Magnetic recording medium

A magnetic recording medium comprising a nonmagnetic support having on one surface thereof a magnetic layer and on the opposite surface thereof a backing layer comprising nonmagnetic particles dispersed in a binder, wherein said binder of said backing layer comprises from about 20 to 90 wt % of polycarbonate polyurethane, and said backing layer comprises from about 40 to 200 parts by weight of said binder per 100 parts by weight of said nonmagnetic particles.

FIELD OF THE INVENTION 
The present invention relates to a magnetic recording medium having an 
improved backing layer, and more particularly it relates to a magnetic 
recording medium having excellent electromagnetic characteristics and 
running durability at a high temperature and a high humidity. 
BACKGROUND OF THE INVENTION 
A magnetic recording medium is widely used for recording electromagnetic 
signals, and there are various shapes of a magnetic recording medium such 
as a tape, a disk or a card according to usage. A magnetic recording 
medium tape is used for audio recording, video recording and for 
computers. Hereinafter, a magnetic recording medium tape is referred to as 
a "magnetic tape". 
A magnetic tape is basically composed of a nonmagnetic support and a 
magnetic layer containing ferromagnetic particles provided thereon. A 
magnetic tape further provided with a backing layer (back coat layer) on 
the opposite surface has been developed to improve running efficiency and 
is now used. 
The backing layer using polyester polyurethane or polyether polyurethane as 
a binder and containing nonmagnetic particles dispersed in the binder has 
conventionally been known. 
Polyester polyurethane has advantages such as a high tear strength, high 
solubility in a selected solvent, or the like, but has the disadvantage of 
extremely poor hydrolysis resistance. Although polyether polyurethane is 
excellent in hydrolysis resistance, it is much poorer than polyester 
polyurethane in mechanical strength and magnetic characteristics necessary 
for a magnetic tape when polyether polyurethane is used as a binder. 
Accordingly, a magnetic recording medium using polyester polyurethane or 
polyether polyurethane as a binder for a backing layer has the defects 
that electromagnetic characteristics and running durability are greatly 
deteriorated at a high temperature and a high humidity. 
One purpose of a backing layer is to improve running efficiency of a 
magnetic recording medium, and it is very important to develop a backing 
layer with good characteristics at a high temperature and a high humidity. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a magnetic recording 
medium excellent in running efficiency and electromagnetic characteristics 
even at a high temperature and a high humidity. 
Another object of the present invention is to provide a binder composition 
for use in a backing layer exhibiting stable and excellent characteristics 
even at a high temperature and a high humidity. 
Other objects and effects of the present invention will be apparent from 
the following description. 
It has now been found that the above objects of the present invention are 
attained by a magnetic recording medium comprising a nonmagnetic support 
having on one surface thereof a magnetic layer and on the opposite surface 
thereof a backing layer comprising nonmagnetic particles dispersed in a 
binder, wherein said binder of said backing layer comprises from about 20 
to 90 wt % of polycarbonate polyurethane, and said backing layer comprises 
from about 40 to 200 parts by weight of said binder per 100 parts by 
weight of said nonmagnetic particles. 
DETAILED DESCRIPTION OF THE INVENTION 
A magnetic recording medium composed of a nonmagnetic support having 
provided on one surface a magnetic layer rand on the other surface a 
backing layer is already known. The magnetic recording medium of the 
present invention provides a backing layer having a novel and improved 
composition, and the magnetic recording medium apart from the backing 
layer can be prepared by a conventional method using conventional 
materials and any conventional shape (as described, e.g., in JP-B-56-26890 
(the term "JP-B" used herein means an "examined Japanese patent 
publication"), West German Patent Application (OLS) 3,314,953, U.S. Pat. 
Nos. 4,411,953, 4,474,843, 4,592,952, 4,442,171, 4,414,270, 4,598,014, and 
British Patent 2,082,477B). 
For example, nonmagnetic supports include plastics such as polyethylene 
terephthalate or the like, metals and ceramics in a form of a tape, a 
disk, a drum and the like, which may be pre-treated if necessary. 
A magnetic layer is a layer containing ferromagnetic particles dispersed in 
a binder. Examples of the ferromagnetic particles include .gamma.-iron 
oxide type ferromagnetic particles, cobalt-containing .gamma.-iron oxide 
type ferromagnetic particles, ferromagnetic metal particles, barium 
ferrite and strontium ferrite. Various known additives can be optionally 
added into the magnetic layer. 
The backing layer of the present invention and the composition of the 
backing layer are hereinafter explained in greater detail. 
The backing layer of the present invention contains nonmagnetic particles 
dispersed in a binder containing from about 20 to 90 wt % of polycarbonate 
polyurethane. 
The nonmagnetic particles can be those generally used as a granular filling 
material for use in a backing layer. 
Example of the nonmagnetic particles include carbon black, graphite and 
inorganic filling materials, which can be used alone or in combination. 
Specific examples of inorganic filling materials include those generally 
used as abrasive agents, such as TiO.sub.2, TiO, ZnO, CaO, SnO.sub.2, 
SiO.sub.2, .alpha.-Fe.sub.2 O.sub.3, Cr.sub.2 O.sub.3, .alpha.-Al.sub.2 
O.sub.3, ZnS, MoS.sub.2, BaSO.sub.4, CaSO.sub.4, MgCO.sub.3, BN and SiC. 
These may be used alone or in combination. 
Of those, carbon black is preferably used. Examples of the carbon black 
includes furnace black, thermal black, black for coloring and acetylene 
black. It is preferred to use carbon black having an average particle size 
of from about 5 to 1,000 m.mu., more preferably from about 10 to 300 
m.mu., (determined by an electron microscope), a specific surface area 
measured by a nitrogen absorption method of from about 1 to 800 m.sup.2 
/g, more preferably from about 5 to 500 m.sup.2 /g,a pH of from about 4 to 
11, more preferably from about 7 to 10 (JIS K6221), and a dibutylphthalate 
oil absorption amount of from about 10 to 800 ml/100 g, more preferably 
from about 20 to 500 ml/100 g, (JIS K6221). (The term "JIS" as used herein 
refers to a published "Japanese Industrial Standard".) As to the carbon 
black size, carbon black having an average diameter of from about 5 to 100 
m.mu. is preferred to decrease the surface electric resistance of a coated 
film; carbon black having an average diameter of about 50 to 1,000 m.mu. 
is preferred to adjust the strength of a coated film; carbon black having 
an average diameter of about 100 m.mu. or less is preferred to control 
surface roughness of a coated film and to smooth the surface for reduction 
of spacing loss; and carbon black having an average diameter of about 50 
.mu.m or more is preferred to decrease the friction coefficient as a 
result of roughing a surface. Fine particle and coarse particle of carbon 
black can be used in combination. Carbon black having a surface which is 
partially graphited or grafted can also be used. 
Particle sizes of other nonmagnetic particles are not limited, and those 
having particle sizes conventionally used can be employed. For example, 
when inorganic filling materials are used, the average particle size is 
preferably in the range of from about 0.01 to 10 .mu.m, more preferably 
from about 0.5 to 5 .mu.m. 
The shapes of the nonmagnetic particles can be spherical, acicular, 
tabular, cubic and the like. 
The binder for use in the backing layer of this invention is a binder 
containing from about 20 to 90 wt % of polycarbonate polyurethane. 
The polycarbonate polyurethane is a copolymer obtained by condensing (1) a 
polyol, (2) a polyisocyanate and optionally (3) a chain-extending agent. 
The above-described (1) polyol is (A) a polycarbonate polyol having a 
molecular weight of from about 300 to 200,000 and a hydroxy value of from 
about 20 to 1,000 synthesized by condensing or ester-exchanging at least 
one polyhydric alcohol represented by formula (I): 
EQU HO--R--OH (I) 
wherein R represents, for example, --(CH.sub.2).sub.n --(wherein n is an 
integer from 3 to 14), 
##STR1## 
with at least one of phosgene, chloroformic ester, dialkylcarbonate or 
diallycarbonate; or (B) a polycarbonate polyester polyol having a 
molecular weight of from about 400 to 200,000 and a hydroxy value of from 
about 5 to 1,000 obtained by condensating the above polycarbonate polyol 
and a dihydric carboxylic acid represented by formula (II): 
EQU HOOC--R.sup.1 --COOH (II) 
wherein R.sup.1 represents an alkylene group having from 3 to 6 carbon 
atoms, a phenylene group or a cyclohexylene group. 
Other polyols such as polyether polyol, polyester ether polyol or polyester 
may be used in combination with the above polyol in an amount of about 90 
wt % or less based on the amount of the above-defined polyol. 
The above (2) polyisocyanate is not particularly limited and those 
generally used can be used in the present invention. Examples of 
polyisocyanates include hexamethylene diisocyanate, tolydine diisocyanate, 
isophorone diisocyanate, xylylene diixocyanate, cyclohexane diisocyanate, 
toluidine diisocyanate, 2,4-tolylene diisocyanate, 2,6-tolylene 
diisocyante, 4,4'-diphenylmethane diisocyanate, p-phenylene diisocyanate, 
m-phenylene diisocyanate, and 1,5-naphthylene diisocyanate. Among these, 
hexamethylene diisocyanate, xylylene diixocyanate, 2,4-tolylene 
diisocyanate and 2,6-tolylene diisocyanate are preferably used. 
The above (3) chain-extending agent as a comonomer may be any commonly used 
conventional substance such as the above described polyhydric alcohols, 
aliphatic polyamines, acyclic polyamines or aromatic polyamines. Among 
these, 1,4-butanediol and Bisphenol A are preferably used. 
The molar ratio of the components (1), (2) and (3) is preferably (0.1 to 
5)/(0.1 to 5)/(0.1 to 5), and more preferably (0.5 to 2)/(0.5 to 2)/(0.5 
to 2). 
The polycarbonate polyurethane obtained from the above described (1) and 
(2) and, if desired (3) preferably has a number average molecular weight 
of from about 10,000 to 250,000. When a polycarbonate polyurethane having 
a lower molecular weight than the above range is used, running durability 
becomes poor. When a polycarbonate polyurethane having a higher molecular 
weight than the above range is used, solubility in a solvent is decreased 
or dispersibility becomes poor, which is undesirable from the industrial 
point of view. 
The polycarbonate polyurethane may contain at least one functional group 
selected from the group consisting of --SO.sub.3 M, --COOM, --NH.sub.2, 
--SH, --OH, a phosphoric acid group and a phosphoric acid ester group, 
wherein M represents a hydrogen atom or an alkali metal atom. Among these, 
--SO.sub.3 M, --COOM, --OH, a phosphoric acid group and a phosphoric acid 
ester group are preferably used. The polycarbonate polyurethane having 
such a functional group can be prepared by a known conventional method. 
The above described polycarbonate polyurethane is commercially available 
under the trade names of, for example, "Polycarbonate polyurethane 9000 
Series, 7000 Series and 5000 Series" manufactured by Dainichiseika Colour 
& Chemicals Mfg. Co., Ltd. and "5740X Series" manufactured by Goodrich 
Co., Ltd, and is described, e.g., in JP-A-62-114120, 61-264509, 61-120338, 
61-289522, 61-222027 (the term "JP-A" used herein means a "published 
unexamined Japanese patent application"), and U.S. Pat. No. 4,643,946. 
The binder for use in the backing layer in this invention is a mixture of 
the above polycarbonate polyurethane in an amount of from about 20 to 90 
wt %, preferably from about 25 to 60 wt % and other binders. When the 
content of the above polycarbonate polyurethane in the backing layer is 
higher than the above ranges, the reproduced output of the recording 
medium decreases, the still reproduction becomes poor and the head 
clogging takes place. When the content is lower than the above ranges, the 
still life and the head clogging are deteriorated. 
Examples of the binders to be used in combination with the above 
polycarbonate polyurethane are not particularly limited, and conventional 
binders for a magnetic recording medium can be used, such as thermoplastic 
resins, thermosetting resins, reactive type resins or a mixture thereof. 
Examples of the above described thermoplastic resins include those having a 
softening point of about 150.degree. C. or lower and an average molecular 
weight of from about 10,000 to 300,000, such as a copolymer of vinyl 
chloride and vinyl acetate, a copolymer of vinyl chloride and vinylidene 
chloride, a copolymer of vinyl chloride and acrylonitrile, a copolymer of 
acrylate and acrylonitrile, a copolymer of acrylate and vinylidene 
chloride, a copolymer of acrylate and styrene, a copolymer of methacrylate 
and acrylonitrile, a copolymer of methacrylate and vinylidene chloride, a 
copolymer of methacrylate and styrene, a urethane elastomer, a 
nylon-silicon type resin, a nitrocellulose-polyamide resin, a polyvinyl 
fluoride, a copolymer of vinylidene chloride and acrylonitrile, a 
copolymer of butadiene and acrylonitrile, a polyamide resin, a polyvinyl 
butyral, cellulose derivatives (e.g, cellulose acetate butyrate, cellulose 
diacetate, cellulose triacetate and cellulose propionate, nitrocellulose), 
copolymer of styrene and butadiene, a polyester resin, a copolymer of 
chlorovinyl ether and acrylate, an amino resin and various synthetic 
rubber type thermoplastic resins. Of those, vinyl chloride polymers and 
copolymers, vinylidene chloride polymers and copolymers and nitrocellulose 
are particularly preferred. 
The above described thermosetting resins or reactive type resins are those 
having a molecular weight of about 200,000 or lower when they are in a 
coating composition and the molecular weight becomes extremely high when 
they are coated, dried and heated. Specific examples thereof include 
phenol resins, phenoxy resins, epoxy resins, polyurethane hardenable 
resins, urea resins, melamine resins, alkyd resins, silicone resins, acryl 
type reactive resins, epoxy-polyamide resins, nitrocellulose melamine 
resins, a mixture of a high molecular weight polyester resin and an 
isocyanate prepolymer, a mixture of a methacrylate copolymer and a 
diisocyanate prepolymer, a mixture of a polyester polyol and a 
polyisocyanate, urea formaldehyde resins, a mixture of a low molecular 
weight glycol, a high molecular weight diol and triphenylmethane 
triisocyanate, polyamine resins and the mixture thereof. Of these, phenoxy 
resins and epoxy resins are particularly preferred. 
Furthermore, these binders may contain conventional functional groups. 
Binders containing --SO.sub.3 M, --SO.sub.2 M, --COOM, --NH.sub.2, 
--N.sup.+ R.sub.4, --OH, a phosphoric acid group or a phosphoric acid 
ester group (wherein M represents a hydrogen atom or an alkali metal atom, 
and R represents a lower alkyl group having 10 or less of carbon atoms) 
are particularly preferred. The content of these functional groups is 
preferably from about 1.times.10.sup.-6 to 1.times.10.sup.-2 equivalent 
per 1 g of the binder. 
The above described phenoxy resins are linear polymers obtained from the 
condensation reaction of bisphenol A and epichlorohydrin and having the 
same structure unit as that of widely used epoxy resins. However, the 
widely used epoxy resins have a degree of polymerization of at most 20 but 
the phenoxy resin has a degree of polymerization of from about 50 to 700 
and is thermoplastic when used alone and can become thermosetting when 
cross-linked with amino resins and the like. 
A compound having at least two isocyanate groups (polyisocyanate) may 
further be incorporated in the above described binder. Examples of such a 
compound includes isocyanates such as tolylene diidocyanate, 
4,4'-diphenylmethane diisocyanate, hexamethylene diisocyanate, xylylene 
diisocyanate, naphthylene-1,5-diisocyanate, o-toluidine diisocyanate, 
isophorone diisocyanate, triphenylmethane triisocyanate or the like, 
reaction products of these isocyanates and polyalchols, and 
polyisocyanates produced by condensing those isocyanates. The above 
polyisocyanates are commercially available under the trade names of 
"Collonate L", "Collonate HL", "Collonate 2030", "Collonate 2031", 
"Millionate MR" and "Millionate MLT" manufactured by Nippon Polyurethane 
Co., Ltd.; "Takenate D-102", "Takenate D-110N", "Takenate D-200", and 
"Takenate D-202" manufactured by Takeda Chemical Industries, Ltd,; and 
"Desmodule L", "Desmodule IL", "Desmodule N" and "Desmodule HL" 
manufactured by Sumitomo Bayer Co., Ltd. 
The preferred amount ratio of the binder and the nonmagnetic particles in 
the backing layer of the present invention is from about 40 to 200 parts 
by weight, more preferably from about 50 to 130 parts by weight, of the 
binder per 100 parts by weight of nonmagnetic particles. When the amount 
of the binder is less than the above range, running durability of the 
magnetic recording medium deteriorates. When it is more than the above 
range, a desired surface electric resistance cannot be obtained or the 
friction coefficient tends to increase due to a low packing density of the 
nonmagnetic particles. 
The backing layer of the present invention can be provided on the surface 
of the nonmagnetic support opposite surface to the magnetic layer by any 
conventional method (as described, e.g., in West German Patent Application 
(OLS) 3,314,953, U.S. Pat. Nos. 4,411,953, 4,474,843, 4,592,952, 
4,442,171, 4,414,270, 4,598,014, and British Patent 2,082,477B). A 
suitable method of providing a backing layer includes, for example: 
dispersing or dissolving the above described nonmagnetic particles and the 
above described binder components and, if necessary, additives in an 
organic solvent to prepare a coating composition, coating the resulting 
coating composition on a surface of a support, drying the thus formed 
layer to evaporate the solvent and hardening the binder components. 
The backing layer is provided so that the thickness of the backing layer of 
the thus obtained magnetic tape is generally from about 0.8 to 3.0 .mu.m, 
preferably from about 1.5 to 2.5 .mu.m. 
As the organic solvents, the dispersing and mixing methods and the 
apparatus therefor, the methods of coating a coating composition on a 
support and the apparatuses therefor, those conventional in the art may be 
used. 
Provision of a the backing layer and the magnetic layer on the support may 
be carried out in an optional order, and both layers may be provided 
simultaneously. Further, any of the magnetic layer or the backing layer 
(preferably the magnetic layer) may be coated, dried, and if necessary 
hardened by heating to harden the coated layer; and then the other layer 
(preferably the backing layer) may be provided, dried and hardened by 
heating to prepare a magnetic tape. 
As the method for producing the magnetic recording medium of the present 
invention and additives and additional layers which can be used in the 
present invention, those described, e.g., in JP-A-59-19230, 59-142741, 
59-165239, West German Patent 3,403,822A, U.S. Pat. Nos. 4,442,159, 
4,450,199, Dutch Patent Application Nos. 8105575, 8105576, JP-B-58-43816 
and 59-46051 may be used. 
Lubricating agents may be provided on the surface of the backing layer of 
the present invention. The lubricating agents can be present on the 
surface of the backing layer by dissolving or dispersing lubricating 
agents in a coating composition for the backing layer, by coating the 
solution of lubricating agents in an organic solvent on the backing layer 
and drying, or by coating the solution of lubrication agents in an organic 
solvent on a surface of the magnetic layer and contacting the backing 
layer with the magnetic layer under a winding tension of preferably about 
200 g/inch width so as to transfer the lubricating agent to the surface of 
the backing layer. 
Examples of the lubricating agents include organic compounds such as higher 
fatty acids, metal soaps, higher fatty acid amides, higher fatty acid 
esters, mineral oils or fatty oils, silicone oils, inorganic fine 
particles, plastic fine particles, .alpha.-olefin polymers, unsaturated 
aliphatic hydrocarbons which are liquid at a normal temperature, 
fluorocarbons and mixtures thereof. The amount of the lubricating agent is 
preferably from about 0.01 to 2.0 parts by weight per 100 parts by weight 
of the nonmagnetic particles.

The present invention will be explained in more detail by the following 
Examples and Comparative Examples but the present invention is not to be 
construed as being limited thereto. In each Example and Comparative 
Example, all parts, percents and ratios are by weight. 
SYNTHESIS EXAMPLE 
A mixture of 1 mol of poly(hexamethylene carbonate)glycol (molecular 
weight: 1,500) prepared in accordance with the disclosure of Example 1 of 
U.S. Pat. No. 4,131,731 and 1 mol of 1,4-butandiol (chain-extending agent) 
was heated at about 100.degree. C. 1 mole of methylene bisdiphenyl 
diisocyanate heated at about 100.degree. C. was added into the mixture and 
reacted by a high temperature random fusion polymerization method. .The 
reaction was an exothermic reaction, and within from about 1 to 5 minutes 
after mixing those substances, the temperature increased to about 
200.degree. to 250.degree. C. It was confirmed by the increase of 
viscosity that polymerization proceeded. A polycarbonate polyurethane 
having a weight average molecular weight of 10.times.10.sup.4, a number 
average molecular weight of 5.5.times.10.sup.4, a viscosity (18%NV) of 9 
poise and an elongation of 1,200% or less was prepared. 
EXAMPLE 1 
The magnetic coating composition prepared by the following method and 
having the following composition was coated on a polyethylene 
terephthalate support (nonmagnetic support) having a thickness of 20 .mu.m 
so that the dry thickness of the coated layer was 6.0 .mu.m, subjected to 
orientation, dried and calendered. 
Then, the coating composition for a backing layer having the following 
composition prepared by the following method was coated on the opposite 
surface of the polyethylene terephthalate support to the magnetic layer so 
that the dry thickness of the backing layer was 2.0 .mu.m. After the 
coating composition was coated, it was dried and subjected to surface 
smoothing treatment and the magnetic recording medium obtained was wound 
up. 
The wound-up medium was heated at 40.degree. C. for 48 hours for hardening 
and the medium was slit to a 1 inch width to prepare a sample tape. 
The thus obtained sample tape was evaluated in the following manner and the 
results are shown in Table 1. 
Magnetic coating composition: 
______________________________________ 
Co containing .gamma.-Fe.sub.2 O.sub.3 particle 
300 parts 
(Nitrogen absorption method: 
specific surface area: 40 m.sup.2 /g, 
Hc: 850 Oe) 
Copolymer of vinyl chloride and 
40 parts 
vinyl acetate ("VMCH", manufactured 
by Union Carbide Co., Ltd., Mw: 60,000) 
Polyurethane resin ("UR 8300", 
17 parts 
manufactured by TOYOBO CO., LTD., 
Mw: 30,000) 
Carbon black 15 parts 
(average particle size: 20 m.mu.) 
Lecithin 1.5 parts 
Oleic acid 3 parts 
Octyl laurate 4 parts 
Lauric acid 3 parts 
Butyl acetate 700 parts 
Methyl ethyl ketone 300 parts 
______________________________________ 
A part of the above composition was put in a ball mill, mixed and kneaded 
and then the rest thereof was added into the ball mill and mixed and 
kneaded, and 24 parts of polyisocyanate compound ("Collonate 3040", 
manufactured by Nippon Polyurethane Co., Ltd.) was added and mixed and 
dispersed to prepare a magnetic coating composition. 
Coating composition of backing layer: 
______________________________________ 
Carbon black ("CONDUCTEX SC", 
100 parts 
average particle size: 20 m.mu.) 
Polycarbonate polyurethane 
75 parts 
(manufactured in Synthesis Example) 
Fhenoxy resin ("PKHH"manufactured 
20 parts 
by Union Carbide Co., Ltd., Mw: 40,000) 
Copper oleate 1 part 
Stearic acid 1 part 
Silicone oil 0.1 part 
"KF 69"manufactured by 
Shin-Etsu Chemical Co., Ltd.) 
Methyl ethyl ketone 700 parts 
Cyclohexanone 300 parts 
______________________________________ 
The above composition was put in a ball mill, mixed and kneaded, and 25 
parts of polyisocyanate compound ("Collonate 2030" manufactured by Nippon 
Polyurethane Co., Ltd.) was added thereto, uniformly mixed, kneaded and 
dispersed, and the viscosity of the composition was adjusted so that the 
coating composition for the backing layer was prepared. 
Evaluation of the sample tapes were carried out by the following manner. 
Decrease of reproduced output: 
Using a video tape recorder "BVH-500" manufactured by Sony Corporation with 
a ferrite head, a tape was made running at a normal running speed and the 
reproduced output of RF was measured. The reproduced output is shown as a 
relative value when the reproduced output of the sample tape of Example 1 
was assumed to be 0 dB. 
Friction coefficient: 
A tape which had made 10 passes on the VTR was passed around a SUS pole 
with a 180.degree. contact at the surface of the magnetic layer at a tape 
tension T.sub.1 of 200 g and a speed of 1.5 cm/sec., and then the tape 
tension T.sub.2 was read. 
The friction coefficient .mu. was calculated by the following equation. 
EQU .mu.=(1/.pi.)ln(T.sub.2 /T.sub.1) 
Still life: 
The recorded signals were reproduced in a still mode and the reproduced RF 
output level was recorded by a recorder and the period of time for signal 
level to decrease to 1/2 the original level was measured. A still life of 
300 minutes or higher was acceptable. 
Head clogging at full length running: 
Repeated running tests of a full length (30 minutes) tape were conducted, 
and whether recorded signals were not faithfully reproduced on a monitor 
due to head clogging was checked. Rank A means that head clogging did not 
take place even after 10 passes, and rank B means that head clogging took 
place in the first pass. 
EXAMPLE 2 
The same procedure as in Example 1 was repeated except that the composition 
of the backing layer was changed so that the amount of polycarbonate 
polyurethane was changed to 50 parts, the amount of the phenoxy resin was 
changed to 20 parts, the amount of the polyisocyanate compound was changed 
to 15 parts, and further 35 parts of polyester polyurethane ("C7209", 
manufactured by DAINIPPON INK AND CHEMICALS, INC., Mw: 100,00) to prepare 
a sample tape. 
The evaluation result of the sample tape is shown in Table 1. 
EXAMPLE 3 
The same procedure as in Example 1 was repeated except that the composition 
of the backing layer was changed so that the amount of the polycarbonate 
polyurethane was changed to 63 parts, the amount of the phenoxy resin was 
changed to 17 parts, and the amount of the polyisocyanate compound was 
changed to 21 parts to prepare a sample tape. 
The evaluation result of the sample tape is shown in Table 1. 
EXAMPLE 4 
The same procedure as in Example 1 was repeated except that the composition 
of the backing layer was changed s that the amount of the polycarbonate 
polyurethane was changed to 38 parts, the amount of the phenoxy resin was 
changed to 10 parts, and the amount of the polyisocyanate compound was 
changed to 13 parts to prepare a sample tape. 
The evaluation result of the sample tape is shown in Table 1. 
EXAMPLE 5 
The same procedure as in Example 1 was repeated except that the amount of 
the polycarbonate polyurethane was changed to 75 parts, the amount of the 
polyisocyanate compound was changed to 25 parts, and the phenoxy resin was 
replaced by 20 parts of vinyl chloride resin ("MR-110", manufactured by 
Nippon Zeon Co., Ltd., Mw: 40,000) to prepare a sample tape. 
The evaluation result of the sample tape is shown in Table 1. 
EXAMPLE 6 
The same procedure as in Example 1 was repeated except that the composition 
of the backing layer was changed so that the amount of the polycarbonate 
polyurethane was changed to 80 parts, the amount of the polyisocyanate 
compound was changed to 20 parts, and the phenoxy resin was replaced by 20 
parts of nitrocellulose (manufactured by DAICEL CHEMICAL INDUSTRIES, LTD., 
Mw: 20,000) to prepare a sample tape. 
The evaluation result of the sample tape is shown in Table 1. 
EXAMPLE 7 
The same procedure as in Example 5 was repeated except that the 
polycarbonate polyurethane in the coating composition for the backing 
layer was replaced by 45 parts of polycarbonate polyurethane having a 
--SO.sub.3 Na group at a concentration of 2.0.times.10.sup.-5 eq/g (which 
was synthesized in the same manner as in the Synthesis Example) to prepare 
a sample tape. 
The evaluation result of the sample tape is shown in Table 1. 
EXAMPLE 8 
The same procedure as in Example 5 was repeated except that the 
polycarbonate polyurethane in the coating composition for the backing 
layer was replaced by 30 parts of polycarbonate polyurethane having a 
--COOH group at a concentration of 5.4.times.10.sup.-4 eq/g (synthesized 
in the same manner as in Synthesis Example) to prepare a sample tape. 
The evaluation result of the sample tape is shown in Table 1. 
EXAMPLE 9 
The same procedure as in Example 5 was repeated except that the 
polycarbonate polyurethane in the coating composition for the backing 
layer was replaced by 50 parts of polycarbonate polyurethane having a 
--OPO(OH).sub.2 group at a concentration of 1.3.times.10.sup.-5 eq/g and 
50 parts of polycarbonate polyurethane having an N--CH.sub.3 group at a 
concentration of 8.0.times.10.sup.-5 eq/g (synthesized in the same manner 
as in Synthesis Example) to prepare a sample tape. 
The evaluation result of the sample tape is shown in Table 1. 
EXAMPLE 10 
The same procedure as in Example 5 was repeated except that the 
polycarbonate polyurethane in the coating composition for the backing 
layer was replaced by 75 parts of polycarbonate polyurethane having --COOH 
group at a concentration of 4.0.times.10.sup.-5 eq/g and having an ester 
bond at a concentration of 2.times.10.sup.-3 eq/g to prepare a sample 
tape. 
The evaluation result of the sample tape is shown in Table 1. 
COMATIVE EXAMPLE 1 
The same procedure as in Example 1 was repeated except that the composition 
of the backing layer was changed so that the amount of the polycarbonate 
polyurethane was changed to 110 parts, the amount of the polyisocyanate 
compound was changed to 10 parts, and the phenoxy resin was not used to 
prepare a sample tape. 
The evaluation result of the sample tape is shown in Table 1. 
COMATIVE EXAMPLE 2 
The same procedure as in Example 1 was repeated except that the composition 
of the backing layer was changed so that the amount of the polycarbonate 
polyurethane was changed to 30 parts, the amount of the phenoxy resin was 
changed 80 parts, and the amount of the polyisocyanate compound was 
changed to 10 parts to prepare a sample tape. 
The evaluation result of the sample tape is shown in Table 1. 
COMATIVE EXAMPLE 3 
The same procedure as in Example 1 was repeated except that the 
polycarbonate polyurethane in the coating composition for a backing layer 
was replaced by 75 parts of polyester polyurethane ("C7209" manufactured 
by DAINIPPON INK AND CHEMICALS INC., Mw: 100,00) to prepare a sample tape. 
The evaluation result of the sample tape is shown in Table 1. 
COMATIVE EXAMPLE 4 
The same procedure as in Example 1 was repeated except that the 
polycarbonate polyurethane in the coating composition for a backing layer 
was replaced by 75 parts of polyester polyurethane ("Pandex 5102A", 
manufactured by DAINIPPON INK AND CHEMICALS INC., Mw: 50,000) to prepare a 
sample tape. 
The evaluation results of the sample tape are shown in Table 1. 
COMATIVE EXAMPLE 5 
The same procedure as in Example 1 was repeated except that the composition 
of the backing layer was changed so that the amount of the polycarbonate 
polyurethane was changed to 19 parts, the amount of the phenoxy resin was 
changed to 5 parts, and the amount of the polyisocyanate compound was 
changed to 6 parts to prepare a sample tape. 
The evaluation result of the sample tape is shown in Table 1. 
COMATIVE EXAMPLE 6 
The same procedure as in Example 1 was repeated except that the composition 
of the backing layer was changed so that the amount of the polycarbonate 
polyurethane was changed to 150 parts, the amount of the phenoxy resin was 
changed to 40 parts, and the amount of the polyisocyanate compound was 
changed to 50 parts to prepare a sample tape. 
The evaluation result of the sample tape is shown in Table 1. 
TABLE 1 
__________________________________________________________________________ 
Decrease of Head clogging at 
reproduced output (dB) 
Friction coefficient 
Still life 
full length running 
Example 
Condition A Condition A 
Condition B 
Condition B 
Condition A 
__________________________________________________________________________ 
Example 1 
0 0.20 0.20 300 min. 
A 
or higher 
Example 2 
0 0.20 0.21 300 min. 
A 
or higher 
Example 3 
0 0.20 0.20 300 min. 
A 
or higher 
Example 4 
0 0.22 0.22 300 min. 
A 
or higher 
Example 5 
0 0.20 0.20 300 min. 
A 
or higher 
Example 6 
0 0.20 0.20 300 min. 
A 
or higher 
Example 7 
0 0.20 0.21 300 min. 
A 
or higher 
Example 8 
0 0.20 0.21 300 min. 
A 
or higher 
Example 9 
0 0.20 0.20 300 min. 
A 
or higher 
Example 10 
0 0.20 0.19 300 min. 
A 
or higher 
Comparative 
-2 0.24 0.21 100 min. 
B 
Example 1 
Comparative 
0 0.20 0.20 60 min. 
B 
Example 2 
Comparative 
-10 0.5 0.35 120 min. 
B 
Example 3 or higher 
Comparative 
-5 0.5 0.30 140 min. 
B 
Example 4 or higher 
Comparative 
0 0.34 0.31 180 min. 
A 
Example 5 
Comparative 
-2 0.31 0.28 180 min. 
B 
Example 6 
__________________________________________________________________________ 
Condition A: 70.degree. C., 80 % RH 
Condition B: 40.degree. C., 80 % RH 
It is clear from the results of each Example and Comparative Examples 3 and 
4 shown in Table 1 that tapes using the polycarbonate polyurethane as a 
binder in the backing layer were excellent in the decrease of reproduced 
output, the friction coefficient, the still life and the head clogging at 
a full length, at a high temperature and a high humidity, when compared 
with tapes using a conventionally known polyester polyurethane. 
It is also clear from the results of the Examples and Comparative Examples 
1 and 2 that both the still life and the head clogging at full length 
running were greatly deteriorated when the content of the polycarbonate 
polyurethane in the binder of the backing layer was outside the ranges of 
from about 20 to 90 wt %. 
It is clear from the comparison between each Example and Comparative 
Examples 5 and 6 that when the amount ratio of the binder to the 
nonmagnetic particles in the baking layer was too high, the friction 
coefficient was high and when it was too low, the still life was 
deteriorated. 
Even when the magnetic recording medium of the present invention is used at 
a high temperature and a high humidity for a long period of term, decrease 
of the reproduced output does not take place, the still durability is 
excellent, the head clogging hardly occurs and the blocking caused by the 
backing layer hardly occurs. Therefore, the magnetic recording .medium of 
the present invention is extremely excellent in electromagnetic properties 
and provides extremely good running durability. 
While the invention has been described in detail and with reference to 
specific examples 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.