Magnetic recording medium

A magentic recording medium in which a thin ferromagnetic film is formed over a strip or sheet of plastic film whose surface is formed with grain-, rumple- or worm-like nodules into which may be dispersed finely divided particles of the diameter ranging from 0.01 to 0.2 .mu.m. In the case of a thin ferromagnetic metal film type recording medium, steep protrusions of the height ranging from 100 to 2,000 .ANG. are distributed over the surface at the average density of 10.sup.4 to 10.sup.6 per mm.sup.2.

BACKGROUND OF THE INVENTION 
The present invention relates to a magnetic recording medium especially 
adapted for use with a rotary head type video tape recorder. 
In the fabrication of thin ferromagnetic film type recording media, an 
vacuum evaporation, sputtering or ion-plating process is employed so that 
iron, cobalt, nickel or their alloy may be deposited over a base of a 
high-weight compound film such as polyester or polyimide film. Such thin 
ferromagnetic film type magnetic recording media are by far advantageous 
over the prior art coating type magnetic recording media consisting of a 
base and a thin magnetic layer in that the recording density can be 
remarkably increased. However, in order to attain a higher degree of 
recording density, the surfaces of a magnetic recording medium must be 
made as flat as possible so that "spacing loss" may be reduced to a 
minimum. But if the surface is extremely smooth or flattened, it makes 
intimate contact with a magnetic head so that the smooth transportation of 
the magnetic tape past the head is hindered. With the rotary head type 
video tape recorder system which has been increasingly popular in the 
market, the use of thin ferromagnetic film type magnetic recording media 
is almost imperative in order to improve the recording denisty. Such 
magnetic recording media or tapes must satisfy various demands in 
practice. For instance, they must make a suitable contact with rotary 
heads so as to avoid excessive wear or abrasion of the heads, thereby 
preventing the clogging of gaps thereof. Furthermore, even when they are 
in contact with a rotary head cylinder, tape guide posts, a stationary 
audio head and so on, they must move past them smoothly. In other words, 
they must have a low coefficient of friction and exhibit a high degree of 
resistance against wear. The magnetic layer of a thin ferromagnetic film 
type magnetic recording tape is extremely thin and is of the order of from 
0.1 to 0.5 .mu.m so that the above-described properties or characteristics 
are all dependent upon the surfaces of the plastic film which is the base 
of the magnetic recording medium. Therefore, various proposals have been 
made in order to provide satisfactory surface textures of the bases. For 
instance, such proposals were disclosed in Japanese Laid Open Patent 
Application Nos. 116115/1978, 128683/1978, 94574/1979, 10455/1981 and 
16937/1981. These proposals are common in that they impart to the surface 
relatively uniform and minute roughness. For instance, the surface is 
formed with rumple- or worm-like nodules or with grain-like nodules. The 
common object of such proposals is, therefore, to maintain suitable 
contact with the magnetic heads, thereby improving the transportability; 
that is, the smooth travel of the magnetic tape past the heads and the 
like. In the case of the rotary head assembly of a video tape recorder, 
the width of contact between a magnetic tape and a head is extremely 
narrow and is of the order of less than hundreds of microns. Furthermore, 
the relative velocity between the magnetic tape and the head is extremely 
high and is of the order of a few meters per second, whereas in the tape 
transport system, the magnetic tape is transported at a relatively slow 
velocity of the order of a few centimeters per second and makes contact 
with various component parts in the transport system with relatively large 
areas. Therefore, the surface texture design becomes a compromise between 
the ability of tape to move smoothly past the rotary head assembly and its 
associated parts and the ability of being smoothly transported through the 
tape transport system. In addition, the surface texture must be so 
designed as to have some abrasive action on the rotary heads in order to 
prevent clogging of the gaps thereof. 
SUMMARY OF THE INVENTION 
In view of the above, the primary object of the present invention is to 
provide a thin ferromagnetic film type magnetic recording medium which 
exhibits a higher degree of resistance against wear when in contact with 
rotary magnetic heads, ensures a higher degree of transportability, 
prevents clogging of the gaps of the rotary magnetic heads and ensures 
steady recording and reproducing of signals. 
To the above and other ends, the present invention provides a magnetic 
recording medium in which a thin ferromagnetic film is formed over the 
surface of a plastic film base which is formed with grain-, rumple- or 
worm-like nodules into which are locally dispersed finely divided 
particles of the diameter of 0.01 to 0.2 .mu.m. Furthermore, the present 
invention provides a thin ferromagnetic metal thin film type magnetic 
recording medium with the surfaces from which are extended steep 
protrusions of the height ranging from 100 to 2,000 .ANG. at an average 
density of 10.sup.4 to 10.sup.6 per mm.sup.2. As described above, since 
the surface of the magnetic recording medium is distributed with steep 
protrusions or formed with grain-, rumple- or worm-like nodules into which 
are locally dispersed finely divided particles, the magnetic recording 
medium in accordance with the present invention can simultaneously satisfy 
not only the requirement for smooth transportability but also capability 
of preventing the clogging of the gaps of the rotary magnetic heads.

EXAMPLE 1 
An aqueous emulsion containing the following emulsions was prepared: 
(A) Epoxy polydimethylsiloxene emulsion, 
(B) Magnesium chloride emulsion, and 
(C) Methylcellulose 
This aqueous emulsion was mixed with the following colloidal solutions: 
(A) Colloidal solution of calcium carbonate (the reaction product between 
milk of lime and CO.sub.2 at high temperature), 
(B) Colloidal solution of titanium oxide (the product of hydrolysis of 
alkoxide), and 
(C) Colloidal solution of iron oxide hydrate (obtained by adding alkali to 
a ferrous salt solution). 
This aqueous emulsion was applied to both surfaces of strips of uniaxially 
elongated polyethyleneterphthalate film and dried. Thereafter, the strips 
of film were further elongated transversely or in cross direction and 
thermally cured, whereby polyester film 12 .mu.m in thickness was 
obtained. Both surfaces of this film were formed with worm-like nodules 
from which were locally extended or protruded finely divided particles. 
These film strips were spliced and rolled. The roll was then placed in an 
oblique vacuum evaporation equipment which contained a small amount of 
oxygen and a Co--Ni alloy containing 20% by weight of Ni was obliquely 
deposited over one major surface of the film, thereby forming a thin 
ferromagnetic film 0.1 .mu.m in thickness. Thereafter, the other major 
surface of the film strip was coated with thin epoxy resin lubricant film 
containing some graphite. Next, the film strip was slit into nine tapes of 
a predetermined width, each having different magnetic surface textures. 
The specifications of such nine samples are listed in TABLE below. 
TABLE 2 
__________________________________________________________________________ 
Surface Textures resulting from 
the treatment of aqueous emulsion 
worm-like 
worm-like particle protru- 
modules, 
nodules, 
particle 
sion density, the 
still-picture- 
degradation in 
Sample 
height in 
spacing 
size in 
number of pro- 
nuclei of 
display life- 
picture quality 
No. .ANG. in .mu.m 
.mu.m 
trusions per mm.sup.2 
particles 
time in minute 
at high humidity 
__________________________________________________________________________ 
1 50 0.2 0.005 
10,000 calcium 
150 observed 
carbonate 
2 " " 0.01 " calcium 
190 not 
carbonate observed 
3 120 1.0 0.05 1,000 calcium 
210 not 
carbonate observed 
4 " " " 5,000 calcium 
220 not 
carbonate observed 
5 220 1.5 0.1 1,000 calcium 
" not 
carbonate observed 
6 " " 0.05 10,000 titanium 
250 not 
oxide observed 
7 " " 0.1 2,000 titanium 
240 not 
oxide observed 
8 150 1.0 " 500 iron oxide 
200 not 
hydrate observed 
9 " " 0.3 " iron oxide 
210 noise is high 
hydrate from the start 
__________________________________________________________________________ 
The sample tapes were housed in cassettes and were subjected to the image 
quality tests as well as the still-picture-display life expectancy tests 
with a home video tape recorder at 25.degree. C. and at 65% R.H. (relative 
humidity) and at 25.degree. C. and 85% R.H. One of the objects of the 
tests was to investigate the degradation in image or picture quality due 
to jitter or the like at relatively high humidity. The test results are 
also listed in the above TABLE. It should be noted that Sample 1 is not of 
the present invention and was tested for the sake of comparison with other 
samples of the present invention. 
From TABLE it is apparent that the magnetic recording media in accordance 
with the present invention have a long still-picture-display lifetime and 
are free from the degradation in picture quality at relatively high 
humidity. 
EXAMPLE 2 
Polyester film was used which was entirely free from minute particles which 
were the residue of polymerization catalysts. When this film was being 
elongated, a modified silicone emulsion containing a thickener was applied 
to both major surfaces of the film and then cured. As a result, the 
surfaces were corrugated or wrinkled and the spacing between the ridges 
was about 1 .mu.m. The surface roughness value was 200 .ANG.. By varying 
the temperature (between 180.degree. and 200.degree. C.) hot air and the 
continuous blowing time interval, minute crystals of polyester oligomer 
were precipitated over the surfaces at various densities. Thereafter, by 
employing an oblique vacuum evaporation process, a thin Co--Ni(Ni=20% by 
weight) film of the thickness of 1,000 .ANG. was deposited over one major 
surface of the film in the presence of a small amount of oxygen. Thus, 
various types of magnetic tapes were prepared. The precipitation of minute 
crystals of oligomer formed steep protrusions whose average height was 
between 300 and 500 .ANG.. These tapes were left at 60.degree. C. and 90% 
R.H. for one month so that rust which would cause the clogging of the gap 
of a magnetic head was produced. At 20.degree. C. and 60% R.H., video 
recording and reproduction were repetitively made in order to determine 
the number of repetition at which the clogging of the gap of the magnetic 
head causes the degradation in picture quality. The results are shown in 
FIG. 7. 
EXAMPLE 3 
Polyester film was used which was almost free from particles which were the 
residue of the polymerization catalysts. A solution containing 1% by 
weight of the stoichiometric mixture of phenol and ethane tetrachloride 
was prepared and applied to the both the surface of the film and dried by 
blowing hot air. The surfaces were imparted with rumple-like ridge nodules 
(spaced apart from each other by 2.5 .mu.m) with the surface roughness 
value of 300 .ANG.. Thereafter, following the procedure of EXAMPLE 2, a 
thin Co--Ni film was formed and an aqueous emulsion of epoxy modified 
silicone containing colloids of titanium oxide (the product of hydrolysis 
of titanium alkoxide) which varied in concentration was applied to the 
thin ferromagnetic film and baked. As a result, several magnetic tapes 
were prepared whose magnetic coating was formed with steep protrusions 
whose nuclei were titanium colloidal particles. The average height of the 
steep protrusions was 400 .ANG.. Following the procedure of EXAMPLE 2, the 
relationship between the protrusion density and the reproduction circuit 
was obtained and the results are shown in FIG. 8. 
In summary, according to the present invention, the magnetic recording 
media can be advantageously used as video recording magnetic tapes which 
can satisfactorily withstand wear and abrasion by the magnetic head and 
ensure a higher degree of transportability. In addition, they can 
effectively avoid the clogging of the gap of the magnetic head.