Magnetic recording medium

A magnetic recording medium is described, comprising a non-magnetic support having coated thereon on opposite surfaces a magnetic layer containing a magnetic substance and a binder-containing backing layer, wherein the magnetic layer has a light transmittance of 2% or more for light having a wavelength of 900 nm, the backing layer has a transmittance of from 1 to 10% for light having a wavelength of 900 nm, the magnetic recording medium as a whole has a transmittance of 1% or less for light having a wavelength of 900 nm, and the backing layer has a center line average roughness (Ra) of 0.05 .mu.m or less at a cut-off value of 0.08 mm.

FIELD OF THE INVENTION 
The present invention relates to a magnetic recording medium, and more 
particularly it relates to a magnetic recording medium having excellent 
electromagnetic properties and running durability. 
BACKGROUND OF THE INVENTION 
In magnetic recording tapes for video and audio recording, the surface of 
the magnetic layer is generally made smooth to improve sensitivity and 
output, particularly in a high frequency region. 
It has also been proposed to provide a backing layer to improve the running 
properties of such magnetic tapes as described, for example, in U.S. Pat. 
No. 4,135,031. However, conventionally used backing layers have rather 
coarse surfaces, and their unevenness tends to be imprinted on the 
magnetic layer. 
In order to produce video tapes having higher S/N and to improve the 
packing density of ferromagnetic particles therein, it has been proposed 
to make the ferromagnetic particles finer and to reduce the amount of 
additive particles other than ferromagnetic particles, e.g., the amount of 
carbon black to a value as small as possible. However, when the 
ferromagnetic particles are made finer and the amount of carbon black in 
the magnetic layer is reduced, the optical density decreases, and the 
magnetic tapes tend to stop running and to become electrically charged, 
resulting in deterioration of the running durability of the tapes. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a magnetic recording 
medium having particularly excellent electromagnetic properties and 
running durability. 
As a result of extensive research as to the physical properties of magnetic 
layers, backing layers, and magnetic tapes as a whole, it has now been 
found that the above-described problems can be solved by adjusting the 
light transmittance of each layer to a predetermined level and by 
adjusting the surface roughness of the backing layer. 
That is, the present invention provides a magnetic recording medium 
comprising a non-magnetic support having coated on each surface thereof a 
magnetic layer containing a magnetic substance and a binder-containing 
backing layer, wherein the magnetic layer has a light transmittance of 2% 
or more for light having a wavelength of 900 nm, the backing layer has a 
transmittance of from 1 to 10% for light having a wavelength of 900 nm, 
the magnetic recording medium as a whole has a transmittance of 1% or less 
for light having a wavelength of 900 nm, and the surface roughness of the 
backing layer as defined by the center line average roughness (Ra) at a 
cut-off value of 0.08 mm is 0.05 .mu.m or less.

DETAILED DESCRIPTION OF THE INVENTION 
The light transmittance of the magnetic layer of the present invention for 
light having a wavelength of 900 nm is 2% or more, and preferably 3% or 
more. 
The light transmittance of the backing layer for light having a wavelength 
of 900 nm according to the present invention is from 1 to 10%, and 
preferably from 1.5 to 8%. 
The backing layer of the present invention preferably has a thickness of 2 
.mu.m or less, and more preferably the thickness is from 0.4 to 1.5 .mu.m. 
The backing layer has a smooth surface, and particularly has a center line 
average roughness (Ra) of 0.05 .mu.m or less, and preferably 0.03 .mu.m or 
less, at a cut-off value of 0.08 mm. 
By adjusting the light transmittance of the magnetic recording medium as a 
whole for light having a wavelength of 900 nm to 1% or less, the magnetic 
tape having provided thereon a magnetic layer and a backing layer can have 
excellent electromagnetic properties, particularly S/N (signal/noise) 
ratio, which conventional video tapes for broadcasting having provided 
thereon a magnetic layer and a backing layer have not been able to attain. 
In a coated tape magnetic layer using Co-containing magnetic iron oxide, 
the S/N ratio is excellent when the magnetic particles have a specific 
surface area S.sub.BET of 35 m.sup.2 /g or more and carbon black is used 
in a small amount, that is, the packing density is high and light 
transmittance is high. The S/N ratio is abruptly improved when the light 
transmittance for light having a wavelength of 900 nm is 2% or more. 
Also, in order to have an excellent S/N ratio, the backing layer preferably 
has a smooth surface so that unevenness of the backing layer is not 
imprinted on a magnetic layer. In order to have excellent running 
durability, the light transmittance for light having a wavelength of 900 
nm is preferably 1% or more. If the light transmittance is not higher than 
1%, the running durability of magnetic tapes is deteriorated, the backing 
layer quickly wears out, and the number of drop out increases. If the 
light transmittance exceeds 10%, the optical density is insufficient and 
magnetic tapes mounted on a video tape recorder (VTR) of the VHS type tend 
to stop running when the magnetic layer has a high S/N ratio. 
Colored particles such as carbon black graphite, titanium oxide, and 
titanium carbide can be added to adjust the light transmittance of the 
magnetic layer and the backing layer. 
The carbon black is not limited and any conventional carbon black can be 
used. The particle size thereof is preferably from 5 to 250 .mu.m and 
carbon black having various particle sizes can be used in combination. 
Titanium oxide particles can be obtained by reducing titanium dioxide 
particles. For example, a detailed explanation is provided in Japanese 
Patent Application (OPI) No. 91037/83 (the term "OPI" as used herein 
refers to a "published unexamined Japanese Patent Application"). 
The particle size of the colored particles is preferably from 5 to 25 
.mu.m. 
The colored particles are preferably used in an amount of 10 parts by 
weight or less and more preferably in an amount of 7 parts by weight or 
less per 100 parts by weight of the magnetic substance in the magnetic 
layer. 
The colored particles are preferably used in the backing layer in an amount 
of 30 parts by weight or more, and more preferably from 30 to 200 parts by 
weight per 100 parts by weight of the binder contained in the backing 
layer. 
The magnetic layer of the present invention can be a conventionally coated 
type magnetic layer containing Co-containing magnetic iron oxide as 
described, e.g., in U.S. Pat. No. 4,135,031, or a thin ferromagnetic metal 
film type magnetic layer prepared, e.g., by vapor deposition method such 
as vacuum evaporation, sputtering or ion plating as described, e.g., in 
U.S. Pat. Nos. 4,097,650 and 4,172,171. When Co-containing magnetic iron 
oxide is used, particles having a specific surface area of 35 m.sup.2 /g 
or more are preferred. 
Conventionally used binders, lubricating agents, abrasive agents and the 
like can be used in the magnetic layer, particularly in the coated type 
magnetic layer. Specific examples of materials and methods for preparing 
the magnetic layer are disclosed, e.g., in U.S. Pat. No. 4,135,016. 
The binders can be those conventionally used in magnetic recording media, 
such as cellulose derivatives (nitrocellulose, cellulose propionate), 
vinyl chloride-vinyl acetate resins, and further including those 
containing a vinyl alcohol or maleic acid, polyurethanes, epoxy resins, 
polyamides, polyisocyanates, and polyesters. The binders can be used in an 
amount of 10 to 50 parts by weight per 100 parts by weight of the magnetic 
substance. The abrasive agents include .alpha.-Al.sub.2 O.sub.3, Cr.sub.2 
O.sub.3, SiC, and .alpha.-Fe.sub.2 O.sub.3. The amount of the abrasive 
which can be used is 0.001 to 20 parts by weight per 100 parts by weight 
of the magnetic substance. A dispersing agent such as lecithin, a 
plasticizer such as TPP or DBP, and a stabilizing agent such as lead 
stearate can also be added, if desired. The amounts of the dispersing 
agent, plasticizer and stabilizing agent are 0 to 10 parts by weight, 0 to 
5 parts by weight and 0 to 5 parts by weight, respectively, per 100 parts 
by weight of the magnetic substance. Inorganic particles such as silica, 
CaCO.sub.3, or talc can be added in an amount of 0 to 100 parts by weight 
per 100 parts by weight of the colored particles in the backing layer. 
In addition to the colored particles, inorganic particles can be mixed and 
kneaded with binders in the backing layer, if desired. Examples of 
preferred inorganic particles include calcium carbonate, barium sulfide, 
ZnS, MgCO.sub.3, ZnO, Fe.sub.3 O.sub.4, SnO.sub.2, SiO.sub.2, Cr.sub.2 
O.sub.3, .alpha.-Al.sub.2 O.sub.3, and SiC. 
The binders used in the backing layer include those conventionally used, 
such as thermoplastic resins, thermosetting resins, reactive type resins, 
or mixtures thereof. 
The thermoplastic resins include a vinyl chloride-vinyl acetate copolymer, 
a vinyl chloride-vinylidene chloride copolymer, a vinyl 
chloride-acrylonitrile copolymer, an acrylate-acrylonitrile copolymer, an 
acrylate-vinylidene chloride copolymer, an acrylate-styrene copolymer, a 
methacrylate-acrylonitrile copolymer, a methacrylate-vinylidene chloride 
copolymer, a methacrylate-styrene copolymer, urethane elastomer, polyvinyl 
fluoride, a vinylidene chloride-acrylonitrile copolymer, a butadiene and 
acrylonitrile copolymer, polyamide resin, polyvinyl butyral, a cellulose 
type resin (cellulose acetate butyrate, cellulose diacetate, cellulose 
propionate, nitrocellulose and the like), a styrene-butadiene copolymer, a 
polyester resin, a chlorovinyl ether-acrylate copolymer, an amide resin, 
and various rubber type resins as described, e.g., in U.S. Pat. No. 
4,135,031. 
Examples of the thermosetting resins or reactive type resins include a 
phenol resin, an epoxy resin, a curable polyurethane resin, a urea resin, 
a melamine resin, an alkyd resin, an acrylic reactive resin, 
polyisocyanate and polyamine. 
The mixing weight ratio of particles to binder used in the backing layer 
(P/B ratio) is preferably from 1/0.5 to 1/3.5 and more preferably from 
1:0.7 to 1:3.0. If the particles are contained in an excessive amount, the 
particles tend to more easily drop off, whereas if the particles are 
contained in a very small amount, the friction coefficient increases. 
The backing layer of the present invention can contain a lubricating agent 
which is generally used for magnetic tapes, or can be coated with the 
lubricating agent. 
The present invention will be illustrated in more detail by examples. In 
the examples, all parts are by weight. 
EXAMPLE 1 
On opposite surfaces of a polyethylene terephthalate film having a 
thickness of 14 .mu.m, a magnetic layer containing Co-containing magnetic 
iron oxide having a predetermined specific surface area and a backing 
layer having a dry thickness of 1 .mu.m were provided. The coating 
compositions for the magnetic layer and the backing layer were as follows. 
______________________________________ 
Coating composition for the magnetic layer: 
Co-Containing Magnetic Iron Oxide 
100 parts 
(predetermined specific surface 
area S.sub.BET are shown in Table 1) 
Vinyl Chloride-Vinyl Acetate 
12 parts 
Copolymer (Molecular Weight: 73000) 
Polyurethane Resin 7 parts 
(Molecular Weight: 75000) 
.alpha.-Al.sub.2 O.sub.3 1 part 
(Average Particle Size: 0.3 .mu.m) 
Carbon Black Amounts shown in 
(Average Particle Size: 40 m.mu.) 
Table 1 
Stearic Acid 1 part 
Oleic acid 0.5 part 
Butyl Stearate 1 part 
Polyisocyanate 11 parts 
("Coronate L", trademark of 
Nippon Polyurethane Co., Ltd.) 
(an adduct of tolylene diisocyanate 
and trimethylolpropane, Molecular 
Weight: 620) 
Butyl Acetate 100 parts 
Methyl Ethyl Ketone 200 parts 
Coating composition for the backing layer: 
Carbon Black Amounts shown in 
(average particle size: 40 m.mu.) 
Table 1 
Nitrocellulose 40 parts 
Polyurethane Resin 30 parts 
(Molecular Weight: 75000) 
Polyisocyanate 
("Coronate L", trademark of 
Nippon Polyurethane Co., Ltd.) 
(an adduct of tolylene diisocyanate 
and trimethylolpropane, Molecular 
Weight: 620) 
Methyl Ethyl Ketone 600 parts 
______________________________________ 
The thus obtained magnetic tapes were slit to a width of 1/2 inch to 
prepare Sample Nos. 1 to 12. 
EXAMPLE 2 
On opposite surfaces of a polyethylene terephthalate film having a 
thickness of 12 .mu.m, a Co-Ni (Ni content: 20 wt%) magnetic layer was 
provided by an oblique vapor deposition method to have a thickness of 0.15 
.mu.m and a backing layer was provided to have a dry thickness of 1 .mu.m. 
The thus obtained magnetic tape was slit to a width of 1/2 inch to prepare 
Sample Nos. 13 to 15. 
The various characteristics of tape samples Nos. 1 to 15 were measured or 
evaluated in the following manner and the results obtained are set forth 
in Table 1. 
(1) Number of Drop Outs: 
The number of drop out having 15 .mu.s per one minute on Samples after 100 
passes was counted by a drop out counter, model "VD-3D", manufactured by 
Victor Company of Japan Ltd. 
(2) Color S/N: 
Sample tapes were replayed using a VTR model "NV-8300", manufactured by 
Matsushita Electric industries Co., Ltd. Color S/N was measured by "Noise 
meter 925C" manufactured by Shibasoku K.K. using the 10 KHz high-pass 
filter and the 500 KHz low-pass filter. Color S/N is shown as a relative 
value based on Sample No. 10 (0 dB) in Sample Nos. 1 to 12 and by a 
relative value based on Sample No. 13 in Sample Nos. 13 to 15. 
(3) Light Transmittance: 
The light transmittance of magnetic tapes was measured using a recording 
spectrophotometer, "EPS-3T", manufactured by Hitachi Ltd. 
TABLE 1 
______________________________________ 
Magnetic Layer 
Specific Surface 
Area S.sub.BET of Co- Carbon Black 
Containing Mag- 
Carbon Black 
in a backing 
Sample netic iron oxide 
(parts by 
layer (parts 
No. (m.sup.2 /g) weight) by weight) 
______________________________________ 
1 55 0.2 30 
2 55 0.2 50 
3 55 0.2 100 
4 55 0.2 150 
5 45 0.5 70 
6 45 0.5 70 
7 40 1.5 70 
8 35 3 40 
9 35 3 120 
10 28 7 Backing layer 
was not provided 
11 28 7 70 
12 28 7 200 
13 Backing layer 
was not provided 
Magnetic layer 
14 prepared by 70 
vapor deposition 
15 150 
______________________________________ 
Center Line 
Average 
Roughness Light Transmittance 
Ra at a cut (%) (.lambda.: 900 nm) 
off of 0.08 mm Magnetic 
Sample 
of backing Magnetic Backing Tape (as 
No. layer (.mu.m) 
Layer Layer a whole) 
______________________________________ 
1 0.021 15.8 15.8 2.5 
2 0.023 15.8 7.1 1.1 
3 0.025 15.8 2.0 0.32 
4 0.035 15.8 0.63 0.10 
5 0.024 10.0 3.2 0.32 
6 0.060* 10.0 3.2 0.32 
7 0.024 4.0 3.2 0.13 
8 0.022 2.5 8.9 0.22 
9 0.030 2.5 1.3 0.032 
10 Backing layer 
0.79 -- 0.79 
was not provided 
11 0.024 0.79 3.2 0.025 
12 0.040 0.79 0.40 0.003 
13 Backing layer 
4.0 -- 4.0 
was not provided 
14 0.024 4.0 3.2 0.13 
15 0.035 4.0 0.63 0.025 
______________________________________ 
Sample 
Color S/N Number of Drop 
No. (dB) out per min. Total Evaluation 
______________________________________ 
1 +3.5 2 D 
2 +3.5 2 D 
3 +3.5 1 A 
4 +3.4 50 D 
5 +2.6 2 A 
6 +1.8 30 D 
7 +1.8 3 A 
8 +1.0 2 A 
9 +1.0 7 B 
10 0 2 C 
11 0 3 C 
12 0 80 D 
13 0 2 D 
14 +1.0 2 A 
15 0 40 D 
______________________________________ 
*As to the backing layer of Sample No. 6, Ra was made a predetermined 
value by shortening the period of time for dispersing a coating 
composition. 
A: Very good 
B: Good 
C: Poor 
D: Very poor 
As is clear from Table 1, Samples Nos. 3, 5, 7, 8, 9, and 14, whose 
magnetic layer, backing layer, and magnetic tape as a whole have light 
transmittance within the range of the present invention and have center 
line average roughness Ra of 0.05 .mu.m at a cut-of value of 0.08 mm are 
excellent in color S/N and the number of drop outs per minute. Samples 
Nos. 1, 2, 4, 6, 10, 11, 12, 13 and 15, which do not meet at least one of 
the above-described requirements, do not exhibit such excellent results. 
While the invention has been described in detail and with reference to 
specific embodiment 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.