Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
1~2~
BAC~GROU~D OF THE I~VENTION
1. FI~I.D OP T~IE INVENTION
This invention relates to a magnetic recording
medium and a process for the production thereo and more
particularly, it is concerned with a magne~ic recording
medium with a low noise level and a process for the produc-
tion of the same.
2. DESCRIPTION OF TH~ PRIOR ART
Ferromagnetic materials used in magnetic recording
substances such as audio tapes, video tapes, memory tapes,
magnetic sheets and magnetic cards are fine powders of
ferromagnetic iron oxides, cobalt ferrite, ferromagnetic
chromium dioxide and ferromagnetic metals or thin films of
ferromagnetic metals. These magnetic recording substances
have been used in a wide variety of technical fields wherein
electric or magnetic signals are recorded and reproduced and,
of late, a system of recording, in particular, a short wave-
length signal in a high density has been watched with keen
i~terest. Accordingly, magnetic recording properties suit-
~0 able for high density recording, for example, a considerably
high coercive force and large residual flux density are
required for ferromagnetic materials. Moreover, it is ~
necessary, for example, for magnetic cards, tha-~ demagnetiza- ; -
tion due to heating or pressing is little. Metallic ferro-
magnetic materials }-ave been considered most promising for
high density recording with low demagnetization.
- On the other hand, the recording wavelength in a
video tape is much shorter than in a sound recording. Por
example, in a VTl~ (video tape recorder) for broadcasting,
a short wave to a minimum wavelength of about 2 microns has
,~.
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~o be recorded. In par~icular, a VTR using a minimum wave-
length of about 0.6 to 1 micron has la~ely been developed.
However, the above described o~ide-type magnetic subs~cances
are not suitable for magnetic recordin~ of a signal of short
recording l~avelength (about 2 microns or less)7 because their
magnetic properties such as particle shape, particle size,
coercive force and residual magnetic flux density are in-
sufficient for high density recording.
Development of ferromagnetic metal powders capable
of satisfying these properties and suitable for high density
recording has lately been carried out actively. The follow-
ing six methods are known as a method of preparing the
ferromagnetic metal powder:
1. A method comprising heat-decomposing an organic
acid salt of a ferromagnetic me~al and reducing with a
reducing gas. This method is described in, for example,
Japanese Patent Publication Nos. 11412/1961, 22230/1961,
14809/1963, 3807/1964, 8026/1965, 8027/1965, 15167/1965,
16899/1965 ~U.S. Pat. No. 3,186,829)~ 12096/1966, 14818/
l'J66 (U.S. Pat. No. 3,190,748), 24032/1967, 3221/1968, 22394
1968, 29268/1968, 4471/1969, 27942/1969, 38755/1971, 38417/
1971, 41158/1972 and 29280.
2. A method comprising reducing a needle-like iron
oxyhydroxide, substance containing metals other than iron
as well as the above oxyhydroxide or needle-like iron oxide
derived from the oxyhydroxide. This method is described in,
for example, Japanese Patent Publication Nos. 3862/1960,
11520/1962, 20335/1964, 20939/1964, 24833/1971, 2(~706/1962,
30477/1972 (U.S. Pat. No. 3,598,563), 39477/1972 and 24952/
aO 1973, Japanese Patent Application (OPI) Nos. 5057/1971, 7153/
2~
1971, 79153/1973 and 82695/1973 and U.S. Pat. Nos. 3,607,220
and 3,702,270.
3. A method comprising evaporating a ferromagnetic
metal in an inert gas at a lo~ pressure. This method is
described in, for example, Japanese Patent Publication Nos.
25620/1971, 4131/1972 and 27718/1972 and Japanese Patent
Application ~OPI) Nos. 25662/1973, 25663/1973, 25664/1973,
25665/1973, 31166/1973~ 55400/1973 and 81092/1973.
4. A method comprising heat-decomposing a metal
carbonyl compound. This method is described in Japanese
Patent Publication Nos. 1004/1964, 3415/1965 and 16868/1970
and U.S. Pat. Nos. 2,983,997, 3,172,776, 3,200,007 and
3,228,882.
5. A method comprising electrodepositing a ferro-
magnetic metal powder using a mercury cathode and then
separating the metal powder from mercury. This method is
described in, for example, Japanese Patent Publication Nos.
12910/1960, 3860/1~61, 5513/1961, 787/1964, 15525/1964,
S123/1965, 9605/1965 ~U.S. Pat. No. 3,198,717), 19661/1970
.S. Pat. No. 3,156,650) and U.S. Pat. No. 3,262,812.
6. A method comprising reducing a solution con-
taining a ferromagnetic metal salt by adding a reducing
agent thereto. This method is described in, for example,
Japanese Patent Publication Nos. 20520/1963, 26555/~9639
20116/1968, 9~69/19709 14934/1970, 7820/197Z9 16052/1972,
4171S/1972, 41719/1972 ~U.S. Pat. No. 3,607,Z18), Japanese
Patent Application ~OPI) Nos. 1353/1972 (U.S. Pat. No.
3,756,866), 1363/1972, 42252/19729 42253/1972, 44194/1973,
79754/1973, 82396/1973 and 41899/1974 and U.S. Pat. Nos.
3~ 3,206,338, 3,494,760, 3,535,104, 3,567,525, 3,661,556,
~2~2~
3,663,31~ 9,6~1~, 3,~72,~67 and ~,726,66~.
In a tape whereii1 a ferromagnetic metal powder
obtai'ned by each o~ these methods is mi~ed or dispersed in
a binder and dispersing agent and coated o~to a support
member, however, the noise level is high due -to that -the
dispersibility of the ferromagnetic is low, which consti-
tutes a bar for the practical use of a magnetic recording
medium using such a ferromagnetic metal powder. This is
considered to be due to the fact that the ferromagnetic
metal powder has an active surface and hy~rop'nilic property
and is hardly miscible with the binder. In particular, a
ferromagnetic metal powder obtained by the reduction with
a phosphinate compound or borohydride compound in'aqueous
solution has a very low miscibility with binders and, there-
fore, it is difficult to prepare a magnetic recording medium
with a high output and low noise level from such a ferro-
ma~netic metal powder.
SU~ ARY OF THE IN~ENTIO~
' It is an object of the invention to provide a
_0 magnetic recording medium with a low noise level. '
It is another object of the invention to provide
a magnetic recording medium having 0l1 a non-magnetic support
a magnetic layer comprising a ferromagnetic fine powder
dispersed in a binder.
It is a further o'bject of the invention to provide
a process for the production of a magnetic recording medium
with a low noise level. , -,~
I-t is ~ still further object o~ the invention to
providc an improved magnetic recording substance whereby the
3n above descrihed disadvantages of the prior art can be overcome.
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1 These objects can be attained by a magnetic recording
medium having on a non-magnetic support a magne~ic layer com-
prising a ferromagnetic fine powder dispersed in a binder, in
which colloidal silica having a diameter of 7 to 50 m~ and methyl
groups on the surface thereof is added during dispersion of the
coating composition of the ferromagnetic find powder.
BRIEF DESCRIPTION OF THE DRAWING
The accompanying drawing is a graph showing the relation
of the quantity of a colloidal silica with the output and noise
o~ a video tape in one embodiment of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
We, the inventors, have made efforts to provide a mag-
netic recording medium having a high output and a low noise level
using a ferromagnetic metal powder. That is to say, the
inventors have made studies on various methods to increase the
dispersibility of alloy find powders of li~uid phase reduction
type and consequently have found that a magnetic recording
medium with a markedly decreased noise level can be obtained by
preparing a magnetic coating composition containing colloidal
silica having a diameter of 7 to 50 m~ and methyl groups on the
surface thereof, added during dispersion of a ferromagnetic fine
powder and coating the magnetic coating composition onto a non-
magnetic support, followed by drying. The present invention
is based on this finding. In summary, the present invention
provides a magnetic recording medium having on anon-magnetic
support a magnetic layer comprising a ferromagnetic find powder
dispersed in a binder, characterized in that the magnetic
5 -
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layer contains colloiclal silica. Furthermore3 in accordance
with the present invention, there is provided a process for
the production of a magnetic recording medium, which com-
prises dispersing a ferromagnetic fine powder in a binder
while simultaneously aclding thereto colloidal silica having
a diameter of 7 to 50 m~ and methyl groups on the surface
thereof to thus prepare a magnetic coating composition,
coating the magnetic coating composition onto a non-magnetic
support and then drying.
In the prior art, addition of colloidal silica to
a magnetic layer is also carried out as disclosed in Japanese.
Patent Application (OPI) Nos. 11909/1974 and 12802/1974,
which aim at strengthening a binder and thus need a step of
mixing previously the binder and colloidal sillca. It is
further found that, if such a strengthened binder prepared
by mixing previously a binder and colloidal silica is used,
the object o the present invention cannot be obtained.
- This is possibly due to that in this step, the binder is
bridged partly by colloidal silica and thus the dispersibility ~;
of the binder is lowered.
The colloidal silica used in the present invention
is silicic anhydride having~a diameter of 7 to 50 m~, pref~
erably 10 to 30 m~ which is gienerally prepared by combustion
of silicon tetrachloride. The silanol groups on the surface~
of colloidal silica are decreased by treatment with methanol,
triemethylmonochlorosllane, dimethyldichlorosilane, etc.
in the present invention. The thus treated colloidal silica
having methyl groups on the surface is herein used. This
methyl group replacement is generally carried out by heating
colloidal silica in methanol at the boiling polnt or treating
:'
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. , . : .~" .
:. : . . .
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colloidal silica ~ h methanol vapor ior 30 rr.inutes to 6
hours or by treating colloidal silica and Se~rachlorosilane
with methanol vapor or 30 minutes tc b hours~ ~lore prefer-
ably, colloidal silica with dimethyldichlorosilane and s~eam
are subjected to heating or reaction at about 400 C in a
fluidi~ed bed using an iner~ gas such as nitrogen gas as a
carrier. In the colloidal silica obtained in thi.s way, 70
or more of the silanol groups on the surface is replaced by
methyl groups. The colloidal silica has one silanol group
~-Si-OI-I) bonded to the surface ~hereof per 28 ~ 33 A (square
Angstrom), and thus, a colloidal. silica with a specific
surface of 200 m2/g has about 6 x 102 silanol groups in lg, ~ ~
which ~lleans that one particle o colloidal silica has about ~;
2,00G silanol groups. ~ilerefore, about 140 silanol groups
in each particle of the above described colloidal silica are
replaced by methyl groups. These si.lanol groups and methyl
gl'OUpS can be analysed by means of an infrared ray spectro-
scope. It is not clear why such a colloidal silica having
methyl groups is preferable, but the reason is possibly that
~0 the surface thereof is hydrophobic and miscible wit'n binders.~ ~-
The colloidal used in the present invention is added
wllen a erromagnetic fine powder and binder are mixed, kneaded
or dispersed. Addition after the dispersion lS not prefer~ble
because the object of the present lnvention canno~ completely
be attained. The quantity of colloidal silica to b~ adcled
is generally 2 to 20 parts by weLght, prelerably ~ to lO parts
by weight, more preferably 4 to 8 parts by weig}lt per 100 `;
parts by weight of a f-erromagnetic fine powder. If the
~uantity is less than 2 parts by weight, the~ effects or
~0 advantages of the present invention cannot be obtained, while
"~
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1 if more than 2Q parts by weight, not only the magnetic flux
densi.t~ of the magnetic recording medïum is low, but also the
adhesiveness of the magnetic recording layer to a suppor-t is
deteriorated and the magnetic recording layer tends to peel off.
Methods of preparing the magnetic coating composition
used in the present invention are described in detail in Japanese
Patent Publication Nos. 186/1968, 28043/1972, 28045/1972,
28046/1972, 2804B/1972 and 31445/1972. The magnetic coating
compositions in these patents mainly comprise a ferromagnetic
powder, a binder and a coating solvent, and in addition, the
composition can contain additives such as a dispersing agent, a
lubricant, an abrasive, an antistatic agent and the like.
The ferromagnetic powder which can be used in the
present învention includes ferromagnetic alloy fine powders, .
ferromagnetic iron oxides, ferromagnetic chromium dioxide, etc.,
and for the purpose of achieving the advantages of the present
invention, it is preferable to use ferromagnetic alloy fine
powders, above all, those obtained by the method (6). The
ferromagnetic metal salt used in this method means a salt con-
~ taining, as a main component, iron, cobalt, nickel, iron-coba}t,
iron-nickel, cobalt-nickel or iron-cobal-nickel and, optionally,
a minor amount of one or more salts of lanthanum, cerium, neo-
dymium samarium, aluminum, sulfur, chromium, manganese, copper,
tin and zinc to improve the magnetic property and oxldation .
sta~ility. Examples of these metal salts are sulfates, chlorides,
ni.trates, formates, acetates, sulfamates and pyrophosphates.
As the reducing agent for effecting the reducing
.
.
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rcaction~ ~here can generally be used one or more of acids
or salts containing hypophosphite ion, borohydride compounds
such as sodium borohydride, borane and borazane or derivatives
thereof, hydrazine or deriva~ives thereof and reducing gases
such as hydrogen and carbon monoxide. The above described
metal salt is reduced by this reducing agent to precipitate
the corresponding ferromagnetic metal or alloy.
The concentration of borohydride compounds or
derivatives thereof used as a reducing agent is preferably
0.0002 to 10 mols/l and the reaction is preferably carried
out within a range of a reducing agent/metal ion molar ratio .
o~ 0.1 to 5.
The reaction conditions in this method are not
particularly limited but, preferably, the reaction pressure
is 0.5 to 5 atmospheres and the reactlon temperature and pH ~-;
depend on a reducing material used. Preferably, the reaction
tcmperature is 65 to 90 C and pH lS 8 to 12 in the~system "
of hypophosphite; the temperature~is -5 to;60 C and pH is
l to 12 in the system of a borohydride compound; and the
~n temperature is 60 to 100 C and~pH is 7.5 to 12 in the syste~
of hydra~ine. The action of a magnetic field is useful at `
several ten oersteds or more and a DC magnetlc field, AC
: ~.
magnetic field or pulse magnetic field is e:ffectl~e.
The concentration o a metal`ion should be adjusted
to such as not to give its supersaturation or less. I the
concentration is too high, various problems arise that the
properties of the resultant powder are deteriorated~ -the
reaction yield is lowered and enlargement of.the reaction ~-
apparatus becomes necessary due to foaming, whilst, if the
~0 concentration is too low, the powder yield is lowered resulting
in lo--ering o:E the production e-iciency on a comnercial.
scale ~ind enlargemen~. o-f ~he reaction appara~us is thus
necessary. In this mechod, ~he concen~ration of a metal
iOIl ia ordinarily in the range of 0.002 to 4 mols/l, pre-f-
el-ably 0.01 to 2 rnols/l.
Sllitable binders which can be used in the present
invention include hi~herto known thermoplastic resi.ns, thermo-
setting resins and mixtures thereol. ~
Suitable tnermoplastic resins are those resins ..
~hich have a softenino; point o~ about 150 C or less, a
mean molecul.ar weight of about 10,000 to ~00,000 and a degree. .:
of poly~llerization of the order of about 200 to 2~000 J for
example, a vinyl chloride-vinyl acetate copolyrner, a vinyl
chlo*ide-vinylidene chioride copolyrner, a vi.ny~ chloride- ;
acrylonitrile copolymer, an acrylale-acrylorlltr;.le copolymer,
an acrylate-vinylidene chloride copolymer, an acrylate-
styrene copolymer, a methacrylate-ilcrylonitrile copolymer,
a methacrylate-vinylidene ch:Loride copolymer, a methacrylate- ~ -
s~yrene copolymer, an urethane elastomer, a polyvinyl ~luoride,
a vinylidene chloride-acrylcnitrile copolymer, a polyamide,
-
a polyvinyl butyral, cellulose derivatives such as cellulose l :~
acetate butyrate, cellulose diacetate, cellulose triacetate, :
cellulose propionate; cellulose and the like, a styrene-
acrylonitrile copolymer5 a polyester resin~ an amino resin
...
various synthetic rubber based thermoplastic resins such as :
polybutadiene, polychloroprene, polyisoprene, styrene-
but~d.iene copolymer, butadiene-a.crylonitrile copolymer and
the like and mixtures thereo~
Examples of these resins which can be used are
30 described in Japanese Pa~ent Pubiic:ation Nos. 6877/1962,
-10-
.
Z~
1 ?528/1~J6~ _82/.1.;.)~ r ~ 53~9/~ ?0907/ 1~S.~ ~ ~4 'rj ,/ 1966 ~ ~
1!10~S!1966 ~ 16~8~/ i.3~, 6~L ~?~/lr1fj7 ~ ~ f~2 jl ~ 7 ~ )23/lr)~
15206/ll36~, 2SS9/l~li9, ï.7~3~,7/].963, l~,2~2/1909 ~ l~rO20/1570 ~
i~500/1970 ~ IS575/19 ~2, 22063/1'~72 ~ 2 ~ 064 /1972 ~ '205~/lr~)72 7
?2069/1972 ~ 22070/1~7~ and 27886/1973 and IJ . S~ Pat. ~os.
3~ ,35~., 3~419,~2()7 3~49997~9 ~d ~,713~7.
Suit~ble tne*mGsetti.ng resins h.lve ~ molecular
~ieight o~ abo~t 20,000 or less as a coating sollltion and
wllen heatecl af~er coat.illg and drying, -the molecul r weight
beco]lles infi.nity due t.o reactions sucn as ccndensa~ion~
addition and the like. Of these resi.rls, preferred resins .
are those resins which do not soften or melt before the
l`eSill tnermally decompose,.
Examples of these resins~are a phenol-Lorn~.alin
novolak resin~ a phenol-formalin resol resin:, a phenol-
furfural resin~ a x~ylene-formaldeh~yde resin, a ulea re~sin, ` : .
a melamine resin~ a drying oil-modi.fied alkyd resin, a
phenol resi.n-.llodified alkyd res~.n9 a mdlei.ç resin-modified ;;`
alkyd resin, all unsaturated polyester res;in~ a.n epoxy resin
~0 alld llardening agent such as polyamine, acid anhydlicle, poly-.
amide res:in and tile like, an isocya.nate-ter~,linated polyes~er
moisture-hardenin~ type resin, an isocyanate-termina~ed :~
pol)~ether moisture-h.lrdening type resin, s polyiiocyallate
prepolymer such as compound having three or moree lsocyanate
groups in one molecular obtained by ieacting a~diisocyanate~ ~.
~it~l a low moleculzr ~.~eight triol, trimer, teLrarlle~ and
pentamer of diisocyana~es, a po].y:ijocya.nate pre-r,ol~rmer a.lLcl
resin 'laving active hydrogen such as polyestor pc)lyol. ~ poly-
etller polyol, acrylic acld copolyn!er~ maleic aci.d copolymer,
-ilydroxyethyl ~ethacrylate copolymer, p-llydroxysty-rene and
~L~Z~Z24L
the like, and Iri~ures thereof.
Suitable examples of these resins are desc-~ibed
in Japanese Patent Publica-tion hos. ~103/1964, 9779/1965,
71~/1966, 801~/1966, 14275/1966, 18179/196~, 120~1/1968,
28023/1969, 1~501/1970, 24902/1970, ~3103/1971~ 22065/1972,
~066/1972, 22067/1972, 22072/1972, 22073/1972, ~8~J45/1972,
28048/1972, 28322/1972, U.S. Pat. ~os. 3,144,353, 3,320,090,
3,437,510, 3,597,2737 3,781,210, 3,781,211, etc.
These binders can be used individually or in combi-
naticn WitII eacil other and other additives can be added to
the binders. The mixing ratio by weight of the Eerromagnetic~ ;
powder ancl thc binder is 10 to 400 parts by weight, preEer-
ably 15 to 200 parts by weight, more preferably 15 to ]00
parts by weight of the binder per 100 parts by weight of
the ferromagnetic powder.
In addition to the above described binder and ferro- ~ ;
magnetic fine powder, additives such as a dispersing agent,
a lubricant, an abrasive, an antistatic agent, and the like
can be used in the magnetic rècording layer.
Suitable dispersing agents are fatty acids contain
ing about 12 to 1~ carbon atoms (e.g.~ having the formula -
Rl COOH wherein Rl is an alkyl group containing about 11 to
17 carbon atoms), for example, caprylic acid, capric acid?
lauric acid, myristic acid, palmitic acid, stearic acid,
linolenic acid, stearolic acid and the like; metallic soaps
comprising the alkali metal (Li, Na, K, etc.) salts or the
alkaline earth metal (Mg, Ca, Ba, etc.) salts o:E the above
described fatty acids; fluorine-containing compounds of the
above describ~d fatty acid esters; amides of the above des-
cribed fatty acids; alkylphosphoric acid esters of polyalkylene
-12-
.
~-~.2~
oxide; leci~l~in; trialkylpolyolef'ino~y quat~rnary ammonium
sal~s ~alkyl grou!~ has 1 to S carbon ator~s ancl olefin means
ethylcne or propyl~ne), etc. III addition, higher alcohols
containing ~bout 12 or mor~ car'bon atoms and -the sul-fat~s
thereof can be used. These dispersing agents are generally
employed in a propor~ion o about 0.5 to 20 parts by weight
per 100 parts by weight of the binder. Sui~able dispersing
agents are described in Japanese Patent Publication Nos. ~
2&369/l;j64~ 17315/1969, 7441/1973~ 15001/1973~ 15002/1973~ .
16363/1973 and 4121/1~75 and U.S.'Pat. Nos. 3~387~993 and ~'""
3~470~021~ The additives of the present invention can be
used togeth~r with these dispersing agen-ts without deterio- ~
rating the effects of these dispersing agents. ' '
Suitable lubricants whlch can be used include in- ~'
organic fine powders SUC}I as molybdenum disulfide, tungsten
disulfide and thc like; plastic fine powders such as poly-
ethylene, polypropylene~ ethylene-vinyl chlorLde copolymer,
polytetraf]uoroethylene and the llke; ~-olefin polymers;
un~aturated aliphatic hydrocarbons liquid at room tempera-
ture (compounds wherein the double bond of ~-olefin is com- ; ~'
;, ~
bined ~ith the terminal car~on, the nulnber of carbons: about ~ .
20); and falty ~cid esters of monobasic Iatty aclds havlng
12 to 20 carban atoms and monohydric alcohols havirlg :S to 12
carbon atoms. These lubricants are generally used~in a
proportion of about 0.2 to 20 parts by weight per 100 parts
by weigllt of t}le binder. I'hese lu'bricants are described in `,
Japanese Patent Publication Nos. 18064/1966, 23889/1968,
40461/1971 ~ 1562i/1972 ~ 18482/1972 ~ 28043/L97.2 ~ 32001/1972
and 5042/1975 and U.S. Pat. Nos. 3,470,021t 3~9Z>235~ 3~497~ `
~0 ~ 3~523~086~ 3~625~760~ 3~630~772~ 3 j6~r2~533~ 'IIBM Technical
,~
:
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L2Z~
Disclosure 13u:Lletin"~ Vol. 9 ~o. 7 page 77~ (I)ecember 1966),
and ~'ELECTRONIK~ o. 12, page 380 (1~61).
Typical abrasi.ve agents whicll can be used inc~ude
materials gene-rally used, e.g~, fused alumina, silicon car-
bide, cllromium o~ide7 corundum, syn~hetic corundu,n, diamond,
synthetic. diamond, garnet, emery (main component: corundum
and magnetite) and ~he like. Those abrasive agen~s are used
~hicll have an average particle size of about 0.05 to 5 l~m,
preferably about 0.1 to 2 ~m. These lubricants are generally
1~ use~ in a proportion of from abou~ 0.5 to 20 parts by weight
per 100 parts of the binder. 'rhese abrasive agents are des-
cribed in, for example, Japanese Patent Publication Nos.
18572/1973, 15003/1973, 15004/197~ ~U.S. Pat. No. 3,617,
37S), 39402/1974 and 9401/1975, UiS. Pat. Nos. 3,007,807,
3,041,196, 3,293,066, 3,630,91Q, and 3,~687,7253 British
Patent No. 1,145,349, and DT-PS 8C3,211. Tlle joint use of
these abrasive agents with the additives of the~present
invention results in decrease of the head abrasiveness due
to the abrasive agents.
~0 Antistatic agents whlch can be used in the present ~ ~
invention include electrically conductive fine pol~ders such ;~ '
as of carbon black, graphite, carbon black gra~ted polymers, ~ -
etc.; natural surface active agents such as saponin; nonionlc '.
surfacc active agents such as alkylene o~ide based, glycerln
based, glycidol based sur~ace active agent and the like;;'
cationic surface active agents SUCIl as heterocycli.c cor,lpounds,
e.g., higher alkylamines, quaternary ammonium salts, pyridine
`~ and the like, phosphoniums,sulfoniun~sancl the like; anionic
surface active agents containi.ng acid groups such as clrboxylic
acid groups, sulfonic acid groups, phosphDric acid groups,
, :
14- ~ '
.
2~
julf2te groups, phosp}late groups a-nd t}-le like; and amp}loteric
surface active agents such as sulfates or phosphates of amino
acids, amino sulfonic acids and amino alcohols and the like;
etc. The above described electrically conductive fine pol~der
is gellerally adde~ in a proporl:ion o 0.2 to 20 parts by
~eight to 100 parts by ~leight of the binder and the surface
ac~ive agents are generally added in a propor~ion o-f 0.1 to
10 parts by weight.
F:xa~ples of the electrically conducti~e fine powder
and surface active agents ~hat can be used as antistatic
agents are describecl in, for example, Japanese Patent Publi- ~;
cation Nos. 22726/1971, 24881/l972, 26882/lg72, 15440/1973
and 26761/1973, U.S. Pat. Nos. 2,2717623, 2,240,4~2, 2,288,
226, 2,676,122, 2~676,924, 2,676,975, 2,691,566, 2,727,860,
2,730,~98, 2,7~273737 2,739,891, 3,068,1017 3,158,484, 3,201,
253, 37210,191, 3,2~,5407 3,415,6497 3,441,~137 37442765~7
3,475,174 and 3,545,9747 West German Patent Application (OLS)
No 1,942,665, British Patent Nos 1,077,317 and 1,198,450~
Ryollei Oda, et al., "Kaimen Kassei Zai no Gosei to so no Oyo
(Synthesis of Surface Active Agents and Their Applicatlons),;
~laki Shoten, Tokyo (1964), A.l~l. Schwarts et al., "Surface
Acti~e Agents", Interscience Publicatlons Colp.,~New York
~1958), J.P. Sislcy et al., i'Encyclopedia of Surface Active
Agents", Vol. 2, Chemical Publishing Co., New York, "Kaimen
Kassei Zai Binran (Handbook of Surface Active Agents)", 6th
Ed., Sangyo Tosho Co., Tokyo7 ~ec 20, 1966, etc.
These surface active agents can be used indivldua]ly
or in combination l~ith each other. These surface active
agents are generally used as antistatic agents~ bu~ in some ~ `cases, they are used for other purposes, for example, for `~
`
-15
- : , , , " ~
24L
improviing disll,ersibi!ity~ magrletic proper~ies, and 'Lubricity,
or as auxiliary coatinn agellts.
Forrnation of -the magnetic recording layer of the
plesent invention is carried ou~ by dissolvinr, kneading or
dispersin~ the above described composition in an organic
solven-t and then coating the resultin~ composi-~ion onto a
non-magne~ic support, -ollowed by drying. After the coating
but before the drying~ this magnelic layer ca~ be subjected
to a treatment for orienting the magnet:ic powder in the layer
and if desired, the magnetic layer can be subjected to a
surface-smoothening treatment after the drying. .
Suitable materials \~hich can be used for producing
the non-magnetic support are plastics, for example', poly-
esters such as polyethylene terephthalate~ polyethylene-2,6-
napllthalate and the like, polyolefins such-as polypropylene ~ -
and the like, cellulose derivatives such as cellulose
triacetate, cellulose diacetate and the like, polycarbonates,
etc., non-magnetic rnetals such as co~pper, aluminum~ ~illC ,
and the like, and ceramics such as glass, porcelain, earthen-
.
_O ~are and the like.
~he shape of such a non-magnetic support can 'be
chosen froln .Llly of tapes, films, sheets, disks, cards and
drums ancl the material thereoE can optionally be chosen
from the above described valious matelials dependlng on the
; shape. The thicknoss of the non-rnagnetic support is about
2 to 50 ~Im, preferably 3 to 25 ~m in the case~of fil~ns,
tapes or sheets and about 0.5 to 10 mrn in the case of disks ~'
or cards. In the case of drums, a suitable cylindrican form
is chosen depending on a recorder used.
' The above described non-magnetic support can be
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1 subjected to the so-called back coating of the surface opposite
to that hav;ng the magnetic layer thereon, for the purpose o
preventing static charging, magnetic print through, wow flutter
and so on in the case of flexible supports such as films, tapes,
sheets, thin flexible disks and the like. Suitable back coating
techniques which can be used are described in, for example,
U.S. Pat. Nos. 2,804,401, 3,293,066, 3,617,378, 3,062,676,
3,734,772, 3,476,596, 2,643,048, 2,803,556, 2~887~462
2,923,642, 2,997,451, 3,115,420, 3,166,688 and 3,761,311.
The ferromagnetic powder, binder, colloidal silica, dis-
persing agent, lubricant, abrasive, antistatic agent, solvent, etc.
are mixed and kneaded to prepare a magnetic coating composition.
In the kneading, the magnetic powder and other components are
charged in a kneading machine simultaneously or separately. For
example, in one method a ferromagnetic powder is added to a sol-
vent containing a dispersing agent and kneaded in a predetermined
time to prepare a magnetic coating compositions. For the kneading
and dispersing of such a magnetic coating composition, there can
be used various kneading machines such for example as two rolls
mills, three rolls mills, ball mills, pebble mills, trommels,
sand grinders, Szegvari attriters, high speed impeller dispersing `~
machines, high speed stone mills, high speed impact mills, dis-
persing kneader~, high speed mixers, homogenizers, ultrasonic
dispersing machines, etc.
The magnetic recording la~er can be coa-ted on the support
using coating methods such as air doctor coating, blade
coating, air knife coating, squeeze coating, dip coating, reverse
`~ roll coating, transfer roll coating, gravure .`
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1 coating, kiss coating, cast coatiny, spray coating and the
like, and other methods can also be used. These methods are
described in, for example, "Coa~ing Kogaku (Coating Engineering)",
pages 253 to 277, published by Asakura Shoten, Tokyo (March ~0,
1971).
In the magnetic recording medium of the present invention,
a magnetic layer is coated on a non-magnetic support by the
above described coating method and then dried. In some cases, two
or more magnetic layers can be provided by a continuous coating :
operation repeating this process or by a simultaneous m~tltilayer
coating method as described in Japanese Patent Application (OPI) .
Nos. 98803/1973 (West German Patent Application (OLS) No
2,30~,159 and 99233/1973 (West German Patent (DT-AS) No. ~ .
2,309,158~.
Typical organic solvents which can be used.in the coat- `~
ing include ketones such as acetone, methyl ethyl ketone, cyclo-
-
hexanone and the like; alcohols such.as methanol, ethanol, pro-. ;~ -
panol, butanol and the like; esters such as methyl acetate-, --~
ethyl acetate, butyl acetate, ethyl lactate, glycol monoethyl
20 ether acetate and the.like; ethers and glycol ethers such as .
diethyl ether, glycol monoethyl ether, glycol dimethyl ether,
dioxane and the like; aromatic hydrocarbons such as benzene, .
toluenej xylene and the like; and chlorinated hydrocarbons such
as methylene chloride, ethylene chlorlde, carbon tetrachloride, :-
chloroform, ethylene chlorohydrin, dichlorobenzene and the like. : ~
These solvents can be used individually or in combination with ~ `
each other.
The maynetic layer coated on the support by the
above described method is dried after, if desired, the coating
30 has been subjected to a treatment for orienting the magnetic ``
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:,
1 powder in the layer. If required, the ma~netic layer can be
subjected to a surface-smoothening treatment or cut to the desired
shape, thereby to form the magnetic recording substance of the
present invention. In particular, it is found in the present
invention that a magnetic recording medium having a smooth
surface and excellent abrasion resis~ance can be obtained through
the surface smoothening treatment of the magnetic recording layer.
In the orienting treatment for the magnetic layer, the
orienting magnetic field can be either an AC or DC magnetic field
with a field strength of about 50 to 2000 gauss. The drying
temperature of the maynetic layer can range from about 50 to 120C, ; --
preferably 70 to 100C, more preferably 80 to 90C, the air flow
rate is 1 to 5 Kl/m3/min, preferably 2 to 3 Kl/m3/min and the
drying time is about 30 seconds to 10 minutes, preferably 1 to-5
minutes. The orienting direction of the magnetic substance is -~
determined depending upon the intended use thereof. That is to say,
the direction is parallel to the lengthwise direction of a tape
in the case of sound tapes, small sized video tapes and memory
tapes and inclined by about 30 to 90 degrees to the length~ise
direction in the case of broadcasting video tapes.
Suitable orienting methods of magnetic powders are
disclosed in U.S. Pat. Nos. 1,949,840, 2,796,359, 3,001,891, ~
3,172,776, 3,416,949, 3,473,960 and 3,681,138, Japanese Patent -~-
Publication No. 3427/1957, 28368/1964, 23624/1965, 23625/1965,
13181/1966, 13043/1973 and 39722/1973 and West German Patent
(DT-AS) No. 1,190,985. The orienting directions of the upper
layer and lower layer can be made different.
- The foregoing surface-smoothening treatment of the
-- lg --
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1 magnetic layer ~efore dr~ing is carried out, as occasion
demands, by means of magnet smoothers, smoothening coils,
smoothening blades, smoothening blankets, etc., as
described in Japanese Patent Publication Nos. 38802/1972 and
11336/1973, Japanese Patent Application (OPI) No. 53631/1974
and British Pat. No. 1,191,424.
The surface-smoothening treatmen~ of the maynetic
layer after drying is carried out by a calendering treatment
or the like. In the case o~ such a calendering treatment, in
particular, the magnetic layer is passed through between a
metal roll and a cotton roll or synthetic resin (e.g., nylon~
roll according to the supercalendering method. The supercalender-
ing conditions are preferably a roll pressure of about 25 to 100
Kg/cm2, preferably 30 to 70 Kg/cm2, a temperature of about 35
to 100C and a treatment speed of 5 to 120 m/min. If ~he pressure ~ ~
and temperature exceed the upper limit thereof, the magnetic `
layer and non-magnetic sup ort are unfavourably affected, while
if the treatment speed is less than about 5 m/min, the surface- ;
smoothening effect cannot be attained and if more than about
120 m/min, the operation is difficult.
These surface-smoothening treatments are described in
U.S. Pat. Nos. 2,688,567, 2,998,325 and 3,783,023, West German ~ ;
Patent Application (DT-OS) No. 2,405,222 and Japanese Patent ~;
Application (OPI) Nos. 53631/1974 and 10337/1975.
According to the present invention, a magnetic record-
ing medium with a low noise can be obtained by adding the
colloîdal silica of the present invention to a magnetic coating
composit;on during the dispersion thereof without deteriorating
the various properties of the magnetic recording
.:
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1 medium using a magnetic substance~ in par-ticular, ferromagnetic
fine powder. It ls found according to the present invention that
the use of a hydrophobic colloidal silica having methyl group
on the surface thereof, in particular, results in marked advant-
ages since when colloidal silica not treated is added, the
magnetic layer is hygroscopic and the magnetic flux density is
deteriorated due to oxidation in a magnetic recording medium
using a ferromagnetic fine powder.
The present invention will be explained in detail with
reference to the following examples, It will be obvious to one
skilled in the art that various changes and modifications can be
made in the components, ratios, operational order and the like
without depar-ting ~rom the spirit of the present inventi.on~
Therefore, the present invention should not be construed as
being limited to the following examples. All parts, percents, :
ratios and the like are by weiyht unless otherwise indicated,
Example l .
In a D~ magnetic field of 1000 gauss, l mol~l of. ~ ~.
sodium borohydride was added to an aqueous solution containing
0.685 mol/l of ferrous s.ulfate, 0.305 mol~l of cobalt sul~ate and
0.010 mol/l of chrome alum to obtain a ferromagnetic alloy fine
powder. The resulting alloy fine powder had such.a shape that,
on the aYerage, lQ to 15 pa~ticles with a diameter o~ about ~ ;
o , :, .
400 A were chained, and had a compos;~tion of Fe 69, Co 30 and .
Cr 1 containing about 3 ~ of B,
Polyestexpolyurethane 20 pa~ts;
- (.Reaction product of polyethylene
adipate and 4,4'-diphenylmethane
.
2~ Z~
1 diisocyanate having a mean molecular
weight of 130,000 as styrene weight)
Synthetic Nondrying Oil Modified Alkyd Resin 25 parts
(Solution containing 70% solid of a reaction
product of glycerine, phthalic anhydride and
glycidyl ester of fatty acid synthesized by
Koch method in methyl isobutyl ketone/xylene,
Oil length: 29%, Hydroxyl value: about 130)
Oleic Acid 3 parts
Silicone Oil (dimethylpolysiloxane)3 parts
Colloidal Silica Variable as shown in Table 1
(AEROSIL R-972, commercial name made by
Nippon Aerosil Co., Mean diameter: 16 m~,
50% of silanol groups being replaced by
methyl groups)
Methyl Isobutyl Ketone (MIBK)600 parts
300 parts of the above described alloy fine powder and this
composition were charged in a ball mill, kneaded and dispersed
for 24 hours, to which 20 parts of a polyisocyanate compound -~
(DESMODUR L-75, commercial name made by Bayer AG, 75% solution
of an adduct of 3 mols of tolylene diisocyanate and 1 mol of
trimethylolpropane in ethyl acetate) was added. The resulting
mixture was then subjected to dispersion under high speed
~ shearing for 1 hour and filtered by means of a filter with a
mean pore diameter of 3 microns, thus obtaining a magnetic
coating composition.
The above described magnetic coating composition was
coated onto a polyethylene terephthalate film of 22 microns
in thickness to give a thickness of 3 microns on dry base by
.l doctor coating, subjected to an orienting treatment in a DC
~ magnetic field of 2500 gauss for 0.02 second, dried at 100C
.
for 2 minutes with an air flow rate of 2 Kl/m3/min and then
subjected to a supercalendering treatment at 60C under a
` 30 pressure of 60 ~g/cm2 at a speed o 40 m/
* Trade Marks
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1 min, thus obtaining a wide magnetic recording film. The result-
in~ film was slit to give a video tape of 1/2 inch in width.
In the following Tahle 1 are shown the quantity oE
colloidal silica added C% by weight ~ased on the weight of the
ferromagnetic fine po~der~ and the properties of the resulting
video tapes ~C Nos: Comparative Examples~.
TABLE
.
Sam- Amount Maximum Square- Out- Noise Dur- Po~der
ple of Col- Magnetic ness put Id~ ahil~ Scraped
No. loidal Flux Den- Ratio ~c~ ~dB~ ity Cel ~f)
Silica sity Ca) (b~ ~dB) ~min~ (mg~
_ ~1 (gauss)
C-l 0 4320 0.88 5.6 5.3 3Q less than 1 ~ `
C-2 1 4300 0.88 5.6 4.5 30
1 2 4280 0.88 5.6 2.9 3~ 1
2 4 4210 0.88 5.6 2.4 3a
3 6 4170 0.88 5.6 2O2 3Q 1 -~
4 8 4~50 0.88 5.6 2.0 3a
1~ 394~ 0~88 5.4 2.0 35~ 2 -
6 15 3870 ~.88 5.2 2.1 35
7 20 3760 ~.~8 5.0 2.0 35 3
20C~3 25 3550 0.88 4.7 1.9 35 11 ` `'-
C-4 , 3~ 3290 0.88 4.1 1.~ 4Q
,
The accompanying drawing is a graph showing the relation~
of the quantity of the colloidal silica with the output and noise
of the video tape, in which the broken line corresponds to the
output and the solid line corresponds to the noise.
Measurement of the properties shown in Tahle 1 i~ carried
~y the following methods:
Ca~ Maximum Magnetic Flux Density: The resulting video
tape i5 su~jected to measurement in an external magnetic field of
30ao Oe using a measuring machine (VSM-III, commercial
..
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224
name manuaoture~1 ~y ~roei Kogyo K.K ).
(b~ ~Sq.lareness Rati.o: Ratio of the residual
magnetic flux c1ensity to the ma~imum magneti.c flux density
measured in an external magne~ic field of 3000 Oe.
(c) Output: Reproduced video output, represented
by a relative value to the standard value of a CrO2 tape,
when a standard siglnal o~ 5 ~Iz is recorded by the optimum
recordillg current using an EIAJ Type I VTR (Electroni~
Industries Association of Japan). Measuring machine: AV-
S700 macle by SON~' Corp.
~ c1) Noise: Represented by a relative value to thestandard value of a CrO2 tape. The output of a noise repro-
duced through modulation in 4 ~IlHz when a standard signal of
5 ~ z is reproduced is measured by means of a frequency
spectrum analyser. Measuring machine: FSA-l B ma~e by Ando
Den~i K.~. ~
(e) Durability: Time until an abnormal condition
appears on a monitor TV when a test pattern is recorded and
reproduced by still mode.
, (f) Powder Scraped: Weight of a magnetic layer
~dhered to a he?d or scraped by a head when a tape of 50 m
ill lengt}l is reciprocated l00 times at a speed of 5.0 m/sec
in a tape re~corde~ provided with a dummy fixed head having
(~
a contact length of 2.0 mm with a~magne~tlc layer. ~ '
It will clear].y be understood :Erom the accompany-
ing drawing and l'able l that, in a magnetic recordlng medium
Usillg a magnetic coating composition to which the colloidal
silica of the present invention is added, the.noise level
is largely decreased in spite of that the output is substnnt-
~O ially the same as that o~ a comparative magnetic recording
' '.
-24- ~ ,
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1 medium without addition of the colloidal silica. If the quantity
of the colloidal silica of the present invention is less than ..
2%, the effect of decreasing the noise level is ~mall, ~hile if
more than 20%, the saturated magnetic. flux density is decreased
thus to lower the output some~hat and to increase the powder
scraped. Moreover, it is apparent that the squareness ratio and
durability are su~stantially independent upon the quantity of the
colloidal silica.
Comparative Example 1
tO The procedures of Example 1 were repeated except that a
colloidal silica having a mean diameter of about 12 m~ but no
methyl group on the surface (Aerosil 20Q, commercial name, manu- .
factured ~y Nippon Aerosil Co.~ was used in place of the colloidal
-silica used in Example 1, thus obtaining a video tape. In the
following Table 2 are sho~n the quantity of the colloidal silica ;. .
used and the properties of the resulting video tape f in whiah the
properties are measured in an analogous manner to Table 1.
TABLE 2
Sam~ Quantity Maximum Square- Out- Noise Dura~..Po~der
ple of Collo Magnetic ness put ~dB). ~ility Scraped
No~ idal Sil- Flux Den~ Ratio CdB~ ~min~ (~g~
ica C~L sity : :~
' Cgauss~ _
C-l Q 432Q 0.88 5.6 5.3 30 less than 1
C-5 4 4170 0.88 5.6 3.1 3Q 12 ;
C~6 8 3980 0.88 5,6 2.g 30 18 :~
C 7 20 3730 0.88 4.9 2.9 3Q more than 30
C~8 30 3240 0.88 4,0 2.7 35 more than 30
It is apparent from Table 1 and Table 2 that the coll-
oidal silica having no methyl group on the surface gives a higher
noise level as compared with that having methyl groups, even if
the particle size is small.
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1 Camparative Example 2
The procedures of Example 1 were repeated except that
a finely powdered silica having a mean particle size of 0.6 ~ but
no methyl group on the surface was used instead of the colloidal
silica used in Example 1, thus obtaining a video tape. In Table
3 are shown the quantity of the finely powdered silica used and
the properties of the resulting video tape.
TABLE 3
Sam~ Quantity Maximum Square- Out- Noise Dura~ Powder -
ple of Collo- Magnetic ness put ~dB~ ~ility Scraped
10 No, idal Silica Flux Den~ Ratio (dB~ ~min) ~mg)
~1 sity
~gauss~
C-l 0 4320 ~.88 5.6 5.3 30 less than 1
C-9 4 4170 0.88 5.2 5.1 30 less than 1
C-10 8 3890 0.88 S.0 5. a 40 less than 1
C-ll 20 3540 0.88 4.6 4.5 45 5
It is apparent from the results of Table 3 that the silica having
a larger particle size and no methyl group on the surface cannot
give the effects according to the present inven~ion.
Comparative Example 3
The procedures of Example 1 were repeated except that
a finel~ po~dered silica having methyl groups on the sur~ace and
a variable particle size as sho~n in Table 4 was u~ed instead of
the colloidal silica used in Example 1~ thus o~taining a video
tape, In the following Ta~le 4 are shown the particle size of
the finely powdered silica with a constant quantity thereof and
the properties of the resulting video tape, in which the proper- `
ties are measured in an analo~ous manner to Tahle 1.
~26-
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1 TABL~ 4
Sam- Parti- Quantity Maximum Square- Out~ Noise Duxa- Powder
ple cle of Silica Magnetic ness put CdB) a~ Scraped
No. size (%) Flux Den- Ratio (dB) ity (mg)
(m) sity (min)
Csuass)
4 16 8 4050 0.88 5.6 2,0 30
8 40 8 4120 0,87 5.6 2.8 35
C-12 ~.6 8 4160 0.87 5.6 4.4 35 1
C-13 2.3 8 420~ 0.87 5.6 5,1 40 1
It is apparent from Ta~le 4 that if the particle size of the
finely powdered 5ilica is large, the noise level is increased and
the effects of the present invention cannot he given even if the
surface is made hydrophobic by replacement of methyl groups.
Comparative Example 4
Polyesterpolyurethane 2Q parts
(same as that of Example 11
Synthetic Mondrying Oil Modified 25 parts
Alkyd Resin - .
(same as that of Example 1~
MIBK lb parts ~ -
Colloidal Silica Variable as shown in Tahle 5
The above described composition was kneaded and dispersed at a
tempexature of 60C for 60 minutes using a kneader. Then, 40 parts ~`
of MIBK was added thereto and the resulting mixture was dispersed :-
two t~mes by means of a three rolls mill.
Fe-Co-Cr Alloy Fine Powder300 parts
(same as that of Example 1)
Oleic Acid 3 parts
Silicone Oil 3 parts :::
~dimethylpolys~loxane~
MIBK 550 parts
After the dispersinq~ the above descibed composition was added
thereto and the resulting mixture was then suhjected to the similar
-27
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224
1 steps of dispersing and coating to those of Example 1, thus ob-
taining a video tape. In the following Table 5, there are shown
the kinds and quan~ities of colloidal silica added and the pro-
perties of the resulting video tapes, in ~hich the properties are
measured in an analogous manner to Table 1.
TABLE 5
Sam- Kinds Quantity Maximum Square- Out- Noise Dura- Powder
ple of Col- of Collo- Magnetic ness put ~dB~ ~ility Scraped
No. loidal idal Sil- Flux Ratio (dBl ~min~ (mg)
Silica ica Density
(parts~ ~gauss~
............ ......... ......... ......... ....... ...... ....... ..... ............. -
10C 1 - ~ 4320 Q.88 5.6 5,3 30 less
than 1
C-14 R-~72 12 4250 0.88 5.6 4.8 45 less
than 1
C-15 R-~72 24 4130 0.88 5.6 4~6 5~ less
than 1
C-16 20a 12 4230 0.87 5.6 4.~ 4Q less
than I
C-17 20~ 24 41a~ 0.87 5.5 4.6 45 less :
than 1 ~ .
It is apparent from Table 5 that, when a colloidal silica is
firstly mixed with a hinder and a ferromagnetic fine po~der is
,
then added thereto, the durability is somewhat improved but the
noise level is not so improved, as compared ~ith.the case of add-
ing no colloidal silica~
Comparative Example 5
The video tapes obtained in Example 1 and Compara~ive
Example 1 were stored ~or one week in an atmosphere at 60~ and
~Q% RH and then subjected to measurement of the deterioration of
the saturated magnetic flux density due to temperature and humidity,.
thus ohtaining results shown in Ta~le 6
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1 TABLE 6
Sam- Kinds Quantity Maximum Magnetic Flux Density Change
ple of Col- of Collo- (gauss) Ratio
No. loidal idal Sil- Before Treatment After Treatment (%)
Silica ica (%)
C-l - 0 4320 406Q -6
2 R-972 4 4210 3960 -6
4 R-972 8 4050 3770 -7
C-5 200 4 4170 3750 -10
C-6 200 8 3980 350Q -12
In this ta~le, the maximum magnetic flux density is measured in
the similar manner to Ta~le 1.
It is apparent from Table 6 that, even if AEROSIL R-972
having methyl groups on the surface is added, the deterioration
of the magnetic properties due to temperature and humidity i5
substantially the same as in the case of no addition, while a
considera~le deterioration takes place in the case of adding :
AEROSIL 200 having no methyl group on the surface,
As can be seen from the foregoing Examples and Compar-
ative Examples, when the colloidal silica having methyl groups on
the surface is added to a magnetic coating composition according
to the present invention, it is possible to lower to a greater
extent the noise level of a magnetic recording medium using a :
ferromagnetic fine powder, in particular, liquid phase reduction :~
type alloy fine powder~ which has hitherto ~een considered to be~
difficult, and this magnetic recording medium is freed from deter-
ioration of the magnetic properties due to temperature and ~umidity
even after the passage of time~ The effect of lo~ering the noise
level according to the present invention can similarly~ be obtained
in the case of using ferromagnetic iron oxides and chromium di- ~i
oxide .
~
.~ ',,.
2 9 ~ ~1
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1 Furthermore~ it is to be noted that the method and
intended use for using the colloidal silica of the present inven-
tion are completely differen.t from those described in Japanese
Patent Application (.OPI~ Nos. ll~Q9/1974 and 128Q2/1874.
1 0 " ,
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