Magnetic recording medium

A magnetic recording medium is disclosed, comprising a non-magnetic support having provided on one side thereof a magnetic recording layer comprising a binder having dispersed therein fine ferromagnetic particles and on the other side thereof a backing layer, wherein at least one of said magnetic layer and backing layer contains (1) a vinyl chloride resin having --SO.sub.3 M (wherein M is Li, Na, or K) groups and epoxy groups, and (2) polyurethane as binders. The medium has reduced output variation and improved running durability.

FIELD OF THE INVENTION 
This invention relates to a magnetic recording medium, and particularly to 
a binder of a magnetic recording medium, such as video tapes, audio tapes, 
computer tapes, floppy disks, and the like. More particularly, it relates 
to a magnetic recording medium which undergoes little variation in output 
power and has improved running durability. 
BACKGROUND OF THE INVENTION 
Binders which have been commonly used for magnetic recording media include, 
for example, vinyl chloride-vinyl acetate resins, alone or as mixtures 
thereof with one or more polymers selected from cellulose derivatives, 
acrylic resin (i.e., copolymers of acrylic acid or methacrylic acid and 
esters thereof), urethane resins, vinylidene chloride copolymers, 
synthetic rubbers, polyesters, etc., and reactive mixtures comprising the 
above-described mixture and a polyisocyanate, which are cured by 
crosslinking. 
However, when these conventional binders are used in video tapes, the vinyl 
chloride-vinyl acetate resin contained in video tapes forms and releases 
hydrochloric acid, which ultimately causes corrosion of metals of the 
running system, e.g., guide poles, or a magnetic head of a video deck or 
ferromagnetic metal particles. 
On the other hand, binders not containing vinyl chloride-vinyl acetate 
resins have a disadvantage of relatively high cost. 
In magnetic recording tapes for audio or video devices or computers, the 
surface of a magnetic recording layer generally has a smooth finish in 
order to ensure good sensitivity, and, in particular, good output power in 
the high frequency region. However, such a smooth magnetic recording tape 
is liable to disorder in winding and rewinding. Use of such a magnetic 
recording tape causes deterioration of running properties and variation in 
output and also tends to induce deformation or damage of the tape. 
In order to overcome this problem, it has been proposed to provide a 
backing layer on the surface of a support opposite to the magnetic 
recording layer. Binders which have conventionally been used for the 
backing layer include vinyl chloride-vinyl acetate copolymers and the 
like. Backing layers using these conventional binders, however, adversely 
affect running durability of the tape due to their poor wear resistance, 
toughness, etc., thus resulting in an increased coefficient of friction, 
folding of the tape, and the like. Therefore, improvements in the backing 
layer has been strongly desired. 
In addition, when a backing layer is provided generally for the purpose of 
improving running characteristics or running durability, it is known that 
when a magnetic medium is rolled up (particularly if being in a tape 
shape) or piled up (if being in a sheet shape), surface roughness of the 
backing layer is transferred to the surface of the magnetic layer to 
impair surface properties of the magnetic layer, which leads to 
deterioration in electromagnetic properties of the magnetic medium, 
especially variation in output power. This is because the surface of the 
backing layer is made too much rough from the standpoint of running 
properties, and such a surface roughness is transferred to the magnetic 
layer surface when the magnetic medium is preserved in storage or allowed 
to stand in roll form or piles. 
As described above, although use of a mixture of a vinyl chloride-vinyl 
acetate resin as a main binder and other resins as a binder for magnetic 
recording media, such as video tapes, is economically advantageous, not 
only such a binder corrodes the running system of a video deck, a 
permalloy head, or a metal magnetic material, but also the dispersibility 
of fine ferromagnetic particles in such binder is too poor to ensure high 
chroma S/N ratios. Moreover, the above-described binder cannot 
sufficiently satisfy other requirements for video tapes, such as running 
properties, still life, and the like. 
In order to overcome these disadvantages, it is described, e.g., in 
Japanese Patent Application (OPI) No. 200426/83 (the term "OPI" as herein 
used means "unexamined published application"), to use a phenoxy resin, a 
polyurethane elastomer, and a polyisocyanate as a binder. This phenoxy 
resin, however, has a difficulty in terms of solubility, and therefore it 
cannot be applied to use with ease and convenience. 
SUMMARY OF THE INVENTION 
Accordingly, an object of this invention is to provide a binder for 
magnetic tapes which serves to prevent a video tape recorder (VTR) running 
system from corroding with rust due to dehydrochlorination, particularly 
under the circumstances of high temperature and humidity. 
Another object of this invention is to provide a binder for magnetic tapes 
which is capable of sufficiently dispersing fine ferromagnetic particles 
and provides high chroma S/N ratios. 
A further object of this invention is to provide a binder for magnetic 
tapes which ensures stable tape running to thus provide stable images for 
an extended period of time, and also exhibits video output stability with 
less drop outs. 
The above objects can be achieved by a magnetic recording medium comprising 
a non-magnetic support having provided on one side thereof a magnetic 
layer comprising a binder having dispersed therein fine ferromagnetic 
particles and on the other side thereof a backing layer, wherein at least 
one of said magnetic layer and backing layer contains (1) a vinyl chloride 
resin having --SO.sub.3 M groups wherein M is Li, Na, or K, and epoxy 
groups, and (2) polyurethanes as binders. 
DETAILED DESCRIPTION OF THE INVENTION 
The vinyl chloride resin which is used in the present invention is a 
copolymer comprising vinyl chloride and other copolymerizable monomers, 
such as vinyl acetate, vinylidene chloride, acrylonitrile, styrene, 
acrylic esters, maleic anhydride, etc., with --SO.sub.3 M groups and epoxy 
groups being bonded thereto. The vinyl chloride resin preferably has a 
number average molecular weight of from 15,000 to 60,000. The vinyl 
chloride resin preferably comprises from 80 to 90% by weight of vinyl 
chloride and from 10 to 20% by weight of at least one copolymerizable 
monomer. The --SO.sub.3 M groups are bonded to the vinyl chloride resin in 
an amount of from 0.1 to 2.0% by weight, and preferably from 0.2 to 1.5% 
by weight, and the epoxy groups are bonded to the vinyl chloride resin in 
an amount of from 0.2 to 10% by weight, and preferably from 0.7 to 5% by 
weight, both based on the total weight of the vinyl chloride resin. 
In the group --SO.sub.3 M, M is selected from lithium, sodium, and 
potassium, with sodium being preferred. 
Polyurethanes which can be used as a binder in combination with the vinyl 
chloride resin may be arbitrarily selected from polymers, either low 
polymeric or high polymeric, prepared from among isocyanates or from an 
isocyanate and a compound having a functional group capable of reacting 
with an isocyanate to form a polymer. Preferred polyurethanes includes 
polyester polyurethane resins or polyether polyurethane resins which are 
obtained by reacting (a) a reactive polyester, preferably the one having 
an average molecular weight of from 600 to 3,000, prepared by reacting an 
organic dibasic acid, e.g., phthalic acid, adipic acid, a linoleic acid 
dimer, maleic acid, etc., with a glycol, e.g., ethylene glycol, propylene 
glycol, butylene glycol, diethylene glycol, etc., or a polyhydric alcohol, 
e.g., trimethylolpropane, hexanetriol, glycerin, trimethylolethane, 
pentaerythritol, etc., or a mixture of such reactive polyesters; or a 
reactive polyether, preferably the one having an average molecular weight 
of from 600 to 3,000, obtained by polymerization of propylene oxide, 
ethylene oxide, etc. or a mixture of such reactive polyethers with (b) an 
isocyanate, e.g., tolylene diisocyanate, 4,4'-diphenylmethane 
diisocyanate, hexamethylene diisocyanate, etc. These polyester 
polyurethane resins or polyether polyurethane resins may be used 
individually or in combination thereof either by mixing or binding. These 
polyurethane resins preferably have a number average molecular weight of 
from 20,000 to 100,000. 
In addition, polyurethanes that can be used include those containing 
therein a 
##STR1## 
ring and having a skeleton represented by formula (II) 
EQU --O--R--OOC--R'--CO).sub.m --OROOCNH--R'--NHCO-- (II) 
wherein m represents an integer of from 5 to 100; R represents a divalent 
group derived from an alicyclic or aromatic compound having at least two 
hydroxyalkyl groups having from 1 to 4 carbon atoms or hydroxyalkoxy 
groups having from 1 to 4 carbon atoms; R' represents 
##STR2## 
and n represents an integer of from 4 to 6. 
These polyurethane resins effectively have a molecular weight of from 5,000 
to 500,000, and preferably from 10,000 to 200,000. Additional details 
regarding such polyurethane resins are described, e.g., in Japanese Patent 
Application (OPI) No. 122234/80. 
A preferred weight proportion of the --SO.sub.3 M-- and epoxy-containing 
vinyl chloride resin to the polyurethane is 95/5 to 50/50. 
In a preferred embodiment according to the present invention, a low 
molecular weight polyisocyanate compound (molecular weight: 150 to 2,000) 
having at least two isocyanate groups is additionally incorporated into 
the magnetic layer and/or backing layer as a binder to thereby form a 
three-dimensional network in the layer, which further improves physical 
strength. 
The polyisocyanate compound which can be used in the above-described 
embodiment includes isocyanates having at least two isocyanate groups and 
their adducts, for example, aliphatic diisocyanates, aliphatic 
diisocyanates having a cyclic group, aromatic diisocyanates, naphthalene 
isocyanates, biphenyl isocyanates, diphenylmethane diisocyanates, 
triphenylmethane diisocyanates, triisocyanates, tetraisocyanates, etc., 
and adducts thereof. 
Specific examples of the polyisocyanates include isocyanates, such as 
ethane diisocyanate, butane diisocyanate, hexane diisocyanate, 
2,2-dimethylpentane diisocyanate, 2,2,4-trimethylpentane diisocyanate, 
decane diisocyanate, .omega.,.omega.'-diisocyanate-1,3-dimethylbenzoyl, 
.omega.,.omega.'-diisocyanate-1,2-dimethylcyclohexane diisocyanate, 
.omega.,.omega.'-diisocyanate-1,4-diethylbenzoyl, 
.omega.,.omega.'-diisocyanate-1,5-dimethylnaphthalene, 
.omega.,.omega.'-diisocyanate-n-propylbiphenyl, 1,3-phenylene 
diisocyanate, 1-methylbenzoyl-2,4-diisocyanate, 
1,3-dimethylbenzol-2,6-diisocyanate, naphthalene-1,4-diisocyanate, 
1,1'-dinaphthyl-2,2'-diisocyanate, biphenyl-2,4'-diisocyanate, 
3,3'-dimethylbiphenyl-4,4'-diisocyanate, 
diphenylmethane-4,4'-diisocyanate, 2,2'-dimethyldiphenylmethane-4,4'-diiso 
cyanate, 3,3'-dimethoxydiphenylmethane-4,4'-diisocyanate, 
4,4'-diethoxydiphenylmethane-4,4'-diisocyanate, 
1-methylbenzol-2,4,6-triisocyanate, 
1,3,5-trimethylbenzol-2,4,6-triisocyanate, 
diphenylmethane-2,4,4'-triisocyanate, 
triphenylmethane-4,4',4"-triisocyanate, tolylene diisocyanate, 
1,5-naphthylene diisocyanate, etc.; dimers or trimers (adducts) of these 
isocyanates; and adducts of these isocyanates with di- or trihydric 
alcohols. The adducts of the isocyanates include an adduct of tolylene 
diisocyanate with trimethylolpropane, an adduct of 3 mols of tolylene 
diisocyanate, an adduct of 3 mols of hexamethylene diisocyanate, an adduct 
of 2 mols of hexamethylene diisocyanate, and adducts formed among the 
above-recited isocyanates. 
The above-described polyisocyanate can be used in an amount of from 5 to 
100% by weight based on the total amount of the binder(s) other than the 
polyisocyanates. 
The magnetic layer according to the present invention may also contain 
polymers, as binder components, other than the above-described vinyl 
chloride resin and polyurethane, in such an amount that does not adversely 
affect various magnetic characteristics of the magnetic layer, namely, in 
an amount of from 1 to 1.5% by weight based on the total amount of the 
binder. 
Such polymers include rubber polymers, such as an acrylonitrile-butadiene 
copolymer, a styrene-butadiene copolymer, etc., cellulose derivatives, 
such as cellulose nitrate, cellulose acetate, etc., and the like. 
A magnetic layer according to the present invention can be formed by 
dispersing known fine ferromagnetic particles in the binder using a 
commonly employed solvent and coating the resulting magnetic dispersion on 
a commonly employed support in a known coating manner. The dispersion may 
contain various conventionally employed additives, such as a dispersing 
agent, a lubricant, an abrasive, an antistatic agent, and the like. 
The fine ferromagnetic particles to be used includes fine particles of 
ferromagnetic iron oxide, ferromagnetic chromium dioxide and ferromagnetic 
alloys, and the like. Ferromagnetic iron oxide may contain divalent 
metals, e.g., Cr, Mn, Co, Ni, Zn, etc., in an amount of up to about 10 
atom%. Ferromagnetic chromium dioxide to be used is CrO.sub.2 which may 
contain up to 20% by weight of metals, e.g., Na, K, Ti, V, Mn, Fe, Co, Ni, 
Te, Ru, Sn, Ce, Pb, etc., semi-metals, e.g., P, Sb, Te, etc., or oxides 
thereof. The fine ferromagnetic particles which can be used in the present 
invention have a length of from about 0.1 to about 1.0 .mu.m and an 
acicular (length/width) ratio of from about 1 to about 15. 
Dispersing agents which can be used include fatty acids having from 12 to 
18 carbon atoms, e.g., caprylic acid, lauric acid, stearic acid, etc., 
alkali metal or alkaline earth metal salts of these fatty acids, and 
esters, amides, etc., of these fatty acids. 
Lubricant which can be used includes the above-described fatty acids and, 
in addition, silicone oil, graphite, molybdenum disulfide, etc. 
Abrasive to be used includes fused alumina, silicon carbide, corundum, etc. 
Antistatic agents to be used includes fine conductive powders, such as 
carbon black, and surface active agents, such as saponin. 
Organic solvents which can be used in the magnetic coating composition of 
the present invention include ketones, e.g., acetone, etc., esters, e.g., 
ethyl acetate, etc., tar solvents, e.g., benzene, etc., chlorinated 
hydrocarbons, e.g., methylene chloride, etc., and the like. 
The weight ratio of the fine ferromagnetic particles to the binder is 
generally from 100/13 to 100/45, and preferably is from 100/17 to 100/30. 
The amount of the solvent to be used is from 3.5 to 4.5 times the weight 
of the ferromagnetic particles. The dispersing agent, the lubricant, the 
abrasive, the surface active agent as an antistatic agent and the 
conductive fine powder as an antistatic agent are generally used in 
amounts of from 0.5 to 20 parts by weight, from 0.2 to 20 parts by weight, 
from 0.5 to 20 parts by weight, from 0 to 3 parts by weight and from 0.2 
to 20 parts by weight, respectively, per 100 parts by weight of the 
binder. 
The ferromagnetic particles, binders and additives are mixed to form a 
magnetic coating composition, and the composition is coated on a 
non-magnetic support, such as polyesters, polyolefins, cellulose 
derivatives, and the like, to form a magnetic layer. The thickness of the 
magnetic coating is determined depending on the end use, shape (films, 
tapes, sheets, etc.) and other specifications of the magnetic recording 
medium, but is usually from 1 to 18 .mu.m on a dry basis. 
The drying temperature and time vary depending on the kind of the solvent 
used, the amount of the solvent in the coating composition and the desired 
amount of the solvent remaining in the magnetic layer, but generally 
ranges from 40.degree. C. to 100.degree. C. The drying is usually carried 
out by gradually elevating the temperature with the progress of the drying 
for a period of from about 5 seconds to about 3 minutes. 
After the drying, the magnetic recording medium may be wound on a roll, 
etc., but is usually subjected to calendering, the latter being preferred 
for improving the S/N ratio. Calendering conditions are optimally selected 
according to the type of tape to be treated. In general, calendering is 
carried out under a nip linear pressure of from 80 to 300 kg/cm, and 
preferably from 100 to 240 kg/cm, at a rate of tape feed of from 30 to 200 
m/min., and preferably from 60 to 150 m/min., and at a roll temperature of 
from 25.degree. to 120.degree. C., and preferably from 45.degree. to 
80.degree. C. 
The backing layer according to the present invention can contain, as a 
binder, polymers other than vinyl chloride-vinyl acetate copolymers in 
addition to the above-described binders of the invention. Such polymers 
include an acrylonitrile-butadiene copolymer, a cellulose derivative, an 
ethylene-butadiene copolymer, and the like. These polymers can be used in 
an amount of from 1 to 20% by weight based on the total amount of the 
binder. 
In the formation of the backing layer, the binder is dispersed in a 
solvent, such as methyl ethyl ketone, tetrahydrofuran, etc. For the 
purpose of controlling surface roughness, electrical resistance, etc., of 
the back surface, various fillers are admixed with the dispersion. 
Examples of the fillers to be used include carbon black powders, graphite, 
tungsten disulfide, molybdenum disulfide, boron nitride and other 
inorganic powders, e.g., SiO.sub.2, CaCO.sub.3, Al.sub.2 O.sub.3, Fe.sub.2 
O.sub.3, TiO.sub.2, MnO, ZnO, CaO, etc., with carbon black and CaCO.sub.3 
being particularly preferred. These fillers are generally used at a weight 
ratio of from 0.7/1 to 4/1, and preferably from 1.1/1 to 3/1 with respect 
to the binder. The fillers preferably have an average particle size of 
from 0.01 to 2 .mu.m. 
For the purpose of reducing the coefficient of friction of the backing 
layer to thereby improve running properties and durability, a fatty acid 
having 12 or more carbon atoms or its ester compound, silicones, paraffin 
wax, etc. may be employed as a lubricant. Of these, fatty acids having 
from 18 to 22 carbon atoms are preferred. 
The thickness of the backing layer is generally not more than 3 .mu.m, and 
preferably not more than 2 .mu.m. In order to minimize transfer to the 
magnetic layer so as not to impair the S/N characteristics of tapes as 
mentioned above, the most preferred thickness of the backing layer is 1 
.mu.m or less. 
It is desirable to reduce surface roughness of the backing layer in order 
to provide a backing layer whose surface unevenness is not transferred to 
the magnetic layer. As a result of extensive investigations, the inventors 
have found it preferable to control the center-line average roughness 
(Ra), at a cut-off value of 0.08 mm, of the backing layer to be not more 
than 0.05 .mu.m, and more preferably not more than 0.024 .mu.m, when the 
magnetic layer has Ra of not more than 0.02 .mu.m.

This invention will now be illustrated in greater detail with reference to 
the following examples, but it should be understood that they are not 
intended to limit the present invention. In these examples, all the parts 
are by weight. 
EXAMPLE 1 
______________________________________ 
(a) Ferromagnetic metal fine powder Fe:Ni 
100 
89:11 by weight; BET specific surface 
(parts) 
area: 50 m.sup.2 /g) 
(b) Vinyl chloride resin (vinyl chloride = 
12* 
87 wt %, vinyl acetate = 13 wt %, number 
average molecular weight: 2.6 .times. 10.sup.4), to 
which an epoxy group and a --SO.sub.3 Na group 
had been bonded in amounts shown in Table 1 
(c) Thermoplastic polyurethane resin (prepared) 
10 
from polybutylene adipate of 2200 in 
molecular weight and diphenylmethane diiso- 
cyanate; number average molecular weight: 
35,000; Tg: -26.degree. C.) 
(d) Behenic acid/butyl stearate (1:1 by weight) 
2 
(e) .alpha.-Al.sub.2 O.sub.3 (mean particle size: 0.4 .mu.m) 
7 
(f) Tetrahydrofuran/methyl ethyl ketone/cyclo- 
320 
hexane (1:1:1 by weight) 
______________________________________ 
*parts of the vinyl chloride resin 
The above components (a) to (f) were kneaded and dispersed for 10 hours, 
and 7 parts of a triisocyanate compound (an ethyl acetate solution 
containing 75 wt% of a 3:1 adduct of tolylene diisocyanate and 
trimethylolpropane having a molecular weight of 656; "Desmodule L-75" 
produced by Bayer A.G.) was added thereto. The mixture was subjected to 
high rate shear dispersion for one hour to prepare a magnetic coating 
composition. 
The resulting coating composition was coated on a 14 .mu.m thick 
polyethylene terephthalate film to a dry thickness of 5 .mu.m. After 
drying, the film was supercalendered and then slit to a width of 1/2 inch 
to produce a video metal tape. The resulting tape samples were designated 
as Samples 1 to 18. Each of the samples was evaluated for various 
characteristics and corrosive action on a permalloy head in accordance 
with the following test methods. The results obtained are shown in Table 
2. 
For comparison, Sample 19 was prepared in the same manner as for Samples 1 
to 18, except for replacing the component (b) with VMCH (vinyl 
chloride/vinyl acetate/maleic acid=86/13/1 by weight). 
TEST METHOD 
(i) Video Output: 
An output at a signal of 4 MHz was measured, as compared with a reference 
tape (VHS standard tape SAG produced by Fuji Photo Film Co., Ltd.) as 0 
dB. The measurement was effected on a VHS type video tape recorder 
"NV-6600" manufactured by Matsushita Electric Industries, Ltd. 
(ii) S/N Ratio: 
An S/N ratio at a frequency in the range of from 10 KHz to 4 MHz. Visual 
correction was conducted. The measurement was effected on "NV-6600". 
(iii) Video Running Properties: 
A sample tape was run on 50 commercially available VHS type video tape 
recorders at conditions of 25.degree. C., 50% RH or 40.degree. C., 80% RH 
to observe jitter and skew. The running properties were evaluated 
according to the following grades: 
A: No jitter or skew was observed. 
B: Jitter or skew sometimes occurred, but are acceptable in practical use. 
C: Jitter or skew occurred with fairly high frequency and are not 
acceptable in practical use. 
(iv) Increase of DO (Drop Outs) 
An increase of DO was measured simultaneously with the measurement of 
running properties. 
A: D.O. increased by less than 10/min. 
B: D.O. increased by less than 20/min. 
C: D.O. increased by less than 30/min. 
D: D.O. increased by 30/min. or more. 
(v) Still Life (Durability at Still Mode): 
A time (min.) until a reproduced still image developed significant defects 
in it and the RF output decreased to one third or less. The measurement 
was effected on "NV-6600". 
(vi) Corrosion of Permalloy Head: 
A same tape was left to stand in contact with a permalloy head for one week 
in an atmosphere of 60.degree. C. and 85% RH, and corrosion of the head 
was observed under a microscope at 200 magnifications. 
TABLE 1 
______________________________________ 
Epoxy Group --SO.sub.3 Na Group 
Sample 
Content Content 
No. (wt. %)* (wt. %)* Remark 
______________________________________ 
1 0.2 0.5 Invention 
2 0.7 0.5 " 
3 1.2 0.5 " 
4 1.7 0.5 " 
5 2.2 0.5 " 
6 2.7 0.5 " 
7 3.5 0.7 " 
8 4.5 0.7 " 
9 6.5 0.7 " 
10 8.5 0.7 " 
11 1.7 0 Comparison 
12 1.7 0.2 Invention 
13 1.7 0.6 " 
14 1.7 0.8 " 
15 1.7 1.5 " 
16 0 0.2 Comparison 
17 0 0.8 " 
18 0 0 " 
19 0 0 " 
______________________________________ 
Note: 
*wt % based on the vinyl chloride resin. 
TABLE 2 
______________________________________ 
Video 
Sam- Video S/N Running 
Still Corro- 
ple Output Ratio Prop- Life sion D.O. 
No. (dB) (dB) erties (min) of Head 
Increase 
______________________________________ 
1 0.3 0.2 A &gt;120 none A 
2 0.4 0.3 A " " A 
3 0.7 0.5 A " " A 
4 0.9 0.6 A " " A 
5 0.4 0.4 A " " A 
6 0.2 0.2 A " " B 
7 0.2 0.2 A " " A 
8 0.3 0.3 A " " A 
9 0.3 0.2 A " " A 
10 0.2 0.3 A " " A 
11 -0.1 -0.5 A " " B 
12 0 0.5 A " " B 
13 0.4 0.5 A " " A 
14 0.4 0.4 A " " A 
15 0.1 0.5 A " " A 
16 0 -0.1 B 50 observed 
C 
17 0 -0.2 B 20 " C 
18 -0.4 -0.1 B 10 " C 
19 0 -0.1 A 75 " D 
______________________________________ 
As can be seen from Tables 1 and 2 above, Samples 11, 18 and 19 wherein 
vinyl chloride resins containing no --SO.sub.3 Na group were used are 
inferior in video output and S/N ratio, and Samples 16, 17, 18 and 19 
wherein vinyl chloride resins containing no epoxy group were used show 
corrosive action on a permalloy head and have low S/N ratios. Further, 
samples 16, 17 and 18 undergo a significant increase of DO. To the 
contrary, samples according to the present invention are excellent in 
various characteristics and cause no corrosion of the permalloy head. 
Thus, video tapes in accordance with the present invention makes it 
possible to effectively prevent corrosion of the VTR running system and to 
provide images excellent in various video characteristics. 
EXAMPLE 2 
______________________________________ 
(a) Vinyl chloride resin (vinyl chloride = 
30* 
87 wt %, vinyl acetate = 13 wt %; number 
(parts) 
average molecular weight: 2.6 .times. 10.sup.4), to 
which an epoxy group and a --SO.sub.3 Na group 
had been bonded in amounts shown in Table 3 
(b) Thermoplastic polyurethane resin (butylene 
10 
adipate/diphenylmethane diisocyanate; 
number average molecular weight: 3.5 .times. 10.sup.4 ; 
Tg: -26.degree. C.) 
(c) Polyisocyanate ("Coronate" 25roduced by 
Nippon Polyurethane Industry (Co., Ltd.) 
(d) Calcium carbonate fine powder (average 
150 
particle size: 0.07 .mu.m) 
(e) Methyl ethyl ketone 650 
______________________________________ 
*parts of the vinyl chloride resin 
The above components (a), (b), (d) and (e) were thoroughly kneaded in a 
ball mill, and the component (c) was added thereto to prepare a coating 
composition. The composition was coated on a back side of a magnetic 
recording tape having a magnetic recording layer prepared according to 
Example 1 above to form a backing layer having a dry thickness of from 0.8 
to 1.2 .mu.m on the side opposite to the magnetic recording layer. The 
resulting samples were designated as Samples A-1 to A-18. 
For comparison, Sample A-19 was prepared in the same manner as described 
above except for replacing the component (a) with VMCH (vinyl 
chloride/vinyl acetate/maleic acid=86/13/1 by weight). 
Each of Samples A-1 to A-19 was evaluated for running durability in 
accordance with the following test methods. 
TEST METHODS 1 
A tape sample was run on a VHS type video tape recorder for 100 passes, and 
the lead-in tension (T.sub.1) and lead-out tension (T.sub.2) at the rotary 
cylinder were measured for each of the virgin tape and the tape after 100 
passes to evaluate running durability. 
Further, output variation after 100 passes was also determined. 
TEST METHOD 2 
A sample tape was run on a VHS type video tape recorder, and the degree of 
wear (scratches) of the backing layer surface and the degree of damage of 
the tape (folds and waving) after 100 passes were observed. In the 
evaluation of wear of backing layer, "very slight" means 0 to 4 scratches 
and "many" means 10 or more scratches. In the evaluation of tape damages, 
"very slight" means 0 to 4 tape damages and "many" means 10 or more tape 
damages. 
The results of these tests are shown in Table 4. 
TABLE 3 
______________________________________ 
Epoxy Group --SO.sub.3 Na Group 
Sample 
Content Content 
No. (wt %)* (wt %)* Remark 
______________________________________ 
1 0.2 0.5 Invention 
2 0.7 0.5 " 
3 1.2 0.5 " 
4 1.7 0.5 " 
5 2.2 0.5 " 
6 2.7 0.5 " 
7 3.5 0.7 " 
8 4.5 0.7 " 
9 6.5 0.7 " 
10 8.5 0.7 " 
11 1.7 0 Comparison 
12 1.7 0.2 Invention 
13 1.7 0.6 " 
14 1.7 0.8 " 
15 1.7 1.5 " 
16 0 0.2 Comparison 
17 0 0.8 " 
18 0 0 " 
19 0 0 " 
______________________________________ 
Note: 
*wt % based on the vinyl chloride resin. 
TABLE 4 
__________________________________________________________________________ 
Sample No. 
A-1 A-2 A-3 A-4 A-5 A-6 A-7 A-8 A-9 A-10 
__________________________________________________________________________ 
Output Variation 
0.4 0.1 0.2 0.1 0.2 0.4 0.2 0.2 0.2 0.2 
After 100 Passes 
(dB) 
T.sub.2 /T.sub.1 (g) of 
75/35 
68/34 
86/39 
69/35 
68/35 
79/35 
80/35 
68/34 
67/35 
75/35 
Virgin Tape 
T.sub.2 /T.sub.1 (g) After 
71/35 
65/34 
84/39 
69/35 
65/35 
77/35 
79/35 
69/35 
66/34 
72/35 
100 Passes 
Wear of Backing 
very 
very 
very 
very 
very 
very 
very 
very 
very 
very 
Layer (scratches) 
slight 
slight 
slight 
slight 
slight 
slight 
slight 
slight 
slight 
slight 
Damage of Tape 
very 
none 
none 
none 
none 
very 
none 
none 
none 
none 
(folds, and 
slight slight slight 
waving) 
__________________________________________________________________________ 
Sample No. 
A-11 
A-12 A-13 
A-14 A-15 
A-16 A-17 
A-18 A-19 
__________________________________________________________________________ 
Output Variation 
1.1 0.3 0.2 0.1 0.4 1.2 1.2 1.1 0.9 
After 100 Passes 
(dB) 
T.sub.2 /T.sub.1 (g) of 
108/39 
76/37 
67/35 
80/38 
77/33 
106/38 
110/37 
115/34 
101/39 
Virgin Tape 
T.sub.2 /T.sub.1 (g) After 
105/39 
74/37 
65/35 
75/38 
76/33 
101/38 
107/37 
110/34 
100/39 
100 Passes 
Wear of Backing 
many 
very very 
very very 
many many 
many many 
Layer (scratches) 
slight 
slight 
slight 
slight 
Damage of Tape 
many 
very none 
none very 
many many 
many many 
(folds, and slight slight 
waving) 
__________________________________________________________________________ 
EXAMPLE 3 
______________________________________ 
(a) Vinyl chloride resin (vinyl chloride = 
30* 
80 wt %, vinylidene chloride = 20 wt %; 
(parts) 
number average molecular weight: 
2.6 .times. 10.sup.4), to which an epoxy group and 
a --SO.sub.3 Na group had been bonded in amounts 
shown in Table 3 
(b) Thermoplastic polyurethane resin (the 
20 
same as used in Example 2) 
(c) Polyisocyanate (the same as used in 
45 
Example 2) 
(d) Carbon black (particle size: 1.7 .mu.m) 
190 
Carbon black (particle size: 280 .mu.m) 
50 
(e) Methyl ethyl ketone 960 
______________________________________ 
*parts of the vinyl chloride resin 
A magnetic tape was produced in the same manner as described in Example 2 
except for using the above composition as a backing layer coating 
composition. The resulting samples were designated as Samples B-1 to B-18. 
Each of the samples was subjected to the same tests as in Example 2, and 
the results obtained are shown in Table 5 below. 
TABLE 5 
__________________________________________________________________________ 
Sample No. 
B-1 B-2 B-3 B-4 B-5 B-6 B-7 B-8 B-9 
__________________________________________________________________________ 
Output Variation 
0.4 0.2 0.2 0.2 0.2 0.3 0.1 0.2 0.1 
After 100 Passes 
(dB) 
T.sub.2 /T.sub.1 (g) of 
77.35 
86/38 
68/34 
68/35 
67/35 
75/35 
77/33 
67/34 
87/37 
Virgin Tape 
T.sub.2 /T.sub.1 (g) After 
75/33 
83/38 
65/34 
66/35 
63/35 
70/35 
74/33 
65/34 
85/37 
100 Passes 
Wear of Backing 
very 
very 
very 
very 
very 
very 
very 
very 
very 
Layer (scratches) 
slight 
slight 
slight 
slight 
slight 
slight 
slight 
slight 
slight 
Damage of Tape 
very 
none 
none 
none 
none 
very 
none 
none 
none 
(folds, and 
slight slight 
waving) 
__________________________________________________________________________ 
Sample No. 
B-10 
B-11 
B-12 
B-13 
B-14 
B-15 
B-16 
B-17 
B-18 
__________________________________________________________________________ 
Output Variation 
0.1 1.0 0.4 0.1 0.2 0.4 1.1 1.2 1.0 
After 100 Passes 
(dB) 
T.sub.2 /T.sub.1 (g) of 
75/35 
80/37 
79/35 
67/35 
86/39 
76/37 
102/34 
108/35 
112/35 
Virgin Tape 
T.sub.2 /T.sub.1 (g) After 
71/34 
75/37 
76/35 
67/35 
84/39 
76/37 
100/34 
107/35 
106/35 
100 Passes 
Wear of Backing 
very 
many 
very 
very 
very 
very 
many 
many 
many 
Layer (scratches) 
slight slight 
slight 
slight 
slight 
Damage of Tape 
none 
many 
very 
none 
none 
very 
many 
many 
many 
(folds, and slight slight 
waving) 
__________________________________________________________________________ 
As is apparent from Tables 4 and 5 above, magnetic tapes having a backing 
layer in which the binders according to the present invention are used 
exhibit excellent running durability. 
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.