Magnetic recording material

A magnetic recording material which comprises a non-magnetic substrate, and a magnetic layer formed on at least one surface of the substrate. The magnetic layer is made of a uniform dispersion of a magnetic powder in a mixture of epoxy and urea resins used in an amount from 40 to 200 wt % of the magnetic powder.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to magnetic recording materials, particularly 
magnetic hard discs which have much improved film properties and magnetic 
characteristics. 
2. Description of the Prior Art 
Magnetic recording materials adapted to use in magnetic disc recording and 
reproducing apparatus are usually made by procedures which comprise mixing 
magnetic powders, binder resins and solvents, applying the resulting 
magnetic paint on a disc substrate, and baking the applied paint for 
curing. The magnetic recording material of this type is now required to 
have improved magnetic and mechanical performances because the recent 
trend toward high recording density causes the disc and magnetic heads to 
come closer to each other. In addition, starting and stopping operations 
of the apparatus under contact with the disc are at present adopted as 
usual practice. More particularly, the magnetic material should have good 
magnetic characteristics such as squareness ratio of BH loop, coersive 
force, residual flux density, and the like. Also, from the mechanical 
standpoint, it is necessary that the magnetic layer be brought to intimate 
contact with the substrate, be small in thickness, and be flat and 
lubricative on the surface thereof. In addition, the number of missing 
pulse errors and extra pulse errors which may be caused on the basis of 
both magnetic and mechanical defects of the magnetic material must be 
small. 
SUMMARY OF THE INVENTION 
It is accordingly an object of the present invention to provide magnetic 
recording materials which comprise a magnetic layer making use of a 
specific combination of binder resins to ensure good dispersability of 
magnetic particles whereby good magnetic characteristics and good 
mechanical or physical properties are attained at the same time. 
According to the present invention, there is provided a magnetic recording 
material of the type which comprises a non-magnetic substrate, and a 
magnetic layer formed on the substrate, the magnetic layer being made of a 
composition which comprises a uniform dispersion of a magnetic powder in a 
mixture of epoxy and urea resins used in an amount from 40 to 200 wt% of 
the magnetic powder. Typical of the magnetic materials to which the 
present invention is directed is a so-called hard disc. 
DETAILED DESCRIPTION AND EMBODIMENTS OF THE INVENTION 
The present invention is characterized in that the binder resin for the 
magnetic layer is a combination of epoxy and urea resins. The epoxy resins 
useful in the practice of the invention are not critically limited but 
bisphenol A-type epoxy resins are preferred from the standpoint of 
mechanical characteristics, particularly hardness. In practice, the 
molecular weight of the epoxy resin generally ranges from 800 to 8000. 
Bisphenol A-type epoxy resins are commercially available, for example, 
from Shell Chemical Co., Ltd., under the designations of Epikote 1001, 
Epikote 1007, etc. 
Urea resins are not also critically limited but any known urea resins may 
be used in the practice of the invention provided that they are miscible 
with epoxy resins. Especially, when used in combination with bisphenol 
A-type epoxy resins, alkylated urea resins such as are preferred. Examples 
of the alkylated urea resins include methylated, ethylated, propylated, 
iso or n-butylated and the like alkylated urea resins, of which 
iso-butylated urea resin is most preferable. In the best mode, bisphenol 
A-type epoxy resins are used in combination with iso-butylated urea resin, 
by which dispersion of magnetic particles is remarkably improved, leading 
to improvements of the resulting magnetic layer with respect to magnetic 
characteristics. 
A variety of alkylated urea resins are commercially sold, for example, from 
Dainippon Ink Chem. Ind. Co., Ltd. under the designations of Beckamin 
G-1850 and Beckamin P-138, from Hitachi Chemical Co., Ltd. under the 
designation of Melan 11, from Mitsui Toatsu Chemical Co., Ltd. under the 
designations of Uban 10S-60 and Uban 10R, and the like. 
The mixing ratio of expoxy to urea resins is generally, from 90:10 to 
10:90, preferably from 70:30 to 50:50, on the weight basis. The total 
amount of epoxy and urea resins are generally from about 40 to 200 wt%, 
preferably about 60 to 120 wt%, of magnetic powder used. 
The magnetic powders useful in the present invention are those powders 
ordinarily used for these purposes and include, for example, magnetic 
oxide powders such as gamma-Fe.sub.2 O.sub.3, Co-gamma-Fe.sub.2 O.sub.3, 
Co--Fe.sub.3 O.sub.4, CrO.sub.2 and the like, and surface-treated magnetic 
metal powders such as Co, Ni, Fe and the like. 
For the manufacture of the magnetic recording materials according to the 
invention, magnetic powders are dispersed in a mixture of epoxy and urea 
resins dissolved in solvents. The resulting dispersion is applied onto at 
least one surface of a non-magnetic substrate and heated for curing at a 
temperature from 150.degree. to 200.degree. C. for about 15 minutes to 2 
hours to form a magnetic layer having a dry thickness of 0.2 to 2 microns 
on the substrate as usual. The above-defined temperature and time 
conditions are sufficient to completely cure the mixture of the binder 
resins. Where the magnetic recording material is a magnetic disc, the 
cured material may be polished such as by a polishing tape made, for 
example, of alumina, silicon carbide or the like to obtain the disc. 
Solvents for the mixed binder include, for example, alcohols such as 
methanol, ethanol, propanol, butanol and the like, aromatic compounds such 
as xylene, toluene, benzene and the like, ketones such as acetone, methyl 
ethyl ketone, methyl isobutyl ketone and the like, and mixtures thereof. 
If necessary, known additives which are ordinarily employed in this field 
may be added to the dispersion. Such additives include, for example, 
dispersants, lubricants, abrasive agents, antistatic agents, surfactants 
and the like. 
The non-magnetic substrates may be sheets or plates metals such as 
aluminum, magnesium, copper and the like, and glass or ceramics.

The present invention is particularly described by way of examples. 
EXAMPLE 1 
Fifty parts by weight of gamma-Fe.sub.2 O.sub.3, about 10 parts by weight 
of alumina powder, about 26 parts by weight of epoxy resin (Epikote 1007, 
by Shell Chemical Co., Ltd.), about 13 parts by weight of urea resin 
(Melan 11), about 1 part by weight of lecithin and about 200 parts by 
weight of a mixed solvent of toluene and butanol were mixed in a ball mill 
for about 24 hours, thereby obtaining a magnetic paint. This magnetic 
paint was applied onto a disc substrate by a spin coating and dried, 
followed by keeping at 200.degree. C. for 30 minutes to bake and cure the 
applied layer and polishing by means of a polishing tape. The thickness of 
the layer had a thickness of 0.8 micron. 
EXAMPLE 2 
The general procedure of Example 1 was repeated using a curing temperature 
of 175.degree. C., thereby obtaining a magnetic disc. 
EXAMPLE 3 
Fifty parts by weight of gamma-Fe.sub.2 O.sub.3, about 10 parts by weight 
of alumina powder, about 26 parts by weight of epoxy resin (Epikote 1007, 
by Shell Chemical Co., Ltd.), about 12 parts by weight of urea resin 
(Melan 11), about 1 part by weight of a curing accelerator (S475, i.e. 50% 
ethylene glycol monoethyl ether solution of morphonyl 
paratoluenesulfonate), about 1 part by weight of lecithin, and about 200 
parts by weight of a mixed solvent of toluene and butanol were mixed in a 
ball mill for about 24 hours to give a magnetic paint. The paint was 
applied onto a non-magnetic disc substrate and dried, followed by keeping 
at 150.degree. C. for 30 minutes for baking and curing and subsequently 
polishing to obtain a magnetic disc. 
EXAMPLE 4 
Fifty parts by weight of magnetic gamma-Fe.sub.2 O.sub.3 powder, about 10 
parts by weight of alumina powder, about 19 parts by weight of epoxy resin 
(Epikote 1001, by Shell Chemical Co., Ltd.), about 20 parts by weight of 
urea resin (Beckamin G-1850), about 1 part by weight of lecithin, and 
about 200 parts by weight of a mixed solvent of toluene and butanol were 
used in the same manner as in Example, thereby obtaining a magnetic disc. 
COMATIVE EXAMPLE 1 
The general procedure of Example 1 was repeated using phenolic resin 
(PC-25, by Sumitomo Chemical Ind. Co., Ltd.) instead of the urea resin, 
thereby obtaining a magnetic disc. 
COMATIVE EXAMPLE 2 
The general procedure of Example 2 was repeated using phenolic resin 
(PC-25, by Sumitomo Chemical Ind. Co., Ltd.) instead of the urea, thereby 
obtaining a magnetic disc. 
The magnetic discs obtained in the examples and comparative examples were 
subjected to measurements of film characteristics and magnetic 
characteristics. The results are shown in Table below. 
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Film Characteristics 
Sur- Number 
face of 
Surface Rough- Surface Ad- Magnetic Characteristics 
hardness 
ness*2 Defects hesive- 
Hc Br 
*1 (.mu.m) *3 ness*4 (Oe) (Gauss) 
Rs 
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Ex. 1 6H 0.010 8 100/100 
420 970 0.83 
Ex. 2 6H 0.010 7 100/100 
421 970 0.83 
Ex. 3 6H 0.008 6 100/100 
420 970 0.83 
Ex. 4 6H 0.009 7 100/100 
418 990 0.85 
Com. 6H 0.014 20 100/100 
421 920 0.80 
Ex. 1 
Com. 3H 0.029 280 42/100 
422 900 0.80 
Ex. 2 
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*1 The surface hardness is expressed in terms of pencil hardness. 
*2 The surface roughness means an average roughness on the center line 
(Ra). 
*3 The number of surface defects means a number of errors on one surface. 
*4 The adhesiveness was determined by a cross cut test in which the 
magnetic layer was cut to have one hundred squares each with 1 mm .times. 
1 mm in dimension, on which a cellophane adhesive tape was attached and 
then peeled off to determine the number of the squares not peeled off. 
The above results revealed that the magnetic recording materials of the 
invention are superior to the materials of Comparative Examples with 
respect to the magnetic characteristics, i.e. coersive force, Hc, residual 
flux density, Br, and squareness ratio, Rs. Moreover, this is true of the 
film characteristics and particularly the number of surface defects.