CA2132710A1 - Flexible magnetic recording medium containing polybenzazole polymers - Google Patents
Flexible magnetic recording medium containing polybenzazole polymersInfo
- Publication number
- CA2132710A1 CA2132710A1 CA002132710A CA2132710A CA2132710A1 CA 2132710 A1 CA2132710 A1 CA 2132710A1 CA 002132710 A CA002132710 A CA 002132710A CA 2132710 A CA2132710 A CA 2132710A CA 2132710 A1 CA2132710 A1 CA 2132710A1
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- Canada
- Prior art keywords
- substrate
- film
- recording medium
- polymer
- magnetizable
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Abandoned
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Classifications
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- 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
-
- C—CHEMISTRY; METALLURGY
- C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
- C08G—MACROMOLECULAR COMPOUNDS OBTAINED OTHERWISE THAN BY REACTIONS ONLY INVOLVING UNSATURATED CARBON-TO-CARBON BONDS
- C08G73/00—Macromolecular compounds obtained by reactions forming a linkage containing nitrogen with or without oxygen or carbon in the main chain of the macromolecule, not provided for in groups C08G12/00 - C08G71/00
- C08G73/06—Polycondensates having nitrogen-containing heterocyclic rings in the main chain of the macromolecule
-
- C—CHEMISTRY; METALLURGY
- C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
- C08G—MACROMOLECULAR COMPOUNDS OBTAINED OTHERWISE THAN BY REACTIONS ONLY INVOLVING UNSATURATED CARBON-TO-CARBON BONDS
- C08G73/00—Macromolecular compounds obtained by reactions forming a linkage containing nitrogen with or without oxygen or carbon in the main chain of the macromolecule, not provided for in groups C08G12/00 - C08G71/00
- C08G73/06—Polycondensates having nitrogen-containing heterocyclic rings in the main chain of the macromolecule
- C08G73/22—Polybenzoxazoles
-
- C—CHEMISTRY; METALLURGY
- C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
- C08G—MACROMOLECULAR COMPOUNDS OBTAINED OTHERWISE THAN BY REACTIONS ONLY INVOLVING UNSATURATED CARBON-TO-CARBON BONDS
- C08G75/00—Macromolecular compounds obtained by reactions forming a linkage containing sulfur with or without nitrogen, oxygen, or carbon in the main chain of the macromolecule
- C08G75/32—Polythiazoles; Polythiadiazoles
-
- 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
- G11B5/739—Magnetic recording media substrates
- G11B5/73923—Organic polymer substrates
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- Chemical & Material Sciences (AREA)
- Health & Medical Sciences (AREA)
- Chemical Kinetics & Catalysis (AREA)
- Medicinal Chemistry (AREA)
- Polymers & Plastics (AREA)
- Organic Chemistry (AREA)
- Compositions Of Macromolecular Compounds (AREA)
- Magnetic Record Carriers (AREA)
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
NO93~19461 ~ 1 ~ Z7 :L O pcr/us93/o22o6 FLEXIBLE MAGNE~IC RECORDING MEDIUM CONTAINING POLYBENZAZOLE POLYMERS
The present invention relates to the art of flexible magnetic recording media, 5 such as floppy disks and magnetic tapes.
Flexible magnetic recording media typically contain a flexible polymer substrateand a magnetizable surf3ce 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 10 made of a continuousthin layer of magnetizable material. Examples of common magneti able 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 magneti2able material to the substrate.
Common substrates can be destroyed by the process conditions needed to directly apply th 15 magn2tic material to the substrate. Typically, the magnetic rnaterial 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 methocds.
One as,oect of the present invention is a flexible magnetic recording 20 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 magnetlzable 25 surface adheres directly to the substrate.
A secona aspect of the present inventlon is a process tO make a flexible magnetlc recording medium comprising the step of contactlng a flexible recording medium substrate that contains a polybenzazole polymer film whicn is stable up to at least 300CC with va30r or WO 93/1~461 ~ 1 3 ~ 7 1 0 PCI /US93/0220h ions that deposit a magnetizable material on the substrate at a temperature of at least 300C
under conditions such that the magnetizable material is deposited on the substrate.
The stability of the polybenzazole polymer is high enough that the magnetizable material can be applied by ordinary sputtering and vapor deposition techniques. The 5 polybenzazole polymer can be selected to provide further advantages, such as high tensile strength, high tensile modulus, etc. The magnetic rnedia 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 polybenzazoie polymer.
10 (Forpurposesofbrevity,theterm"polymer"shallre~ertobothhomopolymersand copolymers, unless otherwise indicated.) Suitable polybenzazole polymers and ,oolymer films are well-known in the art. Polybenzazole polymers are described in references such as Wolfe et al ., Liauid Crvsta! !i ne Polymer Corr~ositions, Process and Products, U .S. Patent 4,703,103 (October 27,1987), Wolfe et al., Liauid ~rvstalline Polvmer ComPositions. Proce~ss and Products, U.S. Patent 4,533,692 (August 6,1985); Wolfe et al., Liquid Crvstalline pol~(2.6-Benzothiazole) ComDositions, Process and Products, U.S. Patent 4 533,724 (August 6,1985); Wolfe, Lic~uld Crvstalline Polvmer ComDositions, Process and Products, U.S. Patent 4,533,693 (August 6,1985);
Evers, Thermooxidative~v Stable Articulated p-8enzobisoxazole and P-~enzobisthiazole .Polvmers, U.S. Patent4,359,567 (November 16,1982); Tsai et al., Method for Makina 20 Heterocvclic Block CoPolvmer, U.S. Patent 4,578,432 (March 25,1986); 11 Ency. Poly. Sci. & Eng., Polvbenzothiazoles and Polybenzoxazoles, 601 (J. Wiley 8 Sons 1988) and W. W. Adams et al.
The Materials Science and Enaineerinq of Riqid-Rod Polvmers (Materials Research Society 989) The palymer may contai n AB-PBZ mer u nits (as represented i n Form u I a 1 (a))25 and/or AA/3B-PBZ mer units ~as represented in Formula 1 (b)) ~r <
1 (a) AB
;WO93/19461 ~3~ ~iO PCr/US93/02206 ~/ Ar 1 ~ ~DM
1(b~ AA/BB
wherei n:
Each Ar represents an aromatic group selected such that the polymer is stable and nonmelting up to at least 300C. The aromatic group may be heterocyclic, such as a pyridinylene group, but it is preferably carbocyclic. The aromatic group rnay be a fused or unfused polycyclic system, but is preferably asingle six-membered ring. Size is not criticai, but the aromatic group preferably contains no more than 18 carbon atoms, more preferably no more thlan 12 carbon atoms and most preferably no more than 6 carbon atoms. Exampies of suitable aromatic groups include phenylene moieties, tolylene moieties, biphenylene moieties and bis-phenylene ether moieties. Arl in AA/BB-mer units is preferably a 1 ,2,4,5-phenylene moiety or an analog thereof. Ar in AB-mer units is preferably a 1,3,4-pbenylene moiety or an analog thereof.
Each ~ is inclependently -O-, -S- or -NR- wherein R is hydrogen, an alkyi group or an aromatic group.
Each DM is independently a bond or a divalent organic moiety selected such that the polymer is stable and nonmelting up tO at least 300C. The divalent organic moiety may contain an aliphatic group, which preferably has no more than 12 carbon atoms, but the divalen~ organic moiety is preferably an aromatic grc3up (Ar3 as previously described. It is most preferably a 1 ,4-phenylene moiety or an analog thereof.
The nitrogen atom and tl1e Z moiety in each azole ring are bonded to adjacent carbon atoms in the arornatic qroup, such that a flve^membered azole ring fused with the aromatic group is formed The azole rings in AA/BB-mer units may be m cls- or trans-position with respect to each other, as illustrated in 1 1 Ency. Poly. Sci. & Eng., supra, at 602 The polymer preferably consists essentialiy of e,ther AB-PBZ mer un~ts or AA/BB-PBZ mer units, and more preferably consists essentially of AAIBB-PBZ mer unlts. The Dolybenzazole polymer may be rigid rod, seml-rigid rod or flexible coil It is preferably rigld rod in the case o~ an AA/BB-PBZ pcslymer or semi-rigid in the case ot an AB-PBZ polymer The 2 1 ~ 2 7 1 0 PCI /IJS93~02206 Y. ~,i``
polymer is preferably a polybenzoxazole ~Z = -O-) or polyben~othiazole (Z = -S-) polymer, and is more preferably polyben~oxazole. The polymer preferably forms anisotropic solutions that contain Iyotropic liquid crystalline domains when its concentration is higher than a critical percentage. The polymer is selected such that it is stable and nonmelting up to at least 300C, 5 preferably at least 400C, and most preferably at least 500C. Stable means that the polymer is still useful after at least 45 minutes under no more tt~an 104 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 10 preferably consists essentially of a number of identical units selected from those illustrated in 2 (a)-~c).
( ) 40 ~ 0 (b) ~0 ~N >{~
: :WO 93/19461 ~ ~ 3 ~ 7 ~ ~ PCl/US93~02~0 ( c ) ~5 ~N >~3--( ~ ~S ~ S>~3 (e ) ~ N,~
Each polymer molecule preferably contains on average at least 2S mer u~its, morepreferably at least 50 mer units and most preferably at least 1 Q0 mer units. The intrinslc viscosity of rigid AA/BB-PB2 polymers in methanesulfonic acid at 25C is ~referably a~ least 10 dUg, more preferably at least 15 dUg and most preferably at least 20 dUg. For some ourposes, an intrinsic viscosity of at least 25 dUg or 30 dUg may be best Intrinslc Viscoslty of 60 d LJo or higher is possible, but the mtrinsic viscosity is preferably no more than 40 dUg The m~rlns~c WO 93/1~461 2 1 3 ~ 7 ~ O PCl`/US93/02206 viscosity r.)f semi-rigid AB-PBZ po!ymers is preferably at least 5 dUg, more preferably at least 10 dUg and most preferably at !east 15 dUg.
Suitable polymers can be synthesizecl by known procedures, such as those described in Wolfe et al., U.S. Patent 4,533,693 (August 6,1985); Sybert et al., U.S. Patent 5 4,772,678 (September 20,1988); Harris, U.S. Patent 4,847,350 (July 11,1989); and Ledbetter et - al., "An Integrated Laboratory Process for Preparing Rigid Rod Fibers from the Monomers," The Materials Science and Enqineerinq of Riqid-Rod Polvmers at 253-64 (Materials Res. Soc.1989).
In summary, suitable monomers IAA-monomers and BB-monomers or AB-monomers) are reacted in a solution of non^oxidizing and dehydrating acid under non-oxidizing atmosphere 10 withvigorousmixingandhighshearatatemperaturethatisincreasedinstep-wiseorramped fashionfromnomorethan 120Ctoatleast190C. ExamplesofsuitableAA-monomersinclude terephthalic acid and analogs thereof. Examples of suitable B8-monomers include 4,6-diarninoresorcinol,2,5-diaminohydroquinone,2,5-diamino- 1,4-dithiobenzene and analogs thereof, typically stored as acid salts. Examples~of suitable AB-monomers include 3-arnino-4--hydroxybenzoic acid,3-hydroxy-4-aminobenzoic acid,3-amino-4-thiobenzoic ac~d,3-thio-4--aminobenzoic acid and analogs thereof, typically stored as acid salts.
Polybenzazole polymers are typically synthesized in an acid solution, known as adope, with a suitable acid such as polyphosphoric acid. The dope should contain a high enough con~entrationofpolymerforthepolymertocoagulatetoformafilmofthedesiredthickness 2~ without substantial flaws. When the polymer is rigid or semi-rigid, then the concentration of polymer in 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 rnost preferably at least 14 weight percent. The maximum concentration is limited primarily by practical factors, such as polymer 25 solubility and dope viscosity The concentration of polymer is seldom more than 30 welght 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 (iii) coagulating the dope by contact with a diluent such 30 as water. The die may be a slit or annular die. The dope film may be stretched by any ordinary means, such as by 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 Patent4,898,924(February6,1990); Harveyetal., U.S Patent4,939,235(July3,1990);
Harvey et al., U.S~ Patent 4,934,285 (October 16,1990); and Pierini et al., U.S Patent Appllcation 35 Ser No.07/670,135(filedMarch15,1991).
In a preferred technique. a dope film is extruded through a slit die. The film is stretched in the machlne direction by rollers and stretched in tne transverse dlrectlon by tentering to achieve desired tensile properties. It is coagulated by contact wlth a non-solvent .-~093/19461 ~ L~ v PCI`/US93/0~206 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 origi nal 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 5 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.
Thetensile modulus of the film is preferably at least 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.~ GPa) ( 1 Msi 10 = 1,000,0Q0 psi), more highly preferably at least 3 Msi (20.7 GPa), and most preferably at least 7 Msi (48 tiPa). The tensile strength of the film is preferably at least 10 ksi (S9 MPa), more preferably at least 25 ksi (170 MPa~, highly preferably at least 50 ksi (340 MPa), more highly preferablyatleast75ksi~sloMpa)~andmostpreferablyatleast1ooksi(69oMpa)~ Thefilm preferably meetsthose 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 meetsthosestrengthandmodulusrequirementsinessentiallyalldirectlonsthatareparallelto the plane of the film. The strength and modulus are sometimes less perpendicutar tO the plane of the film.
The film preferably has no yield point - no point at which stress on the film 20 irreversibly stretches the film from its desired shape without tearing. Its elongation to break is preferably between 1 and 2 percent.
The filrn 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 25 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 ym thick. It IS preferably no more than 200 ~um ~hick, more preferably no more than 100 ~m thick, more highiy preferably no more than Z5 ym thick and most preferably no more than 10 ym thick. It is preferably at least 0.5 }lm thick and more preferably at least 1 }~m thick The other dimensions of the substrate are governed by Dractical consiaerations relating tothe device that it isto be used for. For instance, most magnetlc recording tape substrates are between 4 mm and 50 mm wide, but the taPe may be wlder or narrower if desired. The optimum length of the tape substrate is related to the desired recordlng speed and recording time. for instance, it may be any desired length from 1 meter to greater than 35 10,000 meters. Most magnetic discs are between 35 mm and 135 mm m dlameter, ~ut 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 tenslle modulus aiong the WO 93/19461 ~ 1 ~ 2 ~ 1 0 PCl /US93/02206 `~
length of the film. The higher tensile modulus als)ng the width can help the tape to lay flatter Such a substrate can be made by mechanically stretching the dope film in the transverse direction before coagulation, so that the resultin~ 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 311 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 S percent. However, if the tensile modulus of the film is high enough, then the need for a uniform tensile modulus is less, 10 because the substrate will not stretch significantly out of roundness in any case.
A magnetizable layer adheres to at least one face of the substrate. Magnetizablelayers may optionally adhere to both faces. The magnetizable layer contains a ~ontinuous thin film of magnetizable material.
Examples of suitable magnetizable materials are described previously and in Sharrock, "Particulate ~;ecording 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 oxidealloy. Themagnetizablelayerispreferablyasthinaspractical,butisusuallyatleast100 ~ thick. It is preferably no more than 5000 ~ thick, more preferably no more than 3000 A thick, 20 and most preferably no more than 1500 A thick.
The magnetizable layer is applied di rectly 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 eva~ooration) processes. The 25 optimal temperature for depositing the magneti~able layer varies depending upon the magnetizable material that is being deposited. For instance, barium ferrite can be deposited at temperatures below 300C, bu~ it is less effective for magnetic recording than barium ferrite ~hat is deposited at 2 temperature of 300C or greater. Barium ferrite is preferably sputtered at a temperature of at least 300~, more preferably at least 400C and most preferably at least 30 450~C. Similar temperatures may be used with other magnetizable materials, if desired. The temperature is preferably no more than 650C and more preferably no more than 550C The pressure during sputtering is preferably no more than 10 ~ torr (.13 Pa~ and more oreferably no more than 104 torr (.013 Pa) and most preferably no more than 1 O j torr (.0013 Pa). The minimum pressure is not critical. It is governed by practical conslderat~ons and is usually no less than 10 ~ torr (.000013 Pa).
The magnetic medium may optionally further contaln a third "overcoat" layer over the magnetizable layer to protect the magnelizable 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
3VO 93/19461 ~ 7 1 (1 PCr/US~3/022~)~
thick. The overcoat layer preferably contains materials that are flexible, strong enough to protect the magnetic layer, adhesive on the magnetizable layer, thermal Iy stable and/or have low water uptake. For instance, a car~on layer may be sputtered over a continuous thin magnetizable layer, or aluminum oxide particles and a lubricant such as per~louropolyether 5 may be applied over a particulate and binder layer. Examples of other overcoat layers are described in Kitoo et al., Maanetic Recordin~Medium Havlna Oraanl otective Overlaver, U.S. Patent4,529,651 (July 16,1985).
The coercivity of the finished magnetic recording medium is preferably at least 500 Oe, and more preferably at ieast 90û Oe, and most preferably at least 1 Sû0 Oe.
The polybenzazole subst~ates of the present invention are essen~ially 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 polybenza~ole substrates have high tensi le strength and mod u I us, so that they can be made thi n . Many preferred polymers have essentially no yield poir~t up to the point that they tear, so that 15 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 illustratiYe purposes only. They should not be taken as limiting the scope of either the specification or the claims. U nless otherwise stated, al I
parts and percentages are by weight.
20 ~Bm~
Afilmofcis-PBOwasmadeby(1)extrudingadopefilmfromdopecontaining 14 percent cis-PBO dissolved in polyphosphoric acid that contained about 84 percent P~Os; (2) stretchin~ 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 25 neated to a temperature shown in Table I under a pressure of 0 1 mtorr for ~0 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 barlum ferrite at a pressure of 0.5 mtorr under the temperature and for the time sho~,vn in Table I until the coating thickness is as shown in Table 1.
Sample 1-1 has a coercivity of 970 to 995 Oe as measured by a vibrating sample magnetometer which was commerci al Iy avai lable f rom Digltal Measu rement Systems.
Example 2 A film as described in Example 1 was sputter-coated with a layer of barium ferrlte 35 2000 to 4000 A thick at a temperature of 450C to 500C and a pressure 10 torr The fiim had a coercivity of 1350 Oe, as measure by the equipment in Example 1.
ExamPle 3 WOg3/l9461 ~13~ ~10 PCr/us93/02206 Tabl e Sample TemP ( C) Time (min) r~ickness I-5 500 8. 5 1000 A P80 tape as described in Claim 1 was metallized by contacting it with evaporated cobalt and 10 5 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~o the deposition direction.
.
The present invention relates to the art of flexible magnetic recording media, 5 such as floppy disks and magnetic tapes.
Flexible magnetic recording media typically contain a flexible polymer substrateand a magnetizable surf3ce 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 10 made of a continuousthin layer of magnetizable material. Examples of common magneti able 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 magneti2able material to the substrate.
Common substrates can be destroyed by the process conditions needed to directly apply th 15 magn2tic material to the substrate. Typically, the magnetic rnaterial 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 methocds.
One as,oect of the present invention is a flexible magnetic recording 20 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 magnetlzable 25 surface adheres directly to the substrate.
A secona aspect of the present inventlon is a process tO make a flexible magnetlc recording medium comprising the step of contactlng a flexible recording medium substrate that contains a polybenzazole polymer film whicn is stable up to at least 300CC with va30r or WO 93/1~461 ~ 1 3 ~ 7 1 0 PCI /US93/0220h ions that deposit a magnetizable material on the substrate at a temperature of at least 300C
under conditions such that the magnetizable material is deposited on the substrate.
The stability of the polybenzazole polymer is high enough that the magnetizable material can be applied by ordinary sputtering and vapor deposition techniques. The 5 polybenzazole polymer can be selected to provide further advantages, such as high tensile strength, high tensile modulus, etc. The magnetic rnedia 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 polybenzazoie polymer.
10 (Forpurposesofbrevity,theterm"polymer"shallre~ertobothhomopolymersand copolymers, unless otherwise indicated.) Suitable polybenzazole polymers and ,oolymer films are well-known in the art. Polybenzazole polymers are described in references such as Wolfe et al ., Liauid Crvsta! !i ne Polymer Corr~ositions, Process and Products, U .S. Patent 4,703,103 (October 27,1987), Wolfe et al., Liauid ~rvstalline Polvmer ComPositions. Proce~ss and Products, U.S. Patent 4,533,692 (August 6,1985); Wolfe et al., Liquid Crvstalline pol~(2.6-Benzothiazole) ComDositions, Process and Products, U.S. Patent 4 533,724 (August 6,1985); Wolfe, Lic~uld Crvstalline Polvmer ComDositions, Process and Products, U.S. Patent 4,533,693 (August 6,1985);
Evers, Thermooxidative~v Stable Articulated p-8enzobisoxazole and P-~enzobisthiazole .Polvmers, U.S. Patent4,359,567 (November 16,1982); Tsai et al., Method for Makina 20 Heterocvclic Block CoPolvmer, U.S. Patent 4,578,432 (March 25,1986); 11 Ency. Poly. Sci. & Eng., Polvbenzothiazoles and Polybenzoxazoles, 601 (J. Wiley 8 Sons 1988) and W. W. Adams et al.
The Materials Science and Enaineerinq of Riqid-Rod Polvmers (Materials Research Society 989) The palymer may contai n AB-PBZ mer u nits (as represented i n Form u I a 1 (a))25 and/or AA/3B-PBZ mer units ~as represented in Formula 1 (b)) ~r <
1 (a) AB
;WO93/19461 ~3~ ~iO PCr/US93/02206 ~/ Ar 1 ~ ~DM
1(b~ AA/BB
wherei n:
Each Ar represents an aromatic group selected such that the polymer is stable and nonmelting up to at least 300C. The aromatic group may be heterocyclic, such as a pyridinylene group, but it is preferably carbocyclic. The aromatic group rnay be a fused or unfused polycyclic system, but is preferably asingle six-membered ring. Size is not criticai, but the aromatic group preferably contains no more than 18 carbon atoms, more preferably no more thlan 12 carbon atoms and most preferably no more than 6 carbon atoms. Exampies of suitable aromatic groups include phenylene moieties, tolylene moieties, biphenylene moieties and bis-phenylene ether moieties. Arl in AA/BB-mer units is preferably a 1 ,2,4,5-phenylene moiety or an analog thereof. Ar in AB-mer units is preferably a 1,3,4-pbenylene moiety or an analog thereof.
Each ~ is inclependently -O-, -S- or -NR- wherein R is hydrogen, an alkyi group or an aromatic group.
Each DM is independently a bond or a divalent organic moiety selected such that the polymer is stable and nonmelting up tO at least 300C. The divalent organic moiety may contain an aliphatic group, which preferably has no more than 12 carbon atoms, but the divalen~ organic moiety is preferably an aromatic grc3up (Ar3 as previously described. It is most preferably a 1 ,4-phenylene moiety or an analog thereof.
The nitrogen atom and tl1e Z moiety in each azole ring are bonded to adjacent carbon atoms in the arornatic qroup, such that a flve^membered azole ring fused with the aromatic group is formed The azole rings in AA/BB-mer units may be m cls- or trans-position with respect to each other, as illustrated in 1 1 Ency. Poly. Sci. & Eng., supra, at 602 The polymer preferably consists essentialiy of e,ther AB-PBZ mer un~ts or AA/BB-PBZ mer units, and more preferably consists essentially of AAIBB-PBZ mer unlts. The Dolybenzazole polymer may be rigid rod, seml-rigid rod or flexible coil It is preferably rigld rod in the case o~ an AA/BB-PBZ pcslymer or semi-rigid in the case ot an AB-PBZ polymer The 2 1 ~ 2 7 1 0 PCI /IJS93~02206 Y. ~,i``
polymer is preferably a polybenzoxazole ~Z = -O-) or polyben~othiazole (Z = -S-) polymer, and is more preferably polyben~oxazole. The polymer preferably forms anisotropic solutions that contain Iyotropic liquid crystalline domains when its concentration is higher than a critical percentage. The polymer is selected such that it is stable and nonmelting up to at least 300C, 5 preferably at least 400C, and most preferably at least 500C. Stable means that the polymer is still useful after at least 45 minutes under no more tt~an 104 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 10 preferably consists essentially of a number of identical units selected from those illustrated in 2 (a)-~c).
( ) 40 ~ 0 (b) ~0 ~N >{~
: :WO 93/19461 ~ ~ 3 ~ 7 ~ ~ PCl/US93~02~0 ( c ) ~5 ~N >~3--( ~ ~S ~ S>~3 (e ) ~ N,~
Each polymer molecule preferably contains on average at least 2S mer u~its, morepreferably at least 50 mer units and most preferably at least 1 Q0 mer units. The intrinslc viscosity of rigid AA/BB-PB2 polymers in methanesulfonic acid at 25C is ~referably a~ least 10 dUg, more preferably at least 15 dUg and most preferably at least 20 dUg. For some ourposes, an intrinsic viscosity of at least 25 dUg or 30 dUg may be best Intrinslc Viscoslty of 60 d LJo or higher is possible, but the mtrinsic viscosity is preferably no more than 40 dUg The m~rlns~c WO 93/1~461 2 1 3 ~ 7 ~ O PCl`/US93/02206 viscosity r.)f semi-rigid AB-PBZ po!ymers is preferably at least 5 dUg, more preferably at least 10 dUg and most preferably at !east 15 dUg.
Suitable polymers can be synthesizecl by known procedures, such as those described in Wolfe et al., U.S. Patent 4,533,693 (August 6,1985); Sybert et al., U.S. Patent 5 4,772,678 (September 20,1988); Harris, U.S. Patent 4,847,350 (July 11,1989); and Ledbetter et - al., "An Integrated Laboratory Process for Preparing Rigid Rod Fibers from the Monomers," The Materials Science and Enqineerinq of Riqid-Rod Polvmers at 253-64 (Materials Res. Soc.1989).
In summary, suitable monomers IAA-monomers and BB-monomers or AB-monomers) are reacted in a solution of non^oxidizing and dehydrating acid under non-oxidizing atmosphere 10 withvigorousmixingandhighshearatatemperaturethatisincreasedinstep-wiseorramped fashionfromnomorethan 120Ctoatleast190C. ExamplesofsuitableAA-monomersinclude terephthalic acid and analogs thereof. Examples of suitable B8-monomers include 4,6-diarninoresorcinol,2,5-diaminohydroquinone,2,5-diamino- 1,4-dithiobenzene and analogs thereof, typically stored as acid salts. Examples~of suitable AB-monomers include 3-arnino-4--hydroxybenzoic acid,3-hydroxy-4-aminobenzoic acid,3-amino-4-thiobenzoic ac~d,3-thio-4--aminobenzoic acid and analogs thereof, typically stored as acid salts.
Polybenzazole polymers are typically synthesized in an acid solution, known as adope, with a suitable acid such as polyphosphoric acid. The dope should contain a high enough con~entrationofpolymerforthepolymertocoagulatetoformafilmofthedesiredthickness 2~ without substantial flaws. When the polymer is rigid or semi-rigid, then the concentration of polymer in 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 rnost preferably at least 14 weight percent. The maximum concentration is limited primarily by practical factors, such as polymer 25 solubility and dope viscosity The concentration of polymer is seldom more than 30 welght 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 (iii) coagulating the dope by contact with a diluent such 30 as water. The die may be a slit or annular die. The dope film may be stretched by any ordinary means, such as by 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 Patent4,898,924(February6,1990); Harveyetal., U.S Patent4,939,235(July3,1990);
Harvey et al., U.S~ Patent 4,934,285 (October 16,1990); and Pierini et al., U.S Patent Appllcation 35 Ser No.07/670,135(filedMarch15,1991).
In a preferred technique. a dope film is extruded through a slit die. The film is stretched in the machlne direction by rollers and stretched in tne transverse dlrectlon by tentering to achieve desired tensile properties. It is coagulated by contact wlth a non-solvent .-~093/19461 ~ L~ v PCI`/US93/0~206 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 origi nal 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 5 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.
Thetensile modulus of the film is preferably at least 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.~ GPa) ( 1 Msi 10 = 1,000,0Q0 psi), more highly preferably at least 3 Msi (20.7 GPa), and most preferably at least 7 Msi (48 tiPa). The tensile strength of the film is preferably at least 10 ksi (S9 MPa), more preferably at least 25 ksi (170 MPa~, highly preferably at least 50 ksi (340 MPa), more highly preferablyatleast75ksi~sloMpa)~andmostpreferablyatleast1ooksi(69oMpa)~ Thefilm preferably meetsthose 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 meetsthosestrengthandmodulusrequirementsinessentiallyalldirectlonsthatareparallelto the plane of the film. The strength and modulus are sometimes less perpendicutar tO the plane of the film.
The film preferably has no yield point - no point at which stress on the film 20 irreversibly stretches the film from its desired shape without tearing. Its elongation to break is preferably between 1 and 2 percent.
The filrn 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 25 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 ym thick. It IS preferably no more than 200 ~um ~hick, more preferably no more than 100 ~m thick, more highiy preferably no more than Z5 ym thick and most preferably no more than 10 ym thick. It is preferably at least 0.5 }lm thick and more preferably at least 1 }~m thick The other dimensions of the substrate are governed by Dractical consiaerations relating tothe device that it isto be used for. For instance, most magnetlc recording tape substrates are between 4 mm and 50 mm wide, but the taPe may be wlder or narrower if desired. The optimum length of the tape substrate is related to the desired recordlng speed and recording time. for instance, it may be any desired length from 1 meter to greater than 35 10,000 meters. Most magnetic discs are between 35 mm and 135 mm m dlameter, ~ut 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 tenslle modulus aiong the WO 93/19461 ~ 1 ~ 2 ~ 1 0 PCl /US93/02206 `~
length of the film. The higher tensile modulus als)ng the width can help the tape to lay flatter Such a substrate can be made by mechanically stretching the dope film in the transverse direction before coagulation, so that the resultin~ 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 311 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 S percent. However, if the tensile modulus of the film is high enough, then the need for a uniform tensile modulus is less, 10 because the substrate will not stretch significantly out of roundness in any case.
A magnetizable layer adheres to at least one face of the substrate. Magnetizablelayers may optionally adhere to both faces. The magnetizable layer contains a ~ontinuous thin film of magnetizable material.
Examples of suitable magnetizable materials are described previously and in Sharrock, "Particulate ~;ecording 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 oxidealloy. Themagnetizablelayerispreferablyasthinaspractical,butisusuallyatleast100 ~ thick. It is preferably no more than 5000 ~ thick, more preferably no more than 3000 A thick, 20 and most preferably no more than 1500 A thick.
The magnetizable layer is applied di rectly 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 eva~ooration) processes. The 25 optimal temperature for depositing the magneti~able layer varies depending upon the magnetizable material that is being deposited. For instance, barium ferrite can be deposited at temperatures below 300C, bu~ it is less effective for magnetic recording than barium ferrite ~hat is deposited at 2 temperature of 300C or greater. Barium ferrite is preferably sputtered at a temperature of at least 300~, more preferably at least 400C and most preferably at least 30 450~C. Similar temperatures may be used with other magnetizable materials, if desired. The temperature is preferably no more than 650C and more preferably no more than 550C The pressure during sputtering is preferably no more than 10 ~ torr (.13 Pa~ and more oreferably no more than 104 torr (.013 Pa) and most preferably no more than 1 O j torr (.0013 Pa). The minimum pressure is not critical. It is governed by practical conslderat~ons and is usually no less than 10 ~ torr (.000013 Pa).
The magnetic medium may optionally further contaln a third "overcoat" layer over the magnetizable layer to protect the magnelizable 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
3VO 93/19461 ~ 7 1 (1 PCr/US~3/022~)~
thick. The overcoat layer preferably contains materials that are flexible, strong enough to protect the magnetic layer, adhesive on the magnetizable layer, thermal Iy stable and/or have low water uptake. For instance, a car~on layer may be sputtered over a continuous thin magnetizable layer, or aluminum oxide particles and a lubricant such as per~louropolyether 5 may be applied over a particulate and binder layer. Examples of other overcoat layers are described in Kitoo et al., Maanetic Recordin~Medium Havlna Oraanl otective Overlaver, U.S. Patent4,529,651 (July 16,1985).
The coercivity of the finished magnetic recording medium is preferably at least 500 Oe, and more preferably at ieast 90û Oe, and most preferably at least 1 Sû0 Oe.
The polybenzazole subst~ates of the present invention are essen~ially 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 polybenza~ole substrates have high tensi le strength and mod u I us, so that they can be made thi n . Many preferred polymers have essentially no yield poir~t up to the point that they tear, so that 15 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 illustratiYe purposes only. They should not be taken as limiting the scope of either the specification or the claims. U nless otherwise stated, al I
parts and percentages are by weight.
20 ~Bm~
Afilmofcis-PBOwasmadeby(1)extrudingadopefilmfromdopecontaining 14 percent cis-PBO dissolved in polyphosphoric acid that contained about 84 percent P~Os; (2) stretchin~ 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 25 neated to a temperature shown in Table I under a pressure of 0 1 mtorr for ~0 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 barlum ferrite at a pressure of 0.5 mtorr under the temperature and for the time sho~,vn in Table I until the coating thickness is as shown in Table 1.
Sample 1-1 has a coercivity of 970 to 995 Oe as measured by a vibrating sample magnetometer which was commerci al Iy avai lable f rom Digltal Measu rement Systems.
Example 2 A film as described in Example 1 was sputter-coated with a layer of barium ferrlte 35 2000 to 4000 A thick at a temperature of 450C to 500C and a pressure 10 torr The fiim had a coercivity of 1350 Oe, as measure by the equipment in Example 1.
ExamPle 3 WOg3/l9461 ~13~ ~10 PCr/us93/02206 Tabl e Sample TemP ( C) Time (min) r~ickness I-5 500 8. 5 1000 A P80 tape as described in Claim 1 was metallized by contacting it with evaporated cobalt and 10 5 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~o the deposition direction.
.
Claims (8)
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 that the substrate contains a polybenzazole polymer and the magnetizable surface adheres directly to the substrate.
(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 that the 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 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:
(a) , (b) , (c) , (d) , (e) , (f) , (g) , and (h) .
(a) , (b) , (c) , (d) , (e) , (f) , (g) , and (h) .
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.
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 400°C
and the polybenzazole polymer is stable and nonmelting up to at least 400°C.
(a) , (b) , (c) , (d) , (e) , (f) , (g) , and (h) .
and the polybenzazole polymer is stable and nonmelting up to at least 400°C.
(a) , (b) , (c) , (d) , (e) , (f) , (g) , and (h) .
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US85704392A | 1992-03-24 | 1992-03-24 | |
| US07/857,043 | 1992-03-24 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2132710A1 true CA2132710A1 (en) | 1993-09-30 |
Family
ID=25325052
Family Applications (2)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002132710A Abandoned CA2132710A1 (en) | 1992-03-24 | 1993-03-11 | Flexible magnetic recording medium containing polybenzazole polymers |
| CA002132545A Abandoned CA2132545A1 (en) | 1992-03-24 | 1993-03-11 | Flexible magnetic recording medium containing polybenzazole polymers |
Family Applications After (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002132545A Abandoned CA2132545A1 (en) | 1992-03-24 | 1993-03-11 | Flexible magnetic recording medium containing polybenzazole polymers |
Country Status (6)
| Country | Link |
|---|---|
| EP (1) | EP0632921A1 (en) |
| JP (1) | JPH07504291A (en) |
| KR (2) | KR950701116A (en) |
| CA (2) | CA2132710A1 (en) |
| TW (2) | TW240311B (en) |
| WO (2) | WO1993019461A1 (en) |
Families Citing this family (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JP3669457B2 (en) * | 1996-03-19 | 2005-07-06 | 富士通株式会社 | Magnetic recording medium and method for manufacturing the same |
| EP0927992A4 (en) * | 1997-07-17 | 1999-09-15 | Sony Corp | Magnetic recording medium and magnetic recorder/reproducer comprising the same |
Family Cites Families (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US3449296A (en) * | 1967-05-29 | 1969-06-10 | Du Pont | Polybenzoxazole film |
| JPS58122622A (en) * | 1982-01-18 | 1983-07-21 | Hitachi Ltd | Formation of organic protective film on magnetic recording medium |
| US5124432A (en) * | 1989-08-31 | 1992-06-23 | The Dow Chemical Company | Branched polybenzazole polymer and method of preparation |
-
1992
- 1992-07-09 TW TW081105449A patent/TW240311B/zh active
- 1992-07-09 TW TW081105450A patent/TW216460B/zh active
-
1993
- 1993-03-11 CA CA002132710A patent/CA2132710A1/en not_active Abandoned
- 1993-03-11 CA CA002132545A patent/CA2132545A1/en not_active Abandoned
- 1993-03-11 WO PCT/US1993/002206 patent/WO1993019461A1/en not_active Application Discontinuation
- 1993-03-11 JP JP5516615A patent/JPH07504291A/en active Pending
- 1993-03-11 EP EP93907411A patent/EP0632921A1/en not_active Withdrawn
- 1993-03-11 WO PCT/US1993/002207 patent/WO1993019462A1/en active Application Filing
-
1994
- 1994-09-24 KR KR1019940703310A patent/KR950701116A/en not_active Application Discontinuation
- 1994-09-24 KR KR1019940703309A patent/KR950701115A/en not_active Application Discontinuation
Also Published As
| Publication number | Publication date |
|---|---|
| CA2132545A1 (en) | 1993-09-30 |
| KR950701116A (en) | 1995-02-20 |
| EP0632921A1 (en) | 1995-01-11 |
| WO1993019461A1 (en) | 1993-09-30 |
| KR950701115A (en) | 1995-02-20 |
| JPH07504291A (en) | 1995-05-11 |
| TW240311B (en) | 1995-02-11 |
| WO1993019462A1 (en) | 1993-09-30 |
| TW216460B (en) | 1993-11-21 |
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