Magnetic recording medium

A magnetic recording medium is disclosed which comprises a non-magnetic support having provided thereon a magnetic recording layer consisting ferromagnetic powder and containing graphite and a fatty acid ester.

FIELD OF THE INVENTION 
The present invention relates to a magnetic recording medium, and more 
particularly it relates to a magnetic recording medium having good still 
image like and providing reduced magnetic head wear. 
BACKGROUND OF THE INVENTION 
Almost all video tape recording decks for recording and playback of video 
images (VTR) developed recently are equipped with still mode settings for 
replaying still images. The magnetic recording medium used in such a VTR 
is required to have good still life, i.e., to be durable in a still mode 
operation for a long period of time. It is disclosed, for example, in U.S. 
Pat. No. 368,725 that various abrasives can be incorporated in the 
recording medium to improve the still life. However, when a large quantity 
of abrasives is added to improve still life as described in U.S. Pat. No. 
4,135,016, wear of the magnetic head is greatly increased and the life of 
the head is correspondingly reduced. In known magnetic recording media, it 
has been very difficult to satisfy both requirements of good still life 
and reduced head wear. 
SUMMARY OF THE INVENTION 
A first object of the present invention is to provide a magnetic recording 
medium having good still life. 
A second object of the present invention is to provide a magnetic recording 
medium having reduced head wear. 
As a result of thorough investigations, it has been found that these and 
other objects of the present invention can be attained by a magnetic 
recording medium comprising a non-magnetic support having coated thereon a 
magnetic layer, said magnetic layer containing graphite and an ester of a 
fatty acid. 
DETAILED DESCRIPTION OF THE INVENTION 
The graphite which is used in the present invention is not unduly limited, 
and can be natural graphite or artificial graphite in particular having a 
flake shape, a granular shape, or a lump (plumbago) shape. Among these, 
flake shaped graphite is superior because it provides greater reduction of 
head wear. The particle size of the graphite used can be selected as 
desired, it preferably has an average primary particle size of about 0.01 
to 3.mu., more preferably about 0.05 to 1.mu., and most preferably about 
0.1 to 0.5.mu.. If the particle size is larger than about 3.mu., the 
electromagnetic properties of the resulting magnetic medium are adversely 
affected, and it is smaller than about 0.01.mu., satisfactory lubricating 
effect cannot be obtained. 
The amount of graphite added is from about 0.1 to 10 parts by weight and 
preferably from about 0.5 to 5 parts by weight per 100 parts by weight of 
magnetic particles. If the amount added is smaller, still life is reduced 
and dropouts increase. If the amount is larger, electromagnetic properties 
are adversely affected. 
The fatty acid ester used in the present invention is an ester of a fatty 
acid having 2 to 22 carbon atoms and an alcohol having 2 to 27 carbon 
atoms, preferably an ester of a fatty acid having 6 to 18 carbon atoms and 
an alcohol having 2 to 14 carbon atoms. Both saturated and unsaturated 
fatty acids can be used. Preferred alcohols are monohydric alcohols, but 
dihydric and trihydric alcohols can be also used. 
The amount of fatty acid ester used in the present invention is from about 
0.05 to 10 parts by weight, preferably from about 0.1 to 5 parts by weight 
per 100 parts by weight of magnetic particles. A fatty acid ester can be 
used alone or a mixture of at least two kinds of fatty acid esters can be 
used. 
Examples of suitable fatty acid esters include amyl stearate, butyl 
stearate, ethyl stearate, butyl myristate, oleyl oleate, butyl palmitate, 
etc. Of these, amyl stearate, butyl stearate and ethyl stearate are 
preferred with amyl stearate and butyl stearate being most preferred. 
Still life can be further increased by the addition of abrasives having 
Mohs hardness of 6 or more, preferably 6 to 9 such as Cr.sub.2 O.sub.3, 
alumina, garnet, .alpha.--Fe.sub.2 O.sub.3 or SiC, to the above described 
graphite and fatty acid ester. The amount of the abrasive added is from 
about 0.1 to 30 parts by weight, preferably from about 1 to 20 parts by 
weight per 100 parts by weight of magnetic particles. The average particle 
size of the abrasive is from about 0.05 to 5.mu., preferably from 0.1 to 
2.mu.. 
The electromagnetic properties of the magnetic recording medium can be 
further improved by smoothing the nonmagnetic support to a center line 
average roughness (Ra) of about 0.035.mu. (cut off 0.08 mm) or less, 
preferably about 0.025 or less. However, a support surface thus smoothed 
may adversely affect the running properties of the magnetic recording 
medium on a VTR. In addition, dropouts can be caused by the scraped tips 
of the support putting on the magnetic layer. Both of these problems can 
be avoided by (1) using a support having different surface smoothnesses on 
opposite surfaces, (2) providing a backing layer or (3) providing a layer 
containing a lubricant on the surface of the support opposite to the 
magnetic layer. Among these methods, the third is most effective because 
each layer has a separate function. If a backing layer is used carbon 
black and graphite particles are preferably incorporated into the backing 
layer to prevent electrostatic charging. 
It is preferred that carbon black is additionally included in the magnetic 
layer. Because of its antistatic effect, which is useful for preventing 
dropout of signals caused by attached dust. 
The carbon black which can be used in the present invention has an average 
particle size of about 30 m.mu. or less, preferably about 10 to 30 m.mu. 
and more preferably about 15 to 25 m.mu.. 
Carbon black is added to the magnetic recording layer in an amount of from 
about 0.1 to 10 parts by weight and preferably from about 0.5 to 5 parts 
by weight per 100 parts by weight of magnetic particles. Various 
additives, supports and the methods for preparing the magnetic recording 
tapes are described in U.S. Pat. No. 4,135,016.

The present invention is illustrated in more detail by the following 
Examples, but the present invention is not to be construed as being 
limited thereto. Materials, quantities and the sequence of operation can 
be changed by those skilled in the art so long as the gist of the present 
invention is not essentially changed. 
In the Examples and Comparative Examples, all parts percents and ratios are 
by weight unless otherwise indicated. 
EXAMPLE 1 
______________________________________ 
Co-containing .gamma.-Fe.sub.2 O.sub.3 
100 parts 
Graphite (Flake Shaped, Average 
5 parts 
Primary Particle Size: 0.3.mu.) 
Fatty Acid Ester (Butyl Stearate) 
1 part 
Copolymer of Vinyl Chloride and Vinyl 
20 parts 
Acetate Containing a Carboxylic Group 
(Mol Ratio: 83:9:8; Molecular Weight: 
30,000) 
Polyurethane Resin (Molecular Weight: 
20 parts 
50,000) 
Polyisocyanate (Molecular Weight: 656) 
20 parts 
Abrasive (Cr.sub.2 O.sub.3 ; Average Particle Size: 
6 parts 
0.6.mu.) 
Methyl Ethyl Ketone 150 parts 
Butyl Acetate 150 parts 
______________________________________ 
The above composition was milled in a ball mill for 20 hours, and was 
adjusted to a viscosity of 40 poise by adding a mixed solvent of methyl 
ethyl ketone and butyl acetate (1:1 by weight) and agitating the mixture 
in an agitater and then coated on a polyester support having a thickness 
of 15.mu. and a surface smoothness of Ra 0.035.mu. in a thickness of 5.mu. 
(the same thickness used in the following Examples) to prepare Sample No. 
1. 
COMATIVE EXAMPLE 1 
Comparative Sample No. 2 was prepared in the same manner as in Example 1 
except that graphite and the fatty acid ester were omitted from the 
composition. 
EXAMPLE 2 
______________________________________ 
Co-containing .gamma.-Fe.sub.2 O.sub.3 
100 parts 
Graphite (Flake Shaped, Average 
5 parts 
Primary Particle Size: 0.3.mu.) 
Fatty Acid Ester (Butyl Stearate) 
1 parts 
Vinyl Chloride-Vinyl Acetate Copolymer 
20 parts 
Containing a Carboxylic Group (Mol 
Ratio: 83:9:8; Molecular Weight: 30,000) 
Polyurethane Resin (Molecular Weight: 
20 parts 
50,000) 
Polyisocyanate (Molecular Weight: 656) 
20 parts 
Abrasive (Cr.sub.2 O.sub.3 ; Average Particle Size: 
6 parts 
0.6.mu.) 
Carbon Black (Average Primary Particle 
5 parts 
Size: 30 m.mu.) 
Methyl Ethyl Ketone 150 parts 
Butyl Acetate 150 parts 
______________________________________ 
Sample No. 3 having the above composition was prepared in the same manner 
as in Example 1 except that carbon black was added to the composition. 
Sample Nos. 4, 5 and 6 were prepared in the same manner as above except 
that graphite having a particle size of 0.5.mu., 1.mu. and 3.mu., 
respectively, was used. 
COMATIVE EXAMPLE 2 
Comparative Sample No. 7 was prepared in the same manner as Example 2 
except that graphite and fatty acid ester were omitted from the 
composition of Example 2. Comparative Sample No. 8 was prepared in the 
same manner as Example 2 but only graphite was omitted therefrom and 
Comparative Sample No. 9 was prepared in the same manner but the fatty 
acid ester was omitted. 
EXAMPLE 3 
Sample No. 10 was prepared by coating the same magnetic recording layer as 
in Example 2 on one surface of the same support as in Example 2, and 
further coating a backing layer having the following formulation in a 
thickness of 5.mu. (the same as used in the following Examples) on the 
opposite surface thereof. 
______________________________________ 
Carbon Black (Average Primary Particle 
100 parts 
Size: 90 m.mu.) 
Nitrocellulose (Molecular Weight: 
30 parts 
27,000; Degree of Nitration: 12.0) 
Polyurethane Resin (Molecular Weight: 
5 parts 
25,000) 
Copolymer of Vinylidene Chloride- 
15 parts 
Acrylonitrile (Mol Ratio: 80:20; 
Molecular Weight: 30,000) 
Polyisocyanate (Molecular Weight: 656) 
25 parts 
Calcium Carbonate 50 parts 
.alpha.-Fe.sub.2 O.sub.3 (Average Particle Size: 1.0.mu.) 
2 parts 
Methyl Ethyl Ketone 1,000 parts 
Butyl Acetate 500 parts 
______________________________________ 
COMATIVE EXAMPLE 3 
Comparative Sample Nos. 11, 12 and 13 were prepared in the same manner as 
in Example 2 except that instead of fatty acid ester of Example 2, a fatty 
acid (oleic acid), silicone (dimethyl polysiloxane) and liquid paraffin, 
were used, respectively. 
Comparative Sample No. 14 was prepared by substituting a fatty acid ester 
for a fatty acid (oleic acid) and by increasing the amount of the abrasive 
to 30 parts. 
EXAMPLE 4 
______________________________________ 
Magnetic Recording Layer 
Co-containing FeOx (x = 1.4) (Average 
100 parts 
Particle Size: 0.3.mu. .times. 0.03.mu.; 
Co/Fe = 3/97) 
Graphite (Flake Shaped, Average 
7 parts 
Primary Particle Size: 0.4.mu.) 
Fatty Acid Ester (Amyl Stearate) 
1 part 
Fatty Acid Ester (Ethyl Stearate) 
1 part 
Fatty Acid (Myristic Acid) 
0.5 part 
Silicone Oil (Dimethyl Polysiloxane) 
0.1 part 
Copolymer of Vinyl Chloride-Vinyl 
25 parts 
Acetate-Vinyl Alcohol (Mol Ratio: 
90:3:7; Molecular Weight: 22,000) 
Polyurethane Resin (Molecular Weight: 
15 parts 
50,000) 
Polyisocyanate (Molecular Weight: 656) 
25 parts 
Abrasive (.alpha.-Al.sub.2 O.sub.3 ; Average Particle 
5 parts 
Size: 0.4.mu.) 
Carbon Black (Average Primary 
10 parts 
Particle Size: 20 m.mu.) 
Toluene 150 parts 
Butyl Acetate 150 parts 
Backing Layer 
Carbon Black (Average Primary 
100 parts 
Particle Size: 30 m.mu.) 
Graphite (Flake Shaped, Average 
100 parts 
Primary Particle Size: 0.4.mu.) 
Copolymer of Vinyl Chloride and 
75 parts 
Vinyl Acetate Containing a 
Carboxylic Group) (Mol Ratio: 
83:9:8; Molecular Weight: 30,000) 
Polyurethane Resin (Molecular 
35 parts 
Weight: 30,000) 
Polyisocyanate (Molecular Weight: 656) 
50 parts 
Talc Powder (Average Particle Size: 
50 parts 
1.5.mu.) 
Calcium Carbonate 50 parts 
Goethite (Average Particle Size: 
50 parts 
1.2.mu. .times. 0.2.mu. (Acicular)) 
Methyl Ethyl Ketone 1,000 parts 
Cyclohexanone 300 parts 
______________________________________ 
The above described composition for the magnetic layer was milled in a 
three roll mill (30 passes) and then a sand grinder (5 kg/min, 10 passes) 
and the above described composition for the backing layer was also milled 
and each composition was coated on a polyester support having a thickness 
of 20 .mu.m and a surface smoothness Ra: 0.020.mu., respectively, on 
opposite surfaces to prepare Sample No. 15. 
EXAMPLE 5 
The same procedure as in Example 3 was repeated except that the amount of 
the abrasive (Cr.sub.2 O.sub.3) was changed to 0.05 part, 0.1 part, 1 
part, 10 parts, 20 parts, 30 parts and 40 parts, respectively to prepare 
Sample Nos. 16, 17, 18, 19, 20, 21 and 22. 
The thus-prepared magnetic tape samples were evaluated using a VHS type 
VTR. Still life was determined by measuring the time (min.) required for 
recorded images of the samples replayed in the still mode of the VTR to 
disappear. Head wear is shown by the degree of wear (.mu.) resulting when 
the samples were run on the VTR for 100 hours. Video S/N is shown by the 
relative value (dB) to the standard magnetic tape prepared by Fuji Photo 
Film Co., Ltd. The dropout measurement equals the dropouts (number/min.) 
which were observed on a TV monitor over a period of at least 12 minutes. 
The results are shown in the following Table. 
TABLE 
__________________________________________________________________________ 
Head Drop- 
Sam- 
Example or Amount Size of Still 
wear 
Video 
out 
ple 
Comparative 
Size of 
of Fatty acid 
carbon 
Back- 
Back life 
(.mu./ 
S/N (number/ 
No. 
Example 
graphite 
abrasive 
ester black 
ing coat (min) 
100 h) 
(dB) 
min) 
__________________________________________________________________________ 
1 Example 1 
0.3.mu. Butyl None 
None &gt;400 
3.0 +2.3 
35 bad 
Stearate runability 
2 Comparative 
None None None 
None &lt;1 &gt;30 -1.5 
250 bad 
Example 1 runability 
3 Example 2 
0.3.mu. Butyl 20 m.mu. 
None &gt;400 
1.5 +2.3 
13 
Stearate 
4 Example 2 
0.5.mu. Butyl 20 m.mu. 
None &gt;400 
1.8 +2.2 
17 
Stearate 
5 Example 2 
1.0.mu. Butyl 20 m.mu. 
None &gt;400 
2.3 +1.6 
20 slightly 
Stearate bad filtra- 
tion of 
liquid (*) 
6 Example 2 
3.0.mu. Butyl 20 m.mu. 
None &gt;400 
5.0 -0.5 
35 bad filtra- 
Stearate tion of 
liquid (*) 
7 Comparative 
None None 20 m.mu. 
None &lt;1 20 +0.5 
75 
Example 2 
8 Comparative 
None Butyl 20 m.mu. 
None 25 
8 +1.8 
60 
Example 2 Stearate 
9 Comparative 
0.3.mu. 
6 parts 
None 20 m.mu. 
None 3 15 +0.7 
60 
Example 2 
10 Example 3 
0.3.mu. 
6 parts 
Butyl 20 m.mu. 
Provided 
&gt;400 
1.3 +2.1 
3 
Stearate 
11 Comparative 
0.3.mu. 
6 parts 
None 20 m.mu. 
None 10 
1.3 +2.3 
14 
Example 3 (fatty acid) 
12 Comparative 
0.3.mu. 
6 parts 
None 20 m.mu. 
None 2 0.9 +1.6 
17 
Example 3 (silicone) 
13 Comparative 
0.3.mu. 
6 parts 
None 20 m.mu. 
None 7 1.5 +2.0 
16 
Example 3 (liquid 
paraffin) 
14 Comparative 
None 30 parts 
None 20 m.mu. 
None 250 
&gt;30 +1.9 
90 
Example 3 (fatty acid) 
15 Example 4 
0.3.mu. 
5 parts 
Amyl 20 m.mu. 
Provided 
&gt;400 
0.9 +2.3 
3 
(smooth 
Stearate/ 
surface 
Ethyl 
support) 
Stearate 
16 Example 5 
0.3.mu. 
0.05 
part 
Butyl 20 m.mu. 
Provided 
300 
0.8 +1.0 
4 
Stearate 
17 Example 5 
0.3.mu. 
0.1 
part 
Butyl 20 m.mu. 
Provided 
&gt;400 
0.8 +2.2 
3 
Stearate 
18 Example 5 
0.3.mu. 
1 part 
Butyl 20 m.mu. 
Provided 
&gt;400 
1.0 +2.2 
4 
Stearate 
19 Example 5 
0.3.mu. 
10 parts 
Butyl 20 m.mu. 
Provided 
&gt;400 
1.5 +2.1 
3 
Stearate 
20 Example 5 
0.3.mu. 
20 parts 
Butyl 20 m.mu. 
Provided 
&gt;400 
3.0 +2.0 
3 
Stearate 
21 Example 5 
0.3.mu. 
30 parts 
Butyl 20 m.mu. 
Provided 
&gt;400 
7.0 +2.0 
5 
Stearate 
22 Example 5 
0.3.mu. 
40 parts 
Butyl 20 m.mu. 
Provided 
&gt;400 
15 +1.7 
4 
Stearate 
__________________________________________________________________________ 
(*) Time required for filtering through a filter having a thickness of 3 
.mu.m at a pressure of 2 kg/cm.sup.2 was measured. 
Comparison of Sample No. 3 (invention) with Sample Nos. 7, 8 and 9 
(Comparative Samples) demonstrates that still life and head wear 
resistance are remarkably improved along with video S/N and dropout 
characteristics only when graphite and a fatty acid ester are present in a 
magnetic recording layer according to the invention. 
If the fatty acid ester in Sample No. 3 is replaced by other lubricants, 
the surprising improvements obtained by using a fatty acid ester in 
combination with graphite were not observed (Sample Nos. 11, 12 and 13). 
In addition the results obtained using Sample Nos. 1, 3 and 10 show that 
the addition of carbon black to the magnetic layer and the coating of a 
backing layer was preferable. Sample Nos. 3 to 6 demonstrate that the use 
of graphite having a particle size of not more than 0.5.mu. is preferable. 
Sample Nos. 10, 16, 17, 18, 19, 20, 21 and 22 demonstrate that if the 
amount of abrasive (Cr.sub.2 O.sub.3) is reduced the S/N ratio is 
decreased and still life is reduced, and that if the amount of an abrasive 
is increased, head wear also increases. The amount of the abrasive is 
preferably 1 to 20 parts by weight based on 100 parts by weight of 
magnetic particles. 
While the invention has been described in detail and with reference to 
specific embodiment thereof, it will be apparent to one skilled in the art 
that various changes and modifications can be made therein without 
departing from the spirit and scope thereof.