Magnetic recording medium having an infra-red light transparent balkcoat layer

Flexible magnetic recording media consisting essentially of a web-like nonmagnetic substrate, a magnetic layer applied to one main side of the web-like substrate and a backing layer formed on the opposite main side of the substrate from a polymeric binder, at least one filler, at least one auxiliary pigment and a polyolefin having a spherical particle shape.

The present invention relates to flexible magnetic recording media 
consisting essentially of a web-like nonmagnetic substrate, a magnetic 
layer applied to one main side of the web-like substrate and a backing 
layer formed on the opposite main side of the substrate from a polymeric 
binder and nonmagnetic fillers and auxiliary pigments. 
It is known that flexible magnetic recording media can be provided with 
backing layers containing nonmagnetizable, nonconductive and/or conductive 
substances. 
U.S. Pat. No. 3 293 066 states that electrostatic charges on magnetic 
tapes, which can form in recorders at high tape speeds, can be eliminated 
by applying conductive backing layers, and furthermore the backs of the 
tapes can be made more hard-wearing by means of backing layers. 
Furthermore, GB-A 1 197 661 and U.S. Pat. No. 4,135,031 disclose that the 
winding properties of magnetic tapes can be improved by applying backing 
layers having a predetermined surface roughness. Such backing layers are 
also known for magnetic cards. EP-A 101 020 discloses special binder 
mixtures which, particularly with the addition of carbon black, give 
backing layers which have excellent adhesive strength, are very 
hard-wearing and are stable under conditions of high temperature and 
humidity. 
Such backing layers are of particular importance in video tapes, in 
particular in those for the home video sector. Thus, inter alia, U.S. Pat. 
No. 4,735,325 proposes a backing layer which consists of carbon black of 
different particle sizes and of fillers having a Mohs' hardness of 
.gtoreq.8 dispersed in a polymeric binder, for improving the 
scratch-resistance and for reducing the number of errors. In addition to 
improving the wear properties and reducing the abrasiveness, the:proposed 
backing layers also serve to reduce the transparency of the tape material 
to light, which is necessary particularly when such tapes are used on 
commercial video recorders. EP-A 105 471 proposes for this purpose a 
backing layer based on barium sulfate/.alpha.-iron(III) oxide with or 
without the special addition of carbon black. However, video tapes treated 
in this manner have the disadvantage that they are unsuitable for a 
conventional duplication method, thermomagnetic duplication (TMD). In this 
TMD method, a magnetic recording medium containing chromium dioxide as 
magnetic material is brought into contact with a master tape having a high 
coercive force and provided with the recording, and at the same time the 
chromium dioxide magnetic layer is heated above the Curie point of the 
chromium dioxide. 
During subsequent cooling below the Curie point, the chromium dioxide is 
magnetized according to the information pattern of the master tape. The 
chromium dioxide magnetic layer is generally heated with the aid of a 
laser beam (usually a krypton laser having a wavelength of 1064 nm) 
through the back of the magnetic tape. However, this means that any 
backing layer present must not substantially absorb the energy of the 
laser beam. 
It is an object of the present invention to provide a magnetic recording 
medium whose backing layer meets the requirements with regard to 
mechanical properties, such as resistance to wear and abrasiveness, while 
at the same time has sufficient transparency to light so that a 
corresponding magnetic recording medium can also be used for the TMD 
method. 
We have found that this object is achieved by flexible magnetic recording 
media, essentially consisting of a web-like nonmagnetic substrate, a 
magnetic layer applied to one main side of the web-like substrate and a 
backing layer formed on the opposite main side of the substrate from a 
polymeric binder and nonmagnetic additives when the backing layer is 
obtained by dispersing a mixture essentially consisting of an organic 
polymer and, based on the backing layer, from 2.5 to 25% by volume of a 
filler, from 0.5 to 3% by volume of an auxiliary pigment and from 1 to 10% 
by volume of a polyolefin having a spherical particle shape, a density of 
from 0.9 to 1.0 and a mean particle size of from 1 to 1,000 .mu.m and by 
applying the resulting dispersion to that surface of the substrate which 
is opposite the magnetic layer, in a layer thickness such that, after 
solidification of the layer, a layer thickness of from 0.1 to 2.0 .mu.m 
results. 
Suitable fillers for the purposes of the present invention are particulate 
compounds, such as silica, in particular precipitated silica, calcium 
carbonate, barium sulfate and/or gypsum having a mean agglomerate size of 
from 0.05 to 4 .mu.m. Suitable auxiliary pigments are likewise particulate 
compounds selected from the group consisting of alumina, .alpha.-iron(III) 
oxide, titanium dioxide, zinc ferrite and/or chromium green, having a mean 
agglomerate size of from 0.1 to 0.5 .mu.m. An essential component of the 
backing layer of the novel magnetic recording medium is the polyolefin. 
These polymers have a spherical particle shape with a mean particle size 
of from 1 to 1,000 .mu.m, in particular from 8 to 800 .mu.m, 
advantageously from 30 to 500 .mu.m. Low density polyolefins having an 
average molecular weight of from 3,000 to 25,000 and a density of from 0.9 
to 1.0 g/cm.sup.3 have proven particularly advantageous. 
The dispersion forming the backing layer of the novel magnetic recording 
medium is prepared by conventional process. 
Suitable organic polymers for the backing layer are the binders known for 
the production of magnetic recording media. These are copolyamides, 
polyvinylformals, polyurethane elastomers, mixtures of polyisocyanates and 
relatively high molecular weight polyhydroxy compounds, vinyl chloride 
polymers containing more than 60% of vinyl chloride building blocks, a 
copolymerized vinyl chloride with one or more unsaturated carboxylic acids 
of 3 to 5 carbon atoms as comonomers or hydroxyl-containing vinyl chloride 
copolymers which can be prepared by partial hydrolysis of vinyl 
chloride/vinyl ester copolymers or direct copolymerization of vinyl 
chloride with hydroxyl-containing monomers, such as allyl alcohol or 
4-hydroxybutyl or 2-hydroxyethyl (meth)acrylate, said polymers being 
soluble in conventional solvents. Other suitable binders are mixtures of 
one or more polyurethane elastomers with polyvinylformals, phenoxy resins 
and vinyl chloride copolymers of the abovementioned composition. 
Particularly preferred organic polymers are mixtures of polyurethane 
elastomers with phenoxy resins and of polyurethane elastomers with vinyl 
chloride polymers. 
Cyclic ethers, such as tetrahydrofuran and dioxane, and ketones, such as 
methyl ethyl ketone and cyclohexanone, are preferably used as solvents for 
the preparation and processing of the polymers. Of course, polyurethanes 
can also be dissolved in other strongly polar solvents, such as 
dimethylformamide, pyrrolidone, dimethyl sulfoxide or ethylene glycol 
acetate. It is also possible to mix the stated solvents with aromatics, 
such as toluene or xylene, and esters, such as ethyl or butyl acetate. 
For dispersing, the particulate components are mixed together with the 
dissolved organic polymers and conventional dispersants, such as soybean 
lecithin, saturated and unsaturated, straight-chain and branched fatty 
acids, fatty acid salts, quaternary ammonium compounds and phosphoric acid 
derivatives, and the mixture is processed in known dispersing apparatuses. 
It may also be advantageous to add conventional lubricants, such as fatty 
acids, fatty esters, silicone oils or fluorine-based additives to these 
backing layers. 
The dispersion is prepared in ball mills or vertical or horizontal stirred 
ball mills in a conventional manner. The backing layer is preferably 
applied by means of engraved rollers. For evaporating the solvents and 
drying or curing the backing layer, the latter is passed through a heating 
tunnel. It is possible to apply both magnetic and backing layer 
dispersions in one operation or in succession. The coated films can, if 
required, be calendered and compacted on conventional machines by being 
passed between heated and polished rollers, if necessary under pressure. 
The thickness of the backing layer is less than 2.0 .mu.m, in particular 
less than 1.5 .mu.m, preferably from 0.3 to 0.7 .mu.m. 
In an advantageous embodiment of the novel magnetic recording media, the 
backing layer is composed of from 15 to 25, in particular from 20 to 25, % 
by 15 volume of a precipitated silica having an SiO.sub.2 content of from 
98 to 99.5%, a pH of from 5 to 7 and a density of 1.9 g/cm.sup.3, from 0.5 
to 3, preferably from 1.5 to 2.25, % by volume of a cubic zinc ferrite 
having a mean particle size of from 0.1 to 0.5 .mu.m, of a spherical 
.alpha.-Fe.sub.2 O.sub.3 or of an Al.sub.2 O.sub.3 and from 1 to 10, 
preferably from 2 to 5, % by volume of a spherical low density polyolefin 
having a mean particle diameter of from 8 to 800 .mu.m, preferably from 30 
to 500 .mu.m, and from 20 to 40, preferably from 25 to 35, % by volume of 
a linear polyesterurethane obtained from adipic acid, 1,4-butanediol and 
4,4-diisocyanatodiphenylmethane, from 15 to 25, preferably from 18 to 20, 
% by volume of a polyphenoxy resin obtained from bisphenol and 
epichlorohydrin and from 10 to 25, in particular from 15 to 20, % by 
volume of a polyisocyanate resin. In addition to the amounts, not 
exceeding 2% by volume, of a known dispersant and of a lubricant, it may 
be advantageous also to add not more than 1.5% by volume of a carbon black 
to the dispersion. 
Because of the special backing layer, the novel magnetic recording media 
have extremely advantageous running behavior. Because the back of such 
magnetic recording media is mechanically stable and hard-wearing, 
corresponding video tapes have a greatly reduced number of errors compared 
with those of the prior art. If the magnetic layer of the novel magnetic 
recording media is based on chromium dioxide, such magnetic tapes are 
particularly suitable for the TMD method. The transparency of this backing 
layer to infrared light furthermore ensures that the chromium dioxide in 
the magnetic layer can be heated above the Curie point substantially 
without loss, ie. without a laser current increase required in comparison 
with magnetic tapes without a backing layer.

The Examples which follow illustrate the invention and compare it with 
prior art experiments. In the Examples and Comparative Experiments, parts 
and percentages are by volume, unless stated otherwise. 
EXAMPLE 1 
3,325 parts of zirconium dioxide spheres having a diameter of 1.0-1.25 mm, 
71.5 parts of a precipitated silica having a mean agglomerate size of 3 
.mu.m, 6.8 parts of a cubic zinc ferrite having a mean particle size of 
0.12 .mu.m, 9.2 parts of a polyolefin having an average molecular weight 
of 3,000 and a mean spherical particle diameter of 500 .mu.m, 103 parts of 
a 14.75% strength solution of a vinyl chloride copolymer having an average 
molecular weight of 35,000 and a hydroxyl content of 1.8% by weight in a 
mixture of 45.74 parts of tetrahydrofuran and 39.51 parts of dioxane, 162 
parts of a 10.62% strength solution of a linear polyesterurethane resin, 
prepared from adipic acid, 1,4-butanediol and 
4,4-diisocyanatodiphenylmethane, in a mixture of 47.95 parts of 
tetrahydrofuran and 41.43 parts of dioxane, 2.8 parts of an isomeric 
C.sub.18 -carboxylic acid, and 848 parts of a mixture of 455 parts of 
tetrahydrofuran and 393 parts of dioxane were introduced into a batchwise 
stirred ball mill having a volume of 10,000 parts. The stirred ball mill 
was then closed and the contents were dispersed for 6 hours. Thereafter, 
the mill was opened again and 14.52 parts of 9.26% strength solution of 
dibutyltin laurate in a mixture of 48.68 parts of tetrahydrofuran and 
42.06 parts of dioxane, 5.7 parts of an 8.5% strength solution of a 
fluorine additive in a mixture of 49.10 parts of tetrahydrofuran and 42.4 
parts of dioxane, 751 parts of a 10.62% strength solution of the linear 
polyesterurethane resin described above in a mixture of 47.95 parts of 
tetrahydrofuran and 41.43 parts of dioxane, 414.75 parts of a 14.75% 
strength solution of the vinyl copolymer described above in a mixture of 
45.74 parts of tetrahydrofuran and 39.51 parts of dioxane, 1807.5 parts of 
a mixture of 969.65 parts of tetrahydrofuran and 837.85 parts of dioxane 
were introduced and milling was continued for a further 3 hours. 
The dispersion was then removed from the mill. In order to crosslink the 
layer after application, 30 parts of a 41.6% strength solution of an 
isocyanate resin obtained from 1 mol of trimethylolpropane and 3 mol of 
toluylene diisocyanate in 58.4 parts of a tetrahydrofuran, per 1,000 parts 
of the dispersion, were stirred in for 15 minutes. After filtration 
through a paper filter, the dispersion was applied to a 15 .mu.m thick 
polyethylene terephthalate film by means of an engraved roller and was 
dried in the drying tunnel of the coating machine. The resulting backing 
layer was 0.5 .mu.m thick. 
The backing layer was very uniform and devoid of any stripes. The pigment 
volume concentration of the silica in the layer was 22.48%, that of the 
zinc ferrite was 2,14% and that of the polyolefin was 2.9%. For further 
processing, the magnetic layer containing CrO.sub.z as magnetic pigment 
was then applied in a conventional manner in a thickness of 2.5 .mu.m to 
the film side opposite the backing layer. After calendering, the film web 
was slit into 12.7 mm wide (1/2 inch) tapes. The tapes were then tested as 
follows: 
the laser current required to obtain satisfactory copies was measured. This 
must not be higher than for a tape free of a backing layer since the laser 
current requirement limits the life of the laser lamp. To measure the 
wear-resistance properties of the backing layer, a continuous loop of the 
test tape was passed for 8 minutes, under a tension of 60 p, over a 
cleaning fleece with the back facing the fleece (v=20 cm/s). The number of 
scratches before and after the test was determined optically. The fewer 
scratches present or formed during the test, the more hard-wearing is the 
backing layer. This test is important for the operational reliability of 
the tapes in the TMD copying machine, in which the tapes are constantly 
passed over a fleece. Deposits lead to errors and make continuous changing 
of the cleaning fleece essential. The results are shown in Table 1. 
EXAMPLE 2 
The procedure was as described in Example 1, except that spherical 
.alpha.-Fe.sub.2 O.sub.3 having a mean particle size of 0.3 .mu.m was used 
instead of a cubic zinc ferrite, and the pigment volume concentration of 
the silica was 24.86%, that of the .alpha.-Fe.sub.2 O.sub.3 was 1.05% and 
that of the polyolefin was 2.85%. The backing layer was likewise applied 
in a thickness of 0.5 .mu.m. The results are shown in Table 1. 
EXAMPLE 3 
The procedure was as described in Example 1, except that an 
.alpha.-Al.sub.2 O.sub.3 having a mean particle size of 0.1-0.2 .mu.m was 
used instead of the cubic zinc ferrite. The pigment volume concentration 
of the silica was 24.81%, that of the .alpha.Al.sub.2 O.sub.3 was 1.23% 
and that of the polyolefin was 2.85%. The results are shown in Table 1. 
EXAMPLE 4 
Instead of the VC copolymers stated in Example 1, 107 parts of a 16.83% 
strength solution of a polyphenoxy resin obtained from bisphenol A and 
epichlorohydrin and having 6% by weight of hydroxyl groups in a mixture of 
44.62 parts of tetrahydrofuran and 38.55 parts of dioxane were introduced 
in dispersion phase I and a further 431 parts of the same solution were 
introduced in phase II. The other parameters remained unchanged. The 
pigment volume concentrations in the layer were 21.05% for the silica, 
1.99% for the cubic zinc ferrite and 2.71% for the polyolefin. The results 
are shown in the table. 
EXAMPLE 5 
The procedure was as described in Example 4, except that, instead of the 
polyolefin stated in Example 1 having an average molecular weight of 
3,000, 9.2 parts of a polyolefin having an average molecular weight of 
6,000, a mean spherical particle diameter of 30 .mu.m and a density of 
0.92 g/cm.sup.3 were used. The pigment volume concentrations in the layer 
were 21.06% for the silica, 2.0% for the cubic zinc ferrite and 2.63% for 
the polyolefin. The results are shown in Table 1. 
EXAMPLE 6 
The procedure was as described in Example 4, except that, instead of the 
polyolefin stated in Example 1, 9.2 parts of a polymeric, polyolefinic 
filler having an average molecular weight of 6,500, a mean spherical 
particle diameter of 300 .mu.m and a density of 0.96 g/cm.sup.3 were used. 
The pigment volume concentrations were 21.06% for the silica, 2.0% for the 
cubic zinc ferrite and 2.63% for the polyolefin. The results are shown in 
Table 1. 
EXAMPLE 7 
The procedure was as described in Example 4. Instead of the 9.2 parts of 
the polyolefin used in Example 1, 18.7 parts of the same polyolefin were 
employed. The pigment volume concentrations were 20.49% for the silica, 
1.95% for the cubic zinc ferrite and 5.29% for the polymeric filler. The 
results are shown in Table 1. 
EXAMPLE 8 
The procedure was as described in Example 4, except that, instead of the 
polyolefin stated in Example 1 having a mean spherical particle size of 
500 .mu.m, 9.0 parts of a polyolefin having a mean spherical particle size 
of 8 .mu.m and a density of 0.94 g/cm.sup.3 were used. The pigment volume 
concentration was 21.06% for the silica, 2.0% for the cubic zinc ferrite 
and 2.63% for the polymeric, polyolefinic filler. The results are shown in 
Table 1. 
EXAMPLE 9 
The procedure was as described in Example 4, except that a highly 
conductive carbon black having a specific surface area of 1000 m.sup.2 /g 
was concomitantly used. Instead of the 71.5 parts of the precipitated 
silica, only 67.4 parts of said! silica and 6.8 parts of the cubic zinc 
ferrite and 4.5parts of the highly conductive carbon black were 
introduced. 
The pigment volume concentration was 20.25% for the silica, 2.05% for the 
cubic zinc ferrite, 1.39% for the carbon black and 2.79% for the 
polyolefin. The results are shown in Table 1. 
EXAMPLE 10 
The procedure was as described in Example 4, except that, instead of the 
71.5 parts of the precipitated silica, 31.8 parts of a finely divided 0.11 
.mu.m BaSO.sub.4 having an oil absorption of 24 ml/100 g were used. All 
other parameters remained unchanged. The pigment volume concentration was 
13.84% for the BaSO.sub.4, 2.98% for the cubic zinc ferrite and 4.05% for 
the polymeric, polyolefinic filler. The results are shown in Table 1. 
COMATIVE EXPERIMENT 1 
A backing layer dispersion was prepared according to Example 1 of EP-A 101 
020, except that dispersing was carried out in a batchwise stirred ball 
mill having a capacity of 10,000 parts by volume and containing 3,325 
parts of zirconium dioxide balls having a diameter of 1.0-1.25mm. 126.3 
parts of carbon black, 63.15 parts of a silica gel treated with organic 
substances, 8 parts of cubic zinc ferrite, 2194.5 parts of a mixture of 
1223.6 parts of tetrahydrofuran and 970.9 parts of dioxane, 10.2 parts of 
stearic acid, 553.3 parts of a 16.36% solution of a polyurethane resin 
obtained from 44:56 trimethylolpropane/1,6-hexanediol in a mixture of 
44.87 parts of tetrahydrofuran and 38.77 parts of dioxane, and 838.6 parts 
of a 16.83% strength solution of a polyphenoxy resin obtained from 
bisphenol A and epichlorohydrin and having 6% by weight of hydroxyl groups 
in a mixture of 44.62 parts of tetrahydrofuran and 38.55 parts of dioxane 
were introduced into said mill. 
The batch was then milled for 6 hours, after which 1283 parts of a 10.25% 
strength solution of a saturated polyester resin, prepared from 1:2 
terephthalic/isophthalic acid and ethylene glycol, in a mixture of 48.15 
parts of tetrahydrofuran and 41.6 parts of dioxane, 871.6 parts of a 
10.62% strength solution of a linear polyesterurethane resin, prepared 
from adipic acid, 1,4-butanediol and 4,4'-diisocyanatodiphenylmethane, in 
a mixture of 47.95 parts of tetrahydrofuran and 41.43 parts of dioxane, 
11.6 parts of butyl stearate and 478.5 parts of a 41.6% strength solution 
of an isocyanate resin obtained from 1 mol of trimethylolpropane and 3 mol 
of toluylene diioscyanate in 58.4 parts of tetrahydrofuran were added. 
Stirring was carried out for a further 15 minutes, after which 
homogenization was complete and the backing layer dispersion was filtered 
through a paper filter and applied to a 15 .mu.m polyethylene 
terephthalate film by means of an engraved roller and dried in a drying 
tunnel of the coating machine. The thickness of the backing layer was 0.5 
pm. The pigment volume concentrations in the layer were 14.3% for carbon 
black, 5.81% for the silica and 0.74% for the cubic zinc ferrite. For 
further processing, the magnetic layer was applied in a conventional 
manner in a thickness of 2.5 .mu.m to the film side opposite the backing 
layer. After calendering and slitting of the block into 1/2 inch wide 
tapes, the latter were tested as described under Example 1. The results 
are shown in Table 1. 
COMATIVE EXPERIMENT 2 
A backing layer dispersion was prepared according to Example 1 of EP-A 105 
471 from 93 parts of BaSO.sub.4 having a mean particle size of 0.08 .mu.m, 
20 parts of .alpha.-Fe.sub.2 O.sub.3 having a mean particle size of 0.1 
.mu.m, 85.98 parts of nitrocellulose, 78.33 parts of polyesterurethane, 32 
parts of a trifunctional isocyanate resin, 5.81 parts of n-butyl stearate, 
11.62 parts of myristic acid, 1057 parts of cyclohexanone and 1157.4 parts 
of toluene. The 10 backing layer dispersion was applied to give a 0.5 
.mu.m thick layer. The pigment volume concentrations in the backing layer 
were 31.64% for BaSO.sub.4 and 6.8% for .alpha.-Fe.sub.2 O.sub.3. 
Comparative Experiment 2 was further processed and tested in the same way 
as the other Examples. The results are shown in Table 1. 
COMATIVE EXPERIMENT 3 
The procedure was as described in Example 1, except that, instead Of the 
cubic zinc ferrite and the polyolefin, 81.0 parts of the precipitated 
silica and 8.85 parts of the highly conductive carbon black described in 
Example 9 were used. The 0.5 .mu.m thick layer contained precipitated 
silica in a pigment volume concentration of 21.56% and carbon black in a 
concentration of 2.47%. Further processing and testing were carried out as 
described. The results are shown in Table 1. 
COMATIVE EXPERIMENT 4 
In this Experiment, no backing layer was applied but only the magnetic 
layer containing CrO.sub.2 as magnetic material having a coercive force of 
51 kA/m. Further processing and testing were carried out as for the other 
Examples. The results are shown in Table 1. 
TABLE 1 
__________________________________________________________________________ 
Examples Comparative Experiments 
1 2 3 4 5 6 7 8 9 10 1 2 3 4 
__________________________________________________________________________ 
Laser current in amp 
32 32 32 32 32 32 32 32 33 32 not 
34 35 32 
in standard TMD copying pos- 
method sible 
Scratches 
before test 0 0 0 1 2 2 0 4 1 2 1 0 10 2 
after test 0 1 1 2 3 7 2 11 3 5 3 2 &gt;50 &gt;50 
__________________________________________________________________________ 
EXAMPLE A 
An image was recorded on video tapes obtained according to Example 1, 
Example 4 and Comparative Experiment 4 (magnetic tape without backing 
layer) on a standard TMD copying apparatus. The video properties were 
measured by using the video tape of Comparative Experiment 4 as a 
reference tape, ie. at 0 dB. The signal-to-noise ratio (S/N), the color 
noise modulation (CNM) and the HF output were measured. The results are 
shown in Table 2. 
TABLE 2 
______________________________________ 
Video values* Example A/1 
Example A/4 
______________________________________ 
S/N [dB] +2 +1 
CNM [dB] +2.5 +2 
HF output [dB] +2 +2 
______________________________________ 
TABLE 2