Magnetic recording medium

A magnetic recording medium is described, comprising a nonmagnetic support having provided thereon a first magnetic layer and a second magnetic layer in this order, each containing ferromagnetic particles, and the first and second magnetic layers each comprising at least one polyurethane resin as a binder, where the number average molecular weight of the at least one polyurethane resin contained in the first magnetic layer is 4/5 or less of that of the at least one polyurethane resin contained in the second magnetic layer.

FIELD OF THE INVENTION 
This invention relates to a magnetic recording medium comprising a 
non-magnetic support and a magnetic layer, more particularly, it relates 
to a magnetic recording medium having at least two magnetic layers. 
BACKGROUND OF THE INVENTION 
A magnetic recording medium is widely used as an audio tape, a video tape 
or a floppy disk. Certain magnetic recording media are fundamentally 
composed of a non-magnetic support having thereon a magnetic layer 
containing ferromagnetic particles dispersed in a binder. 
High performance characteristics such as electromagnetic characteristics, 
running durability or running property are required for a magnetic 
recording medium. That is, higher ability for reproducing original sound 
is required for an audio tape for recording and reproducing music. 
Excellent electromagnetic characteristics are required for a video tape to 
ensure ability for reproducing original images. 
It is known that the electromagnetic characteristics of a magnetic 
recording medium comprising ferromagnetic particles remarkably vary 
depending upon the dispersion state of the ferromagnetic particles in the 
magnetic layer. That is, even though ferromagnetic particles having 
excellent magnetic properties are used to improve electromagnetic 
characteristics, the excellent magnetic properties do not yield an 
improvement in electromagnetic characteristics if the dispersion state 
thereof is poor. 
One approach to improving the dispersibility of ferromagnetic particles in 
a magnetic layer is long term mixing, kneading and dispersing in preparing 
the magnetic coating composition used for a magnetic layer. However, there 
is the problem of decrease the magnetic properties of ferromagnetic 
particles if mixing, kneading and dispersing is conducted for too long a 
term. 
Recently, it was proposed that a polar group should be introduced into the 
resin component used to form the binder so that the binder in the magnetic 
layer has good affinity with the ferromagnetic particles. 
For example, it is disclosed in JP-A-59-5424 that resins having a 
predetermined polar group such as a metal sulfonate group should be used 
in the binder in an amount of 50 wt % or more. The term "JP-A" as used 
herein means"an unexamined published Japanese patent application". As 
disclosed above, good dispersibility of ferromagnetic metal particles in a 
magnetic layer can be secured using resins having a polar group as a 
binder, and accordingly, electromagnetic characteristics of a magnetic 
recording medium using such a magnetic layer can be improved 
However, a magnetic layer using a polar group containing resin as a binder 
tends to be hard. Such a hard magnetic layer is likely to adhere 
incompletely to the support, and the smoothness of the magnetic layer 
tends to be deteriorated due to poor moldability during calendering. 
On the other hand, in view of the molecular weight of resins used as 
binders, it is known that among resins having the above described polar 
groups, resins having a lower molecular weight are preferred, because 
dispersibility of ferromagnetic metal particles is better and 
electromagnetic characteristics are improved. However, when resins having 
a low molecular weight are used as a binder in a magnetic layer, the layer 
tends to be fragile. A fragile magnetic layer has insufficient strength 
and surface hardness, and tends to have poor running properties and 
durability. 
Accordingly, even though electromagnetic characteristics are fairly good, 
durability is not satisfactorily excellent. 
SUMMARY OF THE INVENTION 
An object of this invention is to provide a magnetic recording medium such 
as an audio tape, video tape or the like having excellent electromagnetic 
characteristics and improved durability. 
This invention relates to a magnetic recording medium comprising a 
non-magnetic support having provided thereon a first magnetic layer and a 
second magnetic layer in this order each containing ferromagnetic 
particles, and the first and second magnetic layers each comprising at 
least one polyurethane resins as a binder, where the number average 
molecular weight of the polyurethane type resins contained in the first 
magnetic layer is 4/5 or less of that of the at least one polyurethane 
resin contained in the second magnetic layer.

DETAILED DESCRIPTION OF THE INVENTION 
Thus, this invention relates to a magnetic recording medium having at least 
two magnetic layers where the under layer (the first magnetic layer) uses 
a binder composed of at least one polyurethane type resin having a low 
number average molecular weight and the upper layer (the second magnetic 
layer) uses a binder composed of polyurethane type resins having a high 
number average molecular weight which is within a predetermined molecular 
weight range. 
In the above described under layer, the ferromagnetic particles have good 
dispersibility and the surface of the magnetic layer is extremely smooth. 
Accordingly, a magnetic recording medium having good electromagnetic 
characteristics in a low or high wave length region, improved moldability 
during calendering and also good durability and practical property can be 
obtained by providing an upper layer having good durability on the under 
layer having a smooth surface. 
The magnetic recording medium of this invention is essentially composed of 
a non-magnetic support having thereon at least two magnetic layers 
containing ferromagnetic particles dispersed in a binder. 
The non-magnetic supports for use in this invention are conventional and 
include films or sheets composed of polyesters such as polyethylene 
terephthalate (PET) or polyethylene naphthalate, polyolefins such as 
polypropylene, cellulose derivatives such as cellulose triacetate or 
cellulose diacetate, vinyl type resins such as polyvinyl chloride or 
polyvinylidene chloride, synthetic resins such as polycarbonate, 
polyamide, polyamidoimide or polyimide resins; non-magnetic metal foils 
such as aluminum or copper; metal foils such as stainless steel foil; and 
paper or ceramic sheets. 
These supports have thickness of generally from 2.5to 100 .mu.m, and 
preferably from 3 to 70 .mu.m. 
The binder of this invention, which is contained in the first and second 
magnetic layers, is composed of polyurethane type resins, and the number 
average molecular weight of polyurethane type resins(s) contained in the 
first magnetic layer is 4/5 or less that of the polyurethane type resin 
contained in the second magnetic layer. Further, the weight percentage of 
the polyurethane type resins per ferromagnetic particles contained in the 
first magnetic layer is preferably 5/4 or more of that of the polyurethane 
type resin(s) contained in the second magnetic layer. 
The number average molecular weight of the polyurethane type resin(s) 
contained in the second magnetic layer is preferably from 2,000 to 
200,000, more preferably from 10,000 to 100,000. When the molecular weight 
thereof is less than 2,000, still characteristic (i.e., durability in the 
case a video tape is in the still mode) is deteriorated, and when the 
molecular weight exceeds 200,000, the dispersibility of ferromagnetic 
particles is deteriorated. The number average molecular weight of the 
polyurethane type resin(s) contained in the first magnetic layer is 
generally from 1/10 to 4/5 of that of the polyurethane type resin(s) 
contained in the second magnetic layer, preferably from 1/10 to 3/5 and 
more preferably from 1/10 to 1/2. 
The dispersibility of ferromagnetic particles is excellent and the surface 
of a magnetic layer is made extremely smooth by adjusting the number 
average molecular weight of the polyurethane type resin(s) contained in 
the first magnetic layer to 4/5 or less of that of the polyurethane type 
resin(s) contained in the second magnetic layer. Accordingly, by providing 
the second magnetic layer on the first magnetic layer, excellent surface 
smoothness of the composite magnetic layers can be obtained. Also, by 
using a polyurethane type resin(s) having a lower number average molecular 
weight in the first magnetic layer, moldability while calendering of the 
composite magnetic layers is improved and electromagnetic characteristics 
are excellent. 
Regarding the content of polyurethane type resin(s) per ferromagnetic 
particles, it is preferred that the weight percentage of polyurethane type 
resin(s) per ferromagnetic particles contained in the first magnetic layer 
is 5/4 or more of that of the polyurethane resin per the ferromagnetic 
particles contained in the second magnetic layer, and particularly 3/2 to 
5/1 of that. By adjusting the weight percentage to the range as described 
above, the moldability while calendering of the composite magnetic layers 
is improved and, thus, the adhesion of magnetic layers to the support is 
improved. 
The polyurethane type resins for use in this invention are not particularly 
limited. Particularly, the polyurethane type resins as described in U.S. 
Pat. Nos. 4,152,485 and 4,521,486 can be used. For example, polyester type 
polyurethane resins, polyether type polyurethane resins, polyurethane type 
resins having introduced therein a polar group such as hydroxyl group, a 
carboxyl group, a phosphoric acid group, a phosphoric acid ester group, 
--SO.sub.3 Na or --SO.sub.2 Na, and polycarbonate polyurethane resins can 
be used. 
Binder resins for use in each magnetic layer may be used in addition to the 
above polyurethane type resins and they are not particularly limited. The 
binder resins include vinyl chloride type copolymer (e.g., copolymers of 
vinyl chloride and vinyl acetate, copolymers of vinyl chloride, vinyl 
acetate and vinyl alcohol, copolymers of vinyl chloride, vinyl acetate and 
acrylic acid, copolymers of vinyl chloride and vinylidene chloride, 
copolymers of vinyl chloride and acrylonitrile, and copolymers of ethylene 
and vinyl acetate), cellulose derivatives such as a nitrocellulose resins, 
acrylic resins, polyvinyl acetal resins, polyvinyl butyral resins, epoxy 
resins and phenoxy resins. Vinyl chloride type copolymers having a polar 
group such as a hydroxyl group, a carboxyl group, an epoxy group, a 
sulfonic acid metal salt group, a phosphoric acid group or a phosphoric 
acid ester group are particularly preferred. 
The above copolymers and resins can be used alone or in combination. 
The polyurethane type resin(s) contained in the first magnetic layer is 
contained in an amount of preferably 10 wt % or more and more preferably 
from 20 to 80 wt % in the binder and that contained in the second magnetic 
layer is contained in amount of preferably 5 wt % or more and more 
preferably from 10 to 90 wt % in the binder. The effect of this invention 
cannot be obtained if the contents of the polyurethane type resin(s) is 
less than the above ranges. 
When a hardening agent is used, a polyisocyanate compound is generally 
used. The polyisocyanate compounds are generally selected from those 
polyurethane type resins conventionally used as hardening agents for 
polyurethane type resins. The polyisocyanate compounds include a reaction 
product of tolylenediisocyanate and 1 mole of trimethylol propane (e.g., 
"Desmodule L-75", manufactured by Bayer Co., Ltd.), a reaction product of 
3 moles of a diisocyanate such as xylene diisocyanate or hexamethylene 
diisocyanate, a Biuret adduct product of 3 moles of hexamethylene 
diisocyanate, an isocyanurate compound of 5 moles of tolylene 
diisocyanate, an isocyanurate adduct product of 3 moles of tolylene 
diisocyanate and 2 moles of hexamethylene diisocyanate, and polymer of 
isophorondiisocyanate and diphenylmethane diisocyanate. The amount of the 
polyisocyanate compound used is preferably from 1/2 to 4 times as large as 
the amount of the polyurethane type resins used. 
When hardening treatment is conducted with electron beam irradiation as 
described in JP-A-59-58623 and JP-A-59-71130, compounds having a reactive 
double bond (e.g., urethane acrylate) can be used. 
In this invention, it is preferred that resins having softness, such as the 
polyurethane type resins of this invention, and resins having high 
hardness, such as the vinyl chloride type copolymer having the above 
described polar group, be used in combination as resin components. 
With respect to the vinyl chloride type copolymers, the number average 
molecular weight of the vinyl chloride type copolymer contained in the 
first magnetic layer preferably is different from that contained in the 
second magnetic layer. That is, it is particularly preferred that the 
degree of polymerization of the copolymer of the vinyl chloride type 
contained in the first magnetic layer is lower by 20 or more than that 
contained in the second magnetic layer. This invention becomes most 
effective under the above conditions. 
A higher content of binder of binders is/are used in case when 
ferromagnetic metal particle having a low hardness are used as 
ferromagnetic particles than in the case when .gamma.-Fe.sub.2 O.sub.3 
having high hardness is used. In this instance, the soft resins such as 
the polyurethane type resins are generally used in a larger amount. 
The binder tends to soften with increasing amounts of the polyurethane type 
resins. Therefore, the hardness of the binder can be maintained by 
increasing the amount of a hardening agent such as a polyisocyanate 
compound(s). 
In the case polyurethane type resins are used as a resin component and 
polyisocyanate-compounds are used as a hardening agent, the mixing weight 
ratio of the polyurethane type resins/polyisocyanate compounds is 
generally from 1/0.8 to 1/2 (preferably form 1/1 to 1/1.5). Even though 
ferromagnetic metal particles having low hardness are used, the softening 
tendency of the binder due to the use of polyurethane type resins can be 
effectively prevented by limiting the mixing ratio as described above. 
The total weight of the resin component and hardening agents is preferably 
from 5 to 40 parts by weight, and more preferably from 10 to 20 parts by 
weight, per 100 parts by weight of the ferromagnetic particles in the 
layer. 
Ferromagnetic particles for use in this invention include ferromagnetic 
particles of metal oxides such as .gamma.-Fe.sub.2 O.sub.3, ferromagnetic 
particles of metal-doped metal oxides such as co-containing 
.gamma.-Fe.sub.2 O.sub.3, and ferromagnetic metal particles containing 
ferromagnetic metals such as iron, cobalt or nickel. 
When ferromagnetic metal particles are used, the ferromagnetic metal 
particles containing iron, cobalt or nickel preferably have a specific 
surfaces area(B.E.T. method) of generally 42 m.sup.2 /g or higher, and 
preferably 45 m.sup.2 /g or higher. 
Such ferromagnetic metal particles have a metal content of 75 wt % or more, 
and 80 wt % or more of the metal content is at least one kind of 
ferromagnetic metal or alloy, (e.g., Fe, Co, Ni, Fe-Co, Fe-Ni, Co-Ni, 
Co-Ni-Fe, and 20 wt % or less of the metal content is another components 
(e.g., Al, Si, S, Sc, Ti, V, Cr, Mn, Cu, Y, Mo, Rh, Pd, Ag, Sn, Sb, B, Ba, 
Ta, W, Re, Au, Hg, Pb, P, La, Ce, Pr, Nd, Te, Bi). The above described 
ferromagnetic metal can include a slight amount of water, hydroxides or 
oxides. 
The method for preparing such ferromagnetic particles is known and 
conventional methods (for example, methods as described in Chemistry and 
Industry of Magnetic Substance, Tekumato K. K.) can be used in this 
invention. 
The shape of the ferromagnetic particles is not particularly limited, and 
ferromagnetic particles which are acicular, granular, dice-like, 
ellipsoidal and plate-like can be used. Acicular ferromagnetic particles 
are particularly preferred. The acicular ferromagnetic particles have an 
acicular ratio (long axis/short axis) of preferably from 3 to 20 and 
particularly preferably from 4 to 7. 
Further, the average length in the short axis of the ferromagnetic 
particles which are preferably used in the first magnetic layer is from 
300 to 500 .ANG. (Angstrom), and the avere length in the short axis of the 
ferromagnetic particles which are preferably used in the second magnetic 
layer is from 200 to 350 .ANG.. 
The above described resin components, hardening agents and ferromagnetic 
particles are conventionally mixed, kneaded and dispersed with a 
conventional solvent (e.g., methyl ethyl ketone, dioxane, cyclohexanone, 
ethyl acetate) to prepare a magnetic coating composition. 
In addition to the above components, generally used additives such as 
abrasive agents (e.g., .alpha.-Al.sub.2 O.sub.3, Cr.sub.2 O.sub.3), 
antistatic agents (e.g., carbon black), lubricating agents (e.g., fatty 
acids, fatty acid esters, silicon oils) or dispersing agents and fillers 
may be added into the magnetic coating composition. Particularly, the 
above abrasive agents preferably have an average particle size of 0.5 
.mu.m or less and the above antistatic agents preferably have an average 
particle size of 150 m.mu. or less. 
The thus prepared magnetic coating composition is coated on a non-magnetic 
support in a conventional manner. For example, the composition for forming 
the magnetic layer such as the resin(s) for the first magnetic layer, 
ferromagnetic particles, and, if desired, abrasive agents and hardening 
agents or the like, are mixed, kneaded and dispersed with a solvent to 
prepare a magnetic coating composition for the first magnetic layer. Also, 
a magnetic coating composition for the second magnetic layer is prepared 
by the same manner as described above. Using a conventional coating method 
for coating a magnetic coating composition on a nonmagnetic support, the 
first magnetic layer is formed and then the second magnetic layer is 
formed thereon. 
One coating method is a conventional method using, for example, a reverse 
roll. 
The dry thickness of the first magnetic layer is preferably from 0.5 to 8 
.mu.m and the dry thickness of the second magnetic layer is preferably 
from 0.1 to 2 .mu.m. 
The dry thickness of a magnetic layer (i.e. the first and second magnetic 
layer) is generally from 0.5 to 10 .mu.m and preferably from 2 to 6 .mu.m. 
A backing layer may be provided on the surface of the non-magnetic support 
opposite the magnetic layer. A backing layer is generally a layer coated 
on the surface of the non-magnetic support opposite the magnetic layer 
with a conventional coating composition for backing layers containing 
granular components such as abrasive agents or antistatic agents and 
binders dispersed in an organic solvent. A method for preparing the 
backing layer used in the present invention is described in U.S. Pat. No. 
4,567,063. 
An adhesive layer may be provided on both the surfaces of the non-magnetic 
support before the magnetic layer and the backing layer are provided, if 
desired. 
A magnetic layer is generally subjected to magnetic orientation to 
orientate the ferromagnetic particles contained in the magnetic layer and 
then dried. 
The dried magnetic layer is then subjected to a surface smoothing 
treatment, using, for example, super calendering rolls. Voids formed due 
to removal of solvent upon drying are removed by providing a surface 
smoothing treatment, thereby improving the packing density of the 
ferromagnetic particles in the magnetic layer, and, thus, a magnetic 
recording medium having excellent electromagnetic characteristics is 
obtained. 
The obtained magnetic layers are subjected to hardening treatment and are 
the cut to the desired shape. 
Cutting is done in a conventional manner using a slitter and the like. 
The magnetic recording medium having under and upper layers of this 
invention has been described, and as long as two magnetic layers as 
defined in this invention are included, three or more layers may be used. 
The present invention will now be illustrated more specifically by the 
following Examples and Comparative Examples. In each Example and 
Comparative Example, all parts are by weight, unless otherwise mentioned. 
EXAMPLE 1 
______________________________________ 
Magnetic coating composition for first magnetic layer: 
______________________________________ 
Co--.UPSILON.Fe.sub.2 O.sub.3 
100 parts 
(average length in the short axis: 
350 .ANG. A (Angstrom), 
average length in the long axis: 
0.20 .mu.m, 
Hc: 650 Oe, .sigma.s: 74 emu/g, 
S BET (specific surface area): 35 m.sup.2 /g) 
Copolymer of vinyl chloride, vinyl 
acetate and maleic anhydride 
8 parts 
(Composition ratio 87/8/5, degree of 
polymerization:400) 
Polyester polyurethane resin 
8 parts 
(Number average molecular 
weight (Mn): 1.6 .times. 10.sup.4) 
.alpha.-alumina (average particle size: 0.2 .mu.m) 
3 parts 
Butyl stearate 1 part 
Stearic acid 2 part 
Butyl acetate 300 parts 
______________________________________ 
Coating composition for second magnetic layer: 
______________________________________ 
Co--.UPSILON.Fe.sub.2 O.sub.3 
100 parts 
(average length in the short axis: 
290 .ANG. A, 
average length in the long axis: 
0.18 .mu.m 
Hc: 750 Oe, .sigma.s: 74 emu/g 
S BET (specific surface area): 45 m.sup.2 /g) 
Copolymer of vinyl chloride, vinyl 
acetate and maleic anhydride 
8 parts 
(Composition ratio 87/8/5, degree of 
polymerization:400) 
Polyester polyurethane resin 
8 parts 
(Number average molecular 
weight (Mn): 2.0 .times. 10.sup.4) 
.alpha.-alumina (average particle size: 0.2 .mu.m) 
3 parts 
Butyl stearate 1 part 
Stearic acid 2 part 
Butyl acetate 300 parts 
______________________________________ 
Each of the above compositions was mixed, kneaded and dispersed in a sand 
mill, and the resulting dispersion was filtered using a filter having an 
average pore size of 1 .mu.m and collected to obtain a magnetic coating 
composition for the first and the second magnetic layers. 
The thus obtained coating composition for forming a first magnetic layer 
was coated using a reverse roll on a polyethylene terephthalate support 
having a thickness of 15 .mu.m, travelling at a rate of 60 m/min. so that 
the dry thickness was 3.0 .mu.m. Then the coating composition for forming 
a second magnetic layer was coated thereon using a reverse roll in a dry 
thickness of 1.0 .mu.m, the magnetic layers were subjected to orientation 
using magnets having a magnetic force of 3000 gauss, while they were wet, 
and then dried and were subjected to a super calendering treatment, and 
slit to a 1/2inch.multidot.width to prepare a video tape. 
EXAMPLE 2 
By following the same procedure as in Example 1, a video tape was prepared 
except that a polyester polyurethane resin having a lower molecular weight 
(number average molecular weight (Mn): 1.2.times.10.sup.4) was used 
instead of the polyester polyurethane resin (number average molecular 
weight (Mn): 1.6.times.10.sup.4) used in the coating composition for 
forming a first magnetic layer in EXAMPLE 1. 
EXAMPLE 3 
By following the same procedure as in Example 1, a video tape was prepared 
except that a polyester polyurethane resin having a lower molecular weight 
(number average molecular weight (Mn): 0.8.times.10.sup.4) was used 
instead of the.;polyester polyurethane resin (number average molecular 
weight (Mn): 1.6.times.10.sup.4) in the coating composition for forming 
the first magnetic layer. 
EXAMPLE 4 
By following the same procedure as in Example 1, a video tape was prepared 
except that a polyester polyurethane resin having a lower molecular weight 
(number average molecular weight (Mn): 0.4.times.10.sup.4) was used 
instead of the polyester polyurethane resin (number average molecular 
weight (Mn): 1.6.times.10.sup.4) used in the coating composition for 
forming the first magnetic layer. 
EXAMPLE 5 
By following the same procedure as in Example 2, a video tape was prepared 
except that a polyester polyurethane having a higher molecular weight 
(number average molecular weight (Mn): 6.0.times.10.sup.4) was used 
instead of the polyester polyurethane resin (number average molecular 
weight (Mn): 2.0.times.10.sup.4) used in the coating composition for 
forming the second magnetic layer in Example 2. 
EXAMPLE 6 
By following the same procedure as in Example 1, a video tape was prepared 
except that a polyester polyurethane resin having a lower molecular weight 
(number average molecular weight (Mn): 0.6.times.10.sup.4) was used 
instead of the polyester polyurethane resin (number average molecular 
weight (Mn): 1.6.times.10.sup.4) used in the coating composition for 
forming the first magnetic layer and a polyester polyurethane resin having 
a lower molecular weight (number average molecular weight (Mn): 
1.0.times.10.sup.4) was used instead of the polyester polyurethane resin 
(number average molecular weight (Mn): 2.0.times.10.sup.4) used in the 
coating composition for forming the second magnetic layer. 
EXAMPLE 7 
By following the same procedure as in Example 2, a video tape was prepared 
except that 10 parts of the same polyester polyurethane resin (number 
average molecular weight (Mn): 1.2.times.10.sup.4) was used instead of 8 
parts thereof as were used in the coating composition for forming the 
first magnetic layer. 
EXAMPLE 8 
By following the same procedure as in Example 2, a video tape was prepared 
except that 12 parts of the same polyester polyurethane resin (number 
average molecular weight (Mn): 1.2.times.10.sup.4) was used instead of 8 
parts of the same resin as used in the coating composition for forming the 
first magnetic layer. 
EXAMPLE 9 
By following the same procedure as in Example 2, a video tape was prepared 
except that 16 parts instead of 8 parts of the same polyester polyurethane 
resin (number average molecular weight (Mn): 1.2.times.10.sup.4) used in 
the coating composition for forming a first magnetic layer was used. 
EXAMPLE 10 
By following the same procedure as in Example 1, a video tape was prepared 
except that a copolymer of vinyl chloride, vinyl acetate and maleic 
anhydride having a lower degree of polymerization of 300 was used instead 
of the copolymer of vinyl chloride, vinyl acetate and maleic anhydride 
having a degree of copolymerization of 400 used in the coating composition 
for forming the first magnetic layer. 
EXAMPLE 11 
By following the same procedure as in Example 1, a video tape was prepared 
except that a copolymer of vinyl chloride, vinyl acetate and maleic 
anhydride having a lower degree of polymerization of 250 was used instead 
of the copolymer of vinyl chloride, vinyl acetate and maleic anhydride 
having a degree of polymerization of 400 as was used in the coating 
composition for forming a first magnetic layer. 
COMATIVE EXAMPLE 1 
By following the same procedure as in Example 1, a video tape was prepared 
except that a polyester polyurethane resin (number average molecular 
weight (Mn): 2.0.times.10.sup.4) was used instead of the polyester 
polyurethane resin (number average molecular weight (Mn): 
1.6.times.10.sup.4) used in the coating composition for forming the first 
magnetic layer. 
COMATIVE EXAMPLE 2 
By following the same procedure as in Example 1, a video tape was prepared 
except that a polyester polyurethane resin (number average molecular 
weight (Mn): 6.0.times.10.sup.4) was used instead of the polyester 
polyurethane (average molecular weight (Mn): 1.2.times.10.sup.4) used in 
the coating composition for forming the first magnetic layer in Example 5. 
The number average molecular weight (Mn) and the additive amounts of 
polyester polyurethane used in each layer of the video cassette tapes 
obtained in the Examples and Comparative Examples are shown in Table 1, 
and the results of measuring the physical properties of each tape in the 
following manner are shown in Table 2. 
MEASUREMENT 
(1) Surface roughness (Ra) 
The surface roughness of a magnetic layer of a video tape was measured at a 
cut off value of 0.25 mm using a three-dimensional roughness measurement 
device (SE-3AK manufactured by Kosaka Kenkyusho, Co., Ltd.) according to 
JIS B 0601. 
(2) Maximum residual flux density (Bm) 
The maximum residual flux density was measured using a sample oscillating 
flux meter (VSM-III, manufactured by Toei Kogyo Co., Ltd.). 
(3) Y.multidot.S/N (signal to noise ratio of brilliance signal of video) 
The S/N of luminance signals at 4 MHz was measured, assuming that the 
output level of the video tape in Comparative Example 1 is 0 dB. 
(4) Still life 
Signals on each video tape were reproduced to the still mode and the period 
of time for S/N to decrease by 6 dB was measured. Output was measured 
using an output level measurement apparatus "NV-870 HD type" (manufactured 
by Matsushita Electric Industrial Co., Ltd.). 
TABLE 1 
______________________________________ 
Polyurethane in 
Polyurethane in 
the first layer 
the second layer 
Parts by Parts by 
Mn weight Mn weight 
______________________________________ 
Example 
1 1.6 .times. 10.sup.4 
8.0 2.0 .times. 10.sup.4 
8.0 
2 1.2 .times. 10.sup.4 
8.0 2.0 .times. 10.sup.4 
8.0 
3 0.8 .times. 10.sup.4 
8.0 2.0 .times. 10.sup.4 
8.0 
4 0.4 .times. 10.sup.4 
8.0 2.0 .times. 10.sup.4 
8.0 
5 1.2 .times. 10.sup.4 
8.0 6.0 .times. 10.sup.4 
8.0 
6 0.6 .times. 10.sup.4 
8.0 1.0 .times. 10.sup.4 
8.0 
7 1.2 .times. 10.sup.4 
10.0 2.0 .times. 10.sup.4 
8.0 
8 1.2 .times. 10.sup.4 
12.0 2.0 .times. 10.sup.4 
8.0 
9 1.2 .times. 10.sup.4 
16.0 2.0 .times. 10.sup.4 
8.0 
10 1.6 .times. 10.sup.4 
8.0 2.0 .times. 10.sup.4 
8.0 
11 1.6 .times. 10.sup.4 
8.0 2.0 .times. 10.sup.4 
8.0 
Comparative 
Example 
1 2.0 .times. 10.sup.4 
8.0 2.0 .times. 10.sup.4 
8.0 
2 6.0 .times. 10.sup.4 
8.0 6.0 .times. 10.sup.4 
8.0 
______________________________________ 
TABLE 2 
______________________________________ 
Ra Bm Y .multidot. S/N 
Still life 
(.mu.m) 
(G) (dB) (min.) 
______________________________________ 
Example 
1 0.011 1860 0.5 100 
2 0.010 1900 0.7 100 
3 0.009 1920 0.8 100 
4 0.007 1980 1.0 100 
5 0.013 1880 0.2 120 
6 0.008 1940 0.9 100 
7 0.009 1920 0.8 100 
8 0.008 1930 0.9 100 
9 0.007 1950 1.0 100 
10 0.009 1900 0.8 100 
11 0.007 1960 1.0 100 
Comparative 
Example 
1 0.015 1820 0.0 100 
2 0.018 1740 -0.6 120 
______________________________________ 
In Table 1, in Examples 1 to 4, the same polyester polyurethane resin 
(number average molecular weight (Mn): 2.0.times.10.sup.4) for the second 
magnetic layer was used, and the number average molecular weight (Mn) of 
the polyester polyurethane resin for the first magnetic layer was 
decreased from 1.6.times.10.sup.4 (Example 1) and the effect of the 
decrease was checked. It was clearly seen that surface roughness (Ra), Bm, 
Y.S/N were improved by decreasing the molecular weight of the polyester 
polyurethane resin used for the first magnetic layer and that 
electromagnetic characteristics were also improved. 
Example 5 illustrates the case where the difference of number average 
molecular weight of the polyurethane resin used in the first magnetic 
layer and the second magnetic layer is high, and Example 6 illustrate the 
case when the difference is low contrary to Example 5. In Example 5, using 
a polyester polyurethane resin having a high number average molecular 
weight in the second layer, surface roughness (Ra), Bm and Y.S/N are 
slightly deteriorated compared to the other Examples, but still life is 
excellent. 
Examples 7 to 9 illustrate the case where the polyurethane resin content 
per ferromagnetic particles in the first magnetic layer is increased. As 
the content is increased, surface roughness (Ra), Bm and Y.S/N are 
slightly improved and electromagnetic characteristics are improved. 
Examples 10 and 11 illustrate the case where the molecular weight (degree 
of polymerization) of the copolymer of vinyl chloride, vinyl acetate and 
maleic anhydride (used in combination with the polyurethane resins) was 
decreased to see the effect of the decrease. It is clearly seen that 
surface roughness (Ra), Bm and Y.S/N were slightly improved and 
electromagnetic characteristics were improved. 
On the other hand, in Comparative Examples 1 and 2 using the polyurethane 
resins having the same number average molecular weight in the first and 
the second magnetic layer, it is clearly seen that while still life is as 
good as that in the Examples, surface roughness (Ra), Bm and Y.S/N, which 
are related to electromagnetic characteristics, were inferior. 
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.