Magnetic recording medium

A magnetic recording medium comprising a nonmagnetic support and a magnetic recording layer provided on said support, said magnetic recording layer comprising a ferromagnetic metal powder having a specific surface area of not less than 45 m.sup.2 /g and an inorganic powder having a Mohs' scale of hardness of not less than 5 which are dispersed in a binder, wherein PA0 5-40 particles of said inorganic powder are arranged in the magnetic recording layer per (10 .mu.m).sup.2 of the surface area of said recording layer in such manner that a portion of each inorganic particle is exposed on of the surface of the magnetic recording layer; and PA0 the average content of said inorganic powder in a portion of said recording layer within depth of 0.1 .mu.m from the surface thereof is higher than that in other portion of the magnetic recording layer.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a magnetic recording medium. More 
particularly, the invention relates to an improvement of a magnetic 
recording medium comprising a nonmagnetic support and a magnetic recording 
layer. 
2. Description of Prior Arts 
A magnetic recording medium (hereinafter referred to sometimes as a 
magnetic tape) such as an audio tape, a video tape, or a recording medium 
employed in a computer system, basically comprises a nonmagnetic support 
and a magnetic recording layer provided on the support. The magnetic 
recording layer comprises a ferromagnetic metal oxide powder such as a 
needle crystalline powder of .gamma.-Fe.sub.2 O.sub.3 or Co-containing 
.gamma.-Fe.sub.2 O.sub.3. Recently, a demand for a higher density 
recording system has increased, and hence a magnetic tape using a 
ferromagnetic metal powder has been employed in place of the conventional 
oxide-type ferromagnetic powder. Particularly, in an 8 mm-width type video 
system which has been recently employed in practice, a tape width of a 
video tape used therefor is narrower as compared with conventional VHS 
type or .beta. type video tapes, so that much higher density recording is 
desired for the tape. The 8 mm type video tape generally employs the 
ferromagnetic metal powder. 
The ferromagnetic metal powder is high in a coercive force (Hc) and a 
residual flux density (Br). For this reason, the ferromagnetic metal 
powder is suitable for the high density recording system. However, the 
ferromagnetic metal powder inherently has a low hardness, and therefore a 
magnetic recording medium employing said powder is poor in the running 
endurance (or running property). That is, in the magnetic recording medium 
using the ferromagnetic metal powder, a magnetic recording layer is liable 
to be damaged on its surface, or the ferromagnetic metal powder is apt to 
drop off from the magnetic recording layer. Particularly in the case of 
the video tape, the magnetic recording layer shows only a short still life 
in the still mode in which a still video image is continuously reproduced. 
It is known that an abrasive (i.e., hard particles) such as corundum, 
silicon carbide or chromium oxide can be incorporated into the magnetic 
recording layer to improve the running endurance of the magnetic recording 
medium using the ferromagnetic metal powder. However, for obtaining 
prominent effect of incorporation of the abrasive, the abrasive is 
required to be contained in the magnetic recording layer in a large 
amount. The increase of an amount of the abrasive contained in the 
magnetic recording layer eventually brings about decrease of the amount of 
the ferromagnetic metal powder incorporatable in the same layer. In other 
words, in the art of a magnetic recording medium conventionally employed, 
a magnetic recording medium is unavoidably decreased in the 
electromagnetic conversion characteristics in exchange for the improvement 
of the running endurance. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a novel magnetic 
recording medium. 
It is another object of the invention to provide a magnetic recording 
medium which is excellent in both of the electromagnetic conversion 
characteristics and the running endurance. 
There is provided by the present invention a magnetic recording medium 
comprising a nonmagnetic support and a magnetic recording layer provided 
on the support, said magnetic recording layer comprising a ferromagnetic 
metal powder having a specific surface area of not less than 45 m.sup.2 /g 
and an inorganic powder having a Mohs' scale of hardness of not less than 
5 which are dispersed in a binder, 
wherein 
5-40 particles of said inorganic powder are arranged in the magnetic 
recording layer per (10 .mu.m).sup.2 of the surface area of said recording 
layer in such manner that a portion of each inorganic particle is exposed 
on of the surface of the magnetic recording layer; and 
the average content of said inorganic powder in a portion of said recording 
layer within depth of 0.1 .mu.m from the surface thereof is higher than 
that in other portion of the magnetic recording layer. 
The present invention is based on the finding that only a portion of the 
abrasive contained in the magnetic recording layer, which resides on the 
surface of the magnetic recording layer, contributes to the improvement of 
the running endurance, and other portion thereof does not participate in 
the improvement of the running endurance. The magnetic recording medium of 
the invention is mainly characterized in that the abrasive is unevenly 
distributed in the magnetic recording layer, particularly distributed on 
the surface side of the magnetic recording layer, under the specific 
conditions. 
The magnetic recording medium of the invention shows satisfactory 
electromagnetic conversion characteristics as well as high running 
endurance. That is, the present medium shows higher electromagnetic 
conversion characteristics as compared with a conventional medium when the 
running endurance of the recording medium of the invention is the same 
level as that of the conventional one, and shows higher running endurance 
as compared with a conventional medium when the electromagnetic conversion 
characteristic is the same level as that of the conventional one. Further, 
according to the present invention, both of the electromagnetic conversion 
characteristics and running endurance are well balanced to obtain a 
magnetic recording medium being more satisfactory in the both properties 
as compared with the conventional magnetic recording medium. 
DETAILED DESCRIPTION OF THE INVENTION 
A magnetic recording medium of the invention comprises a nonmagnetic 
support and a magnetic recording layer provided on the support. The 
magnetic recording layer comprises a ferromagnetic metal powder and an 
inorganic powder dispersed in a binder. 
As a material of the nonmagnetic support, there can be employed those 
conventionally employed. Examples of the nonmagnetic support material 
include synthetic resin films such as films of polyethylene terephthalate, 
polyethylene, polypropylene, polycarbonate, polyethylene naphthalate, 
polyamide, polyamideimide, polyimide, polysulfone and polyether sulfone; 
and metallic foils such as aluminum foil and stainless steel foil. The 
thickness of the support generally is in the range of 3 to 50 .mu.m, 
preferably in the range of 5 to 30 .mu.m. 
The nonmagnetic support may have a back layer (or backing layer) on the 
opposite side of the side where a magnetic recording layer is to be 
coated. 
The magnetic recording medium of the invention has the above-described 
nonmagnetic support coated thereupon with a magnetic recording layer 
comprising a ferromagnetic metal powder dispersed in a binder, as 
described hereinbefore. 
The ferromagnetic metal powder employable in the invention contains iron, 
cobalt or nickel, and has a specific surface area (S-BET) of not less than 
45 m.sup.2 /g, preferably not less than 50 m.sup.2 /g. When the specific 
surface area of the ferromagnetic metal powder is less than 45 m.sup.2 /g, 
satisfactory electromagnetic conversion characteristics cannot be obtained 
in the resulting magnetic recording medium. 
As a typical ferromagnetic metal powder, there can be mentioned a 
ferromagnetic alloy powder containing a metal component of at least 75 
wt.% in which at least 80 wt.% of the metal component comprises at least 
one ferromagnetic metal or metal alloy (e.g., Fe, Co, Ni, Fe-Co, Fe-Ni, 
Co-Ni, Fe-Zn-Ni or Co-Ni-Fe) and the remaining metal component, if 
present, comprises other atom(s) (e.g., Al, Si, S, Sc, Ti, V, Cr, Mn, Cu, 
Zn, Y, Mo, Rh, Pd, Ag, Sn, Sb, Te, Ba, Ta, W, Re, Au, Hg, Pb, Bi, La, Ce, 
Pr, Nd, B, or P). The ferromagnetic metal component may contain a small 
amount of water, hydroxide, or oxide. Methods of the preparation of these 
ferromagnetic metal powders are already known, and the ferromagnetic metal 
powder employed in the invention can be prepared by the known methods. 
There is no specific limitation on the shape of the ferromagnetic metal 
powder employable in the invention, but normally used is a ferromagnetic 
metal powder in a needle shape, grain shape, dice shape, rice shape or 
plate shape. 
The binder material employable for the formation of the magnetic recording 
layer in the invention can be selected from those conventionally employed. 
Examples of the binder material include cellulose derivatives (e.g., 
nitrocellulose and cellulose acetate), vinyl chloride/vinyl acetate 
copolymer resins, (e.g., vinyl chloride/vinyl acetate copolymers, vinyl 
chloride/vinyl acetate/vinyl alcohol copolymers, and vinyl chloride/vinyl 
acetate/maleic anhydride copolymers), vinylidene chloride resins (e.g., 
vinylidene chloride/vinyl chloride copolymers and vinylidene 
chloride/acrylonitrile copolymers), polyester resins (e.g., alkyl resin 
and linear polyester), acrylic resins (e.g., acrylic acid/acrylonitrile 
copolymer and methyl acrylate/acrylonitrile copolymer), polyvinyl acetal 
resin, polyvinyl resin, phenoxy resin, epoxy resin, 
butadiene/acrylonitrile copolymer resin, polyurethane resin and urethane 
epoxy resin. 
It is preferred to employ as the binder a combination of a resin having a 
high hardness such as a vinyl chloride/vinyl acetate/maleic anhydride 
copolymer and a resin having a low hardness such as a polyurethane resin, 
which is further added with a curing agent such as a polyisocyanate 
compound. The employment of such binder composition (i.e., cured 
composition) is very advantageous for enhancing the running endurance 
because it has a high hardness. 
The binder is contained in the magnetic recording layer in the amount of 
10-100 parts by weight, preferably 20-40 parts by weight, based on 100 
parts by weight of the ferromagnetic metal powder. 
The magnetic recording layer of the medium of the invention further 
contains an inorganic powder (i.e., abrasive) having a Mohs' scale of 
hardness of not less than 5. 
There is no specific limitation on the inorganic powder provided that the 
inorganic powder has a Mohs' scale of hardness of not less than 5. 
Examples of the inorganic powder employable in the invention include 
.alpha.-Al.sub.2 O.sub.3 (a Mohs' scale of hardness: 9), TiO (the same: 
6), TiO.sub.2 (the same: 6.5), SiO.sub.2 (the same: 7), SnO.sub.2 (the 
same: 6.5), Cr.sub.2 O.sub.3 (the same: 9), and .alpha.-Fe.sub.2 O.sub.3 
(the same: 5.5). 
The inorganic powder can be employed singly or in combination. 
The average content of the inorganic powder in the whole magnetic recording 
layer is not more than 10 parts by weight per 100 parts by weight of the 
ferromagnetic metal powder. 
In the magnetic recording medium of the present invention, it is necessary 
that 5-40 particles of the above-mentioned inorganic powder are arranged 
in the magnetic recording layer per (10 .mu.m).sup.2 of the surface area 
of the magnetic recording layer in such a manner that a portion of each 
inorganic particle is exposed on the surface of the magnetic recording 
layer. The inorganic particle(s) arragned under such condition is 
sometimes referred to simply as "exposed particle(s)" hereinafter. The 
number of the exposed particles per (10 .mu.m).sup.2 of the surface area 
of the magnetic recording layer is preferably in the range of 10-30. When 
the number of the exposed particles per (10 .mu.m).sup.2 of the surface 
area of the magnetic recording layer is less than 5, the running endurance 
of the resulting medium is unsatisfactory. When the number of the exposed 
particles per (10 .mu.m).sup.2 of the surface area of the magnetic 
recording layer is more than 40, the electromagnetic conversion 
characteristics are hardly improved, for instance, the reproduction output 
of a recorded signal having short wavelength decreases. 
The above-described condition, namely, a condition in which the inorganic 
powder is fixed in such a manner that a portion of each particle is 
exposed outside of the surface of the magnetic recording layer, means that 
the inorganic powder is fixedly arranged in the magnetic recording layer 
not so as to readily drop off from the layer in such a manner that a 
portion of each particle of the inorganic powder is exposed outside of the 
surface of the recording layer to serve as an abrasive. Accordingly, it 
should be understood that inorganic particles under the following 
conditions cannot be employed in the invention. For instance, an inorganic 
particle is attached to the surface of the magnetic recording layer and 
almost wholly exposed outside of the layer so as to easily drop off from 
the layer by applying a slight external force thereonto. Otherwise, a very 
small portion of an inorganic particle is exposed on of the recording 
layer, so that the inorganic particle hardly functions as an abrasive. 
In the recording medium of the invention, it is further required that the 
average content of the inorganic powder in a portion of said recording 
layer within depth of 0.1 .mu.m from the surface thereof is higher than 
that in other portion of the magnetic recording layer. 
The electromagnetic conversion characteristics can be highly improved by 
decreasing the average content of the inorganic powder in the deeper 
portion of the magnetic recording layer as described above. The reason is 
as follows. An inorganic powder contained in such deeper portion of the 
magnetic recording layer hardly serves as abrasive in the magnetic 
recording medium. According to the present invention, however, a portion 
of the inorganic powder not serving as abrasive can be replaced with a 
ferromagnetic metal powder, whereby the average content of the 
ferromagnetic metal powder in the whole magnetic recording layer can be 
increased. 
In the present invention, preferred is a magnetic recording medium 
essentially not containing the inorganic powder in a portion of 1/2 of the 
magnetic recording layer in thickness from the surface of the support and 
containing almost all of the inorganic powder in the upper portion of the 
recording layer. Such favorable magnetic recording medium can be prepared, 
for instance, by a multiple-coating method (superposition coating method) 
which will be described hereinafter. The magnetic recording medium having 
the above-described structure is excellent in both of the electromagnetic 
conversion characteristics and the running endurance. 
The exposed particles of the inorganic powder can be observed on the 
surface of the magnetic recording layer by the use of a scanning electron 
microscope, etc. at approx. 100,000 magnifications. 
The mean particle diameter of the inorganic particle generally ranges from 
0.1 to 1 .mu.m. 
The magnetic recording layer may further contain other additives 
conventionally employed in the preparation of a magnetic recording layer 
such as a lubricant, an antistatic agent, a filler, and a dispersing 
agent. 
The magnetic recording medium of the present invention can be produced by a 
process comprising the steps of first preparing two kinds of magnetic 
paints (or dispersions), that is, a magnetic paint containing the 
inorganic powder and a magnetic paint containing no inorganic powder; then 
applying onto the nonmagnetic support the latter (magnetic paint 
containing no inorganic powder), upon which applying the former (magnetic 
paint containing the inorganic powder) by utilizing a multiple coating 
method. 
The magnetic paint can be prepared by using the known methods. The 
above-mentioned ferromagnetic metal powder, binder, inorganic powder and 
other additives such as a filler if necessary are kneaded with a solvent 
to prepare a magnetic paint containing an inorganic powder. Those 
components except for the inorganic powder are likewise kneaded with a 
solvent to prepare a magnetic paint containing no inorganic powder. The 
binders used for those two magnetic paints are generally as the same as 
each other. 
The solvent employable for kneading in the present invention can be 
selected from those conventionally used for the preparation of a magnetic 
paint. There is no specific limitation on the kneading method. The order 
of addition of each component can be appropriately selected. 
The magnetic paint can be prepared using a conventional kneading apparatus 
such as a two-roll mill, a three-roll mill, a ball mill, a pebble mill, a 
thoron mill, a sand grinder, a Szegvari attritor, a high-speed impeller 
dispersing machine, a high-speed stone mill, a high-speed impact mill, a 
disper, a kneader, a high-speed mixer, a homogenizer and a ultrasonic 
dispersing machine. 
The magnetic paints prepared as above are then subjected to a coating 
procedure. 
The magnetic paint containing no inorganic powder is first coated on the 
surface of the nonmagnetic support, and then on thus coated magnetic paint 
is coated with the magnetic paint containing the inorganic powder. The 
thickness of the magnetic paint containing no inorganic powder preferably 
is larger than 1/2 of the thickness of the resulting magnetic recording 
layer, and the thickness of the the magnetic paint containing the 
inorganic powder preferably is not larger than 1/2 of the thickness of the 
resulting magnetic recording layer (preferably not larger than 1 .mu.m). 
Examples of the coating method employable in the invention include air 
doctor coating, blade coating, rod coating, extrusion coating, air knife 
coating, squeeze coating, impregnation coating, reverse roll coating, 
transfer roll coating, gravure coating, kiss coating, cast coating, spray 
coating and spin coating. Other methods can be also employed in the 
invention. 
The whole thickness of the magnetic recording layer generally ranges from 
0.2 to 10 .mu.m, preferably from 0.5 to 7.0 .mu.m, after dryness. 
The magnetic paints coated as above are generally subjected to a magnetic 
orienting treatment for orienting the ferromagnetic metal powder, and 
dried to form a magnetic recording layer on the support. If desired, a 
surface smoothing treatment is further carried out. The magnetic recording 
medium having been subjected these treatments is subsequently cut to give 
a medium having a desired shape. 
The magnetic recording medium of the present invention can be prepared by a 
method other than the above-described multiple coating method. For 
instance, a magnetic paint containing an inorganic powder is prepared in a 
conventional manner. Thus prepared magnetic paint is coated on the 
nonmagnetic support by a known method, and instantaneously a surface of 
the nonmagnetic support not coated with the magnetic paint (back surface 
of the support) is allowed a magnet to get close thereto so as to 
compulsively precipitate the ferromagnetic metal powder in the magnetic 
paint, whereby the inorganic powder (i.e., abrasive) is distributed mainly 
on the surface side of the resulting magnetic recording layer. 
The magnetic recording medium of the invention shows high electromagnetic 
conversion characteristics, and is very advantageous particularly in the 
case that it is applied to an 8 mm type video tape. The 8 mm type video 
tape according to the invention shows remarkably high reproduction output. 
Moreover, this video tape shows an improved running endurance, and hence 
it shows a relatively long still life. 
The examples and the comparison examples of the present invention are given 
below. In the following examples, the expression "parts" means "parts by 
weight", otherwise specified.

EXAMPLE 1 
The components indicated below were kneaded for 48 hours in a ball mill to 
give a dispersion. 
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Ferromagnetic metal alloy powder (Fe--Ni alloy, 
100 parts 
Ni: 5 wt. %, specific surface 
area (S--BET): 50 m.sup.2 /g) 
Vinyl chloride/vinyl acetate/maleic anhydride 
11 parts 
copolymer (400 .times. 110 A available from Nippon 
Geon Co., Ltd., Japan, 
polymerization degree: 400) 
Polyurethane resin (N-2301 available from Nippon 
2 parts 
Polyurethane Co., Ltd., Japan) 
.alpha.-Fe.sub.2 O.sub.3 (inorganic powder, 
10 parts 
mean particle size: 0.2 .mu.m) 
Carbon black (mean particle size: 40 m.mu., available 
2 parts 
from Asahi Carbon Co., Ltd., Japan) 
Oleic acid 1 part 
Stearic acid 1 part 
Butyl stearate 1 part 
Methyl ethyl ketone 500 parts 
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To the dispersion was added a curing agent containing 8 parts of 
polyisocyanate compound (Colonate L, trade name, available from Nippon 
Polyurethane Co., Ltd.) dissolved in 100 parts of methyl ethyl ketone, and 
the mixture was kneaded for one hour to give a dispersion. The dispersion 
was filtered using a filter having a mean pour size of 1 .mu.m to prepare 
a magnetic paint containing an inorganic powder. 
The above-described procedure was repeated except for not using the 
inorganic powder to prepare a magnetic paint containing no inorganic 
powder. 
On a surface of a polyethylene terephthalate support (thickness: 10 .mu.m) 
was coated the magnetic paint containing no inorganic powder to give a 
coated layer of thickness of 2.0 .mu.m (thickness in dry state), on which 
was then coated the magnetic paint containing inorganic paint to give a 
coated layer of thickness of 1.0 .mu.m (thickness in dry state), by using 
a reverse roll. The support having been coated with the magnetic paints 
was treated with an electromagnet at 3,000 gauss under wet condition to 
give a magnetic orientation. After the coated magnetic paints was dried, 
the layer was subjected to supercalendering. The resulting sheet was 
slitted to give a video tape (8 mm type video type) having a width of 8 
mm. 
The obtained video tape was examined with respect to reproduction output, 
still life and the number of exposed inorganic particles. 
A signal of 5 MHz was input into the obtained video tape in a video tape 
recorder (FUJIX-8), and then the signal was reproduced from the video 
tape. A relative reproduction output of the video tape was measured by 
comparing a reproduction output given by a reference video tape (video 
tape prepared in Comparison Example 1) in which a video output of the 
reference tape recorded with a signal of the same 5 MHz was set to 0 dB. 
Still life of the video tape was examined by continuously carrying out the 
reproduction procedure using the above-mentioned video tape and video 
recorder under a still mode. The examination was made to determine the 
term (i.e., still life) at the end of which one-third of the reproduced 
video image diminished. 
The surface of the magnetic recording layer of the obtained video tape was 
observed by a scanning electron microscope at 100,000 magnifications, to 
examine the number of inorganic particles exposed outside of the surface 
of the magnetic recording layer per 10 .mu.m.sup.2 of the surface area. 
The results are set forth in Table 1. 
The reproduction output, still life and the number of exposed inorganic 
particles with respect to the video tapes prepared in the following 
examples and comparison examples were examined in the same manner as 
described above. 
EXAMPLE 2 
The procedure of Example 1 was repeated except for varying the amount of 
.alpha.-Fe.sub.2 O.sub.3 in the magnetic paint containing inorganic powder 
to 20 parts to prepare an 8 mm type video tape. 
The obtained video tape was examined with respect to reproduction output, 
still life, and the number of exposed inorganic particles. The results are 
set forth in Table 1. 
EXAMPLE 3 
The procedure of Example 1 was repeated except that 10 parts of 
.alpha.-Fe.sub.2 O.sub.3 in the magnetic paint containing inorganic powder 
was replaced with 7.5 parts of .alpha.-Fe.sub.2 O.sub.3 and 2.5 parts of 
.alpha.-Al.sub.2 O.sub.3 (mean particle size: same as that of 
.alpha.-Fe.sub.2 O.sub.3) to prepare an 8 mm type video tape. 
The obtained video tape was examined with respect to reproduction output, 
still life, and the number of exposed inorganic particles. The results are 
set forth in Table 1. 
COMISON EXAMPLE 1 
The procedure of Example 1 was repeated except that only the magnetic paint 
containing inorganic powder was coated in such a manner that the resulting 
layer would have a thickness of 3 .mu.m in dry state, to prepare an 8 mm 
type video tape. 
The obtained video tape was examined with respect to reproduction output, 
still life, and the number of exposed inorganic particles. The results are 
set forth in Table 1. 
COMSION EXAMPLE 2 
The procedure of Example 1 was repeated except for varying the amount of 
.alpha.-Fe.sub.2 O.sub.3 in the magnetic paint containing inorganic powder 
to 4 parts to prepare an 8 mm type video tape. 
The obtained video tape was examined with respect to reproduction output, 
still life, and the number of exposed inorganic particles. The results are 
set forth in Table 1. 
COMISON EXAMPLE 3 
The procedure of Example 1 was repeated except for varying the amount of 
.alpha.-Fe.sub.2 O.sub.3 in the magnetic paint containing inorganic powder 
to 45 parts to prepare an 8 mm type video tape. 
The obtained video tape was examined with respect to reproduction output, 
still life, and the number of exposed inorganic particles. The results are 
set forth in Table 1. 
COMISON EXAMPLE 4 
The procedure of Example 1 was repeated except that the magnetic paint not 
containing inorganic paint was replaced with the magnetic paint containing 
inorganic powder prepared in Example 2 which contains 20 parts of 
.alpha.-Fe.sub.2 O.sub.3, to prepare an 8 mm type video tape. 
The obtained video tape was examined with respect to reproduction output, 
still life, and the number of exposed inorganic particles. The results are 
set forth in Table 1. 
TABLE 1 
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Exposed Reproduc- 
particle tion output 
(per (10 .mu.m).sup.2) 
(dB) Still life 
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Example 1 10 +6 more than 60 min. 
Example 2 20 +5 more than 60 min. 
Example 3 10 +5 more than 60 min. 
Com. Example 1 
3 0 10 min. 
Com. Example 2 
4 +5 10 min. 
Com. Example 3 
45 -2 more than 60 min. 
Com. Example 4 
10 -3 more than 60 min. 
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Note: The term "more than 60 min." in still life shown in Table 1 means 
that twothird of the recorded video image remained even after a lapse of 
60 min. under a still mode.