Magnetic recording medium

A magnetic recording medium having good running stability and high electromagnetic characteristics which comprises a base film, a magnetic layer provided on one surface of the base film and a back coat layer provided on the other surface of the base film, said back coat layer comprising non-magnetic oxide particles at least one of complex oxide or oxide solid solutions of the SiO.sub.2 or TiO.sub.2 system.

The present invention relates to a magnetic recording medium. More 
particularly, it relates to a magnetic recording tape comprising a base 
film, a magnetic layer provided on one surface (i.e. the major surface) of 
the base film and a back coat layer provided on the other surface (i.e. 
the back surface) of the base film. 
In general, a magnetic recording tape comprises a base film and a magnetic 
layer provided on the major surface of the base film. In order to assure 
good magnetic characteristics, the surface of the magnetic layer is 
finished in order to be smooth. For improvement of the running stability, 
the back coat layer is formed by the use of non-magnetic particles 
uniformly dispersed in a resinous binder so as to roughen the back 
surface. However, when the back surface has been roughened excessively, 
the surface state at the back coat layer is transferred to the smooth 
surface of the magnetic layer, and as a result, the electromagnetic 
characteristics are excessively deteriorated. For this reason, the 
non-magnetic particles for the back coat layer are required to be so fine 
so as to be able to impart an appropriate roughness to the back surface 
while maintaining the improvement of the running stability. Examples of 
the non-magnetic particles which are readily available, are BaSO.sub.4, 
CaCO.sub.3, etc. An attempt has also been made to incorporate such 
non-magnetic oxide particles heretofore used for improvement of the 
abrasion resistance of the magnetic layer as Al.sub.2 O.sub.3 and TiO.sub. 
2 into said non-magnetic particles. 
In general, the non-magnetic particles as heretofore used for the back coat 
layer do not have good compatibility with the resinous binder therein, and 
their dispersibility in the back coat layer is insufficient. For this 
reason, the surface state of the back coat layer becomes inferior and 
affords an unfavorable influence on the smooth surface of the magnetic 
layer. Also, the non-magnetic particles are apt to be eliminated from the 
back coat layer during the running, and the abrasion resistance is thus 
lowered. As a result, the electromagnetic characteristics and the running 
stability are frequently deteriorated. 
In order to overcome the above drawback, an extensive study has been made, 
and it has been found that certain specific complex oxides or oxide solid 
solutions have good compatibility with the resinous binder in the back 
coat layer and maintain high dispersibility in the back coat layer. Thus, 
the surface of the back coat layer can be maintained in a state which does 
not unfavorably effect the magnetic layer. In addition, said non-magnetic 
oxide particles are hardly eliminated, and the back coat layer can be kept 
highly abrasion-resistant. It is therefore possible to provide a magnetic 
recording medium having a back coat layer which can maintain not only the 
running stability but also satisfactorily maintain the electromagnetic 
characteristics. 
The magnetic recording medium of the present invention comprises a base 
film, a magnetic layer provided on one surface of the base film and a back 
coat layer provided on the other surface of the base film, said back coat 
layer comprising non-magnetic oxide particles of complex oxides or oxide 
solid solutions of the SiO.sub.2 or TiO.sub.2 system. 
As understood from the above, the magnetic recording medium of the 
invention is characteristic in comprising non-magnetic particles of 
complex oxides or oxide solid solutions of the SiO.sub.2 or TiO.sub.2 
system. The complex oxides of the SiO.sub.2 or TiO.sub.2 system may 
constitute at least one selected from SiO.sub.2 or TiO.sub.2 and at least 
one other oxide in a constant or definite proportion. Examples of such 
complex oxides are aluminum titanate (Al.sub.2 O.sub.3.TiO.sub.2), 
forsterite (2MgO.SiO.sub.2), enstatite (MgO.SiO.sub.2), zircon 
(ZrO.sub.2.SiO.sub.2), mullite (3Al.sub.2 O.sub.3.2SiO.sub.2), cordierite 
(2MgO.2Al.sub.2 O.sub.3.5SiO.sub.2), sapphirine (4MgO.5Al.sub.2 
O.sub.3.2SiO.sub.2), spodumene (Li.sub.2 O.Al.sub.2 O.sub.3.4SiO.sub.4), 
eucryptite (Li.sub.2 O.Al.sub.2 O.sub.3.2SiO.sub.2), petalite (Li.sub.2 
O.Al.sub.2 O.sub.3.8SiO.sub.2), beryl (3BeO.Al.sub.2 O.sub.3.6SiO.sub.2), 
celsian (BaO.Al.sub.2 l O.sub.3.2SiO.sub.2), etc. Among those, preferred 
are mullite, aluminum titanate, cordierite, zircon, spodumene, etc. Those 
particularly preferred are mullite, aluminum titanate, cordierite, etc. 
The oxide solid solutions of the SiO.sub.2 or TiO.sub.2 system may 
likewise constitute at least one selected from SiO.sub.2 or TiO.sub.2 and 
at least one other oxide, but the proportion of these oxide components are 
not constant or definite. Examples of the oxides which constitute said 
oxide solid solutions may be those which constitute the complex oxides. 
The average particle size of the non-magnetic oxide particles is normally 
not more than about 0.2 micron, preferably from about 0.02 to 0.1 micron. 
When the average particle size exceeds said upper limit, the surface 
property of the back coat layer becomes inferior, and the surface 
smoothness of the magnetic layer is unfavorably influenced and the 
electromagnetic characteristics are deteriorated. When the average 
particle size is smaller than said lower limit, the surface of the back 
coat layer is too smooth, and the running stability is hardly improved. 
The non-magnetic oxide particles having said average particle size can be 
readily prepared by pulverizing the particles by the aid of a conventional 
pulverization machine such as centrifugal mill or ball mill. The hardness 
of the non-magnetic oxide particles depends on the kind of the particle 
and is usually not less than about 4 in Moh's hardness, preferably not 
less than about 6 in Moh's hardness. 
The kind of the non-magnetic oxide particles is not necessarily limited to 
single. Two or more kinds may be employed. In general, the non-magnetic 
oxide particles may be used from about 20 to 80% by weight, preferably 
from about 35 to 70% by weight based on the total weight of the 
non-magnetic oxide particles and the resinous binder. When the amount is 
too small, it is not possible to impart an appropriate roughness to the 
back coat layer, and good running stability is hardly obtained. When the 
amount is too large, the non-magnetic oxide particles tend to be 
eliminated during the running. The eliminated particles attach on the 
surface of the magnetic layer so that the electromagnetic characteristics 
are unfavorably influenced. Further, the running stability may be 
deteriorated. 
The magnetic recording medium of the invention may be prepared by a 
conventional procedure. For instance, it may be prepared by applying, a 
magnetic coating composition comprising magnetic particles and a resinous 
binder dispersed or dissolved in a liquid medium, onto one surface (i.e. 
the major surface) of a base film, such as a polyester (e.g. polyethylene 
terephthalate) film usually having a thickness of about 4 to 15 microns, 
followed by drying to form a magnetic layer usually having a thickness of 
about 1.5 to 10 microns. Alternatively, the magnetic layer may be formed 
by deposition of a vaporized magnetic metal onto the major surface of the 
base film. Onto the other surface (i.e. the back surface) of the base 
film, a non-magnetic coating composition comprising the non-magnetic oxide 
particles and a resinous binder dispersed or dissolved in a liquid medium 
is applied, followed by drying to make a back coat layer usually having a 
thickness of about 0.3 to 3 microns. 
The magnetic coating composition comprises magnetic particles and a 
resinous binder therefor dispersed or dissolved in a liquid medium. As the 
magnetic particles, there may be used particles of gamma-Fe.sub.2 O.sub.3 
or intermediary oxides thereto, particles of Fe.sub.3 O.sub.4 or 
intermediary oxides thereto, particles of Co-containing gamma-Fe.sub.2 
O.sub.3 or intermediary oxides thereto, particles of Co-containing 
Fe.sub.3 O.sub.4, CrO.sub.2 particles, Sb-containing CrO.sub.2 particles, 
Fe particles, Co particles, Fe-Ni particles, Fe-Co-Ni particles, 
barium-ferrite particles, strontium-ferrite particles, etc. These magnetic 
particles have usually an average particle size (longer) of about 0.05 to 
1 micron. Examples of the resinous binder are polyvinyl chloride, vinyl 
chloride/vinyl acetate copolymer, polybutyral resin, polyacetal resin, 
polyurethane resin, polyester resin, acrylic resin epoxy resin, phenol 
resin, polyol resin, amino resin, synthetic rubber resin, cellulose resin, 
isocyanate compounds, etc. As the liquid medium, there may be usually 
employed one or more organic solvents chosen from ketones (e.g. 
cyclohexanone, methylethylketone, methylisobutyl ketone), esters (e.g. 
ethyl acetate, butyl acetate), aromatic hydrocarbons (e.g. benzene, 
toluene, xylene), alcohols (e.g. isopropanol), acid amides (e.g. 
dimethylformamide), sulfoxides (e.g. dimethylsulfoxide), ethers (e.g. 
tetrahydrofuran, dioxane), etc. In addition to or in place of the organic 
solvent, water and an emulsifier may be also used so as to give an 
emulsion type coating composition. In any event, other additives such as 
dispersing agents, lubricating agents, polishing agents and antistatic 
agents may be optionally incorporated into the coating composition. The 
contents of the magnetic particles and the resinous binder in the magnetic 
coating composition are not limitative, but usually the weight proportion 
of the magnetic particles and the resinous binder may be from about 6:4 to 
9:1. 
The non-magnetic coating composition comprises non-magnetic oxide particles 
and a resinous binder therefor dispersed or dissolved in a liquid medium. 
As the resinous binder and the liquid medium, there may be used those as 
exemplified with respect to the magnetic coating composition. In addition, 
the coating composition may comprise optionally any other non-magnetic 
oxide particles (e.g. TiO.sub.2, Al.sub.2 O.sub.3, BaSO.sub.4, CaCO.sub.3) 
as conventionally employed for the back coat layer. The coating 
composition may also comprise optionally conventional additives such as 
dispersing agents, lubricating agents and antistatic agents. 
As understood from the above, the back coat layer in the magnetic recording 
medium of the present invention is characteristic in comprising complex 
oxides or oxide solid solutions of the SiO.sub.2 or TiO.sub.2 system, and 
such back coat layer can assure a sufficient running stability without 
affording any unfavorable influence onto the magnetic layer. The back coat 
layer has high abrasion resistance, and the non-magnetic oxide particles 
are hardly removed therefrom. As the result, the magnetic recording medium 
can show excellent electromagnetic performances. 
Practical and presently preferred embodiments of the invention are 
illustratively shown in the following examples wherein parts and % are by 
weight.

EXAMPLE 1 
(A) Magnetic coating composition:- 
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Materials Part(s) 
______________________________________ 
Co-containing magnetic 
250 
iron oxide particles 
Carbon black 12 
alpha-Iron oxide particles 
10 
Nitrated cotton 22 
Polyurethane resin 19 
Trifunctional low molecular 
7 
weight isocyanate compound 
n-Butyl stearyl 3 
Myristic acid 2 
Liquid paraffin 2 
Cyclohexanone 340 
Toluene 340 
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(B) Non-magnetic coating composition:- 
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Materials Part(s) 
______________________________________ 
Fine particles of mullite 
210 
(3Al.sub.2 O.sub.3.2SiO.sub.2) (average 
particle size, 0.07 micron) 
Carbon black ("Black Pearl L" 
90 
manufactured by Cabbot; 
volatile component, 5%) 
Cellulose resin (nitrated 
100 
cotton) 
Polyurethane resin 70 
Trifunctional low molecular 
30 
weight isocyanate compound 
n-Butyl stearate 3 
Myristic acid 2 
Liquid paraffin 3 
Cyclohexanone 750 
Toluene 750 
______________________________________ 
The materials under the item (A) were mixed together in a ball mill for 50 
hours to make a magnetic coating composition. The composition was applied 
onto the major surface of a polyester film of 14 microns thick having good 
smoothness, followed by drying to make a magnetic layer of 5 microns 
thick, which was then subjected to surface treatment. 
The materials under the item (B) were mixed together in a ball mill for 100 
hours to make a non-magnetic coating compostion. The composition was 
applied onto the back surface of said polyester film, followed by drying 
to make a non-magnetic layer of 0.8 micron thick, which was then subjected 
to surface treatment and cut in a predetermined width to give a magnetic 
recording tape. 
EXAMPLE 2 
In the same manner as in Example 1 but using 210 parts of fine particles of 
aluminum titanate (Al.sub.2 O.sub.3.TiO.sub.2) (average particle size, 
0.07 micron) for the non-magnetic coating composition in place of 210 
parts of fine particles of mullite, there was prepared a magnetic 
recording tape. 
EXAMPLE 3 
In the same manner as in Example 1 but using 210 parts of fine aprticles of 
zircon (ZrO.sub.2.SiO.sub.2) (average particle size, 0.06 micron) for the 
non-magnetic coating composition in place of 210 parts of fine particles 
of mullite, there was prepared a magnetic recording tape. 
EXAMPLE 4 
In the same manner as in Example 1 but using 210 parts of fine particles of 
cordierite (2MgO.2Al.sub.2 O.sub.3.5TiO.sub.2) (average particle size, 
0.07 micron) for the non-magnetic coating composition in place of 210 
parts of fine particles of mullite, there was prepared a magnetic 
recording tape. 
EXAMPLE 5 
In the same manner as in Example 1 but using 150 parts of fine particles of 
mullite and 60 parts of fine particles of aluminum titanate (Al.sub.2 
O.sub.3.TiO.sub.2) (average particle size, 0.07 micron) for the 
non-magnetic coating composition in place of 210 parts of fine particles 
of mullite, there was prepared a magnetic recording tape. 
EXAMPLE 6 
In the same manner as in Example 1 but using 150 parts of fine particles of 
mullite and 60 parts of fine particles of titanium oxide (TiO.sub.2) 
(average particle size, 0.07 micron) for the non-magnetic coating 
composition in place of 210 parts of fine particles of mullite, there was 
prepared a magnetic recording tape. 
COMATIVE EXAMPLE 1 
In the same manner as in Example 1 but using 150 parts of fine particles of 
barium sulfate (BaSO.sub.4) (average particle size, 0.07 micron) and 60 
parts of fine particles of titanium oxide (average particle size, 0.07 
micron) for the non-magnetic coating composition in place of 210 parts of 
fine particles of mullite, there was prepared a magnetic recording tape. 
With respect to the magnetic recording tapes as prepared in Examples 1 to 6 
and Comparative Example 1, the surface smoothness (surface roughness at 
the centerline, averaged), the color S/N ratio (relative value taking the 
standard tape as O), the abrasion resistance of the back coat layer (the 
decrease of the S/N ratio after 100 times running in comparison with that 
before 100 times running) and the running stability (wow-flutta) were 
measured. The results are shown in the following table: 
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Surface Abrasion Color Running 
roughness resistance S/N ratio 
stability 
Example (micron) (dB) (dB) (%) 
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1 0.02 0.5 +4.5 0.06 
2 0.02 0.5 +4.5 0.06 
3 0.02 0.7 +4.0 0.08 
4 0.02 0.5 +4.2 0.07 
5 0.02 0.6 +4.0 0.08 
6 0.03 0.8 +4.0 0.10 
Compar- 0.04 1.0 +3.8 4.0 
ative 
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As understood from the above table, the magnetic recording tape of the 
invention is provided with a back coat layer having excellent abrasion 
resistance and appropriate surface property and being satisfactory in both 
running stability and electromagnetic characteristics.