms a beam and proceeds to the adjacent filter section.

  The filter unit is provided with a 45 degree vent type filter equipped with a filter coil. The carbon ions and electrons are bent by the magnetic field and go to the film formation chamber, but the neutral elements and microparticles cannot be bent and are trapped. Further, the film thickness is distributed uniformly in the film surface by swinging the beam up, down, left and right with a raster magnet.

The degree of vacuum in the film formation chamber affects the beam stability, but is usually set in the range of 0.8 to 10 ⁻⁴ Pa. The current / voltage setting values are as follows.
・ Arc current: 50-100A
・ Cathode current: 5-10A
Filter coil current: 5-10A, 3-8A
・ Raster coil current: X = 0 A, Y = 10 A
・ Bias voltage: 2 to 2000 kV

  These set values have optimum values depending on the apparatus and the carbon layer to be formed. However, the proportion of C—C bonds in the carbon layer is determined by the bias voltage, and the film thickness is determined by the processing time. Also, in the conventional DLC method, the temperature at the time of film formation is as high as 200 ° C., so even if the cooling treatment is performed with a roll or the like, problems such as thermal deformation of the non-magnetic support may be caused. However, the FCVA method does not have the above problem because the film forming temperature can be lowered to 80 ° C.

The proportion of sp ³ bonds among the C—C bonds in the carbon layer can be analyzed at a peak appearing at 280 to 288 eV of EELS (Electron Energy Loss Spectroscopy).

  The magnetic recording medium of the present invention has a magnetic layer containing magnetic powder and a binder resin (that is, a coating-type magnetic layer), and in some cases, a nonmagnetic material is provided between the nonmagnetic support and the magnetic layer. Since it can be set as the structure which provides an undercoat layer, the lubricant mentioned later can be included in these layers. Therefore, unlike the layer structure of the metal thin film type magnetic recording medium, it is not always necessary to provide the lubricant layer on the carbon layer.

<Non-magnetic support>
The thickness of the magnetic support varies depending on the application, but usually 1.5 to 11.0 μm is used. More preferably, it is 2.0-7.0 micrometers. Nonmagnetic supports with a thickness in this range are used when film thickness is less than 1.5 μm, and film strength is difficult, and tape strength decreases. This is because the recording capacity per unit becomes small.

Longitudinal Young's modulus of the nonmagnetic support is preferably 5.8GPa (590kg / mm ²⁾ or more, 7.1GPa (720kg / mm ²⁾ or more is more preferable. The non-magnetic support preferably has a Young's modulus in the longitudinal direction of 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 a specific range of 0.60 to 0.80, more preferably in the range of 0.65 to 0.75. preferable. The reason why the Young's modulus in the longitudinal direction / Young's modulus in the width direction is preferably in a specific range of 0.60 to 0.80 is outside this range, the mechanism is currently unknown. This is because the variation (flatness) in output between the incoming side and the outgoing side becomes large. This variation is minimized when the Young's modulus in the longitudinal direction / Young's modulus in the width direction is around 0.70. 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 11.30, although the reason is not clear.

The temperature expansion coefficient in the width direction of the nonmagnetic support is preferably −10 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 falls outside this range, off-track occurs due to changes in temperature and humidity, and the error rate increases.

  Nonmagnetic supports satisfying such properties include biaxially stretched polyethylene terephthalate film, polyethylene naphthalate film, aromatic polyamide film, aromatic polyimide film and the like.

<Magnetic layer>
The thickness of the magnetic layer is preferably 0.01 μm or more and 3.5 μm or less. If the magnetic layer thickness is less than 0.01 μm, the output obtained is small and it is difficult to apply a uniform magnetic layer. If it exceeds 3.5 μm, the total thickness of the tape becomes too large and one roll of tape This is because the hit capacity is small.

  In order to further improve the short wavelength recording characteristics, it is preferable to provide an undercoat layer between the magnetic layer and the nonmagnetic support, and to make the thickness of the magnetic layer 0.01 to 0.1 μm. 0.02 to 0.08 μm is more preferable, and 0.02 to 0.06 μm is most preferable.

  The coercive force of the magnetic layer is preferably 80 to 320 kA / m, more preferably 100 to 320 kA / m, and still more preferably 120 to 320 kA / m. This range is preferable because if the recording wavelength is shorter than 80 kA / m, the output is reduced due to demagnetization, and if it exceeds 320 kA / m, recording by the magnetic head becomes difficult.

  The binder resin (binder resin) used for the magnetic layer (the same applies to the undercoat layer described later) includes vinyl chloride resin, vinyl chloride-vinyl acetate copolymer resin, vinyl chloride-vinyl alcohol copolymer resin, and chloride. Selected from vinyl-vinyl acetate-vinyl alcohol copolymer resins, vinyl chloride-vinyl acetate-maleic anhydride copolymer resins, vinyl chloride-hydroxyl group-containing alkyl acrylate copolymer resins, and cellulose resins such as nitrocellulose. What combined at least 1 sort (s) and a polyurethane resin etc. are mentioned. 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, and polyester polycarbonate polyurethane resin.

-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′R ″, —N + R ′ ″ R ″ ″ R ′ ″ ″ [in these formulas, R ′, R ″, R ″ ', R ″ ″, R ′ ″ ″ represent hydrogen or a hydrocarbon group], and a binder resin such as a urethane resin made of a polymer having an epoxy group is used. The reason why such a binder resin is used is that the dispersibility of the 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 among them, a combination of —SO ₃ M groups is preferable.

  These binder resins are used in the range 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 to 30 parts by weight of a vinyl chloride resin and 2 to 20 parts by weight of a polyurethane resin as a binder resin.

  It is preferable to use together with these binder resins a thermosetting crosslinking agent that is bonded to a functional group contained in the binder resin to crosslink. 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 at a ratio of 1 to 30 parts by weight with respect to 100 parts by weight of the binder resin. More preferably, it is 5 to 20 parts by weight. However, 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. Is done.

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

  As the radiation curable resin used as a binder resin for curing each coating film (magnetic layer, undercoat layer, backcoat layer) of the magnetic tape, conventionally known ones are used. From these known examples, the constitution of the binder resin using the radiation curable resin can be classified as follows.

(1) Thermoplastic resin + radiation curable resin (monomer)
(2) Thermoplastic resin + radiation curable resin (polymer (oligomer))
(3) Thermoplastic resin + radiation curable resin (monomer) + radiation curable resin (oligomer, polymer)
(4) Radiation curable resin (monomer)
(5) Radiation curable resin (polymer (oligomer))
(6) Radiation curable resin (monomer) + radiation curable resin (polymer (oligomer))

  Each of these uses has its characteristics, and it is preferable to use them according to the required specifications of the magnetic tape. For example, in the case of the above (1) to (3), since a thermoplastic resin that is provided with a large number of excellent dispersibility with respect to magnetic powder, non-magnetic powder, etc. can be used, magnetic properties with excellent recording / reproduction characteristics can be used. Easy to design layers. However, when radiation curing treatment is performed, a crosslinked network is formed between the molecules of the radiation curable resin, and the coating film can be cured by the network. However, a crosslinked network is formed between the molecules of the thermoplastic resin and the radiation curable resin. Therefore, in order to design a coating film with excellent durability, the selection of binder resin, lubricant, and non-magnetic powder is not possible. Ingenuity is necessary.

  In the above (4) to (6), a radiation curable resin is used as a binder resin, and a crosslinked network is formed between all resin molecules. Cheap. However, radiation curable resins with excellent dispersibility for magnetic powder, non-magnetic powder, etc. have not yet been sufficiently provided. To design a magnetic layer with excellent recording / reproduction characteristics However, it is necessary to devise the surface treatment and dispersion method of magnetic powder.

As the radiation curable resin, acrylic acid esters, acrylamides, methacrylic acid esters, methacrylic acid amides, allyl compounds, vinyl ethers usually having two or more radiation-sensitive double bonds in one molecule, A vinyl ester is used, and a radiation curable resin having a weight average molecular weight of 50 to 4000 per double bond is preferable. Examples of such radiation curable resins include 1,3-butanediol diacrylate, 1,4-butanediol diacrylate, 1,6-hexanediol diacrylate, ethylene glycol diacrylate, diethylene glycol diacrylate, and triethylene glycol diacrylate. Acrylate, tetraethylene glycol diacrylate, polyethylene glycol diacrylate, propylene glycol diacrylate, dipropylene glycol diacrylate, tripropylene glycol diacrylate, ethoxylated bisphenol A diacrylate, novolac diacrylate, propoxylated neopentyl glycol diacrylate, etc. And the same difunctional methacrylate as the above acrylate, tris (2-hydroxy Cyl) isocyanurate triacrylate, trimethylolpropane triacrylate, ethoxylated trimethylolpropane triacrylate, pentaerythritol triacrylate, propoxylated trimethylolpropane triacrylate, propoxylated glyceryl triacrylate, caprolactone modified trimethylolpropane triacrylate Functional acrylates and trifunctional methacrylates similar to those described above, pentaerythritol tetraacrylate, ditrimethylolpropane tetraacrylate, ethoxylated pentaerythritol tetraacrylate, dipentaerythritol hydroxypentaacrylate, dipentaerythritol hexaacrylate and the like Above Similar to tetra- or higher-functional monomer acrylates (methacrylates) and oligomers and polymers with skeletons such as ethers, esters, carbonates, epoxies, vinyl chlorides, urethanes, etc. Those containing bonds are used. As a polymer containing a radiation-sensitive double bond, a radiation curable vinyl chloride copolymer (manufactured by Toyobo Co., Ltd .: TB0246) (degree of polymerization = 300, polar group: -OSO ₃ K = 1.5) There are specific examples such as radiation curable polyurethane resin (manufactured by Toyobo Co., Ltd .: TB0242) (number average molecular weight Mn = 25000, polar group: phosphorus compound = 1 molecule / molecule).

  The average particle size of the magnetic powder contained in the magnetic layer is preferably in the range of 10 to 100 nm, more preferably in the range of 15 to 80 nm. This range is preferred because if the average particle size is less than 10 nm, the surface energy of the particles becomes large and dispersion becomes difficult, and if the average particle size exceeds 100 nm, noise increases. As the magnetic powder, ferromagnetic iron-based metal magnetic powder, iron nitride magnetic powder, plate-shaped hexagonal Ba-ferrite magnetic powder, and the like are preferable.

  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, cerium, neodymium, samarium, terbium, and yttrium are preferable because high coercive force can be obtained when these are used. The amount of rare earth elements is 0.2 to 20 atomic%, preferably 0.3 to 15 atomic%, more preferably 0.5 to 10 atomic%, based on iron.

  Boron may be included in the ferromagnetic iron-based metal magnetic powder. By containing boron, granular or elliptical ultrafine particles having an average particle diameter of 50 nm or less can be obtained. The amount of boron to be included is 0.5 to 30 atom%, preferably 1 to 25 atom%, more preferably 2 to 20 atom%, based on iron in the entire magnetic powder. Both the atomic% are values measured by fluorescent X-ray analysis (see JP 2001-181754 A).

  As the iron nitride magnetic powder, known powders can be used, and those having an irregular particle shape such as a spherical shape or a cubic shape can be used in addition to the needle shape. Regarding the particle size and specific surface area, in order to clear the required characteristics as magnetic powder for magnetic recording, it is necessary to make the manufacturing conditions of the limited magnetic powder (refer to International Publication WO 03/079333 A1 pamphlet).

The coercive force of the ferromagnetic iron-based metal magnetic powder and the iron nitride magnetic powder is preferably 80 to 320 kA / m, and the saturation magnetization is preferably 80 to 200 A · m ² / kg (80 to 200 emu / g), 100 to 180 A · m ² / kg (100 to 180 emu / g) is more preferable.

The average particle size of the ferromagnetic iron-based metal magnetic powder and the iron nitride magnetic powder is preferably 10 to 100 nm, and more preferably 15 to 80 nm. This range is preferable because when the average particle size is less than 10 nm, the coercive force is reduced or the surface energy of the particles is increased, which makes it difficult to disperse in the paint. This is because the particle noise based on the particle size becomes large when the particle size is larger than 100 nm. BET specific surface area of the ferromagnetic powder is preferably at least 35m ² / g, more preferably at least 40 m ² / g, most preferably at least 50 m ² / g. Usually, it is 100 m ² / g or less.

  The ferromagnetic iron metal temporal powder and the iron nitride magnetic powder may be used after being surface-treated with Al, Si, P, Y, Zr or an oxide thereof.

The coercive force of the hexagonal Ba-ferrite magnetic powder is preferably 120 to 320 kA / m, and the saturation magnetization is preferably 40 to 70 A · m ² / kg (40 to 70 emu / g). The particle size (size in the plate surface direction) of the hexagonal Ba-ferrite magnetic powder is preferably 10 to 50 nm, more preferably 10 to 30 nm, and still more preferably 10 to 20 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) of the hexagonal Ba-ferrite magnetic powder is preferably 2 to 10, more preferably 2 to 5, and still more preferably 2 to 4. The BET specific surface area of the hexagonal Ba-ferrite magnetic powder is preferably 1 to 100 m ² / g.

  Note that the magnetic properties of these ferromagnetic powders all refer to values measured with an external magnetic field of 1273.3 kA / m (16 kOe) using a sample vibration 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.

  In the magnetic recording medium of the present invention, it is preferable not to contain nonmagnetic powder in the magnetic layer, but a conventionally known abrasive may be added if necessary. 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 higher is used alone or in combination. As the particle size of the abrasive, it is usually preferable that the average particle diameter is 10 to 150 nm in a thin magnetic layer having a thickness of 0.01 to 0.09 μm.

  Furthermore, if necessary, conventionally known carbon black may be added for the purpose of improving conductivity and improving surface lubricity. As carbon black, acetylene black, furnace black, thermal black, etc. can be used. In this case, the average particle size of carbon black is preferably 10 to 100 nm. This range is preferable because when the average particle size is 10 nm or less, it is difficult to disperse the carbon black. When the average particle size is 100 nm or more, it is necessary to add a large amount of carbon black. In any case, the surface becomes rough and the output decreases. This is because it causes. Moreover, you may use two or more types of carbon black of a different average particle diameter as needed.

<Undercoat layer>
When the thickness of the magnetic layer is 0.5 μm or less in order to improve the short-wavelength recording characteristics of the magnetic recording medium, the nonmagnetic support and the magnetic layer can be used to reduce the unevenness of the magnetic layer and improve the durability. It is preferable to provide a nonmagnetic undercoat layer between the layers.

  The thickness of the undercoat layer is preferably 0.2 μm or more and 1.5 μm or less, more preferably 1.0 μm or less, and even more preferably 0.8 μm or less. If the thickness of the undercoat layer is less than 0.2 μm, the effect of reducing the uneven thickness of the magnetic layer and the effect of improving the durability are small, and if the thickness exceeds 1.5 μm, the total thickness of the magnetic tape becomes too thick, and the per tape roll The recording capacity is reduced.

  Nonmagnetic powder is added to the undercoat layer. Examples of the nonmagnetic powder to be added 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 200 nm. preferable. Non-magnetic iron oxide powder is mainly used, and carbon black having a particle size of 0.01 to 0.1 μm and aluminum oxide having a particle size of 0.05 to 0.5 μm are often supplemented as necessary. When the undercoat layer is applied and formed, in order to smooth the surface and reduce thickness unevenness, the nonmagnetic particles and carbon black preferably have a sharp particle size distribution.

  It is preferable to add nonmagnetic plate-like powder having an average particle diameter of 10 to 100 nm to the undercoat layer. As the component of the non-magnetic plate-like powder, rare earth elements such as cerium, oxides or complex oxides of elements such as zirconium, silicon, titanium, manganese, and iron are 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 non-magnetic plate-like powder, film thickness uniformity, surface smoothness, rigidity, and dimensional stability are improved.

  The binder resin used for the undercoat layer can be the same as that used in the magnetic layer.

<lubricant>
In the magnetic layer and the undercoat layer, 0.5 to 3.0% by weight of fatty acid amide and 0.5 to 5.0% by weight of the total powder contained in the magnetic layer and the undercoat layer, respectively. It is preferable to contain 0.2 to 3.0% by weight of higher fatty acid esters of fatty acids. Fatty acid amides in the above range are preferable when the amount is less than 0.5% by weight, and direct contact at the head / magnetic layer interface is likely to occur, and the effect of preventing seizure is small. This is because defects such as out may occur. The addition of higher fatty acids within the above range is preferable when the amount is less than 0.5% by weight, and the effect of reducing the friction coefficient is small. When the amount exceeds 5.0% by weight, the undercoat layer may be plasticized and the toughness may be lost. Because. Addition of higher fatty acid esters within the above range is preferable when the content is less than 0.2% by weight, since the effect of reducing the friction coefficient is small, and when the content exceeds 3.0% by weight, the amount transferred into the magnetic layer is too large. This is because there is a risk of causing side effects such as sticking. 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 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. Note that this does not exclude the mutual movement of the lubricant in the magnetic layer and the lubricant in the undercoat layer. If the lubricant is contained in the undercoat layer, the magnetic layer may not contain a 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.

  Moreover, you may supply the lubrication agent used for a magnetic layer and undercoat from the top of a carbon layer by application | coating as needed.

<Dispersant>
The non-magnetic powder, carbon black, and magnetic powder contained in the undercoat layer and magnetic layer are, for example, caprylic acid, capric acid, lauric acid, myristic acid, palmitic acid, stearic acid, behenic acid, oleic acid. Fatty acids having 12 to 18 carbon atoms such as elaidic acid, linoleic acid, linolenic acid, stearolic acid (RCOOH: R is an alkyl group or alkenyl group having 11 to 17 carbon atoms), alkali metals or alkalis of the fatty acids Metal soap made of earth metal, compound containing fluorine of fatty acid ester, amide of fatty acid, polyalkylene oxide alkyl phosphate ester, lecithin, trialkylpolyolefinoxy quaternary ammonium salt (alkyl has 1 to 5 carbon atoms) Olefin, ethylene, propylene, etc.), sulfate, sulfone Salts, phosphate salts, and may be surface treated with a conventionally known dispersant such as copper phthalocyanine may be performed paint manufacturing process with a dispersant. 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.

<Back coat layer>
A back coat layer is provided on the other surface (the surface opposite to the surface on which the magnetic layer is formed) of the non-magnetic support constituting the magnetic recording medium of the present invention for the purpose of improving running performance. Can do. The backcoat layer usually contains a nonmagnetic powder mainly composed of carbon black and a binder resin.

  The thickness of the back coat layer is preferably 0.2 to 0.8 μm. This range is good because if the thickness is less than 0.2 μm, the effect of improving running performance is insufficient, and if it exceeds 0.8 μm, the total thickness of the tape becomes thick and the recording capacity per roll becomes small. 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 black, those having an average particle size of 5 to 200 nm are used, and those having an average particle size of 10 to 100 nm are more preferable. This range is more preferable if the average particle size is less than 10 nm, it is difficult to disperse the carbon black, and if the average particle size is 100 nm or more, it is necessary to add a large amount of carbon black, and in either case the surface is rough. This is because it causes the back-off (embossing) of the magnetic layer. When a large particle size carbon black having an average particle size of 200 to 400 nm is used as the large particle size carbon black at a ratio of 5 to 15% by weight of the small particle size carbon black, the surface is not roughened, and the effect of improving running performance is great. Become. 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. The center line average surface roughness Ra of the backcoat layer is preferably 3 to 8 nm, and more preferably 4 to 7 nm. Since the magnetic signal of the magnetic recording layer may be disturbed if the backcoat layer is magnetic, the backcoat layer is usually nonmagnetic.

  A nonmagnetic plate-like powder having an average particle size 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 whose average particle diameter is 0.1-0.6 micrometer as needed. The addition amount is preferably 2 to 40% by weight, more preferably 5 to 30% by weight, based on the weight of the total inorganic powder in the backcoat layer. It is preferable to add alumina having an average particle size of 0.1 μm to 0.6 μm because durability is further improved.

  For the back coat layer, the same resin as that used for the magnetic layer and the undercoat layer described above can be used as the binder resin. Among these, in order to reduce the coefficient of friction and improve the runnability, cellulose resin and polyurethane are used. It is preferable to use in combination with a resin. The content of the binder resin is usually 40 to 150 parts by weight, preferably 50 to 120 parts by weight, more preferably 60 to 110 parts by weight with respect to 100 parts by weight of the total amount of the carbon black and the inorganic nonmagnetic powder. Parts, more preferably 70 to 110 parts by weight. The above range is preferable because if the amount is less than 50 parts by weight, the strength of the backcoat layer is insufficient, and if it exceeds 120 parts by weight, the friction coefficient tends to increase. It is preferable to use 30 to 70 parts by weight of cellulose resin and 20 to 50 parts by weight of polyurethane resin. Furthermore, it is preferable to use a crosslinking agent such as a polyisocyanate compound in order to cure the binder resin.

  For the backcoat layer, a crosslinking agent similar to the crosslinking agent used for the magnetic layer and the undercoat layer described above can be used. The amount of the crosslinking agent is usually 10 to 50 parts by weight, preferably 10 to 35 parts by weight, and more preferably 10 to 30 parts by weight with respect to 100 parts by weight of the binder resin. The above range is preferable because if less than 10 parts by weight, the coating strength of the backcoat layer tends to be weak, and if it exceeds 35 parts by weight, the dynamic friction coefficient against SUS increases.

  In the backcoat layer, an electron beam curable resin similar to the magnetic layer or the undercoat layer can be used as a crosslinking agent in order to crosslink and cure the coating film. As the electron beam curable resin to be combined, a resin excellent in curability is particularly preferable, and an electron beam curable resin having a weight average molecular weight per double bond of 50 to 300 is preferable. The electron beam curable resin having a weight average molecular weight of 50 to 300 per double bond is more preferably a monomer type. The amount of the electron beam curable resin is usually used in a ratio of 5 to 30 parts by weight, preferably 7 to 25 parts by weight, and more preferably 10 to 20 parts by weight with respect to 100 parts by weight of the binder resin. . The above range is preferable, if less than 7 parts by weight, the coating strength of the backcoat layer tends to be weak, and if it exceeds 25 parts by weight, the coefficient of dynamic friction with respect to the guide rollers and guide pins provided in the running system of the tape drive increases. Because.

  As the binder resin in the back coat layer, a combination of two or more kinds of electron beam curable resins can be used. In this case, it is preferable to combine the polymer type and the monomer type. The polymer type preferably has a weight average molecular weight of 10,000 to 100,000, and the monomer type preferably has a weight average molecular weight of 100 to 2,000. More preferably, the monomer type electron beam curable resin has a weight average molecular weight of 50 to 300 per double bond.

  In the present invention, since the carbon layer is provided on the magnetic layer, the lubricant component supplied from the magnetic layer and the undercoat layer may be difficult to be supplied to the surface on the magnetic layer side. 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. Specifically, 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% by weight of higher fatty acids.

  Further, a carbon layer similar to the carbon layer provided on the magnetic layer may be provided instead of the back coat layer as described above. In this case, the carbon layer can be provided on the other surface (the surface opposite to the surface on which the magnetic layer is provided) of the nonmagnetic support in the same manner as the carbon layer provided on the magnetic layer.

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

<Manufacturing process>
As a method for forming the magnetic recording medium of the present invention, a method of applying a magnetic layer on a nonmagnetic support using an extrusion coater, a slide coater or a curtain coater is preferable. When an undercoat layer is provided between the magnetic layer and the nonmagnetic support, it is preferable to apply the undercoat layer and the magnetic layer simultaneously using an extrusion coater, a slide coater, or a curtain coater. After applying an undercoat layer on a non-magnetic support using a gravure roll coater, an extrusion type coater, a slide type coater or a curtain type coater, the magnetic layer is applied after undried or dried, or after smoothing after drying. You may apply using an extrusion type coater, a slide type coater, or a curtain type coater. It is preferable to apply a primer layer and a magnetic layer on a non-magnetic support, dry, and smooth the surface with a calendar, and then form the carbon layer by the above-mentioned conventionally known method. If not, the undercoat layer and the magnetic layer may be coated on the nonmagnetic support, and the carbon layer of the uppermost nonmagnetic layer may be formed after drying, followed by smoothing with a calendar. After forming the nonmagnetic carbon layer, the lubricant may be applied using an extrusion type coater, a slide type coater, a curtain type coater or even a spray type coater. After forming the carbon layer, the magnetic recording medium is The back coat layer is applied to the opposite side of the nonmagnetic support to be formed using a gravure roll coater, extrusion coater, slide coater or curtain coater, dried, smoothed with a calendar, and then wound up. Thus, the lubricant may be transferred to the magnetic layer side from the backcoat layer.

[Example]
Examples of the present invention will be described below. Unless otherwise specified, “parts” in the following Examples and Comparative Examples means “parts by weight”, and “average particle diameter” means “number average particle diameter”.

<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 (Co—Fe—Al—Y) 100 parts (Co / Fe: 24 at%,
Al / (Fe + Co): 4.7 wt%
Y / (Fe + Co): 7.9 at%
σs: 119 A · m ² / kg (119 emu / g),
Hc: 181.4 kA / m (2280 Oe),
Average particle diameter: 60 nm, axial ratio: 5)
-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)
-Methyl acid phosphate (MAP) 2 parts-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) Blending process ・ Polyisocyanate 1.5 parts ・ Methyl ethyl ketone / cyclohexanone (MEK / A) 29 parts

  In the above primer coating component, after kneading (1) with a batch kneader, (2) is added and stirred, and then dispersion treatment is performed with a sand mill for a residence time of 60 minutes. After the filtration, an undercoat paint (undercoat layer paint) was obtained.

  Apart from this, in the components of the magnetic paint described above, (1) the mixing process component is previously mixed at high speed, the mixed powder is kneaded with a continuous biaxial kneader, and (2) the dilution process component is further mixed. In addition, the mixture was diluted in at least two stages with a continuous twin-screw kneader, dispersed in a sand mill with a residence time of 45 minutes, and (3) the blending process components were added thereto, followed by stirring and filtration to obtain a magnetic paint.

  On the nonmagnetic support (base film) made of an aromatic polyamide film (thickness 3.9 μm, MD = 11 GPa, MD / TD = 0.7, trade name: Miktron, Toray Industries, Inc.) The undercoat layer after drying and calendering is applied so that the thickness of the undercoat layer becomes 0.9 μm, and the magnetic coating is further applied onto the coating layer (undercoat layer) after the magnetic field orientation treatment, drying and calendering treatment. The magnetic layer was coated with a wet-on-wet with an extrusion coater so that the thickness of the magnetic layer was 0.04 μm, and after magnetic field orientation treatment, it was dried using a dryer and far infrared rays to obtain a magnetic sheet.

《Paint component for back coat layer》
Carbon black (average particle size: 25 nm) 80 parts Carbon black (average particle size: 350 nm) 10 parts Nonmagnetic plate-like iron oxide powder (average particle size: 50 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

  After dispersing the coating component for the backcoat layer with a sand mill with a residence time of 45 minutes, after adding 15 parts of polyisocyanate to adjust the coating for the backcoat layer and filtering, on the opposite side of the magnetic layer of the magnetic sheet prepared above, It was applied and dried so that the thickness after drying and calendering was 0.5 μm.

  The magnetic sheet thus obtained was mirror-finished under the conditions of a temperature of 100 ° C. and a linear pressure of 196 kN / m with a seven-stage calendar made of a metal roll. Time-aging was performed to obtain a magnetic sheet with a backcoat layer.

《Carbon layer》
The carbon layer provided on the magnetic layer was produced by the FCVA method. As a means for producing the carbon layer, an FCVA apparatus including a conveyance unit, a film formation chamber, a filter unit, and a discharge chamber for running a magnetic sheet web with a backcoat layer was used.

The degree of vacuum in the film formation chamber was set to 10 ⁻² Pa, the bias voltage was 400 V, the processing time was 10 seconds, and a carbon layer having a sp ³ bond ratio of 65% and a thickness of 5 nm was formed on the magnetic layer. The obtained magnetic sheet with a carbon layer was cut into a width of 1/2 inch (about 12.65 mm) by a slit machine.

  A slit machine (an apparatus that cuts the magnetic tape original into a magnetic tape having a predetermined width) was used with the components improved as described below. 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.

  The magnetic tape obtained as described above was incorporated into a cartridge to produce a computer tape (magnetic tape for computer data backup).

A carbon layer having a bias voltage of 400 V, a treatment time of 22 seconds and a sp ³ bond ratio of 65% and a thickness of 10 nm was produced under the carbon layer production conditions of Example 1. Otherwise, the computer tape of Example 2 was produced in the same manner as in Example 1.

A carbon layer having a bias voltage of 400 V, a processing time of 5 seconds, and a sp ³ bond ratio of 65% and a thickness of 2 nm was produced under the carbon layer production conditions of Example 1. Other than that was carried out similarly to Example 1, and produced the computer tape of Example 3. FIG.

A carbon layer having a bias voltage of 100 V, a processing time of 10 seconds, and a sp ³ bond ratio of 85% and a thickness of 5 nm was produced under the carbon layer production conditions of Example 1. Other than that was carried out similarly to Example 1, and produced the computer tape of Example 4. FIG.

A carbon layer having a bias voltage of 2000 V, a processing time of 22 seconds, a sp ³ bond ratio of 25%, and a thickness of 10 nm was produced under the carbon layer production conditions of Example 1. Other than that was carried out similarly to Example 1, and produced the computer tape of Example 5. FIG.

A carbon layer having a bias voltage of 3000 V, a processing time of 22 seconds, a sp ³ bond ratio of 23%, and a thickness of 10 nm was produced under the carbon layer production conditions of Example 1. Otherwise, the computer tape of Example 6 was made in the same manner as Example 1.

As a method for producing the carbon layer, a microwave frequency of 2.45 GHz, an input power of 3 kW, a vacuum degree of a transfer chamber of 3 × 10 ⁻⁵ Torr, a vacuum degree of a film forming chamber of 0.02 Torr, and methane as an introduced gas are used. A carbon layer having a sp ³ bond ratio of 20% and a thickness of 10 nm was formed on the magnetic layer under the conditions of a volume ratio of hydrogen and oxygen of 1: 2: 0.2, a bias voltage of 200 V, and a treatment time of 15 seconds. . Other than that was carried out similarly to Example 1, and produced the computer tape of Example 7. FIG.

  Example 1 except that 0.8 parts of palmitic acid amide (PA), 0.8 parts of n-butyl stearate (SB), and 1.0 part of stearic acid (SA) were added to the paint for the backcoat layer. In the same manner, a computer tape of Example 8 was produced.

  After forming a carbon layer on the magnetic layer by the FCVA apparatus, a lubricant comprising 0.8 parts of n-butyl stearate (SB), 1.0 part of stearic acid (SA), and 200 parts of methyl ethyl ketone is formed on the carbon layer. The solution was applied by a gravure coating method and dried. Otherwise, the computer tape of Example 9 was made in the same manner as Example 1.

[Comparative Example 1]
In the magnetic coating material of Example 1, 8 parts of alumina powder (average particle size: 80 nm) was added to the components of the kneading step to form a magnetic layer, but no carbon layer was formed. Except for these points, the computer tape of Comparative Example 1 was produced in the same manner as in Example 1.

[Comparative Example 2]
A carbon layer having a bias voltage of 400 V, a treatment time of 28 seconds, and a sp ³ bond ratio of 65% and a thickness of 12 nm was produced under the carbon layer production conditions of Example 1. Other than that was carried out similarly to Example 1, and produced the computer tape of the comparative example 2. FIG.

[Comparative Example 3]
A carbon layer having a bias voltage of 400 V, a treatment time of 3 seconds, and a sp ³ bond ratio of 65% and a thickness of 1 nm was produced under the carbon layer production conditions of Example 1. Other than that was carried out similarly to Example 1, and produced the computer tape of the comparative example 3. FIG.

[Comparative Example 4]
A computer tape of Comparative Example 4 was produced in the same manner as in Example 1 except that no carbon layer was produced in Example 1.

  Each of the obtained computer tapes was subjected to the following measurements to evaluate the characteristics.

<C / N measurement>
A drum tester was used for measuring the electromagnetic conversion characteristics (C / N measurement) of the tape. The drum tester was equipped with an electromagnetic induction head (track width 25 μm, gap 0.2 μm) and MR head (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.

  For the output and noise, a rectangular wave was input to the 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 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 both was taken as C / N, and the relative value with the value of the tape of Comparative Example 1 was determined.

<Still durability>
The still durability indicating the durability on the magnetic layer side in the computer tape was similarly evaluated using a drum tester. The tape is mounted as described above, a 0.9 μm carrier signal is written by the same writing method, and the output is continuously measured with both heads being applied. The time after which the initial output value dropped to 80% was defined as the still life.

<Coefficient of friction>
The friction coefficient on the magnetic layer side of the obtained computer tape was measured according to ECMA-320. That is, a sample tape is put on a round bar of 25.4 mm in diameter of calcium titanate ceramic (surface roughness Ra = 0.05 μm) so that the magnetic layer side is in contact, a load of 0.64 N is applied to one end, and a stress sensor is applied to the other end. The friction coefficient γ was obtained from the following equation, with F2N as the stress generated in the sensor when pulled at a speed of 1 mm / sec in the horizontal direction.
γ = (2 / π) ln (F2 / 0.64)

<Percentage of sp ³ bonds in carbon layer>
The sp ³ bond ratio of the carbon layer is determined by attaching an EELS (Electron Energy Loss Spectroscopy) detector to Auger Electron Spectroscopy (AES), irradiating 300 eV low-energy electrons, and inelastic scattering. The measured electron energy distribution was measured and obtained from the peak height appearing at 280 to 288 eV. A peak height of graphite C-C bonds is made of 100% sp ² bond, C-C bonds with reference to the peak height of diamond consisting of 100% sp ³ bonds, C-C bonds of sp ³ carbon layer The percentage of binding was determined.

  Tables 1 and 2 show the evaluation results of each computer tape. In these tables, the carbon layer preparation conditions (bias voltage / treatment time) in each example and comparative example, the presence / absence of nonmagnetic powder in the magnetic layer, and the like are also described. In these tables, “CB layer” indicates a carbon layer, and “BC layer” indicates a backcoat layer.

  As is apparent from Tables 1 and 2, each of the computer tapes of Examples 1 to 9 does not contain nonmagnetic powder in the magnetic layer, and therefore each of Comparative Examples 1 to 4 that does not satisfy the requirements of claim 1. Compared to computer tape, the C / N is good and the carbon layer is the uppermost layer, so the durability is high. Further, each of the computer tapes of Examples 1 to 5, 8, and 9 satisfying the requirement of claim 2 was compared with each of the computer tapes of Examples 6 and 7 that did not satisfy the requirement of claim 2. It can be seen that the durability is higher because of the large proportion of SP bonds. Further, in computer tapes (Example 8) in which a lubricant is contained in the backcoat layer and computer tapes (Example 9) in which the lubricant is applied to the carbon layer and dried, these layers are lubricated. It can be seen that the still durability is slightly higher and the coefficient of friction is slightly lower than the computer tape of Example 1 having the same configuration except that it is not contained or coated.

Claims (3)

A magnetic recording medium having a magnetic layer made of a coating containing magnetic powder and a binder resin on one surface of a nonmagnetic support, and having a backcoat layer on the other surface,
A magnetic recording medium, wherein a carbon layer having a thickness of 2 nm to 10 nm is provided on the magnetic layer. The magnetic recording medium according to claim 1, wherein a sp ³ bond ratio in a carbon layer in the carbon layer is 25 to 85%. A method for producing a magnetic recording medium having a magnetic layer comprising a coating film containing magnetic powder and a binder resin on one surface of a nonmagnetic support, and having a backcoat layer on the other surface. ,
Applying a magnetic paint containing magnetic powder and a binder resin to one surface of the non-magnetic support;
Drying the magnetic layer formed by this coating;
A step of smoothing the surface of the magnetic layer after this drying;
And a step of forming a carbon layer having a thickness of 2 nm to 10 nm on the surface of the smoothed magnetic layer.