Magnetic recording medium and process for preparing the same

Disclosed is a magnetic recording medium comprising, a non-magnetic support having thereon (i) a lower non-magnetic layer comprising a non-magnetic powder and a binder, and (ii) an upper magnetic layer comprising a ferromagnetic powder and a binder, wherein an extract, obtained by extracting with tetrahydrofuran the binder present in the area between the outer surface of the upper magnetic layer and a position 0.1 .mu.m in depth from the outer surface, has a weight-average molecular weight of 15,000 or more, and wherein the reaction ratio of the curing agent reacted with the binder in the area between the outer surface of the upper magnetic layer and a position 0.1 .mu.m in depth from the outer surface is at least 1.3 times as much as the reaction ratio of the curing agent reacted with the binder containing a polyurethane in the lower nonmagnetic and upper magnetic layers as a whole. Also disclosed is a process for making this magnetic recording medium.

FIELD OF THE INVENTION 
The invention relates to a magnetic recording medium, and more particularly 
to a magnetic recording medium which is excellent in running durability. 
It also relates to a process for preparing a magnetic recording medium 
which is excellent in running durability. 
BACKGROUND OF THE INVENTION 
Generally, in magnetic recording mediums obtained by dispersing 
ferromagnetic powder, a binder, a curing agent and other additives in an 
organic solvent, and coating the resulting coating composition on a 
nonmagnetic support and drying it, the magnetic layer thereof may be 
scraped as it slides by a magnetic head, etc. Accordingly, it is necessary 
that the magnetic layer is scarcely abraded and has excellent durability. 
To meet this need, it has been conventional to use a polyurethane binder 
which is excellent in abrasion resistance and toughness. Alternatively, 
the strength of the layer is increased by adding a curing agent to the 
binder and carrying out a crosslinking reaction, or an adsorptive polar 
group, such as a sulfonic acid group, a phosphoric acid group, a carboxyl 
group or a derivative thereof, is introduced into the binder to form 
adhesion between the ferromagnetic powder and the binder. 
The sliding of the magnetic head on the magnetic recording medium is 
greatly affected by the binder present on the surface of the magnetic 
layer and in the vicinity of the surface thereof. Even when ferromagnetic 
powder is subjected to an adsorption treatment or when a crosslinking 
reaction of the binder is carried out, the low-molecular components in the 
uncrosslinked or unadsorbed binder migrate to areas in the vicinity of the 
surface of the magnetic layer. As a result, problems such as clogging of 
the head and staining of the guide pole are caused. 
To cope with the problem, the present inventor has previously proposed 
that, for example, in a multi-layer structured magnetic recording medium 
comprising a non-magnetic support having thereon a lower magnetic layer 
and an upper magnetic layer, the binder used in the upper magnetic layer 
has a molecular weight which is higher than that of the binder used in the 
lower magnetic layer. As a result, the strengths of the layer can be 
increased and at the same time, running durability can be improved [see, 
JP-A-1-263925 (the term "JP-A" as used herein means an "unexamined 
published Japanese patent application")]. Further, JP-A-1-205723 discloses 
that in a magnetic recording medium having the structure described above, 
the amount of the polyisocyanate contained in the upper magnetic layer is 
larger than that of the polyisocyanate contained in the lower magnetic 
layer, to thereby improve running durability. 
In the former method, however, since a binder having a high molecular 
weight is used, there is the problem that the magnetic layer is 
excessively hardened and as a result, there is a difficulty in 
satisfactorily calendering the magnetic layer. Additionally, since the 
molecular weight of the binder is originally too high, the ferromagnetic 
powder is poorly dispersed, and, as a result, good surface properties can 
not be obtained and it is difficult to obtain a high RF output. 
In the latter method, since the amount of the polyisocyanate to be 
contained in the upper magnetic layer and the molecular weight of the 
binder to be reacted with the polyisocyanate must be properly controlled, 
there is the problem that it is difficult to prepare an upper magnetic 
layer having optimum hardness. 
The problem of running durability in particular has become a serious 
problem with the development of higher-density recording and higher 
performance of the magnetic recording medium in recent years. 
Accordingly, there is a need for an effective means for obtaining stable, 
good running durability in the magnetic recording mediums. 
The present inventors have made studies with the view to solve the problems 
mentioned above. In the studies, materials by which the head was clogged 
and the reaction system of the resin component of the binder with the 
curing agent component in the magnetic layer were quantitatively analyzed 
by means of ESCA, FT-IR, EPMA, GPC, etc. As a result, an effective means 
of solving the above problems has been found. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a magnetic recording 
medium which prevents clogging of the head and staining of the guide pole, 
is excellent in running durability and in the surface properties of its 
magnetic layer, has a high reproducing output, and is excellent in 
electromagnetic conversion characteristics. 
Another object of the present invention is to provide a process for 
preparing the above-described magnetic recording medium. 
The present invention provides in one aspect a magnetic recording medium 
comprising a non-magnetic support having thereon at least two layers of a 
lower non-magnetic layer comprising at least a non-magnetic powder and a 
binder, and an upper magnetic layer comprising at least a ferromagnetic 
powder and a binder. An extract, obtained by extracting with 
tetrahydrofuran the binder present in the area between the outer surface 
of the upper magnetic layer and a position 0.1 .mu.m in depth from the 
outer surface, has a weight-average molecular weight of 15,000 or more. 
The reaction ratio of a curing agent reacted with the binder present in 
the area between the outer surface of the upper magnetic layer and a 
position 0.1 .mu.m in depth from the outer surface is at least 1.3 times 
as much as the reaction ratio of the curing agent reacted with the binder 
containing a polyurethane present in the coated layers containing the 
lower non-magnetic and upper magnetic layers as a whole. 
The present invention provides, in another aspect, a process for preparing 
a magnetic recording medium which comprises the steps of: 
coating a non-magnetic coating composition containing at least a 
non-magnetic powder, a binder and a polyisocyanate on a non-magnetic 
support to form a lower non-magnetic layer; 
simultaneously or successively coating a magnetic coating composition 
containing at least a ferromagnetic powder, a polyurethane having a polar 
group of a weight-average molecular weight of 5,000 to 50,000 and a 
polyisocyanate on the lower non-magnetic layer while the lower 
non-magnetic layer (hereinafter referred to as a lower layer or a 
non-magnetic layer) is still in a wet state; and 
drying the non-magnetic layer and the magnetic coating composition to 
obtain a magnetic recording medium comprising the non-magnetic support 
having thereon the non-magnetic layer and the magnetic layer, wherein the 
reaction ratio of the polyisocyanate reacted with the binder present in 
the area between the surface of the dried magnetic layer and a position 
0.1 .mu.m in depth from the outer surface of the magnetic layer is at 
least 1.3 times as much as the reaction ratio of the polyisocyanate 
reacted with the binder contained in the coated layers as a whole.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention is illustrated in more detail below. 
In the present invention, the structural form of the binder, which has a 
direct effect on running durability, particularly the reaction system of a 
resin component with a curing agent component is specified quantitatively, 
the binder being present in the vicinity of the surface of the magnetic 
layer, that is, in the area between the outer surface of the magnetic 
layer and a position 0.1 .mu.m in depth from the outer surface of the 
magnetic layer. The term "binder" as used herein refers to a 
high-molecular resin (or resin component) excluding a low-molecular curing 
agent (or curing agent component) which is capable of crosslinking with 
the binder, unless otherwise stated. 
Namely, in the present invention, the binder formed by the reaction of the 
resin component of the binder with the curing agent component is specified 
for the area in the vicinity of the surface of the magnetic layer and for 
the whole of the magnetic layer. In other words, the frequency of the 
reaction between the curing agent component and the resin component in the 
vicinity of the surface of the magnetic layer is 1.3 times (in terms of 
probability) the frequency of the reaction between the curing agent 
component and the resin component in the coated layers as a whole and in 
turn in the entire magnetic layer. Thus, the binder formed by the reaction 
between the curing agent component and the resin component in the vicinity 
of the magnetic layer, that is, in the area between the outer surface of 
the magnetic layer and a level 0.1 .mu.m in depth from the outer surface 
of the magnetic layer, has an average molecular weight which is higher 
than that of the binder formed in the whole of the magnetic layer. The 
present inventors have found that the problems associated with prior art 
can be solved and a magnetic recording medium having excellent running 
durability and good surface properties can be obtained when in addition to 
the above, the layer in the vicinity of the outer surface of the magnetic 
layer is extracted with tetrahydrofuran (THF), the extract of the binder 
has a weight-average molecular weight of 15,000 or more. 
In the present invention, the reaction ratio of the curing agent reacted 
with the binder present in the area between the outer surface of the upper 
magnetic layer and a position 0.1 .mu.m in depth from the outer surface is 
at least 1.3, preferably 1.3 to 2.2, times as much as the reaction ratio 
of the curing agent reacted with the binder containing a polyurethane 
present in the lower non-magnetic and upper magnetic layers as a whole. 
In the present invention, the reaction ratio of the polyisocyanate reacted 
with the binder present in the area between the surface of the dried 
magnetic layer and a position 0.1 .mu.m in depth from the outer surface of 
the magnetic layer is at least 1.3 times, preferably 1.3 to 2.2, as much 
as the reaction ratio of the polyisocyanate reacted with the binder 
contained in the coated layers as a whole. 
In the present invention, unreacted or unadsorbed binder, which is present 
in the vicinity of the surface of the magnetic layer and conventionally 
causes staining of the head and the guide pole, is properly reacted with 
the curing agent to increase the molecular weight of the binder and to 
improve running durability. Further, since the molecular weight of the 
binder can be increased by the reaction between the resin component and 
the curing agent component, a resin component having a relatively low 
molecular weight can be used. Hence, the dispersibility of the 
ferromagnetic powder in the magnetic coating composition can be improved 
and the calendering of the magnetic layer can be improved. As a result, 
the smoothness of the surface of the magnetic layer can be improved. 
Accordingly, satisfactory electromagnetic characteristics can be obtained. 
Further, in the present invention the flexibility of the whole of the 
magnetic layer and the whole of the coated layers can be maintained and 
head touch can be improved by setting the reaction percentage of the 
curing agent component. 
In the magnetic recording medium of the present invention, the reaction 
ratio of the curing agent component or the existence ratio of the curing 
agent component reacted with the resin component in the whole of the 
coated layers or in the magnetic layer can be determined by various 
analytical methods. For example, such methods as ESCA, FT-IR, EPMA and GPC 
can be used either alone or in combination. A method for measuring the 
curing agent component wherein a polyisocyanate is used as the curing 
agent component is illustrated below by the way of example. It should be 
understood that the measuring method is not limited to the polyisocyanate, 
but can also be applied to other curing agent components. Namely, when the 
curing agent component is a polyisocyanate, a reaction between isocyanato 
group (--NCO) and the functional group of the resin component takes place. 
Accordingly, the reaction ratio can be measured on the basis of the N 
atom. 
A magnetic recording medium is extracted with n-hexane at room temperature 
for 30 minutes, and the N/Fe present in the vicinity of the surface of the 
magnetic layer of the magnetic recording medium is determined by means of 
ESCA. Since the analysis depth by ESCA is very shallow (100.ANG. or less), 
it is necessary to remove the lubricant layer on the surface of the 
magnetic layer. A higher fatty acid or an ester thereof on the coated 
layer, particularly on the surface thereof is removed by an extraction 
treatment with n-hexane to thereby uncover the ferromagnetic powder or the 
binder. Uncured curing agent is not dissolved in hexane. 
Further, the magnetic recording medium used in the extraction treatment 
with n-hexane is drawn, and the whole of the coated layers is peeled off 
therefrom, thoroughly crushed and pelletized into 10 mm.phi. pellets. An 
N/Fe is determined from the pellets by means of ESCA. 
The aforesaid ratio can be obtained by dividing the former (i.e., the N/Fe 
of the vicinity of the surface of the magnetic layer) by the latter (i.e., 
the N/Fe of the whole of the coated layers). ESCA is an abbreviation of 
Electron Spectroscopy for Chemical Analysis and has the same meaning as 
XPS and X-ray photoelectron spectroscopy. The measuring conditions are as 
follows. 
Apparatus for ESCA: PHI-5400 MC (manufactured by ULBACPHI Co., Ltd.) 
Conditions: 400 W (15 kV) 
Mg anode 
Measuring time: 10 minutes 
N/Fe: ratio of intensity of peak of 1S of N to intensity of peak of 2P3/2 
of Fe. 
When the ratio is less than 1.3, the ratio is too small to achieve the 
object of the present invention and running durability can not be 
improved, while when the ratio is larger than 2.2, the surface is hard and 
brittle and head touch becomes poor. 
The measuring range of the N/Fe ratio in the vicinity of the outer surface 
of the magnetic layer is such that the upper limit thereof is usually a 
depth of 100.ANG. at its maximum from the outer surface of the magnetic 
layer. Accordingly, the measurement by means of ESCA shows a distribution 
of N and Fe present in the vicinity of the outer surface of the magnetic 
layer. The magnetic recording medium is then cut into a thin piece in the 
form of a layer in the direction of the surface thereof by an 
ultramicrotome provided with a glass knife. A cut of 0.1 .mu.m in depth 
from the surface of the upper magnetic layer is subjected to a measurement 
by means of ESCA, whereby the ratio of N/Fe present in the vicinity of the 
outer surface of the magnetic layer can be confirmed. 
The weight-average molecular weight of an extract obtained by extracting 
the binder with tetrahydrofuran can be measured in the following manner, 
that binder being present in the area between the outer surface of the 
magnetic layer and a position of 0.1 .mu.m in depth from the outer surface 
of the magnetic layer. 
A layer of about 0.1 .mu.m between the outer surface of the magnetic layer 
and a position of about 0.1 .mu.m in depth from the surface of the 
magnetic layer is scraped off by an abrasive tape (#700). The resulting 
powder of the magnetic layer is extracted with tetrahydrofuran. The 
weight-average molecular weight of the soluble matter obtained by GPC can 
be determined from a calibration curve by means of spectrophotometry such 
as with a UV detector, the calibration curve being previously prepared by 
using polystyrene having a known molecular weight. An example of a 
suitable measuring device is the HLC-8020 manufactured by Tosoh Corp. 
In the present invention, the weight-average molecular weight of the 
extracted resin obtained by the above measurement is preferably in the 
range of 15,000 to 60,000, more preferably 20,000 to 40,000. The binder 
extracted in the above measurement comprises the oligomers of the binder 
which is reacted with the curing agent and has a still solubility, as well 
as unreacted binder. It contains a very small amount of unreacted curing 
agent. 
Accordingly, when the above requirements of the reaction ratio between the 
curing agent component and the resin component and the above extraction 
condition are met in the present invention, the low-molecular components 
of the binder present in the vicinity of the magnetic layer are eliminated 
as much as possible, and a binder having a high molecular weight is 
formed. As a result, the strength of the layer can be increased, and the 
dispersibility of ferromagnetic powder can be improved so that the surface 
properties of the magnetic layer allow the center line average surface 
roughness to be preferably 5.0 nm or below, more preferably 2 to 4 nm. The 
"center line average surface roughness" is world-wide used to express 
surface roughness and is defined by JIS B 0601. 
The above definition for the reaction ratio of the curing agent component 
and the THF extract in the present invention (hereinafter referred to as 
the definition of the present invention) is a concrete useful index for 
choosing a blending ratio of the resin component of the binder and the 
curing agent component in the magnetic coating composition and in the 
non-magnetic coating composition or the molecular weight of the resin 
component. 
The resin component in the non-magnetic component of the present invention 
may have a weight-average molecular weight of 5,000 to 50,000, preferably 
10,000 to 30,000. The amount of the curing agent component contained in 
the non-magnetic composition is 10 parts by weight or more, preferably 
from 20 to 70 parts by weight, more preferably from 30 to 50 parts by 
weight, per 100 parts by weight of the resin component. 
The resin component in the magnetic coating composition of the present 
invention may have a weight-average molecular weight of 5,000 to 50,000, 
preferably 10,000 to 30,000. The amount of the curing agent component 
contained in the upper magnetic layer is 10 parts by weight or more, 
preferably from 20 to 70 parts by weight, more preferably from 30 to 50 
parts by weight, per 100 parts by weight of the resin component. 
An example of the composition of the binder used in the coating 
compositions for the lower non-magnetic layer and the upper magnetic layer 
is the combination of a resin component comprising a polyurethane resin 
(i.e., having the same meaning as polyurethane), preferably polyurethane 
having a polar group, and a vinyl chloride copolymer resin with a 
polyisocyanate as the curing agent component. 
Examples of effective means for meeting the requirements of the present 
invention include, but are not limited to, the following factors: 
(1) The use of a wet-on-wet coating system as the coating means described 
in U.S. Pat. No. 4,844,946: 
The upper magnetic layer is simultaneously or successively coated on the 
lower non-magnetic layer while the lower non-magnetic layer is still in a 
wet state. When the coating is conducted by this coating system, the 
curing agent component in the lower non-magnetic layer can easily migrate 
to the area in the vicinity of the surface of the upper magnetic layer, 
and hence the molecular weight of the resin component in the coating 
composition for the upper magnetic layer can be lowered, whereby the 
dispersibility of ferromagnetic powder can be improved. 
Further, with the wet-on-wet coating system, the thickness of the magnetic 
layer can be further reduced. 
(2) The choice of drying conditions: 
For example, when a sharp temperature gradient is applied during drying, 
the migration of the curing agent component from the lower layer to the 
upper layer can be accelerated. Hence, a similar effect to that described 
above can be obtained. 
(3) The choice of the blending ratio of the curing agent component in each 
of the coating compositions for the lower non-magnetic layer and for the 
upper magnetic layer: 
A larger amount of the curing agent component can be contained in the lower 
non-magnetic layer when the above means (1) and (2) are applied (the ratio 
is as described above). 
(4) The choice of a resin component and a curing agent component which are 
well compatible with each other: 
The above-described means (1) to (3) become more effective when the means 
(4) is applied. 
The binders which can be used in the present invention include a 
polyurethane having a polar group as the resin component. Any resin 
component which is reactive with the curing agent component can be used, 
so long as it has a functional group capable of forming a covalent bond 
with the functional group of the curing agent component. 
When the curing agent component has an isocyanate group (--NCO group), 
examples of the functional group of the resin component include --OH, 
--COOH, --NH.sub.2, --NH-- and --NRH groups (wherein R represents an alkyl 
group). 
A particularly preferred resin component which is reactive with the curing 
agent component is polyurethane. 
Any conventional polyurethane structure can be used. Examples of the 
polyurethane structures which can be used in the present invention include 
polyester polyurethane, polyether polyurethane, polyether polyester 
polyurethane, polycarbonate polyurethane, polyester polycarbonate 
polyurethane and polycaprolactone polyurethane. 
Trifunctional polyisocyanates are particularly preferred as the curing 
agent component. 
When polyurethane as the reactive resin is used in combination with the 
polyisocyanate in the present invention, the number of the functional 
groups per molecular of polyurethane can be properly chosen from among the 
conditions described below. 
Polyurethane having a glass transition temperature of from -50.degree. to 
100.degree. C., an elongation at break of 100 to 2,000%, a breaking stress 
of 0.05 to 10 kg/cm.sup.2 and a yield point of 0.05 to 10 kg/cm.sup.2 can 
be preferably used in the present invention. 
Vinyl chloride resins which can be preferably used in combination with 
polyurethane in the present invention may be reactive with the curing 
agent component and include conventional resins. 
Examples of the vinyl chloride resins include polyvinyl chloride and 
copolymers of vinyl chloride with other comonomers such as at least one of 
vinyl acetate, vinyl alcohol, maleic acid and acrylic acid. 
When the vinyl chloride resins are used as the reactive resin in 
combination with the polyisocyanate in the present invention, the number 
of the functional groups per molecule may be properly chosen from among 
the conditions of the present invention described below. 
The proportion of the polyurethane based on the total amount of the binder 
is in the range of 10 to 80% by weight, preferably 20 to 60% by weight. 
When the amount is less than 10% by weight, sufficient durability can not 
be imparted. 
The amount of the binder used in the lower non-magnetic layer and the upper 
magnetic layer of the present invention is in the range of preferably 10 
to 40% by weight based on the amount of non-magnetic powder or 
ferromagnetic powder. 
Examples of other resins which can be used as the resin component in the 
present invention include conventional thermoplastic resins, thermosetting 
resins, reactive resins and mixtures thereof. 
The thermoplastic resins are those having a glass transition temperature of 
-100.degree. to 150.degree. C., a number average molecular weight of 1,000 
to 200,000, preferably 10,000 to 100,000, and a degree of polymerization 
of 50 to 1,000. Examples of the thermoplastic resins include homopolymers 
and copolymers of vinyl chloride, vinyl acetate, vinyl alcohol, maleic 
acid, acrylic acid, acrylic esters, vinylidene chloride, acrylonitrile, 
methacrylic acid, methacrylic esters, styrene, butadiene, ethylene, vinyl 
butyral, vinyl acetal and vinyl ether; polyurethane resins; and various 
rubber resins. Examples of the thermosetting resins or the reactive resins 
include phenolic resins, epoxy resins, polyurethane curing type resins, 
urea resins, melamine resins, alkyd resins, acrylic reactive resins, 
formaldehyde resins, silicone resins, epoxy-polyamide resins, mixtures of 
polyester resins and isocyanate prepolymers, mixtures of polyester-polyols 
and polyisocyanates, and mixtures of polyurethanes and polyisocyanates. 
The details of these resins are described in Plastic Handbook (published by 
Asakura Shoten). 
Further, conventional electron beam-curable resins can be used in the lower 
or upper layer. Examples thereof and a process for preparing them are 
described in JP-A-62-256219. 
A polyurethane having at least one polar group may be used as the resin 
component in the present invention. Examples of the polar group include 
--COOM, --SO.sub.3 M, --OSO.sub.3 M, --P.dbd.O(OM').sub.2, 
--O--P.dbd.O(OM').sub.2 (wherein M represents a hydrogen atom, an alkali 
metal or an ammonium group, and M' represents a hydrogen atom, an alkali 
metal, an ammonium or alkyl group), --OH, --NR.sub.2, --N.sup.+ R.sub.3 
(wherein R.sub.2 and R.sub.3 each represents a hydrocarbon group), an 
epoxy group, --SN and --CN. It is preferred that all of the 
above-described resin components (reactive or non-reactive with the curing 
agent component) have at least one member selected from the group 
consisting of the above-described polar groups to impart excellent 
dispersibility as well as excellent durability. These polar groups can be 
introduced into the resin components by a copolymerization reaction or an 
addition reaction. The amount of the polar group to be introduced is 
1.times.10.sup.-1 to 1.times.10.sup.-8 eq/g, preferably 1.times.10.sup.- 2 
to 1.times.10.sup.-6 eq/g. 
Specific examples of the binders which can be used in the present invention 
include VAGH, VYHH, VMCH, VAGF, VAGD, VROH, VYES, VYNC, VMCC, XYHL, XYSG, 
PKHH, PKHJ, PKHC and PKFE (manufactured by Union Carbide Co., Ltd.); 
MPR-TA, MPR-TA5, MPR-TAL, MPR-TSN, MPR-TMF, MPR-TS, MPR-TM and MPR-TAO 
(manufactured by Nisshin Chemical Industry Co., Ltd.); 1000W, DX80, DX81, 
DX82, DX83 and 100FD (manufactured by Denki Kagaku Kogyo K.K.); MR105, 
MR110, MR100 and 400X110A (manufactured by Nippon Zeon Co., Ltd.); 
Nipporan N2301, N2302 and N2304 (manufactured by Nippon Polyurethane Co., 
Ltd.); Pandex T-5105, T-R3080 and T-5201, Burnock D-400 and D-210-80, 
Crisvon 6109 and 7209 (manufactured by Dainippon Ink & Chemicals, Inc.); 
Vylon UR8200, UR8300, UR8600, UR5500, UR4300, RV530 and RV280 
(manufactured by Toyobo Co., Ltd.); Daipheramine 4020, 5020, 5100, 5300, 
9020, 9022 and 7020 (manufactured by Dainichiseika Color & Chemicals Mfg. 
Co., Ltd.); MX5004 (manufactured by Mitsubishi Kasei Corporation); 
Sunprene SP-150 (manufactured by Sanyo Chemical Industries, Ltd.); and 
Salan F310 and F210 (manufactured by Asahi Chemical Industry Co., Ltd.). 
The magnetic recording medium of the present invention comprises a support 
having thereon a lower non-magnetic layer and a upper magnetic layer. 
However, the non-magnetic layer and/or the magnetic layer may optionally 
have a multi-layer structure, so long as the above-described conditions 
regarding the binder extract and the reacted .curing agent are met. Each 
layer of the multi-layer structure can be a proper composition. 
Accordingly, these layers may differ in the amounts of ferromagnetic 
powder, non-magnetic powder and the binder, the amounts of the vinyl 
chloride resin and the polyurethane resin in the binder, the amount of the 
polyisocyanate, the amount of another resin, the molecular weight of each 
resin in the magnetic layer, the amount of the polar group and the 
above-described characteristics of the resins. 
Examples of the polyisocyanate which can be used in the present invention 
include isocyanates such as tolylene diisocyanate, 4,4'-diphenylmethane 
diisocyanate, hexamethylene diisocyanate, xylylene diisocyanate, 
naphthylene-1,5-diisocyanate, o-toluidine diisocyanate, isophorone 
diisocyanate and triphenylmethane triisocyanate; products of these 
isocyanates with polyhydric alcohols; and polyisocyanates formed by the 
condensation of isocyanates. Particularly preferred are trifunctional 
polyisocyanates. 
Examples of the isocyanates which are commercially available include 
Coronate L (trifunctional), Coronate HL, Coronate 2030, Coronate 2031, 
Millionate MR and Millionate MTL (manufactured by Nippon Polyurethane Co., 
Ltd.); Takenate D-102 (trifunctional), Takenate D-110N, Takenate D-200 and 
Takenate D-202 (manufactured by Takeda Chemical Industries, Ltd.); and 
Desmodule L (trifunctional), Desmodule IL, Desmodule N and Desmodule HL. 
These polyisocyanates may be used either alone or in combination by 
utilizing a difference in curing reactivity in the lower non-magnetic 
layer and in the upper magnetic layer. 
The non-magnetic powder which can be used in the non-magnetic layer of the 
present invention includes inorganic and organic powders. Carbon black can 
also be used. 
Examples of the non-magnetic inorganic powder which can be used in the 
non-magnetic layer of the present invention include metals, metal oxides, 
metal carbonates, metal sulfates, metal nitrides, metal carbides and metal 
sulfides. Specific examples of the non-magnetic inorganic powder include 
TiO.sub.2 (rutile, anatase), TiOx, cerium oxide, tin oxide, tungsten 
oxide, ZnO, ZrO.sub.2, SiO.sub.2, Cr.sub.2 O.sub.3, .alpha.-alumina having 
an .alpha.-conversion of 90% or more, .beta.-alumina, .gamma.-alumina, 
.alpha.-iron oxide, goethite, corundum, silicon nitride, titanium carbide, 
magnesium oxide, boron nitride, molybdenum disulfide, copper oxide, 
MgCO.sub.3, CaCO.sub.3, BaCO.sub.3, SrCO.sub.3, BASO.sub.4, silicon 
carbide and titanium carbide. These compounds may be used either alone or 
in combination. 
These inorganic powders have a particle size of preferably 0.01 to 2 .mu.m. 
The shape thereof is not critical. Two or more inorganic powders having 
different particle sizes may be used in combination. When a single 
non-magnetic powder is used, the particle size distribution thereof may be 
properly chosen. These non-magnetic: powders have a tap density of 0.05 to 
2 g/ml, preferably 0.2 to 1.5 g/ml, a water content of 0.1 to 5%, 
preferably 0.2 to 3%, a pH of 2 to 11, a specific surface area of 1 to 100 
m.sup.2 /g, preferably 5 to 50 m.sup.2 /g, more preferably 7 to 40 m.sup.2 
/g, a crystallite size of preferably 0.01 to 2 .mu.m, a DBP oil absorption 
of 5 to 100 ml/100 g, preferably 10 to 80 ml/100 g, more preferably 20 to 
60 ml/100 g, and a specific gravity of 1 to 12, preferably 2 to 8. The 
shape of the nonmagnetic powder may be an acicular spherical, cubic or 
plate-like shape. It is not required that non-magnetic powder is always 
100% pure. The surface of non-magnetic powder may be treated with another 
compound such as Al, Si, Ti, Zr, Sn, Sb or Zn according to purpose, to 
form an oxide thereon. When non-magnetic powder has a purity of not lower 
than 70%, the effect thereof is not reduced. For example, when titanium 
oxide is used, the surface thereof is generally treated with alumina. It 
is preferred that ignition loss is 20% or less. It is also preferred that 
the above inorganic powders have a Mohs' scale of hardness of 4 or more. 
Examples of non-magnetic powder which can be used in the non-magnetic layer 
of the present invention include UA 5600 and UA 5605 (manufactured by 
Showa Denko K.K.); AKP-20, AKP-30, AKP-50, HIT-55, HIT-100 and ZA-G1 
(manufactured by Sumitomo Chemical Co., Ltd.); G5, G7 and S-1 
(manufactured by Nippon Chemical Industrial Co., Ltd.); TF-100, TF-120, 
TF-140 and R516 (manufactured by Toda Kogyo Corp.); TTO-51B, TTO-55A, 
TTO-55B, TTO-55C, TTO-55S, TTO-55D, FT-1000, FT-2000, FTL-100, FTL-200, 
M-1, S-1 and SN-100 (manufactured by Ishihara Sangyo Kaisha, Ltd.); 
ECT-52, STT-4D, STT-30D, STT-30 and STT-65C (manufactured by Titan Kogyo 
K.K.); T-1 (manufactured by Mitsubishi Materials Corp.); NS-O, NS-3Y and 
NS-8Y (manufactured by Japan Catalytic Chemical Industry Co., Ltd.); 
MT-100S, MT-100T, MT-150W, MT-500B, MT-600B and MT-100F (manufactured by 
Teika KK); FINEX-25, BF-1, BF-10, BF-20, BF-1L and BF-10P (manufactured by 
Sakai Chemical Industry); DEFIC-Y and DEFIC-R (manufactured by Dowa Mining 
Co., Ltd.); and Y-LOP (manufactured by Titan Kogyo K.K.). 
Examples of carbon black which can be used in the non-magnetic layer of the 
present invention include furnace black for rubber, thermal black for 
rubber, carbon black for color and acetylene black. Carbon black has a 
specific surface area of 100 to 500 m.sup.2 /g, preferably 150 to 400 
m.sup.2 /g, a DBP oil absorption of 20 to 400 ml/100 g, preferably 30 to 
200 ml/100 g, an average particle size of 5 to 80 m.mu., preferably 10 to 
50 m.mu., more preferably 10 to 40 m.mu., a pH of 2 to 10, a water content 
of 0.1 to 10% and a tap density of preferably 0.1 to 1 g/ml. 
Specific examples of carbon black which can be used in the non-magnetic 
layer the present invention include BLACK PEARLS 2000, 1300, 1000, 900, 
800, 880 and 700, VULCAN XC-72 (manufactured by Cabot Co., Ltd.); #3050B, 
#3150B, #3250B, #3750B, #3950B, #950, #650B, #970B, #850B and MA-600 
(manufactured by Mitsubishi Kasei Corporation); CONDUCTEX SC, RAVEN 8800, 
8000, 7000, 5750, 5250, 3500, 2100, 2000, 1800, 1500, 1255 and 1250 
(manufactured by Columbia Carbon Co., Ltd. ; and Ketjen Black EC 
(manufactured by Akzo Co., Ltd.). The surface of carbon black may be 
treated with dispersants. Carbon black may be grafted onto a resin. A part 
of the surface of carbon black may be graphitized. Carbon black may be 
previously dispersed in the binder before it is added to the non-magnetic 
coating composition. 
These carbon black powders are used in an amount of 50% or less based on 
the amount of inorganic powder and in an amount of 40% or less based on 
the total weight of the non-magnetic layer. These carbon black powders may 
be used either alone or in combination. 
Carbon black can be used in the present invention according to Carbon Black 
Handbook edited by Carbon Black Association of Japan. 
Examples of non-magnetic organic powder which can be used in the 
non-magnetic layer the present invention include acrylic styrene resin 
powder, benzoguanamine resin powder, melamine resin powder, phthalocyanine 
pigment, polyolefin resin powder, polyester resin powder, polyamide resin 
powder, polyimide resin powder and polyfluoroethylene resin powder. These 
resin powders can be prepared by the methods described in JP-A-62-18564 
and JP-A-60-255827. 
These non-magnetic powders are usually used in a ratio by weight of 
non-magnetic powder to binder of 20 to 0.1 and in a ratio by volume of 
non-magnetic powder to binder of 10 to 0.1. 
General-purpose magnetic recording mediums are provided with an 
undercoating layer. The undercoating layer is provided to improve adhesion 
between the support and the magnetic layer, etc. The undercoating layer 
has a thickness of 0.5 .mu.m or less and is different from the lower 
non-magnetic layer of the present invention. It is preferred that an 
undercoating layer is provided to improve adhesion between the support and 
the lower non-magnetic layer in the present invention. 
Examples of ferromagnetic powder which can be used in the magnetic layer of 
the present invention include conventional ferromagnetic powder such as 
.gamma.-FeOx (x=1.33 to 1.5), Co-modified .gamma.-FeOx (x=1.33 to 1.5), 
CrO.sub.2, Fe-, Ni- or Co-based ferromagnetic alloy powder (Fe, Ni or Co 
content being 75% or more) (e.g., Co--Ni--P alloy, Co--Ni--Fe--B alloy, 
Fe--Ni--Zn alloy, Ni--Co alloy, Co--Ni--Fe alloy), barium ferrite and 
strontium ferrite. These ferromagnetic powders may contain, in addition to 
the above-described elements, other elements such as Al, Si, S, Sc, Ti, V, 
Cr, Cu, Y, Mo, Rh, Pd, Ag, Sn, Sb, Te, Ba, Ta, W, Re, Au, Hg, Pb, Bi, La, 
Ce, Pr, Nd, P, Co, Mn, Zn, Ni, Sr and B. These ferromagnetic powders may 
be treated with dispersant, lubricant, surfactant, antistatic agent, etc., 
as described hereinafter, before dispersion. The treatment is concretely 
described in JP-B-44-14090 (the term "JP-B" as used herein means an 
"examined Japanese patent publication"). 
Among the above-described ferromagnetic powders, ferromagnetic alloy 
powders may contain a small amount of hydroxides or oxides. Ferromagnetic 
alloy powders prepared by conventional methods can be used. Examples of 
the methods for preparing ferromagnetic powders include the following: 
1. a method wherein a composite organic acid salt (mainly an oxalate) is 
reduced with a reducing gas such as hydrogen; 
2. a method wherein iron oxide is reduced with a reducing gas such as 
hydrogen to obtain Fe or Fe--Co particles; 
3. a method wherein a metal carbonyl compound is heat-decomposed, and a 
method wherein a reducing agent such as sodium boron hydride, a 
hypophosphite or hydrazine is added to an aqueous solution of a 
ferromagnetic metal to carry out reduction; 
4. a method wherein metal is evaporated in an inert gas under low pressure 
to obtain fine powder. 
The thus-obtained ferromagnetic alloy powders may be subjected to the 
following conventional slow oxidation treatments: 
5. a method wherein the powder is immersed in an organic solvent and then 
dried; 
6. a method wherein after the powder is immersed in an organic solvent, an 
oxygen-containing gas is introduced into the solution to form an oxide 
film on the surface of the powder, and the powder is dried; 
7. a method wherein partial pressure of an oxygen-containing gas and that 
of an inert gas are controlled to form an oxide film on the surface of the 
powder without using any organic solvent. 
Ferromagnetic powder contained in the upper magnetic layer of the present 
invention has a specific surface area of 25 to 80 m.sup.2 /g, preferably 
35 to 70 m.sup.2 /g, as measured by BET method. When the specific surface 
area is less than 25 m.sup.2 /g, noise becomes high, and when the specific 
surface area is more than 80 m.sup.2 /g, it is difficult to obtain good 
surface properties. 
Ferromagnetic powder contained in the upper magnetic layer of the present 
invention has a crystallite size of 100 to 450.ANG., preferably 100 to 
350.ANG., when determined from the spread of half width by X-ray 
diffractometry. 
Magnetic iron oxide powder has a saturation magnetization (.rho..sub.s) of 
50 emu/g or more, preferably 70 emu/g or more. Ferromagnetic metallic 
powder has a saturation magnetization of preferably 100 emu/g or more, 
more preferably from 110 to 170 emu/g. Ferromagnetic powder has a coercive 
force of preferably from 1100 to 2500 Oe (oersted), more preferably from 
1400 to 2000 Oe. Ferromagnetic powder has an acicular ratio of preferably 
18 or less, more preferably 12 or less. 
The .gamma.1500 of ferromagnetic powder is preferably not higher than 1.5, 
more preferably not higher than 1.0. The term ".gamma.1500" as used herein 
refers to the percentage of magnetization amount still remaining without 
inversion when a magnetic field of 1500 Oe in the opposite direction is 
applied to a magnetic recording medium after the magnetic recording medium 
is saturation-magnetized. 
The water content of ferromagnetic powder is controlled to preferably 0.01 
to 2%. It is preferred that the water content of ferromagnetic powder is 
properly controlled according to the type of the binder. When 
ferromagnetic powder is .gamma.-iron oxide, the tap density thereof is 
preferably 0.5 g/ml or more, more preferably 0.8 g/ml or more. In the case 
of alloy powder, the tap density is preferably from 0.2 to 0.8 g/ml. When 
the tap density is more than 0.8 g/ml, the oxidation of ferromagnetic 
powder is apt to proceed and it is difficult to obtain sufficient 
magnetization (.rho..sub.s), and when the tap density is less than 0.2 
g/ml, dispersion is likely to be poor. 
When .gamma.-iron oxide is used, the ratio of iron(II) to iron(III) is 
preferably 0 to 20%, more preferably 5 to 10%. The ratio of cobalt atoms 
to iron atoms is 0 to 15%, preferably 2 to 8%. 
It is preferred that the pH of ferromagnetic powder is properly chosen 
according to the type of the binder to be used in combination therewith. 
The pH is in the range of 4 to 12, preferably 6 to 10. If desired, the 
surface of ferromagnetic powder may be treated with Al, Si, P or an oxide 
thereof. The amount of the surface treating agent is 0.1 to 10% based on 
the amount of ferromagnetic powder. Such a surface treatment is preferred 
because a lubricant such as a fatty acid adsorbed is 100 mg/m.sup.2 or 
less when the surface treatment is carried out. Ferromagnetic powder often 
contains soluble inorganic ions such as Na, Ca, Fe, Ni and Sr ions. 
However, when the content of the inorganic ions is 500 ppm or less, the 
characteristics of ferromagnetic powder are not affected thereby. 
It is preferred that the ferromagnetic powder used in the present invention 
has a lower void content. The void content is preferably 20% by volume or 
less, more preferably 5% by volume or less. When characteristics with 
regard to particle size can be met, the shape of the ferromagnetic powder 
may be an acicular, granular, ellipsoidal or plate-like shape. In the case 
of needle ferromagnetic powder, the acicular ratio thereof is preferably 
12 or less. It is necessary that Hc distribution of ferromagnetic powder 
is made narrow to achieve SFD (switching field distribution) of 0.6 or 
less. To achieve this value, one may use a method wherein the particle 
size distribution of goethite is properly made, a method wherein 
.gamma.-hematite is prevented from being sintered, and a method wherein 
the deposition rate of cobalt is retarded in comparison with conventional 
deposition rates in the preparation of Co-modified iron oxide. 
Further, in the present invention, there can be used platy hexagonal 
ferrites, for example, substituted ferrites such as barium ferrite, 
strontium ferrite, lead ferrite and calcium ferrite; ,Co-substituted 
ferrites; and hexagonal Co powder. Specific examples thereof include 
magnetoplumbite type barium ferrite and strontium ferrite, and partially 
spinel structural magnetoplumbite type barium ferrite and strontium 
ferrite. Particularly preferred are barium ferrite and strontium ferrite. 
Elements such as Co--Ti, Co--Ti--Zr, Co--Ti--Zn, Ni--Ti--Zn or Ir--Zn may 
be added to the hexagonal ferrite to control the coercive force thereof. 
Usually, hexagonal ferrite comprises hexagonal platy particles. The 
particle size thereof is the width of the plate of the particle and is 
measured by using an electron microscope. In the present invention, the 
particle size is controlled to preferably from 0.01 to 0.2 .mu.m, 
particularly preferably from 0.03 to 0.1 .mu.m. The average thickness 
(thickness of plate) of the fine particles is from 0.001 to 0.2 .mu.m, 
particularly preferably from 0.003 to 0.05 .mu.m. Further, the plate ratio 
(]particle size/thickness of plate) is from 1 to 10, preferably from 3 to 
7. The specific surface area (S.sub.BET) Of these hexagonal ferrite fine 
powders is preferably from 25 to 70 m.sup.2 /g. The specific surface area 
is a value obtained by making measurement with BET one point method 
(partial pressure: 0.30) using Quantarsorb (manufactured by US 
Quantarchrom, Co., Ltd.) after dehydration at 30.degree. C. in a nitrogen 
gas atmosphere for 30 minutes. 
Examples of carbon black which can be used in the upper magnetic layer of 
the present invention include furnace black for rubber, thermal black for 
rubber, carbon black for color and acetylene black. Carbon black has 
preferably a specific surface area of 5 to 500 m.sup.2 /g, a DBP oil 
absorption of 10 to 400 ml/100 g, an average particle size of 5 to 300 
m.mu., a pH of 2 to 10, a water content of 0.1 to 10% and a tap density of 
0.1 to 1 g/ml. Specific examples of carbon black which can be used in the 
present invention include BLACK PEARLS 2000, 1300, 1000, 900, 800 and 700, 
VULCAN XC-72 (manufactured by Cabot Co., Ltd.); #80, #60, #55, #50 and 
#35 (manufactured by Asahi Carbon Co., Ltd.); #2400B, #2300, #900, #1000, 
#30, #40 and #10B (manufactured by Mitsubishi Kasei Corporation); and 
CONDUCTEX SC, RAVEN 150, 50, 40 and 15 (manufactured by Columbia. Carbon 
Co., Ltd.). The surface of carbon black may be treated with dispersants. 
Carbon black may be grafted onto a resin. A part of the surface of carbon 
black may be graphitized. Carbon black may be previously dispersed in the 
binder before carbon black is added to the magnetic coating composition. 
These carbon black may be used either alone or in combination. Carbon 
black is used in an amount of preferably 0.1 to 30% based on the amount of 
ferromagnetic powder. Carbon black is capable of imparting antistatic 
properties to the magnetic layer, reducing a coefficient of friction of 
the magnetic layer, imparting light-screening properties to the magnetic 
layer and improving the strength of the magnetic layer. These functions 
vary depending on carbon black to be used. Accordingly, the carbon black 
is used in the lower and upper layers by taking into consideration the 
types, amounts and combinations of carbon black, the above-described 
characteristics such as particle size, oil absorption, 
electric=conductivity, pH, etc. For example, carbon black which can be 
used in the upper layer can be properly chosen by referring to Carbon 
Black Handbook edited by Carbon Black Association of Japan. 
If desired, the magnetic layer of the present invention may contain 
non-magnetic organic powder which can be used in the non-magnetic layer. 
Examples of the abrasive which can be used in the upper magnetic layer of 
the present invention include conventional materials having a Mohs' scale 
of hardness of 6 or more such as .alpha.-alumina having an a conversion of 
90% or more, .beta.-alumina, silicon carbide, chromium oxide, cerium 
oxide, .alpha.-iron oxide, corundum, artificial diamond, silicon nitride, 
silicon carbide, titanium carbide, titanium oxide, silicon dioxide and 
boron nitride. These compounds may be used either alone or in combination. 
Composite materials (obtained by treating the surface of an abrasive with 
other abrasive) may be used. These abrasives often contain other compounds 
or element than the principal component. However, when the content of the 
principal component is 90% or more, the effect thereof is not reduced. The 
abrasives have a particle size of preferably 0.01 to 2 .mu.m. If desired, 
abrasives having different particle sizes may be used in combination, or 
when an abrasive alone is used, the particle size distribution thereof may 
be widened to provide a similar effect to that of the composite material. 
The abrasives have preferably a tap density of 0.3 to 2 g/ml, a water 
content of 0.1 to 5%, a pH of 2 to 11 and a specific surface area of 1 to 
30 m.sup.2 /g. The abrasives used in the present invention may be in any 
form of a needle, a sphere and a die. However, abrasives having a partly 
edgy form are preferred because they have a high abrasive effect. 
Specific examples of the abrasives which can be used in the present 
invention include AKP-20A, AKP-30, AKP-50, HIT-50 and HIT-100 
(manufactured by Sumitomo Chemical Co., Ltd.); G5, G7 and S-1 
(manufactured by Nippon Chemical Industrial Co., Ltd.); and TF-100 and 
TF-140 (manufactured by Toda Kogyo Corp.). The abrasives can be properly 
used in the lower and upper layers by varying the types, amounts and 
combinations thereof according to purpose. These abrasives may be 
previously dispersed in the binder and then added to the magnetic coating 
composition. The amount of the abrasive present on the surface and edge 
surface of the upper magnetic layer of the magnetic recording medium of 
the present invention is preferable 5 particles per 100 .mu.m.sup.2 or 
more. 
Further, compounds having a lubricating effect, an antistatic effect, a 
dispersion effect, a plasticizing effect, etc., can be used as additives 
in the present invention. Examples of the additives include molybdenum 
disulfide, tungsten disulfide, graphite, boron nitride, fluorinated 
graphite, silicone oil, silicone having a polar group, fatty acid-modified 
silicone, fluorinated silicone, fluorinated alcohols, fluorinated esters, 
polyolefins, polyglycols, alkylphosphoric esters and alkali metal salts 
thereof, alkylsulfuric esters and alkali metal salts thereof, polyphenyl 
ethers, fluorinated alkylsulfuric esters and alkali metal salts thereof, 
monobasic fatty acids (which may have unsaturated bonds or branched 
chains) having from 10 to 24 carbon atoms and metal salts (e.g., Li, Na, 
K, Cu, etc.) thereof; monohydric to hexahydric alcohols having 12 to 22 
carbon atoms (which may have unsaturated bonds or branched-chains); 
alkoxyalcohols having from 12 to 22 carbon atoms; mono-, di- or tri-fatty 
acid esters derived from monobasic fatty acids having 10 to 24 carbon 
atoms (which may have unsaturated bonds or branched chains) and any 
monohydric to hexahydric alcohols having from 2 to 12 carbon atoms (which 
may have unsaturated bonds or branched chains); fatty acid esters derived 
from monoalkyl ethers of alkylene oxide polymers; fatty acid amides having 
8 to 22 carbon atoms; and aliphatic amines having 8 to 22 carbon atoms. 
Specific examples of the fatty acids and derivatives thereof and the 
alcohols which can be used as the additives in the present invention 
include lauric acid, myristic acid, palmitic acid, stearic acid, behenic 
acid, butyl stearate, oleic acid, linoleic acid, linolenic acid, elaidic 
acid, octyl stearate, amyl stearate, isooctyl stearate, octyl myristate, 
butoxyethyl stearate, anhydrosorbitan monostearate, anhydrosorbitan 
distearate, anhydrosorbitan tristearate, oleyl alcohol and lauryl alcohol. 
Examples of surfactants which can be used in the present invention include 
nonionic surfactants such as alkylene oxides, glycerins, glycidols and 
ethylene oxide adducts of alkyl phenols; cationic surfactants such as 
cyclic amines, ester amides, quaternary ammonium salts, hydantoin 
derivatives, heterocyclic compounds, phosphonium salts and sulfonium 
salts; anionic surfactants having an acid group such as a carboxyl, sulfo, 
phosphoric, sulfuric ester or phosphoric ester group; and ampholytic 
sUrfactants such as amino acids, aminosulfonic acids, aminoalcohol 
sulfates or phosphates and alkylbetaines. The details of these surfactants 
are described in Surfactant Handbook (published by Sangyo Tosho KK). It is 
not required that these lubricants and surfactants are always 100% pure. 
They may contain, in addition to the principal component, impurities such 
as isomers, unreacted compound, by-products, decomposition products, 
oxides, etc. The amount of these impurities is preferably 30% or less, 
more preferably 10% or less. 
These lubricants and surfactants can be properly used in the lower 
non-magnetic layer and the upper magnetic layer by varying the types and 
amounts thereof. For example, the lower non-magnetic layer contains a 
fatty acid having a melting point which is different from that of a fatty 
acid used in the upper magnetic layer to thereby control the oozing 
thereof onto the surface. Esters having different boiling points or 
polarities are used to control the oozing thereof onto the surface. The 
amount of the surfactant is controlled to improve the stability of 
coating. The lower nonmagnetic layer contains the lubricant in an amount 
which is larger than that of the lubricant contained in the upper magnetic 
layer to thereby improve the lubricating effect. 
Embodiments are not limited to the above examples, but various other 
embodiments can be made. 
The whole or a part of the additives which can be used in the present 
invention may be added at any stage during the course of the preparation 
of the magnetic coating composition and the non-magnetic coating 
composition. For example, the additives may be mixed with the 
ferromagnetic powder before the kneading stage. The additives may be added 
to the kneading stage where the ferromagnetic powder, the binder and the 
solvent are kneaded. The additives may be added during or after the 
dispersion stage. The additive may be added just before coating. 
Examples of commercially available lubricants which can be used in the 
present invention include NAA-102, NAA-415, NAA-312, NAA-160, NAA-180, 
NAA-174, NAA-175, NAA-222, NAA-34, NAA-35, NAA-171, NAA-122, NAA-142, 
NAA-160, NAA-173K, caster oil-hardened fatty acids, NAA-42, NAA-44, Cation 
SA, Cation MA, Cation AB, Cation BB, Nymeen L-201, Nymeen L-202, Nymeen 
S-202, Nonion E-208, Nonion P-208, Nonion S-207, Nonion K-204, Nonion 
NS-202, Nonion NS-210, Nonion HS-206, Nonion L-2, Nonion S-2, Nonion S-4, 
Nonion O-2, Nonion LP-20R, Nonion PP-40R, Nonion SP-60R, Nonion OP-80R, 
Nonion OP-85R, Nonion LT-221, Nonion ST-221, Nonion TO-221, Monoguri MB, 
Nonion DS-60, Anon BF, Anon LG, butyl stearate, butyl laurate and erucic 
acid (manufactured by Nippon Oils & Fats Co., Ltd.); oleic acid 
(manufactured by Kanto Chemical Co., Ltd.); FAL-205 and FAL-123 
(manufactured by Takemoro Yushi Co., Ltd.); Enujerubu LO, Enujerubu IPM 
and Sansosyzer E-4030 (manufactured by Shin Nippon Rika Co., Ltd.); TA-3, 
KF-96, KF-96L, KF-96H, KF410, KF420, KF965, KF54, KF50, KF56, KF-907, 
KF-851, X-22-819, X-22-822, KF-905, KF-700, KF-393, KF-857, KF-860, 
KF-865, X-22-980, KF-101, KF-102, KF-103, X-22-3710, X-22-3715, KF-910 and 
KF-3935 (manufactured by Shin-Etsu Chemical Co., Ltd.); Armide P, Armide C 
and Armoslip CP (manufactured by Lion Ahmer Co., Ltd.); Duomin TDO 
(manufactured by Lion Fat and Oil Co., Ltd.); BA-41G (manufactured by 
Nissin Oil Mills, Co., Ltd.); and Profan 2012E, Newpole PE61, Ionet 
MS-400, Ionet MO-200, Ionet DL-200, Ionet DS- 300, Ionet DS-1000 and Ionet 
DO-200 (manufactured by Sanyo Chemical Industries, Ltd.). 
Organic solvents may be used in an arbitrary ratio. Examples of the organic 
solvents which can be used in the present invention include ketones such 
as acetone, methyl ethyl ketone, methyl isobutyl ketone, diisobutyl 
ketone, cyclohexanone, isophorone and tetrahydrofuran; alcohols such as 
methanol, ethanol, propanol, butanol, isobutyl alcohol, isopropyl alcohol 
and methylcyclohexanol; esters such as methyl acetate, butyl acetate, 
isobutyl acetate, isopropyl acetate, ethyl lactate and glycol acetate; 
glycol ethers such as glycol dimethyl ether, glycol monoethyl ether and 
dioxane; aromatic hydrocarbons such as benzene, toluene, xylene, cresol 
and chlorobenzene; chlorinated hydrocarbons such as methylene chloride, 
ethylene chloride, carbon tetrachloride, chloroform, ethylene 
chlorohydrin, dichlorobenzene; N,N-dimethylformamide; and hexane. It is 
not required that these organic solvents are 100% pure. The organic 
solvents may contain, in addition to the principal component, impurities 
such as isomers, unreacted materials, by-products, decomposition products, 
oxides, water, etc. The content of the impurities is preferably 30% by 
weight or less, more preferably 10% by weight or less. If desired, organic 
solvents used in the upper magnetic layer may be different in type and 
amount from those used in the lower non-magnetic layer. For example, 
organic solvents which are higher volatile are used in the upper magnetic 
layer to improve surface properties. Organic solvents having a high 
surface tension (e.g., cyclohexanone, dioxane) are used in the lower 
non-magnetic layer to improve the stability of coating. Organic solvents 
having a high solubility parameter are used in the magnetic layer to 
improve the degree of loading. Embodiments are not limited to these 
specific examples, and other various embodiments can be made. 
The magnetic recording medium of the present invention has such a thickness 
profile in which the nonmagnetic support has a thickness of 1 to 100 
.mu.m, preferably 6 to 20 .mu.m, the lower non-magnetic layer has a 
thickness of 0.5 to 10 .mu.m, preferably 1 to 5 .mu.m, and the upper 
magnetic layer has a thickness of preferably 1.5 .mu.m or less, more 
preferably 1.0 .mu.m or less, particularly preferably 0.5 .mu.m or less. 
The combined thickness of the lower non-magnetic layer and the upper layer 
is in the range of 1 to 100 times to twice the thickness of the 
non-magnetic support. An undercoating layer may be provided between the 
non-magnetic support and the lower non-magnetic layer to improve adhesion 
therebetween. The undercoating layer has a thickness of 0.01 to 2 .mu.m, 
preferably 0.05 to 0.5 .mu.m. Further, a back coat layer may be provided 
on the opposite side of the support to the magnetic layer. The back coat 
layer has a thickness of 0.1 to 2 .mu.m, preferably 0.3 to 1.0 .mu.m. The 
undercoating layer and the back coat layer may be conventional layers. 
Examples of the non-magnetic support which can be used in the present 
invention include conventional films such as films of polyesters (e.g., 
polyethylene terephthalate, polyethylene naphthalate), polyolefins, 
cellulose triacetate, polycarbonates, polyamides, polyimides, 
polyamide-imides, polysulfones, aramid and aromatic polyamides. These 
support may be subjected to a corona discharge treatment, a plasma 
treatment, an undercoating treatment, a heat treatment or a dust removal 
treatment. It is necessary that the nonmagnetic support having a center 
line average surface roughness of 0.03 .mu.m or less, preferably 0.02 
.mu.m or less, more preferably 0.01 .mu.m or less, is used to achieve the 
object of the present invention. Further, it is preferred that not only 
the center line average surface roughness of the non-magnetic support is 
small, but also that the support does not have any coarse protrusion of 1 
.mu.m or more. The roughness of the surface thereof can be arbitrarily 
controlled by the size and amount of a filler which is optionally added to 
the support. Examples of the filler include powders of oxides and 
carbonates of Ca, Si and Ti and acrylic organic fine powder. 
The F-5 (i.e., the load at the 5% elongation) value of the non-magnetic 
support of the present invention is preferably from 5 to 50 kg/mm.sup.2 in 
the tape running direction and from 3 to 30 kg/mm.sup.2 in the tape width 
direction. Generally, the F-5 value in the tape running direction is 
higher than that in the tape width direction. However, the F-5 value in 
the tape width direction may be higher than that in the running direction 
when the strength of the tape in the width direction is to be increased. 
The heat shrinkage ratio of the non-magnetic support in the tape running 
direction and in the tape width direction at 100.degree. C for 30 minutes 
is preferably 3% or less, more preferably 1.5% or less, and that at 
80.degree. C. for 30 minutes is preferably 1% or less, more preferably 
0.5% or less. The breaking strength of the non-magnetic support in the 
tape running direction and in the tape width direction is preferably 5 to 
10 kg/mm.sup.2, and the modulus thereof is preferably 100 to 2,000 
kg/mm.sup.2. 
The manufacturing stage of the magnetic coating composition and the 
non-magnetic coating composition for the preparation of the magnetic 
recording medium of the present invention comprises at least the kneading 
stage and the dispersion stage. In addition thereto, the mixing stage is 
optionally provided before or after the kneading stage and the dispersion 
stage. Each stage may be divided into two or more steps. All of the 
materials such as the ferromagnetic powder, the non-magnetic powder, the 
binder, the carbon black, the abrasive, the antistatic agent, the 
lubricant and the solvent may be added to the first stage or may be 
separately added during the preparation of the coating compositions. Each 
material may be added portionwise at two or more stages. For example, 
polyurethane may be added in portions to the kneading stage, the 
dispersion stages and the mixing stage for the controlling of viscosity 
after dispersion. 
Conventional manufacturing processes may be used as a part of the stages of 
the present invention, to achieve the object of the present invention. A 
kneader having an intense kneading force such as a continuous kneader or a 
pressure kneader can be used in the kneading stage, whereby a magnetic 
recording medium which gives high Br (residual magnetic flux density) can 
be obtained. When the continuous kneader or the pressure kneader is used, 
the whole or a part (preferably at least 30% by weight of the binder) of 
the binder is kneaded in an amount of 15 to 500 parts by weight per 100 
parts by weight of ferromagnetic powder. The details of the kneading 
treatment are described in JP-A-1-106338 and JP-A-1-79274. 
Examples of apparatuses and methods for carrying out coating by means of a 
wet-on-wet coating system to prepare a magnetic recording medium having a 
multi-layer structure as in the present invention include the following 
apparatuses and methods: 
1. the lower layer is first coated by means of gravure coating, roll 
coating, blade coating or extrusion coating apparatus. While the lower 
layer is still in a wet state, the upper layer is then coated on the lower 
layer by using a support pressing type extrusion coater as described in 
JP-B-1-46186, JP-A-60-238179 and JP-A-2-265672. 
2. the lower layer and the upper layer are substantially simultaneously 
coated by using a coating head provided with two slits through which the 
coating compositions are passed as described in JP-A-63-88080, 
JP-A-2-17971 and JP-A-2-265672. 
3. the lower layer and the upper layer are substantially simultaneously 
coated by using an extrusion coater provided with back-up roller as 
described in JP-A-2-174965. 
It is desirable that shear is applied to the coating composition within the 
coating head by a method described in JP-A-62-95174 or JP-A-1-236968 to 
prevent the electromagnetic characteristics, etc. of the resulting 
magnetic recording medium from being lowered by the agglomeration of 
ferromagnetic powder. Further, it is necessary that the viscosity of the 
coating composition is in the range of numerical values described in 
JP-A-3-8471. 
Intense orientation is made to obtain the magnetic recording medium of the 
present invention. It is preferred that a solenoid of 1,000 G (gauss) or 
more and a cobalt magnet of 2,000 G or more are used in combination. 
Further, it is preferred that a suitable drying stage is provided before 
orientation so that orientation after drying reaches the highest level. 
When the present invention is applied to disk type mediums, it is 
preferred that an orientation method is used to randomize orientation. 
Rolls of heat-resistant plastics such as epoxy resin, polyimide, polyamide 
and polyimide-amide are used as calendering rolls. Metallic rolls may be 
used. Treating temperature is preferably 70.degree. C. or more, more 
preferably 80.degree. C. or more. Linear pressure is preferably 200 kg/cm 
or more, more preferably 300 kg/cm or more. 
The upper layer of the magnetic recording medium of the present invention 
and the opposite surface thereto have a coefficient of friction of 
preferably 0.5 or less, more preferably 0.3 or less, against SUS 420J. The 
magnetic layer has a surface resistivity of preferably 10.sup.4 to 
10.sup.11 .OMEGA./sq. When the lower layer alone is coated, the surface 
resistivity is preferably 10.sup.4 to 10.sup.8 .OMEGA./sq. The back layer 
has a surface resistivity of preferably 10.sup.3 to 10.sup.9 .OMEGA./sq. 
The upper layer has a modulus at 0.5% elongation of preferably 100 to 2,000 
kg/mm.sup.2 in the running direction and in the width direction and a 
breaking strength of 1 to 30 kg/cm.sup.2. The magnetic recording medium 
has a modulus of preferably 100 to 1,500 kg/mm.sup.2 in the running 
direction and in the width direction, a residual elongation of preferably 
0.5% or less and a heat shrinkage ratio of preferably 1% or less, more 
preferably 0.5% or less, still more preferably 0.1% or less, over a 
temperature range of 100.degree. C. or less. 
The amount of the solvent left behind in the upper layer is preferably 100 
mg/m.sup.2 or less, more preferably 10 mg/m.sup.2 or less. It is preferred 
that the amount of the solvent left in the upper layer is less than that 
of the solvent left in the lower layer. 
Each of the upper layer and the lower layer has a void content of 
preferably 30% by volume or less, more preferably 20% by weight or less. A 
lower void content is preferred to obtain a high output. However, it is 
sometimes preferred that the content of the non-magnetic layer is 
increased according to purpose. For example, good running durability can 
be often achieved when the non-magnetic layer has a higher void content in 
the magnetic recording medium for data recording in which repeated use is 
important. 
The magnetic recording medium of the present invention has such magnetic 
characteristics that the squareness ratio thereof in the tape traveling 
direction is 0.70 or more, preferably 0.80 or more, more preferably 0.90 
or more, when measured at a magnetic field of 5 kOe. The squareness ratio 
in two directions perpendicular to the tape running direction is 
preferably 80% or less of that in the tape running direction. The SFD of 
the magnetic layer is preferably 0.6 or less. 
The center line average surface roughness Ra of the magnetic layer is from 
2 to 20 nm, preferably 5.0 nm or less. However, the value of Ra should be 
properly set according to purpose. It is preferred that Ra is small when 
electromagnetic characteristics is to be improved. On the other hand, a 
large Ra value is preferred when running durability is to be improved. It 
is preferred that RMS surface roughness R.sub.RMS as determined by 
evaluation of STM is in the range of 3 to 16 nm. 
The magnetic recording medium of the present invention has a lower layer 
and a upper layer. The lower and upper layers may be different in physical 
characteristics from each other according to purpose. For example, the 
modulus of the magnetic layer is increased to improve running durability, 
and at the same time, the modulus of the non-magnetic layer is set to a 
value which is lower than the modulus of the magnetic layer to improve the 
head touch of the magnetic recording medium. 
The present invention is further illustrated in greater detail by reference 
to the following examples and comparative examples. In the following 
examples and comparative examples, all parts are by weight. 
EXAMPLE 1 
______________________________________ 
Non-magnetic coating composition 
Non-magnetic inorganic powder 
80 parts 
TiO.sub.2 
crystal structure: rutile type 
average primary particle size: 0.035 .mu.m 
TiO.sub.2 content: 90% or more 
specific surface area: 40 m.sup.2 /g 
DBP oil absorption: 27-38 ml/100 g 
pH: 7 
Carbon black 20 parts 
average primary particle size: 16 m.mu. 
Specific surface area (BET): 250 m.sup.2 /g 
DBP oil absorption: 80 ml/100 g 
pH: 8.0 
volatile matter content: 1.5% 
Vinyl chloride-vinyl acetate-vinyl 
8 parts 
alcohol copolymer 
composition ratio (molar ratio) = 
86:13:1 
a degree of polymerization: 400 
--N(CH.sub.3).sub.2.sup.+ Cl.sup.- group content: 5 .times. 10.sup.-4 
eq/g 
Polyurethane 6 parts 
neopentyl glycol/caprolactone 
polyol/MDI = 0.9/2.6/1 
--SO.sub.3 Na group content: 1 .times. 10.sup.-4 eq/g 
weight-average molecular weight: 15,000 
Butyl stearate 1 part 
Myristic acid 1 part 
Methyl ethyl ketone 100 parts 
Cyclohexanone 50 parts 
Toluene 50 parts 
Magnetic coating composition 
Ferromagnetic metallic fine powder 
100 parts 
composition: Fe/Zn/Ni = 92/6/2 
Hc: 1600 Oe 
specific surface area (BET): 60 m.sup.2 /g 
crystallite size: 195.ANG. 
particle size (long axis): 0.20 .mu.m 
acicular ratio: 10 
saturation magnetization (.sigma..sub.s): 130 emu/g 
Vinyl chloride copolymer 8 parts 
--SO.sub.3 Na group content: 1 .times. 10.sup.-4 eq/g 
a degree of polymerization: 300 
Polyurethane 6 parts 
neopentyl glycol/caprolactone 
polyol/phthalic acid/MDI system 
polar group: --SO.sub.3 Na 
weight-average molecular 
weight: 26,000 
.alpha.-Alumina 2 parts 
average particle size: 0.3 .mu.m 
Carbon black 0.5 parts 
average particle size: 0.10 .mu.m 
Butyl stearate 1 part 
Myristic acid 1 part 
Stearic acid 1 part 
Methyl ethyl ketone 90 parts 
Cyclohexanone 50 parts 
Toluene 60 parts 
______________________________________ 
The above ingredients for each of the above coating compositions were 
kneaded in a continuous kneader and dispersed in a sand mill. To the 
resulting dispersion for the non-magnetic coating composition, there was 
added 6 parts of a trifunctional low-molecular polyisocyanate compound. To 
the resulting dispersion for the magnetic coating composition, than was 
added 6 parts of the trifunctional low-molecular polyisocyanate compound. 
Thus, there was prepared a non-magnetic coating composition and a magnetic 
coating composition. 
The non-magnetic coating composition and the magnetic coating composition 
were coated on a polyethylene terephthalate support of 10 .mu.m in 
thickness by means of simultaneous multi-layer coating in the following 
manner. 
The non-magnetic coating composition was coated on the support in such an 
amount so as to give a dry thickness of 2.5 .mu.m to form a lower 
non-magnetic layer. Immediately thereafter, the magnetic coating 
composition was coated on the lower non-magnetic layer in such an amount 
as to give a dry thickness of 0.5 .mu.m to form an upper magnetic layer. 
While both layers were still in a wet state, the coated product was passed 
through a magnetic field to carry out orientation. After drying, the 
resulting product was subjected to a surface smoothing treatment at 
90.degree. C. by using a 7-stage calender constructed from only metallic 
rolls, and then slitted into a 1/2-inch magnetic tape. 
Further, other samples given in Table 1 were prepared in the same manner as 
described above. In Table 1, P in P/H ratio represents the proportion of 
polyurethane, that is, [100.times.parts of polyurethane/(parts of 
polyurethane+parts of vinyl chloride resin+parts of polyisocyanate)], and 
H represents the proportion of polyisocyanate, that is, [100.times.parts 
of polyisocyanate/(parts of polyurethane+parts of vinyl chloride 
resin+parts of polyisocyanate)]. 
TABLE 1 
__________________________________________________________________________ 
Thickness (.mu.m) 
Magnetic coating Non-Magnetic coating of coated layer 
composition composition Magnetic 
Non-magnetic 
Mw of 
Polar group Mw of 
Polar group layer 
layer 
Sample poly- 
of poly- 
P/H poly- 
of poly- 
P/H Coating (upper 
(lower 
No. urethane 
urethane 
ratio 
urethane 
urethane 
ratio 
system layer) 
layer) 
__________________________________________________________________________ 
Example 1 
2.6 .times. 10.sup.4 
attached 
30/30 
1.5 .times. 10.sup.4 
attached 
30/30 
simultaneously 
0.5 2.5 
multi-layer 
coating 
Example 2 
3.8 .times. 10.sup.4 
attached 
40/20 
1.3 .times. 10.sup.4 
attached 
40/20 
simultaneously 
0.5 2.5 
multi-layer 
coating 
Comparative 
5.0 .times. 10.sup.4 
attached 
50/10 
1.0 .times. 10.sup.4 
attached 
50/10 
simultaneously 
0.5 2.5 
Example 1 multi-layer 
coating 
Comparative 
2.6 .times. 10.sup.4 
attached 
30/30 
1.5 .times. 10.sup.4 
attached 
30/30 
successive 
0.5 2.5 
Example 2 multi-layer 
coating 
Comparative 
2.6 .times. 10.sup.4 
omitted 30/30 
1.5 .times. 10.sup.4 
omitted 30/30 
simultaneously 
0.5 2.5 
Example 3 multi-layer 
coating 
Comparative 
2.6 .times. 10.sup.4 
omitted 50/10 
1.5 .times. 10.sup.4 
omitted 50/10 
simultaneously 
0.5 2.5 
Example 4 multi-layer 
coating 
Comparative 
2.6 .times. 10.sup.4 
omitted 30/30 
-- -- -- single layer 
3.0 -- 
Example 5 
Comparative 
2.6 .times. 10.sup.4 
attached 
30/30 
-- -- -- single layer 
3.0 -- 
Example 6 
Comparative 
1.0 .times. 10.sup.4 
attached 
30/30 
1.5 .times. 10.sup.4 
attached 
30/30 
simultaneously 
0.5 2.5 
Example 7 multi-layer 
coating 
Comparative 
2.6 .times. 10.sup.4 
attached 
60/0 
1.5 .times. 10.sup.4 
attached 
60/0 
simultaneously 
0.5 2.5 
Example 8 multi-layer 
coating 
__________________________________________________________________________ 
EXAMPLE 2 
The procedure of Example 1 was repeated except that the compositions of the 
magnetic coating composition and the non-magnetic coating composition were 
changed as shown in Table 1. 
COMATIVE EXAMPLE 1 
The procedure of Example 1 was repeated except that the compositions of the 
magnetic coating composition and the non-magnetic coating composition were 
changed as shown in Table 1. 
COMATIVE EXAMPLE 2 
The procedure of Example 1 was repeated except that the compositions of the 
magnetic coating composition and the non-magnetic coating composition were 
changed as shown in Table 1, and coating was carried out by a successive 
multi-layer coating system wherein the magnetic layer was coated after the 
non-magnetic layer was coated. 
COMATIVE EXAMPLES 3 AND 4 
The procedure of Example 1 was repeated except that the compositions of the 
magnetic coating composition and the non-magnetic coating composition were 
changed as shown in Table 1, and polyurethane having no polar group was 
used. 
COMATIVE EXAMPLE 5 
The procedure of Example 1 was repeated except that only the magnetic layer 
was coated, the non-magnetic layer was not provided and the resin 
component having no polar group was used. 
COMATIVE EXAMPLE 6 
The procedure of Example 1 was repeated except that only the magnetic layer 
was coated and the non-magnetic layer was not coated. 
COMATIVE EXAMPLE 7 
The procedure of Example 2 was repeated except that the composition of the 
magnetic coating composition and the weight-average molecular weight of 
polyurethane were changed as shown in Table 1. 
COMATIVE EXAMPLE 8 
The procedure of Example 1 was repeated except that the! curing agent 
component was eliminated from the magnetic coating composition and the 
non-magnetic coating composition. 
The magnetic tapes obtained by Examples and Comparative Examples were 
tested to examine surface roughness Ra, reproducing RF output and the 
staining of guide pole. The ratio of the curing agent component present in 
the area between the surface of the magnetic layer and a position of about 
0.1 .mu.m in depth from the surface of the magnetic layer to the total 
amount of the curing agent component present in the magnetic layer (the 
ratio of curing agent component in Table 2). The weight-average molecular 
weight of polyurethane as the resin component extracted with TMF (Mw of 
THF extract in Table 2) was determined, the THF extract being obtained by 
extracting polyurethane as the resin component, present in an area between 
the outer surface of the magnetic layer and a position of 0.1 .mu.m in 
depth from the outer surface of the magnetic layer, with THF. The 
evaluation methods were as follows. 
1. Surface roughness Ra 
The surface roughness was measured by using a contact surface roughness 
tester (Surfcom 800A type manufactured by Tokyo Seimitsu Co., Ltd.). The 
cut off value was 0.08 mm. 
2. Reproducing RF output 
Video signals of image signals 501RE (The Institute of Radio Engineers) 
were recorded with reference image transcription current. The mean value 
of the envelope of the reproducing RF output thereof was measured. The 
reproducing RF output was calculated from the following formula: 
EQU Reproducing RF output (dB)=20log.sub.10 V/V.sub.0 
V: mean value 
V.sub.0 : reference value 
3. Staining of guide pole 
The resulting tape was run 50 times. The guide pole was taken out from the 
actually run in-cassette and inspected through a stereomicroscope. The 
level of staining was evaluated. 
4. Ratio of curing agent component 
The magnetic recording medium was extracted with n-hexane at room 
temperature for 30 minutes. The N/Fe of the surface of the magnetic layer 
of the magnetic recording medium was determined by ESCA. The magnetic 
recording medium used in the extraction with n-hexane was drawn, and the 
whole coated layer was peeled off, thoroughly crushed in an agata mortar 
and pelletized into 10 mm.phi. pellets. The N/Fe of the whole coated layer 
was determined from the pellets by ESCA. The ratio was determined by 
dividing the former (i.e., the N/Fe of the surface of the magnetic layer) 
by the latter (i.e., the N/Fe of the whole coated layer). 
5. Mw of THF extract 
A layer of about 0.1 .mu.m between the outer surface of the magnetic layer 
and a position of about 0.1 .mu.m in depth from the surface of the 
magnetic layer was scraped off with an abrasive tape (#700). The resulting 
magnetic layer powder was extracted with tetrahydro-furan. The 
weight-average molecular weight of the soluble matter was determined by 
means of GPC. To determine the molecular weight of polyurethane, a 
measurement with an UV detector was made by utilizing the fact that since 
polyurethane contained MDI, polyurethane had absorption in the ultraviolet 
region and the vinyl chloride copolymer had no absorption in the 
ultraviolet region. The measuring device used was HLC-8020 manufactured by 
Tosoh Co., Ltd. A calibration curve was prepared using polystyrene. 
The results are shown in Table 2. 
The evaluation is made as follows. 
Staining of guide pole 
The mark O: not stained 
The mark X: stained 
Overall evaluation 
The mark O: good 
The mark .DELTA.: poor 
The mark X: bad 
TABLE 2 
__________________________________________________________________________ 
Analysis of 
finished magnetic tape 
Reproducing 
Mw of Ratio of curing 
Ra RF output 
Staining of 
Overall 
Sample No. 
THF extract 
agent component 
(nm) 
(dB) guide pole 
evaluation 
__________________________________________________________________________ 
Example 1 
3.6 .times. 10.sup.4 
1.7 2.5 
+2.0 .largecircle. 
.largecircle. 
Example 2 
3.5 .times. 10.sup.4 
1.4 4.5 
+1.5 .largecircle. 
.largecircle. 
Comparative 
3.5 .times. 10.sup.4 
1.1 6.2 
+0.5 .largecircle. 
.DELTA. 
Example 1 
Comparative 
1.8 .times. 10.sup.4 
1.1 9.7 
-0 X X 
Example 2 
Comparative 
4.0 .times. 10.sup.4 
1.7 7.2 
-0.5 .largecircle. 
X 
Example 3 
Comparative 
2.0 .times. 10.sup.4 
1.1 7.5 
-1.0 .largecircle. 
X 
Example 4 
Comparative 
3.8 .times. 10.sup.4 
1.9 6.3 
-0.8 .largecircle. 
X 
Example 5 
Comparative 
3.5 .times. 10.sup.4 
1.8 2.2 
.largecircle. 
.largecircle. 
X 
Example 6 
Comparative 
1.3 .times. 10.sup.4 
1.8 2.0 
+2.3 X X 
Example 7 
Comparative 
1.2 .times. 10.sup.4 
-- 4.3 
+0.8 X X 
Example 8 
__________________________________________________________________________ 
It is apparent from Tables 1 and 2 that the samples of the present 
invention have good surface roughness and improved RF output, do not cause 
the staining of guide pole, and have improved running durability. These 
effects are thought to be due to the fact that the amount of the curing 
agent component reacted in the surface layer of the magnetic layer is 
increased, and as a result, the rate of reaction of polyurethane is 
increased and the strength of the surface of the magnetic layer is 
increased. 
In the present invention, the average molecular weight of the binder formed 
by the reaction of the curing agent component with the binder present in 
an area in the vicinity of the outer surface of the magnetic layer, that 
is, in an area between the surface of the magnetic layer and a position of 
0.1 .mu.m in depth from the surface of the magnetic layer is controlled so 
that the average molecular weight of the binder formed is higher than that 
of the binder formed in the whole of the magnetic layer, whereby the 
amounts of unreacted or unadsorbed binder which causes the staining of 
head and guide pole is reduced and running durability is improved. The 
flexibility of the whole of the magnetic layer and the whole of the coated 
layers is kept and head touch is improved by ensuring the ratio of the 
curing agent component reacted in the range defined above. 
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.