Magnetic recording medium and magnetic recording method

A magnetic recording medium comprises a support and provided thereon, a magnetic recording component layer which comprises an uppermost layer containing a ferromagnetic metal powder and a binder, a thickness of said magnetic recording component layer being from 0.2 to 3.0 .mu.m, a thickness of the support being from 3.5 to 10.5 .mu.m, an average center line roughness of the surface of the support on said magnetic recording component layer side being not more than 10 nm, and an average center line roughness of the surface of said uppermost magnetic layer being not more than 4 nm, wherein E.sub.T (in terms of kg/mm.sup.2), Young's modulus in the transverse direction of the support and t.sub.s (in terms of mm), a thickness of the support satisfy the following inequality: EQU E.sub.T .gtoreq.0.03/t.sub.s.sup.2

FIELD OF THE INVENTION 
The present invention relates to a magnetic recording medium, particularly 
to a magnetic recording medium having an improved head touch. 
BACKGROUND OF THE INVENTION 
In the recent years, a tape-shaped magnetic recording medium (hereinafter 
referred to as a tape) are required to be capable of being recorded in 
high density and providing high outputs. Concretely, it is required to 
improve magnetic layers of the medium, and make the tape thinner to be 
capable of recording for a long time, which includes making thinner a 
component layer comprising the magnetic layers and a non-magnetic support. 
However, even if a high output is achieved by making improvements in 
magnetic layer, head touch failures are liable to occur and thereby the 
output is lowered as long as a thin non-magnetic support is used. Though a 
high-output capability is not easily compatible with decrease in tape 
thickness as mentioned above, many studies have so far been made to 
reconcile them. Particularly, enhancement of tape stiffness in transverse 
direction (hereinafter referred to as TD) is known as a useful means to 
prevent head touch failures and achieve high outputs in thin tapes, is 
studied in various manners. 
For example, Japanese Ptent Publication open to Public Inspection 
(hereinafter referred to as Pat. O.P.I. Pub.) No. 146518/1992 proposes a 
magnetic recording medium having a total thickness of 11 .mu.m or less and 
comprising a polyethylene terephthalate support having a TD Young's 
modulus of 700 kg/mm.sup.2 or more and a machine direction (hereinafter 
referred to as MD) Young's modulus of 800 kg/mm.sup.2 or less. 
Japanese Pat. O.P.I. Pub. No. 146519/1992 proposes a magnetic recording 
medium having a TD stiffness/MD stiffness ratio of 0.65 to 1.00. 
Further, Japanese Pat. O.P.I. Pub. No.271010/1992 proposes a magnetic 
recording medium having a total thickness of 11.mu.m or less and 
comprising a polyethylene terephthalate support having a TD Young's 
modulus of 700 kg/mm.sup.2 or more and an MD Young's modulus of 450 to 720 
kg/mm.sup.2. 
The TD stiffness of a tape is expressed in Et.sup.3 when the TD Young's 
modulus of a tape is shown by E and the tape thickness by t. Accordingly, 
when E is kept constant, the TD stiffness sharply decreases as the tape 
becomes thinner. However, the methods proposed so far specify only the TD 
and MD Young's moduli or the strength balance of TD/MD, and do not specify 
at all the relationship between these factors and the tape thickness. 
With the increasing demand for recording of higher density and longer time, 
the tape thickness will be reduced to 6 .mu.m or so in the near future. 
For that reason, there is demanded a technique to maintain a high 
outputting capability even in such a thin tape, or a technique to 
manufacture, without lowering the output, a tape with uniform magnetic 
layer structure even in a tape thickness of 6 to 13 .mu.m. 
SUMMARY OF THE INVENTION 
Accordingly, an object of the invention is to provide a magnetic recording 
medium having an adequate head touch and a high outputting capability 
irrespective of its thickness. 
The above object of the invention is achieved by a magnetic recording 
medium having at least one magnetic recording component layer formed on 
one side of a non-magnetic support, a magnetic layer constituting the 
uppermost layer and containing a ferromagnetic metal powder and a binder, 
and a back coating layer provided when necessary on the other side of the 
support, wherein the thickness of the magnetic recording component layer 
t.sub.r is 0.2 to 3.0 .mu.m and the thickness of the non-magnetic support 
t.sub.s is 3.5 to 10.5 .mu.m in the total thickness of the magnetic 
recording medium T, the TD Young's modulus E.sub.T (kg/mm.sup.2) of the 
non-magnetic support satisfies E.sub.T .gtoreq.0.03/t.sub.s.sup.2 (t.sub.s 
is in mm), the center line average roughness on the component layer side 
surface of the non-magnetic support R.sub.a S is 10 nm or less, and the 
center line average roughness on the surface of the component layer 
R.sub.a M is 4 nm or less. 
In a preferred embodiment of the invention, the non-magnetic support is a 
polyethylene terephthalate film having a TD Young's modulus E.sub.T of 550 
to less than 700 kg/mm.sup.2, a polyethylene naphthalate film having a TD 
Young's modulus E.sub.T of 750 to 1500 kg/mm.sup.2 or a aramid film having 
a TD Young's modulus E.sub.T of 1000 to 2500 kg/mm.sup.2. 
Further, the above object of the invention is achieved by a magnetic 
recording method which uses, in recording and playing back, the magnetic 
recording medium of the invention under the condition which satisfies the 
following relationship: 
EQU r.ltoreq.500T 
where r (mm) is a curvature radius of an arc formed by a projecting head of 
the section cut by a plane containing a head drum rotation axis and the 
center of a head gap of a magnetic recorder and facing a sliding magnetic 
recording medium, and T (mm) is a total thickens of the magnetic recording 
medium.

DETAILED DESCRIPTION OF THE INVENTION 
As the result of a study conducted using a variety of polyethylene 
terephthalate (hereinafter referred to as PET) supports, polyethylene 
naphthalate (hereinafter referred to as PEN) supports and aramid 
(hereinafter referred to as PAD) supports different in MD and TD Young's 
moduli, as well as a study made for finding out an optimum magnetic head 
shape for tapes comprising these supports, it has proved that a magnetic 
recording medium having an adequate head touch and a high outputting 
capability can be obtained, irrespective of the tape thickness, by making 
the ratio of support's TD Young's modulus to support's thickness not less 
than a specific value, and that much better effects can be obtained by 
keeping the head shape suited to the tape thickness. 
The TD stiffness of a tape is determined by the TD Young's moduli of a 
support and component layers including a magnetic layer as well as the 
thicknesses thereof. Among them, the magnetic layer usually contains a 
large amount of needle-like magnetic powder oriented in the longitudinal 
direction of the tape. And the degree of the orientation becomes higher 
with the increase in output of a tape. Accordingly, the strength of a 
magnetic layer is large in MD and small in TD; this tendency can be seen 
more apparently in tapes of higher outputs. Further, the thickness of 
component layers including a magnetic layer is smaller than that of a 
support. Therefore, the TD stiffness of a tape is virtually determined by 
the nature of a support, and improvement in support's TD Young's modulus 
becomes indispensable for the enhancement of tape stiffness. 
However, when the TD stiffness of a tape expressed as E.sub.T t.sub.s.sup.3 
(where E.sub.T is a TD Young's modulus of a tape and t.sub.s is a total 
thickness of a support) is looked at, it is obvious that a 6-.mu.m thick 
support needs, to be the same in stiffness, an E.sub.T about 10 times that 
of a 13-.mu.m thick support or a Young's modulus too large to be achieved 
in a support. Hence it seems that the same output cannot be obtained in a 
thickness range of 13 .mu.m to 6 .mu.m without changing the structure of a 
magnetic layer. However, as the result of a study using PET, PEN and PAD 
supports different in MD and TD moduli and a study on the shape of a 
magnetic head suited to tapes using these supports, the present inventors 
have succeeded in obtaining nearly the same head touch and output over the 
support's thickness range of 13 .mu.to 6 .mu.m, without changing the 
structure of a magnetic layer, by setting E.sub.T t.sub.s.sup.2 (where 
E.sub.T is a TD Young's modulus of a support and t.sub.s is a thickness of 
a support), or the TD strength of a support, to be not less than a 
specific value. 
TD Young's modulus E.sub.T (kg/mm.sup.2) necessary for a support is 
expressed as 
EQU E.sub.T .gtoreq.0.03/t.sub.s.sup.2 
where t.sub.s (mm) is a support thickness. The material of a support is not 
particularly limited as long as the above equation is satisfied. Further, 
since TD Young's modulus E.sub.T of a support achieved in the near future 
is regarded to be about 900 kg/mm.sup.2 for PET, about 1500 kg/mm.sup.2 
for PEN and about 2500 kg/mm.sup.2 for PAD, serviceable ranges of these 
materials can be determined by introducing these values as the upper 
limits of E.sub.T into the above equation. 
However, the strength of PET is weaker than those of PEN and PAD; 
accordingly, MD Young's modulus E.sub.M of a PET support sharply decreases 
to 500 kg/mm.sup.2 or less when TD Young's modulus E.sub.T is raised to 
700 kg/mm.sup.2 or more. A sharp drop in MD Young's modulus E.sub.M of a 
tape not only heavily lowers the running durability of a tape but 
increases the residual elongation due to the tension during manufacture or 
use of a tape and causes skewing. 
Judging from the above facts, it is preferred that a PET support having an 
MD Young's modulus E.sub.M maintained at 500 kg/mm.sup.2 or more and a TD 
Young's modulus E.sub.T enriched to 550 to less than 700 kg/mm.sup.2 be 
used. Therefore, the applicable thickness of a PET support is limited up 
to about 6.5 .mu.m; this necessitates improvements in a magnetic layer to 
maintain a high output using a PET support having a thickness of 6.5 .mu.m 
or less. 
PEN and PAD supports can maintain an adequate MD Young's modulus even if 
the TD Young's modulus is substantially raised and, therefore, these can 
be used as thin supports of 6.5 .mu.m or less without making any 
improvement in a magnetic layer. 
The study on the shape of a magnetic head was conducted, using a Sony Hi 8 
Deck EV-S900 equipped with an unused head, by running a sample tape for 10 
hours on the deck to wear the head to a shape optimum for the tape and 
measuring the shape of the head after it was stabilized. The measurement 
was made, using a WYKO light-interference type non-contact surface 
roughness meter, measuring the cross-sectional shape of the arc of a 
head's tip, which was on a plane containing the head rotation axis and the 
head gap center and projecting against a sample tape, approximating the 
measured shape to a circle passing through three points including both 
ends and a peak of the worn head, and determining a curvature radius r 
(mm) of the circle. The curvature radius of the unused head was processed 
to a very small dimension of about 1 mm. While a tape was run, r became 
larger, as the head was gradually worn, and the head was nearly stabilized 
in about 4 hours running and stabilized into a virtually fixed shape in 10 
hours. Repetition of this wearing procedure with various tapes have proved 
that stabilized r is smaller as a tape becomes thinner. In the wearing 
procedure, TD Young's modulus E.sub.T of a support used in a sample tape 
did not exert a great influence, and r became nearly the same value for 
tapes having the same thickness. Further, when the curvature radius of a 
head was larger than that of a stabilized head for a tape, head touch 
failures occurred. Such head touch failures increased as the curvature 
radius became larger. On the contrary, use of a head having a curvature 
radius smaller than that of a stabilized head for a tape gave an adequate 
head touch and did not cause troubles in any other respects. 
To bring out the feature of the magnetic recording medium of the invention 
more fully, it is required, judging from the above results, that the 
relationship between curvature radius r (mm) of the arc of a magnetic head 
projecting against a tape and the tape thickness T (mm) be 
EQU r.ltoreq.500T 
and preferably 
EQU r.ltoreq.500(T-0.001). 
In the embodiment of the invention, conventional techniques of magnetic 
recording media can be used as long as the above constituents of the 
invention are satisfied. 
Next, the magnetic recording medium of the invention is described. 
Non-magnetic Support 
Materials suitable for the non-magnetic support include polyesters such as 
PET, PEN, polyolefins such as polypropylene, cellulose derivatives such as 
cellulose triacetate, cellulose diacetate, and other thermoplastic resins 
such as polyamide, PAD such as poly-p-phenylene terephthalamide (PPTA) and 
poly-p-benzamide (PBA), polycarbonate. Preferred are PET, PEN and PAD 
supports having TD Young's moduli of 550 to less than 700, 750 to 500 and 
1000 to 2500 kg/mm.sup.2 respectively 
The thickness of the non-magnetic support is in the range of 3.5 to 10.5 
.mu.m, and center line average roughness R.sub.a of the surface is 10 nm 
or less. 
The non-magnetic support may be one subjected to surface treatment such as 
corona discharge. 
For the purpose of improving running property, antistatic property of 
magnetic recording media and preventing transfer of a magnetic layer, a 
back coating layer may be provided on one side of a support where no 
magnetic layer is formed (reverse side); further, a subbing layer may be 
provided between the magnetic layer and the non-magnetic layer. 
Magnetic Recording Component Layer 
The total thickness of the component layers ranges from 0.2 to 3.0 .mu.m. 
The uppermost layer of the component layers is a magnetic layer containing 
a ferromagnetic metal powder. The center line average roughness of its 
surface R.sub.a '0 is 4 nm or less. 
The ferromagnetic metal powder used in the uppermost magnetic layer 
includes ferromagnetic powders such as Fe type, Co type, Fe--Al type, 
Fe--Al--Ni type, Fe--Al--Zn type, Fe--Al--Co type, Fe--Al--Ca type, Fe--Ni 
type, Fe--Ni--Al type, Fe--Ni--Co type, Fe--Ni--Si--Al--Mn type, 
Fe--Ni--Si--Al--Zn type, Fe--Al--Si type, Fe--Ni --Zn type, Fe--Ni--Mn 
type, Fe--Ni--Si type, Fe--Mn--Zn type, Fe--Co --Ni--P type, Ni--Co type, 
and metal magnetic powders whose principal components are Fe, Ni and Co. 
Of them, Fe type metal powders are excellent in electrical properties. 
When corrosion-resistance and dispersibility are taken into consideration, 
preferred are Fe--Al type metal powders including Fe--Al type, Fe--Al--Ca 
type, Fe--Al--Ni type, Fe--Al--Zn type, Fe--Al--Co type, 
Fe--Ni--Si--Al--Zn type and Fe--Ni--Si--Al--Mn type. 
The ferromagnetic metal powders used in the invention comprise particles 
having an average major axial length of 0.5 .mu.m or less, preferably 0.01 
to 0.4 .mu.m and more preferably 0.01 to 0.3 .mu.m when measured with a 
transmission electron microscope, and their axial ratio (average major 
axial length/average minor axial length) is 12 or less, preferably 10 or 
less. 
One preferred example of the ferromagnetic metal powders used in the 
invention is an Fe--type ferromagnetic metal powder, for example, one 
having an Fe:Al weight ratio of 100:5, an average major axial length of 
0.16 .mu.m, a coercive force (Hc) of 1780 Oe, and a saturation 
magnetization quantity (.sigma..sub.s) of 120 emu/g. 
Irrespective of being ferromagnetic metal powders having an average major 
axial length and an axial ratio within the ranges specified above or 
tabular ferromagnetic alloy powders having an axis of easy magnetization 
perpendicularly to particle face, it is usually preferred that their 
saturation magnetization quantity (.sigma..sub.s), an essential magnetic 
property, be 70 emu/g or more. When the saturation magnetization quantity 
is less than 70 emu/g, the electromagnetic conversion property is liable 
to be lowered. 
In the embodiment of the invention, ferromagnetic metal powders having a 
specific surface area of 45 m.sup.2 /g or more by the BET method are 
advantageously used in high density recording. 
When the magnetic layer is provided as a laminated layer, conventional 
ferromagnetic powders employed in magnetic recording media can be used as 
ferromagnetic powders. Examples thereof include oxide magnetic substances 
such as .gamma.-Fe.sub.2 O.sub.3, Co-containing .gamma.-Fe.sub.2 O.sub.3, 
Co-coated .gamma.-Fe.sub.2 O.sub.3 and CrO.sub.2, and ferrites represented 
by magnetite, namely Fe.sub.3 O.sub.4, Co-containing Fe.sub.3 O.sub.4, 
Co-coated Fe.sub.3 O.sub.4. 
Among the above ferrites, tabular ones having an easy axis for 
magnetization perpendicularly to particle face can be employed as 
preferable ferromagnetic powders. Such ferromagnetic powders include, for 
example, hexagonal ferrites. 
In the invention, there may be provided a non-magnetic layer, a high 
magnetic permeability layer and, if necessary, a conductive layer, as 
magnetic recording component layers other than the magnetic layer. 
Suitable non-magnetic powders for a non-magnetic layer include, for 
example, carbon black, graphite, TiO.sub.3, barium sulfate, ZnS, 
MgCO.sub.3, CaCO.sub.3, ZnO, CaO, tungsten disulfide, molybdenum 
disulfide, boron nitride, MgO, SnO.sub.2, SiO.sub.2, Cr.sub.2 O.sub.3, 
.alpha.-Al.sub.2 O.sub.3, .alpha.-Fe.sub.2 O.sub.3, .alpha.-FeOOH, SiC, 
cerium sulfide, corundum, artificial diamond, .alpha.-iron oxide, garnets, 
garnet, silica, silicon nitride, silicon carbide, molybdenum carbide, 
boron carbide, tungsten carbide, titanium carbide, tripoli, diatom earth, 
dolomite. 
Among them, preferred are inorganic powders such as carbon black, 
TiO.sub.2, barium sulfate, .alpha.-Al.sub.2 O.sub.3, .alpha.-Fe.sub.2 
O.sub.3, .alpha.-FeOOH, Cr.sub.2 O.sub.3. 
In the invention, non-magnetic powders comprising needle-like particles can 
be advantageously used. Using such needle-like non-magnetic powders 
improves the surface smoothness of the non-magnetic layer and, further, 
improves the surface smoothness of the magnetic layer laminated thereon as 
the uppermost layer. 
The major axial length of the non-magnetic powder particles is usually 0.50 
.mu.m or less, preferably 0.40 .mu.m or less and more preferably 0.30 
.mu.m or less. 
In embodying the invention, it is preferred that the above non-magnetic 
powders be subjected to surface treatment using a Si-containing compound 
and/or an Al-containing compound. By use of such surface-treated 
non-magnetic powders, the surface condition of a magnetic layer 
constituting the uppermost layer can be improved. The content of the Si 
and/or Al is preferably 0.1 to 10% by weight of the non-magnetic powder 
for Si and 0.1 to 10 wt % for Al. 
The content of non-magnetic powder in the non-magnetic layer is usually 50 
to 99% preferably 60 to 95% and more preferably 70 to 95% by weight of the 
total components to form the non-magnetic layer. When the content is 
within the above range, the surface condition can be improved in the 
non-magnetic layer as well as in the magnetic layer which forms the 
uppermost layer. 
Preferred high magnetic permeability materials are those of which coercive 
force Hc is within the range of 0&lt;Hc.ltoreq.1.0.times.10.sup.4 (A/m), 
preferably 0&lt;Hc.ltoreq.5.0.times.10.sup.3 (A/m). When the coercive force 
is within the above range, the uppermost layer's magnetization is 
effectively stabilized in its magnetization area. In contrast with this, 
when the coercive force is larger than that specified above, properties as 
a magnetic material are developed, making it difficult to provide desired 
properties. 
In a preferred embodiment of the invention, high magnetic permeability 
materials are properly selected from those having a coercive force in the 
above range; such high magnetic permeability materials include metallic 
soft-magnetic materials and oxide soft-magnetic materials. 
Examples of the metallic soft-magnetic materials include Fe--Si alloys, 
Fe--Al alloys (Alperm, Alfenol, Alfer), Permalloys (Ni--Fe type binary 
alloy and multi-component alloys obtained by adding Mo, Cu, Cr, etc. 
thereto), Sendust (Fe--Si--Al, composition: 9.6 wt % Si, 5.4 wt % Al and 
Fe as the balance) and Fe--Co alloys. Of them, Sendust is preferred. 
Usable metallic soft-magnetic materials are not limited to those 
illustrated above; other metallic soft-magnetic materials can also be 
employed as high magnetic permeability materials. These high magnetic 
permeability materials can be used singly or in combination of two or more 
kinds. 
Examples of the oxide soft-magnetic materials include spinal type ferrites 
such as MnFe.sub.2 O.sub.4, Fe.sub.3 O.sub.4, CoFe.sub.2 O.sub.4, 
NiFe.sub.2 O.sub.4, MgFe.sub.2 O.sub.4, Li.sub.0.5 Fe.sub.2.5 O.sub.4, 
Mn--Zn type ferrites, Ni--Zn type ferrites, Ni--Cu type ferrites, Cu--Zn 
type ferrites, Mg--Zn type ferrites, Li--Zn type ferrites. Among them, 
Mn--Zn type ferrites and Ni--Zn type ferrites are preferred. These oxide 
soft-magnetic materials can be used singly or in combination. 
These high magnetic permeability materials are pulverized with a ball mill 
or other pulverizing apparatus to a fine powder, of which particle size is 
1 m.mu. to 1,000 m.mu., preferably 1 m.mu. to 500 m.mu.. In order to 
obtain such a fine powder, when a metallic soft-magnetic material is used, 
a molten alloy is sprayed into a vacuum atmosphere. Oxide soft-magnetic 
materials can be finely pulverized by use of the glass crystallization 
method, coprecipitation firing method, hydrothermal synthesis method, flux 
method, alkoxide method or plasma jet method. 
In a layer containing such a high magnetic permeability material, the 
content of the high magnetic permeability material is 10 to 100 wt %, 
preferably 50 to 100 wt % and more preferably 60 to 100 wt %. When the 
content of the high magnetic permeability material is within the range 
specified above, the magnetization in the uppermost layer is effectively 
stabilized. When the content is less than 50 wt %, the effect as a high 
magnetic permeability layer cannot be obtained in many cases. 
This layer which contains the high magnetic permeability material may 
contain non-magnetic particles. 
As non-magnetic powders to be added to the conductive layer, at least one 
selected from those illustrated below (these may be jointly used) is used 
in amounts of 10 to 80 wt % of the total weight of the conductive layer. 
Examples of the non-magnetic powders include powders of tin oxide, 
tin-oxide-containing indium oxide, indium oxide, zinc oxide, silicon 
carbide, titanium oxide, barium oxide, molybdenum oxide or magnesium 
oxide. There can also be added as binders, in amounts of 10 to 90 wt %, 
conductive polymers such as .pi.-conjugated conductive polymers 
(polypyrrole, polyaniline, poly-p-phenylene, polyphenylene vinylene, 
polythienylene, polythiophene, poly-2,5-pyridinediyl, 
polyisothianaphthene), Li-salt-containing polyvinyl alcohols, and 
Li-salt-containing polyethylene oxides. 
Using the means described above, the surface specific resistance of the 
lower layer is made 109 .OMEGA./sq or less. 
By forming the magnetic recording component layers as described above, the 
conductivity of the component layers is enhanced and their antistatic 
property is improved; as a result, the number of dropouts is decreased and 
discharge noises are prevented. 
Binders Used in the Invention 
Typical binders used in the invention are polyurethanes, polyesters, and 
vinyl chloride type resins such as vinyl chloride copolymers. Preferably, 
these resins contain repeated units having at least one polar group 
selected from --SO.sub.3 M, --OSO.sub.3 M, --COOM and 
--PO(OM.sup.1).sub.2. 
In the above polar groups, M represents a hydrogen atom or an alkali metal 
atom such as Na, K or Li, M.sup.1 represents a hydrogen atom, an alkali 
metal atom such as Na, K or Li, or an alkyl group. 
These polar groups have a function to enhance dispersibility of 
ferromagnetic powders and are contained in the resin at a rate ranging 
from 0.1 to 8.0 mol %, preferably from 0.5 to 6.0 mol %. When the content 
is less than 0.1 mol %, the dispersibility of ferromagnetic powders is 
lowered. On the contrary, a content larger than 8.0 mol % causes a 
magnetic paint to gel. Meanwhile, the weight average molecular weight of 
the above resins is preferably in the range of 15,00 to 50,000. 
These binders are used in the magnetic layer in amounts of usually 10 to 20 
parts, preferably 15 to 30 parts per 100 parts by weight of ferromagnetic 
powder. 
The binders can be used either singly or in combination of two or more 
kinds; when these are used in combination, the ratio of polyurethane 
and/or polyester to vinyl chloride type resin is within the range of 
usually 90:10 to 10:90, preferably 70:30 to 30:70 in weight ratio. 
Besides the above resins, other resins of which weight average molecular 
weight ranges from 10,000 to 200,000 can also be used. Examples thereof 
include vinyl chloride-vinyl acetate copolymers, vinyl chloride-vinylidene 
chloride copolymers, vinyl chloride-acrylonitrile coplymers, 
butadien-acrylonitrile copolymers, polyamide resins, polyvinyl butyrals, 
cellulose derivatives including nitrocellulose, styrene-butadiene 
copolymers, phenolic resins, epoxy resins, phenoxy resins, silicone 
resins, acrylic resins, urea-formamide resins, and various resins of 
synthetic rubber type. Other Ingredients 
In embodying the invention, other ingredients such as durability improvers, 
dispersing agents, lubricants, abrasive materials, antistatic agents and 
fillers can be used to improve the properties of the magnetic layer. 
Suitable durability improvers are polyisocyanates such as aromatic 
polyisocyanates including adducts of active-hydrogen-containing compounds 
with tolylene diisocyanate (TDI) and aliphatic polyisocyanates including 
adducts of active-hydrogen-containing compounds with hexamethylene 
diisocyanate (HMDI). The weight average molecular weight of these 
polyisocyanates is preferably in the range of 100 to 3,000. 
Examples of the dispersing agents include fatty acids having 12 to 18 
carbon atoms such as caprylic acid, capric acid, lauric acid, myristic 
acid, palmitic acid, stearic acid, oleic acid; alkali metal salts, alkali 
earth metal salts or amides of these fatty acids; alkyl phosphates of 
polyalkylene oxide; lecithin; trialkyl polyolefinoxy quaternary ammonium 
salts; and azo compounds having a carboxyl group or a sulfonic acid group. 
These dispersing agents are employed usually within the range of 0.5 to 5 
wt % of ferromagnetic powder. 
As the lubricants, fatty acids and/or fatty esters are used. When fatty 
acids are used, the addition amount is 0.2 to 10 wt %, preferably 0.5 to 5 
wt % of ferromagnetic powder. An addition amount less than 0.2 wt % tends 
to lower the running property; an addition amount larger than 10 wt % is 
liable to cause bleeding of fatty acids and lowers the output. The 
addition amount of fatty esters is also 0.2 to 10 wt %, preferably 0.5 to 
10 wt % of ferromagnetic powder. When the addition amount is less than 0.2 
wt %, the still durability becomes poor; when it exceeds 10 wt %, fatty 
esters migrate to the surface of a magnetic layer, causing output drop. 
When a fatty acid and a fatty ester are jointly used to enhance the 
lubricating property much more, the ratio of fatty acid to fatty ester is 
preferably 10:90 to 90:10 by weight. 
Further, as lubricants other than the above fatty acids and fatty esters, 
silicone oils, graphite, carbon fluoride, molybdenum disulfide, tungsten 
disulfide, fatty amides and .alpha.-olefin oxides can also be used. 
Examples of the abrasive materials include .alpha.-alumina, molten alumina, 
chromium oxide, titanium oxide, .alpha.-iron oxide, silicon oxide, silicon 
nitride, tungsten carbide, molybdenum carbide, boron carbide, corundum, 
zinc oxide, cerium oxide, magnesium oxide, boron nitride. Preferred are 
those having an average particle size of 0.05 to 0.6 .mu.m, especially 
preferred are those having an average particle size of 0.1 to 0.3 .mu.m. 
Antistatic agents usable in the invention are conductive powders such as 
carbon black and graphite; other usable ones include cationic surfactants 
such as quaternary ammonium salts, anionic surfactants having an acid 
group such as sulfonic acid, sulfuric acid, phosphoric acid, phosphate or 
carboxylic acid, amphoteric surfactants such as aminosulfonic acid, and 
natural surfactants such as saponins. These antistatic agents are usually 
employed in the range of 0.01 to 40 wt % of binder. 
FIG. 1 is a cross-sectional view showing one example of the magnetic 
recording medium which meets the requirements of the invention, in which 1 
shows a support, 2 component layers, 3 a back coating layer, 21 a magnetic 
layer and 22 a non-magnetic layer. 
Manufacture of Magnetic Recording Medium 
In kneading and dispersing components to form a magnetic layer, a variety 
of kneaders and dispersers can be used. 
Suitable examples include two-roll mills, three-roll mills, ball mills, 
pebble mills, coball mills, Tron mills, sand mills, sand grinders, 
Sqegvari attritor, high-speed impeller dispersers, high-speed stone mills, 
high-speed impact mills, dispersers, high-speed mixers, homogenizers, 
supersonic dispersers, open kneaders, continuous kneaders, and pressure 
kneaders. Among these kneaders and dispersers, those which can provide a 
power consumption load of 0.05 to 0.5 KW (per Kg magnetic powder) are 
pressure kneaders, open kneaders, two-roll mills and three-roll mills. 
Conventional coating methods can be used to form a magnetic layer and an 
intermediate layer on a non-magnetic support. To carry out double-layer 
coating, use of the extrusion method, particularly the wet-on-wet 
extrusion coating method, is preferred. This wet-on-wet coating is 
practiced, as shown in FIG. 2, by providing coating solutions for the 
respective component layers double-layeredly in wet-on-wet mode with 
extrusion coaters 10 and 11 on film support 1 delivered with feed roll 32, 
passing the coated 1 through orienting magnet or vertically orienting 
magnet 33, and introducing it into dryer 34 where it is dried with hot air 
blown from nozzles arranged up and down. Support 1 bearing dried layers is 
then led to supercalender 37 comprising calender rolls, calendered there, 
and wound on wind-up roll 39. The magnetic film so prepared is cut into 
tapes of desired widths to obtain, for example, 8-mm wide magnetic 
recording tapes for video camera. 
In the above procedure, the coating solutions may be fed to extrusion 
coaters 10 and 11 through unillustrated in-line mixers. In FIG. 2, arrow D 
indicates the direction in which the non-magnetic base film is conveyed. 
Extrusion coaters 10 and 11 are provided with reservoirs 13 and 14, 
respectively, so that coating solutions from the two coaters are coated in 
wet-on-wet mode; that is, immediately after coating the coating solution 
for lower layer (while it is wet), the coating solution for upper layer is 
coated thereon. 
As the coater head, head (c) shown in FIG. 3 is advantageously used in the 
embodiment of the invention. 
Solvents blended in the above paints or diluting solvents used at the 
coating of the paints are, for example, ketones such as acetone, methyl 
ethyl ketone, methyl isobutyl ketone, cyclohexanone; alcohols such as 
methanol, ethanol, propanol, butanol; esters such as methyl acetate, ethyl 
acetate, butyl acetate, ethyl lactate, ethylene glycol monoacetate; ethers 
such as glycol dimethyl ether, glycol monoethyl ether, dioxane, 
tetrahydrofuran; aromatic hydrocarbons such as benzene, toluene, xylene; 
and halogenated hydrocarbons such as methylene chloride, ethylene 
chloride, carbon tetrachloride, chloroform, dichlorobenzene. These 
solvents can be used singly or in combination of two or more kinds. 
The magnetic field set up by the above orienting magnet or vertically 
orienting magnet is about 20 to about 5,000 gausses, the drying 
temperature in the dryer is about 30.degree. to about 120.degree. C., and 
the drying time is about 0.1 to about 10 minutes. 
In the above double-layer coating carried out in wet-on-wet mode, the 
uppermost magnetic layer is coated while the layer located thereunder is 
wet. Therefore, the surface of the lower layer, or the interface with the 
uppermost layer, becomes smooth and, at the same time, the surface 
property of the uppermost layer is improved and, in addition, the adhesion 
between the upper and lower layers is enhanced. As the result, 
requirements for magnetic tapes, namely high outputs and low noises 
necessary for high density recording, are satisfied and, further, 
delamination is prevented, coating strength is improved, and thereby high 
durability is attained. Moreover, wet-on-wet double-layer coating brings 
about additional advantages such as decrease in dropouts and rise in 
reliability. 
Surface Smoothing 
Preferably, the magnetic recording medium of the invention is calendered 
for the improvement of surface smoothness. 
In the invention, the surface roughness of the component layers or the 
uppermost magnetic layer is given by center line roughness R.sub.a 
measured by use of a feeler. 
One preferred means to make the above R.sub.a 4 nm or less is to control 
properly the surface smoothness of the magnetic layer by setting the 
calendering conditions in the foregoing manufacturing process. That is, in 
the surface smoothing of the invention, factors which exert strong 
influences upon calendering conditions are temperature, linear pressure 
and line speed. Other influencing factors are the kneading conditions and 
surface treatment of magnetic powder as well as the size and amount of 
particles added to the magnetic layer. 
Preferably, calendering is carried out under the conditions of temperature: 
50.degree. to 140.degree. C., linear pressure: 50 to 400 kg/cm and line 
speed: 20 TO 600 m/min. Particularly preferred is use of a steel/steel 
calender which can provide a linear pressure higher than 1000 kg/cm. 
After calendering, the magnetic film so obtained is burnished or bladed 
according to a specific requirement and then slitted into tapes. 
EXAMPLES 
The invention is hereunder described with examples in which the coating 
layer on the magnetic layer side is composed only of a 2.0 -.mu.m thick 
single layer containing a magnetic metal powder, but the scope of the 
invention is by no means limited to them. Particularly, when the coating 
layer on the magnetic layer side is formed in double-layer structure by 
providing a 0.20 -.mu.m thick upper layer using the same magnetic coating 
and a 1.80 -.mu.m thick lower layer using a non-magnetic coating prepared 
by dispersing a needle-like hematite (DPN-250BX) in the same binder as 
that used in the magnetic coating, though both output and C/N are 
respectively raised by about 2 dB, overall properties are improved, and 
thereby the effect of the invention is maintained. 
The following magnetic layer composition was thoroughly kneaded with a 
pressure kneader, followed by mixing and dispersing. The resultant 
magnetic coating was coated on a support as shown in Table 1 so as to give 
a dry thickness of 2.0 .mu.m after calendering and dried in a drying oven 
of 80.degree.C. while subjected to magnetic orientation using a 5000-G 
electromagnet. After calendering, a back coating solution of the following 
composition was coated to a dry thickness of 0.5 .mu.m. The magnetic film 
so prepared was slitted into 8-mm wide tapes, and then loaded in a 
cassette to use as samples for evaluation. 
All the components, amounts and procedures shown in the following example 
can be varied within the range not deviating from the scope of the 
invention. In the example, all "parts" are parts by weight. 
The following magnetic composition was prepared. 
______________________________________ 
Magnetic Composition 
______________________________________ 
Fe--Al type magnetic metal powder 
100 parts 
(Fe:Al ratio in number of atoms: overall 
average = 100:4, surface layer = 50:50, 
BET value = 53 m.sup.2 /g, Hc = 1760 Oe, 
average major axial length = 0.14 .mu.m) 
Vinyl chloride resin 10 parts 
(MR110 made by Nippon Zeon Co., 
Ltd.) 
Metal-sulfonate-containing polyurethane 
10 parts 
resin (UR8700 made by Toyobo Co., 
Ltd.) 
.alpha.-Alumina 8 parts 
Stearic acid 1 part 
Butyl stearate 1 part 
Mixed solvent X parts* 
(mixture of methyl ethyl ketone, toluene 
and cyclohexanone in equal volumes) 
______________________________________ 
*added in parts during kneading and dispersing 
After kneading and dispersing the composition, 5 parts of polyisocyanate 
compound (Coronate L) was added thereto to make a magnetic coating 
solution. 
______________________________________ 
Back Coating 
______________________________________ 
Carbon black (RAVEN 1035) 
40 parts 
Barium sulfate (average particle size: 
10 parts 
300 nm) 
Nitrocellulose 25 parts 
Polyurethane resin 25 parts 
(N-2301 made by Nippon Polyurethane 
Ind. Co.) 
Polyisocyanate compound 10 parts 
(Coronate L made by Nippon Polyurethane 
Ind. Co.) 
Cyclohexanone 400 parts 
Methyl ethyl ketone 250 parts 
Toluene 250 parts 
______________________________________ 
Thus, magnetic tape samples shown in Table 1 were obtained. 
TABLE 1 
__________________________________________________________________________ 
Young's Moduli Magnetic 
Material 
Thickness 
Support Support: 
Layer: 
of of Support 
TD MD R.sub.a S 
R.sub.a M 
Support 
.mu.m! 
kg/mm.sup.2 ! 
kg/mm.sup.2 ! 
nm! nm! 
__________________________________________________________________________ 
Example 1 
PET 9.8 650 550 6.0 2.5 
Example 2 
PET 7.5 650 550 6.0 2.5 
Example 3 
PEN 8.5 800 700 7.0 2.7 
Example 4 
PEN 6.0 1200 600 7.0 2.7 
Example 5 
PEN 5.5 1200 600 7.0 2.7 
Example 6 
PPTA 5.0 1300 1330 7.0 2.7 
Example 7 
PPTA 4.0 1960 860 7.0 2.7 
Comp. PET 9.8 450 750 6.0 2.5 
Example (1) 
Comp. PET 7.5 450 750 6.0 2.5 
Example (2) 
Comp. PEN 7.5 800 700 12.0 5.0 
Example (3) 
Comp. PEN 6.0 600 980 7.0 2.7 
Example (4) 
Comp. PEN 5.5 600 980 7.0 2.7 
Example (5) 
Comp. PPTA 5.0 860 1960 7.0 2.7 
Example (6) 
Comp. PPTA 4.0 1300 1330 7.0 2.7 
Example (7) 
__________________________________________________________________________ 
In Table 1, center line average roughness R.sub.a S, R.sub.a M were 
measured using a Talystep at a cut-off of 0.25 mm. 
The following measurements were made using the samples. The results are 
shown in Table 2. 
Measuring Items 
Electromagnetic Characteristics 
The output and C/N were measured at 7 MHz on a Sony EV-S900. 
Measurement of r 
In the measurement, each of the tapes specified in the second row of Table 
2 was used as a reference tape for a head shape. That is, prior to the 
measurement, each reference tape was run for 10 hours using an unused head 
to stabilize the shape of the head, and then r was measured. Further, 
every time a sample tape was measured, the output and C/N of the reference 
tape were measured to ascertain that these values were kept unchanged. 
TABLE 2 
__________________________________________________________________________ 
Electrical Properties: 
Electrical Properties: 
Electrical Properties: 
Electrical Properties: 
Measuring measuring Measuring measuring 
Condition 1 
Condition 2 
Condition 3 
Condition 4 
Reference Tape: 
Reference Tape: 
Reference Tape: 
Reference Tape: 
Example 7 Example 5 Example 3 Example 1 
R of Head: 2.7 mm 
R of Head: 3.4 mm 
R of Head: 4.5 mm 
R of Head: 5.7 mm 
Output 
C/N Output 
C/N Output 
C/N Output 
C/N 
dB! dB! dB! dB! dB! dB! dB! dB! 
__________________________________________________________________________ 
Example 1 1.0 0.5 1.0 0.5 0.8 0.4 0.7 0.4 
Example 2 0.0 0.0 0.0 0.0 -0.1 -0.1 -0.2 -0.1 
Example 3 0.3 0.1 0.3 0.1 0.0 0.0 -0.1 -0.1 
Example 4 0.3 0.1 0.2 0.1 0.0 0.0 -0.4 -0.3 
Example 5 0.0 0.0 0.0 0.0 -0.2 -0.1 -0.6 -0.3 
Example 6 -0.0 0.0 -0.1 0.0 -0.3 -0.1 -0.8 -0.4 
Example 7 -0.2 -0.1 -0.4 -0.2 -0.7 -0.3 -1.2 -0.7 
Comp. Example (1) 
0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 
Comp. Example (2) 
-0.7 -0.4 -0.7 -0.4 -0.8 -0.5 -1.1 -0.7 
Comp. Example (3) 
-1.2 -1.0 -1.2 -1.0 -1.4 -1.1 -1.6 -1.2 
Comp. Example (4) 
-1.4 -1.2 -1.5 -1.2 -1.7 -1.3 -2.0 -1.6 
Comp. Example (5) 
-2.0 -1.8 -2.2 -1.8 -2.5 -2.0 -2.7 -2.1 
Comp. Example (6) 
-1.4 -1.3 -1.7 -1.5 -2.0 -1.7 -2.3 -1.9 
Comp. Example (7) 
-2.1 -1.8 -2.3 -2.0 -2.7 -2.5 -3.2 -3.0 
__________________________________________________________________________