Magnetic recording medium

A magnetic recording medium comprises a substrate coated with a magnetic layer wherein a magnetic powder is orientated in a binder and a polyoxyethylenesorbitane higher fatty acid ester surfactant.

BACKGROUND OF THE INVENTION 
1. Field of the Invention: 
The present invention relates to an improvement of a magnetic recording 
medium such as a magnetic tape and a flexible disc. More particularly, it 
relates to a magnetic recording medium which has low coefficience of 
friction and small adhesion of a magnetic powder on a head of a recorder. 
2. Description of the Prior Arts: 
The magnetic recording medium is usually obtained by coating a magnetic 
powder composition as a mixture of a magnetic powder and a binder such as 
polymers on a substrate made of a polymer such as polyester. In the 
conventional magnetic recording medium, the magnetic powder is peeled off 
by friction resistance between the magnetic powder membrane and a head of 
a recorder so as to adhere the magnetic powder on a head of a recorder 
whereby reproduction characteristics in high tone region are deteriorated. 
In order to overcome such disadvantages, a lubricant is incorporated into 
a magnetic powder composition so as to increase the effect of lubrication 
of the magnetic powder composition and to decrease a friction resistance 
between the magnetic powder membrane and the head of the recorder. The 
effect of lubrication of the incorporation of the conventional lubricant 
at a low temperature is unsatisfactorily low. It has been required to 
improve such characteristics for a magnetic recording medium used at a low 
temperature in winter such as a magnetic tape used for a car stereo 
recorder. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a magnetic recording 
medium which can be smoothly run in highly stable condition even at a low 
temperature under remarkably reducing the adhesion of the magnetic powder 
on the head of the recorder. 
The foregoing and other objects of the present invention have been attained 
by providing a magnetic recording medium which comprises a substrate 
coated with a magnetic layer wherein a magnetic powder is orientated in a 
binder and a polyoxyethylenesorbitane higher fatty acid ester surfactant.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS: 
The polyoxyethylenesorbitane higher fatty acid esters used in the present 
invention are condensates of ethyleneoxide 
##STR1## 
and a sorbitane higher fatty acid ester. It is preferable to use such 
polyoxyethylenesorbitane higher fatty acid esters which have hydrophilic 
property to be water soluble and especially have HLB of 7 to 18. 
The polyoxyethylenesorbitane higher fatty acid ester is usually 
incorporated at a ratio of 1 to 30 wt.% preferably 2 to 20 wt.% based on a 
total of the binder. 
The binder can be the conventional binder for the magnetic powder 
composition coated on the substrate of the magnetic recording medium. 
Suitable binders include the conventional binders such as polyurethane, 
nitrocellulose and copolymers of vinyl chloride-vinyl acetate and/or vinyl 
alcohol and the other thermoplastic or thermosettable resins. The magnetic 
powders can be the conventional magnetic powders. These binders and 
magnetic powders are described in detail in the prior patent applications 
and accordingly, the description of the binders and the magnetic powders 
is not repeated. 
The following prior patents and applications are referred as the 
descriptions of the binders and the magnetic powders. U.S. Pat. No. 
4,115,290, G.B. Patent Publications Nos. 2,017,117A, 2,021,126A and Patent 
Applications U.S. Ser. No. 109,032, West German Pat. No. 3,001,451, U.K. 
Pat. No. 8001322 and Netherland Pat. No. 800061 and Japanese Patent 
Application No. 79937/1979. 
The polyoxyethylenesorbitane higher fatty acid ester surfactant can be 
incorporated in a preparation of the resin solution or in a mixing of the 
magnetic powder or after a preparation of the magnetic powder composition 
though it is usually incorporated with the binder. 
The present invention will be further illustrated by certain examples and 
references which are provided for purposes of illustration only and are 
not intended to be limiting the present invention. 
The following magnetic powder and solvent were commonly used in both of 
examples and references. 
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Magnetic Powder: 
.gamma.-Fe.sub.2 O.sub.3 : 
100 wt. parts 
Solvent: 
Methyl ethyl ketone: 
100 wt. parts 
Methyl isobutyl ketone: 
100 wt. parts 
Toluene: 100 wt. parts 
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The following binders were respectively incorporated in the mixture of the 
magnetic powder and the solvent. A is a binder of vinyl chloride-vinyl 
acetate copolymer; B is a binder of polyurethane resin; and C is 
polyoxyethylenesorbitane higher fatty acid ester surfactant. In the 
references and examples, the following kinds of the binders and the 
surfactants were used together with the above-mentioned magnetic powder 
and solvent. Only the binder and the surfactant added to said magnetic 
powder and the solvent are stated. 
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REFERENCE 1: 
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A: vinyl chloride-vinyl acetate copolymer: 
23 wt. parts 
(VAGH manufactured by UCC) 
B: polyurethane: 10 wt. parts 
(Nippolan 5033 manufactured by 
Nippon Polyurethane Co.) 
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A: vinyl chloride-vinyl acetate-vinyl alcohol: 
26 wt. parts 
copolymer (10% vinyl alcohol component) 
B: polyurethane: 7 wt. parts 
(Estane 5703 manufactured by 
B.F. Goodrich Co.) 
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EXAMPLE 1 
The same kinds and amounts of components A and B of Reference 1 were used 
together with the following surfactant. 
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C: polyoxyethylenesorbitane trioleate: 
2 wt. parts 
(Sorbon T-85 manufactured by Toho Kagaku 
Kogyo K.K.) 
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EXAMPLE 2 
The same kinds and amounts of components A and B of Reference 2 were used 
together with the following surfactant. 
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C: polyoxyethylenesorbitane tristearate: 
2 wt. parts 
(Tween 65 manufactured by Kao-Atlas Co.) 
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EXAMPLE 3 
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A: vinyl chloride-vinyl acetate copolymer: 
23 wt. parts 
(S-LECA manufactured by Sekisui Kagaku 
Kogyo K.K.) 
B: polyurethane: 10 wt. parts 
(Estane 5715 manufactured by B.F. Goodrich 
Co.) 
C: polyoxyethylenesorbitane monooleate: 
2 wt. parts 
(Tween 80 manufactured by Kao-Atlas Co.) 
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EXAMPLE 4 
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A: vinyl chloride-vinyl acetate copolymer: 
26 wt. parts 
(VAGH manufactured by UCC) 
B: polyurethane: 7 wt. parts 
(Nippolan 5032 manufactured by Nippon 
Polyurethane Co.) 
C: polyoxyethylenesorbitane monopalmitate: 
2 wt. parts 
(Tween 40 manufactured by Kao-Atlas Co.) 
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In each preparation of each magnetic tape, the binder and the surfactant 
were incorporated into the magnetic powder and the solvent as stated in 
the references and the examples, the mixture was thoroughly mixed to 
disperse in a ball mill and the mixture was coated on a polyester film 
having a thickness of 12.mu. and dried to give a thickness of the membrane 
of 6.mu. and the surface of the coated film was processed and the film was 
cut to obtain each magnetic tape. 
Coefficients of friction of the magnetic tapes obtained in References 1 and 
2 and Examples 1 and 2 were measured in a temperature range of -20.degree. 
to 30.degree. C. The results are shown in FIG. 1. 
As it is clearly found in FIG. 1, the coefficients of friction in Examples 
1 and 2 were remarkably lower than those of References 1 and 2. The 
coefficients of friction of the examples were lower depending upon the 
decrease of the temperature. In accordance with the present invention, the 
coefficients of friction at the low temperature region are remarkably 
lower than those of the conventional ones. At the low temperature region, 
excellent lubrication effect is shown. Although the data are not shown in 
FIG. 1, the coefficients of friction in Examples 3 and 4 were 
substantially the same. Adhesions of the magnetic powder on a head by 
using the magnetic tapes obtained in References 1 and 2 and Examples 1 and 
2 were measured in a temperature range of -20.degree. to 30.degree. C. The 
results are shown in FIG. 2. 
In the tests, a car stereo tape recorder was used and the magnetic tapes 
were respectively, continuously runned for 100 hours. The amount of the 
magnetic powder (mg) peeled off from the magnetic membrane which was 
adhered on the head at each temperature was measured. 
As it is clearly found in FIG. 2, the adhesions of the magnetic powder in 
Examples 1 and 2 were remarkably smaller than those of References 1 and 2. 
The adhesions of the magnetic powder at the low temperature region are 
remarkably lower than those of the conventional ones. Therefore, the 
magnetic tape of the present invention is especially preferable to use it 
at a low temperature ambient in view of the low coefficient of friction 
and the small adhesion of the magnetic powder at the low temperature 
region. 
Although the data are not shown in FIG. 2, the adhesions of the magnetic 
powder in Examples 3 and 4 were substantially the same. 
In accordance with the preparation of the magnetic recording medium of the 
present invention, a polyoxyethylenesorbitane higher fatty acid ester 
surfactant is incorporated in the magnetic powder composition and the 
composition is coated on a substrate, whereby a stability in running at a 
low temperature is remarkably high and an adhesion of the magnetic powder 
on a head is small. Therefore, the magnetic recording medium having high 
quality and high reliability can be obtained.