Magnetic recording medium having binder cured by electron beam radiation

A magnetic recording medium comprising a support having coated thereon a magnetic layer is disclosed. The magnetic layer is comprised of a binder composition having ferromagnetic particles dispersed therein. The binder composition is cured by an electron beam and comprises a copolymer, a urethane resin and a compound having two or more acryloyl groups or methacryloyl groups. The copolymer may be a vinyl chloride-vinyl acetate-vinyl alcohol copolymer having a vinyl alcohol content of 1 to 15 wt % or a vinyl chloride-vinyl acetate-maleic acid copolymer having a maleic acid content of 1 to 10 wt %. The recording medium has excellent electromagnetic properties and the magnetic layer has excellent dynamic properties.

FIELD OF THE INVENTION 
The present invention relates to magnetic recording media such as video 
tapes, audio tapes or computer tapes. More specifically, it relates to a 
magnetic recording medium having a magnetic layer cured by an electron 
beam. 
BACKGROUND OF THE INVENTION 
Binders which have been commonly used for magnetic recording media include 
thermoplastic resins such as vinyl chloride-vinyl acetate resins, vinyl 
chloride-vinylidene chloride resins, cellulose resins, acetal resins, 
urethane resins and acrylonitrile-butadiene resins used alone or in 
combination. However, with such binders the magnetic layer generally has 
poor wear resistance and the tape guiding systems which contact with 
magnetic tapes are stained during tape running. 
Various improvements have been proposed where a thermosetting resin such as 
melamine resins or urea resins is used as a binder, or a binder which can 
be cross-linked by a chemical reaction such as an isocyanate compound or 
an epoxy compound is added to the above-described theremoplastic resins. 
However, disadvantages occur when using such a cross-linking type binder. 
Firstly, the storage stability of the resin solution in which 
ferromagnetic particles are dispersed is low, i.e., the pot life is short 
and the physical properties of the resulting magnetic coating composition 
cannot be kept homogeneous, and accordingly, magnetic tapes cannot be 
uniformly prepared. Secondly, after coating and drying the magnetic 
coating composition, heat treatment is necessary to cure a coating layer 
thereby requiring a long period of time to manufacture magnetic recording 
media. 
In order to eliminate the above disadvantages, a method has been proposed 
where a magnetic recording medium is prepared by using a combination of an 
acrylate oligomer and an acrylate type monomer as a binder and curing the 
binder by electron beam radiation after drying, as disclosed in Japanese 
Patent Publication No. 12423/72, Japanese Patent Application (OPI) Nos. 
77433/75 and 25231/81 (the term "OPI" as used herein refers to a 
"published unexamined Japanese patent application"), U.S. Pat. Nos. 
3,871,908 and 4,368,239, German Pat. No. 2,100,037 and Dutch Pat. No. 
7,118,222. However, a magnetic recording medium having satisfactory 
physical properties and electric properties cannot be obtained in 
accordance with the above-described conventional methods. 
Recently, it has become necessary that a support for magnetic tapes be made 
thinner for long-time recording and that the physical strength of the 
magnetic layer be improved. Further, as video cassette recorders have 
become to have multi-functions, video tapes used therefor are required to 
withstand use under severe conditions. However, it has hitherto been 
impossible to produce a magnetic recording medium having physical strength 
which satisfies the above requirements. 
The inventors have attained the present invention as a result of extensive 
studies for improvements of conventional methods such as a method of using 
thermoplastic resins and thermosetting resins as disclosed in U.S. Pat. 
Nos. 3,634,137, 4,238,548 and 4,307,154, a method of adding a binder 
cross-linkable by a chemical reaction as disclosed in U.S. Pat. Nos. 
4,333,988, 4,154,895 and 4,049,871, or a method of using a curable binder 
cured by electron beams, as disclosed in U.S. Pat. Nos. 3,871,908, 
4,004,997 and 4,343,831 (Tsuji et al). 
SUMMARY OF THE INVENTION 
A primary object of the present invention is to provide a magnetic 
recording medium having an excellent electromagnetic properties. 
Another object of the present invention is to provide a magnetic recording 
medium having a magnetic layer excellent in dynamic properties. 
A further object of the present invention is to provide a magnetic 
recording medium having a magnetic coating composition excellent in 
storage stability and homogeneous physical properties. 
A still further object of the present invention is to provide a magnetic 
recording medium having excellent wear resistance. A yet further object of 
the present invention is to provide a magnetic recording medium prepared 
without heat treatment which is normally necessary for curing a magnetic 
coating layer. 
The above objects can be accomplished with a magnetic recording medium 
comprised of a binder composition containing (1) at least one of vinyl 
chloride-vinyl acetate-vinyl alcohol copolymer (content of vinyl alcohol: 
1 to 20 wt %) and vinyl chloride-vinyl acetate-maleic acid copolymer 
(content of maleic acid: 1 to 10 wt %), (2) a urethane resin, and (3) a 
compound having two or more acryloyl groups or methacryloyl groups, the 
amount of each of the copolymer and the urethane resin being 20 to 80 
parts by weight and the amount of said compound (3) being 50 to 400 parts 
by weight based on 100 parts by weight of the mixture of the copolymer and 
the urethane resin, and said binder composition being cured by electron 
beam radiation. 
DETAILED DESCRIPTION OF THE INVENTION 
The binder composition of the present invention is comprised of a vinyl 
chloride-vinyl acetate type copolymer, a urethane resin and a compound 
having two or more of acryloyl groups or methacryloyl groups in one 
molecule. The binder composition is irradiated with an electron beam 
whereby these resins function synergistically to provide a magentic 
recording medium excellent in electromagnetic properties, running 
properties and durability. 
The vinyl alcohol content gf vinyl chloride-vinyl acetate-vinyl alcohol 
copolymer is preferably 1 to 20 wt % and more preferably 5 to 15 wt %. 
The content of maleic acid of vinyl chloride-vinyl acetate-maleic acid 
copolymer is preferably 1 to 10 wt % and more preferably 2 to 8 wt %. 
Outside the above ranges, electromagnetic properties are markedly 
decreased. 
The vinyl chloride-vinyl acetate type copolymers used in the present 
invention preferably have a breaking strength more than about 5 
kg/mm.sup.2 and an average molecular weight more than about 15,000. 
Urethane resins include both polyether type and polyester type resins. The 
preferred range of the molecular weight is 5,000 to 100,000, more 
preferably 10,000 to 50,000. Outside the above range, dispersion of 
ferromagnetic particles is degraded. 
Examples of urethane resins which can be used in the present invention 
include polyester polyurethane resins obtained from a diisocyanate 
compound and a polyester polyol (prepared by reacting a dibasic acid such 
as phthalic acid, adipic acid, dimerized linoleic acid, maleic acid or the 
like with a polyhydric alcohol such as a glycol, for example, ethylene 
glycol, propylene glycol, butyrene glycol or diethylene glycol, 
trimethylolpropane, hexanetriol, glycerin, trimethylolethane, 
pentaerythritol or the like), and polyether polyurethanes obtained from a 
diisocyanate compound and a polyether polyol (prepared by reacting 
propylene oxide, ethylene oxide or the like with a polyhydric alcohol as 
described above). Examples of diisocyanate compounds useful for urethane 
formation are tolylene diisocyanate, 4,4'-diphenylenemethane diisocyanate, 
hexamethylene diisocyanate, metaxylylene diisocyanate, cyanate, cyclohexyl 
diisocyanate and the like. 
Examples of compound having two or more acryloyl groups or methacryloyl 
groups in the molecule include acrylates such as diethylene glycol 
diacrylate, triethylene glycol diacrylate, tetraethylene glycol 
diacrylate, trimethylolpropane triacrylate or pentaerythritol 
tetraacrylate; methacrylates such as diethylene glycol dimethacrylate, 
triethylene glycol trimethacrylate, tetraethylene glycol dimethacrylate, 
trimethylolpropane trimethacrylate or pentaerythritol tetramethacrylate; 
esters of polyols and acrylic acids or methacrylic acids; and compounds 
having two or more acryloyl groups or methacryloyl groups at the terminal 
groups of the main polymer chain or in the side chains thereof. 
The compounds having acryloyl group or methacryloyl group at the terminal 
groups of the main chain or in the side chains thereof are referred to in 
A. Vrancken, Fatipec Congress, 11, 19 (1972). For example, these compounds 
can be shown by the following structure. 
##STR1## 
wherein n is an integer. The polyester skeleton of the above compounds can 
be a polyurethane skeleton, an epoxy resin skeleton, a polyether skeleton 
or a polycarbonate skeleton or a mixture thereof. The molecular weight is 
preferably in the range of from 1,000 to 20,000, but is not particularly 
limited. 
Preferred examples of the compounds having two or more acryloyl groups or 
methacryloyl groups are diethylene glycol diacrylate, triethylene glycol 
diacrylate, tetraethylene glycol diacrylate, trimethylolpropane 
triacrylate, pentaerythritol tetraacrylate and the acrylate compound 
represented by the above formula. 
The above-described compounds may be used alone or as a mixture thereof. 
Further, a compound having one carbon-carbon unsaturated bond in its 
molecule which can be polymerized by electron beams can also be added to 
the composition of the present invention. 
Examples of compounds having one carbon-carbon unsaturated bond are acrylic 
acid, itaconic acid, methyl acrylate and homologues thereof such as alkyl 
acrylates, styrene and homologues thereof such as .alpha.-methylstyrene, 
.beta.-chlorostyrene, etc., acrylonitrile, acrylamide, vinyl acetate, 
vinyl propionate, N-vinylpyrrolidone and the like. Other examples include 
compounds disclosed in "Kankosei Jushi Data-shu (A List of Data on 
Photosensitive Resins)", published by Sogo Kagaku Kenkyusho, December 
1968, pp. 235-236. 
The mixing ratio of vinyl chloride-vinyl acetate type copolymer and 
urethane resin is preferably 20 to 90/80 to 10 by weight, more preferably 
40 to 80/60 to 20 by weight. Sufficient durability of the magnetic 
recording medium cannot be obtained if the copolymer or the urethane resin 
is less than the above ratio. The amount of the compound having two or 
more of acryloyl groups or methacryloyl groups in one molecule is 
preferably in the range of from 50 to 400 parts by weight, more preferably 
in the range of from 80 to 250 parts by weight, per 100 parts by weight of 
a mixture of the vinyl chloride-vinyl acetate type copolymer and the 
urethane resin. Use of the compound in an amount more than the upper limit 
is not preferred since the dose of electron beam required for 
polymerization becomes high. On the other hand, use of the compound in an 
amount less than the lower limit does not give rise to sufficient 
crosslinking which results in poor durability of the magnetic recording 
medium. 
Ferromagnetic powders used in the present invention include ferromagnetic 
iron oxide fine powders, Co-doped ferromagnetic iron oxide fine powders, 
ferromagnetic chromium dioxide fine powders, ferromagnetic alloy fine 
powders and barium ferrite. The acicular ratio of ferromagnetic iron oxide 
and chromium dioxide is about 2/1 to about 20/1, preferably more than 5/1. 
An average length of the ferromagnetic alloy powders is about 0.2 to about 
2.0 .mu.m. The ferromagnetic alloy fine powders generally have a metal 
content more than 75 wt %, with more than 80 wt % of the metal content 
being a ferromagnetic metal (i.e., Fe, Co, Ni, Fe-Co, Fe-Ni, Co-Ni, 
Fe-Co-Ni), and has a longer diameter of less than about 1.0 .mu.m. 
Examples of organic solvents which can be used for the magnetic coating 
composition include ketones such as acetone, methyl ethyl ketone, methyl 
isobutyl ketone, cyclohexanone; esters such as methyl acetate, ethyl 
acetate, butyl acetate, ethyl lactate, and monoethyl ether glycol acetate; 
glycol ethers such as diethyl ether, glycol dimethyl ether, glycol 
monoethyl ether and dioxane; aromatic hydrocarbons such as benzene, 
toluene and xylene; chlorinated hydrocarbons such as methylene chloride, 
ethylene chloride, carbon tetrachloride, chloroform, ethylene chlorohydrin 
and dichlorobenzene. The organic solvent can be used in an amount of about 
400 to about 1,500 parts by weight, preferably 600 to 1,200 parts by 
weight, per 100 parts by weight of the resin composition. 
Additives such as a lubricant, an abrasive, a dispersing agent, a rust 
preventing agent or an antistatic agent can be added to the magnetic 
coating composition of the present invention. Examples of lubricants are 
saturated or unsaturated higher fatty acids, fatty acid esters, higher 
fatty acid amides, higher alcohols, each containing 10 or more carbon 
atoms, preferably 12 or more carbon atoms, silicone oils, mineral oils, 
edible oils or fluoride type compounds. Examples of rust preventing agents 
are salicylanilide, tin oxide, mercury phenyloleate, copper naphthenate, 
zinc naphthenate, trichlorophenol, p-dinitrophenol, sorbic acid, butyl 
p-oxybenzoate, dihydroacetoacetic acid and the like. Some of these 
additives are disclosed in U.S. Pat. No. 4,135,016. These additives can be 
added when a coating composition is prepared, or can be coated or sprayed 
on a surface of a magentic layer with or without an organic solvent after 
drying, smoothing the magnetic layer or curing the magnetic layer by 
electron beam radiation. 
Materials for the support on which the magnetic coating composition is 
coated include polyesters such as polyethylene terephthalate or 
polyethylene-2,6-naphthalate; polyolefines such as polyethylene or 
polypropylene; cellulose derivatives such as cellulose triacetate; 
plastics such as polycarbonate, polyimide or polyamide imide; non-magnetic 
metals such as aluminum, copper, tin, zinc or non-magnetic alloys 
including the above metals; and a kind of paper or a paper coated or 
laminated with polyolefines. Preferred examples of the supports include 
polyethylene terephthalate, polyethylene-2,6-naphthalate and polyimide 
films. The thickness of these supports can be generally in the range of 
about 5 to about 100 .mu.m, preferably 7 to 20 .mu.m, but can vary widely 
over the above range depending upon the utility of magnetic recording 
media. 
The non-magnetic support may be in the shape of film, a tape, a sheet, a 
disc, a card or a drum, and various materials can be selected depending 
upon the above shapes. 
A backcoat can be provided on the support on the side opposite the magnetic 
layer for the purposes of preventing charging, transferring, wow flutter, 
improving the strength of the magnetic recording medium and for making the 
back side of the support a matted layer. 
A canning method, a double scanning method, a curtain beam method or a 
broad beam curtain method can be used for accelerating electron beams. 
The electron beam used has an accelerating voltage of 100 to 1,000 kv, 
preferably 150 to 300 kv. The absorption dose is 0.5 to 20 megarads, 
preferably 3 to 15 megarads. If the accelerating voltage is less than 100 
kv, the transmitted amount of energy is insufficient and if the 
accelerating voltage is more than 1,000 kv, the energy efficiency used for 
polymerization is lowered making the process uneconomical. If the 
absorption dose is less than 0.5 megarad, the curing reaction is 
insufficient to obtain a magnetic layer having a satisfactory mechanical 
strength and if the absorption dose is more than 20 megarads, the energy 
efficiency used for curing reaction is lowered or a radiated object 
generates heat and the support, particularly a plastic support, may be 
deformed.

The present invention is further illustrated in greater detail by the 
following examples and comparative examples, but the present invention is 
not limited thereto. In examples and comparative examples, all parts and 
ratios are by weight unless otherwise indicated. 
EXAMPLE I 
______________________________________ 
Fe.sub.2 O.sub.3 400 parts 
Vinyl chloride-vinyl acetate 
50 parts 
vinyl alcohol copolymer 
(Copolymerization ratio: 89:3:8) 
Urethane resin 15 parts 
(Condensation product of adipic acid, 
butane diol and tolylene diisocyanate) 
Ester acrylate oligomer 30 parts 
("Aronix M6100" manufactured by 
Toagosei Chemical Industry Co., Ltd.) 
Diethylene glycol diacrylate 
5 parts 
Lecithin 4 parts 
Stearic acid 4 parts 
Butyl stearate 4 parts 
Methyl ethyl ketone 1000 parts 
______________________________________ 
The above composition was kneaded in a ball mill for 50 hours to obtain a 
magnetic coating composition, which was then coated by a doctor blade in a 
dry thickness of 5 .mu. on a polyethylene terephthalate support having a 
thickness of 15 .mu.. The coated layer was subjected to orientation with a 
cobalt magnet and the solvent was evaporated at 100.degree. C. for 1 min. 
The coated layer was subjected to a smoothing treatment with calender 
rolls composed of five pairs of rolls, with each pair including a cotton 
roll and a mirror roll (a roll temperature: 60.degree. C., pressure: 100 
Kg/cm.sup.2) An electron beam with an accelerating voltage of 200 kV and a 
beam current of 10 mA was radiated so that the absorption dose was 10 
Mrad. The resulting magnetic recording medium was identified as Sample No. 
1. 
COMATIVE EXAMPLE 1 
The same procedure as described in Example 1 was repeated except that the 
following binder composition was used instead of the binder used in 
Example 1 and that the roll temperature of the calender treatment was 
30.degree. C. to prepare a magnetic recording medium. The resulting sample 
was identified as Sample No. 2. 
______________________________________ 
Urethane resin 65 parts 
Ester acrylate oligomer 
30 parts 
(Aronix M6100) 
Diethylene glycol diacrylate 
5 parts 
______________________________________ 
COMATIVE EXAMPLE 2 
The same procedure as described in Example 1 was repeated except that the 
following binder composition was used instead of the binder used in 
Example 1 and that the roll temperature of the calender treatment was 
50.degree. C. to prepare a magnetic recording medium. The resulting sample 
was identified as Sample No. 3. 
______________________________________ 
Vinyl chloride-vinyl acetate- 
65 parts 
vinyl alcohol copolymer 
Ester acrylate oligomer 
30 parts 
(Aronix M6100) 
Diethylene glycol diacrylate 
5 parts 
______________________________________ 
COMATIVE EXAMPLE 3 
The same procedure as described in Example 1 was repeated except that the 
following binder composition was used instead of the binder used in 
Example 1 to prepare a magnetic recording medium. The resulting sample was 
identified as Sample No. 4. 
______________________________________ 
Vinyl chloride-vinyl acetate- 
65 parts 
vinyl alcohol copolymer 
Urethane resin 35 parts 
______________________________________ 
EXAMPLE 2 
The same procedure is described in Example 1 was repeated except that the 
following binder composition was used instead of the binder used in 
Example 1 to prepare a magnetic recording medium. The resulting sample was 
identified as Sample No. 5. 
______________________________________ 
Vinyl chloride-vinyl acetate- 
50 parts 
maleic acid copolymer 
(Vinyl chloride: 92, vinyl acetate: 
4, maleic acid: 4) 
Urethane resin 15 parts 
Urethane acrylate oligomer 
30 parts 
("Aronix M1100" manufactured by 
Toagosei Chemical Industry Co., Ltd.) 
Trimethylolpropane triacrylate 
5 parts 
______________________________________ 
EXAMPLE 3 
The same procedure as described in Example 1 was repeated except that the 
following binder composition was used instead of the binder used in 
Example 1 to prepare a magnetic recording medium. The resulting sample was 
identified as Sample No. 6. 
______________________________________ 
Vinyl chloride-vinyl acetate- 
35 parts 
maleic acid copolymer 
(vinyl chloride: 94, vinyl acetate: 
4, maleic acid: 2) 
Urethane resin 25 parts 
Diethylene glycol diacrylate 
40 parts 
______________________________________ 
Samples of Examples 1 to 3 and Comparative Example 1 to 3 were run on a 
video tape recorder for 100 passes in order to measure their dynamic 
friction coefficient, time of durability at still mode and squareness 
ratios (residual flux density/maximum flux density). The results obtained 
are shown in Table 1. 
TABLE 1 
______________________________________ 
Dynamic Time of 
Square- Friction Durability 
Sample 
ness Coefficient after 
at Still 
No. Ratio Repeated Use*1 
Mode (min)*2 
Remarks 
______________________________________ 
1 0.83 0.21 more than 
Invention 
60 
2 0.65 The tape run 
3 Comparative 
stopped due to 
adhesion to a 
magnetic head. 
3 0.75 0.50 15 " 
4 0.75 The magnetic 
1 " 
layer was 
stripped out. 
5 0.83 0.19 more than 
Invention 
60 
6 0.81 0.21 more than 
" 
60 
______________________________________ 
*1: Dynamic friction coefficient was indicated in terms of ".mu." value 
calculated by the following equation, using a VHS type video tape recorde 
(trade name: "Macroad 88" (NV8200 type) manufactured by Matsushita 
Electric Industrial Co., Ltd.) at 40.degree. C. and 65% relative humidity 
T.sub.2 /T.sub.1 = e.sup..mu..pi. where T.sub.1 is a tape tension at the 
supply side of the rotary cylinder of VTR and T.sub.2 is a tape tension 
at the takeup side of the rotary cyli nder of VTR, whereby the effect of 
T.sub.1 is removed from T.sub.2 when T.sub.2 is for larger than T.sub.1. 
Running tension was evaluated in terms of .mu.. Values indicated in Table 
1 were obtained after 100 passes. 
*2: The time of durability at still mode was determined by recording 
predetermined video signals on video tapes (each samples) using a VHS typ 
video tape recorder (trade name: "NV8200 type" manufactured by Matsushita 
Electric Industrial Co., Ltd.), and measuring the period of time until th 
reproduced still images loose their clearness at 5.degree. C. and 65% 
relative humidity. 
As is apparent from the results shown in Table 1, Sample Nos. 1, 5 and 6 
show excellent results in squareness, dynamic friction coefficient and 
still durability. Sample No. 2 prepared by using only a urethane resin 
shows low squareness ratio and poor running properties and still 
durability. Sample No. 3 using only a vinyl chloride-vinyl acetate type 
copolymer and Sample No. 4 containing no compound having (meth)acryloyl 
groups show poor results in running properties and still life as compared 
with Samples according to the present invention. 
While the invention has been described in detail and with reference to 
specific embodiment thereof, it will be apparent to one skilled in the art 
that various changes and modifications can be made therein without 
departing from the spirit and scope thereof.