Magnetic recording medium comprising a specific type of copolymer binder

A magnetic recording medium comprising a non-magnetic support, and a magnetic recording layer formed on the support. The layer is made of a composition which comprises magnetic powder dispersed in a resin binder comprising a thermosetting resin and a copolymer of maleic anhydride and an olefinically unsaturated monomer. The monomer is selected from alkyl esters of acrylic and methacrylic acids, alkenes, vinyl chloride, styrene and mixtures thereof.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to magnetic recording mediums such as magnetic 
tapes, floppy disks, hard disks and allied articles and more particularly, 
to magnetic recording mediums particularly suitable for use in magnetic 
disk apparatus such as computers. 
2. Description of the Prior Art 
Magnetic recording mediums which are used in information recording and 
reproducing apparatus such as, for example, magnetic disk apparatus, are 
of the type obtained by applying a magnetic paint of magnetic powder, 
dispersed in a solvent dissolving suitable resin binders, onto a 
non-magnetic substrate, and baking and curing the applied paint. 
As information recording and reproducing apparatus are now becoming high in 
performance, there is an increasing demand for magnetic recording mediums 
of higher density. In order to meet the demand, there have been proposed a 
number of magnetic recording mediums including, for example, a medium 
whose magnetic recording layer has a smaller thickness than was used for 
ordinary purposes, a medium having a magnetic recording layer which is 
made of a completely uniform dispersion of magnetic powder, and magnetic 
recording mediums using magnetic powders having high coercive force, such 
as magnetic Co-modified gamma-Fe.sub.2 O.sub.3 powder, magnetic Cr.sub.2 
O.sub.3 powder, and magnetic metallic powders. 
When the magnetic recording layer is made thin, the amount of magnetic 
powder is also reduced. It is important not to reduce the amount of 
magnetic powder, but to reduce the amount of other ingredients such as, 
for example, a binder. If, however, the amount of a binder is reduced, 
necessary properties of the magnetic recording medium, e.g. durability, 
will be sacrificed. 
On the other hand, magnetic powders with high coercive force are usually 
available as small-size particles and have a large acicular ratio, so that 
they have a large specific surface area. This is disadvantageous in that 
such powders are less likely to disperse than conventional gamma-Fe.sub.2 
O.sub.3 powder. It undesirably takes a longer time before the magnetic 
powders are dispersed to obtain uniform magnetic paints. The long 
dispersion operation results in a lowering of coercive force due to the 
breakage of the magnetic powder. 
SUMMARY OF THE INVENTION 
It is an object of the invention to provide a magnetic recording medium 
which makes use of a specific type of copolymer resin as part of a binder 
for dispersing magnetic powder therein whereby any magnetic powders are 
well dispersed in a magnetic paint without kneading for a long time. 
It is another object of the invention to provide a high density magnetic 
recording medium comprising a magnetic recording layer which has good 
mechanical strength even when the layer is made thin. 
It is a further object of the invention to provide a high density magnetic 
recording medium which ensures good magnetic characteristics because of 
the good dispersion of magnetic powder. 
The present invention is characterized in that there is used, as a binder 
of a magnetic recording layer, a copolymer of maleic anhydride and an 
olefinically unsaturated monomer copolymerizable with maleic anhydride in 
combination with a thermosetting resin. The copolymer permits good 
dispersion of any type of magnetic powder in a resin binder. 
DETAILED DESCRIPTION AND EMBODIMENTS OF THE INVENTION 
The copolymers useful in the present invention are produced from maleic 
anhydride and olefinically unsaturated monomers by any known procedures. 
The olefinically unsaturated monomers include alkenes having from 2 to 18 
carbon atoms, preferably 2 to 8 carbon atoms, e.g. ethylene, propylene, 
butene, pentene, hexene, heptene, octene and the like; alkyl esters of 
acrylic and/or methacrylic acid in which the alkyl moiety has from 1 to 
18, preferably from 1 to 8 carbon atoms, e.g. methyl, ethyl, propyl, 
butyl, pentyl, hexyl, heptyl, octyl, nonyl, decyl, and the like; and vinyl 
chloride and styrene. These olefinically unsaturated monomers may be used 
singly or in combination, e.g. acrylates may be used singly or in 
combination thereof or with methacrylates, alkenes, styrene and/or vinyl 
chloride. 
In view of compatibility with other resins, the copolymers have generally a 
molecular weight of from about 500 to 50,000, preferably from about 1000 
to 10,000. The content of maleic anhydride in the copolymer is in the 
range of from about 5 to 80 mole%, preferably from 20 to 50 mole%. Within 
the above range, dispersability of magnetic powder in a resin binder 
comprising the copolymer is significantly improved. 
In view of surface characteristics of a magnetic recording medium using a 
binder of the copolymer mentioned above, the copolymer should be used in 
combination with thermosetting resins ordinarily used for these purposes. 
The resins used in combination are thermosetting resins having functional 
groups which are able to undergo a condensation reaction with the reactive 
group of the maleic anhydride units in the copolymer. The functional 
groups may be a glycidyl group in epoxy resins, methylol and/or hydroxyl 
groups in phenolic, urea, polybutyral resins and the like. Specific and 
preferable examples of the thermosetting resins include epoxy resins, 
phenolic resins, melamine resins, urea resins, acrylic resins, butyral 
resins, and mixtures thereof. Of these, the copolymer is preferably used 
in combination with epoxy resins and more preferably, with mixtures of 
epoxy resins and phenolic resins, epoxy resins and melamine resins, epoxy 
resins and acrylic resins, and epoxy resins, phenolic resins and acrylic 
resins or polyvinyl butyral. Such mixtures should be predominantly made of 
epoxy resins. In practice, the copolymer is used in an amount of from 1 to 
50 wt%, preferably from 5 to 20 wt%, of the total binder. 
The binder comprising the maleic anhydride copolymer is used in an amount 
of from 30 to 200 parts by weight, preferably from 50 to 120 parts by 
weight, per 100 parts of magnetic powder. 
The magnetic powders used in the present invention may be any magnetic 
metal powders ordinarily used in this art and include, for example, 
ferromagnetic iron oxides such as gamma-Fe.sub.2 O.sub.3 and Fe.sub.3 
O.sub.4 with or without being deposited with Co, Ni, Mn and the like, 
ferromagnetic metals such as Co, Ni, Fe and alloys thereof such as Fe--Co, 
Fe--Ni, Co--Ni, Fe--Co--Ni, and other ferromagnetic materials such as 
CrO.sub.2, barium ferrite and the like. 
Even though these magnetic powders have a very small size, they are readily 
dispersed in the binder resin comprising the maleic anhydride-based 
copolymer. The size of the powder is generally in the range of from 0.05 
to 2 .mu.m. 
For the manufacture of a magnetic recording medium, a magnetic powder is 
dispersed in a resin binder comprising the copolymer discussed before and 
a solvent for the binder by the use of a suitable mixing or kneading 
means. The resulting magnetic paint is coated onto a non-magnetic support 
at least on one side thereof, and is dried and cured under conditions of a 
temperature of from 50.degree. to 250.degree. C. for a time sufficient for 
the curing and/or the condensation reaction between the copolymer and a 
thermosetting resin. The coating may be effected by any known techniques 
such as spin coating, air knife coating, blade coating, dip coating, 
various roll coatings, spray coating and the like. Because of the use of 
the copolymer, the dry thickness of the recording layer may be reduced 
even to about 0.5 .mu.m though it is generally in the range of from 1 to 
10 .mu.m. 
Non-magnetic supports may be disks, films, foils or sheets of a variety of 
materials including, for example, synthetic or semi-synthetic resins such 
as polyesters, polyolefins, cellulose derivatives and the like, metals 
such as aluminum, magnesium, copper and the like, glasses and ceramics. 
Aside from magnetic powders, various additives may be added to the magnetic 
paint. Such additives include lubricants, dispersants, stabilizers, 
plasticizers, and the like. 
Use of the copolymers of maleic anhydride and olefinically unsaturated 
monomers has a number of advantages. 
(1) Dispersability of magnetic powder is significantly improved, and 
breakage of the magnetic powder during the dispersion operation is avoided 
to an extent, thus preventing lowerings of coercive force and squareness 
ratio of the resulting magnetic recording medium. 
(2) Milling or kneading time for a magnetic paint can be reduced greatly: 
the time is reduced to half the time required for conventional magnetic 
paints for equivalent surface characteristics of magnetic recording 
medium. This leads to a great reduction of manufacturing time and cost. 
(3) Not only high mechanical strength, but also good adherence, wear 
resistance and solvent resistance of a magnetic recording layer are 
obtained because of the crosslinkage of thermosetting resins with the 
copolymer as explained before. With magnetic disks, the wear resistance is 
evaluated by the number of contact-start-stop (CSS) cycles and the 
specification limit prescribed in TC97/SC10N228 of ISO is over 10,000. The 
magnetic disk obtained according to the invention has a value much larger 
than the limit value. The drop of magnetic powder is rarely experienced in 
the magnetic recording medium of the invention. In addition, the magnetic 
recording layer comprising the maleic anhydride-base copolymer is very 
resistant to solvents such as alcohols which are used for washing of the 
magnetic disk on the surface thereof, and is very durable.

The present invention is more particularly described by way of examples, in 
which parts are by weight. 
EXAMPLE 1 
One hundred parts of Co-deposited gamma-Fe.sub.2 O.sub.3 magnetic powder, 
10 parts of alpha-Al.sub.2 O.sub.3 powder, 45 parts of an epoxy resin 
(Epikote 1007, by Shell Petrochem. Inc.), 20 parts of a phenolic resin 
(Sumirac PC-25, Sumitomo Bakelite Co., Ltd.), 10 parts of butyl 
acrylate/maleic anhydride alternating copolymer having a value by mole of 
butyl acrylate/maleic anhydride of 50/50 and a weight average molecular 
weight of about 5000, and 620 parts of a mixed solvent of toluene and 
ethylene glycol monobutyl ether were placed in a ball mill and kneaded for 
24 hours to obtain a magnetic paint. 
The magnetic paint was applied onto an aluminum alloy substrate, followed 
baking and curing at 200.degree. C. for 1 hour and polishing as usual to 
obtain a magnetic disk. 
EXAMPLE 2 
The general procedure of Example 1 was repeated using a kneading time of 12 
hours, thereby obtaining a magnetic disk. 
EXAMPLE 3 
One hundred parts of Co-deposited gamma-Fe.sub.2 O.sub.3 magnetic powder, 
10 parts of alpha-Al.sub.2 O.sub.3 powder, 35 parts of an epoxy resin 
(Epikote 1001, by Shell Petrochem. Inc.), 25 parts of an acrylic resin 
(Dyanal SE-5437, by Mitsubishi Rayon Co., Ltd.), 15 parts of ethyl 
acrylate/maleic anhydride random copolymer, in which maleic anhydride 
molecules are not directly bonded to each other and which has a value by 
mole of ethyl acrylate/maleic anhydride of 60/40 and a weight average 
molecular weight of about 2000, and 620 parts of a mixed solvent of 
toluene and ethylene glycol monobutyl ether were used to make a magnetic 
disc in the same manner as in Example 1. 
Comparative Example 
One hundred parts of Co-deposited gamma-Fe.sub.2 O.sub.3, 10 parts of 
alpha-Al.sub.2 O.sub.3, 50 parts of Epikote 1007, 25 parts of Sumirac 
PC-25, and 620 parts of a mixed solvent of toluene and ethylene glycol 
monobutyl ether were used to make a magnetic disc in the same manner as in 
Example 1. 
The magnetic disks obtained in Examples 1 through 3 and Comparative Example 
were subjected to measurement of magnetic characteristics, i.e. coercive 
force and squareness ratio, surface defects, and were resistance. The 
results are shown in Table 1. 
TABLE 1 
______________________________________ 
Wear 
Coercive Squareness 
Surface Resistance 
Force (Oe) Ratio Defects (CSS Cycles) 
______________________________________ 
Example 
1 680 0.86 0 &gt;30000 
2 685 0.88 2 &gt;30000 
3 690 0.89 0 &gt;30000 
Com. Ex. 
660 0.80 8 18000 
______________________________________ 
As will be apparent from the above results, the magnetic disks of the 
present invention have better magnetic characteristics than the 
comparative disk. This is considered due to good dispersability of the 
magnetic powder by the presence of the copolymer, by which the magnetic 
powder suffers less breakage at the time of kneading. 
The surface defects are also much reduced and the wear resistance is so 
great that the medium may be used semipermanently. 
Moreover, even when the kneading time is reduced to half the conventionally 
required time, the resulting disk is excellent as particularly seen from 
the results of Example 2. 
EXAMPLE 4 
The general procedure of Example 1 was repeated using butyl 
methacrylate/maleic anhydride alternating copolymer having a butyl 
methacrylate/maleic anhydride value by mole of 50/50 and a weight average 
molecular weight of about 5000, thereby obtaining a magnetic disk. 
EXAMPLE 5 
The general procedure of Example 4 was repeated except that the kneading 
time in the ball mill was reduced to half, i.e. 12 hours, thereby 
obtaining a magnetic disk. 
EXAMPLE 6 
The general procedure of Example 3 was repeated using methyl 
methacrylate/maleic anhydride random copolymer having a methyl 
methacrylate/maleic anhydride value by mole of 60/40 and a weight averge 
molecular weight of about 2000, thereby obtaining a magnetic disc. 
EXAMPLES 7 THROUGH 9 
The general procedure of Example 1 was repeated using pentene/maleic 
anhydride alternating copolymer having a pentene/maleic anhydride value by 
mole of 50/50 and a weight average molecular weight of about 5000 (Example 
7), vinyl chloride/maleic anhydride alternating copolymer having a vinyl 
chloride/maleic anhydride value by mole of 50/50 and a weight average 
molecular weight of about 5000 (Example 8), and styrene/maleic anhydride 
alternating copolymer having styrene/maleic anhydride value by mole of 
50/50 and a weight average molecular weight of about 5000, thereby 
obtaining magnetic disks. 
EXAMPLES 10-12 
The general procedure of Examples 7 through 9 was repeated except that the 
kneading time in the ball mill was reduced to half, i.e. 12 hours, thereby 
obtaining magnetic disks. 
EXAMPLES 13-15 
The general procedure of Example 3 was repeated using ethylene/maleic 
anhydride random copolymer having an ethylene/maleic anhydride value by 
mole of 60/40 and a weight average molecular weight of about 2000 (Example 
13), vinyl chloride/maleic anhydride random copolymer having a vinyl 
chloride/maleic anhydride value by mole of 60/40 and a weight average 
molecular weight of about 2000 (Example 14), and styrene/maleic anhydride 
random copolymer having a sytrene/maleic anhydride value by mole of 60/40 
and a weight average molecular weight of about 2000 (Example 15), thereby 
obtaining magnetic disks. 
The magnetic disks obtained in Examples 4 through 15 were subjected to 
measurement in the same manner as in Examples 1 through 3. The results are 
shown in Table 2 below. 
TABLE 2 
______________________________________ 
Wear 
Coercive Squareness 
Surface Resistance 
Force (Oe) Ratio Defects (CSS Cycles) 
______________________________________ 
Example 
4 680 0.86 0 &gt;30000 
5 685 0.87 3 &gt;30000 
6 690 0.89 0 &gt;30000 
7 680 0.86 0 &gt;30000 
8 675 0.87 0 &gt;30000 
9 680 0.86 0 &gt;30000 
10 690 0.88 1 &gt;30000 
11 680 0.88 2 &gt;30000 
12 685 0.86 3 &gt;30000 
13 685 0.87 1 &gt;30000 
14 680 0.87 1 &gt;30000 
15 685 0.87 1 &gt;30000 
Com. Ex. 
660 0.80 8 18000 
______________________________________ 
When alkyl methacrylates, alkenes, vinyl chloride and styrene are used as 
the olefinically unsaturated monomer, similar results are obtained as in 
Examples 1 through 3.