Classifications

• • G—PHYSICS
  • G11—INFORMATION STORAGE
  • G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
  • G11B5/00—Recording by magnetisation or demagnetisation of a record carrier; Reproducing by magnetic means; Record carriers therefor
  • G11B5/62—Record carriers characterised by the selection of the material
  • G11B5/73—Base layers, i.e. all non-magnetic layers lying under a lowermost magnetic recording layer, e.g. including any non-magnetic layer in between a first magnetic recording layer and either an underlying substrate or a soft magnetic underlayer
  • G11B5/733—Base layers, i.e. all non-magnetic layers lying under a lowermost magnetic recording layer, e.g. including any non-magnetic layer in between a first magnetic recording layer and either an underlying substrate or a soft magnetic underlayer characterised by the addition of non-magnetic particles

Definitions

• This invention relates generally to a high recording density magnetic recording medium for use with a light-transmitting servo-tracking system, and more specifically to an undercoating layer used in such media.
• the practice of projecting a beam of light on the surface of the magnetic recording medium and utilizing the light reflected on the surface for effecting servo tracking with a view to improving the recording density has been in done. This is because the improvement of recording density requires the recording surface to be accurately tracked.
• the disc type magnetic recording device of this operating principle is disclosed in Japanese Unexamined Patent Publication (KOKAI) No. 2-14,436, for example.
• the magnetic recording layer should be decreased based on the recording wavelength to enhance the recording density of the magnetic disc.
• the recording medium is notably degraded in strength and durability in proportion as the magnetic recording layer is reduced in thickness.
• the interposition of an undercoating layer between the nonmagnetic supporting member and the magnetic layer to enhance the durability of the magnetic recording medium is disclosed in Japanese Unexamined Patent Publication (KOKAI) No. 63-317,926. Since this undercoating layer uses carbon black for antistatic effect, however, the magnetic recording medium is incapable of acquiring the light transmission ratio necessary for the light transmission servo tracking.
• Japanese Unexamined Patent Publication (KOKAI) No. 61-214, 127 discloses the idea of adding tin oxide or white electroconductive titanium dioxide to the undercoating layer for enhancing the light transmission while maintaining high electric conductivity.
• the magnetic recording medium is not allowed to acquire sufficiently high durability because the amount of the inorganic powder which is to be added to the undercoating layer is small and the thickness of the undercoating layer is small.
• the present invention is aimed at providing a magnetic recording medium which attain light transmission servo tracking and effects high density recording by interposing between the nonmagnetic supporting member and the magnetic layer an undercoating layer which is electrically conductive and transparent to light.
• the present inventors have discovered that interposition of an undercoating layer containing an electrically conductive, inorganic, nonmagnetic substance between the nonmagnetic supporting member and the magnetic layer permits production of a magnetic recording medium which exhibits sufficient durability and antistatic property, and provides light transmittance sufficient for the sake of light- transmitting servo tracking when the magnetic layer is not more than 1 ⁇ m thick.
• This invention is directed to a high recording density magnetic recording medium using the light-transmitting servo-tracking system and comprising a nonmagnetic supporting member and a magnetic layer, which magnetic recording medium is characterized in that an undercoating layer containing an electrically conductive, inorganic, nonmagnetic substance is interposed between the nonmagnetic supporting member and the magnetic layer and the magnetic layer not more than 1 ⁇ m thick.
• the magnetic recording medium of this invention comprises a nonmagnetic supporting member, an undercoating layer, and a magnetic layer.
• the nonmagnetic supporting member of the present invention may be any of the light- transmitting supporting members which are conventionally used for supporting magnetic recording media.
• the light-transmitting supporting members may include organic and inorganic materials such as, for example, polyethylene terephthalate film, polyethylene naphthalate film, acetate film, polyimide film, polyamide film, and glass, which have a light transmission ratio of at least 40% .
• This invention is characterized by providing an undercoating layer containing an electrically conductive, inorganic, magnetic substance for a magnetic recording medium. Generally, carbon black is generally used as an conductivity- imparting material.
• the carbon black however owing to the blackness thereof, notably degrades light transmittance of a layer using it.
• the carbon black generally disperses in a binder only with difficulty, tends to cohere and form an aggregate, and may disrupt the transparency of a magnetic recording medium.
• the electrically conductive organic nonmagnetic substance is preferably generally white and is easily dispersed uniformly in a binder. The undercoating layer which has this substance dispersed therein, therefore, is allowed to acquire sufficient light transmittance.
• Preferred electrically conductive, inorganic, nonmagnetic substances include conductive tin oxide, titanium dioxide, zinc oxide, indium oxide, zinc sulfide, barium sulfate, silicon oxide, and magnetic carbonate, for example.
• electrically conductive means that when the inorganic nonmagnetic substance of the quality mentioned above is compressed with a pressure of 100 kg /cm 2 , the compressed mass exhibits a specific resistance of not more than 300 ⁇ cm.
• the method for imparting conductivity to the inorganic nonmagnetic substance is not specifically restricted, this is accomplished by doping or coating the surface of the substance with Ga, In, Sn, or CuS or by plating the surface therewith, for example.
• the average particle diameter of the electrically conductive, inorganic, nonmagnetic substance must be suitably defined.
• the average particle diameter thereof must fall in the range of from 0.02 to 0.1 ⁇ m. If the average particle diameter is larger than 0.1 ⁇ m, the undercoating layer fails to acquire ample light transmittance and exhibits poor surface smoothness and, as a result, the produced magnetic recording medium suffers from deficiency in electromagnetic conversion property. Conversely, if the average particle diameter falls shown of the lower limit of 0.02 ⁇ m, the particles tend to cohere, disperse in a binder only with difficultly, and fail to produce an undercoating layer having good surface smoothness.
• the undercoating layer comprises the electrically conductive, nonmagnetic particles of the nature described above and a binder.
• the binder in this undercoating layer can be any known binder such as, for example, urethane resin, polyvinyl chloride resin, phenoxy resin, polyester resin, and copolymers of vinyl chloride monomer with other vinyl monomers which are popularly used in magnetic recording media.
• the binder is preferably a binder resin which, when incorporated in the electrically conductive undercoating layer, allows the layer to acquire a light transmission ratio in the range of from 40 to 90% .
• the mixing ratio of the electrically conductive, inorganic, nonmagnetic substance and the binder contained in the undercoating layer is preferably in the range of from 50/50 to 85/15 by weight.
• the undercoating layer fails to manifest sufficient electroconductivity if the proportion of the electrically conductive, inorganic, nonmagnetic substance is smaller than the lower limit of this range. This insufficient supply of the nonmagnetic substance is also disadvantageous in that the formability of servo tracks, the durability of the formed servo tracks, and the electromagnetic conversion characteristic thereof are degraded.
• the proportion of the inorganic, electrically conductive, nonmagnetic substance exceeds the upper limit of the range mentioned above, the adhesiveness of the undercoating layer to the nonmagnetic supporting member is so inferior as to jeopardize the practical utility of the magnetic recording medium.
• the high recording density magnetic recording medium using the light- transmitting servo tracking system is required to form on the surface of the magnetic layer thereof a plurality of grooves i.e., servo tracks.
• a notable improvement in recording density may be realized by effecting servo tracking with the "light” by virtue of these "grooves” and consequently improving the track accuracy.
• These servo tracks may be formed by pressing a die (protuberances) of servo tracks against the surface of the magnetic layer, thereby depressing the magnetic layer. If the magnetic layer has an unduly small thickness or unduly high hardness, however, the magnetic layer will not readily succumb to depression and will prevent efficient formation of servo tracks.
• the thickness of the undercoating layer therefore, should be in the range of from 0.2 to 5 ⁇ m.
• the thickness of the undercoating layer is less than 0.2 ⁇ , the efficiency with which the servo tracks are formed is notably impaired and the servo tracking no longer functions normally. This creates a peculiar problem for the high recording density magnetic recording medium of the servo tracking type which makes use of light.
• Another problem results if the thickness exceeds 5 ⁇ m: the magnetic layer is not easily superposed on the undercoating layer by spreading and the magnetic layer if produced at all, fails to acquire a sufficiently smooth surface and exhibits an inferior electromagnetic conversion characteristic. This phenomenon may be explained by theorizing that when the undercoating layer is excessively thick as compared with the magnetic layer, the undercoating layer excessively absorbs the solvent of the magnetic layer.
• the undercoating layer encounters difficulty in bringing the required supply of the reserved lubricant to the magnetic layer and the magnetic tape suffers from a decline in durability.
• These problems are particularly conspicuous in the high recording density magnetic recording medium which comprises the magnetic layer not exceeding 1.0 ⁇ m in thickness and the undercoating layer.
• the electrical conductivity of the undercoating layer must be controlled within a certain range, although dependent on the particular kind of the magnetic recording medium which incorporates the undercoating layer therein.
• the conductivity (surface resistance) of the magnetic layer is required to be controlled within the range of from 10 6 to 10 9 ohms per square.
• non-electrically conductive, inorganic, magnetic substance such as titanium dioxide can be used as mixed with the aforementioned conductive nonmagnetic member.
• non-electrically conductive titanium dioxide identical in shape and average particle diameter with the aforementioned conductive nonmagnetic particles so that the undercoating layer is enabled to maintain the properties (transparency, surface smoothness, etc.) other than the conductivity.
• the magnetic recording medium of the present invention is provided on the aforementioned undercoating layer with the magnetic layer.
• the present invention aims to provide a light-transmitting servo tracking type magnetic recording medium having a capacity for high recording density.
• the magnetic layer should be not more than 1.0 ⁇ m thick.
• any of magnetic paniculate substances in conventional use such as needle-shaped iron oxide type magnetic substances (such as 7-Fe*-.O 3 and F- ⁇ O.) , cobalt-containing needle-shaped iron oxide type magnetic substances, metallic ferromagnetic substances (metallic magnetic substances), hexagonal magnetic substances (such as barium ferrite), and iron carbide type magnetic substances can be used.
• the magnetic layers formed of such magnetic particles having the same thickness may exhibit different light transmission ratios.
• the magnetic layer of a metallic ferromagnetic substance tends to exhibit a higher light-shielding property than that of a needle-shaped iron oxide type magnetic substance.
• This problem of difference in tinting strength can be solved, however, by adjusting the thickness of the magnetic layer and the thickness of the undercoating layer within the possible ranges so as to control the light transmission ratio.
• the method which comprises increasing the light transmission ratio by dispersing in the binder of the magnetic layer a superfine powder of SiO 2 or Al 2 O 3 having a larger refractive index than the binder is capable of attaining the solution.
• the method which attains the control of the light transmission ratio of the magnetic layer by combining a plurality of different magnetic layers in a superposed construction can be adapted for the purpose of the solution.
• the magnetic recording medium of this invention is produced as follows:
• the coating material for the undercoating layer is produced by kneading and dispersing electrically conductive tin oxide conforming to this invention in combination with a binder, a lubricant, a solvent, etc.
• the components mentioned above are simultaneously placed in the kneading and dispersing devices either wholly at once or as split in several fractions.
• the electroconductive tin oxide is placed and then kneaded therein continuously.
• the blend is transferred into the dispersing device and thoroughly dispersed.
• the dispersed liquid is combined with a lubricant to complete the coating material for the undercoating layer.
• kneading and dispersing operations which are involved in the preparation of the coating material for the undercoating layer
• various known devices adopted for the purpose of this operation may be used.
• kneading devices as kneader, planetary mixer, extruder, homogenizer, and high speed mixer can be used for the kneading operation.
• dispersing devices as sand mill, ball mill, attriter, tornado dispersing device, and high speed shock mill can be used for the dispersing operation.
• the coating material for the undercoating layer when necessary, may further incorporate therein various known additives such as curing agent, antifungal agent, and surfactant heretofore popularly used in magnetic layers and similar additives heretofore used in backcoat layers.
• various known additives such as curing agent, antifungal agent, and surfactant heretofore popularly used in magnetic layers and similar additives heretofore used in backcoat layers.
• the devices which are effectively usable herein for the work of applying the coating material to the supporting member include various known devices such as air doctor coater, blade coater, air knife coater, squeeze coater, reverse roll coater, gravure coater, kiss coater, spray coater, and die coater, for example.
• the coating material for the overcoating layer is produced by kneading and dispersing magnetic particles of a varying kind in combination with a binder, a lubricant, an abrasive, and a solvent. This coating material for the overcoating layer is superposed by spreading on the undercoating layer.
• the overcoating layer is superposed on the undercoating layer after the coating material for the undercoating layer has been applied to the supporting member and then dried until the undercoating layer is formed completely. It is superposed on the undercoating layer before the latter is thoroughly dried. In the alternative, the undercoating layer and the overcoating layer may be simultaneously applied and dried.
• the drying temperature for the undercoating layer and the overcoating layer is variable depending on the type of solvent and supporting member used.
• the drying operation is preferably carried out at a temperature in the range of from 40 to 120°C for a period in the range of from 30 seconds to 10 minutes, with the drying air being fed at a flow rate in the range of from 1 to 5 kl/m 2 s.
• the drying operation may be performed by irradiation with an infrared ray, far infrared ray, or electron beam.
• the magnetic particles Before the overcoating layer has been dried, the magnetic particles may be oriented or disoriented as occasion demands.
• the orientation of the magnetic particles is effected by exerting a magnetic field longitudinally, vertically, or obliquely (at an angle of 45° relative to the plane of the supporting member, for example) on the overcoating layer by the use of a permanent magnet or an electromagnet and drying the overcoating layer outside or inside the magnetic field.
• the disorientation is effected by randomly changing the direction of the magnetic member in a horizontal plane or three-dimensionally by the use of an alternating magnetic field or a rotary magnetic field.
• the orientation or disorientation may be attained by known methods.
• the dried undercoating layer and overcoating layer may be subjected to an appropriate calendaring treatment.
• the calendaring treatment is carried out by the conventional method using a metal-plated roll or an elastic roll, for example.
• the conditions for the calendaring treatment are variable with the kinds of materials (binder, supporting member, etc.) to be used in the undercoating layer and the overcoating layer, the treatment should be performed at a temperature in the range of from 30 to 90 °C at a pressure in the range of from 500 to 4,000 pounds per lineal inch (pli).
• the various materials to be used in this invention other than the electrically conductive, inorganic, nonmagnetic substance, and non-electrically conductive, inorganic, nonmagnetic substance which are specifically defined for this invention may be selected from among those materials known heretofore in the art.
• the servo tracks are then formed by stamping, for example.
• the light-transmitting servo tracking type high recording density magnetic recording medium which is obtained by this invention enjoys sufficient durability, resistance to electrification, and light-transmitting property.
• Example An undercoating material was produced by kneading and dispersing the raw materials indicated in Table 1 as follows.
• a high speed mixer By the use of a high speed mixer, the entire amount of a nonmagnetic substance indicated in the Table 1 was kneaded for about 10 minutes in a sclution having the entire amount of urethane resin and the entire amount of vinyl resin dissolved in the entire amount of solvent.
• the kneading in the high speed mixture was continued for about 50 minutes to obtain a blend.
• the blend was transferred into a sand mill and dispersed therein for 20 hours, to obtain a dispersion.
• the entire amount of oleic acid, the entire amount of isocetyl stearate, and the entire amount of polyisocyanate were stirred by the use of a high speed mixer for about 30 minutes, to obtain the undercoating material in its finished form.
• Urethane resin (molecular resin [Mw] about 8,000, (Mixing ratio produced by Takeda Chemical Industries, Ltd. and of urethane marketed under product code of "XE-148") resin to vinyl resin 1: 1)
• Vinyl resin (molecular weight [Mw] about 34,000, produced by Sekisui Chemical Co., Ltd. and marketed under product code of "C-130")
• Oleic acid produced by Kao Soap Co. , Ltd. and marketed 1 under trademark designation of "Lunac-OA"
• Isocetyl stearate produced by Kokyu Alcohol K.K. and 2 marketed under product code of "ICS-R"
• a coating material for a magnetic layer was produced by kneading and dispersing the raw materials indicated in Table 2 as follows. Barium ferrite was added piecemeal while stirring with a speed mixer to a solution having the entire amounts of Dispersant 1 and Dispersant 2 dissolved in the entire amount of a solvent. Thirty minutes after the introduction of the entire amount of barium ferrite, the entire amounts of urethane resin and vinyl resin were introduced and stirred continuously. Several minutes thereafter, the entire amount of alumina was introduced and continuously stirred for about 10 minutes to obtain a blend. This blend was transferred into a sand mill and dispersed therein for about 30 hours to obtain a dispersion.
• Alumina (BET value 8.4 m 2 /g, produced by Sumitomo 8 Chemical Co. , Ltd. and marketed under product code of "HIT-50")
• Dispersant 1 [phosphorylated polyoxyalkyl polyol disclosed 4 in KOKAI (Japanese Unexamined Patent Publication) No. 63-14,326]
• Dispersant 2 N,N-dialkyl-N-hydroxyalkylpolyoxyalkylene 2 ammonium salt disclosed in U.S. Patent No. 3, 123,641
• Urethane resin polyether polyurethane having a molecular 4 weight [Mw] of about 10,000
• Oleic acid produced by Kao Soap Co., Ltd. and marketed 3 under trademark designation of "Lunac-O-A"
• Isocetyl stearate (Kokyu Alcohol K.K. and marketed under 2 product code of "ICS-R")
• the undercoating material mentioned above was applied with a gravure cater to a base film 2 ⁇ m thick (produced by Teijin Limited and marketed under product code of IIAXP-54/6") and the applied layer of the coating material was dried at 40 °C for 40 seconds and then at 100°C for 30 seconds.
• the dried layer of the coating material was calendared with a metalplated roll at a temperature of 45°C at a pressure of 1,500 pli.
• the coating material for the magnetic layer mentioned above was applied on the undercoating layer by the use of a gravure cater and then dried at 40 °C for 30 seconds and at 80 °C for 30 seconds.
• the dried magnetic layer was calendared with a metal -plated roll at a temperature of 45° C at a pressure of 1 ,500 pli.
• the thickness of the undercoating layer and that of the magnetic layer were measured after the respective layers had undergone the calendaring treatment.
• the following samples were prepared:
• Sample No. 1 an undercoating layer 2.0 ⁇ m thick formed of conductive tin oxide (produced by Ishihara Sangyo Kaisha, Ltd. and marketed under product code of "SN-100") and a binder in a gravimetric mixing ratio of 85: 15.
• Sample No. 2 a magnetic layer superposed directly on a supporting member without interposition of an undercoating layer.
• Sample No. 3 an undercoating layer comprising carbon black, iron oxide (average particle diameter 0.1 ⁇ m), and a binder, with the mixing ratio of the total of nonmagnetic substances to the binder at 85: 15.
• Sample No. 4 an undercoating layer comprising non-conductive titanium dioxide, the same tin oxide as used in Sample 1 , and a binder, with the mixing ratio of the total of nonmagnetic substances to the binder at 85: 15.
• Sample No. 5 an undercoating layer comprising the same tin oxide as used in Sample 1 and a binder, with the mixing ratio of the tin oxide to the binder at 60:40.
• Sample No. 6 an undercoating layer comprising the same tin oxide as used in Sample 1 and a binder, with the mixing ratio of the tin oxide to the binder at 40:60.
• Sample No. 7 an undercoating layer of the same composition as used in
• Sample 1 except that the thickness of the undercoating layer was changed to 0.3 ⁇ m.
• Sample No. 8 an undercoating layer of the same composition as used in Sample 1, except that the thickness of the undercoating layer was changed to 0.1 ⁇ m.
• Sample No. 9 an undercoating layer 1.5 ⁇ m thick formed in the same composition as in Sample 1, except that granular electroconductive titanium dioxide (produced by Ishihara Sangyo Kaisha, Ltd. and marketed under product code of "ET-300W") was used in the place of tin oxide.
• granular electroconductive titanium dioxide produced by Ishihara Sangyo Kaisha, Ltd. and marketed under product code of "ET-300W" was used in the place of tin oxide.
• Sample No. 10 provided with an undercoating layer 1.5 ⁇ m thick formed in the same composition as in Sample 1, excepting granular electroconductive titanium dioxide of a large particle diameter (produced by Ishihard Sangyo Kaisha, Ltd. and marketed under product code of "ET-500W”) was used in the place of tin oxide.
• granular electroconductive titanium dioxide of a large particle diameter produced by Ishihard Sangyo Kaisha, Ltd. and marketed under product code of "ET-500W" was used in the place of tin oxide.
• Sample No. 11 an undercoating layer 2.5 ⁇ m thick formed in the same composition as in Sample 1, except that needle shaped electroconductive titanium dioxide (produced by Ishihara Sangyo Kaisha, Ltd. and procured as sample) was used in the place of tin oxide.
• needle shaped electroconductive titanium dioxide produced by Ishihara Sangyo Kaisha, Ltd. and procured as sample
• Sample No. 12 an undercoating layer 2.5 ⁇ m thick formed in the same composition as in Sample 1, excepting needle shaped conductive titanium dioxide having a large major diameter (sample for composition) was used in the place of tin oxide.
• Sample No. 13 an equivalent of Sample 1 , except that zinc oxide (produced by Mitsui Mining and Smelting Co., Ltd.) was used in the place of tin oxide.
• Sample No. 14 an equivalent of Sample 1 , except that indium oxide
• Sample No. 15 an equivalent of Sample 1 , except that zinc sulfide (produced by Mitsui mining and Smelting Co. , Ltd.) was used in the place of tin oxide.
• Sample No. 16 an equivalent of Sample 1, except that barium sulfate
• Sample No. 17 an equivalent of Sample 1 , except that silicon oxide (produced by Tokuyama Soda Co. , Ltd.) was used in the place of tin oxide.
• Sample No. 18 an equivalent of Sample 1, excepting magnesium carbonate (produced by Tokuyama Soda Co. , Ltd.) was used in the place of tin oxide.
• the tin oxide-containing undercoating layer in this invention acquired sufficient transparency (not less than 40% and not more than
• this undercoating layer is used as an undercoating layer in a magnetic disc for a magnetic disc device using a light- transmitting optical servo tracking, therefore, the produced magnetic disc acquires high durability without a sacrifice of the performance of servo tracking.

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Citations (4)

• 1993
  • 1993-02-08 JP JP5020173A patent/JPH06236541A/ja active Pending
• 1994
  • 1994-02-07 WO PCT/US1994/001354 patent/WO1994018673A1/en active Application Filing

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