MAGNETIC RECORDING MEDIUM

Abstract: A magnctic recording medium comprising a non-­ magnetic support having provided thereon a magnetic layer mainly composed of needle-like iron oxide magnetic powder and a binder, the magnetic powder being fine powder having specific surface area of not less than 35 m²/g (BET method) improving S/N. To improve dispersibility of the magnetic powder to thereby improve durability and squareness a resin having a molecular weight of not more than 50,000 and having at least one polar group selected from the group consisting of (I), (wherein M represents H, Li, Na or K, and M₁, and M₂ each represent H, Li, Na, K or an alkyl group) is incorporated in the binder.

Description: This invention relates to magnetic recording media. With the rapid advance in the field of magnetic recording, further improvements in magnetic recording media are required. High reproduction output is required in short-wave recording, especially in ε magnetic recording medium for a video tape recorder (VTR). In view of this, finely divided magnetic particles are used to improve the signal-to-noise (S/N) ratio, and to make the surface of the magnetic layer smoother. However, when the magnetic powder includes such fine particles, the coating viscosity increases, so there is an additional problem during manufacture of degraded dispersibility of the magnetic powder. The increased viscosity of the magnetic coating also causes other problems. For example, in the coating transfer step during the manufacture of the magnetic recording medium, the transfer of the coating cannot be smoothly performed. This tends to cause local retention of the coating in the transport path, and to result in hardening of the coating. This may also lead to drop-out when the coating is used to form a magnetic layer. Such a coating is difficult to smooth after being transferred onto a non-magnetic support. As a result, good surface properties cannot be obtained. Furthermore, an increase in the viscosity of the coating makes it difficult to form a thin layer. When the magnetic powder is finely divided, that is, when the specific surface area of the powder is increased, the contact interface between the magnetic powder and a coating or a binder increases. This means that increased energy is required for dispersing the magnetic powder. This problem can be partially resolved by using an improved mixing/dispersing unit. However, the degree of improvement that is possible is limited. In view of this, a surfactant has been used to improve the dispersibility. However, with this method, when the amount of the surfactant added is increased to improve the dispersibility, the properties of the layer tend to be impaired. Japanese patent specification JP-A-57 092 422 discloses the use of a fine oxide powder in a magnetic recording medium, and UK patent specification GB-A-2 054 622 discloses a binder for a magnetic recording medium comprising a polyester resin having -SO₃Na groups. According to the present invention there is provided a magnetic recording medium having on a non-magnetic support a magnetic recording laymer comprising an acicular iron oxide magnetic powder and a binder; characterised in that: said iron oxide magnetic powder has a specific surface area as determined by the BET method, of not less than 35 m²/g; said binder contains not less than 30% of a resin with a single polar group having the general formula: where M is hydrogen, lithium, sodium or potassium; and M₁ and M₂ are hydrogen, lithium, sodium, potassium or alkyl groups; and said resin has a molecular weight of 2,000 to 50,000; the remaining resin binder constituent being nitrocellulose, a vinyl chloride-vinyl acetate copolymer, vinyl chloride-vinyl acetatevinyl alcohol copolymer, vinyl chloride-vinyl propionate copolymer, vinylidene chloride-vinyl chloride copolymer, vinylidene chloride-acrylonitrile copolymer, acrylonitrile-butadiene copolymer, acetal resin, butyral resin, formal resin, polyester resin, polyurethane resin, polyamide resin, epoxy resin, phenoxy resin, or mixtures thereof. The invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a graph showing the relationship between the molecular weight of a polyurethane resin and the coating viscosity of an embodiment of magnetic recording medium according to the present invention, using the specific surface area of the magnetic powder as a parameter; and Figure 2 is a graph showing the relationship between the specific surface area of the magnetic powder and the squareness ratio of the magnetic layer. An iron oxide-based magnetic powder to be used herein may be y-Fe₂O₃, a weakly reduced y-Fe₂O₃ An iron oxide-based magnetic powder to be used herein may be y-Fe₂O₃, a weakly reduced y-Fe₂O₃ (a mixed crystal of y-Fe₂O₃ and Fe₃O₄, or a Co-coated iron oxide in which a Co compound is deposited on the surface of such an iron oxide magnetic powder. The y-Fe₂O₃ and the weakly reduced y-Fe₂O₃ can be prepared by various methods. However, the following method is in general employed. A water-containing goethite or lepidocrocite as a starting material is reduced into a magnetite, which is oxidized to provide a desired, ferromagnetic ypFe₂O₃ or weakly reduced y-Fe₂O₃ of the acicular crystal-type. The Co-coated iron oxide is prepared by suspending in water the y-Fe₂O₃ or weakly reduced y-Fe₂O₃ thus prepared, adding to the suspension a Co compound such as cobalt chloride, and an excess amount of an alkali such as NaOH so as to deposit the Co compound onto the surfaces of the y-Fe₂O₃ particles, recovering the particles from the suspension water, and, if required, heat-treating the particles at a relatively low temperature of about 120°C. The specific surface area of the oxide iron magnetic powders as measured according to the BET method is selected to be not less than 35 m²/g. This is because, when the specific surface area is less then 35 m²/g, a satisfactory improvement in the S/N ratio cannot be expected. Ferromagnetic particles having a specific surface area of not less than 35 m²/g can be obtained by properly selecting the size or specific surface area of the water-containing oxide iron particles as the starting material. In order to improve the dispersibility of the magnetic powder, a resin is used which has at least one polar group with the general formula: where M is hydrogen, lithium, sodium or potassium; and M₁ and M₂ are hydrogen, lithium, sodium, potassium or alkyl groups. Resins having different polar groups can be used in admixture. In the general formulae above, the alkyl groups are preferably saturated straight or branched chain aliphatic hydrocarbon residues to 23 carbon atoms. Examples of such alkyl groups include a tricosyl group, docosyl group, eicosyl group, nonadecyl group, octadecyl group, hexadecyl group, tridecyl group, undecyl group, decyl group, nonyl group, octyl group, hexyl group, pentyl group, butyl group, t-butyl group, propyl group, isopropyl group, ethyl group, and methyl group. Note that M₁ and M₂ may be the same or different. The resin having a polar group as described above can be prepared by introducing a polar group into a polyester resin, a polyurethane resin or a vinyl chloride-based resin. Various methods may be used to introduce the polar group into such a resin. For example, in the case of the polyester resin, there may be adopted a method wherein a dicarboxylic acid having a polar group is used as part of a dicarboxylic acid component to be used as a starting material of the polyester, and such a dicarboxylic acid having a polar group and a dicarboxylic acid without a polar group are subjected to the condensation reaction with a diol. In the case of the polyurethane resin, there may be employed a method wherein these three types of polyester resin starting materials and a diisocyanate are subjected to the condensation reaction and the addition reaction. When a polyester resin, a polyurethane resin and a vinyl chloride-based resin are modified, there may be adopted a method wherein a polar group is introduced by dehydrochlorination reaction between the active hydrogen group contained in these resins and a chlorine-containing compound such as Examples of dicarboxylic acid components which have no polar group for polyester resins which may be used herein include aromatic dicarboxylic acids such as terephthalic acid, isophthalic acid, orthophthalic acid or 1,5-naphthalic acid; aromatic hydroxycarboxylic acids such as p-hydroxybenzoic acid or p-(hydroxyethoxy) benzoic acid; aliphatic dicarboxylic acids such as succinic acid, adipic acid, azelaic acid, sebacic acid or dodecane dicarboxylic acid; or tri- or tetracarboxylic acids such as trimellitic acid, trimesic acid or pyromellitic acid. Terephthalic acid, isophthalic acid, adipic acid, and sebacic acid are particularly preferable. Dicarboxylic acid components having metal sulfonate groups may include 5-sodium sulfoisophthalate, 2-sodium sulfoterephthalate, or 2-potassium sulfoterephthalate. Diol components which may be used for preparing a polyester resin may, for example, be ethylene glycol, propylene glycol, 1,3-propanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, neopentyl glycol, diethylene glycol, dipropylene glycol, 2,2,4-trimethyl-1,3-pentanediol, 1,4-cyclohexanedimethanol, an ethylene oxide adduct of bisphenol A, an ethylene oxide adduct and propylene oxide adduct of hydrogenated bisphenol A, polyethylene glycol, polypropylene glycol or polytetramethylene glycol. Additionally, tri-or tetraols such as trimethylolethane, trimethylolpropane, glycerin or pentaerythritol may be used. Diisocyanate components for a polyurethane resin which may be used herein may be 2,4-tolylenediisocyanate, 2,6-tolylenediisocyanate, p-phenylenediisocyanate, diphenylmethanediisocyanate, m-phenylenediisocyanate, hexamethylenediisocyanate, tetramethylenediisocyanate, 3,3′-dimethoxy-4,4′-biphenylenediisocyanate, 2,4-naphthalenediisocyanate, 3,3′-dimethyl-4,4′-biphenylenediisocyanate, 4,4′-diphenylenediisocyanate, 4,4′-diisocyanate-diphenyl ether, 1,3-naphthalenediisocyanate, p-xylylenediisocyanate, m-xylylenediisocyanate, 1,3-diisocyanatemethylcyclohexane, 1,4-diisocyanatemethylcyclohexane, 4,4′-diisocyanatedicyclohexylmethane or isophoronediisocyanate. When a metal sulfonate group is introduced by way of the modification of vinyl chloride-based resins, a vinyl chloride-based resins may be a vinyl chloride-vinyl acetate-vinyl alcohol copolymer, vinyl chloride-vinyl propionate-vinyl alcohol copolymer, vinyl chloride-vinyl acetate-vinyl maleate-vinyl alcohol copolymer or vinyl chloride-vinyl propionate-vinyl maleate-vinyl alcohol copolymer. The OH group of the vinyl alcohol contained in such a vinyl chloride-based resin is reacted with chlorine of a metal sulfonate containing chlorine such as Cl-CH₂CH₂SO₃M or Cl-SO₃M in an organic polar solvent such as dimethylformamide or dimethylsulfoxide in the presence of dehydrochlorinating agents such as amines, for example, pyridine, picoline or triethylamine, or ethylene oxide or propylene oxide. The resin having a polar group as described above is contained in the binder in an amount of not less than 30% by weight on the total binder content when the amount of the resin is less than this value, a satisfactory dispersion effect cannot be obtained. A binder to be used in combination with such a resin is defined above. The resin has a molecular weight up to 50,000. If the molecular weight of the resin exceeds 50,000, when the specific surface area as determined by the BET method, of a magnetic powder as shown ir Figure 1 becomes large, the viscosity of the coating becomes too high, exceeding an acceptable value. The resin has a molecular weight of not less than 2,000. if the molecular weight of the resin is less than 2,000, when the magnetic coating is applied on a support and is hardened with a hardener, an unreacted portion is produced, providing a low molecular weight, residual component. This will degrade the physical properties of the coating layer. The molecular weight of the resin is particularly preferably not less than 10,000. To a magnetic layer may further be added, for example, aluminium oxide, chromium oxide or silicon oxide as a reinforcing agent; olive oil as a lubricant; carbon black as an antistatic agent; and lecithin as a dispersant. The magnetic layer is generally formed by dissolving components of the magnetic layer in an organic solvent to prepare a magnetic coating, and then by applying the magnetic coating on a non-magnetic support. Suitable organic solvents which may be used herein are, for example, ketones (such as acetone, methyl ethyl, methyl isobutyl ketone or cyclohexanone), alcohols (such as methanol, ethanol, propanol or butanol), esters (such as methyl acetate, ethyl acetate, butyl acetate, ethyl lactate or glycol acetate), monoethyl ether, glycol ethers (such as ethylene glycol dimethyl ether, ethylene glycol monoethyl ether or dioxane), aromatic hydrocarbons (such as benzene, toluene or xylene), aliphatic hydrocarbons (such as hexane or heptane), or nitropropane. Non-magnetic supports on which a magnetic coating is applied may be materials such as polyesters (such as polyethylene terephthalate), polyoletins (such as a polypropylene), cellulose derivatives (such as cellulose triacetate or cellulose diacetate), polycarbonate, polyvinyl chloride polyimide, polyamide, polyhydrazide, metals (such as aluminium or copper) or paper. The invention will now be described by way of the following examples. The composition described above was mixed and dispersed in a ball mill for 48 hours to prepare a magnetic coating. Thereafter, an isocyanate compound ("Desmodur L": manufactured by Bayer Co., Ltd.) was added to the mixture in an amount of 5 parts by weight. After the mixture had been applied on a polyester film having a thickness of 15 µm to a dried thickness of about 6 µm, the coating film was subjected to a surface treatment by calendering. The film was cut into a width of 12.7 mm (1/2 inch) to prepare a magnetic tape. Examples 2 to 7 and Comparative Example Magnetic tapes were prepared using the compositions described below for Examples 2 to 7 and Comparative Example. In the respective compositions, since the types and amounts of the antistatic agent, abrasive, lubricant and solvent were the same as those of Example 1, they are not shown in the following composition. The table below shows the measurements of the various properties of the magnetic tapes obtained in Examples 1 to 6 and Comparative Examples 1 and 2. The viscosity of the coating was measured at 30 rotations with a No. 4 rotor using a Brookfield rotational viscometer. The rubbing-off rating is the rubbing-off onto a drive system (guide roll or the like) after running the tape, which is evaluated in accordance with the low-point system of scoring (-5(poor)-0 (good)). The Y-S/N represents the S/N ratio which is obtained when a monochrome image is recorded on a sample and reproduced therefrom. The durability is the time taken for an output signal to decrease to a predetermined level when the same position on a magnetic tape is scanned with a home VTR. Figure 1 shows the relationship between the molecular weight and the viscosity of the coating when the molecular weight of a polyurethane resin containing a sodium sulfonate group is varied in Example 6, taking a specific surface area of the magnetic powder (75 m²/g: curve A; 60 m²/g: curve B; 45 m²/g: curve C) as a parameter. Figure 2 similarly shows the relationship between the specific surface area and the squareness ratio of the magnetic tape when the specific surface area of the Co-coated iron oxide magnetic powder is varied in Example 6. It is apparent from the table above that embodiments of the present invention provide a magnetic recording medium which has good squareness ratio, durability, and S/N ratio.

Claims: A magnetic recording medium having on a non-magnetic support a magnetic recording laymer comprising an acicular iron oxide magnetic powder and a binder; characterised in that: said iron oxide magnetic powder has a specific surface area as determined by the BET method, of not less than 35 m²/g; said binder contains not less than 30% of a resin with a single polar group having the general formula: where M is hydrogen, lithium, sodium or potassium; and M₁ and M₂ are hydrogen, lithium, sodium, potassium or alkyl groups; and said resin has a molecular weight of 2,000 to 50,000; the remaining resin binder constituent being nitrocellulose, a vinyl chloride-vinyl acetate copolymer, vinyl chloride-vinyl acetate-vinyl alcohol copolymer, vinyl chloride-vinyl propionate copolymer, vinylidene chloride-vinyl chloride copolymer, vinylidene chloride-acrylonitrile copolymer, acrylonitrile-butadiene copolymer, acetal resin, butyral resin, formal resin, polyester resin, polyurethane resin, polyamide resin, epoxy resin, phenoxy resin, or mixtures thereof. A magnetic recording medium according to Claim 1 wherein the alkyl group has up to 23 carbon atoms. A magnetic recording medium according to Claim 1 wherein said iron oxide magnetic powder is -Fe₂O₃, a weakly reduced -Fe₂O₃, a Co-coated -Fe₂O₃ or a weakly reduced Co-coated -Fe₂O₃. A magnetic recording medium according to Claim 1 wherein a main chain of said resin having the polar group is a polyester, polyurethane or a vinyl chloride-based copolymer. A magnetic recording medium according to Claim 1 wherein said resin has a molecular weight of 10,000 to 50,000.

Inventors: HASHIMOTO, AKIRA SONY CORPORATION, NINOMIYA, YOSHINOBU SONY CORPORATION
Applicants: SONY CORPORATION
Application year: 1983
Main classification: G11B5
IPC codes: G11B5, C09D127, C09D5, C09D175