Magnetic recording medium

A magnetic recording medium improved in lubricity and abrasion resistance, which comprises a base material and a magnetic layer provided thereon, said magnetic layer comprising magnetic particles and a binder, characterized in that said magnetic layer further comprises at least one silicone compound of the formula: ##STR1## wherein X is the same or different and --A--COOH, --A--OH, ##STR2## or --A--NH.sub.2 in which A is a divalent hydrocarbon group having not more than 26 carbon atoms, Y and Y' are each --X or --CH.sub.3, p is an integer of 0 to 1,000 and q is an integer of 1 to 500 but p+q is not more than 1,000 in an amount of 0.1 to 5% by weight based on the weight of the magnetic particles in the magnetic layer.

The present invention relates to a magnetic recording medium. More 
particularly, it relates to a magnetic recording medium improved in 
lubricity and abrasion resistance at the magnetic layer. 
A magnetic recording medium such as a magnetic recording tape, which is 
prepared by applying a magnetic coating composition comprising magnetic 
particles, a binder and an organic solvent onto the surface of a base 
material (e.g. polyester film), travels slidably on a magnetic recording 
head, a guide part and the like during recording and reproducing and 
therefore its magnetic layer is apt to be abraded. Because of this reason, 
the magnetic layer of a magnetic recording medium is desired to be small 
in coefficient of friction and excellent in running stability. Also, it is 
required to have high abrasion resistance with good durability. 
In order to improve the lubricity and abrasion resistance of the magnetic 
layer, it has been proposed to incorporate a lubricating agent into the 
magnetic layer. A typical example of the lubricating agent is silicone 
oil. Silicone oil is quite excellent in lubricity but inferior in 
compatibility with a binder. Thus, the friction resistance of the magnetic 
layer can be sufficiently decreased by incorporation of silicone oil 
therein, but the magnetic layer is apt to become uneven whereby pinholes 
are produced. Further, bleeding of silicone oil to the surface of the 
magnetic layer is frequently observed. 
For improvement of the compatibility, it was proposed to use a modified 
silicone compound of the following formula [U.S. Pat. No. 4,135,016]: 
##STR3## 
wherein R, R' and R" are each a hydrocarbon group having 7 to 21 carbon 
atoms, and m and n are each an appropriate integer. The modified silicone 
compound can afford a magnetic recording medium, such as a magnetic 
recording tape, excellent in lubricity and durability. However, after 
storage over a long period of time, the lubricity is gradually lost, and 
the running during reproduction becomes unstable. These defects are 
probably caused by gradual hydrolysis of the modified silicone compound 
with the moisture present in the air. Since hydrolysis of the modified 
silicone compound takes place already in the magnetic recording 
composition before application, its incorporation produces the gelation of 
the magnetic coating composition and lowers the dispersibility of the 
magnetic coating composition. Thus, the magnetic layer wherein magnetic 
particles are evenly dispersed and which shows a high sensitivity is 
hardly obtainable. Because of this reason, the incorporation of the 
modified silicone compound into the magnetic layer is desirably achieved 
by spraying a dispersion comprising the modified silicone compound onto 
the surface of the magnetic layer or by dipping the surface of the 
magnetic layer into the said dispersion. The thus obtained magnetic 
recording medium has the modified silicone compound in a concentrated 
state at the surface of the magnetic layer. Therefore, it shows excellent 
lubricity at the initial stage of running for reproduction but, since the 
modified silicone compound is transferred to the recording head or the 
roller as the result of running under the sliding contact, said lubricity 
cannot be maintained over a long period of time. 
As a result of an extensive study, it has now been found that a certain 
silicone compound can be uniformly dispersed into a magnetic coating 
composition without prevention of the even and stable dispersion of 
magnetic particles and a binder therein while assuring excellent lubricity 
and durability for a magnetic recording medium having a magnetic layer 
comprising the same over a long period of time. 
According to the present invention, there is provided a magnetic recording 
medium comprising a base material and a magnetic layer containing magnetic 
particles and a binder, characterized in that the magnetic layer further 
contains at least one silicone compound of the formula: 
##STR4## 
wherein X is the same or different and --A--COOH, --A--OH, 
##STR5## 
or --A--NH.sub.2 (in which A is a divalent hydrocarbon group having not 
more than 26 carbon atoms), Y and Y' are each --X or --CH.sub.3, p is an 
integer of 0 to 1,000 and q is an integer of 1 to 500 but p+q is not more 
than 1,000. 
The characteristic feature of the invention resides in inclusion of the 
silicone compound (I) in the magnetic layer of a magnetic recording 
medium. By the presence of the silicone compound (I), the magnetic layer 
shows good lubricity and high abrasion resistance, and such good lubricity 
and high abrasion resistance can be maintained over a long period of time 
without any material deterioration. Advantageously, the dispersibility of 
various components in a magnetic coating composition for formation of a 
magnetic layer is not materially influenced by the presence of the 
silicone compound (I) therein. In addition, the silicone compound (I) is 
well compatible with a binder in a magnetic coating composition. 
Therefore, neither bleeding nor pinholes are produced on the magnetic 
layer formed by the use of the magnetic coating composition comprising the 
same. 
The silicone compound (I) is known and per se available on the commercial 
market. In the formula (I), the hydrocarbon group represented by A may be 
saturated or unsaturated and has not more than 26 carbon atoms. When the 
number of carbon atoms exceeds 26, the lubricity is rather lowered. When 
the symbol A is not present, the improvement of abrasion resistance is 
insufficient. Specific examples of the substituent of the formula: --A--X 
are as follows: 
##STR6## 
Y and Y' may be each --CH.sub.3 or --X, and at least one of them is 
preferred to be --X, because a better compatibility with the binder is 
assured and higher lubricity and abrasion resistance are realized. 
Usually, p and q satisfy the following relationships: 
0.ltoreq.p.ltoreq.1,000; 1.ltoreq.q.ltoreq.500; p+q.ltoreq.1,000. When p 
and q are too large, the silicone compound (I) becomes hardly soluble in 
organic solvents, and the preparation of a magnetic coating composition 
containing the silicone compound (I) as evenly dispersed may be difficult. 
One or more kinds of the silicone compound (I) may be used. The amount of 
the silicone compound (I) to be incorporated into the magnetic layer may 
be usually from 0.1 to 10% by weight, preferably from 0.1 to 5% by 
weight, based on the weight of the magnetic particles in the magnetic 
layer. When the amount is smaller than 0.1% by weight, the improving 
effect is not materially produced. When larger than 10% by weight, the 
recording head is stained whereby the output may be lowered. 
In order to make a magnetic layer comprising the silicone compound (I), it 
may be blended with magnetic particles, a binder and an organic solvent, 
followed by application of the resulting magnetic coating composition onto 
the surface of a base material such as an elastic sheet (e.g. polyester 
film) to form a magnetic layer. The content of the silicone compound (I) 
in the magnetic coating composition may be from 0.1 to 5% by weight, 
preferably from 0.1 to 3% by weight, based on the weight of the magnetic 
particles. When the content is more than 5% by weight, the dispersibility 
of the magnetic particles will become inferior. Naturally, the amount of 
the silicone compound (I) in the magnetic layer is limited to be from 0.1 
to 5% by weight based on the weight of the magnetic particles, when the 
magnetic layer is formed by the use of the above magnetic coating 
composition. 
Alternatively, the silicone compound (I) may be dispersed in an organic 
solvent, followed by application of the resulting dispersion onto the 
surface of a magnetic layer as previously formed on a base material. In 
this case, the content of the silicone compound (I) in the dispersion may 
be optional so that its use can provide a magnetic layer containing the 
silicone compound (I) in an amount of even more than 5% by weight based on 
the weight of the magnetic particles. As stated above, however, the amount 
of the silicone compound (I) in the magnetic layer is usually kept at not 
more than 10% by weight based on the weight of the magnetic particles in 
order to avoid the lowering of the output. 
The application may be effected by a conventional procedure such as 
brushing, spraying or dipping. The thickness of the magnetic layer thus 
formed may be usually from 1 to 20.mu. (after drying) depending upon the 
thickness of the elastic film, which is ordinarily from 3 to 100.mu.. In 
the magnetic layer, the weight proportion of the magnetic particles and 
the binder is usually from 50:50 to 90:10. 
Examples of the magnetic particles are .gamma.-Fe.sub.2 O.sub.3 particles, 
Fe.sub.3 O.sub.4 particles, Co-containing .gamma.-Fe.sub.2 O.sub.3 
particles, Co-containing Fe.sub.3 O.sub.4 particles, CrO.sub.2 particles, 
Fe particles, Co particles, Fe-Ni particles, etc. As the binder, there may 
be used, for instance, vinyl chloride/vinyl acetate resin, vinylidene 
chloride resin, acrylonitrile/butadiene resin, vinyl butyral resin, 
urethane resin, cellulose resin, isocyanate compounds or the like. Among 
them, the use of vinyl chloride/vinyl acetate resin, cellulose resin, etc. 
is preferred. No particular limitation is present on the organic solvent, 
and there may be used one or more chosen from ketones (e.g. acetone, 
methyl isobutyl ketone, methyl ethyl ketone, cyclohexanone), esters (e.g. 
ethyl acetate, butyl acetate), aromatic hydrocarbons (e.g. benzene, 
toluene, xylene), alcohols (e.g. isopropyl alcohol), acid amines (e.g. 
dimethylformamide), ethers (e.g. tetrahydrofuran, dioxane), etc. 
Practical and presently preferred embodiments of the invention are 
illustratively shown in the following Examples and Reference Example 
wherein part(s) and % are by weight.

EXAMPLE 1 
______________________________________ 
Materials Part(s) 
______________________________________ 
Co-containing .gamma.-Fe.sub.2 O.sub.3 
80 
magnetic particles 
Vinyl chloride/vinyl acetate/ 
10 
vinyl alcohol copolymer ("VAGH" 
manufactured by Union Carbide 
Corp.) 
Urethane elastomer ("Pandex 
8 
T-5250" manufactured by Dainippon 
Ink K.K.) 
Trifunctional low molecular 
2 
isocyanate compound ("Colonate L" 
manufactured by Nippon Poly- 
urethane Kogyo K.K.) 
Cyclohexanone 50 
Methyl ethyl ketone 50 
Silicone compound (I) (Nos. 1 
1 
to 11) 
______________________________________ 
The above materials were mixed together in a ball mill for 70 hours to make 
a magnetic coating composition. The composition was applied onto a 
polyester film of 11.mu. in thickness, followed by drying to make a 
coating layer of 5.mu. in thickness. The resulting film was calendered at 
the surface and slitted in a width of 3.8 mm to obtain a magnetic 
recording tape (Specimen Nos. 1-1 to 1-11). 
The silicone compound (I) used in this Example was chosen from the 
following ones: 
__________________________________________________________________________ 
No. 
Chemical Structure 
__________________________________________________________________________ 
##STR7## 
2 
##STR8## 
3 
##STR9## 
4 
##STR10## 
5 
##STR11## 
6 
##STR12## 
7 
##STR13## 
8 
##STR14## 
9 
##STR15## 
10 
##STR16## 
11 
##STR17## 
__________________________________________________________________________ 
EXAMPLE 2 
In the same manner as in Example 1 but using nitrocellulose 
("Nitrocellulose H 1/2" manufactured by Asahi Chemical Industry Co., Ltd.) 
in place of "VAGH", a magnetic recording tape was prepared (Specimen Nos. 
2-1 to 2-11). 
EXAMPLE 3 
In the same manner as in Example 1 but omitting the use of "VAGH" and using 
18 parts of "Pandex T-5250" in place of 8 parts of the same, a magnetic 
recording tape was prepared (Specimen Nos. 3-1 to 3-11). 
REFERENCE EXAMPLE 
In the same manner as in Example 1 but using 0.07 part of the silicone 
compound (I) (No. 1) in place of 1 part of the same, a magnetic recording 
tape was prepared (Specimen No. R-1). 
In the same manner as in Example 1 but not using the silicone compound (I), 
a magnetic recording tape was prepared (Specimen No. R-2). 
Each of the magnetic recording tapes prepared in Examples 1 to 3 and 
Reference Example 1 was subjected to tests for durability and also for 
determination of the coefficient of friction. The results are shown in 
Table 1. 
In general, the tape immediately after production was subjected to test. 
Exceptionally, the tape prepared in Example 1 (Specimen Nos. 1-1 to 1-11) 
was subjected to test immediately after production and also after storage 
at a temperature of 60.degree. C. under a relative humidity of 80% for 30 
days. 
The durability test was carried out as follows: a signal of 333 Hz was 
recorded on the tape with a head load of 10 g and a running speed of 4.8 
cm/sec at a temperature of 35.degree. C. under a relative humidity of 80%; 
the recorded signal was reproduced; and the reproduction time until the 
output of 3 dB below the initial reproduction output was shown was 
counted. 
The coefficient of friction was determined as follows: the tape was run 
under the same conditions as adopted in the durability test; and after 
running of 200 times, the dynamic coefficient of friction with the 
recording head at a relative speed of 4.8 cm/sec was measured. 
Besides, for evaluation of the dispersibility of the magnetic layer, some 
of the tapes obtained in Example 1 (Specimen Nos. 1-1, 1-3, 1-5, 1-7, 1-9 
and 1-11) were subjected to a test for determination of the sensitivity 
(dB) during reproduction of the recorded signal according to the standard 
set up by the Japanese Association of Magnetic Recording Tape Industry 
(MTS-102). The results are shown in Table 2. 
TABLE 1 
______________________________________ 
Tape immediately after 
Tape after storage for 
production 30 days 
Specimen 
Durability 
Coefficient 
Durability 
Coefficient 
No. (times) of friction 
(times) of friction 
______________________________________ 
1-1 1300 0.18 1280 0.18 
1-2 1400 0.17 1300 0.18 
1-3 1100 0.20 1100 0.20 
1-4 1200 0.19 1150 0.20 
1-5 1100 0.20 1070 0.20 
1-6 1200 0.19 1100 0.20 
1-7 1300 0.18 1300 0.18 
1-8 1400 0.17 1350 0.18 
1-9 600 0.25 300 0.30 
1-10 550 0.25 200 0.29 
1-11 500 0.25 500 0.25 
2-1 1400 0.16 
2-2 1500 0.15 
2-3 1200 0.18 
2-4 1300 0.17 
2-5 1200 0.18 
2-6 1300 0.17 
2-7 1400 0.16 
2-8 1500 0.15 
2-9 700 0.25 
2-10 500 0.28 
2-11 600 0.23 
3-1 1400 0.18 
3-2 1500 0.17 
3-3 1200 0.20 
3-4 1300 0.19 
3-5 1200 0.20 
3-6 1300 0.19 
3-7 1400 0.18 
3-8 1500 0.17 
3-9 700 0.25 
3-10 640 0.25 
3-11 600 0.26 
R-1 530 0.24 
R-2 210 0.30 
______________________________________ 
TABLE 2 
______________________________________ 
Specimen No. Sensitivity (dB) 
______________________________________ 
1-1 +2.4 
1-3 +2.1 
1-5 +2.3 
1-7 +2.2 
1-9 +1.8 
1-11 +1.1 
______________________________________ 
From the results in Table 1, it is understood that the magnetic recording 
tape of the invention has excellent durability and low coefficient of 
friction. It is also understood that the magnetic recording tape does not 
show any material increase in the coefficient of friction even after 
storage at a high temperature under a high humidity for a long period of 
time and can maintain good lubricity over a long period of time. In 
comparison of Examples 1, 2 and 3, it may be understood that the use of 
vinyl chloride resin or nitrocellulose resin as the binder in the magnetic 
layer can produce particularly excellent durability. From Example 4, it 
may be understood that when the amount of the silicone compound (I) is 
less than 0.1% by weight, the improvement of durability and lubricity is 
not significantly produced. 
From the results in Table 2, it is understood that the silicone compound 
(I) does not afford any material influence on the dispersibility of the 
magnetic coating composition and can provide a magnetic recording tape 
having a high sensitivity.