FLEXIBLE MAGNETIC RECORDING MEDIUM CONTAINING POLYBENZAZOLE POLYMERS

Abstract: A magnetic recording medium, such as a tape or a floppy disc, is made using a substrate of high modulus polybenzazole polymer film. Suitable films can be coagulated from anisotropic polybenzazole dopes. A continuous layer of magnetizable material is deposited upon the substrate at high temperature.

Description: FLEXIBLE MAGNETIC RECORDING MEDIUM CONTAINING POLYBENZAZOLE POLYMERS The present invention relates to the art of flexible magnetic recording media, such as floppy disks and magnetic tapes. Flexible magnetic recording media typically contain a flexible polymer substrate and a magnetizable surface layer. Examples of common substrates include poly(ethylene terephthalate) (PET) polymers and poly(ethylene naphthalate) (PEN) polymers (commercially sold as Teonex™ film by Teijin). In recent years, the best magnetizable surface layers have been made of a continuous thin layer of magnetizable material. Examples of common magnetizable materials include oxides of iron and/or chromium, metallic particles and ferrite compounds of barium, lead and strontium. Unfortunately, it is difficult to apply the magnetizable material to the substrate. Common substrates can be destroyed by the process conditions needed to directly apply the magnetic material to the substrate. Typically, the magnetic material must be applied using complex techniques or using adhesives. What are needed are new recording media in which a continuous magnetizable layer is adhered directly to the substrate by simple and ordinary methods. One aspect of the present invention is a flexible magnetic recording medium comprising: (a) a substrate; and (b) a magnetizable surface layer, that contains a continuous thin film of magnetizable material, characterized in that the substrate contains a polybenzazole polymer and the magnetizable su rf ace ad heres d i rectl y to the su bstrate . A second aspect of the present invention is a process to make a flexible magnetic recording medium comprising the step of contacting a flexible recording medium substrate that contains a polybenzazole polymer film which is stable up to at least 300 C C with vapor or ions that deposit a magnetizable material on the substrate at a temperature of at least 300°C under conditions such thatthe magnetizable material is deposited on the substrate. The stability of the polybenzazole polymer is high enough thatthe magnetizable material can be applied by ordinary sputtering and vapor deposition techniques. The polybenzazole polymer can be selected to provide further advantages, such as high tensile strength, high tensile modulus, etc- The magnetic media can be used in an ordinary manner to record electronic data. The present invention relates to magnetic recording media. Each magnetic recording medium contains a substrate that has a film containing a polybenzazole polymer. (For purposes of brevity, the term "polymer" shall referto both homopolymers and copolymers, unless otherwise indicated.) Suitable polybenzazole polymers and polymer films are well-known in the art. Polybenzazole polymers are described in references such as Wolfe etal.. Liquid Crystalline Polymer Compositions, Process and Products, U.S. Patent 4,703, 103 (October 27, 1987); Wolfe et al.. Liquid Crystalline Polymer Compositions, Process and Products, U.S. Patent 4,533,692 (August 6, 1985); Wolfe et al., Liquid Crystalline Poly(2.6-Benzothiazole) Compositions. Process and Products, U.S. Patent 4,533,724 (August 6, 1985); Wolfe, Liquid Crystalline Polymer Compositions, Process and Products, U.S. Patent 4,533,693 (August 6, 1985); Evers, Thermooxidatively Stable Articulated p-Benzobisoxazole and p-Benzobisthiazole Polymers, U.S. Patent 4,359,567 (November 16, 1982); Tsai et al., Method for Making Heterocvclic Block Copolymer. U.S. Patent 4,578,432 (March 25, 1986); 11 Ency. Poly. Sci. & Eng., Polvbenzothiazoles and Polybenzoxazoles, 601 (J. Wiley & Sons 1988) and W. W. Adams et al.. The Materials Science and Engineering of Rigid-Rod Polymers (Materials Research Society 1989). The polymer may contain AB-PBZ mer units (as represented in Formula 1 (a)) and/or AA/BB-PBZ mer units (as represented in Formula 1(b)) Ka) AB K b ) AA/BB wherein: Each Ar represents an aromatic group selected such that the polymer is stable and nonmelting up to at least 300°C. The aromatic group may be heterocyclic, such as a pyridinylene group, but it is preferably carbocyclic. The aromatic group may be a fused or unfused polycyclic system, but is preferably a single six-membered ring. Size is not critical, but the aromatic group preferably contains no more than 18 carbon atoms, more preferably no more than 12 carbon atoms and most preferably no more than 6 carbon atoms. Examples of suitable aromatic groups include phenylene moieties, tolylene moieties, biphenylene moieties and bis-phenylene ether moieties. A in AA/BB-mer units is preferably a 1 ,2,4,5-phenyleπe moiety or an anaiog thereof. Ar in AB-mer units is preferably a 1 ,3,4-phenylene moiety or an analog thereof. Each Z is independently -0-, -S- or -NR- wherein R is hydrogen, an alkyl group or an aromatic group. Each DM is independently a bond or a divalent organic moiety selected such thatthe polymer is stable and nonmelting up to at least 300°C. The divalent organic moiety may contain an aliphatic group, which preferably has no more than 12 carbon atoms, but the divalent organic moiety is preferably an aromatic group (Ar) as previously described. It is most preferably a 1 ,4-phenylene moiety or an analog thereof. The nitrogen atom and the Z moiety in each azole ring are bonded to adjacent carbon atoms in the aromatic group, such that a five-membered azole ring fused with the aromatic group is formed. The azole rings in AA/BB-mer units may be in cis- or trans-position with respect to each other, as illustrated in 1 1 Ency. Poly. Sci. & Eng., supra, at 602. The polymer preferably consists essentially of either AB-PBZ mer units or AA/BB- PBZ mer units, and more preferably consists essential ly of AA/BB-PBZ mer units. The polybenzazole polymer may be rigid rod, semi-rigid rod or flexible coil. It is preferably rigid rod in the case of an AA BB-PBZ polymer or semi-rigid in the case of an AB-PBZ polymer. The polymer is preferably a polybenzoxazole (Z = -0-) or polybenzothiazo!e (Z = -S-) polymer, and is more preferably polybenzoxazole. The polymer preferably forms anisotropic solutions that contain lyotropic liquid crystalline domains when its concentration is higherthan a critical percentage. The polymer is selected such that it is stable and nonmelting up to at least 300°C, preferably at least 400°C, and most preferably at least 500°C. Stable means that the polymer is still useful after at least 45 minutes under no more than lO^ torr (.013 Pa) pressure at the stated temperature. Preferred mer unitsare illustrated in Formulae 2 (a)-(h). The polymer more preferably consists essentially of mer units selected from those illustrated in 2 (a)-(h), and most preferably consists essentially of a number of identical units selected from those illustrated in 2 (a)-(c). Each polymer molecule preferably contains on average at least 25 mer units, more preferably at least 50 mer units and most preferably at least 100 mer units. The intrinsic viscosity of rigid AA/BB-PBZ polymers in methanesulfonic acid at 25°C is preferably at least 10 dL/g, more preferably at least 15 dL/g and most preferably at least 20 dL/g. For some purposes, . an intrinsic viscosity of at least 25 dL/g or 30 dL/g may be best. Intrinsic viscosity of 60 dL/g or higher is possible, but the intrinsic viscosity is preferably no more than 40 dL/g. The intrinsic viscosity of semi-rigid AB-PBZ polymers is preferably at least 5 d L/g, more preferably at least 10 dL/g and most preferably at least 15 dL/g. Suitable polymers can be synthesized by known procedures, such as those described in Wolfe etal., U.S. Patent ,533,693 (August 6, 1985); Sybert et al., U.S. Patent 4,772,678 (September 20, 1988); Harris, U.S. Patent 4,847,350 (July 1 1 , 1989); and Ledbetter et al., "An Integrated Laboratory Process for Preparing Rigid Rod Fibers from the Monomers," The Materials Science and Engineering of Rigid-Rod Polymers at 253-64 (Materials Res. Soc. 1989). In summary, suitable monomers (AA-monomers and BB-monomers or AB-monomers) are reacted in a solution of non-oxidizing and dehydrating acid under non-oxidizing atmosphere with vigorous mixing and high shear at a temperature that is increased in step-wise or ramped . fashion from no more than 120°Cto at least 190°C. Examples of suitable AA-monomers include terephthalic acid and analogs thereof. Examples of suitable BB-monomers include 4,6- diamϊnoresorcinol, 2,5-diaminohydroquϊnoπe, 2,5-diamino-1 ,4-dithiobenzene and analogs thereof, typically stored as acid salts. Examples of suitable AB-monomers include 3-amino-4- -hydroxybenzoicacid, 3-hydroxy-4-aminobenzoic acid, 3-amϊno-4-thiobenzoic acid, 3-thio-4- -aminobenzoic acid and analogs thereof, typically stored as acid salts. Polybenzazole polymers are typically synthesized in an acid solution, known as a dope, with a suitable acid such as polyphosphoric acid. The dope should contain a high enough concentration of polymer for the polymer to coagulate to form a film of the desired thickness . without substantial flaws. When the polymer is rigid or semi-rigid, then the concentration of polymerin the dope is preferably high enough to provide an anisotropic dope that contains liquid-crystalline domains. The concentration of the polymer is preferably at least 7 weight percent, more preferably at least 10 weight percent and most preferably at least 14 weight percent. The maximum concentration is limited primarily by practical factors, such as polymer solubility and dope viscosity. The concentration of polymer is seldom more than 30 weight percent, and usually no more than 20 weight percent. The dope may be converted to a polybenzazole film or sheet by known methods, such as by: (i) extruding the dope out of a die; (ii) orienting or stretching the dope film uniaxially, biaxially or multiaxially; and (ϊii) coagulating the dope by contact with a diluent such as water. The die may be a siitorannulardie. The dope film may be stretched by any ordinary means, such as b tentering or by a bubble process. Examples of suitable film manufacturing techniques are described in: Chenevey, U.S. Patent 4,487,735 (December 11, 1984); Chenevey, U.S. Patent 4,898,924 (February 6, 1990); Harvey et al., U.S. Patent 4,939,235 (July 3, 1990); Harvey etal., U.S. Patent 4,934,285 (October 16, 1990); and Pierini etal., U.S. Patent Application Ser. No.07/670,135 (filed March 15, 1991). In a preferred technique, a dope film is extruded through a slit die. The film is stretched in the machine direction by rollers and stretched in the transverse direction by tentering to achieve desired tensile properties. It is coagulated by contact with a non-solvent under restraint to minimize shrinkage, washed to remove the remaining solvent, and dried under restraint to minimize shrinkage. The film is preferably stretched to at least 150 percent of its original length and width. It may be stretched up to 7 or more times its original length and/or width. The ratio of stretching in the machine direction (along the length of the film) to stretching in the transverse direction (along the width of the film) may be selected to provide a film with higher tensile strength or modulus along its length, higher tensile strength or modulus along its width, or equal properties in all directions. The tensiie modulus of the film is preferably at Ieast 200 ksi (1.4 GPa) (1 ksi = 1000 psi), more preferably at least 500 ksi (3.4 GPa), highly preferably at least 1 Msi (6.9 GPa) (1 Msi = 1 ,000,000 psi), more highly preferably at least 3 Msi (20.7 GPa), and most preferably at least 7 Msi (48 GPa). The tensile strength of the film is preferably at least 10 ksi (69 MPa), more preferably at least 25 ksi (170 MPa), highly preferably at least 50 ksi (340 MPa), more highly preferably at least 75 ksi (510 MPa), and most preferably at least 100 ksi (690 MPa). The film preferably meets those strength and modulus requirements in at least two directions that are parallel to the plane of the film and perpendicular to each other. The film more preferably meets those strength and modulus requirements in essentially all directions that are parallel to the plane of the film. The strength and modulus are sometimes less perpendicular to the plane of the film. The film preferably has no yield point- no point at which stress on the film irreversibly stretches the film from its desired shape without tearing. Its elongation to break is preferably between and 2 percent. The film may be cut into any form that is useful for a magnetic media substrate. It is preferably formed into a tape or a disc. The thickness of the substrate is preferably minimized, but the substrate must be at least thick enough to substantially retain its shape under the ordinary stresses that it faces when it is used. The substrate is usually between 0.1 and 1000 μm thick. It is preferably no more than 200 μm thick, more preferably no more than 100 μm thick, more highly preferably no more than 25 μm thick and most preferably no more than 10 μm thick. It is preferably at least 0.5 μm thick and more preferably at least 1 μm thick. The other dimensions of the substrate are governed by practical considerations relating to the device that it is to be used for. For instance, most magnetic recording tape substrates are between 4 mm and 50 mm wide, but the tape may be wider or narrower if desired. The optimum length of the tape substrate is related to the desired recording speed and recording time. For.instance, it may be any desired length from 1 meter to greater than 10,000 meters. Most magnetic discs are between 35 mm and 135 mm in diameter, but the disc may be larger or smaller if desired. When the substrate is in the form of a tape, then its average tensile modulus across the width of the film is preferably higher than its average tensile modulus along the length of the film. The higher tensile modulus along the width can hel p the tape to lay flatter. Such a substrate can be made by mechanically stretching the dope film in the transverse direction before coagulation, so thatthe resulting polymer film has a higher strength in the transverse direction. The film can then be slit longitudinally into suitable tapes. When the substrate is in the form of a disc, then it is more important that the tensile modulus be about uniform in all directions along the plane of the film. The variation of tensile modulus in the substrate is preferably no more than 20 percent, more preferably no more than 10 percent and most preferably no more than 5 percent. However, if the tensile modulus of the film is high enough, then the need for a uniform tensile modulus is less, because the substrate will not stretch significantly out of roundness in any case. A magnetizable layer adheres to at I east one face of the substrate. Magnetizable layers may optionally adhere to both faces. The magnetizable layer contains a continuous thin film of magnetizable material. Examples of suitable magnetizable materials are described previously and in Sharrock, "Particulate Recording Media," MRS Bulletin Volume XV, No. 3 at 53-61 (Materials Research Society March 1990). The magnetizable layer may be, for instance, barium ferrite or a nickel-cobalt oxide alloy. The magnetizable layer is preferably as thin as practical, but is usually at least 100 A thick. It is preferably no more than 5000 A thick, more preferably no more than 3000 A thick, and most preferably no more than 1500 A thick. The magnetizable layer is applied directly to the substrate, preferably by methods such as sputtering or vapor deposition. Unlike most common substrates, the polybenzazole substrates of the present invention can withstanding the temperatures needed to deposit materials by ordinary sputtering and vapor deposition (metal evaporation) processes. The optimal temperature for depositing the magnetizable layer varies depending upon the magnetizable material that is being deposited. For instance, barium ferrite can be deposited at temperatures below 300°C, but it is less effective for magnetic recording than barium ferrite that is deposited at a temperature of 300°C or greater. Barium ferrite is preferably sputtered at a temperature of at least 300°C, more preferably at least 400°C and most preferably at least 450°C. Similar temperatures may be used with other magnetizable materials, if desired. The temperature is preferably nb more than 650°C and more preferably no more than 550°C. The pressure during sputtering is preferably no more than 10 "3 torr (.13 Pa) and more preferably no more than 10 "4 torr (.013 Pa) and most preferably no more than 10 '5 torr (.0013 Pa). The minimum pressure is not critical. It is governed by practical considerations and is usually no less than 10 '7 torr (.000013 Pa). The magnetic medium ma optionallyfurthercontain a third "overcoat" layer overthe magnetizable layerto protect the magnetizable layer and/or smooth the surface of the medium. The overcoat layer is usually at least 250 A thick. It is usually no more than 5000 A thick. The overcoat layer preferably contains materials that are flexible, strong enough to protect the magnetic layer, adhesive on the magnetizable layer, thermally stable and/or have low water uptake. For instance, a carbon layer may be sputtered over a continuous thin magnetizable layer, or aluminum oxide particles and a lubricant such as perflouropolyether may be applied over a particulate and binder layer. Examples of other overcoat layers are described in Kitoo et al.. Magnetic Recording Medium Having Organic Protective Overlayer, U.S. Patent 4,529,651 (July 16, 1985). The coercivity of the finished magnetic recording medium is preferably at least 500 Oe, and more preferably at least 900 Oe, and most preferably at least 1500 Oe. The polybenzazole substrates of the present invention are essentially non¬ melting and thermally stable, so that exposure to high temperature, even those suitable for sputtering, does not harm the substrate. As additional advantages, many of the polybenzazole substrates have high tensile strength and modulus, so that they can be made thin. Many preferred polymers have essentially nδ yield point up tothe point that they tear, so that stresses below the tensile strength frequently will not permanently distort the shape of the substrate. The magnetic medium can be used in an ordinary manner for magnetic media. The following examples are for illustrative purposes only. They should not be taken as limiting the scope of either the specification or the claims. Unless otherwise stated, all parts and percentages are by weight. Example 1 A film of cis-PBO was made by (1 ) extruding a dope film from dope containing 14 percent cis-PBO dissolved in poly phosphoric acid that contained about 84 percent P 2 O c ; (2) stretching the dope film to three times its length and five times width; (3) coagulating and washing in water and drying in an oven. The film thickness was about 0.2 mil. The film was heated to a temperature shown in Table I under a pressure of 0.1 mtorr for 30 minutes. It was plasma cleaned at the temperature at a pressure of 0.5 mtorr with a gas composition of 25 percent oxygen and 75 percent argon. The sample was then sputter-coated with barium ferrite at a pressure of 0.5 mtorr under the temperature and for the time shown in Table I until the coating thickness is as shown in Table I. Sample 1-1 has a coercivity of 970 to 995 Oe as measured by a vibrating sample magnetometer which was commercially available from Digital Measurement Systems. Example 2 A film as described in Example 1 was sputter-coated with a layer of barium ferrite 2000 to 4000 A thick at a temperature of 450°C to 500°C and a pressure 10 "5 torr. The f i I m had a coercivity of 1350 Oe, as measure by the equipment in Example 1. Example 3 A PBO tape as described in Claim 1 was metallized by contacting it with evaporated cobalt and 10 s torr oxygen for a period of time sufficient to deposit a layer about 2000 A thick on average. The metallized film had a coercivity of 1677 Oe parallel to the deposition direction and 865 Oe perpendicular to the deposition direction.

Claims: CLAIMS 1. A flexible magnetic recording medium comprising: (a) a substrate that contains a polybenzazole polymer film; and (b) a magnetizable surface layer, that contains a continuous thin film of magnetizable material, adhered to at least one surface of the polybenzazole polymer characterized in thatthe substrate contains a polybenzazole polymer and the magnetizable surface adheres directly to the substrate. 2. A process to make a flexible magnetic recording medium comprising the step of contacting a flexible recording medium substrate that contains a polybenzazole polymer film which is stable up to at least 300°C with vapor or ions that deposit a magnetizable material on the substrate at a temperature of at least 300 C C under conditions such that the magnetizable material is deposited on the substrate. 3. An invention as described in any one of Claims 1 or 2 wherein the polybenzazole polymer consists essentially of mer units selected from the group: 4. The invention as described in any one of Claim 1 or 2 wherein the polybenzazole polymer is stable and nonmelting up to at least 400 C C. 5. The invention as described in any one of Claim 1 or 2 wherein the polybenzazole polymer is stable and nonmelting up to at least 500°C. 6. The invention as described in any one of Claim 1 or 2 wherein the magnetizable material is barium ferrite. 7. The invention as described in Claim 2 wherein the pressure is no more than .13 Pa. 8. The invention as described in Claim 2 wherein the temperature is at least 4Q0°C and the polybenzazole polymer is stable and nonmelting up to at least 400 C C.

Inventors: MCCARTHY TIMOTHY M, PERETTIE DONALD J, PIERINI PETER E, MCCARTHY, TIMOTHY, M., PERETTIE, DONALD, J., PIERINI, PETER, E.
Applicants: DOW CHEMICAL CO, THE DOW CHEMICAL COMPANY
Application year: 1993
Main classification: G11B5
IPC codes: C08G73, C08G75, G11B5