Note: Descriptions are shown in the official language in which they were submitted.
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
WO 93/19461 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
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.-~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
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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
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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- 1 450 20 3000
I-2 455 40 6000
I-3 465 40 6000
I-4 470 21 3000
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.
.
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