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
  • G11B5/68—Record carriers characterised by the selection of the material comprising one or more layers of magnetisable material homogeneously mixed with a bonding agent
  • G11B5/70—Record carriers characterised by the selection of the material comprising one or more layers of magnetisable material homogeneously mixed with a bonding agent on a base layer
  • G11B5/71—Record carriers characterised by the selection of the material comprising one or more layers of magnetisable material homogeneously mixed with a bonding agent on a base layer characterised by the lubricant
• • 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
  • G11B5/68—Record carriers characterised by the selection of the material comprising one or more layers of magnetisable material homogeneously mixed with a bonding agent
  • G11B5/70—Record carriers characterised by the selection of the material comprising one or more layers of magnetisable material homogeneously mixed with a bonding agent on a base layer
  • G11B5/702—Record carriers characterised by the selection of the material comprising one or more layers of magnetisable material homogeneously mixed with a bonding agent on a base layer characterised by the bonding agent
  • G11B5/7021—Record carriers characterised by the selection of the material comprising one or more layers of magnetisable material homogeneously mixed with a bonding agent on a base layer characterised by the bonding agent containing a polyurethane or a polyisocyanate

Definitions

• the present invention relates to a magnetic recording medium comprising a specific binder, magnetic particles and a lubricant; this medium has specific characteristics of hardness, cohesion, surface roughness, resistance to abrasion and scratches, abrasiveness and slippage making it particularly suitable, without overcoating or external protection, for uses on semi supports -rigid such as discs or cards.
• Most magnetic recording media have either an overlay to protect the magnetic layer during handling, scratches, abrasions and other environmental attacks, or protection by pockets or cassettes.
• this medium must be super hard here enough to resist scratches and other attacks from the environment, have a sufficient cohesion to resist wear and tearing, not being brittle or cracking; it must withstand weathering, solvents, the temperatures of use of the magnetic support, as well as those of any finishing treatments for these supports; finally, it must generate a reduced surface friction coefficient between itself and the write / read head, in order to reduce wear and tear on both, and have good sliding properties.
• Binders commonly used for magnetic recording media are formed from a mixture (a) of a high molecular weight elastomeric polyurethane obtained by reaction of a polyol with isocyanates and containing chain extenders which give the. polyurethane its elasticity and (b) a vinyl copolymer based on vinyl chloride containing crosslinkable OH groups, which serves both as a dispersing agent for magnetic particles and participates in crosslinking with isocyanates. This set gives the layer the desired rigidity.
• binders are not, however, hard enough for use without an overcoat or protection, as they would not withstand scratches and other environmental aggressions sufficiently to guarantee the total conservation of the information.
• Another kind of binder for magnetic recording medium comprises a non-elastomeric polyurethane, crosslinked by an isocyanate prepolymer containing at least 3 functional groups so as to obtain a three-dimensional network.
• This type of binder is too hard and tends to break and crack.
• French patent 1311588 describes the binder of French patent 1020808 associated with a copolymer based on highly polymerized vinyl chloride which is used here as an elastomeric compound; in this case it does not contain a crosslinkable OH group and it is used in an amount of 10 to 20% in the mixture.
• This type of binder also lacks strength and does not meet the requirements mentioned above.
• a magnetic medium has been found according to the invention having the required properties, comprising a specific binder.
• the subject of the present invention is a magnetic medium comprising magnetic particles and a binder formed from a crosslinked three-dimensional polyurethane, and it is characterized in that the crosslinked three-dimensional polyurethane is formed from a branched polyol comprising at least 8% by weight of OH groups, having an average molecular weight of less than 4000, and of an isocyanate prepolymer comprising at least 3 isocyanate functions.
• Polyester polyols are compounds containing hydroxyl groups prepared from dicarboxylic acids or derivatives of these acids such as their anhydrides, such as adipic acid, terephthalic, phthalic, isophthalic, tetrahydrophthalic, hexahydrophthalic acids, maleic acid, and polyfunctional alcohols such as glycols, glycerol, hexanediol, hexanetriol, neopentylglycol, trimethylolpropane, pentaerythrol, etc.
• dicarboxylic acids or derivatives of these acids such as their anhydrides, such as adipic acid, terephthalic, phthalic, isophthalic, tetrahydrophthalic, hexahydrophthalic acids, maleic acid, and polyfunctional alcohols such as glycols, glycerol, hexanediol, hexanetriol, neopentylglycol,
• Polyether polyols are also compounds containing hydroxyl groups prepared from alkylene oxides and the same alcohols as those described above for polyesters.
• the amount of -COOH groups from acids and -OH groups from alcohols is such that the OH / COOH ratio is greater than 1.
• the polyols used according to the invention comprise at least 8% by weight of OH groups.
• the dicarboxylic acid is a hydrogenated phthalic acid, optionally in admixture with other acids
• the polyfunctional alcohol is trimethylopropane, optionally in admixture with other alcohols.
• isocyanate prepolymer commonly used in polyurethane chemistry, denotes a compound with at least three isocyanate functions, that is to say a functionality of at least three.
• Such compounds can be obtained, for example, from 3 moles of an aliphatic or aromatic diisocyanate and 1 mole of a trivalent alcohol, such as glycerol, hexanetriol or trimethylol propane.
• the respective amounts of polyol and isocyanate are such that the total NCO / OH ratio is greater than 0.9, and preferably equal to or greater than 1.
• total OH means that if the magnetic medium comprises another source of OH groups as polyol, it must be taken into account for the calculation of the isocyanate prepolymer content, so that the medium is fully crosslinked.
• the recording medium according to the invention can comprise any known magnetic particles, doped or not, such as metal oxides, such as iron, chromium oxides, metal particles such as iron particles, hexaferrites of Ba or of Sr, etc.
• the magnetic particles are particles of barium or strontium hexaferrite having a coercive field greater than 4000 Oe (300 kA / m) and preferably greater than 5000 Oe (400 kA / m) a saturation magnetization greater than 55 emu / g and of preferably greater than 60 emu / g and a ratio of the distribution of the switching fields to the coercive field (SFD / Hc) less than 0.45 and preferably less than 0.40.
• a coercive field greater than 4000 Oe (300 kA / m) and preferably greater than 5000 Oe (400 kA / m) a saturation magnetization greater than 55 emu / g and of preferably greater than 60 emu / g and a ratio of the distribution of the switching fields to the coercive field (SFD / Hc) less than 0.45 and preferably less than 0.40.
• the SFD characterizes the width of the distribution of the switching fields. From the hysteresis cycle
• the magnetization and the coercive field are measured with VSM (Vibrating Sa ple Magnetometer) under a maximum magnetizing field of 20,000 Oe (1590 kA / m).
• the recording medium according to the invention can contain the additives commonly used in such a medium.
• lubricants commonly used in magnetic compositions, in order to reduce the coefficient of friction between the layer and the head, to reduce the wear of both and to improve the sliding properties .
• lubricants include, for example, fine particles of graphite or molybdenum sulfide or tungsten, silicone oils, unsaturated aliphatic hydrocarbons, fatty acid esters of monoalcohols, carbonic esters such as those described in French patent 2,094,663, fluorinated hydrocarbons or fine polymer particles such as polyethylene, polypropylene, polytetrafluoroethylene, etc.
• solid lubricants are used, in which at least 90% and preferably at least 99% of the particles have a size equal to or less than 3 ⁇ m , at a rate of less than 2% by mass relative to the mass of the magnetic particles.
• liquid lubricants tend to exude from the magnetic medium by raising the temperature, and in particular when the final treatment of the magnetic support requires high temperatures.
• Fine particles of graphite, molybdenum or tungsten sulfide, or polyethylene can be used as the solid lubricant.
• polytetrafluoroethylene PTFE
• PTFE polytetrafluoroethylene
• PTFE is sold in the form of powders by different manufacturers. One can find it for example at Du Pont de Nemours under the trade name of Vydax.
• the solid lubricant is compatible with the other constituents of the recording medium, and that it does not contain more than 10% of particles having a size greater than 3 ⁇ and, preferably not more than 1%, d on the one hand, and on the other hand, that its content in the magnetic medium does not exceed 2% relative to the mass of the magnetic particles.
• crosslinked structure of the binder is such that it makes it possible to trap the particles of solid lubricant with dimensions less than 3 ⁇ m.
• the solid lubricant based on polytetrafluoroethylene can be combined with an inert polymer such as a polyolefin, for example polyethylene.
• lubricants such as silicone oils
• One or more surfactants can be used in the magnetic recording medium according to the invention to ensure the wettability, the dispersibility and the spreading of the dispersion. It is possible to use, for example, a polycarboxylic acid ester or a complex organic ester of phosphoric acid, lecithin, fatty acids such as palmitic acid or oleic acid, or mixtures of these surfactants. assets.
• a vinyl copolymer based on vinyl chloride comprising OH groups can also be used as a dispersing agent.
• Such a copolymer generally comprises from 70 to 90% of vinyl chloride, the other monomers being chosen from vinyl acetate, polyvinyl alcohol, and acrylic or methacrylic esters optionally carrying hydroxyl groups.
• account must be taken of the OH groups of the vinyl copolymer, which must also be completely crosslinked in the three-dimensional polyurethane network and the amount of isocyanate prepolymer must be adjusted accordingly.
• a alkylphenoxypolyethoxyethyl ester of phosphoric acid at a rate of 3 to 8 g per 100 g of magnetic particles and a vinyl copolymer at a rate of less than 10% by mass relative to the total binder.
• the binder composition can comprise the usual additives such as abrasive powders such as Al 2O3 or Cr2O3, pigments, in the form of a fine powder or colloid, such as carbon black for coloring the layer and silica for improve the quality and stability of the dispersion.
• abrasive powders such as Al 2O3 or Cr2O3, pigments, in the form of a fine powder or colloid, such as carbon black for coloring the layer and silica for improve the quality and stability of the dispersion.
• it may optionally contain agents to promote crosslinking, depending on the binder used and the kinetics that are desired.
• a coating dispersion is formed by mixing the magnetic particles, the various components of the binder, and the additives, in the presence of solvents. Solvent mixtures known in the art of magnetic dispersions are used.
• esters such as methyl acetate, ethyl acetate, isopropyl acetate, tetrahydrofuran, ketones such as methyl ethyl ketone, methyl isobutyl ketone and cyclohexanone, nitromethane, nitroethane, dichloroethane, benzene solvents such as toluene, etc.
• the useful quantities of solvents can be determined by the technician depending on the desired results, the particular compounds of the dispersion and the equipment used. Particularly useful dispersions are described in the examples.
• the magnetic particles are first impasted with the surfactant and the vinyl copolymer, this paste is ground for example in a ball mill, then all the other constituents of the medium formula are added magnetic.
• a magnetic coating composition is obtained which is applies to a support in a known manner.
• the crosslinking forming the polyurethane takes place after coating by any known means and, in general, by heating, with, as indicated above, a crosslinking promoter, if necessary.
• the magnetic medium obtained is very hard; this hardness can be evaluated by the scratch test described below. It has excellent cohesion, has both high wear resistance and low abrasiveness, and is resistant to solvents. In general, it itself presents, without overlay, wear corresponding to a loss of electrical signal equal to or less than 5% for 2000 passes, an abrasiveness less than 600 ⁇ m and a scratch resistance less than 1.5 ⁇ . In addition, in the case where PTFE is used as described above, the magnetic medium is insensitive to temperature variations between -20 ° C and 130 ° C.
• the recording medium according to the invention can be used in all cases where. We wish a rigid or semi-rigid medium without overlay, for example for discs or cards.
• the magnetic recording medium according to the invention is used in a large capacity magnetic memory card such as that described in French patent application 8814217 entitled: "Large capacity magnetic memory card and manufacturing process”.
• the following examples particularly relate to a card comprising a high energy and high density recording medium, having a coercive field of between 4500 and 6000 Oe, a magnetization at saturation greater than 60 emu / cm, and preferably greater than 80 emu / cm, and a ratio of the distribution of the switching fields to the coercive field less than 0.50, and preferably less than 0.30.
• the magnetization and the coercive field are measured at VSM under a maximum magnetizing field of 20,000 Oe (1590 kA / m).
• the invention is not however limited to such a card.
• the results given in the examples relate to the electrical wear, the abrasiveness, the roughness and the scratch resistance of the magnetic recording medium. These parameters are measured by the following tests: Electrical wear test
• a number of tracks are chosen which are distributed uniformly over the entire magnetic layer.
• the amplitude of the read signal is measured using an oscilloscope.
• the loss of signal corresponds to the difference between the first reading and the last, and it is expressed in% of the signal collected at the first reading.
• the magnetic medium to be measured is displaced at 40 ⁇ m / s over a distance of 2 mm.
• the displacement of a sensor is measured, the part of which in contact with the medium is a ball with a radius of 5 ⁇ m.
• the force applied to this ball is 0.03 mN.
• the analog voltage at the output of the converter, proportional to the oscillations of the sensor, is processed digitally using a computer which calculates the roughness criteria.
• a magnetic dispersion comprising: Ba hexaferrite particles 100.0 g
• Example 2 A magnetic dispersion is prepared containing:
• Methylisobutyl ketone 10 A card is prepared as in the previous example, and the electrical wear test is carried out under the same conditions as in the previous example.
• a magnetic dispersion is prepared containing: Particles of barium hexaferrite 100 g
• Example 4 A magnetic dispersion is prepared containing:
• Toluene 9 A card is prepared as in Example 1, and the electrical wear test is carried out under the same conditions, except that the number of passes is 24,000.
• a magnetic dispersion is prepared containing: Particles of barium hexaferrite 100 g 3.0g
• a card is prepared as in Example 1, and the electrical wear test is carried out under the same conditions, except that the number of passes is 24,000.
• a magnetic dispersion is prepared containing:
• a card is prepared as in Example 1, and the electrical wear test is carried out under the same conditions, except that the number of passes is 24,000.

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• Paints Or Removers (AREA)
• Magnetic Record Carriers (AREA)
• Liquid Crystal (AREA)
• Electrochromic Elements, Electrophoresis, Or Variable Reflection Or Absorption Elements (AREA)
• Compositions Of Macromolecular Compounds (AREA)

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Publications (1)

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• 1988
  • 1988-10-18 FR FR8814219A patent/FR2639647B1/fr not_active Expired - Fee Related
• 1989
  • 1989-10-17 ES ES89420400T patent/ES2043077T3/es not_active Expired - Lifetime
  • 1989-10-17 EP EP89420400A patent/EP0365448B1/de not_active Expired - Lifetime
  • 1989-10-17 AT AT89420400T patent/ATE92662T1/de not_active IP Right Cessation
  • 1989-10-17 DE DE89420400T patent/DE68908077T2/de not_active Expired - Fee Related
  • 1989-10-17 WO PCT/FR1989/000538 patent/WO1990004842A1/fr not_active Application Discontinuation
  • 1989-10-17 EP EP89912248A patent/EP0440716A1/de active Pending
  • 1989-10-17 JP JP1511374A patent/JPH04501334A/ja active Pending

Non-Patent Citations (1)

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