Magnetic recording medium having a magnetic layer comprising iron powder and an underlayer comprising non magnetic powders having specified particle size ratios

A magnetic recording medium comprising a non-magnetic support, an intermediate layer which is formed on the non-magnetic support containing carbon black and non-magnetic particles other than carbon black, a particle size ratio between the carbon black and non-magnetic particles being from 1.0 to 4.0 in terms of a ratio of a diameter or a short axis size of the non-magnetic particles to a particle size of carbon black, and a magnetic layer which is formed on the intermediate layer and has a thickness of 0.2 to 1.0 .mu.m, in which magnetic layer a difference of an intensity at 102 mm.sup.-1 and that at 205 mm.sup.-1, in a frequency analysis of non-contact surface roughness is from 9 dB to 15 dB, in which a total thickness of the non-magnetic support, the intermediate layer and the magnetic layer is not greater than 13 .mu.m, which recording medium is excellent in electromagnetic conversion characteristics and durability.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a magnetic recording medium comprising a 
magnetic layer having a thickness of 1.0 .mu.m or less. In particular, the 
present invention relates to a magnetic recording medium which has 
excellent electromagnetic conversion characteristics and durability, and 
is suitably used in an apparatus which records and reproduces digitized 
signals by a helical scan system. 
2. Description of the Related Art 
A magnetic recording medium is usually produced by coating a magnetic paint 
comprising magnetic powder, a binder component, an organic solvent and 
other necessary components on a non-magnetic support, such as a polyester 
film, and drying it. To satisfy the requirement for high density 
recording, the thickness of the magnetic layer is gradually decreased. 
Recently, with the progress of signal processing and digital circuit 
technologies, the signals are digitized and recorded by a video recorder. 
Presently, since a thickness of the magnetic layer is still thick, 
unsaturated digital recording is employed. But, for recording and 
reproducing the digital signals, saturated digital recording which can be 
overwritten as in the case of a magnetic disc apparatus for a computer is 
preferred. To effect the saturated digital recording the thickness of the 
magnetic layer is preferably made thin. 
However, when the magnetic layer is made thin to increase the recording 
density and improve the overwriting characteristics, surface conditions of 
the non-magnetic support tend to influence the properties of the magnetic 
layer, so that the electromagnetic conversion characteristics deteriorate, 
for example, the reproducing output is lowered as the thickness of the 
magnetic layer is decreased. 
To prevent influence on the surface conditions of the non-magnetic support 
and to improve surface smoothness of the magnetic layer, an intermediate 
layer comprising a non-magnetic powder is provided between the 
non-magnetic support and the magnetic layer. However, while the 
conventional intermediate layer can prevent the influence on the surface 
conditions of the non-magnetic support, pin holes and/or coating streaks 
are formed and the durability is not improved sufficiently. 
From the rheological view point of the intermediate layer when a magnetic 
paint is coated, it in order to increase the durability a improvement of 
the composition of the intermediate layer and/or coating method have been 
attempted. For example, it has been proposed to provide a magnetic layer 
having a thickness of less than 1.0 .mu.m while the intermediate layer is 
still wet, whereby the electromagnetic conversion characteristics and also 
the durability are improved as seen in U.S. Pat. No. 5,258,223. 
However, while the formation of pin holes and coating streaks on the 
magnetic layer surface was prevented, the surface smoothness of the 
magnetic layer was not improved satisfactorily, and the electromagnetic 
conversion characteristics may be worse than those of the magnetic 
recording medium in which the magnetic layer was directly formed on the 
non-magnetic support, in the absence of the intermediate layer. 
SUMMARY OF THE INVENTION 
An object of the present invention is to increase the surface smoothness of 
a magnetic layer having a thickness of 1.0 .mu.m or less without forming 
pin holes or coating streaks sufficiently improving the electromagnetic 
conversion characteristics and durability of the resulting magnetic 
recording medium. 
According to the present invention, there is provided a magnetic recording 
medium comprising a non-magnetic support, an intermediate layer which is 
formed on the non-magnetic support and which comprises carbon black and 
non-magnetic particles other than carbon black (hereinafter referred to as 
"non-magnetic particles"), a particle size ratio between the two 
components being from 1.0 to 4.0 in terms of a ratio of a diameter or a 
short axis size of the non-magnetic particles to a particle size of the 
carbon black, and a magnetic layer which is formed on the intermediate 
layer and has a thickness of 0.2 to 1.0 .mu.m and in which magnetic layer, 
a difference of an intensity at 102 mm.sup.-1 and that at 205 mm.sup.-1, 
in a frequency analysis of non-contact surface roughness, is from 9 dB to 
15 dB, wherein a total thickness of the non-magnetic support, the 
intermediate layer and the magnetic layer is not larger than 13 .mu.m. By 
a structure of the magnetic recording medium, the surface smoothness of 
the magnetic layer is sufficiently increased, and the electromagnetic 
conversion characteristics and durability of the magnetic recording medium 
are satisfactorily improved. 
When the intermediate layer contains, as binder resins, a polyvinyl 
chloride resin and a polyurethane resin which have functional groups 
having the same polarity, the dispersibility of the non-magnetic particles 
and carbon black contained in the intermediate layer is improved, so that 
the surface smoothness of magnetic layer is further increased. 
When iron powder containing 0.1 to 3.0% by weight of aluminum based on the 
weight of iron is used as the magnetic powder contained in the magnetic 
layer and a ratio of a Dx value (X-ray particle size) of the iron powder 
to-the particle size of the non-magnetic particles in the intermediate 
layer is from 0.2 to 1.0 in terms of a ratio of the Dx value to a diameter 
or a short axis size of the non-magnetic particles in the intermediate 
layer, or when the magnetic layer contains alumina particles, and a ratio 
of a particle size of the alumina particles to the particle size of the 
nonmagnetic particles in the intermediate layer is from 3.0 to 12.0 in 
terms of a ratio of a diameter of alumina particles to the diameter or the 
short axis size of the non-magnetic particles in the intermediate layer, 
the durability of the magnetic recording medium is further increased.

DETAILED DESCRIPTION OF THE INVENTION 
In the present invention, the intermediate layer, which is formed between 
the non-magnetic support and the magnetic layer, is formed using both the 
carbon black and the non-magnetic particles the particle size ratio 
between which is from 1.0 to 4.0 in terms of a ratio of a diameter or a 
short axis size of the non-magnetic particles to a particle size of carbon 
black. Thereby, the surface smoothness of the intermediate layer during 
drying is improved, and the resulting good surface conditions have an 
influence on the surface conditions of the magnetic layer which is formed 
on the intermediate layer, and the surface smoothness of the magnetic 
layer having a difference of an intensity at 102 mm.sup.-1 and that at 205 
mm.sup.-1 in a frequency analysis of non-contact surface roughness of from 
9 dB to 15 dB is sufficiently improved. Herein, the diameter and the short 
axis size are averaged values. 
According to the present invention, the surface smoothness of the 
intermediate layer during drying is sufficiently improved, and the 
magnetic layer has a difference of an intensity at 102 mm.sup.-1 and that 
at 205 mm.sup.-1 in a frequency analysis of non-contact surface roughness 
of from 9 dB to 15 dB, whereby the magnetic recording medium has well 
improved electromagnetic conversion characteristics and durability. 
When the particle size ratio of the non-magnetic particles to the carbon 
black which are used together is less than 1.0 or exceeds 4.0 in terms of 
a ratio of a diameter or a short axis size of the non-magnetic particles 
to a particle size of carbon black, the filling of these powders 
deteriorates and the surface smoothness of the intermediate layer is 
insufficient, so that the difference of an intensity at 102 mm.sup.-1 and 
that at 205 mm.sup.-1 in the frequency analysis of non-contact surface 
roughness on the magnetic layer formed on the intermediate layer is less 
than 9 dB or exceeds 15 dB. Consequently, a magnetic recording medium 
excellent in electromagnetic conversion characteristics and durability is 
not obtained. 
When the difference between the intensities in the frequency analysis is 
less than 9 dB, while the number of unevenness the short period increases, 
a height difference of the unevenness decreases, so that a gap between the 
magnetic recording medium and a magnetic head is narrowed and the contact 
area between the magnetic recording medium and the magnetic tape 
increases, whereby the durability of the magnetic recording medium is 
negatively affected. When the difference between intensities in the 
frequency analysis exceeds 15 dB, while the number of unevenness of long 
period decreases, a height difference of the unevenness increases, so that 
the gap between the magnetic recording medium and the magnetic head 
increases, whereby the output is reduced. 
A volume ratio of the non-magnetic particles to the carbon black is 
preferably from 70:30 to 60:40. When the amount of carbon black is too 
small or too large, the coating property of the paint deteriorates, so 
that some defects such as coating streaks appear on the magnetic layer 
surface, and a magnetic recording medium with good electromagnetic 
conversion characteristics or durability is not attainable. 
As the binder resin to be used in the intermediate layer, a polyvinyl 
chloride resin and a polyurethane resin can be used. Preferably, a 
polyvinyl chloride resin and a polyurethane resin which have functional 
groups having the same polarity are used together. When the intermediate 
layer containing the non-magnetic particles, the carbon black and such 
binder resins is formed, the surface smoothness of the magnetic layer is 
satisfactorily improved. 
When a polyvinyl chloride resin and a polyurethane resin which have 
functional groups having different polarity are used, those functional 
groups attract each other, so that the dispersion effects of the resins 
offset each other, and therefore, the surface smoothness is not 
satisfactorily improved. 
A weight ratio of the total weight of the non-magnetic particles and carbon 
black to the binder resin is preferably from 80:20 to 60:40. When the 
amount of the powders is too large, shelf stability of the intermediate 
layer paint deteriorates, so that it is difficult to produce a uniform 
elongate magnetic recording medium. When it is too small, a degree of 
so-called curling of the magnetic tape increases, so that the running 
property of the magnetic recording medium during recording and reproducing 
is worsened and data errors are generated during recording and 
reproducing. 
As the non-magnetic particles to be contained in the intermediate layer, 
those having any shape such as spheres, needles, cubes, etc. may be used. 
Preferably, the particles have a particle size of 20 to 100 nm. Specific 
examples of the non-magnetic particles are alumina, .alpha.-hematite, 
titanium oxide, chromium oxide, zinc oxide, barium sulfate, calcium 
carbonate, and the like. Examples of the commercially available products 
are NANOTITE (manufactured by Showa Denko Co., Ltd.); TF-100, TF-120, 
TF-140 and DPN-245 (manufactured by Toda Industries, Ltd.); TAIPAKE, TTO 
55 and CR-50 (manufactured by Ishihara Industries, Ltd.); AKP-50, AOS-50 
and HIT-50 (manufactured by Sumitomo Chemical Co., Ltd.); and the like. 
As the carbon black, furnace black, thermal black, acetylene black and the 
like can be used. Preferably, the carbon black having a particle size of 
10 to 50 nm is used. Neutral carbon black is preferred, and one having pH 
of 6 to 8.5 is preferably used. Examples of the commercially available 
carbon black are MONARCH 800, MONARCH 900, MONARCH 460, MONARCH 280, 
MONARCH 230, MONARCH 120, REGAL 330R, REGAL 415R, REGAL 250R and REGAL 99R 
(all manufactured by Cabot); LABEN 1250 (manufactured by Columbian 
Carbon); and the like. 
As the binder resins contained in the intermediate layer, the polyvinyl 
chloride resin and the polyurethane resin having an anionic functional 
group such as --SO.sub.3 M wherein M is H or an alkali metal, 
--PO(OH).sub.2, --PO(OH)(ONR.sub.1 R.sub.2 R.sub.3 H) wherein R.sub.1, 
R.sub.2 and R.sub.3 are each H, OH or an alkyl group, --CO.sub.2 M wherein 
M is H or an alkali metal, and the like; or a cationic functional group 
such as --NR.sub.1 R.sub.2 R.sub.3 .sup.X wherein R.sub.1, R.sub.2 and 
R.sub.3 are each an alkyl group and X is a halogen atom are used in 
combination, preferably. More preferably, the polyvinyl chloride resin and 
the polyurethane resin which have the functional groups having the same 
polarity are used together. 
Examples of the polyvinyl chloride resin are MR110, MR113 and MR116 
(manufactured by Nippon Zeon Co., Ltd.); ESLEK E C130 and ESLEK E C110 
(manufactured by Sekisui Chemical Industries, Ltd.); MPR-TAO (manufactured 
by Nisshin Chemical Industries, Ltd.); VMCH, MXR-527, MXR-536 and MXR-535 
(manufactured by UCC); and the like. 
Examples of the polyurethane resin are UR 8300, UR 8700 and UR 8200 
(manufactured by Toyobo Co., Ltd.), and the like. 
In addition to the polyvinyl chloride resin and the polyurethane resin, a 
polyisocyanate compound may be used in combination therewith. Further, 
other binder resins which are used in magnetic recording media may be 
used. 
To the intermediate layer, a lubricant may be added. In particular, an 
aliphatic acid ester base lubricant, such as n-butyl stearate, is 
preferred since it improves the wettability with the magnetic layer which 
is formed on the intermediate layer. Further, an aliphatic acid, such as 
stearic acid, myristic acid, palmitic acid, etc. may be added. 
In the above case, an amount of the aliphatic acid ester base lubricant is 
preferably from 0.5 to 5.0% by weight based on the total weight of the 
non-magnetic particles and the carbon black, since when the amount is less 
than 0.5% by weight, the intended effects are not achieved, while when the 
amount exceeds 5.0% by weight, the durability of the intermediate layer is 
negatively affected. 
An amount of the aliphatic acid is preferably not larger than 4.0% by 
weight based on the total weight of the non-magnetic particles and the 
carbon black, since this amount exceeds 4.0% by weight, the durability of 
the magnetic layer is deteriorated. 
The amount of the lubricant is not determined only from the view point of 
durability of the intermediate layer. Also taken into consideration is the 
durability of the magnetic recording medium. From such view point, when 
the amount of the lubricant is limited from an extracted amount described 
in Japanese Patent KOKAI Publication No. 128495/1993, an extracted amount 
is preferably in the range between 20 and 50 mg/m.sup.2 in the case of the 
aliphatic acid ester, or 10 mg/m.sup.2 or less in the case of the 
aliphatic acid, when the magnetic recording medium is extracted with 
n-hexane as a solvent at room temperature for 16 hours. 
Such intermediate layer is formed by mixing and dispersing the non-magnetic 
particles and the carbon black in the polyvinyl chloride resin and the 
polyurethane resin which have the functional groups having the same 
polarity and an organic solvent to prepare an intermediate layer paint, 
coating the prepared intermediate layer paint on the non-magnetic support, 
and drying it. 
As the organic solvent used in the preparation of the intermediate layer 
paint, any of the conventional solvents which are used in the production 
of the magnetic recording medium may be used. For example, ketones such as 
methyl ethyl ketone, cyclohexanone, methyl isobutyl ketone, etc.; ethers 
such as tetrahydrofuran, dioxane, etc.; acetates such as ethyl acetate, 
butyl acetate, etc., or mixtures thereof can be used. They may be used in 
combination with toluene and the like. 
The magnetic layer on the intermediate layer is formed by mixing and 
dispersing a conventional magnetic powder such as Fe powder, Co powder, 
Fe--Ni powder, barium ferrite, strontium ferrite and the like in the 
binder resin and the organic solvent to prepare the magnetic paint, 
coating the magnetic paint on the intermediate layer which has been formed 
on the non-magnetic support and drying it. 
When the binder resins having functional groups having different polarities 
are used between the intermediate layer and the magnetic layer, for 
example, a resin having a cationic functional group used as the binder 
resin in the intermediate layer and a resin having an anionic functional 
group used as the binder resin in the magnetic layer, the adhesion between 
the intermediate layer and the magnetic layer is increased due to a slight 
attracting force between the different polarities, so that the durability 
is improved. Therefore, the above combination of the binder resins is 
preferred. 
As the magnetic powder, one containing alumina is preferred to improve the 
durability sufficiently. In the case of Fe powder containing alumina, 
alumina is contained in an amount of 0.1 to 3.0% by weight in terms of 
aluminum based on the weight of iron, since when this amount is less than 
0.1% by weight, the durability is not improved, while when it is larger 
than 3.0% by weight, the electromagnetic conversion characteristics are 
deteriorate. In particular, the magnetic powder containing 0.5 to 2.0% by 
weight of aluminum based on the Fe weight is preferably used. 
Preferably, the particle size of the magnetic powder containing alumina has 
the ratio of a Dx value (X-ray particle size) of the iron powder to the 
particle size of the non-magnetic particles in the intermediate layer in 
the range between 0.2 and 1.0, more preferably between 0.4 and 1.0, in 
terms of a ratio of the Dx value to a diameter or a short axis size of the 
non-magnetic particles in the intermediate layer, since when this ratio is 
less than 0.2, the durability cannot be improved sufficiently, while when 
it is larger than 1.0, it is difficult to maintain the reproducing output. 
As an abrasive to be contained in the magnetic layer, alumina is preferably 
used, since it improves the durability sufficiently. In such case, a ratio 
of the particle size of the alumina particles to the particle size of the 
non-magnetic particles in the intermediate layer is from 3.0 to 12.0, more 
preferably from 3.3 to 10, in terms of a ratio of a diameter of alumina 
particles to the diameter or the short axis size of the non-magnetic 
particles in the intermediate layer, since when this ratio is less than 
3.0, the durability cannot be improved sufficiently, while when it is 
larger than 12.0, the magnetic head is flawed. 
An amount of alumina to be contained in the magnetic layer is preferably 
from 6.5 to 15.0%. by weight in terms of an amount of aluminum based on 
the weight of iron. When the amount of aluminum based on the iron is less 
than 6.5% by weight, the durability is not improved sufficiently, while 
when it is larger than 15.0% by weight, a content of non-magnetic 
components becomes too large so that the reproducing output decreases. 
Alumina may be added to the magnetic layer or contained in the magnetic 
powder as described above. In either case, the abrading function is 
facilitated by the presence of alumina so that the durability is 
increased. 
The magnetic recording medium of the present invention is produced by 
laminating the intermediate layer and the magnetic layer on the 
non-magnetic support. To achieve the sufficient recording capacity and 
also increase the electromagnetic conversion characteristics and the 
durability of the magnetic recording medium, preferably, a total thickness 
of the non-magnetic support, the intermediate layer and the magnetic layer 
is not larger than 13 .mu.m, a squareness ratio of the magnetic layer is 
at least 0.7, and a residual magnetic flux density is at least 3000 Gauss. 
The magnetic layer having the squareness ratio of at least 0.78 and the 
residual magnetic flux density of at least 3000 Gauss can be formed by 
coating the magnetic paint after coating the intermediate layer paint and 
before drying it and subjecting the magnetic paint to a magnetic field 
orientation treatment during the drying step of the magnetic paint. As the 
orientation of the magnetic powder particles is improved by the magnetic 
field orientation treatment, the magnetic layer becomes dense, so that the 
durability is increased. When the squareness ratio is 0.78 or larger and 
the residual magnetic flux density is 3000 Gauss or larger, the 
orientation of the magnetic powder particles is sufficiently good and the 
magnetic layer is dense, so that the durability is further increased. 
As a packing density of the magnetic powder increases, the magnetic layer 
becomes more dense and the durability becomes better. Therefore, it is 
preferable to increase the packing property of the magnetic powder in 
addition to its orientation. The packing property of the magnetic powder 
can be adjusted by subjecting the magnetic layer to a surface smoothing 
treatment by calendering after the formation of the magnetic layer. 
Conditions of the surface smoothing treatment depend on the composition of 
the magnetic layer. For example, the treatment may be carried out by 
running the magnetic recording medium between metal rolls which are heated 
to 80.degree. C. while applying a linear pressure of 150 kg/cm on the 
medium. 
In addition, in the case where the intermediate layer and the magnetic 
layer are formed on the non-magnetic support according to the present 
invention, the sufficient durability is achieved when the total thickness 
of the non-magnetic support, the intermediate layer and the magnetic layer 
is in the range between 7.5 and 20.0 .mu.m. When the total thickness of 
the non-magnetic support, the intermediate layer and the magnetic layer is 
less than 13 .mu.m, the sufficient durability is achieved as well as the 
sufficient recording capacity is obtained. 
Accordingly, when the total thickness of the non-magnetic support, the 
intermediate layer and the magnetic layer is less than 13 .mu.m, the 
squareness ratio of the magnetic layer is at least 0.78 and the residual 
magnetic flux density is at least 3000 Gauss, the electromagnetic 
conversion characteristics and the durability are sufficiently improved as 
well as the sufficient recording capacity is obtained. 
The intermediate layer paint and the magnetic layer paint can be coated by 
a per se conventional method such as a method disclosed in Japanese Patent 
Publication No. 59491/1994. For example, to coat the intermediate layer 
paint, roll coating, gravure coating, extrusion die coating, curtain 
coating and the like can be used. To coat the magnetic paint, extrusion 
die coating can be used. 
PREFERRED EMBODIMENTS 
EXAMPLES. 
The examples of the present invention will be explained. 
Example 1 
______________________________________ 
Parts by 
weight 
______________________________________ 
Titanium oxide (TTO 55 manufactured by 
60 
Ishihara Industries, particle size of 35 nm) 
Carbon black (MONARCH 800 manufactured by 
15 
Cabot, particle size of 17 nm) 
MR 110 (SO.sub.3 K-containing polyvinyl chloride 
12.5 
resin manufactured by Nippon Zeon) 
UR 8300 (SO.sub.3 Na-containing polyurethane 
7.5 
resin manufactured by Toyobo) 
n-Butyl stearate 1.5 
Cyclohexanone 110 
Toluene 110 
______________________________________ 
The above components were mixed and dispersed in a sand grinding mill. To 
the mixture, 5 parts by weight of a polyisocyanate compound (COLONATE L 
manufactured by Nippon Polyurethane Industries) was added and mixed, 
followed by filtration through a filter having an average pore size of 0.5 
.mu.m to obtain an intermediate layer paint. 
Separately, a composition of: 
______________________________________ 
Parts by 
weight 
______________________________________ 
Fe-magnetic powder (Dx 15 nm, 
80 
Al/Fe = 0.5 wt. %) 
MR 110 (SO.sub.3 K-containing polyvinyl chloride) 
8 
resin manufactured by Nippon Zeon) 
OR 8300 (SO.sub.3 Na-containing polyurethane 
8 
resin manufactured by Toyobo) 
Alumina (particle size of 200 nm) 
8 
Carbon black 4 
n-Butyl stearate 1.2 
Myristic acid 1.2 
Cyclohexanone 72.5 
Methyl ethyl ketone 72.5 
Toluene 72.5 
______________________________________ 
was mixed and dispersed in a sand grinding mill. To the mixture, 4 parts by 
weight of a polyisocyanate compound (COLONATE L manufactured by Nippon 
Polyurethane Industries) was added and mixed, followed by filtration 
through a filter having an average pore size of 0.5 .mu.m to obtain a 
magnetic paint. 
The prepared intermediate layer paint was coated on a polyester film having 
a thickness of 8 .mu.m with a gravure coater to a thickness of 2.0 .mu.m 
after drying. While the intermediate layer paint was still in the wet 
state, the prepared magnetic paint was coated with a die coater to a 
thickness of 0.5 .mu.m after drying, subjected to the magnetic field 
orientation treatment and dried. 
Then, the magnetic layer was subjected to the surface smoothing treatment 
and kept at 60.degree. C. for 24 hours. Thereafter, on a reverse surface 
of the film, a back coating layer was formed, and the film was cut to a 
width of 8 mm to obtain a 8 mm video tape. 
A particle size ratio of the titanium oxide particles to the carbon black 
in the intermediate layer of the produced video tape was 2.06. A particle 
size ratio of the alumina particles in the magnetic layer to the titanium 
oxide particles in the intermediate layer was 5.71. 
Example 2 
In the same manner as in Example 1 except that .alpha.-hematite (NANOTITE 
60 manufactured by Showa Denko, particle size of 60 nm) was used in the 
same amount in place of titanium oxide in the composition of the 
intermediate layer paint, a 8 mm video tape was produced. 
A particle size ratio of the .alpha.-hematite particles to the carbon black 
in the intermediate layer of the produced video tape was 3.53. A particle 
size ratio of the alumina particles in the magnetic layer to the 
.alpha.-hematite particles in the intermediate layer was 3.33. 
Example 3 
In the same manner as in Example 1 except that .alpha.-hematite (DPN-245 
manufactured by Toda Industries, particle size of 185 nm.times.31 nm) was 
used in the same amount in place of titanium oxide in the composition of 
the intermediate layer paint, a 8 mm video tape was produced. 
A particle size ratio of the .alpha.-hematite particles to the carbon black 
in the intermediate layer of the produced video tape was 1.82. A particle 
size ratio of the alumina particles in the magnetic layer to the 
.alpha.-hematite particles in the intermediate layer was 6.45. 
Example 4 
In the same manner as in Example 1 except that REGAL 250 (manufactured by 
Cabot, particle size of 35 nm) was used in the same amount in place of 
MONARCH 800 as the carbon black in the composition of the intermediate 
layer paint, a 8 mm video tape was produced. 
A particle size ratio of the titanium oxide particles to the carbon black 
in the intermediate layer of the produced video tape was 1.00. A particle 
size ratio of the alumina particles in the magnetic layer to the titanium 
oxide particles in the intermediate layer was 5.71. 
Example 5 
In the same manner as in Example 2 except that REGAL 250 (manufactured by 
Cabot, particle size of 35 nm) was used in the same amount in place of 
MONARCH 800 as the carbon black in the composition of the intermediate 
layer paint, a 8 mm video tape was produced. 
A particle size ratio of the .alpha.-hematite particles to the carbon black 
in the intermediate layer of the produced video tape was 1.71. A particle 
size ratio of the alumina particles in the magnetic layer to the short 
axis of .alpha.-hematite particles in the intermediate layer was 3.33. 
Example 6 
In the same manner as in Example 1 except that the thickness of the 
magnetic layer was changed from 0.5 .mu.m to 0.2 .mu.m, a 8 mm video tape 
was produced. 
Example 7 
In the same manner as in Example 1 except that the thickness of the 
magnetic layer was changed from 0.5 .mu.m to 1.0 .mu.m, a 8 mm video tape 
was produced. 
Example 8 
In the same manner as in Example 1 except that ESLEK E C130 
(N(CH.sub.3).sub.3 Cl-containing polyvinyl chloride resin manufactured by 
Sekisui Chemical Industries) was used in the same amount in place of MR 
110 and a N(CH.sub.3).sub.3 Cl-containing polyurethane resin (backbone: 
butylene-adipate/4,4-diphenylmethane diisocyanate, the molecular weight of 
23,000, the content of functional group of 0.2 meq/g) in the composition 
of the intermediate layer paint, a 8 mm video tape was produced. 
Example 9 
In the same manner as in Example 1 except that the same amount of Fe 
magnetic powder having a Dx value of 15 nm and containing alumina in a 
varying content was used in the composition of the magnetic paint, a 8 mm 
video tape was produced. 
Example 10 
In the same manner as in Example 1 except that the same amount of Fe 
magnetic powder having a different Dx value and the content of alumina was 
same in the composition of the magnetic paint, a 8 mm video tape was 
produced. 
FIG. 1 shows a relationship between an aluminum content (Al/Fe) in the iron 
magnetic powder in the magnetic layer and the durability and 
electromagnetic conversion characteristics of the video tape produced in 
Example 9, and FIG. 2 shows a relationship between a ratio of the Dx value 
of the iron magnetic powder to the particle size of titanium oxide in the 
intermediate layer and the durability and electromagnetic conversion 
characteristics of the video tape produced in Example 10. 
Example 11 
In the same manner as in Example 1 except that alumina having a particle 
size of 400 nm in the same amount was used in place of alumina having a 
particle size of 200 nm in the magnetic paint, a 8 mm video tape was 
produced. 
A particle size ratio of the alumina particles in the magnetic layer to the 
titanium oxide particles in the intermediate layer of the produced video 
tape was 11.4. 
Example 12 
In the same manner as in Example 1 except that alumina having a particle 
size of 170 nm in the same amount was used in place of alumina having a 
particle size of 200 nm in the magnetic paint, a 8 mm video tape was 
produced. 
A particle size ratio of the alumina particles in the magnetic layer to the 
titanium oxide particles in the intermediate layer of the produced video 
tape was 4.86. 
Comparative Example 1 
In the same manner as in Example 1 except that .alpha.-hematite (TF-100 
manufactured by Toda Industries, particle size of 100 nm) was used in the 
same amount in place of titanium oxide in the composition of the 
intermediate layer paint, a 8 mm video tape was produced. 
A particle size ratio of the .alpha.-hematite particles to the carbon black 
in the intermediate layer of the produced video tape was 5.88. A particle 
size ratio of the alumina particles in the magnetic layer to the 
.alpha.-hematite particles in the intermediate layer was 2.00. 
Comparative Example 2 
In the same manner as in Comparative Example 1 except that alumina having a 
particle size of 170 nm in the same amount was used in place of alumina 
having a particle size of 200 nm and CEBACALB MT-Cl (manufactured by 
Columbian Chemistry, particle size of 370 nm) in the same amount was used 
in place of MONARCH 800 in the magnetic paint, a 8 mm video tape was 
produced. 
A particle size ratio of the .alpha.-hematite particles to the carbon black 
in the intermediate layer of the produced video tape was 0.27. A particle 
size ratio of the alumina particles in the magnetic layer to the 
.alpha.-hematite particles in the intermediate layer was 0.59. 
Comparative Example 3 
In the same manner as in Example 2 except that alumina having a particle 
size of 1500 nm in the same amount was used in place of alumina having a 
particle size of 200 nm in the magnetic paint, a 8 mm video tape was 
produced. 
A particle size ratio of the .alpha.-hematite particles to the carbon black 
in the intermediate layer of the produced video tape was 3.53. A particle 
size ratio of the alumina particles in the magnetic layer to the 
.alpha.-hematite particles in the intermediate layer was 25.0. 
In the same manner as in Example 1 except that the thickness of the 
magnetic layer was changed from 0.5 .mu.m to 0.18 .mu.m, a 8 mm video tape 
was produced. 
Comparative Example 5 
In the same manner as in Example 1 except that the thickness of the 
magnetic layer was changed from 0.5 .mu.m to 1.1 .mu.m, a 8 mm video tape 
was produced. 
Comparative Example 6 
In the same manner as in Example 1 except that ESLEK E C130 
(N(CH.sub.3).sub.3 Cl-containing polyvinyl chloride resin manufactured by 
Sekisui Chemical Industries) was used in the same amount in place of MR 
110 in the composition of the intermediate layer paint, a 8 mm video tape 
was produced. 
With each of the video tapes produced in Examples and Comparative Examples, 
the surface roughness was measured by the frequency analysis using the 
non-contact type surface roughness meter, and the electromagnetic 
conversion characteristics and durability were measured as follows: 
Electromagnetic conversion characteristics 
Using a 8 mm VCR manufactured by SONY, 100% white signal is recorded, and 
the output is measured on an oscilloscope when the signal is reproduced. 
Durability 
Using a reciprocal sliding type tester with a bearing steel ball having a 
diameter of 6.35 as a slider, a surface of the 8 mm video tape is rubbed 
by the steel ball at a sliding speed of 30 mm/sec. under a load of 20 g, 
and the number of reciprocal sliding times till a coefficient of friction 
suddenly increases. 
The results are shown in the following Table. 
Frequency analysis with non-contact type surface roughness meter 
An object head PX-4 is attached to a non-contact type surface roughness 
meter TOPO-3D (manufactured by WYCO), and a surface roughness of each 
video tape is measured. Using the same apparatus, a frequency analysis is 
carried out under the following conditions 
Hanning Window: OFF 
Sample Interval: 3 
Reference Level: 1.000 nm 
Maximum Y Value: Automatic 
Integration Limit: 0.00/mm to 1000.00/mm 
Integration Area: Absolute FFT. 
Then, a difference between an intensity at the frequency of 102 mm.sup.-1 
and that at the frequency of 205 mm.sup.-1 is calculated. 
TABLE 
______________________________________ 
Measured value 
Electromagnetic- 
Exam- in frequency characteristics 
Durability 
ple No. 
analysis (dB) 
(dB) (times) 
______________________________________ 
1 10 +4.5 &gt;2000 
2 12 +3.0 &gt;2000 
3 13 +3.5 &gt;2000 
4 9 +4.5 &gt;2000 
5 10 +4.5 &gt;2000 
6 10 +4.5 &gt;2000 
7 10 +4.5 &gt;2000 
8 10 +5.5 &gt;2000 
9 10 See FIG. 1 
10 10 See FIG. 2 
11 15 +3.0 &gt;2000 
12 9 +4.5 &gt;2000 
C. 1 20 0 1000 
C. 2 18 0 500 
C. 3 12 -1.0 500 
C. 4 7 +5.0 500 
C. 5 18 0 &gt;2000 
C. 6 20 -1.0 50 
______________________________________ 
As seen from the above Table, the 8 mm video tapes produced according to 
the present invention (Examples 1 through 12) had better electromagnetic 
conversion characteristics and durability than those produced in 
Comparative Examples 1 through 6. As seen from FIGS. 1 and 2, the 8 mm 
video tapes produced according to the present invention had good 
electromagnetic conversion characteristics and durability. From these 
results, it is understood that the magnetic recording medium of the 
present invention has improved electromagnetic conversion characteristics 
and durability. 
The present invention being thus described, it will be obvious that the 
same may be varied in many ways. Such variations are not to be regarded as 
a departure from the spirit and scope of the invention, and all such 
modifications as would be obvious to one skilled in the art are intended 
to be included within the scope of the following claims.