Magnetic recording medium of ferromagnetic particles and binder

A magnetic recording medium having a non-magnetic base overlaid with a magnetic layer containing ferromagnetic particles and a binder is disclosed. The ferromagnetic particles are metal particles and the binder is composed of (a) carboxyl-containing vinyl chloride/vinyl acetate copolymer, (b) a polyurethane resin and (c) a polyisocyanate compound.

FIELD OF THE INVENTION 
This invention relates to a magnetic recording medium, and more 
particularly, to the composition of a binder effective for use in 
combination with ferromagnetic metal particles. Still more particularly, 
the invention relates to a magnetic recording medium having an improved 
dispersion of ferromagnetic metal particles and hence good operating 
characteristics. 
BACKGROUND OF THE INVENTION 
A magnetic recording medium basically consists of a non-magnetic base made 
of a material such as a polyester film, and an overlying magnetic layer 
principally made of ferromagnetic particles and a binder. Today, 
ferromagnetic metal particles having high saturation magnetization and 
coercive force are used for the purpose of providing increased magnetic 
recording density and reproduction output. Ferromagnetic metal particles 
have good magnetic properties, but because of their high saturation 
magnetization (.sigma.s), the interaction between the individual particles 
is great, a dispersion of the particles is not easily formed, and a 
dispersion once formed does not remain stable for a sufficient period of 
time. As a further disadvantage, ferromagnetic metal particles of this 
type are inherently easily oxidized, so their magnetic properties are more 
easily impaired with time than oxide magnetic particles. For a magnetic 
layer containing ferromagnetic metal particles, improvement in the 
recording density is given the top priority, so the need for shorter 
recording wavelength and smaller loss in spacing with the head unavoidably 
requires a smoother tape surface than that of a magnetic layer containing 
oxide magnetic particles. However, as the tape surface is made smoother, 
the area of contact with the tape transport system increases and hence the 
friction coefficient is increased, whereas the durability of the tape is 
decreased. Because of these disadvantages, the magnetic recording medium 
obtained often has poor magnetic properties and electro-magnetic 
conversion characteristics, as well as poor running property and low 
durability. 
Conventionally known vinyl chloride/vinyl acetate resins, polyurethane 
resins, cellulosic resins, polyester resins and mixtures thereof form a 
good dispersion of magnetic particles and can be stored for an extended 
period, as disclosed in Japanese Patent Publication Nos. 19282/64, 5349/65 
and 22063/72, and Japanese Patent Application (OPI) Nos. 51704/78, 
51705/78, 51706/78, 51707/78, 51708/78 and 84705/79 (the term "OPI" as 
used herein refers to a "published unexamined Japanese patent 
application"). However, they do not always have satisfactory running 
property and durability. 
SUMMARY OF THE INVENTION 
Therefore, one object of this invention is to provide a novel magnetic 
recording medium. 
Another object of the invention is to provide a magnetic recording medium 
having good magnetic properties. 
A further object of the invention is to provide a magnetic recording medium 
having good running property. 
A still further object of the invention is to provide a magnetic recording 
medium having great durability. 
Still another object of the invention is to provide a magnetic recording 
medium whose characteristics are not greatly changed with time. 
As mentioned above, vinyl chloride/vinyl acetate resins form a good 
dispersion of magnetic particles and can be stored for an extended period. 
Therefore, using these resins, various studies were made to provide a 
magnetic recording medium having improved running property and durability 
without reducing its magnetic properties and electro-to-magnetic 
conversion properties. As a result, it has been found that the desired 
magnetic recording medium can be produced by using a binder composed of 
(a) a carboxyl-containing vinyl chloride/vinyl acetate copolymer, (b) a 
polyurethane resin, and (c) a polyisocyanate compound. Therefore, this 
invention provides a magnetic recording medium having a non-magnetic base 
overlaid with a magnetic layer mainly comprising ferromagnetic metal 
particles and a binder which comprises (a) a carboxyl-containing vinyl 
chloride/vinyl acetate copolymer, (b) a polyurethane resin and (c) a 
polyisocyanate compound. 
DETAILED DESCRIPTION OF THE INVENTION 
The carboxyl-containing vinyl chloride/vinyl acetate copolymer used in this 
invention is a copolymer of vinyl chloride, vinyl acetate and a 
polymerizable unsaturated carboxylic acid. Examples of the polymerizable 
unsaturated carboxylic acid include acrylic acid, methacrylic acid, 
crotonic acid, isocrotonic acid and maleic acid. 
The copolymerization ratio of the component (a) is 80-95:3-20:1-5 for vinyl 
chloride:vinyl acetate:polymerizable unsaturated carboxylic acid. 
Examples of the polyurethane resin used in this invention include polyester 
polyurethane resins produced by the reaction of polyester polyols or 
lactone polyester polyols produced from lactones (e.g., 
.epsilon.-caprolactone and .gamma.-butyrolactone) with diisocyanate 
compounds, and polyether polyurethane resins produced by the reaction of 
polyether polyols derived from ethylene oxide, propylene oxide or butylene 
oxide with diisocyanate compounds. The polyester polyols are produced by 
reacting organic dibasic acids such as saturated or unsaturated 
dicarboxylic acids (e.g., maleic acid and adipic acid), alicyclic 
dicarboxylic acids (e.g., norbornene dicarboxylic acid) or aromatic 
dicarboxylic acids (e.g., phthalic acid) with one or more polyols selected 
from glycols (e.g., ethylene glycol, propylene glycol, diethylene glycol 
and polyethylene glycol), polyvalent alcohols (e.g., trimethylolpropane, 
hexanetriol, glycerine and pentaerythritol) and polyvalent phenols (e.g., 
hydroquinone and bisphenol A). Examples of the diisocyanate compounds 
include aromatic diisocyanates (e.g., tolylene diisocyanate, xylylene 
diisocyanate and 4,4'-diphenylmethane diisocyanate) and aliphatic 
diisocyanates (e.g., hexamethylene diisocyanate). These polyurethane 
resins may be terminated with an isocyanate group, hydroxyl group or 
carboxyl group or mixtures thereof. The polyurethane resins have a 
molecular weight of from 10,000 to 200,000. 
The weight ratio of the carboxyl-containing vinyl chloride/vinyl acetate 
copolymer to the polyurethane resin is from 85:15 to 10:90, preferably 
from 70:30 to 50:50. A binder containing too much carboxyl-containing 
vinyl chloride/vinyl acetate copolymer forms a brittle magnetic layer that 
is not firmly adhered to the non-magnetic base. A binder containing too 
much polyurethane resin forms a poor dispersion of magnetic particles and 
hence provides a magnetic layer that has poor surface quality and low S/N 
ratio. 
Examples of the polyisocyanate compound are an adduct of 3 mols of a 
diisocyanate compound such as tolylene diisocyanate and 1 mol of a 
trivalent polyol such as trimethylolpropane; a trimer of tolylene 
diisocyanate; and polyisocyanate and polyphenylmethane polyisocyanate. The 
weight ratio of the polyisocyanate compound to the sum of the 
carboxyl-containing vinyl chloride/vinyl acetate copolymer and the 
polyurethane resin can vary within the range of from 40:60 to 10:90. 
The ferromagnetic metal particles used in this invention mainly consist of 
iron-cobalt or iron-nickel-cobalt, and they can be prepared by any of the 
following non-limiting methods: 
(1) an organic acid salt of ferromagnetic metal is hydrolyzed and then 
reduced with a reducing gas; p1 (2) an acicular oxyhydroxide of a 
ferromagnetic metal, an acicular oxyhydroxide of a ferromagnetic metal and 
another metal, or acicular iron oxide derived from these oxyhydroxides is 
reduced; 
(3) a ferromagnetic matal is vaporized in a low-pressure inert gas; 
(4) a metal carbonyl compound is thermally decomposed; 
(5) particles of a ferromagnetic metal are electro-deposited on a mercury 
cathode from which the particles are then separated; and 
(6) a metal salt capable of forming a ferromagnetic material in aqueous 
solution is reduced with a reducing material (e.g., borohydride compound, 
hypophosphite or hydrazine) to form ferromagnetic particles. 
The ferromagnetic metal particles produced by the methods (2), (3) and (6) 
are particularly preferred because they have good stability against 
oxidation, as well as good magnetic properties and electro-to-magnetic 
conversion characteristics and they achieve good results in the magnetic 
recording medium of this invention. 
The resulting ferromagnetic metal particles may be provided with an oxide 
coating to improve their chemical stability, as described in Japanese 
Patent Publication Nos. 3862/60 and 11724/81 and Japanese Patent 
Application (OPI) Nos. 54998/77, 85054/77 and 30707/81. 
The ferromagnetic metal particles and the binder are blended with 
dispersants, lubricants, abrasives, antistats and a coating solvent to 
obtain a magnetic paint for application onto the non-magnetic base. The 
coating solvent is an organic solvent examples of which include ketones 
such as methyl ethyl ketone and cyclohexanone, alcohols, esters such as 
ethyl acetate and butyl acetate, cellosolves, ethers, aromatic solvents 
such as toluene, and chlorinated hydrocarbon solvents such as carbon 
tetrachloride and chloroform. 
Specific examples of dispersants which can be employed in the magnetic 
recording layer in accordance with this invention are aliphatic carboxylic 
acids having 12 to 18 carbon atoms (e.g., of the formula R.sub.1 COOH, 
wherein R.sub.1 is an alkyl or alkenyl group having 11 to 17 carbon atoms) 
such as caprylic acid, capric acid, lauric acid, myristic acid palmitic 
acid, stearic acid, oleic acid, elaidic acid, linolic acid, linoelic acid, 
stearolic acid, and the like; metallic soaps comprising alkali metal (Li, 
Na, K, etc.) or alkaline earth metal (Mg, Ca, Ba, etc.) salts of the 
above-described aliphatic carboxylic acids; fluorine-containing compounds 
of the above-described aliphatic carboxylic acid esters; amides of the 
above-described aliphatic carboxylic acids; polyalkylene oxide alkyl 
phosphates; lecithin; trialkyl polyolefinoxy quaternary ammonium salts 
(wherein the alkyl group has 1 to 5 carbon atoms, and the olefin is 
exemplified by ethylene, propylene, etc.); and the like. In addition, 
higher alcohols having more than 12 carbon atoms and the sulfuric acid 
esters thereof and the like can also be employed. These dispersants are 
employed in an amount of from about 0.5 to about 20 parts by weight per 
100 parts by weight of the binder. These dispersants are described in 
detail in Japanese Patent Publication Nos. 28369/64, 17945/69, 7441/73, 
15001/73, 15002/73, 16363/73 and 4121/75, U.S. Patents 3,387,993 and 
3,470,021, etc. 
Typical lubricants which can be employed in the magnetic recording layer in 
accordance with this invention include finely divided electrically 
conductive powders such as graphite, etc.; finely divided inorganic 
powders such as molybdenum disulfide, tungsten disulfide and the like; 
finely divided synthetic resin powders such as those of polyethylene, 
polypropylene, polyethylenevinyl chloride copolymers, 
polytetrafluoroethylene and the like; .alpha.-olefin polymers; unsaturated 
aliphatic hydrocarbons which are liquid at normal temperature (compounds 
in which an n-olefin double bond is positioned at the terminal thereof, 
with about 5 to about 20 carbon atoms); aliphatic acid esters of aliphatic 
monocarboxylic acids having 12 to 20 carbon atoms and monovalent alcohols 
having 3 to 12 carbon atoms, and the like. These lubricants can be 
employed at about 0.2 to about 20 parts by weight per 100 parts by weight 
of the binder, and are described in detail in Japanese Patent Publication 
Nos. 18064/66, 23889/68, 40461/71, 15621/72, 18482/72, 28043/72, 32001/72 
and 5042/75, U.S. Pat. Nos. 3,470,021, 3,492,235, 3,497,411, 3,523,086, 
3,625,760, 3,630,772 and 3,642,539, IBM Technical Disclosure Bulletin, 
Vol. 9, No. 7, page 779 (December, 1966), ELEKTRONIK, 1961, No. 12, page 
380, etc. 
Specific examples of abrasives which can be employed in the magnetic 
recording layer in accordance with this invention are those generally 
employed which include fused alumina, silicon carbide, chromium dioxide, 
corundum, artificial corundum, diamond, artificial diamond, garnet, emery 
(with the main components being corundum and magnetite), and the like. 
These abrasives have a Mohs' hardness above about 5. Preferably the 
abrasives employed have an average particle size of about 0.05 to about 
5.mu., more preferably 0.1 to 2.mu.. These abrasives are employed in an 
amount of about 0.5 to about 20 parts by weight per 100 parts of the 
binder. These abrasives are described in detail in Japanese Patent 
Publication Nos. 18572/72, 15003/73, 15004/73 (corresponding to U.S. 
Patent No. 3,617,378), 39402/74 and 9401/75, U.S. Pat. Nos. 3,007,807, 
3,041,196, 3,293,066, 3,630,910 and 3,687,725, British Pat. No. 1,145,349, 
German Patent Application (DT-PS) Nos. 853,211 and 1,101,000, and the 
like. 
Typical examples of antistats which can be employed in the magnetic 
recording layer in accordance with this invention include finely divided 
electrically conductive powders such as carbon black, carbon black graft 
polymers and the like; natural surface active agents such as saponin, 
etc.; nonionic surface active agents such as alkylene oxide type, glycerin 
type, glycidol type and like surface active agents; cationic surface 
active agents such as higher alkyl amines, quaternary ammonium salts, 
pyridines or other heterocyclic compounds, phosphoniums or sulfoniums and 
the like; anionic surface active agents containing an acidic group derived 
from a carboxylic acid group, a sulfonic acid group or a phosphoric acid 
group, a sulfuric acid ester group, a phosphoric acid ester group and the 
like; amphoteric surface active agents such as sulfuric acid esters or 
phosphoric acid esters, etc., of amino acids, aminosulfonic acids, amino 
alcohols and the like. 
The above-described finely divided electrically conductive powders can be 
employed in an amount of about 0.2 to about 20 parts by weight per 100 
parts of the binder, and the surface active agents can be used in an 
amount of from about 0.1 to about 10 parts by weight per 100 parts by 
weight of the binder. 
These electrically conductive finely divided powders which can be employed 
as antistats and some surface active agents are described in Japanese 
Patent Publication Nos. 22726/71, 24881/72, 26882/72, 15440/73 and 
26761/73, U.S. Pat. Nos. 2,271,623, 2,240,472, 2,288,226, 2,676,122, 
2,676,924, 2,676,975, 2,691,566, 2,727,860, 2,730,498, 2,742,379, 
2,739,891, 3,068,101, 3,158,484, 3,201,253, 3,210,191, 3,294,540, 
3,415,649, 3,441,413, 3,442,654, 3,475,174 and 3,545,974, German Patent 
Application (OLS) No. 1,942,665, British Pat. Nos. 1,077,317 and 
1,198,450, etc., and in references such as Ryohei Oda, et al., Kaimen 
Kassezai no Gosei to sono Oyo (Synthesis and Application of Surface Active 
Agents), published by Maki Shoten, Tokyo (1964), A. M. Schwartz and J. W. 
Perry, Surface Active Agents, published by Interscience Publications Inc. 
(1958), J. P. Sisley, Encyclopedia of Surface Active Agents, Vol. 2, 
published by Chemical Publishing Co. (1964), Kaimen Kasseizai Binran 
(Handbook of Surface Active Agents), Sixth Ed., published by Sangyo Tosho 
Kabushiki Kaisha, Japan (Dec. 20, 1966) etc. 
The non-magnetic base may be made of a synthetic resin (e.g., polyester, 
polyamide, polyolefin, cellulose derivative), non-magnetic metal, glass, 
ceramics and paper. The base is used in the form of a film, tape, sheet, 
card, disc, drum or any other suitable form, but it is generally used in 
the form of a tape or sheet. 
The magnetic paint is prepared by charging a mixer with the magnetic 
particles and all other ingredients simultaneously or sequentially. 
Various mixers may be used to achieve thorough mixing of the ingredients. 
For details of the mixers, see T. C. Patton, Paint Flow and Pigment 
Dispersion, John Wiley & Sons, 1964. A magnetic layer is formed from the 
magnetic paint on the base by various methods that are specifically 
described in Coating Kogaku (Coating Engineering), published by Asakura 
Shoten, 1971. The magnetic layer thus-formed on the base is dried after 
the magnetic particles in the layer are optionally oriented. The magnetic 
layer may be passed through a smoothing step for improving its magnetic 
properties (e.g., smoothing before drying or calendering after drying).

This invention is now described in greater detail by reference to the 
following example and comparative examples which are given here for 
illustrative purposes only and are by no means intended to limit the scope 
of the invention. In the examples, all parts are by weight. 
EXAMPLE 1 
Acicular particles of .alpha.-FeOOH containing 5 wt% cobalt were decomposed 
with heat to produce .alpha.-Fe.sub.2 O.sub.3 particles which were reduced 
with hydrogen to form ferromagnetic metal particles. The particles had a 
coercive force (Hc) of 1,400 Oe, a saturation magnetization (.sigma.s) of 
150 emu/g and a squareness ratio (.sigma.s/.sigma.r) of 0.52. 
Three hundred parts of the ferromagnetic metal particles and a composition 
having the following formulation were placed in a ball mill where they 
were mixed thoroughly for 48 hours. 
______________________________________ 
parts 
______________________________________ 
Maleic acid-containing vinyl 
30 
chloride/vinyl acetate copolymer 
(degree of polymerization: ca. 400, 
vinyl chloride/vinyl acetate/ 
maleic acid ratio = 86/13/1) 
Polyester polyurethane (reaction 
20 
product of ethylene adipate and 
2,4-tolylene diisocyanate, 
wt. av. m.w. for polystyrene: 
ca. 110,000) 
Palmitic acid 5 
Butyl stearate 2 
Aluminum oxide (.alpha.-Al.sub.2 O.sub.3) 
6 
Butyl acetate 500 
Methyl isobutyl ketone 
400 
______________________________________ 
To the resulting dispersion, 25 parts of a 75 wt% ethyl acetate solution of 
a triisocyanate compound ("Desmodule L-75" of Bayer A. G., an adduct of 3 
mols of tolylene diisocyanate and 1 mol of trimethylolpropane) was added, 
and the mixture was stirred by a high-speed disperser for 1 hour to 
provide a magnetic coating solution. 
The coating solution was applied to one surface of a polyethylene 
terephthalate film, placed in a magnetic field for orientation, dried, 
calendered, and slit to form a magnetic video tape 1/2 inch wide. 
COMATIVE EXAMPLE 1 
A magnetic tape was produced as in Example 1 except that 20 parts of the 
polyurethane was replaced by 20 parts of a maleic acid-containing vinyl 
chloride/vinyl acetate copolymer. 
COMATIVE EXAMPLE 2 
A magnetic tape was produced as in Example 1 except that 25 parts of the 
ethyl acetate solution of triisocyanate compound was replaced by 15 parts 
of a maleic acid-containing vinyl chloride/vinyl acetate copolymer and 10 
parts of polyurethane. 
The squareness ratio, electro-to-magnetic conversion characteristics, 
time-dependent change in magnetic properties, still mode durability, 
stability to repeated running, and the friction coefficient of the three 
tapes are indicated in Table 1. 
TABLE 1 
__________________________________________________________________________ 
Electro-to- 
Magnetic 
Time-Dependent 
Conversion 
Change in Magnetic 
Properties.sup.2 
Properties.sup.3 
Stability 
Squareness 
(output at 
(percent Still Mode 
to Repeated 
Friction 
Ratio.sup.1 
4 MHz) 
demagnetization) 
Durability.sup.4 
Running.sup.5 
Coefficient.sup.6 
__________________________________________________________________________ 
Example 1 
0.84 8.0 dB 
- 5.0% More than 
100 Passes 
0.36 
90 minutes 
or more 
(no damage) 
Comparative 
0.85 8.0 dB 
- 6.0% 5 Minutes 
30-40 Passes 
0.35 
Example 1 (magnetic 
particles 
dislodged) 
Comparative 
0.84 7.8 dB 
- 6.0% 30 Minutes 
10-20 Passes 
0.40 
Example 2 (foul head) 
__________________________________________________________________________ 
.sup.1 The Bm/Br value as measured by a vibrating flux meter (Model VSMII 
of Toei Kogyo K.K.). 
.sup.2 The reproduction output as measured at 4 MHz by a VHS video tape 
recorder (Model NV8800 of Matsushita Electric Industrial Co., Ltd.) whose 
record/reproduce head was especially made of "Sendust" (alloy). As a 
reference tape, Fuji Video Cassette T120E of Fuji Photo Film Co., Ltd. wa 
used. 
.sup.3 Percent reduction of the saturation flux density (Bm) of the tape 
exposed to 60.degree. C. and 90% RH for 10 days. 
.sup.4 In a still mode, the rotary video head of the VTR of .sup.2 was 
rotated in contact with the same area of a standing tape. The time for th 
output reproduced from the recorded signal by the video head to be reduce 
to substantially zero as a result of tape abrasion was measured. 
.sup.5 A given length of tape (e.g., 10 m) was threaded in a VHS cassette 
half, and a predetermined signal was recorded on the tape and repeatedly 
reproduced with a VHS video tape recorder, and the resulting tape damage 
was checked. 
.sup.6 The friction coefficient as measured at 25.degree. C. and 65% RH 
when the tape was caused to run in contact with a stainless steel pole 
(surface roughness: 0.15.mu. , diameter: 5 mm) at an angle of 180.degree. 
and at a speed of 3.3 cm/sec under a load of 50 g. 
As is clear from Table 1, the tape of Example 1 had a squareness ratio, 
video output and percent demagnetization almost equal to those of the 
tapes of Comparative Examples 1 and 2, which indicates that the former was 
as good as the latter with respect to the dispersibility of magnetic 
particles, electro-to-magnetic conversion properties and long-term 
stability. As for the still mode durability and stability to repeated 
running, the tape of Example 1 was better than the tapes of Comparative 
Examples; magnetic particles were dislodged from the tape of Comparative 
Example 1 in the area of contact with the video head, and the sample of 
Comparative Example 2 fouled the video tape, but the tape of Example 1 
could be passed through the VTR more than 100 times without causing these 
problems. The friction coefficient of the tape of Example 1 was equal to 
that of the tape of Comparative Example 1, but smaller than that of the 
tape of Comparative Example 2. These results show that the combination of 
the carboxyl-containing vinyl chloride/vinyl acetate copolymer of Example 
1 with the polyurethane resin and polyisocyanate compound provides a more 
durable and runnable magnetic recording medium than the combination of 
said copolymer with either the polyurethane resin or polyisocyanate 
compound. 
While the invention has been described in detail and with reference to 
specific embodiments 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.