Magnetic recording medium

Disclosed herein is a magnetic recording medium, (1) in which a magnetic layer is formed by coating a magnetic paint containing a magnetic powder and a binder as its main components on a non-magnetic base material; (2) which is featured by containing as said binder a polymer having an alkenyl phenol as one of the components of the polymer; and (3) which has markedly improved effects in the dispersibility of the magnetic powder in said medium and in the abrasion resistance and flexibility of the magnetic layer.

BACKGROUND OF THE INVENTION 
This invention relates to a magnetic recording medium in which a magnetic 
layer is formed by coating on a non-magnetic base material a magnetic 
paint containing a magnetic powder and a binder as its main components. 
More specifically, this invention relates to a magnetic recording medium 
having high dispersibility of the magnetic powder and improved abrasion 
resistance and flexibility of the magnetic layer. 
Recently, despite advances in the field of magnetic recording media, there 
is in particular, a strong demand for both high-density recording and 
short wave-length recording and also for improved strength of the magnetic 
layer. To achieve these recording goals, it is necessary to improve the 
degree of filling of the magnetic powder, i.e., the dispersibility of the 
magnetic powder. On the other hand, the improved strength of the magnetic 
layer is dependent on the kind of resin used as the binder. 
There have been proposed a wide variety of binders with a view to improving 
the dispersibility of the magnetic material and the strength of the 
magnetic layer. For example, compositions comprising a thermosetting resin 
such as phenolic resin, epoxy resin and melamine resin or a thermoplastic 
resin such as polyvinyl butyral have heretofore been proposed. When a 
thermosetting resin such as a phenolic resin is used as the binder, the 
dispersibility and the abrasion resistance are likely to be improved. 
However, phenolic resins are still inadequate in that they are generally 
low in molecular weight; are unavoidably admixed with low molecular weight 
components which are partially responsible for the generation of drop-out 
in recording play-back; are inferior in flexibility after curing; and are 
almost insoluble in solvents of low polarity which are widely used in 
magnetic paints. 
BRIEF SUMMARY OF THE INVENTION 
The present invention proposes a method of overcoming the aforesaid 
problems. Specifically, the present invention provides a novel binder 
comprising a polymer which contains an alkenyl phenol as one of its 
components, and further provides a novel magnetic recording medium using 
said binder. 
The present invention specifically provides a magnetic recording medium by 
coating a magnetic paint containing a magnetic powder and a binder as its 
main components on a non-magnetic base material, said binder comprising a 
polymer containing an alkenyl phenol (hereinafter referred to as a PG 
polymer) as one of its components. 
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The PG polymer which can be used in the present invention is a polymer 
obtained by copolymerizing an alkenyl phenol and at least one monomer 
polymerizable therewith. 
Specific examples of the alkenyl phenol include vinyl phenol, n-propenyl 
phenol, isopropenyl phenol, n-butenyl phenol and sec-butenyl phenol 
As monomers polymerizable with alkenyl phenol, there may generally be 
mentioned aromatic vinyl monomers and aliphatic vinyl monomers. The 
aromatic vinyl monomers include, for example, styrene, vinyl toluene, 
vinyl xylene, chlorostyrene, dichlorostyrene, .alpha.-methylstyrene and 
p-methylstyrene. The aliphatic vinyl monomers include, for example, 
acrylonitrile, methacrylonitrile, acrylic acid, methacrylic acid, acrylic 
esters such as methyl acrylate, ethyl acrylate, propyl acrylate, butyl 
acrylate, hexyl acrylate and hydroxyethyl acrylate, methacrylic esters 
such as methyl methacrylate, ethyl methacrylate, butyl methacrylate and 
hydroxyethyl methacrylate, acrylamide, methacrylamide, isoprene and 
butadiene. 
The PG polymer of the present invention can be prepared with ease by 
conventional polymerization such as radical polymerization, ionic 
polymerization and charge-transfer polymerization. In these 
polymerizations, such processes as emulsion polymerization, suspension 
polymerization and solution polymerization can be applied. 
The molecular weight of the PG polymer can readily be controlled, and is 
preferably in the range of 5,000-200,000 or more preferably in the range 
of 10,000-100,000 in practice. 
The PG polymer should contain at least 2% by weight or preferably at least 
5% by weight of an alkenyl phenol. If the content is less than 2% by 
weight, the effect of improving the dispersibility of the magnetic powder, 
one of the features of the present invention, is almost impossible to 
obtain. The PG polymer, can be prepared as a binary copolymer comprising 
an alkenyl phenol and one of the above-described polymerizable monomers, 
or as a multicomponent copolymer comprising at least one alkenyl phenol 
and at least two of the above-described polymerizable monomers. Thus, a 
variety of combinations are possible depending on the purpose. A PG 
polymer which is in the form of a viscous liquid, semi-solid or solid can 
be obtained by an appropriate combination of an alkenyl phenol or alkenyl 
phenols and one or more of said monomers. 
The PG polymer thus obtained is a polymer comprising, in combination, a 
phenol group-containing portion which corresponds to a thermosetting resin 
and a thermoplastic or elastic portion derived from a copolymerizable 
vinyl monomer or monomers. The drawbacks described above can be remedied 
by containing the PG polymer in the binder. In other words, by containing 
the PG polymer in the binder, it becomes possible to improve the 
dispersibility of the magnetic powder and the abrasion resistance of the 
magnetic layer. Further, the containing of the PG polymer in the binder 
enables generation of drop-out due to the presence of low molecular 
components to be prevented; flexibility, owing to the inclusion of a 
thermoplastic or elastic portion in the molecule, to be imparted; 
solubility in solvents of low-polarity to be improved; and compatibility 
with conventional binders to be increased. 
As the magnetic powder in the magnetic layer, there may be used 
.gamma.-Fe.sub.2 O.sub.3, Fe.sub.3 O.sub.4, Co-doped oxides, oxides 
surface-treated with a Cobalt compound, ferromagnetic CrO.sub.2 and iron, 
or a magnetic alloy powder, such as, iron-cobalt or iron-cobalt-nickel 
alone or in combination. 
In the present invention, conventional thermoplastic or thermosetting 
resins may be used with the PG polymer. In particular, thermosetting 
resins are so compatible with the PG polymer that the aforesaid 
disadvantages in the blend application with conventional thermoplastic 
resins can be overcome. Conventional thermoplastic resins capable of being 
used with the PG polymer include vinyl chloride-vinyl acetate copolymers, 
vinyl chloride-vinylidene chloride copolymers, acrylic ester-acrylonitrile 
copolymers, vinylidene chloride-acrylonitrile copolymers, polyvinyl 
butyral resins, polyamide resins, polyester resins and cellulose 
derivatives, which may be used alone or in combination. Specific examples 
of thermosetting resins capable of being used with the PG polymer include 
phenolic resins, thermosetting polyurethane resins, amino rsins, silicone 
resins and isocyanate prepolymer mixtures, which may be used alone or in 
combination. 
The content of the PG polymer in the whole binder is preferably in the 
range of 5-90% by weight. If the content is less than 5% by weight, it is 
almost impossible to improve the abrasion resistance. On the other hand, 
if the content is over 90% by weight, running of the recording medium is 
liable to be unstable. 
The magnetic layer may contain, if desired, additives such as dispersing 
agents, lubricants, abrasives and antistatic agents. 
Specific examples of dispersing agents include fatty acids having 12-18 
carbon atoms such as capric acid, lauric acid, stearic acid, linolic acid 
and oleic acid, salts of these fatty acids with alkali metals or alkaline 
earth metals and lecithins. The dispersing agent may be added to the 
binder in an amount of 1-15 parts by weight per 100 parts by weight of the 
binder. 
Useful lubricants for the above purpose include solid lubricants such as 
molybdenum disulfide, tungsten disulfide, silicone oils and fatty acid 
esters, which may be added to the binder in an amount of 0.1-15 parts by 
weight per 100 parts by weight of the binder. 
As abrasives, there may used silicon carbide, chromium oxide, alumina and 
corundum, each preferably having a particle diameter of less than 2 .mu.m 
on average. 
Useful antistatic agents for the above purpose include natural surfactants 
such as carbon black and saponin, nonionic surfactants derived from 
alkylene oxides or glycerol, cationic surfactants such as higher 
alkylamines, ammonium salts and heterocyclic compounds, anionic 
surfactants containing acidic radicals such as carboxylic acid, sulfonic 
acid, phosphoric acid, sulfuric acid ester and phosphoric acid ester, and 
amphoteric surfactnats such as amino acids, aminosulfonic acids, and 
sulfuric or phosphoric acid esters of aminoalcohols. These surfactants may 
be used alone or in combination. 
As the non-magnetic base material, there may be used polyesters such as 
polyethylene terephthalate and polyethylene 2, 6 - naphthalate, 
polyolefins such as polypropylene, cellulose derivatives such as cellulose 
triacetate and cellulose diacetate, polycarbonates, polyamides and 
polyimides. 
In the practice of the magnetic recording medium according to the present 
invention, there are a variety of embodiments such as the application of 
top-coat to impart lubricating properties or of back-layer for antistatic 
purposes.

The present invention is further illustrated by the following examples. It 
should be understood that these examples are given only for the purpose of 
illustration of the present invention and do not limit its scope. In the 
examples, all parts and percentages are by weight unless otherwise 
specified. 
Preparation of PG polymer 
EXAMPLE 1 
One hundred parts of p-isopropenyl phenol, 130 parts of acrylonitrile, 770 
parts of butyl acrylate and, as a solvent, 670 parts of ethyl acetate were 
fed into a reactor. Six parts of azobisisobutyronitrile was added thereto 
in two fractions three hours apart as a polymerization initiator. The 
resulting mixture was subjected to polymerization for ten hours under the 
reflux of ethyl acetate. Thereafter unreacted monomers and the solvent 
were removed to obtain 955 parts of a PG polymer (hereinafter referred to 
as PG polymer A). The content of p-isopropenyl phenol in the polymer was 
10%, measured by .sup.13 C-NMR analysis. The average molecular weight of 
the polymer was about 19,000. 
EXAMPLE 2 
Three hundred parts of p-isopropenyl phenol, 100 parts of acrylonitrile, 
500 parts of butyl acrylate, 100 parts of butyl methacrylate and, as a 
solvent, 700 parts of methyl ethyl ketone were fed into a reactor. 
Thereafter, 960 parts of a PG polymer (referred to as PG polymer B) was 
obtained in the same manner as in Example 1. The content of p-isopropenyl 
phenol in the polymer was 30% and the average molecular weight of the 
polymer was about 22,000. 
EXAMPLE 3 
Fifty parts of p-isopropenyl phenol, 50 parts of acrylonitrile, 900 parts 
of butyl acrylate and, as a solvent, 650 parts of methyl ethyl ketone were 
fed into a reactor. Thereafter, 965 parts of a PG polymer (called PG 
polymer C) was obtained in the same manner as in Example 1. The polymer 
contained 5% of p-isopropenyl phenol and had an average molecular weight 
of about 21,000. 
Preparation of Magnetic Recording Medium (Magnetic Recording Tape) 
EXAMPLE 4 
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Co-coated .gamma.-Fe.sub.2 O.sub.3 (major axis diameter 0.6 
100.m, parts 
coercive force Hc 640 Oe) 
binder (a mixture of 75 parts of PG polymer A 
100 parts 
and 25 parts of a novolac-type phenolic resin) 
lecithin 2 parts 
Cr.sub.2 O.sub.3 2 parts 
carbon black 2 parts 
fatty acid ester 1 part 
methyl ethyl ketone-toluene-cyclohexanone 
150 parts 
mixed solvent (1+1+1) 
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After kneading the foregoing starting materials in a ball mill for 18 hours 
in the proportions described above, 2.5 parts of an isocyanate compound 
(trade name Desmodur L; a product of Farbenfabriken Bayer A.G.) was added 
thereto and the resulting mixture was subjected to high shear-rate 
dispersion for 35 minutes to obtain a magnetic paint. The paint was coated 
on a polyethylene terephthalate film having a thickness of 160 .mu.m in 
such a manner that its thickness became 4 .mu.m after drying. The 
resulting film was subjected to magnetic orientation and thereafter dried 
and wound. This was calendered and cut into tapes having a width of 0.5 
inch (12.7 mm) to obtain a magnetic tape (hereafter called Tape No. 1). 
EXAMPLE 5 
The procedures of Example 4 were repeated to obtain a magnetic tape, except 
that the binder was composed of a mixture of 70 parts of PG polymer B and 
30 parts of a conventional thermosetting polyurethane resin. This tape was 
called Tape No. 2. 
EXAMPLE 6 
The procedures of Example 4 were repeated to obtain a magnetic tape, except 
that the binder was composed of a mixture comprising 35 parts of PG 
polymer C, 15 parts of a novolac-type phenolic resin and 50 parts of a 
vinyl chloride-vinyl acetate-vinyl alcohol copolymer (a trade name VAGH; a 
product of Union Carbide Corporation. This tape was called Tape No. 3. 
EXAMPLE 7 
A magnetic tape was prepared in the same manner as in Example 4, except 
that the binder was composed of a mixture comprising 40 parts of PG 
polymer C, 20 parts of a novolac-type phenolic resin and 40 parts of a 
thermosetting polyurethane resin. This tape was called Tape No. 4. 
EXAMPLE 8 
A magnetic tape was prepared in the same manner as in Example 4, except 
that the binder was composed of a mixture of 50 parts of PG polymer B and 
50 parts of VAGH. This tape was called Tape No. 5. 
EXAMPLE 9 
A magnetic tape was prepared in the same manner as in Example 4, except 
that the binder was formed of a mixture comprising 55 parts of PG polymer 
A, 15 parts of a novolac-type phenolic resin and 30 parts of a 
thermosetting polyurethane resin. This tape was called Tape No. 6. 
COMATIVE EXAMPLE 1 
A magnetic tape was obtained in the same manner as in Example 4 except for 
using 100 parts of a novolac-type phenolic resin as the binder. This tape 
was called Tape No. 7. 
COMATIVE EXAMPLE 2 
A magnetic tape was obtained in the same manner as in Example 4 except that 
the binder was composed of 50 parts of a novolac-type phenolic resin and 
50 parts of polyvinyl butyral. This tape was called Tape No. 8. 
COMATIVE EXAMPLE 3 
A magnetic tape was prepared in the same manner as in Example 4, except 
that the binder was composed of 50 parts of a novolac-type phenolic resin 
and 50 parts of polyacrylonitrile. This tape was called Tape No. 9. 
The characteristics of these magnetic tapes are shown in Table 1. 
In Table 1, the surface gloss was determined by a glossmeter such that a 
light was reflected on the surface of the magnetic layer at an angle of 
incidence of 75.degree. and the amount of reflection was measured. The 
falling-off of magnetic powder was determined by measuring the amount of 
powder adhering to the running parts of a home VTR after each tape was run 
for a one hundred hours and is shown as the amount compared with that of 
Tape No. 1 as 1.0. The durability still in minutes was determined by the 
time duration in which the performance of recording play-back was lowered 
to 80% level as compared to its initial value in a home VTR. The degree of 
scratching of the magnetic layer was evaluated visually after the tape was 
run repeatedly one hundred times in a home VTR. 
TABLE 1 
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Tape Surface gloss 
Powder falling- 
Durability still 
Scratch- 
No. (%) off (minutes) ing 
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1 90 1.0 1100 o 
2 85 1.0 1200 o 
3 85 1.5 900 o 
4 75 2.0 1100 o 
5 80 1.0 1100 o 
6 80 1.5 1100 o 
7 70 1.5 400 x 
8 80 2.0 400 x 
9 65 2.5 800 o 
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