Magnetic recording medium

The magnetic recording medium has superposed on at least one surface of a non-magnetic support member a magnetic layer possessing a surface shape such that the space-average wavelength (2.alpha.a) is in the range of 15 .mu.m to 30 .mu.m and the surface-average roughness (sRa) in the range of 0.008 .mu.m to 0.025 .mu.m. The magnetic recording medium combines highly satisfactory property of media and magnetic interface with high durability and high reproducing output.

The present application claims priority of Japanese Patent Application No. 
62-116079 filed on May 13, 1987. 
FIELD OF THE INVENTION AND RELATED ART STATEMENT 
This invention relates to a magnetic recording medium. More particularly 
this invention relates to a magnetic recording medium excelling in 
property of media and magnetic interface and durability and possessing a 
high recording density. 
Generally, the magnetic recording medium is produced by preparing a 
magnetic coating material having a magnetic particle uniformly dispersed 
in a solution of a binder resin in an organic solvent, applying the 
magnetic coating material on the surface of a non-magnetic supporting 
member such as polyester film, optionally subjecting the layer of coating 
material to an orienting treatment, and then processing the resultant 
composite in a drying step and a surface smoothening step. 
As universally known the magnetic recording medium finds extensive utility 
in recording information on VTR, various audio devices,, and various data 
recording devices. In recent years, studies are being devoted to the 
improvement of recording systems and the impartation of improved quality 
of the magnetic recording medium itself, both for increasing the recording 
density of the magnetic recording medium. Particularly concerning the 
magnetic recording medium itself, studies are being made in search of a 
method for effecting high-density recording by materializing perpendicular 
magnification recording with a recording medium using a microfine magnetic 
particle of hexagonal ferrite. 
Magnetic recording media have varying design concept to suit purposes for 
which they are used. Particularly, the magnetic recording medium to be 
utilized in a floppy disc is designed with more emphasis on the 
improvement in reliability of performance rather than on the improvement 
in recording density of the magnetic recording medium itself. 
Where a magnetic recording medium of this sort is to be given an increased 
capacity for recording density, it requires the surface quality to be 
notably improved to offer an increased reproducing output. The improvement 
in the surface quality entails a disadvantage that since the area of 
contact between the recording-reproducing head and the surface of the 
magnetic recording medium is consequently increased, the property of media 
and magnetic interface and durability of the magnetic recording medium are 
degraded. As a means of improving this property and durability, the method 
which resorts to combined use of different lubricants has been known to 
the art. This method, however, is not so effective as to improve this 
property and durability to a fully sufficient extent. 
SUMMARY AND OBJECT OF THE INVENTION 
As parameters indicative of the surface quality, average roughness along 
the center line (Ra), ten-point average roughness (Rz), maximum roughness 
(Rmax), rout-meansquare roughness (Rr.m.s.) and the like have been 
generally known. Particularly for this invention, with due respect to the 
relation between the magnetic recording medium and the magnetic head, the 
surface-average roughness (sRa) of three-dimensional representation and 
the space average wavelength (s.lambda.a) indicative of shape are adopted. 
The surface roughness is defined in Japanese Industrial Standard (JIS) B 
0601 titled "Definition and Designation of Surface Roughness." Here, it is 
modified so as to define a three-dimensional roughness. The 
surface-average roughness is a parameter corresponding to the average 
roughness (Ra) along the straight line and, in the orthogonal coordinate 
system having the X axis and the Y axis thereof laid from the rough 
surface on the center surface and the Z axis thereof laid vertically to 
the center surface, is represented by the following formula. 
##EQU1## 
This surface-average roughness (sRa) is found as follows. 
First, from the starting points set at fixed intervals in the Y direction, 
surface roughness is measured in a length L, toward the X direction. Then, 
an integral value is found from the lines of rough surface f(X) between O 
and L.sub.x. This calculation is carried out with respect to the lines of 
rough surface between O and L.sub.y in the O and L.sub.y. The average is 
found of the values consequently obtained. The average thus obtained 
represents the surface-average roughness (sRa). 
The space-average wavelength (s.lambda.a) is such that in a surface form 
represented by a sine function, y=Asin .omega. ox, wherein WO stands for 
spatial angular vibration and A for amplitude, the space wavelength will 
be found by the following formula: 
##EQU2## 
X stands for the axis taken along the center line of the roughness of the 
surface subjected to the measurement mentioned above and Y for the 
function f(X), and the space-average wavelength (s.lambda.a) for the 
function f(X) will be the spatial wavelength, .lambda.a, of the sine 
function which satisfies the formula (2) with respect to the f(X). The 
average wavelength of the three-dimensional surface roughness, therefore, 
is found from the following formula: 
##EQU3## 
wherein s.DELTA.a stands for three-dimensional surface-average roughness 
and sRa for three-dimensional average gradient 
From the surface roughness of a medium, the three-dimensional 
surface-average roughness s.DELTA.a is found by cutting sections in the 
surface shape obtained by the aforementioned method, measuring areas of 
particles revealed in the sections and taking count of particles, 
calculating average circles severally in the sections finding an 
increment, .DELTA.r, in each of the radial of the average circles, and 
determining the three-dimensional surface-average roughness s.DELTA.a 
based on the ratio of .DELTA.z/.DELTA.r. Then, the increment 
.DELTA..lambda.a is calculated by the formula (3) using the value of sRa 
which has been derived from the formula (1). 
The inventors prepared a number of magnetic recording media differing in 
surface condition and tested them for surface quality, durability, and 
reproducing output. From the experiment, they have found that a 
correlation exists between the space-average wavelength (s.lambda.a) and 
the surface-average roughness (sRa) as one part and the durability, the 
friction coefficient, and the reproducing output as the other part and 
that a magnetic recording medium vested with high property of media and 
magnetic interface, high durability, and high reproducing output is 
obtained by adjusting the profile of the surface of the magnetic recording 
medium. 
Specifically, as illustrated in FIG. 1, when the surface-average roughness 
(sRa) is fixed and the space-average wavelength (s.DELTA.a) is varied, the 
durability decreases in proportion as the space-average wavelength 
(s.lambda.a) increases. Conversely, when the space-average wavelength 
(s.lambda.a) is fixed and the surface-average roughness (sRa) is varied, 
the durability increases in proportion as the surface-average roughness 
(sRa) increases. 
As illustrated in FIG. 2, the friction coefficient (.mu.K) and the 
reproducing output (dB) both increase in proportion as the space-average 
wavelength (s.lambda.a) increases. 
An attempt at decreasing the surface-average roughness (sRa) tends to 
increase the space-average wavelength (s.lambda.a). The range in which 
these properties prove to be optimum for all the other properties, 
therefore, is relatively narrow. 
An object of this invention, therefore, is to provide a magnetic recording 
medium which is capable of manifesting highly desirable property of media 
and magnetic interface, high durability, and high reproducing output. 
Another object of this invention is to provide a magnetic recording medium 
which fits high density recording. 
The other objects of this invention will become apparent from the further 
description of this invention to be given below. 
The objects of this invention described above are accomplished in a first 
embodiment in a magnetic recording medium provided on at least one surface 
of a nonmagnetic supporting member with a magnetic layer by adapting the 
surface quality of the magnetic layer so that the space-average wavelength 
(s.lambda.a) will fall in the range of 15.mu.m to 30.mu.m and the 
surface-average roughness (sRa) in the range of 0.008.mu.m to 0.025 .mu.m. 
In the above first embodiment of the present invention, the value of the 
space-average wavelength (s.lambda.a) is limited to the aforementioned 
range for the following reason. If the surface shape is such that the 
space-average wavelength (s.lambda.a) exceeds 30 .mu.m, the magnetic 
recording medium has a disadvantage that the durability thereof is 
insufficient, the surface thereof is so heavily undulated as to render it 
is difficult to obtain necessary head touch, and the modulation is 
inevitably suffered to increase. An attempt at forming a magnetic layer 
having a wavelength of less than 15 .mu.m results in an extreme decrease 
in the efficiency of the calendering treatment such that the breaking 
strength and the packing density of the applied layer will be degraded and 
high durability will not be attained without any sacrifice in the 
characteristic of high density. 
In the above first embodiment of the present invention, the surface-average 
roughness (sRa) of the magnetic recording medium is limited to the range 
of 0.008 .mu.m to 0.025 .mu.m for the following reason, If the value of 
sRa is less than 0.008 .mu.m, the magnetic recording medium betrays 
notable deficiency in friction coefficient and durability, though the 
reproducing output is high. In another embodiment of the present invention 
in the case where a high density magnetic recording medium having an 
increasing output is expected at some sacrifice of high durability, the 
value of sRa may be in the range of 0.006 .mu.m to 0.008 .mu.m. But it is 
not practical that the magnetic recording medium having the 
surface-average roughness (sRa) less than 0.006 .mu.m has concurrently 
with the space-average wavelength (s.lambda.a) in the range of 15 .mu.m to 
30 .mu.m, because it is difficult to produce the medium with good 
reproducibility, and the durability of the medium results in a rapid 
decrease. Conversely, if this value exceeds 0.025 .mu.m, the magnetic 
recording medium suffers from deficiency in output, though it enjoys 
stable property of media and magnetic interface and durability. In another 
embodiment of the present invention, a magnetic recording medium such as a 
floppy disc, comprises a nonmagnetic supporting member and a magnetic 
layer formed on at least one surface of the supporting member. The 
magnetic layer comprises a hexagonal ferrite magnetic particle, a 
nonmagnetic particle and a binder resin with the nonmagnetic particle 
having an average particle diameter in the range of 200 nm to 400 nm and 
possessing a surface shape such that the space-average wavelength 
(s.lambda.a) falls in the range of 15 .mu.m to 30 .mu.m and the 
surface-average roughness (sRa) is in the range of 0.006 .mu.m to 0.008 
.mu.m. A suitable nonmagnetic particle is an electroconductive carbon 
black particle. A hexagonal ferrite magnetic particle of this embodiment 
has an average particle diameter in the range of 0.001 .mu.m to 0.2 .mu.m. 
As examples of the material for the magnetic particle for effective use in 
the magnetic layer of the invention, various magnetic metals such as 
.gamma.-Fe.sub.2 O.sub.3, cobalt-absorbed Co-.gamma.-Fe.sub.2 O.sub.3, 
CrO.sub.3 and Ba-ferrite are illustrative. Particularly, magnetic metal 
particle and hexagonal ferrite particle are suitable for high-density 
recording. 
As examples of the hexagonal magnetic particle, hexagonal ferrite particles 
represented by the following general formula are illustrative. 
EQU M.sub.1 O.multidot.n(Fe.sub.1-m M.sub.m).sub.2 O.sub.3 
(wherein M.sub.1 stands for one element selected from the group consisting 
of Ba, Sr, Ca and Pb, n for a number in the range of 5.4 to 6.0, M for one 
metal element selected from the group consisting of Ti, Co, Zn, In, Mn, 
Cu, Ge, Ta, Nb, Te, Zr, V, Al and Sn, and m for a number in the range of 0 
to 2). 
Particularly for this invention, a substituted hexagonal ferrite particle 
possessing an average particle diameter in the range of 0.001 to 0.2 .mu.m 
and a coercive force in the range of 200 to 2,000 Oe and having part of 
the Fe atom thereof substituted with at least one metallic element 
selected from the group consisting of Ti, Co, Zn, In, Mn, Cu, Ge, Ta, Nb, 
Te, Zr, V, Al and Sn is suitable. 
The relation of the amount of the element used for the substitution will be 
described below with reference to a magneto-plumbite type Ba-ferrite. The 
substituent in this case is represented by the following chemical formula. 
EQU BaFe.sub.12- (x+y(+z))M.sub.IIx M.sub.Vy (M.sub.IVz)O.sub.19 
In the formula, x, y and z stand for the amounts respectively of M.sub.II, 
M.sub.V and M.sub.IV elements per chemical formula. The symbols M.sub.II, 
M.sub.V and M.sub.IV stand respectively for divalent, pentavalent, and 
tetravalent elements and the Fe atom subjected to substitution is a 
trivalent element. Thus the relation, y=(x-y)/2, is established. The 
amount of M.sub.IV used for substitution is definitely fixed by the 
amounts of M.sub.II and M.sub.V used for substitution and the amounts of 
M.sub.II and M.sub.V are in such a relation that one of them is 
automatically fixed when the other is fixed. 
The amounts of these elements for substitution are empirically selected so 
that the coercive force of the produced magnetic layer will fall in the 
range of 200 to 2,000 Oe. 
As the binder resin for use in the present invention, either polyurethane 
resin or a mixture of polyurethane resin with other resin is employed. 
The polyurethane resin mentioned above is chiefly of a grade having a 
molecular weight the range of 20,000 to 50,000. If a molecular weight of 
the polyurethane resin is less than 20,000, the magnetic recording medium 
can not enjoy high durability. 
When a polyurethane resin having a molecular weight of not more than 50,000 
and a polyurethane resin having a molecular weight in the range of 80,000 
to 100,000 are used as mixed, the produced magnetic layer enjoys enhanced 
durability. The dispersibility of magnetic particle in this mixture is 
lowered as the mixing ratio of the polyurethane resin having a molecular 
weight of not less than 80,000 is increased. The mixing ratio of the 
polyurethane resin, therefore, is desired to fall approximately in the 
range of 10 to 50 parts by weight, based on 100 parts by weight of the 
polyurethane resin having a molecular weight of not more than 50,000. 
As concrete examples of the polyurethane resin having molecular weight of 
not more than 50,000, the products of Nippon Polyurethane Industry Co., 
Ltd. marketed under product codes of N-3135 and N-3127 and the products of 
Dainichiseika Colour & Chemical Mfg. Co., Ltd. marketed under product 
codes of MAU-7300 and MAU-2360 are illustrative. As concrete examples of 
the polyurethane resin having molecular weight of not less than 80,000, 
the products of Nippon Polyurethane Industry Co., Ltd. marketed under 
product codes of N-2302 and N-3022 are illustrative. 
The other resin which is used as mixed with the polyurethane resin may be 
at least one member selected from the group consisting of cellulose 
derivative resin, polyester resin, polycarbonate resin, polyacrylate 
resin, polyamide resin, epoxy resin, phenol resin, polyether resin, 
phenoxy resin, melamine resin, vinyl butyral resin, furan resin, vinyl 
chloride resin, vinyl acetate resin, vinyl alcohol resin, copolymers 
thereof, and mixtures thereof. Particularly, the cellulose derivative 
resin or the vinyl chloride-vinyl acetate resin is used advantageously 
because the magnetic particle exhibits high dispersibility in the resin. 
Generally, the additional resin is used in an amount in the range of 0 to 
50 parts by weight, based on 100 parts by weight of the polyurethane 
resin. 
The magnetic coating material, when necessary, may incorporate therein a 
lubricant, a dispersant, an abradant, or a conductivity imparting agent 
such as carbon black. 
Examples of the lubricant usable herein include higher saturated fatty 
acids, higher unsaturated fatty acids, and esters thereof each having not 
less than 14 carbon atoms, silicone type compounds, fluorinated 
hydrocarbons, and mixtures thereof. 
Examples of the aforementioned higher fatty acids and esters thereof, 
include saturated fatty acids such as pentadecylic acid, palmitic acid, 
heptadecylic acid, stearic acid, and nonadecanoic acid, unsaturated fatty 
acids such as oleic acid, eleidic acid, linolic acid, and linolenic acid, 
and higher alkyl esters thereof. 
These fatty acids and esters thereof having not less than 14 carbon atoms 
exhibit satisfactory affinity for the binder resin and produce a more 
desirable lubricating effect on the surface of the magnetic recording 
medium than lower fatty acids. 
As examples of the dispersant, anionic surfactants (surface active agents), 
cationic surfactants, nonionic surfactants, silane coupling agents, and 
titanium coupling agents are illustrative. 
As examples of the abradant, powders of such inorganic compounds as 
chromium oxide, alumina, silicone carbide, titania, and Zirconia which 
have degrees of not less than 5 in Mohs' scale are illustrative. 
The amounts of such additives as mentioned above to be used herein are 
desired as small as permissible to ensure manifestation of amply high 
reproducing output. Suitably, the amount of the lubricant is from 1 to 5 
parts by weight and that of the dispersant is not more than 4 parts by 
weight, the amount of the abradant is 2 to 6 parts by weight, and the 
amount of carbon black not more than 3 parts by weight, each based on 100 
parts by weight of the magnetic particle. Further, the addition of 
nonmagnetic particles having an average diameter in the range of 200 nm to 
400 nm makes it easily possible to produce a magnetic recording medium 
having a surface-average roughness (sRa) in the range of 0.006 .mu.m to 
0.008 .mu.m and a space-average roughness (s.lambda.a) in the range of 15 
.mu.m to 30 .mu.m, thereby making it possible to obtain a higher 
reproducing output power. Therefore, the magnetic recording medium 
obtained by such method is most suitable for a floppy disc requiring high 
recording density. 
The magnetic recording medium of the present invention is obtained by 
dissolving or dispersing the binder resin, the magnetic particle, the 
lubricant, and other additives in a solvent, adding a curing agent to the 
resultant solution or dispersion, applying the resultant coating material 
on the non-magnetic medium, subjecting the resultant composite to an 
orienting treatment and a drying treatment, giving the produced layer a 
surface treatment as by calendering with a super calendering device under 
conditions fit for the magnetic layer, and crosslinking the magnetic layer 
at a suitable temperature. Then, the resultant composite is punched or 
slit to suit the particular application and is given a smoothened surface 
to be finished. Optionally, an electroconductive layer may be formed on a 
non-magnetic substrate and the magnetic layer supperposed on this layer. 
In this case, the magnetic coating does not require incorporation of any 
electroconductive carbon therein.

DESCRIPTION OF PREFERRED EMBODIMENTS 
Now, the present invention will be described below with reference to 
preferred embodiments. It should be noted, however, that this invention is 
not limited to these preferred embodiments. 
EXAMPLE 1 
______________________________________ 
Ba-ferrite powder (Ti, Co-substituted 
666 parts by weight 
powder having average diameter 600 to 
800 .ANG. and coercive force of 700 Oe) 
Nitrocellulose resin*.sup.1 
80 " 
Polyurethane resin (molecular weight 
120 " 
45,000)*.sup.2 
Alumina (average diameter 0.15 .mu.m) 
45 " 
Electroconductive carbon 
50 " 
Lecithin 14 " 
Stearic acid 8 " 
Butyl stearate 17 " 
Methylethyl ketone 500 " 
Toluene 500 " 
Cyclohexanone 500 " 
______________________________________ 
*.sup.1 Product of Daicel Chemical Industry Co., Ltd. marketed under 
product code of HC100. 
*.sup.2 Product of Nippon Polyurethane Industry Co., Ltd. marketed under 
product code of N3127. 
The components indicated above were dispersed in a sand mill to produce a 
magnetic coating material. This magnetic coating material was filtered to 
remove flocks and foreign particles and then mixed by stirring with 13 
parts by weight of polyisocyanate compound (product of Nippon Polyurethane 
Industry Co., Ltd. and marketed under product code of "C-3041"). The 
magnetic coating material was applied on both sides of a polyester film 75 
.mu.m in thickness and dried, to produce magnetic layers. Then, in a super 
calendering device, the produced composite was treated at a temperature of 
70.degree. C. under a line pressure of 100kg/cm, then held at a 
temperature of 50.degree. C. for 2 days to cure the magnetic layers. A 
disc 3.5 inches in diameter was punched out of the resultant composite to 
produce a magnetic disc. This disc was tested for surface roughness with a 
three-dimensional roughness tester (produced by Kosaka Kenkyusho and 
marketed under product code of "SE-3AK") and an analyzing device (produced 
by Kosaka Kenkyusho and marketed under product code of "SPA-11"), using a 
stylus of R 2 .mu.m, 30 mgf. 
EXAMPLE 2 
A magnetic disc was produced by following the procedure of Example 1 except 
that the magnetic coating material obtained with the composition of 
Example 1 was applied on both sides of a polyester film 75 .mu.m in 
thickness, dried to form magnetic layers, and treated with a super 
calendering device at a temperature of 70.degree. C. under a line pressure 
of 200 kg/cm. 
EXAMPLE 3 
A magnetic disc was produced by following the procedure of Example 1 except 
that the magnetic coating material obtained with the composition of 
Example 1 was applied on both sides of a polyester film 75 .mu.m in 
thickness, dried to form magnetic layers, and treated with a super 
calendering device at 70.degree. C. under a line pressure of 300 kg/cm. 
EXAMPLE 4 
A magnetic disc was produced by following the procedure of Example 1 except 
that the same amount of vinyl chloride-vinyl acetate-vinyl alcohol 
copolymer resin (produced by U.C.C. and marketed under trademark 
designation of "VAGH") was used in the place of the nitrocellulose resin 
in the composition of Example 1. 
EXAMPLE 5 
A magnetic disc was produced by following the procedure of Example 4 except 
that the magnetic coating material obtained with the composition of 
Example 4 was applied on both sides of a polyester film 75 .mu.m in 
thickness, dried to form magnetic layers, and treated with a super 
calendering device at a temperature of 70.degree. C. under a line pressure 
of 200 kg/cm. 
EXAMPLE 6 
A magnetic disc was produced by following the procedure of Example 4 except 
that that the magnetic coating material obtained with the composition of 
Example 4 was applied on both sides of a polyester film 75 .mu.m in 
thickness, dried to form magnetic layers, and then treated with a super 
calendering device at a temperature of 70.degree. C. under a line pressure 
of 300 kg/cm. 
EXAMPLE 7 
A magnetic disc was produced by following the procedure of Example 1 except 
that the magnetic coating material obtained by using 133 parts by weight 
of polyurethane resin having a molecular weight of 50,000 (produced by 
Nippon Polyurethane Industry Co., Ltd. and marketed under product code of 
"N-3135") in the place of the nitrocellulose resin in the composition of 
Example 1, applied on both sides of a polyester film 75 .mu.m in 
thickness, and dried to form magnetic layers. 
EXAMPLE 8 
A magnetic disc was produced by following the procedure of Example 7 except 
that the magnetic coating material obtained with the composition of 
Example 7 was applied on both sides of a polyester film 75 .mu.m in 
thickness, dried to form magnetic layers, and treated with a super 
calendering device at a temperature of 70.degree. C. under a line pressure 
of 200 kg/cm. 
COMATIVE EXPERIMENT 1 
A magnetic disc was produced by following the procedure of Example 1 except 
that the magnetic coating material obtained with the composition of 
Example 1 was applied on both sides of a polyester film 75 .mu.m in 
thickness, dried to form magnetic layers, and treated with a super 
calendering device at a temperature of 85.degree. C. under a line pressure 
of 200 kg/cm. 
COMATIVE EXPERIMENT 2 
A magnetic disc was produced by following the procedure of Example 1 except 
that the magnetic coating material obtained with the composition of 
Example 1 was applied on both sides of a polyester film 75 .mu.m in 
thickness, dried to form magnetic layers, and treated with a super 
calendering device at a temperature of 50.degree. C. under a line pressure 
of 300 kg/cm. 
COMATIVE EXPERIMENT 3 
A magnetic disc was produced by following the procedure of Example 4 except 
that the magnetic coating material obtained with the composition of 
Example 4 was applied on both sides of a polyester film 75 .mu.m in 
thickness, dried to form magnetic layers, and treated with a super 
calendering device at a temperature of 85.degree. C. under a line pressure 
of 250 kg/cm. 
COMATIVE EXPERIMENT 4 
A magnetic disc was produced by following procedure of Example 7 except 
that the magnetic coating material obtained with the composition of 
Example 7 was applied on both sides of a polyester film 75 .mu.m in 
thickness, dried to form magnetic layers, and treated with a super 
calendering device at a temperature of 50.degree. C. under a line pressure 
of 200 kg/cm. 
EXAMPLE 9 
A magnetic disc was produced by following the procedure of Example 1 except 
that 36 parts by weight of a polyurethane resin having a molecular weight 
of 85,000 (produced by Nippon Polyurethane Industry Co., Ltd. and marketed 
under product code of "N-3022") was used to substitute for part of the 
polyurethane resin having a molecular weight of 45,000 (produced by Nippon 
Polyurethane Industry Co., Ltd. and marketed under product code of 
"N-3127") in the composition of Example 1 
EXAMPLE 10 
A magnetic disc was produced by following the procedure of Example 4 except 
that a polyurethane resin having a molecular weight of 83,000 (produced by 
Nippon Polyurethane Industry Co., Ltd. and marketed under product code of 
"N-2302") was used in the place of 36 parts by weight of the polyurethane 
resin (produced by Nippon Polyurethane Industry Co., Ltd. and marketed 
under product code of "N-3127"). 
EXAMPLE 11 
A magnetic disc was produced by following the procedure of Example 7 except 
that a polyurethane resin having a molecular weight of 85,000 (produced by 
Nippon Polyurethane Industry Co., Ltd. and marketed under product code of 
"N-3022") was used in the place of 60 parts by weight of the polyurethane 
resin having a molecular weight of 45,000 (produced by Nippon Polyurethane 
Industry Co., Ltd. and marketed under product code of "N-3127") in the 
composition of Example 7. 
A magnetic disc was produced by following the procedure of Example 1, 
except that 12 parts by weight of polyurethane resin having a molecular 
weight of 85,000 (N 3022) was used to substitute for part of the 
polyurethane resin having a molecular weight of 45,000 and 30 parts be 
weight of electroconductive carbon having an average particle diameter of 
400 nm in the composition of Example 1. 
EXAMPLE 13 
A magnetic disc was produced by following the procedure of Example 1, 
except that 12 parts by weight of polyurethane resin having a molecular 
weight of 83,000 (N 2302) was used to substitute for part of the 
polyurethane resin having a molecular weight of 45,000 and 30 parts by 
weight of electroconductive carbon having an average particle diameter of 
200 nm in the composition of Example 1. 
EXAMPLE 14 
A magnetic disc was produced by following the procedure of Example 1, 
except that 60 parts be weight of polyurethane resin having a molecular 
weight of 83,000 (N 2302) was used to substitute for part of the 
polyurethane resin having a molecular weight of 45,000 in the composition 
of Example 1, and a super-calendering was carried by using the condition 
of Example 2. 
COMATIVE EXPERIMENT 5 
A magnetic disc was produced by following the procedure of Example 12, 
except that an electroconductive carbon having an average particle 
diameter of 50 nm was used in place of the electroconductive carbon having 
an average particle diameter of 400 nm in the composition of Example 12. 
The magnetic discs obtained in the working examples and comparative 
experiments described above were tested for reproducing output, friction 
coefficient, and durability by the following methods. 
Reproducing Output 
On a single-sided head type disc drive provided with a head having an 
effective gap of 0.29 .mu.m, a given magnetic disc was driven at a 
rotational speed of 300 r.p.m. to measure the reproducing output of 35 
kBPI at a track position 32 mm from the disc center. 
Friction Coefficient 
On a double-sided head type disc drive, a given magnetic disc was driven to 
find the torque produced consequently by the motor of the disc drive and 
determine the friction coefficient by calculation using the magnitude of 
the torque. 
Durability 
On a double-sided head type disc drive, a given magnetic disc was driven 
and caused to record a recording signal at a track position 70. Under the 
condition which the temperature reciprocated between 5.degree. C. and 
60.degree. C. over a cycle of 24 hours, the driving was continued until 
the reproducing output decreased to 70% of the initial level to take count 
of passes. 
The results of the test are shown in the following table. These test 
results represent averages each obtained of five samples and reported with 
all figures below million reduced to zero. 
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Average 
wave- Surface- Friction Dura- Re- 
length average co- bility 
producing 
(s.lambda.a) 
roughness efficient 
(10,000 
output* 
(.mu.m) (sRa) (.mu.m) 
(.mu.K) passes) 
(dB) 
______________________________________ 
Exam- 
ple 
1 20 0.020 0.25 1500 +1.5 
2 27 0.013 0.23 1400 +2.6 
3 30 0.014 0.24 1350 +2.6 
4 25 0.013 0.21 1700 +2.6 
5 29 0.013 0.28 1350 +2.6 
6 30 0.020 0.26 1300 +1.6 
7 27 0.013 0.26 1450 +2.8 
8 30 0.012 0.30 1420 +3.0 
9 29 0.012 0.22 1800 +2.0 
10 30 0.013 0.27 1900 +2.2 
11 28 0.011 0.29 1600 +2.0 
12 18 0.007 0.20 1450 +3.4 
12 24 0.006 0.23 1600 +3.0 
14 15 0.009 0.20 1900 +3.7 
Compar- 
ative 
Experi- 
ment 
1 46 0.008 0.34 800 +3.2 
2 15 0.030 0.25 1100 0 
3 45 0.009 0.43 700 +3.3 
4 32 0.047 0.24 1500 -2.6 
5 45 0.004 0.40 100 +3.7 
______________________________________ 
*Relative values, based on the result of Comparative Experiment 2 taken 
as 0 dB.