Flexible magnetic recording tape having low-friction backside coating

Magnetic recording tape having a nonmagnetizable backside coating including smooth equant particles having a Mohs hardness of at least 7. When the tape is used in a belt-driven cartridge, the presence of the equant particles in the backside coating provides improved uniformity of tape speed.

BACKGROUND OF THE INVENTION 
U.S. Pat. No. 3,692,255 (Von Behren) discloses a magnetic recording tape 
cartridge for storing data wherein the tape is driven by a pretensioned 
belt. The specific embodiment of the patent employs magnetic recording 
tape which is 25 micrometers in thickness. The use of thinner tape has 
sometimes resulted in excessive variations in tape speed. 
It was discovered that the excessive variations in tape speed resulted 
whenever the following two conditions occurred simultaneously: (1) the 
tension on the wound tape substantially exceeded the unwinding tension and 
(2) the tape tended to stick to the underlying convolution so that the 
windup tension was being released spasmodically. Under the worst 
conditions, there would be an audible "popping" noise as the tape was 
unwound. 
It is not known whether tape drives other than belt-driven tapes involve 
this problem. While the belt of the Von Behren patent holds each 
convolution of recording tape tightly against the underlying convolution, 
the outer convolutions on an unbelted spool have more freedom to move 
circumferentially and thus to release windup tensions gradually before 
being unwound from the spool. Nevertheless, the continuing trend to 
thinner magnetic recording tapes increases the likelihood that the problem 
may also occur in other types of drives without anyone being aware of it. 
OTHER PRIOR ART 
The magnetic recording tape of German Offenlegungsschrift No. 2,364,878 
(filed Dec. 28, 1973 and laid open for inspection on July 3, 1975) has a 
backside coating which is said to provide improved friction 
characteristics during rapid starting and stopping and also to provide 
improved scratch resistance. The backside coating consists of 30 to 60 
weight-percent of a polymeric binder and correspondingly 70 to 40 
weight-percent of a powder mixture which may consist of by weight 
(a) 15-60 parts of conductive carbon black, 
(b) 15-85 parts of a mixture comprising 
(1) 15-90 percent of flat silicate particles having a Mohs hardness of less 
than 6 (preferably 1-4 micrometers in diameter and 0.02 to 0.5 micrometer 
in thickness) and 
(2) correspondingly 85-10 percent of cube-to-sphere-shaped particles of an 
oxide having a Mohs hardness of more than 7 such as aluminum oxide 
(preferably 0.5-2 micrometers in diameter) and 
(c) 20-55 parts of an inorganic filler such as talc or aluminum oxide of a 
size selected to provide a preferred surface roughness of 0.8 to 1.2 
micrometers (its particle size being said to be more important than its 
chemical composition). 
U.S. Pat. No. 3,617,378 (Beck) concerns magnetic recording cards and shows 
apparatus for dispensing the cards from a stack into a reading device. A 
rubber picker roll contacts the backside of the lowermost card and 
frictionally drives it into the reading device. This requires a durable 
backside coating having a high coefficient of friction with the picker 
roll and a low coefficient of friction with each of the magnetic recording 
surface of the adjacent card and the aluminum deck of the reading device. 
The backside coating said to be most suitable comprises 8-40% by weight of 
nonmagnetic alpha-Fe.sub.2 O.sub.3 particles. Although the patent does not 
mention the shape of those particles, the commercial product identified at 
column 5, line 19 is said to be acicular. 
U.S. Pat. No. 4,135,031 (Akashi et al.) is directed to the problem of 
obtaining better winding characteristics in video tape by reducing the 
coefficient of friction between the magnetizable coating and the backside 
of the tape. This is achieved by a backside coating comprising a mixture 
of a major proportion of small inorganic particles and a minor proportion 
of larger inorganic particles which are called "spike grains". Preferred 
inorganic particles are "talc, lithopone and complex of zinc sulfide and 
zinc sulfate" (column 2, lines 61-62). Less preferred is aluminum oxide. 
THE PRESENT INVENTION 
The flexible magnetic recording tape of the present invention, like the 
tapes of the aforementioned Offenlegungsschrift, has a magnetizable 
frontside coating and a nonmagnetizable backside coating, each coating 
comprising a nonmagnetizable polymeric binder containing inorganic 
particles. As in the Offenlegungsschrift, some of the inorganic particles 
of the backside coating which have an average diameter of at least 0.1 
micrometer may be of equant shape and have a Mohs hardness of at least 7. 
By "equant shape" is meant particles having a ratio of maximum diameter to 
minimum diameter of less than 2. As in the Offenlegungsschrift, the 
backside coating preferably also contains conductive carbon black 
particles of less than 0.1 micrometer in size and in sufficient quantity 
to bleed off static charges, an especially effective amount being 20-25 
percent by volume of the backside coating. Such quantity of carbon black 
by itself provides a surface roughness of at least 0.2 micrometer. Carbon 
black particles may additionally provide a desirable reinforcing of the 
polymeric binder. 
The novel tape differs from that of the Offenlegungsschrift in that said 
equant particles of its backside coating comprise substantially all of the 
particles in the backside coating which have an average diameter greater 
than 0.1 micrometer. Said equant particles are about 0.1 to 0.5 micrometer 
in average diameter, not more than about 10% of said equant particles have 
an average diameter exceeding about 0.8 micrometer, and substantially all 
of said equant particles are essentially free from sharp edges. 
Furthermore, the backside coating of the novel tape should have a surface 
roughness of 0.2-0.8 micrometer (as measured peak-to-peak with a 0.1-mil 
stylus), preferably 0.3-0.6 micrometer. The Offenlegungsschrift indicates 
a preferred surface roughness of 0.8-1.2 micrometers without indicating 
how this was measured. A surface indicating a given roughness value when 
measured peak-to-valley or with a stylus larger than 0.1 mil would 
indicate a higher roughness value when measured peak-to-peak with a 
0.1-mil stylus. 
If the backside coating of the novel tape had a surface roughness above 0.8 
micrometer (peak-to-peak, 0.1-mil stylus), it would not be suitable for 
the tape cartridge of the aforementioned Von Behren U.S. Pat. No. 
3,692,255 or other data tapes. Its equant particles of high Mohs hardness 
would emboss the facing magnetizable layer during storage in roll form, 
thus debasing its original smoothness, a surface roughness of not 
exceeding about 0.1 micrometer often being required for the magnetizable 
surface of a data tape. 
The smooth equant particles should comprise about 0.5-10% by volume of the 
backside coating, which range is within the useful range of the 
cube-to-sphere-shaped particles of high Mohs hardness in the 
aforementioned Offenlegungsschrift. Preferably the smooth equant particles 
comprise 1-3% by volume of the backside coating, because a larger 
proportion might have an undesirable degree of abrasiveness, in spite of 
the fact that they are essentially free from sharp edges. At only 1-3% by 
volume, the smooth equant particles do not significantly change the degree 
of surface roughness provided by the conductive carbon black as mentioned 
above. At much more than 3% by volume, it is preferred that the average 
diameter of the smooth equant particles be toward the lower end of the 
useful 0.1-0.5 micrometer range in order to keep the surface roughness of 
the backside coating within the range of 0.2-0.8 micrometer. 
Because of their high Mohs hardness, the smooth equant particles would be 
abrasive if they had sharp edges. Sharp particles would not only tend to 
damage the tape guides but would tend to pick magnetizable material out of 
the facing magnetizable coating, and eventually some of that magnetizable 
material would transfer back to the faceside of the tape. The smooth 
equant particles preferably are of uniform size and very few exceed twice 
the average diameter. If the average particle size were much larger than 
0.5 micrometer of if a number of particles were significantly larger than 
the average, the backside coating might tend to abrade the tape guides. 
The presence of the smooth equant particles in the backside coating has 
provided surprisingly improved uniformity of tape speed in magnetic 
recording tape systems in which the tape is driven by a pretensioned belt. 
The improvement in uniformity of tape speed has been observed only when 
the overall thickness of the tape has been 20 micrometers or less. 
An especially useful material for the smooth equant particles is aluminum 
oxide which has a Mohs hardness of 9. It is readily available at 
reasonable cost while possessing each of the properties mentioned above. 
Suitable aluminum oxide particles can be obtained from Products Chimiques 
Ugine Kuhlmann, France, under the designation "Alumine `Exal` Extra Pure 
Alpha A-6". These are essentially free from sharp edges, are of equant 
shape, and have an average diameter of about 0.15-0.2 micrometer. A 
photomicrograph did not reveal any particles larger than 0.5 micrometer. 
Also useful are: 
______________________________________ 
Mohs Hardness 
______________________________________ 
Silicon carbide 9 
Tungsten carbide 9 
Boron nitride 10 
Quartz 7 
______________________________________ 
Preferably the Mohs hardness of the equant particles is at least 8, because 
the guides of many tape drives may have a Mohs hardness of about 8. If the 
backside coating were repeatedly drawn across guides that were harder than 
a significant portion of the backside particles, the coating would 
gradually become smooth and would no longer deter tape-speed variations. 
Any crosslinkable polymeric binder which is useful in magnetizable coatings 
should be useful in the backside coating of the invention. Preferred are 
polyurethanes, especially polyurethane block copolymers, comprising at 
least 15 mole percent 4,4'-diphenylmethane diisocyanate. The polyurethanes 
may be blended with other resins which can be crosslinked together with 
the polyurethane. Especially useful are high-molecular-weight copolymers 
of bisphenol A and epichlorohydrin such as "Phenoxy PKHH" and vinyl resins 
such as "VAGH" and "VROH", all from Union Carbide. 
The polymeric binder from the backside coating need not be crosslinked if 
it performs as if it were crosslinked, such as the segmented copolyesters 
of U.S. Pat. No. 4,025,694 (Vermillion et al.). The polymeric binder 
either should be crosslinked or act as if it were crosslinked in order to 
withstand the high speeds and abrupt starts and stops such as are 
encountered in data processing uses. 
Used as the binder polymer in the following example was a 
hydroxyl-terminated polyester of adipic acid and butane diol-1,4 with 
4,4'-diphenylmethane diisocyanate and was obtained as "Estane" 5707 from 
B. F. Goodrich. The aluminum oxide particles were the "Alumine `Exal` 
Extra Pure Alpha A-6" mentioned above. In the example, all parts are by 
weight.

EXAMPLE 
The following were milled in a vibro energy mill ("Sweco") until a smooth 
dispersion was obtained: 
______________________________________ 
Parts 
______________________________________ 
Aluminum oxide 6.8 
Binder polymer .45 
Dispersant .33 
Tetrahydrofuran solvent 
7.35 
______________________________________ 
A second dispersion was obtained by sandmilling the following until smooth: 
______________________________________ 
Parts 
______________________________________ 
Conductive carbon black 
(average particle size 
0.04 micrometer) 15 
Binder polymer 30 
Dispersant 1 
Tetrahydrofuran solvent 
227 
Toluene 23 
______________________________________ 
The second dispersion was diluted with tetrahydrofuran solvent to 10% 
solids and charged to a disk-type high-speed mixer together with an amount 
of the first dispersion such that the alumina comprised 2.2 parts of the 
charged solids. After mixing for ten minutes, the entire charge was 
transferred to a paddle-type high-speed mixer, and 5 parts (3 parts 
solids) of a 60% solution of a toluene diisocyanate adduct of 
trimethylopropane ("Mondur" CB-60) was added as a crosslinking agent for 
the binder. After mixing for 15 minutes, this was knurl-coated onto one 
side of biaxially-oriented polyethylene terephthalate T-film of 43 gauge 
(11 micrometers) and dried in an oven to provide a backside coating of 
approximately 1.2 micrometers in thickness. The aluminum oxide particles 
comprised about 1.7% by volume of the backside coating, and the carbon 
black, about 22% by volume. 
Subsequently, a magnetizable coating of acicular .gamma.-Fe.sub.2 O.sub.3 
particles in the same binder was knurl-coated onto the frontside of the 
film, the magnetizable particles were oriented longitudinally by a 
magnetic field, and the coating was dried in an oven. The magnetizable 
coating was super-calendered to polish it to a surface roughness of about 
0.1 micrometer (measured as described above), a procedure which inherently 
provides a degree of polishing to the backside coating. The surface 
roughness of the final backside coating was 0.35 micrometer. The overall 
thickness of the finished magnetic recording tape was about 19 
micrometers. 
450 feet of the tape were loaded into a cartridge as illustrated in the 
aforementioned Von Behren U.S. Pat. No. 3,692,255 and recorded with a 3200 
FCI (flux changes per inch) signal while driving the tape in the reverse 
direction until fully wound onto the supply reel. The output signal on 
playback was used to control a phase lock loop running at the same 
frequency. The voltage controlled oscillator (VCO) control voltage was fed 
to an oscilloscope while a digital electronic counter monitored the VCO 
control voltage for any changes of frequency (spikes) greater than 2.4% at 
a frequency modulated rate of greater than 1000 Hz. No such changes of 
frequency were counted. When tape which was identical except for the 
omission of the aluminum oxide from the backside coating was tested, many 
spikes were detected in the VCO control voltage. The spikes occurred 
approximately every 9 mm and in a fairly regular manner until an 
appreciable proportion of the tape had been unwound from the supply reel. 
Thereafter the spikes occurred less frequently.