U.S. patent application number 14/530235 was filed with the patent office on 2015-02-26 for magnetic tape, its cleaning method, and optical servotrack forming/cleaning apparatus.
This patent application is currently assigned to HITACHI MAXELL, LTD.. The applicant listed for this patent is HITACHI MAXELL, LTD.. Invention is credited to Haruhiko FUJISAWA, Shigeo FUJITANI, Satoru FUKIAGE, Naoki MUKAI, Hiroyuki OTA, Kenji SANO.
Application Number | 20150056474 14/530235 |
Document ID | / |
Family ID | 26607227 |
Filed Date | 2015-02-26 |
United States Patent
Application |
20150056474 |
Kind Code |
A1 |
OTA; Hiroyuki ; et
al. |
February 26, 2015 |
MAGNETIC TAPE, ITS CLEANING METHOD, AND OPTICAL SERVOTRACK
FORMING/CLEANING APPARATUS
Abstract
A magnetic tape which comprises a nonmagnetic support, a
magnetic layer which is formed on one surface of the nonmagnetic
support, and a backcoat layer which comprises a binder and
nonmagnetic powder containing carbon black as a component and which
is formed on the other surface of the nonmagnetic support, having
pits for optical servo formed thereon, characterized in that the
average of the reflectance on the flat portion of the backcoat
layer is 8.5% or higher, and that the maximum rate of fluctuation
of the reflectance on the flat portion, depending on a position of
the magnetic tape: [Maximum of absolute value of
(Reflectance-Average reflectance)].times.100/(Average reflectance)
is 10% or lower. This magnetic tape is high in the initial S/N of
the servo signal, and also high in the S/N of the servo signal
found after the magnetic tape is run twice.
Inventors: |
OTA; Hiroyuki; (Kyoto-shi,
JP) ; FUJISAWA; Haruhiko; (Otokuni-gun, JP) ;
MUKAI; Naoki; (Takatsuki-shi, JP) ; FUKIAGE;
Satoru; (Ueno-shi, JP) ; SANO; Kenji;
(Nagaokakyo-shi, JP) ; FUJITANI; Shigeo;
(Mishima-gun, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
HITACHI MAXELL, LTD. |
Osaka |
|
JP |
|
|
Assignee: |
HITACHI MAXELL, LTD.
Osaka
JP
|
Family ID: |
26607227 |
Appl. No.: |
14/530235 |
Filed: |
October 31, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
12862504 |
Aug 24, 2010 |
|
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|
14530235 |
|
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|
10343432 |
Mar 31, 2003 |
7803471 |
|
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PCT/JP2001/011610 |
Dec 28, 2001 |
|
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12862504 |
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Current U.S.
Class: |
428/836 |
Current CPC
Class: |
G11B 5/78 20130101; G11B
5/735 20130101; G11B 23/502 20130101; G11B 5/584 20130101 |
Class at
Publication: |
428/836 |
International
Class: |
G11B 5/735 20060101
G11B005/735; G11B 23/50 20060101 G11B023/50; G11B 5/78 20060101
G11B005/78 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 28, 2000 |
JP |
2000-403216 |
Dec 28, 2000 |
JP |
2000-403217 |
Claims
1. A magnetic tape comprising a nonmagnetic support, a magnetic
layer which is formed on one surface of the nonmagnetic support,
and a backcoat layer which comprises a binder and nonmagnetic
powder containing carbon black as a component, which is formed on
the other surface of the nonmagnetic support, and which has pits
for optical servo formed thereon, wherein a content of the
nonmagnetic powder in the backcoat layer is from 50 wt. % to 60 wt.
% based on the total weight of the nonmagnetic powder and the
binder in the backcoat layer; an average surface roughness Ra of a
flat portion of the backcoat layer, which is measured with an
atomic force microscope, is 30 nm or less; and an average of
reflection on a flat portion of the backcoat layer is from 8.5% to
14.9%.
2. The magnetic tape according to claim 1, wherein a maximum rate
of fluctuation of the reflectance on the flat portion of the
backcoat layer depending on a position of the magnetic tape, which
is defined by the following equation, is from 2.8% to 10%: [Maximum
of absolute value of (Reflectance-Average of
reflectance)].times.100/(Average of reflectance).
3. The magnetic tape according to claim 1, wherein a half width of
fluctuation of the surface roughness Ra, depending on a site of the
magnetic tape, is 5 nm or less.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a Divisional of co-pending application
Ser. No. 12/862,504 filed on Aug. 24, 2010. Application Ser. No.
12/862,504 is a Divisional of co-pending application Ser. No.
10/343,432 (U.S. Pat. No. 7,803,471) filed on Mar. 31, 2003, which
is the National Phase of PCT International Application No.
PCT/JP2001/11610 filed on Dec. 28, 2001, which claims priority
under 35 U.S.C. 119(a) to Patent Application No. 2000-403216 filed
in Japan on Dec. 28, 2000 and Patent Application No. 2000-403217
filed in Japan on Dec. 28, 2000. The entire contents of all of the
above applications are hereby incorporated by reference into the
present application.
TECHNICAL FIELD
[0002] The present invention relates to a magnetic tape in which
pits for optical servo tracks are formed on a backcoat layer, a
method for cleaning a magnetic tape, and an apparatus for forming
and cleaning optical servo tracks.
BACKGROUND ART
[0003] Magnetic tapes have found various applications in audio
tapes, videotapes, data backup tapes for computers, etc. In
particular, in the field of magnetic tapes for data-backup (or
backup tapes), tapes having memory capacities of several tens GB or
more per one reel are commercialized in association with increased
capacities of hard discs for back-up. Therefore, it is inevitable
to increase the capacity of this type of a tape for data-backup so
as to correspond to a further increased capacity of a hard disc. It
is also necessary to increase the feeding speed of tape and a
relative speed between the tape and heads in order to quicken the
access speed and the data transfer speed.
[0004] To increase the capacity of tape for data-backup per one
reel, the following are necessary: (1) the length of a tape per
reel is increased by decreasing the total thickness of the tape;
(2) the thickness demagnetization is decreased to shorten the
recording wavelength by forming a magnetic layer with a thickness
as very thin as 0.3 .mu.M or less; and (3) the recording density in
the tape widthwise direction is increased by narrowing the widths
of the tracks to 15 .mu.m or less.
[0005] When the thickness of the magnetic layer is reduced to 0.3
.mu.m or less, the durability of the tape tend to lower. Therefore,
at least one primer layer is provided between a nonmagnetic support
and the magnetic layer. When the recording wavelength is shortened,
the influence of spacing between the magnetic layer and the
magnetic heads becomes serious. Thus, if the magnetic layer has
large projections or dents, an output decreases due to spacing
loss, and thus an error rate increases.
[0006] When the magnetic layer is formed with a thickness so thin
as 0.3 .mu.m or less and concurrently the recording wavelength is
decreased, magnetic flux leakage from the magnetic recording medium
is decreased. Therefore, it is preferable to use reproducing heads
which make use of megnetoresistance elements capable of achieving
high output from very small magnetic fluxes (hereinafter, referred
to as MR heads). When the recording density in the tape-widthwise
direction is increased by narrowing the width of the tracks (the
width of data tracks on which signals are recorded) to 15 .mu.m or
less, reproduction output decreases due to off-track. To overcome
such a problem, track servo becomes necessary.
[0007] One of such track servo systems is an optical track servo
system, in which pits for optical servo are formed by irradiation
with laser beams or by depression with a stamper, and such pits are
optically detected for servo tracking.
[0008] As other optical track servo systems of this type,
JP-A-03-141087 discloses the formation of pits for optical servo on
the magnetic layer of a floptical disc (an optical servo track type
floppy disc), and JP-A-11-339254 and JP-A-11-213384 disclose the
formation of pits for optical servo are formed on the backcoat
layer of a magnetic tape.
[0009] In the optical track servo systems in which pits for optical
servo are formed on the backcoat layer, the track servo is
performed by detecting a difference in reflectance between the pits
and the flat portion of the backcoat layer. In particular, when the
backcoat layer having such pits is irradiated with light, light
randomly reflects on the pits, and therefore, the intensity of
reflected light which enters an optical detector is low. On the
other hand, light regularly reflects on the flat portion, and thus,
the intensity of reflecting light is high. This system makes use of
such a difference to trace the servo tracks formed as the pits.
Specifically, interlocking with the servo tracking on the backcoat
layer, the magnetic head which records or reproduces signals on or
from the magnetic layer is moved to perform servo tracing on
magnetically recording tracks.
[0010] According to this system, if the pits for optical servo are
formed by irradiation with conventional laser beams, the intensity
of light randomly reflecting on the pits can be sufficiently
lowered. However, the intensity of light which reflects on the flat
portion of the backcoat layer of a conventional magnetic tape is
low, and the reflectance on the flat portion largely fluctuates
depending on a site of the magnetic tape. Thus, it is impossible to
sufficiently increase the ratio of S/N of optical servo signals.
The reason therefor is that keen attentions are paid to only the
tape running performance on the backcoat layer of the a
conventional magnetic tape, but not to the reflectance thereon.
[0011] In case where pits are formed in a backcoat layer by
irradiation with laser beams or by depression with a stamper, the
peripheries (or the edges) of the pits are inevitably raised, which
causes the following problems. In case where the total thickness of
a magnetic tape is 6 .mu.m or less, the rigidity of the tape (i.e.,
ET.sup.3 in which E is a Young's modulus of a tape, and T is a
total thickness of the tape) decreases, and therefore, it is needed
to decrease the winding tension for the tape which is running. In
this case, if the specific positions of the magnetic tape are
raised as described above, the track-formed portion of the tape
wound onto a reel becomes extremely high, which results in a
disorder in the winding of the tape.
[0012] In addition, if the tape has the raised portions as
described above, they are pressed against the side of the recording
layer (magnetically recording surface) of the magnetic tape, so
that the surface of the recording layer becomes uneven, which
results in low reproduction output. In case of a magnetic disc
employing the optical servo track system, such winging as is made
on the tape is unnecessary, and therefore, such disorder in the
winding or the pressing by the raised portions do not occur, even
though the peripheries of the pits for optical servo are raised.
That is, these problems are peculiar to the magnetic tapes. To
solve these problems, it is desirable to decrease the height of the
raised portions of the tape to not higher than the height of the
maximum projection of the flat portion thereof.
[0013] When the pits are formed on the backcoat layer by
irradiation with laser beams, the coating surface of the backcoat
layer is baked off by the energy of laser beams so as to form a
pattern of pits. This method provides higher productivity, however,
has a problem in that the numerous particles of burnt residues as
the result of the laser baking for forming a pattern, undesirably,
adhere to the pits and their peripheries. If the burnt residues are
left as they are, they cause not only contamination of the
tape-running system but also decrease in the ratio of S/N of
optically read signals on the backcoat layer and the dropping-out
of the magnetic layer due to the adhesion of the burnt residues.
Further, the reflectance on the flat portion of the backcoat layer
decreases, so that the reflectance in the lengthwise direction of
the tape largely fluctuates. This fluctuation also decreases the
ratio of S/N of optically read signals. Therefore, the removal of
such burnt residues is necessary.
[0014] It is known that burnt residues which remain after the
formation of pits for optical servo by irradiation with laser beams
are removed using solid CO.sub.2, which has been used for removing
such burnt residues from a floptical disc having pits for optical
servo formed thereon (U.S. Pat. No. 5,419,733). In case of a
floptical disc, the surface area to be cleaned is limited, and the
solid CO.sub.2 can easily be sprayed to clean the surface by
rotating the disk a number of times at a high velocity.
[0015] If this method is applied to clean a magnetic tape, the
total surface area of the lengthy tape to be cleaned is enormous,
and the amount of solid CO.sub.2 blown onto the tape a lot of times
becomes far larger as compared with the disc. Therefore, the
cleaning efficiency is poor. The magnetic tape confronts a further
problem from which the floptical disc has never suffered: that is,
the burnt residues remaining after the formation of the servo
pattern by irradiation with laser beams adhere and transfer when
the magnetic tape is again wound, and such burnt residues, in turn,
adhere to the drive guide roller and the magnetic heads.
[0016] Alternatively, the surface of the magnetic tape is cleaned,
for example, by allowing a tissue cleaning tape to contact with the
front and back surfaces of a magnetic tape. This method is
unsatisfactory, because the cleaning of the flat portion of the
backcoat layer is insufficient, and also the effect of cleaning the
interiors of servo dots formed as pits by laser beams is poor. The
above cleaning treatment in combination with a blade treatment is
also possible. However, a strong blade treatment may damage the
backcoat layer, since the backcoat layer has a lower strength than
the magnetic layer. On the contrary, if a weak blading treatment is
made on the backcoat layer, the burnt residues thereon cannot be
removed. Thus, this method is unsuitable for large-scale
production, because selection of the conditions for the cleaning is
difficult. Still worse, this method has substantially no effect of
cleaning the interiors of the servo dots formed as the pits.
[0017] The present invention has been completed to solve the
foregoing problems of the prior art.
SUMMARY OF THE INVENTION
[0018] The present inventors have intensively researched a magnetic
tape on which optical servo signals having a high S/N ratio (signal
to noise) can be recorded. As a result, they have discovered that
the ratio of S/N is increased by setting an average of the
reflectance on the flat portion of a backcoat layer at 8.5% or
higher, and also by decreasing the rate of fluctuation of the
reflectance on the flat portion depending on a position of the
magnetic tape (a site on the magnetic tape), which is defined by
the following equation, to 10% or lower:
[Maximum of absolute value of (Reflectance-Average of
reflectance)].times.100/(Average of reflectance)
[0019] According to the first aspect, the present invention relates
to a magnetic tape comprising a nonmagnetic support; a magnetic
layer which is formed on one surface of the nonmagnetic support;
and a backcoat layer which contains a binder and nonmagnetic powder
containing carbon black as one component and which is formed on the
other surface of the nonmagnetic support, having pits for optical
servo formed thereon, wherein the average of the reflectance on the
flat portion of the backcoat layer is 8.5% or higher, and wherein
the maximum rate of fluctuation of the reflectance on the flat
portion depending on a position of the magnetic tape, determined by
the following equation, is 10% or lower.
[Maximum of absolute value of (Reflectance-Average of
reflectance)].times.100/(Average of reflectance)
[0020] To set the average of the reflectance on the flat portion of
the backcoat layer at 8.5% or higher, and also to lower the
fluctuation of the reflectance on the flat portion depending on a
site of the magnetic tape to 10% or lower, the content of the
nonmagnetic powder in the backcoat layer:
(Weight of nonmagnetic powder).times.100/(Weight of nonmagnetic
powder+Weight of binder)
is controlled to 50 wt. % or more; the surface roughness Ra of the
flat portion of the backcoat layer, measured with an atomic force
microscope (AFM), is controlled to 30 nm or less; and the half
width of the fluctuation of the surface roughness Ra depending on a
site of the magnetic tape is controlled to 5 nm or less.
[0021] If the ratio of carbon black in the nonmagnetic powder is
increased to 80 wt. % or more, it becomes easy to form the pits for
optical servo (i.e., servo holes) by irradiation with laser beams.
The addition of 20 wt. % or less of iron oxide (e.g., red iron
oxide) in combination with carbon black is effective to improve the
strength of the backcoat layer.
[0022] The present inventors also have carefully researched the
solution of the above problem of a disorder in the winding of a
magnetic tape having a total thickness as thin as 6 .mu.m or less.
As a result, the inventors have found out that the disorder in the
winding of the tape can be prevented by setting the value of H/T at
1/50 or less, preferably 1/100 or less, wherein T is the total
thickness of the magnetic tape, and H is the average height of 100
raised portions around the peripheries of the pits of the backcoat
layer (the peripheral portions of the pits for optical servo).
[0023] Another object of the present invention is to provide a
method and an apparatus for efficiently removing the burnt residue
(powder, etc.) which form when pits for optical servo are formed on
the backcoat layer of a magnetic tape by irradiation with laser
beams and which adhere to the interiors of the pits and their
peripheries on the backcoat layer, and also to provide a magnetic
tape having a low error rate.
[0024] As a result of the researches of a method for efficiently
removing such burnt residues, the following methods are found to be
effective: (1) cleaning by using CO.sub.2, and (2) cleaning by
using a raised fabric or the like. The method (1) using CO.sub.2
requires a relatively large-scale apparatus, while it is relatively
low in running cost, since only consumable is CO.sub.2. On the
other hand, the method (2) using a raised fabric requires a
relatively simple apparatus, although consuming raised fabrics.
[0025] Therefore, according to the second aspect, the present
invention relates to a method for cleaning a magnetic tape which
comprises a nonmagnetic support, a magnetic layer formed on one
surface of the nonmagnetic support, and a backcoat layer which
contains nonmagnetic powder and a binder and which is formed on the
other surface of the nonmagnetic support, having pits for optical
servo, formed thereon by irradiation with laser beams, which method
comprises the step of spraying solid CO.sub.2 onto the surface of
the backcoat layer, thereby removing burnt residues which forms
after the laser irradiation and adheres to the pits for optical
servo and their peripheries, so as to clean the backcoat layer.
[0026] According to the third aspect, the present invention relates
to an apparatus for forming and cleaning optical servo tracks of a
magnetic tape, which apparatus comprises a tape-feeding mechanism
for feeding a reeled magnetic tape in a predetermined direction; a
unit for forming optical servo tracks by forming pits on the
surface of the backcoat layer of the fed magnetic tape by
irradiation with laser beams; a unit for cleaning the surface of
the backcoat layer after the formation of the pits; and a mechanism
for winding the magnetic tape after cleaning. The cleaning unit
comprises a section for spraying solid CO.sub.2 onto the pits for
optical servo and their peripheries on the backcoat layer; a
section for sucking the burnt residues which are blown off by the
solid CO.sub.2 and adhere to the pits and their peripheries; and a
section for wiping the surface of the backcoat layer after the
suction of the burnt residues.
[0027] According to the fourth aspect, the present invention
relates to a method for cleaning a magnetic tape which comprises a
nonmagnetic support, a magnetic layer formed on one surface of the
nonmagnetic support, and a backcoat layer which contains
nonmagnetic powder and a binder and which is formed on the other
surface of the nonmagnetic support, having pits for optical servo
formed thereon by irradiation with laser beams, which method
comprises the steps of allowing a raised fabric or a woven or
nonwoven fabric having raising fibers thereon, to contact with the
surface of the backcoat layer having the pits thereon, and removing
the burnt residues adhered to the pits and their peripheries.
[0028] According to the fifth aspect, the present invention relates
to an apparatus for forming and cleaning optical servo tracks of a
magnetic tape, which apparatus comprises a tape-feeding mechanism
for feeding a reeled magnetic tape in a predetermined direction; a
unit for forming optical servo tracks by forming pits on the
surface of the backcoat layer of the fed magnetic tape by
irradiation with laser beams; a unit for cleaning the surface of
the backcoat layer after the formation of the pits; and a winding
means for winding the magnetic tape after cleaning. The cleaning
unit comprises a section for allowing a raised fabric or a woven or
nonwoven fabric having raising fibers thereon to contact with the
surface of the backcoat layer so as to clean the same, and a
section for wiping and removing unwanted particles adhered to the
surface of the backcoat layer.
BRIEF DESCRIPTION OF DRAWINGS
[0029] FIG. 1 is a schematic diagram illustrating the spraying of
solid CO.sub.2 onto the surface of the backcoat layer of a magnetic
tape.
[0030] FIG. 2 a schematic diagram illustrating an angle at which a
nozzle for spraying CO.sub.2 is set.
[0031] FIG. 3 is a perspective view of the essential portion of an
apparatus for forming and cleaning optical servo tracks,
illustrating a CO.sub.2-spraying section and a sucking section.
[0032] FIG. 4 is a schematic diagram illustrating an example of a
sucking means (a suction nozzle).
[0033] FIG. 5 is a schematic diagram illustrating another example
of a sucking means (a suction nozzle).
[0034] FIG. 6 is a perspective view of a magnetic tape,
illustrating an example of servo patterns formed on the surface of
the backcoat layer.
[0035] FIG. 7 is a schematic diagram illustrating the whole
structure of an apparatus for forming and cleaning optical servo
tracks, used in Examples of the present invention.
[0036] FIG. 8 is a perspective view of a contact-removing section
and its periphery in an apparatus for forming and cleaning optical
servo tracks, used in Examples of the present invention.
[0037] FIG. 9 is a schematic diagram illustration the construction
of a whole of another apparatus for forming and cleaning optical
servo tracks, used in Examples of the present invention.
BEST EMBODIMENTS FOR CARRYING OUT INVENTION
[0038] Firstly the cleaning method (1) using CO.sub.2 is
described.
[0039] A magnetic tape is caused to run in the lengthwise
direction, while the backcoat layer thereof is being irradiated
with laser beams to form pits for optical servo on the backcoat
layer. After this step, solid CO.sub.2 is sprayed onto the surface
of the backcoat layer having the pits formed thereon. Thereby, the
burnt residues adhered to the interiors of the pits and their
peripheries are removed only by causing the magnetic tape to run
once.
[0040] As shown in FIGS. 1 to 3, the magnetic tape (1) to be
cleaned is run at a high speed, for example, about 10 m/sec., while
solid CO.sub.2 (which has been in a liquid phase when sprayed, and
is changed to a solid state immediately after sprayed) is sprayed
onto the surface of the backcoat layer (2), to efficiently remove
the particles adhered to the surface of the backcoat layer (2). In
FIGS. 1 to 3, numeral 3 refers to a nonmagnetic support; 4, to a
magnetic layer; 5, to a servo pattern consisting of a plurality of
pits for optical servo; and 6, to a primer layer.
[0041] Although not bound by any theory, the reason why the burnt
residues can be efficiently removed by spraying such solid CO.sub.2
may be considered as follows. Carbon dioxide (CO.sub.2) sprayed
onto the surface of the backcoat layer is in a liquid state at a
specific temperature or lower under a specific pressure or higher.
However, after the spraying, the pressure rapidly lowers, so that
carbon dioxide is changed from a liquid state to a solid state to
form dry ice fine particles. These dry ice particles are sprayed
from a spray nozzle (15) and then struck onto a part of the surface
of the backcoat layer (2) of the magnetic tape (1) to fly and
spread over the peripheral area of such a part of the backcoat
layer (the dry ice particles forms carbon dioxide gas in short
time). The dry ice particles adsorb the particles (mainly the burnt
residues) adhered to the surface of the backcoat layer when struck
thereto. Thus, the burnt residues on the surface of the backcoat
layer are separated and removed. As shown in FIG. 3, a sucking
means such as a suction nozzle (16) sucks the gas in the
CO.sub.2-sprayed region or its periphery in this step, so that the
burnt residues can be more efficiently removed.
[0042] In the mode shown in FIG. 3, the suction nozzle (16) having
a suction port (16a) with a width larger than the tape width is
arranged above the backcoat layer. The sucking means is not limited
to this type, and it may be such a sucking means (16) as shown in
FIG. 4, which has a suction port (16a) surrounding the surface and
both edge portions of the backcoat layer (2) of the magnetic tape
(1) (both end portions of the magnetic tape (1) along the tape
lengthwise direction), viewed from a direction reverse to the
tape-running direction. Alternatively, it may be such a sucking
means (16) as shown in FIG. 5, which has a suction port (16a)
enclosing a whole of a tape, viewed from a direction reverse to the
tape-running direction. Further, although not shown herein, it may
be such a sucking means that encloses a whole of the
CO.sub.2-sprayed region including a CO.sub.2-spraying nozzle (15),
but is arranged so as not to hinder the running of a magnetic
tape.
[0043] FIG. 6 shows an example of a pattern (a servo pattern) for
arraying pits for optical servo, which is formed by irradiation
with laser beams. The servo pattern (5) shown in FIG. 6 is formed
on a magnetic tape (1) having a width of 12.64 mm (1/2 in.). In
this example, four bands (5a) extending along the tape lengthwise
direction are formed on the tape in the widthwise direction. The
width of one band (5a) is about 0.4 mm. Microscopically, each one
of the bands (5a) is composed of one row of pits for optical servo,
arrayed along the tape lengthwise direction, and a plurality of
such rows of the pits are arranged at intervals in the tape
widthwise direction. The burnt particles resulting from the laser
irradiation most abundantly adhere to the interiors of the pits
within the servo pattern. Therefore, the most efficient spray
nozzle (15) has a plurality of spray orifices (15a) which
correspond to the bands (5a), one to one, as shown in FIG. 3. The
spray nozzle (15) shown in FIG. 3 has four spray orifices (15a)
corresponding to a pattern of four bands. The solid CO.sub.2 (which
has been in a liquid phase when sprayed, as described above) is
uniformly sprayed from the spray orifices (15a), so that the pits
forming the servo pattern (5) and their peripheries are surely
cleaned.
[0044] In another mode, solid CO.sub.2 is sprayed from a
CO.sub.2-spraying nozzle inclined at a certain angle, toward a
direction reverse to a magnetic tape which is running at a high
speed (e.g., 10 m/sec.). In more particular, as shown in FIGS. 2, 3
and 7, the spray nozzle (15) inclined, for example, at an angle of
30 to 90.degree., preferably 30 to 60.degree. relative to the
surface of the backcoat layer (2), is arranged in the front of a
portion of the surface of the backcoat layer onto which the solid
CO.sub.2 is sprayed (i.e., solid CO.sub.2-receiving portion), when
viewed from the magnetic tape-running direction A. The solid
CO.sub.2 is sprayed therefrom in a direction reverse to the
tape-running direction A, and is struck to the CO.sub.2-receiving
portion. Thereby, the burnt residues adhered to the pits and their
peripheries on the surface of the backcoat layer are blown off
Since this method makes it possible to increase the relative
CO.sub.2-spraying speed while the flow of CO.sub.2 onto the pits
for optical servo is being maintained, the cleaning effect can be
improved.
[0045] To efficiently form a servo pattern, such an apparatus as
described below is effectively used. That is, the apparatus can
form pits for optical servo on the surface of the backcoat layer by
irradiation with laser beams, while running several thousands
meters of a reeled magnetic tape, and then, the apparatus cleans
and wipes the surface of the backcoat layer, followed by rewinding
the magnetic tape in good order. In the present invention, as the
apparatus used is such an apparatus that forms and cleans optical
servo tracks on a magnetic tape as shown in FIG. 7. This apparatus
comprises a tape-feeding mechanism (11) for feeding a reeled
magnetic tape (1) in a predetermined direction; an optical servo
track-forming unit (12) for forming pits for optical servo on the
surface of the backcoat layer of the fed magnetic tape (1) by
irradiation with laser beams; a cleaning unit (13) for cleaning the
surface of the backcoat layer after the formation of the pits; and
a tape-winding mechanism (14) for winding the magnetic tape (1)
after cleaning. The cleaning unit (13) comprises a
CO.sub.2-spraying section equipped with a spray nozzle (15) for
spraying solid CO.sub.2 onto the above pits and their peripheries;
a sucking section equipped with a suction nozzle (16) for sucking
burnt residues which are blown off by the spraying of the solid
CO.sub.2 and adhered to the pits and their peripheries; and a
wiping section (17) for wiping the surfaces of the backcoat layer
and the magnetic layer, for example, by means of tissue after the
suction of the burnt residues.
[0046] In the above apparatus, a tension loss occurs in each of the
optical servo track-forming unit (12), and the CO.sub.2-spraying
section and the wiping section in the cleaning unit (13), so that
the magnetic tape may be held under a tension which exceeds an
optimal tension (e.g., 70 to 200 g) to the tape. Accordingly, it is
preferable to provide tension controlling means for individually
controlling the tension of the magnetic tape in each of the unit
and the sections. As will be described later in the part of
Examples, the first to third suction rolls (22 to 24) are provided
so as not to transmit the tension, while the values of tension
detectors (27 and 28) which are provided in the respective units
are feedback-controlled via a servo motor for rotating the suction
rolls (22 to 24), so that the magnetic tape (1) is run under an
optimal tension maintained.
[0047] Next, the cleaning method (2) using a raised fabric or the
like is described.
[0048] A method for efficiently removing burnt residues, which are
formed by irradiation with laser beams and adhered to the interiors
of pits for optical servo and their peripheries, with a relatively
simple apparatus has been researched. As a result, a method which
comprises a step of allowing a raised fabric or a woven or nonwoven
fabric having raising fibers thereon (preferably velvet) to contact
with a magnetic tape running in the lengthwise direction is found
to be very effective to remove the burnt residues only by running
the magnetic tape once. According to the present invention, a
raised fabric or a woven or nonwoven fabric having raising fibers
thereon (preferably velvet) is allowed to contact with the surface
of the backcoat layer of a magnetic tape which is being run at a
high speed, for example, about 10 m/sec. so as to clean the surface
of the backcoat layer. Thereby, the burnt residues (powder, etc.)
adhered to the interiors of the pits for optical servo and their
peripheries on the surface of the backcoat layer are efficiently
removed.
[0049] To carry out the above method, an apparatus for forming and
cleaning optical servo tracks on a magnetic tape as shown in FIG. 9
may be used in the present invention. The apparatus comprises a
tape-feeding mechanism (11) for feeding a reeled magnetic tape (1)
in a predetermined direction; an optical servo track-forming unit
(12) for forming pits for optical servo on the surface of the
backcoat layer of the fed magnetic tape (1) by irradiation with
laser beams; a cleaning unit (13) for cleaning the surface of the
backcoat layer after the formation of the pits; and a tape-winding
mechanism (14) for winding the magnetic tape (1) after cleaning.
The cleaning unit (13) comprises a contact-removing section (15b)
for allowing a raised fabric or a woven or nonwoven fabric having
raising fibers thereon, to contact with the surface of the backcoat
layer so as to clean the same; and a wiping section (17) for wiping
and removing unwanted particles adhered to the surface of the
backcoat layer. This apparatus has high productivity, because the
apparatus can perform the formation of pits for optical servo on
the backcoat layer of a magnetic tape and the cleaning thereof by
removing the burnt residues, on one line. In this case,
tension-controlling means are separately provided in each of the
optical servo track-forming unit, and the contact-removing section
and the wiping section in the cleaning unit, so that the tension of
the magnetic tape can be controlled in each of the unit and the
sections by such tension-controlling means. By this arrangement,
the productivity is improved.
[0050] The reason why the burnt residues in the pits and their
peripheries on the surface of the backcoat layer are efficiently
removed may be considered as follows. The raising fibers of the
woven or nonwoven cloth have appropriate lengths and rigidity, and
thus, such raising fibers enter the pits and efficiently rake out
the burnt residues from the pits.
[0051] The diameter of a single fiber out of the raising fibers is
preferably 0.5 to 10 .mu.m, more preferably 1 to 8 .mu.m,
particularly 2 to 6 .mu.m. If the diameter of such a single fiber
is smaller than 0.5 .mu.m, the rigidity (toughness) of the fiber is
insufficient, and thus, such a fiber hardly rakes out the burnt
residues. On the other hand, if the diameter of a single fiber
exceeds 10 .mu.m, such a fiber is hard to enter a pit.
[0052] The length of the single fiber is preferably 0.5 to 5 mm,
more preferably 1 to 4 mm, particularly 1 to 3 mm. If the length of
the single fiber is shorter than 0.5 mm, such a fiber hardly enters
a pit. On the other hand, if the length of the single fiber exceeds
5 mm, the rigidity (toughness) of the fiber becomes poor, and thus
hardly rakes out the burnt residue. It is also effective to split
the tip end portion of a thick single fiber for use, so as to
obtain a raking effect while maintaining the rigidity of the
fiber.
[0053] The raising fibers are of at least one selected from natural
fibers such as cotton and hemp, and synthesized fibers such as
rayon and polyester. The fibers may be of a single kind or of a
blended kind. The fibers may be of a single fiber or of a twisted
yarn of at least two fibers.
[0054] Preferably, a material for such raising fibers contains at
least cotton, since cotton has a proper rigidity (toughness) and
thickness. For example, a blended fiber containing 30 to 70% of
cotton and 70 to 30% of rayon can be used.
[0055] As already described with reference to FIG. 6, the fine
particles formed as a result of the laser beam irradiation most
abundantly adhere to the interiors of the pits of the servo
pattern. As shown in FIG. 8, the raised fabric or the woven or
nonwoven fabric (32) having raising fibers thereon may be wrapped
around a rotary drum (31), and such a fabric may be replaced for
each one reel of a magnetic tape (1) having a continuous length of
several thousands meters. Otherwise, such a fabric may be
continuously fed to the drum. Herein, the former type is employed,
because the apparatus to be used is more simple than the latter
type.
[0056] As shown in FIG. 8, the wrapped drum (30) (the rotary drum
(31) wrapped at its outer circumference with the raised fabric
(32)) is allowed to contact with the surface of the backcoat layer
(2) of a magnetic tape (1) which is running at a high speed, for
example, 10 m/sec., at a contact angle of 90 to 140.degree., while
the wrapped drum (30) is being rotated at a certain velocity [30 to
50 rps (1,800 to 3,000 rpm)] in a direction reverse to the
tape-running direction. The tension applied to the magnetic tape on
the side of the inlet is set at 50 to 100 g, while the tension
applied to the magnetic tape on the side of the outlet is set at
170 to 260 g, so that the tension of the magnetic tape is adjusted
to 1.7 to 2.5 N. Thereby, the burnt residue-removing effect is
improved.
[0057] If the contact angle is less than 90.degree., it is needed
to reduce the tape-feeding speed, so that longer time is required
to remove the burnt residues. In case where the treating time is
short, the burnt residues drop from the pits of the backcoat layer
and again adhere to the magnetic layer and the flat portion of the
backcoat layer while recording/reproducing is repeatedly performed
on or from the magnetic tape. As a result, the error rate
increases, and the ratio of SN of servo signals decreases. If the
contact angle exceeds 140.degree., the components of the apparatus
are arranged in a cramped state. In general, the contact angle is
preferably at 90 to 120.degree..
[0058] The rotating velocity of the wrapped drum is preferably from
188.4 to 314 radian/sec. (1,800 to 3,000 rpm). If the rotating
velocity is lower than 188.4 radian/sec. (1,800 rpm), it is needed
to reduce the tape-feeding speed, so that longer time is required
to remove the burnt residues. If the rotating velocity exceeds 314
radian/sec. (3,000 rpm), an expensive motor is needed.
Alternatively, two or more wrapped drums may be arranged. However,
in this case, the dimensions of the apparatus become larger.
[0059] To efficiently form a servo pattern, a magnetic tape with a
continuous length of several thousands meters or longer wound onto
a reel is being run, while pits for optical servo are being formed
on the surface of the backcoat layer by irradiation with laser
beams; the backcoat layer is subjected to a cleaning treatment and
a wiping treatment; and then, the magnetic tape is again wound in
good order. The apparatus to be used in the above steps may be an
optical servo track-forming and -cleaning apparatus as shown in
FIG. 9, which comprises a tape-feeding mechanism (11) for feeding a
reeled magnetic tape (1) in a predetermined direction; an optical
servo track-forming unit (12) for forming pits for optical servo on
the surface of the backcoat layer of the fed magnetic tape (1) by
irradiation with laser beams; a cleaning unit (13) for cleaning the
surface of the backcoat layer after the formation of the pits; and
a tape-winding mechanism (14) for winding the magnetic tape (1)
after cleaning. The cleaning unit (13) comprises a contact-removing
section (15b) for allowing a raised fabric or a woven or nonwoven
fabric having raising fibers thereon, to contact with the surface
of the backcoat layer so as to clean the same; and a wiping section
(17) for wiping and removing unwanted particles adhered to the
surface of the backcoat layer, using, for example, a tissue.
However, in the apparatus of this type, a tension loss occurs in
each of the optical servo track-forming unit (12), and the
contact-removing section (15b) and the wiping section (17) in the
cleaning unit (13), so that the tension applied to the tape
sometimes exceeds an optimal tension to the tape (e.g., 70 to 200
g). To overcome this problem, it is preferable to provide
tension-controlling means in each of the unit and the sections so
as to separately control the tension of the magnetic tape in each
unit or section. This is described in more detail. As will be
described later in the part of Examples, the first to third suction
rolls (22 to 24) are provided so as not to transmit the tension,
and the values of tension detectors (27 and 28) provided in the
respective units are feedback-controlled via a servo motor for
rotating the suction rolls (22 to 24), so that the magnetic tape
(1) can be run under an optimal tension maintained.
[0060] Hereinafter, the respective elements of a magnetic recording
medium are described.
<Nonmagnetic Support>
[0061] The thickness of a nonmagnetic support is preferably 7.0
.mu.m or less, more preferably from 2.0 to 7.0 .mu.m. When the
thickness of the nonmagnetic support is less than 2 .mu.m, it is
difficult to form a film. Furthermore, the strength of the
resultant magnetic tape decreases. When the thickness of the
nonmagnetic support exceeds 7.0 .mu.m, the total thickness of the
magnetic tape increases so that the recording capacity per one reel
of the magnetic tape decreases.
[0062] The Young's modulus of the nonmagnetic support in the
lengthwise direction depends on the thickness of the support, and
is usually at least 4.9 GPa (500 kg/mm.sup.2). When the thickness
of the support is 5.0 .mu.m or less, the Young's modulus is
preferable at least 9.8 GPa (1,000 kg/mm.sup.2). If the Young's
modulus of the nonmagnetic support is lower than 4.9 GPa (500
kg/mm.sup.2), the strength of the magnetic tape becomes poor, or
the running of the magnetic tape becomes unstable.
[0063] The ratio of Young's modulus MD in the lengthwise direction
to Young's modulus TD in the widthwise direction (MD/TD) of the
nonmagnetic support is preferably from 1.0 to 1.8, more preferably
from 1.1 to 1.7. When the ratio of MD/TD is within this range, the
head touch is improved.
[0064] Examples of such a nonmagnetic support include a
polyethylene terephthalate film, a polyethylene naphthalate film,
an aromatic polyamide film, an aromatic polyimide film, etc.
<Primer Layer>
[0065] A primer layer may be formed between a nonmagnetic support
and a magnetic layer. The thickness of the primer layer is
preferably from 0.3 to 3.0 .mu.m, more preferably from 0.3 to 2.5
.mu.m, particularly 0.3 to 2.0 .mu.m. When the thickness of the
primer layer is less than 0.3 .mu.m, the durability of the magnetic
tape may become poor. When the thickness of the primer layer
exceeds 3.0 .mu.m, the effect to improve the durability of the
magnetic tape is saturated, and the total thickness of the magnetic
tape increases. Accordingly, the length of the tape per one reel
decreases, so that the recording capacity decreases.
[0066] The primer layer may contain carbon black (CB) to improve
the conductivity, and contain nonmagnetic particles to control the
viscosity of a paint and the stiffness of the magnetic tape.
Examples of the nonmagnetic particles to be contained in the primer
layer include titanium oxide, iron oxide, alumina, etc. The
addition of iron oxide alone, or a mixture of iron oxide and
alumina is preferable.
[0067] The surface roughness of the magnetic layer, which is formed
on the primer layer by a wet-on-wet method, can be reduced, when
the primer layer contains 15 to 35 wt. % of carbon black having a
particle size of 10 to 100 nm, 35 to 83 wt. % of nonmagnetic iron
oxide having a major axis length of 0.05 to 0.20 .mu.m and a minor
axis length of 5 to 200 nm, and optionally 0 to 20 wt. % of alumina
having a particle size of 10 to 100 nm, based on the weight of the
total inorganic particles contained in the primer layer.
[0068] The nonmagnetic iron oxide particles may be of a needle
shape, or particulate or random shape. When particulate or random
nonmagnetic iron oxide is used, its particle size is preferably
from 5 to 200 nm.
[0069] The present invention does not avoid the addition of large
size carbon black (CB) having a particle size of 100 nm or more,
provided that the surface smoothness is not impaired. In this case,
preferably, the sum of the small size carbon black (CB) and the
large size carbon black (CB) is within the above range.
[0070] Examples of carbon black (CB) to be added to the primer
layer are acetylene black, furnace black, thermal black, etc. Such
carbon black usually has a particle size of 5 to 200 nm, preferably
10 to 100 nm. When the particle size of carbon black is less than
10 nm, it may be difficult to disperse the carbon black particles,
since carbon black has a structure. When the particle size of
carbon black exceeds 100 nm, the surface smoothness of the primer
layer degrades.
[0071] The amount of carbon black to be contained in the primer
layer varies depending on the particle size of carbon black, and it
is preferably from 15 to 35 wt. %. When the amount of carbon black
is less than 15 wt. %, the conductivity may not be sufficiently
improved. When the amount of carbon black exceeds 35 wt. %, the
effects of the addition of carbon black may saturate. More
preferably, carbon black having a particle size of 15 to 80 nm is
used in an amount of 15 to 35 wt. %, and particularly, carbon black
having a particle size of 20 to 50 nm is used in an amount of 20 to
30 wt. %. When carbon black having the above particle size is used
in the above-defined amount, the electrical resistance is
decreased, and the feeding irregularity is lessened.
[0072] The nonmagnetic iron oxide to be added to the primer layer
preferably has a major axis length of 0.05 to 0.20 .mu.m and a
minor axis length (particle diameter) of 5 to 200 nm in the case of
the needle-shape particles, or it has a particle size of 5 to 200
nm, preferably 5 to 150 nm, more preferably 5 to 100 nm, in the
case of the particulate or random shape particles. In particular,
the needle-shape iron oxide particles are preferable, since the
orientation of the magnetic layer can be improved. The amount of
the nonmagnetic iron oxide to be added to the primer layer is
preferably from 35 to 83 wt. %, more preferably from 40 to 80 wt.
%. When the particle size of the nonmagnetic iron oxide (the minor
axis length in case of the needle shape particle) is less than 5
nm, the iron oxide particles may not be uniformly dispersed. When
the particle size exceeds 200 nm, the unevenness of the interface
between the primer layer and the magnetic layer may increase. When
the amount of the nonmagnetic iron oxide is less than 35 wt. %, the
effect to improve the strength of the primer layer is poor. When
the amount of the iron oxide exceeds 83 wt. %, the strength of the
primer layer may rather decrease.
[0073] The primer layer may contain alumina in addition to iron
oxide. The particle size of alumina is preferably from 10 to 100
nm, more preferably from 20 to 100 nm, particularly from 30 to 100
nm. When the particle size of alumina is less than 10 nm, the
alumina particles may not be uniformly dispersed. When the particle
size of alumina exceeds 100 nm, the unevenness of the interface
between the primer layer and the magnetic layer tends to increase.
The amount of alumina to be added to the primer layer is usually
from 0 to 20 wt. %, preferably from 2 to 10 wt. %.
<Lubricant>
[0074] A coating layer comprising the primer layer and the magnetic
layer may contain a lubricant having a different function. The
coefficient of dynamic friction of the magnetic tape against the
guide of the feeding system or the like can be decreased, for
example, when the primer layer contains 0.5 to 4.0 wt. % of a
higher fatty acid and 0.2 to 3.0 wt. % of a higher fatty acid
ester, based on the weight of the entire powder components in the
primer layer. When the amount of the higher fatty acid is less than
0.5 wt. %, the effect to decrease the coefficient of dynamic
friction is insufficient. When the amount of the higher fatty acid
exceeds 4.0 wt. %, the primer layer may be plasticized and thus the
toughness of the primer layer may be lost. When the amount of the
higher fatty acid ester is less than 0.5 wt. %, the effect to
decrease the coefficient of friction is insufficient. When the
amount of the higher fatty acid ester exceeds 3.0 wt. %, the amount
of the higher fatty acid ester which migrates to the magnetic layer
may become too large, so that the magnetic tape may stick to the
guide or the like of the feeding system.
[0075] As the fatty acid, higher fatty acids such as lauric acid,
myristic acid, palmitic acid, stearic acid, behenic acid, oleic
acid, linoleic acid, etc. can be used. As the fatty acid ester,
butyl stearate, octyl stearate, amyl stearate, isooctyl stearate,
octyl myristate, butoxyethyl stearate, anhydrous sorbitan
monostearate, anhydrous sorbitan distearate, anhydrous sorbitan
tristearate, etc. can be used.
[0076] The coefficient of dynamic friction of the magnetic tape
against the guide roller of the feeding system or the slider of the
MR head can be decreased, when the magnetic layer contains 0.2 to
3.0 wt. % of a fatty acid amide and 0.2 to 3.0 wt. % of a higher
fatty acid ester, based on the weight of the ferromagnetic powder.
When the amount of the fatty acid amide is less than 0.2 wt. %, the
coefficient of dynamic friction between the head slider and the
magnetic layer tends to increase. When the amount of the fatty acid
amide exceeds 3.0 wt. %, the fatty acid amide bleeds out and causes
a defect such as dropout.
[0077] When the amount of the higher fatty acid ester is less than
0.2 wt. %, the coefficient of dynamic friction is hardly decreased.
When the amount of the higher fatty acid ester exceeds 3.0 wt. %,
the magnetic tape sticks to the guide of the feeding system.
[0078] As the fatty acid amide, the amides of the above higher
fatty acids such as palmitic acid, stearic acid and the like can be
used.
[0079] The intermigration between the lubricant of the magnetic
layer and the lubricant of the primer layer may be allowed.
[0080] The coefficient of dynamic friction (.mu..sub.msL) between
the magnetic layer and the slider of the MR head is preferably 0.30
or less, more preferably 0.25 or less. When this coefficient of
dynamic friction exceeds 0.30, the spacing loss tends to arise due
to the contamination on the slider. The coefficient of dynamic
friction of less than 0.10 is hardly realized.
[0081] The coefficient of dynamic friction (.mu..sub.msus) between
the magnetic layer and SUS is preferably from 0.10 to 0.25, more
preferably from 0.12 to 0.20. When this coefficient of dynamic
friction is less than 0.10, the tape is so slidable on the guide
portion that the tape cannot be stably run. When this coefficient
of dynamic friction exceeds 0.25, the guide rollers may easily be
contaminated.
[0082] The ratio of [(.mu..sub.mSL)/(.mu..sub.mSUS)] is preferably
from 0.7 to 1.3, more preferably from 0.8 to 1.2. In this preferred
range, dislocation from a track (off-track) because of the
tape-meandering becomes smaller.
<Magnetic Layer>
[0083] The thickness of a magnetic layer is usually 0.3 .mu.m or
less, preferably from 0.01 to 0.3 .mu.m, more preferably from 0.01
to 0.25 .mu.m, particularly from 0.01 to 0.10 .mu.m.
[0084] When the thickness of the magnetic layer is less than 0.01
.mu.m, it is difficult to form an uniform magnetic layer. When the
thickness of the magnetic layer exceeds 0.3 .mu.m, the reproducing
output may decrease due to the thickness loss, or the product of
the residual magnetic flux density and the thickness becomes too
large, so that the reproducing output tends to be skewed due to the
saturation of the MR head.
[0085] The coercive force of the magnetic layer is preferably from
120 to 320 kA/m, more preferably from 140 to 320 kA/m. When the
coercive force of the magnetic layer is less than 120 kA/m, less
recording wavelength causes output decrease due to the
demagnetizing field demagnetization, when the recording wavelength
is shortened. When the coercive force exceeds 320 kA/m, the
recording with the magnetic head may become difficult.
[0086] The product of the residual magnetic flux density in the
lengthwise direction and the thickness is preferably from 0.0018 to
0.06 .mu.Tm, more preferably from 0.0036 to 0.050 .mu.Tm. When this
product is less than 0.0018 .mu.Tm, the reproducing output with the
MR head may be low. When this product exceeds 0.06 .mu.m, the
reproducing output with the MR head tends to be skewed.
[0087] The contact between the magnetic tape and the MR head can be
improved, and the reproducing output with the MR head increases,
under the following conditions: the average surface roughness Ra of
the magnetic layer is from 3.2 nm to 1.0 nm; and the value of
(P.sub.1-P.sub.0) is from 30 nm to 10 nm, and the value of
(P.sub.1-P.sub.20), 5 nm or less, wherein P is the center value of
the unevenness of the magnetic layer; P.sub.1 is the height of the
highest projection of the magnetic layer; and P.sub.20 is the
height of the 20th highest projection.
[0088] As the magnetic powder to be added to the magnetic layer,
ferromagnetic iron metal powder or hexagonal barium ferrite powder
may be used. The coercive force of the ferromagnetic iron metal
powder or hexagonal barium ferrite powder is preferably from 120 to
320 kA/m. The saturation magnetization is preferably from 120 to
200 Am.sup.2/kg (120 to 200 emu/g), more preferably from 130 to 180
Am.sup.2/kg (130 to 180 emu/g) in case of the ferromagnetic iron
metal powder. It is preferably from 50 to 70 Am.sup.2/kg (50 to 70
emu/g) in case of the hexagonal barium ferrite powder.
[0089] The magnetic characteristics of the magnetic layer and the
ferromagnetic powder are measured with a vibration sample
magnetometer in an external magnetic field of 1.28 MA/m (16
kOe).
[0090] An average major axis length of the ferromagnetic iron metal
powder is preferably from 0.03 to 0.2 .mu.m, more preferably from
0.03 to 0.18 .mu.m, particularly from 0.03 to 0.10 .mu.m. When the
average major axis length is less than 0.03 .mu.m, the dispersion
of the powder in the paint is difficult since the agglomeration
force of the magnetic powder increases. When the average major axis
length exceeds 0.2 .mu.m, the coercive force decreases, or the
particle noise due to the particle size increases. For the same
reason, the plate size of the hexagonal barium ferrite powder is
preferably from 5 to 200 nm, more preferably 10 to 100 nm,
particularly 10 to 50 nm.
[0091] The average major axis length and the particle size are
obtained by actually measuring the particle sizes on a photograph
taken with a scanning electron microscope (SEM) and averaging the
measured values of 100 particles.
[0092] The BET specific surface area of the ferromagnetic iron
metal powder is preferably at least 35 m.sup.2/g, more preferably
at least 40 m.sup.2/g, particularly at least 50 m.sup.2/g as the
best. The BET specific surface area of the hexagonal barium ferrite
powder is preferably 1 to 100 m.sup.2/g.
[0093] A binder to be contained in the primer layer or the magnetic
layer may be a combination of a polyurethane resin and at least one
resin selected from the group consisting of a vinyl chloride resin,
a vinyl chloride-vinyl acetate copolymer resin, a vinyl
chloride-vinyl alcohol copolymer resin, a vinyl chloride-vinyl
acetate-vinyl alcohol copolymer resin, a vinyl chloride-vinyl
acetate-maleic anhydride copolymer resin, a vinyl chloride-hydroxyl
group-containing alkyl acrylate copolymer resin, nitrocellulose,
and the like. Among them, a mixture of a vinyl chloride-hydroxyl
group-containing alkyl acrylate copolymer resin and a polyurethane
resin is preferably used. Examples of the polyurethane resin
include polyesterpolyurethane, polyetherpolyurethane,
polyetherpolyesterpolyurethane, polycarbonatepolyurethane,
polyestrepolycarbonatepolyurethane, etc.
[0094] Preferably, a binder resin such as an urethane resin formed
from a polymer having a functional group such as COOH, SO.sub.3M,
OSO.sub.2M, P.dbd.O(OM).sub.3, O--P.alpha.O(OM).sub.2 [wherein M is
a hydrogen atom, an alkali metal ion or an amine salt], OH,
NR.sup.1R.sup.2, N.sup.+R.sup.3R.sup.4R.sup.5 [wherein R.sup.1,
R.sup.2, R.sup.3, R.sup.4 and R.sup.5 are each a hydrogen atom or a
hydrocarbon group], or an epoxy group is used. The reason why such
a binder is used is that the dispersibility of the magnetic powder
or the like is improved. When two or more resins are used in
combination, it is preferable that the polarities of the functional
groups of the resins are the same. In particular, the combination
of resins both having --SO.sub.3M groups is preferable.
[0095] The binder is used in an amount of 7 to 50 parts by weight,
preferably from 10 to 35 parts by weight, based on 100 parts by
weight of the ferromagnetic powder. In particular, the combination
of 5 to 30 parts by weight of a vinyl chloride-based resin and 2 to
20 parts by weight of the polyurethane resin is best.
[0096] It is preferable to use a thermally curable crosslinking
agent, which bonds with the functional groups in the binder to
crosslink the binder. As the crosslinking agent, the following are
preferably used: tolylene diisocyanate, hexamethylene diisocyanate
and isophorone diisocyanate; reaction products of these isocyanates
with compounds having plural hydroxyl groups such as
trimethylolpropane; condensation products of these isocyanates, and
the like.
[0097] The crosslinking agent is used in an amount of 10 to 50
parts by weight, preferably 10 to 35 parts by weight, based on 100
parts by weight of the binder. When the amount of the crosslinking
agent to be contained in the magnetic layer is about 50% (for
example, 30 to 60%) of that contained in the primer layer, the
coefficient of dynamic friction of the magnetic layer against the
slider of the MR head is preferably decreased. When the amount of
the crosslinking agent is less than 30%, the film strength of the
magnetic layer tends to decrease, while, when it exceeds 60%, the
LRT treatment conditions (the conditions for the wiping treatment
using tissue) should be selected severely so as to decrease the
coefficient of dynamic friction against the slider, which leads to
the increase of cost.
[0098] The magnetic layer may contain conventional carbon black
(CB) to improve the conductivity and the surface lubricity. As
carbon black, acetylene black, furnace black, thermal black, etc.
may be used. Carbon black having a particle size of 5 to 200 nm is
generally used, and carbon black having a particle size of 10 to
100 nm is preferable. When the particle size of carbon black is
less than 5 nm, the dispersion of carbon black particles is
difficult. When the particle size of carbon black exceeds 200 nm, a
large amount of carbon black should be added. In either case, the
surface of the magnetic layer becomes coarse and thus the output
tends to decrease.
[0099] The amount of carbon black is preferably from 0.2 to 5 wt.
%, more preferably from 0.5 to 4 wt. %, based on the weight of the
ferromagnetic powder. When the amount of carbon black is less than
0.2 wt. %, the effect of the addition of carbon black is
insufficient. When the amount of carbon black exceeds 5 wt. %, the
surface of the magnetic layer tends to be rough.
<Backcoat Layer>
[0100] The thickness of a backcoat layer is preferably from 0.25 to
0.8 .mu.m, more preferably from 0.4 to 0.8 .mu.m, particularly from
0.4 to 0.6. When the thickness of the backcoat layer is less than
0.25 .mu.m, the conditions for forming pits for optical servo (the
power of the laser, etc.) are hardly controlled. When the thickness
of the backcoat layer exceeds 0.8 .mu.m, the total thickness of the
magnetic tape increases, so that the recording capacity of the tape
per one reel decreases.
[0101] The coefficient of dynamic friction (.mu.) between the
backcoat layer and SUS is preferably from 0.10 to 0.30, more
preferably from 0.10 to 0.25. When this coefficient of dynamic
friction is less than 0.10, the magnetic tape becomes excessively
slidable on the guide rollers, so that the running of the tape
becomes unstable. When this coefficient of dynamic friction exceeds
0.30, the guide rollers tend to be contaminated. The ratio of
[(.mu..sub.mSL)/(.mu..sub.BSUS)] is preferably from 0.8 to 1.5,
more preferably from 0.9 to 1.4. Within this range, dislocation
from a track (off-track) on the magnetic tape due to the
tape-meandering becomes smaller.
[0102] The average of the reflectance on the flat portion of the
backcoat layer is preferably 8.5% or more, more preferably 9.0% or
more, particularly 10% or more. When the average of the reflectance
is less than 8.5%, servo signals (S) become low, which causes
tracking failure. The practical upper limit of the average of the
reflectance of a backcoat layer is usually 15%. When the average of
the reflectance of the backcoat layer exceeds 15%, the durability
generally may degrade in case of an uniform backcoat layer. In case
where a backcoat layer whose average of the reflectance exceeds 15%
is used, the average of the reflectance on the flat portion other
than the portion where pits for optical servo are formed is
controlled below 15% so that the durability of the backcoat layer
cannot degrade.
[0103] It is preferable that the average of the reflectance is
controlled above 8.5%, and also that the rate of fluctuation of
reflectance on the flat portion depending on a site of the magnetic
tape (a position on the magnetic tape), determined by the following
equation, is controlled below 10%, preferably below 5%, more
preferably below 3%, particularly 0% as the best:
(Maximum of absolute value of fluctuation from average of
reflectance).times.100/(Average of Reflectance)
[0104] When the rate of fluctuation exceeds 10%, the S/N of servo
signals decreases, which induces a tracking error.
[0105] To evaluate the rate of fluctuation of reflectance depending
on a site of a magnetic tape, the fluctuation of reflectance per 40
mm length of the magnetic tape is investigated. This is because the
fluctuation of reflectance per 40 mm length of the magnetic tape is
substantially equal to the fluctuation of reflectance over the
entire length of the magnetic tape.
[0106] To control the average of the reflectance on the flat
portion above 8.5% and simultaneously to control the rate of
fluctuation of reflectance on the flat portion, depending on a site
of the magnetic tape, below 10%, the content of the nonmagnetic
powder in the backcoat layer, calculated by the following equation,
is controlled to 50 wt. % or more:
(Weight of nonmagnetic powder).times.100/(Weight of nonmagnetic
powder+Weight of binder),
and also the surface roughness Ra of the flat portion of the
backcoat layer, measured with an AFM, is controlled to 30 nm or
less; and the half width of the fluctuation of the surface
roughness Ra depending on a site of the magnetic tape is controlled
to 5 nm or less. The surface roughness Ra of the flat portion is
preferably 10 nm or more, more preferably 20 nm or more. When the
Ra is less than 10 nm, the durability of the magnetic tape tends to
degrade. Therefore, it is necessary that, in case where a backcoat
layer having a surface roughness Ra of less than 10 nm at the flat
portion is used, the flat portion of the backcoat layer, other than
the pit-formed portion, should have a surface roughness Ra of 10 nm
or more. In this regard, when the surface roughness Ra is measured
at 100 points per an area of 40 .mu.m.times.40 .mu.m of a magnetic
tape with the AFM, the results are substantially equal to the Ra
per a whole length of the magnetic tape and the fluctuation of Ra
of the same.
[0107] As described above, the reflectance of the flat portion of
the backcoat layer increases, when the content of the nonmagnetic
powder is 50 wt. % or more, and when the surface thereof is smooth.
However, if the content of the nonmagnetic powder in the backcoat
layer is 60 wt. % or more, it is difficult to control the surface
roughness Ra of the flat portion to less than 30 nm. Thus, when the
surface roughness Ra of the flat portion is adjusted to 30 nm or
less by making the calendering conditions severe, the resultant
backcoat layer tends to have a poor durability. For such reasons,
the content of the nonmagnetic powder in the backcoat layer is
practically from 50 to 60 wt. %, preferably from 50 to 58 wt. %,
more preferably from 50 to 56 wt. %, particularly from 53 to 56 wt.
%.
[0108] Preferably, the proportion of carbon black in the
nonmagnetic powder is 80 wt. % or more, because the pits for
optical servo can be easily formed by irradiation with laser beams.
More preferably, the proportion of carbon black is 85 wt. % or
more. The addition of 20 wt. % or less of iron oxide (e.g., red
iron oxide) in combination with carbon black is more preferable,
because the strength of the backcoat layer is enhanced.
[0109] As carbon black (CB) to be contained in the backcoat layer,
acetylene black, furnace black, thermal black, etc. can be used. In
general, carbon black with a small particle size and carbon black
with a large particle size are used. The particle size of small
particle size carbon black is usually from 5 to 200 nm, preferably
from 10 to 100 nm. When the particle size of small particle size
carbon black is less than 10 nm, it is difficult to disperse the
carbon black particles. When the particle size of small particle
size carbon black exceeds 100 nm, a large amount of carbon black
should be added. In either case, the surface roughness Ra of the
backcoat layer is 30 nm or more, and the reflectance on the flat
portion decreases.
[0110] When the large particle size black carbon having a particle
size of 200 to 400 nm is used in an amount of 5 to 15 wt. % of the
whole amount of carbon black (total of the small particle size
carbon black and the large particle size carbon black), the surface
of the backcoat is not roughened and the effect to increase the
tape-running performance is increased. When the amount of the large
particle size black carbon is less than 5 wt. %, the
durability-improving effect is poor. When it exceeds 15 wt. %, the
reflectance on the flat portion largely fluctuates. The total
amount of the small particle size carbon black and the large
particle size carbon black is preferably from 80 to 100 wt. %, more
preferably from 85 to 100 wt. %, based on the weight of nonmagnetic
powder. The surface roughness Ra of the backcoat layer, measured
with the AFM, is preferably 30 nm or less, and it is generally 10
nm or more, as described above.
[0111] To enhance the strength of the backcoat layer, it is
preferable to add 20 wt. % or less of inorganic additives such as
iron oxide (e.g., iron oxide and alumina which are usually added to
the backcoat layer) in total, based on the weight of the inorganic
powder. The addition amount of the inorganic additives is
preferably from 2 to 20 wt. %, more preferably 5 to 15 wt. %. When
this amount is less than 2 wt. %, the strength of the backcoat
layer is not effectively improved. If it exceeds 20 wt. %, the
formation of pits for optical servo by irradiation with laser beams
becomes difficult. In this regard, an oxide mainly containing iron
oxide is preferably used, while iron oxide may be added together
with alumina at the same time. In the latter case, the addition
amount of alumina is preferably 20 wt. % or less based on the
weight of the iron oxide. When the addition amount of alumina
exceeds 20 wt. % based on the weight of the iron oxide, it is
needed to strictly select the conditions for cleaning the burnt
residues. The particle size of iron oxide (particles) is preferably
from 0.05 to 0.4 .mu.m, more preferably from 0.07 to 0.35 .mu.m.
When the particle size of iron oxide is less than 0.05 .mu.m, the
strength of the backcoat layer is hardly improved. On the other
hand, when it exceeds 0.4 .mu.m, the reflectance on the flat
portion largely fluctuates.
[0112] The binder to be contained in the backcoat layer may
comprise the same resin as used in the magnetic layer and the
primer layer. Among these resins, the combination of a cellulose
resin and a polyurethane resin is preferably used so as to decrease
the coefficient of friction and to improve the tape-running
performance.
[0113] The amount of the binder in the backcoat layer is usually
from 40 to 150 parts by weight, preferably from 50 to 120 parts by
weight, more preferably from 50 to 110 parts by weight,
particularly from 50 to 100 parts by weight, based on the total 100
parts by weight of carbon black and the inorganic nonmagnetic
powder in the backcoat layer. When the amount of the binder is less
than 50 parts by weight, the strength of the backcoat layer is
insufficient. When the amount of the binder exceeds 120 parts by
weight, the coefficient of friction increases. Preferably, 30 to 70
parts by weight of a cellulose resin and 20 to 50 parts by weight
of a polyurethane resin are used. To cure the binder, a
crosslinking agent such as a polyisocyanate compound is preferably
used.
[0114] The crosslinking agent to be contained in the backcoat layer
may be the same as those used in the magnetic layer and the primer
layer. The amount of the crosslinking agent is usually from 10 to
50 parts by weight, preferably from 10 to 35 parts by weight, more
preferably from 10 to 30 parts by weight, based on 100 parts by
weight of the binder. When the amount of the crosslinking agent is
less than 10 parts by weight, the film strength of the backcoat
layer tends to decrease. When the amount of the crosslinking agent
exceeds 50 parts by weight, the coefficient of dynamic friction of
the backcoat layer against SUS increases.
<LRT (Lapping/Rotary/Tissue) Treatment>
[0115] The magnetic layer is subjected to a LRT treatment so as to
optimize the surface smoothness, the coefficient of dynamic
friction against the slider of the MR head and the cylinder
material, the surface roughness and the surface shape. Thus, the
running performance of the magnetic tape and the reproducing output
with the MR head are improved, and the spacing loss is reduced.
[0116] The respective steps of the LRT treatment are described
below.
[0117] (1) Lapping:
[0118] An abrasive tape (lapping tape) is moved by a rotary roll at
a constant rate (standard: 14.4 cm/min.) in a direction opposite to
the tape-feeding direction (standard: 400 m/min.), and is allowed
to contact with the surface of the magnetic layer of the magnetic
tape while being pressed under the guide block. In this step, the
magnetic layer is polished while the unwinding tension of the
magnetic tape and the tension of the lapping tape being maintained
constant (standard: 100 g and 250 g, respectively).
[0119] The abrasive tape (lapping tape) (3) used in this step may
be an abrasive tape (lapping tape) with fine abrasive particles
such as M20000, WA10000 or K10000. It is possible to use an
abrasive wheel (lapping wheel) in place of or in combination with
the abrasive tape (lapping tape). In case where frequent
replacement is needed, the abrasive tape (lapping tape) alone is
used.
[0120] (2) Rotary Treatment
[0121] A rotary wheel having air-bleeding grooves (standard: width
of 1 inch (25.4 mm); diameter of 60 mm.phi.; air-bleeding groove
width of 2 mm; groove angle of 45 degrees, manufactured by KYOWA
SEIKO Co., Ltd.) is rotated at a constant revolution rate (usually
200 to 3,000 rpm; standard: 1,100 rpm) in a direction opposite to
the feeding direction of the magnetic layer, while being allowed to
be in contact with the magnetic layer of the magnetic tape at a
constant contact angle (standard: 90 degrees). Thus, the surface of
the magnetic layer is treated.
[0122] (3) Tissue treatment
[0123] Tissue (a woven fabric, for example, Traysee manufactured by
Toray) is fed at a constant rate (standard: 14.0 mm/min.) by rotary
rods, in a direction opposite to the feeding direction of the
magnetic tape, so as to clean the surfaces of the backcoat layer
and the magnetic layer of the magnetic tape, respectively.
[0124] A cassette tape including a magnetic tape of the present
invention shows a high S/N with respect to optical servo signals,
and thus is excellent in servo tracking performance. Therefore,
such a cassette tape can be used as a backup tape for a hard disc
drive, with high reliability.
EXAMPLES
[0125] The present invention will be explained in detail by way of
the following Examples, which do not limit the scope of the
invention in any way. In Examples and Comparative Examples, "parts"
are "parts by weight", unless otherwise specified.
Example 1
Components of a Paint for a Primer Layer
TABLE-US-00001 [0126] (1) Iron oxide powder (particle size: 0.11
.times. 0.02 .mu.m) 68 parts .alpha.-Alumina (particle size: 0.07
.mu.m) 8 parts Carbon black (particle size: 25 nm; 24 parts oil
absorption: 55 g/cc) Stearic acid 2.0 parts Vinyl
chloride-hydroxypropyl acrylate copolymer 8.8 parts (--SO.sub.3Na
group content: 0.7 .times. 10.sup.-4 eq./g) Polyesterpoyurethane
resin 4.4 parts (Tg: 40.degree. C., --SO.sub.3Na group content: 1
.times. 10.sup.-4 eq./g) Cyclohexanone 25 parts Methyl ethyl ketone
40 parts Toluene 10 parts (2) Butyl stearate 1 part Cyclohexanone
70 parts Methyl ethyl ketone 50 parts Toluene 20 parts (3)
Polyisocyanate 4.4 parts (Colonate L manufactured by Nippon
Polyurethane) Cyclohexanone 10 parts Methyl ethyl ketone 15 parts
Toluene 10 parts
<Components of a Paint for a Magnetic Layer>
TABLE-US-00002 [0127] (A) Ferromagnetic iron metal powder 100 parts
(Co/Fe: 30 atomic %, Y/(Fe + Co): 3 atomic %, Al/(Fe + Co): 5 wt.
%, Ca/Fe: 0; .sigma.s: 155 Am.sup.2/kg, Hc: 188.2 kA/m, pH: 9.4,
major axis length: 0.10 .mu.m) Vinyl chloride-hydroxypropyl
acrylate copolymer 12.3 parts (--SO.sub.3Na group content: 0.7
.times. 10.sup.-4 eq./g) Polyesterpolyurethane resin 5.5 parts
(--SO.sub.3Na group content: 1 .times. 10.sup.-4 eq./g)
.alpha.-Alumina (particle size: 0.12 .mu.m) 8 parts .alpha.-Alumina
(particle size: 0.07 .mu.m) 2 parts Carbon black (particle size: 75
nm; 1.0 part DBP oil absorption: 72 cc/100 g) Methyl acid phosphate
2 parts Palmitic acid amide 1.5 parts n-Butyl stearate 1.0 part
Tetrahydrofuran 65 parts Methyl ethyl ketone 245 parts Toluene 85
parts (B) Polyisocyanate (Colonate L manufactured by 2.0 parts
Nippon Polyurethane Kogyo K.K.) Cyclohexanone 167 parts
[0128] A paint for a primer layer was prepared by kneading the
components of Group (1) with a kneader, adding the components of
Group (2) to the mixture, and stirring them, dispersing the mixture
in a sand mill in residence time of 60 minutes, and adding the
components of Group (3), followed by stirring and filtering the
mixture.
[0129] Separately, a paint for a magnetic layer was prepared by
kneading the components of Group (A) with a kneader, dispersing the
mixture in a sand mill in residence time of 45 minutes, and adding
the components of Group (B), followed by stirring and filtering the
mixture.
[0130] The paint for primer layer was applied on a nonmagnetic
support composed of a polyethylene naphthalate film (thickness of
6.2 .mu.m, MID=6.08 GPa, MD/TD=1.1; manufactured by TEIJIN) so that
the primer layer could have a thickness of 1.8 .mu.m after dried
and calendered, and then, the paint for magnetic layer was applied
on the primer layer by a wet-on-wet method so that the magnetic
layer could have a thickness of 0.15 .mu.m after oriented in a
magnetic field, dried and calendered. After the orientation in the
magnetic field, the magnetic layer was dried with a drier to obtain
a magnetic sheet. The orientation in the magnetic field was carried
out by arranging N-N opposed magnets (5 kG) in front of the drier,
i.e., arranging two N-N opposed magnets (5 kG) spaced 50 cm from
each other, at a position 75 cm before a position where the dryness
of the layer was confirmed by one's fingers within the drier. The
coating rate was 100 m/min.
<Components of a Paint for a Backcoat Layer>
TABLE-US-00003 [0131] Carbon black (particle size: 25 nm) 78 parts
(41.5 wt. %) Carbon black (particle size: 350 nm) 10 parts (5.3 wt.
%) [Total carbon black: 88 parts (46.8 wt. %)] Red iron oxide A 10
parts (5.3 wt. %) (particle size: 0.1 .mu.m) Red iron oxide B 2
parts (1.1 wt. %) (particle size: 0.27 .mu.m) [Total nonmagnetic
powder: 100 parts (53.2 wt. %)] Nitrocellulose (NC) 44 parts (23.4
wt. %) Polyurethane resin 31 parts (16.4 wt. %) (containing
--SO.sub.3Na groups) Cyclohexanone 260 parts Toluene 260 parts
Methyl ethyl ketone 525 parts
[0132] The components of a paint for a backcoat layer were
dispersed in a sand mill in residence time of 45 minutes and a
polyisocyanate (13 parts, 6.9 wt. %) was added to the mixture to
obtain a paint for backcoat layer. After filtration, the paint was
coated on a surface of the above magnetic sheet opposite to the
magnetic layer so that the backcoat layer could have a thickness of
0.5 .mu.m after dried and calendered, and then, the backcoat layer
was dried to finish the magnetic sheet.
[0133] The magnetic sheet obtained was planished by seven-stage
calendering using metal rolls at a temperature of 100.degree. C.
under a linear pressure of 147 kN/m (150 kgf/cm), and wound onto a
core and aged at 70.degree. C. for 72 hours. The magnetic sheet was
cut into a plurality of magnetic tapes with a width of 1/2 inch.
Then, the magnetic tape was subjected to LRT treatment under the
following conditions. Then, pits for optical servo were formed on
the backcoat layer, using the apparatus for forming and cleaning
optical servo tracks shown in FIG. 7. The backcoat layer was
cleaned by spraying solid CO.sub.2 thereto. The magnetic tape thus
obtained was set in a cartridge to provide a tape for use in a
computer. The apparatus for forming and cleaning optical servo
tracks, and the treatment using the same apparatus will be
described later.
<LRT (Lapping/Rotary/Tissue) Treatment>
[0134] (1) Lapping
[0135] An abrasive tape (lapping tape) was moved by rotary rolls at
a rate of 14.4 cm/min. in a direction opposite to the feeding
direction of the magnetic tape (400 m/min.), while being pressed
down from above by a guide block (4) to contact with the surface of
the magnetic layer of the magnetic tape. In this step, the magnetic
layer was polished while the unwinding tension of the magnetic tape
being maintained at 100 g and the tension of the lapping tape, at
250 g.
[0136] (2) Rotary Aluminum Wheel Treatment
[0137] An aluminum rotary wheel which had a width of 1 inch. (25.4
mm), a diameter of 60 mm, and air-bleeding grooves with a width of
2 mm (the angle of groove: 45 degrees, manufactured by KYOWA SEIKO
Co., Ltd.;) was rotated at a revolution rate of 1,100 rpm in a
direction opposite to the feeding direction of the magnetic tape,
in contact with the magnetic layer of the magnetic tape at a
contact angle of 90 degrees. Thus, the surface of the magnetic
layer was treated.
[0138] (3) Tissue Treatment
[0139] A tissue (a woven fabric: Toraysee manufactured by Toray)
was fed at a rate of 14.0 mm/min. in a direction opposite to the
feeding direction of the magnetic tape by rotary bars to clean the
surfaces of the backcoat layer and the magnetic layer of the
magnetic tape.
[0140] Hereinafter, the treatments using the apparatus for forming
and cleaning optical servo tracks, as mentioned above, are
described.
[0141] As shown in FIG. 7, the apparatus for forming and cleaning
optical servo tracks comprises a tape-feeding mechanism (11) for
feeding a reeled magnetic tape (1) in a predetermined direction; an
optical servo track-forming unit (12) for forming pits for optical
servo on the surface of the backcoat layer of the fed magnetic tape
(1) by irradiation with laser beams; a cleaning unit (13) for
cleaning the surface of the backcoat layer after the formation of
the pits; and a tape-winding mechanism for winding the magnetic
tape (1) after cleaning.
[0142] In the cleaning unit (13), there are arranged a
CO.sub.2-spraying section equipped with a spray nozzle (15) for
spraying solid CO.sub.2 onto the above pits and their peripheries;
a sucking section equipped with a suction nozzle (a sucking means)
(16) for sucking the burnt residues which have been blown by the
solid CO.sub.2-spraying and adhered to the pits and their
peripheries; and a wiping section (17) for wiping the surface of
the backcoat layer with a tissue cleaner after the suction of the
burnt residues.
[0143] The spray nozzle (15) provided in the CO.sub.2-spraying
section has CO.sub.2-spraying orifices (15a) which correspond to a
pattern for tracks of pits for optical servo, arranged in the
widthwise direction of the magnetic tape (1), as shown in FIG. 3.
The spray nozzle (15) is set above the surface of the backcoat
layer (2) of the magnetic tape (1), inclining by 30.degree. thereto
(see FIG. 3). The solid CO.sub.2 is obliquely sprayed onto a
CO.sub.2-receiving portion (B) of the backcoat layer (2), toward a
direction reverse to the tape-running direction. The suction nozzle
(16) provided in the sucking section has suction ports (16a) which
are located in the vicinity of the above CO.sub.2-receiving portion
(B) of the backcoat layer. The nozzle (16) sucks the burnt residues
which have been separated from the surface of the backcoat layer by
the spraying of the solid CO.sub.2, through its suction ports (16a)
and removes them from the surface of the backcoat layer.
[0144] The wiping section (17) comprises tissue cleaners (18, 19)
arranged so as to contact with the surfaces of the magnetic layer
and the backcoat layer of the magnetic tape (1), and a respective
pair of rollers (20, 21) which hold and wind the tissue cleaners
(18, 19) at predetermined speeds. The tissue cleaners (18, 19) are
pressed against the surfaces of the magnetic layer and the backcoat
layer of the magnetic tape (1), respectively, to wipe and remove
unwanted particles adhered thereto.
[0145] In addition, the apparatus shown in FIG. 7 is provided with
tension-controlling means as follows. That is, a first suction roll
(22) is arranged between the optical servo track-forming unit (12)
and the suction nozzle (16); a second suction roll (23), between
the spray nozzle (15) and the wiping section (17); and a third
suction roll (24), between the wiping section (17) and the winding
unit (14). A tension arm (25) is arranged between the tape-feeding
mechanism (11) and the optical servo track-forming unit (12); and a
tension arm (26), between the third suction roll (24) and the
winding mechanism (14). The tension arms (25, 26) are to control
the tension of the magnetic tape (1). Further, a tension detector
(27) is arranged between the second suction roll (23) and the spray
nozzle (15); and a tension detector (28), between the second
suction roll (23) and the wiping section (17). The tension
detectors (27, 28) are provided to detect the tension of the
magnetic tape (1) and control the tension thereof. Thus, an optimal
tension is separately applied to the magnetic tape (1) at each of
the optical servo track-forming unit (12), and the
CO.sub.2-spraying section and the wiping section (17) in the
cleaning unit (13), by interrupting the transmission of the tension
of the magnetic tape (1) via each of the suction rolls (22 to 24),
and feedback-controlling the values of the tension detectors (27,
28) via a servo motor which rotates each of the suction rolls (22
to 24).
[0146] In the Examples of the present invention, the above
apparatus was used to run the magnetic tape at 10 m/sec. while
maintaining the tension of the magnetic tape at 150 g, so as to
form a pattern of pits for optical servo as described below, on the
magnetic tape, to spray solid CO.sub.2, and to clean the tape by
removing the burnt residues.
<Formation of Pattern of Pits for Optical Servo>
[0147] In the optical servo track-forming unit (12) of the
apparatus shown in FIG. 7, the surface of the backcoat layer of the
magnetic tape (1) was irradiated with laser beams to form pits for
optical servo thereon. As a pattern of pits for optical servo,
tracks of pits were formed as four bands each having a width of
about 0.4 mm, and such four bands were widthwise arranged on the
tape which had a width of 12.64 mm, as shown in FIG. 6.
<Solid CO.sub.2-Spraying Treatment>
[0148] Next, the solid CO.sub.2-spraying nozzle (15) and the
suction nozzle (16) were used to remove substantially all of burnt
residues which had been formed by the above irradiation with laser
beams. The spray nozzle (15) was inclined by 30.degree. relative to
the surface of the backcoat layer, as described above.
<Cleaning Treatment>
[0149] Finally, the tissue cleaners (18, 19) provided in the wiping
section (17) were used to completely remove the residual burnt
residues. Thus, the finished magnetic tape could have a Br.delta.
of 0.045 .mu.Tm (the product of the residual magnetic flux density
and the thickness of the magnetic layer in the tape lengthwise
direction), and a coercive force He of 192 kA/m.
Example 2
[0150] A magnetic tape was obtained substantially in the same
manner as in Example 1, except that the calendering conditions were
changed, that is, the temperature and the linear pressure were
changed from 100.degree. C. and 147 kN/m (150 kgf/cm) to 90.degree.
C. and 294 kN/m (300 kgf/cm).
Example 3
[0151] A magnetic tape was obtained substantially in the same
manner as in Example 1, except that the calendering conditions were
changed, that is, the temperature and the linear pressure were
changed from 100.degree. C. and 147 kN/m (150 kgf/cm) to
120.degree. C. and 294 kN/m (300 kgf/cm).
Example 4
[0152] A magnetic tape was obtained substantially in the same
manner as in Example 1, except that the thickness of the backcoat
layer was changed from 0.5 .mu.m to 0.4 .mu.m.
Example 5
[0153] A magnetic tape was obtained substantially in the same
manner as in Example 1, except that the thickness of the backcoat
layer was changed from 0.5 .mu.m to 0.6 .mu.m.
Example 6
[0154] A magnetic tape having a total thickness of 5.7 .mu.m,
Br.delta. of 0.030 .mu.Tm, and a coercive force He of 192 kA/m was
obtained substantially in the same manner as in Example 1, except
that a nonmagnetic support with a thickness of 4.0 .mu.m, a primer
layer with a thickness of 1.0 .mu.m and a magnetic layer with a
thickness of 0.1 .mu.m were used, and that the thickness of the
backcoat layer was changed from 0.5 .mu.m to 0.6 .mu.m.
Examples 7 to 10
[0155] Magnetic tapes were obtained substantially in the same
manner as in Example 1, except that backcoat layers having the
composition ratios shown in Table 1 were used.
Reference Example 1
[0156] A magnetic tape was obtained substantially in the same
manner as in Example 1, except that the solid CO.sub.2-spraying
treatment was omitted.
Comparative Examples 1 to 6
[0157] Magnetic tapes were obtained substantially in the same
manner as in Example 1, except that backcoat layers having the
thickness and the composition ratios shown in Table 2 were
used.
Comparative Example 7
[0158] A magnetic tape having a total thickness of 5.7 .mu.m was
obtained substantially in the same manner as in Comparative Example
3, except that a nonmagnetic support with a thickness of 4.0 .mu.m,
a primer layer with a thickness of 1.0 .mu.m and a magnetic layer
with a thickness of 0.1 .mu.m were used, and that the thickness of
the backcoat layer was changed from 0.5 .mu.m to 0.6 .mu.m.
TABLE-US-00004 TABLE 1 Ex. 1 Ex. 7 Ex. 8 Ex. 9 Ex. 10 Small
particle size CB 41.5 wt. % 40.0 wt. % 42.1 wt. % 47.0 wt. % 47.3
wt. % Particle size (nm) 25 25 25 17 25 Large particle size CB 5.3
wt. % 5.0 wt. % 7.4 wt. % 2.5 wt. % 6.8 wt. % Particle size (nm)
350 370 370 280 280 Ratio of large particle 11.3 wt. % 11.1 wt. %
11.2 wt. % 5.0 wt. % 12.5 wt. % size CB*.sup.1 Ratio of CB*.sup.2
88.0 wt. % 90.0 wt. % 89.9 wt. % 89.3 wt. % 90.9 wt. % Red iron
oxide A 5.3 wt. % 5.0 wt. % 4.9 wt. % 5.4 wt. % 4.5 wt. % Particle
size (.mu.m) 0.1 0.1 0.1 0.1 0.1 Red iron oxide B 1.1 wt. % -- 1.0
wt. % 1.5 wt. % 0.9 wt. % Particle size (.mu.m) 0.27 0.20 0.20 0.20
BaSO.sub.4 -- -- -- -- -- Particle size (.mu.m) Ratio of
nonmagnetic 53.2 wt. % 50.0 wt. % 55.4 wt. % 55.4 wt. % 59.5 wt. %
powder NC 23.4 wt. % 25.0 wt. % 22.3 wt. % 22.3 wt. % 20.3 wt. %
Polyurethane 16.4 wt. % 17.5 wt. % 14.8 wt. % 14.9 wt. % 13.5 wt. %
Crosslingking agent 6.9 wt. % 7.5 wt. % 7.4 wt. % 7.4 wt. % 6.8 wt.
% Notes: *.sup.1The ratio of large particle size carbon black in a
whole of carbon black. *.sup.2The ratio of carbon black in
nonmagnetic powder.
TABLE-US-00005 TABLE 2 Com. Ex. 1 Com. Ex. 2 Com. Ex. 3 Com. Ex. 4
Com. Ex. 5 Com. Ex. 6 Small particle size CB 38.6 wt. % 33.8 wt. %
35.9 wt. % 42.8 wt. % 38.7 wt. % 38.7 wt. % Particle size (nm) 17
17 17 17 17 17 Large particle size CB 1.6 wt. % -- -- 0.09 wt. %
7.8 wt. % 3.9 wt. % Particle size (nm) 270 280 280 280 Ratio of
large particle 4.0 wt. % -- -- 2.0 wt. % 16.8 wt. % 9.2 wt. % size
CB*.sup.1 Ratio of CB*.sup.2 90.3 wt. % 100.0 wt. % 100.0 wt. %
99.7 wt. % 95.9 wt. % 87.8 wt. % Red iron oxide A -- -- -- -- --
3.9 wt. % Particle size (.mu.m) 0.1 Red iron oxide B -- -- -- 0.04
wt. % 0.03 wt. % 0.1 wt. % Particle size (.mu.m) 0.2 0.2 0.2
BaSO.sub.4 4.3 wt. % -- -- -- 1.9 wt. % 1.9 wt. % Particle size
(.mu.m) 0.05 0.05 0.05 Ratio of nonmagnetic 44.5 wt. % 33.8 wt. %
35.9 wt. % 43.0 wt. % 48.5 wt. % 48.5 wt. % powder NC 27.9 wt. %
20.2 wt. % 19.6 wt. % 42.8 wt. % 38.7 wt. % 38.7 wt. % Polyurethane
18.4 wt. % 33.8 wt. % 32.7 wt. % 12.9 wt. % 11.6 wt. % 11.6 wt. %
Crosslingking agent 9.3 wt. % 12.2 wt. % 11.8 wt. % 1.3 wt. % 1.2
wt. % 1.2 wt. % Notes: *.sup.1The ratio of large particle size
carbon black in a whole of carbon black. *.sup.2The ratio of carbon
black in nonmagnetic powder.
Example 11
[0159] Pits for optical servo were formed on the backcoat layer of
a magnetic tape, using an apparatus for forming and cleaning
optical servo tracks shown in FIG. 9. After that, a tape for a
computer was obtained substantially in the same manner as in
Example 1, except that a contact-removing treatment using a raised
fabric or the like, and a cleaning treatment, as described below,
were carried out on the magnetic tape.
[0160] The apparatus shown in FIG. 9 and the treatments using this
apparatus are explained.
[0161] The apparatus used in Example 11 comprises, as shown in FIG.
9, a tape-feeding mechanism (11) for feeding a reeled magnetic tape
(1) in a predetermined direction; an optical servo track-forming
unit (12) for forming pits for optical servo on the surface of the
backcoat layer of the fed magnetic tape (1) by irradiation with
laser beams; a cleaning unit (13) for cleaning the surface of the
backcoat layer (2) after the formation of the pits; and a
tape-winding mechanism (14) for winding the magnetic tape (1) after
cleaning.
[0162] The cleaning unit (13) comprises a contact-removing section
for allowing raised cloth such as a raised fabric or woven or
nonwoven fabric having raising fibers thereon, to contact with the
surface of the backcoat layer, in order to clean the surface of the
backcoat layer, and a wiping section (17) for wiping the surfaces
of the backcoat layer and the magnetic layer with tissue
cleaners.
[0163] In the contact-removing section (15b), a wrapped drum (30)
as shown in FIG. 8 is arranged. The wrapped drum (30) comprises a
rotary drum (31) (having a diameter of 100 mm in this Example)
which is rotated in a direction reverse to the running direction of
the magnetic tape (1) and is wrapped with raised cloth (32) at its
circumference. A pair of guide rollers (41, 41) are arranged before
and after the wrapped drum (30) so as to allow the drum (30) to
contact with the surface of the backcoat layer of the magnetic tape
(1) in a predetermined condition.
[0164] The wiping section (17) includes tissue cleaners (18, 19)
which are arranged so as to contact with the surfaces of the
magnetic layer and the backcoat layer of the magnetic tape (1),
respectively, and two pairs of rollers (20, 21) which hold the
tissue cleaners (18, 19), respectively, so that the tissue cleaners
can be wound at predetermined velocities. The tissue cleaners (18,
19) are pressed against the surfaces of the magnetic layer and the
backcoat layer, respectively, so as to wipe and remove the unwanted
particles thereon.
[0165] In addition, the apparatus shown in FIG. 9 is provided with
means for controlling the tension of the tape described below. That
is, a first suction roll (22) is arranged between the optical servo
track-forming unit (12) and the contact-removing section (15b); a
second suction roll (23), between the contact-removing section
(15b) and the wiping section (17); and a third suction roll (24),
between the wiping section (17) and the winding mechanism (14). A
tension arm (25) is arranged between the tape-feeding mechanism
(11) and the optical servo track-forming unit (12); and a tension
arm (26), between the third suction roll (24) and the winding
mechanism (14). The tension arms (25, 26) are provided to control
the tension of the magnetic tape (1). Further, a tension detector
(27) is arranged between the second suction roll (23) and the
contact-removing section (15b); and a tension detector (28),
between the second suction roll (23) and the wiping section (17).
The tension detectors (27, 28) are provided to detect the tension
of the magnetic tape (1) and control the tension thereof. An
optimal tension is separately applied to the magnetic tape (1) at
each of the optical servo track-forming unit (12), and the
contact-removing section (15b) and the wiping section (17) in the
cleaning unit (13), by interrupting the transmission of the tension
of the magnetic tape (1) via each of the suction rolls (22 to 24),
and feedback-controlling the values of the tension detectors (27,
28) via a servo motor which rotates each of the suction rolls (22
to 24).
[0166] In the Examples of the present invention, the above
apparatus was used to run the magnetic tape at 10 m/sec. while
maintaining the tension of the magnetic tape constant, so as to
form a pattern of pits for optical servo on the magnetic tape, to
treat the magnetic tape by contacting the wrapped drum (30), and to
wipe the same with the tissue cleaners (18, 19) for cleaning.
<Formation of Pattern of Pits for Optical Servo>
[0167] In the optical servo track-forming unit (12) of the
apparatus shown in FIG. 9, the surface of the backcoat layer of the
magnetic tape (1) was irradiated with laser beams so as to form a
predetermined pattern of pits for optical servo thereon. As the
pattern of pits for optical servo, tracks of pits were formed as
four bands each having a width of about 0.4 mm, and such four bands
were widthwise arranged on the tape which had a width of 12.64 mm,
as shown in FIG. 6.
<Contact-Removing Treatment using Wrapped Drum>
[0168] Next, in the contact-removing section (15b), the wrapped
drum (30) was rotated at a rate of 314 radian/sec. (3,000 rpm) in a
direction reverse to the tape-running direction, while the raised
cloth (32) wrapping the outer circumference of the drum (30) was
allowed to contact with the surface of the backcoat layer (2) of
the magnetic tape (1) under a tension of 2.0 N to remove
substantially all of the burnt residues which had formed by baking
with laser beams in the step of forming the servo pattern, from the
interiors of the pits and their peripheries on the backcoat layer.
As the raised cloth (32) wrapping the drum (30), velvet on which
2.5 mm yarns obtained by twisting 4 cotton single fibers having a
diameter of 4 .mu.m were flocked was used. The tension applied on
the side of the inlet was 86 g, and that on the side of the outlet,
208 g. The contact angle between the wrapped drum (30) and the
magnetic tape (1) was 120.degree..
<Cleaning Treatment>
[0169] Finally, the tissue cleaners (18, 19) provided in the wiping
section (17) were used to completely remove the residual burnt
residues. Thus, the finished magnetic tape could have a Br.delta.
of 0.045 .mu.Tm, and a coercive force He of 192 kA/m. The average
reflectance of the magnetic tape was 9.0%; the rate of fluctuation
thereof, 3.0%; the surface roughness Ra thereof, measured with AFM,
25.1 nm; and the half width of Ra, 3.3 nm. The S/N of the servo
signal of Example 11 was 1.5 dB, on the assumption that the S/N of
the servo signal of Reference Example 2 was 0 dB, and it was 6.1
dB, on the assumption that the S/N of the servo signal of
Comparative Example 1 was 0 dB.
Example 12
[0170] A magnetic tape was obtained substantially in the same
manner as in Example 11, except that the contact angle between the
magnetic tape (1) and the wrapped drum (30) was changed to
90.degree..
Example 13
[0171] A magnetic tape was obtained substantially in the same
manner as in Example 11, except that the rotation velocity of the
wrapped drum (30) was changed to 188.4 radian/sec. (1,800 rpm). The
tension of the tape on the side of the inlet was 95 g, and that on
the side of the outlet, 188 g, in the step of the contact-removing
treatment (treatment at the contact-removing section).
Example 14
[0172] A magnetic tape was obtained substantially in the same
manner as in Example 11, except that the tension of the tape being
subjected to the contact-removing treatment with the wrapped drum
was changed to 1.8 N. The tension of the tape on the side of the
inlet was 80 g, and that on the side of the outlet, 188 g, in the
step of the contact-removing treatment.
Example 15
[0173] A magnetic tape having a total thickness of 5.7 .mu.m, a
Br.delta. of 0.030 .mu.Tm, and a coercive force of 192 kA/m was
obtained substantially in the same manner as in Example 11, except
that a nonmagnetic support with a thickness of 4.0 .mu.m, a primer
layer with a thickness of 1.0 .mu.m and a magnetic layer with a
thickness of 0.1 .mu.m were used, and that the thickness of the
backcoat layer was changed from 0.5 .mu.m to 0.6 .mu.m.
Examples 16 to 19
[0174] Magnetic tapes were obtained substantially in the same
manner as in Example 11, except that raised fabrics, or woven or
nonwoven cloths having raising fibers thereon, shown in Table 3,
were used as the raised cloth, and that the number of wrapped drums
used were changed as shown in Table 8.
Reference Example 2
[0175] A magnetic tape was obtained substantially in the same
manner as in Example 11, except that the contact-removing treatment
using the wrapped drum (30) was omitted. The reflectance on the
backcoat layer of the resultant magnetic tape was 8.5%; the rate of
fluctuation thereof, 4.0%; the surface roughness Ra thereof,
measured with AFM, 25.2 nm; and the half width of Ra, 4.5 nm.
TABLE-US-00006 TABLE 3 Ex. 11-15 Ex. 16 Ex. 17 Ex. 18 Ex. 19 Raised
fabric Cotton Cotton Rayon Polyester Polyester pile long pile short
short short fiber long fiber fiber fiber fiber Kind of fiber Cotton
Cotton Rayon Polyester Polyester Diameter of 4 .mu.m 4 .mu.m 5
.mu.m 2 .mu.m 10 .mu.m* single fiber Number of 2 4 1 1 1 fibers
twisted Length of fiber 2.5 mm 1.65 mm 1.5 mm 0.5 mm 4 mm *The tip
end of the single fiber was split into 8 pieces.
[0176] The measurement and the evaluation were conducted as
follows.
<Reflectance>
[0177] The reflectance on the flat portion of a magnetic tape was
evaluated using a spectrometer (UNISOF), on condition that the
incident angle was 20.degree., and the reflection angle,
20.degree.. A light emitting diode (or LED) of a wavelength of 880
nm was used as a light source. The spot diameter was 100 .mu.m. The
reflectance was measured at 400 points per 40 mm of the magnetic
tape to determine the average reflectance and the maximum rate of
fluctuation thereof. The average reflectance was a simple average
value of the reflectance, and the maximum rate of fluctuation was
the percentage of a value which was found by dividing the maximum
of the deviation from the average reflectance by the average
reflectance. The reflectance on the flat portion of the magnetic
tape which had been run, and the maximum rate of fluctuation
thereof were measured by running the magnetic tape twice with a LTO
drive, and cutting the magnetic tape for the measurement.
<Evaluation of Surface Roughness Ra with AFM>
[0178] The average surface roughness Ra of a magnetic tape was
measured using an AFM (Dimension TM 3100 manufactured by
Digital-Instrument Co., Ltd.). The scanning mode was a tapping mode
AFM. In the tapping mode, a cantilever equipped at its tip end with
a probe was vibrated with around a resonant frequency (about 50 to
about 500 kHz) using a piezo-vibrator, while the probe was being
allowed to intermittently and softly touch (or tap) the surface of
the tape sample so as to scan the tape. A change in the amplitude
of the cantilever due to the unevenness of the surface of the
sample was evaluated using laser beams. The field of view for the
measurement was 40 .mu.m.times.40 .mu.m. The fluctuation of the
surface roughness Ra depending on a site of the magnetic tape was
determined from the half width of the fluctuation of Ra as follows:
the surface roughness Ra was measured at 100 points which were
spaced at regular intervals per 40 mm length of the magnetic tape,
and Ra at each point was plotted on the axis of abscissa, and the
frequency (1 nm pitch), on the axis of ordinate, so that the half
width of the fluctuation of Ra was determined from the resultant
graph.
<S/N of Servo Signal on Servo Track>
[0179] Light with a center wavelength of 880 nm was caused to emit
onto the backcoat layer at an incident angle of 20.degree., and the
S/N of a servo signal was measured from the reflecting light, using
the servo signal-measuring section of a floptical drive. The S/N of
the servo signals of Examples 1 to 10, Reference Example 1 and
Comparative Examples 2 to 7 were represented as relative values
based on the S/N of Comparative Example 1 as a standard (0 dB). The
S/N of the servo signals of Examples 11 to 19 were represented as
relative values based on the S/N of Reference Example 2 as a
standard (0 dB).
<Measurement of Error Rate>
[0180] The error rate (or ERT) was measured by recording and
reproducing signals on and from a magnetic tape (recording
wavelength: 0.37 .mu.m) using a LTO drive which was improved so as
to be used on a thinner tape. The ERT was a value obtained in the
test mode.
<Evaluation of Magnetic Properties>
[0181] The magnetic properties of a magnetic layer and
ferromagnetic powder were evaluated using a vibration sample
magnetometer (manufactured by Toei Kogyo Co., Ltd.). The external
magnetic field was 1.28 MA/m (16 kOe).
[0182] The results of the evaluation of the magnetic tapes of
Examples 1 to 10 and Comparative Examples 1 to 7 are shown in
Tables 4 to 7.
TABLE-US-00007 TABLE 4 Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Reflectance
(initial) Av. reflectance (%) 9.0 10.2 11.5 8.8 9.5 Rate of 3.0 3.4
5.0 3.5 2.8 fluctuation (%) Reflectance after twice tape-running:
Av. reflectance (%) 9.2 9.8 10.0 8.6 9.4 Rate of 3.5 3.8 7.5 3.7
3.1 fluctuation (%) Ratio of nonmagnetic 53.2 wt. 53.2 wt. 53.2 wt.
53.2 wt. 53.2 wt. powder Surface roughness with AFM (initial) Ra
(nm) 25.2 23.4 21.5 29.5 24.1 Half width of Ra 3.0 3.8 4.8 4.1 2.7
(nm) Surface roughness after twice tape- running: Ra (nm) 25.1 23.8
25.2 29.5 23.9 Half width of Ra 3.3 4.0 8.5 4.5 3.0 (nm) Servo
signal (initial) S/N (relative value) 6.1 6.1 5.0 5.4 6.6 Servo
signal after twice tape-running: S/N (relative value) 5.5 5.5 2.6
5.0 6.2 Error rate (initial) .times. 0.5 -- -- -- -- 10.sup.-8
TABLE-US-00008 TABLE 5 Ex. 6 Ex. 7 Ex. 8 Ex. 9 Ex. 10 Reflectance
(initial) Av. reflectance (%) 9.1 8.5 10.5 12.3 14.9 Rate of 3.1
4.5 4.2 3.2 4.5 fluctuation (%) Reflectance after twice
tape-running: Av. reflectance (%) 9.1 8.6 10.4 11.2 13.4 Rate of
3.6 5.0 4.7 4.9 5.0 fluctuation (%) Ratio of nonmagnetic 53.2 wt.
50.0 wt. 55.4 wt. 55.4 wt. 59.5 wt. powder Surface roughness with
AFM (initial) Ra (nm) 25.0 22.5 27.5 24.7 29.4 Half width of Ra 2.9
4.9 4.4 3.9 4.7 (nm) Surface roughness after twice tape- running:
Ra (nm) 24.9 23.1 27.2 28.2 29.9 Half width of Ra 3.1 4.0 4.5 5.5
5.0 (nm) Servo signal (initial) S/N (relative value) 6.0 4.1 5.3
7.2 6.5 Servo signal after twice tape-running: S/N (relative value)
5.4 3.7 4.8 4.9 5.6
TABLE-US-00009 TABLE 6 Ref. Com. Com. Com. Com. Ex. 1 Ex. 1 Ex. 2
Ex. 3 Ex. 4 Reflectance (initial) Av. reflectance (%) 8.5 7.7 6.8
7.0 7.4 Rate of 4.0 10.5 10.4 10.0 10.4 fluctuation (%) Reflectance
after twice tape-running: Av. reflectance (%) 8.7 7.2 6.4 6.8 7.2
Rate of 12.5 15.6 16.1 15.4 13.9 fluctuation (%) Ratio of
nonmagnetic 53.2 wt. 44.5 wt. 33.8 wt. 35.9 wt. 43.0 wt. powder
Surface roughness with AFM (initial) Ra (nm) 25.2 19.7 32.4 25.6
21.9 Half width of Ra 4.5 7.1 10.5 7.0 7.7 (nm) Surface roughness
after twice tape- running: Ra (nm) 25.1 19.9 33.1 27.2 23.1 Half
width of Ra 7.2 8.3 12.6 8.2 8.0 (nm) Servo signal (initial) S/N
(relative value) 4.6 0.0 -0.5 -0.2 -0.1 Servo signal after twice
tape-running: S/N (relative value) -0.2 -2.0 -2.7 -2.2 -1.5 Error
rate (initial) .times. 1200 -- -- -- -- 10.sup.-8
TABLE-US-00010 TABLE 7 Com. Ex. 5 Com. Ex. 6 Com. Ex. 7 Reflectance
(initial) Av. reflectance (%) 8.0 7.8 7.1 Rate of 8.2 8.6 10.4
fluctuation (%) Reflectance after twice tape-running: Av.
reflectance (%) 8.1 7.8 6.7 Rate of 9.8 9.9 16.2 fluctuation (%)
Ratio of nonmagnetic 48.5 wt. 48.5 wt. 35.9 wt. powder Surface
roughness with AFM (initial) Ra (nm) 22.1 21.9 25.9 Half width of
Ra 7.5 7.7 7.2 (nm) Surface roughness after twice tape- running: Ra
(nm) 22.0 22.3 27.9 Half width of Ra 7.6 7.8 8.1 (nm) Servo signal
(initial) S/N (relative value) 1.2 0.9 -0.3 Servo signal after
twice tape-running: S/N (relative value) 0.5 0.3 -2.5
[0183] As is apparent from the results of Examples 1 to 10 and
Comparative Examples 1 to 7 shown in Tables 4 to 7, the magnetic
tapes were high in the initial S/N of the servo signals, and also
high in the S/N thereof found after the magnetic tapes had been run
twice, when their average reflectances were 8.5% or higher on the
flat portions, and their maximum coefficients of fluctuation of
reflectance on the flat portions, depending on positions of the
magnetic tapes, i.e., [maximum of the absolute value of
(reflectance-average reflectance)].times.100/(average reflectance),
were 10% or lower. Also, as is apparent from the results of Example
1 and Reference Example 1, by carrying out the solid CO.sub.2
spraying treatment, the error rate was decreased and the S/N of the
servo signal found after the magnetic tape had been run twice was
increased. Thus, this treatment is found to be effective to remove
the burnt residues in the pits of the backcoat layers.
[0184] The results of the evaluation of the magnetic tapes of
Examples 11 to 19 and Reference Example 2 are shown in Table 8.
TABLE-US-00011 TABLE 8 Ex. 11 Ex. 12 Ex. 13 Ex. 14 Ex. 15 Error
rate (initial) .times. 10.sup.-8 0.5 1.0 2.0 5 0.6 Number of
wrapped 1 1 1 1 1 drums Ref. Ex. 16 Ex. 17 Ex. 18 Ex. 19 Ex. 2
Error rate (initial) .times. 10.sup.-8 0.5 2.0 1.0 0.5 1200 Number
of wrapped 2 4 2 2 0 drums
[0185] As is apparent from the results of Examples 11 to 19 and
Reference Example 2 shown in Table 8, the magnetic tapes having low
error rates can be obtained by carrying out the step of allowing
the raised fabrics or woven or nonwoven cloths having raising
fibers thereon to contact with the surfaces of the backcoat layers
of the magnetic tapes, thereby removing the burnt residues adhered
to the pits for optical servo and their peripheries on the backcoat
layers, in the course of cleaning the surfaces of the backcoat
layers of the magnetic tapes.
[0186] In other words, a magnetic tape as follows is high in the
initial S/N of the servo signal, and also high in the S/N of the
servo signal found after the magnetic tape has been run twice: that
is, such a magnetic tape comprises a nonmagnetic support; a
magnetic layer which is formed on one surface of the nonmagnetic
support; and a backcoat layer which contains nonmagnetic powder and
a binder and which is formed on the other surface of the
nonmagnetic support, having pits for optical servo formed thereon,
and the magnetic tape is characterized in that the average
reflectance on the flat portion of the backcoat layer is 8.5% or
higher, and that the maximum rate of fluctuation of the reflectance
on the flat portion depending on a position of the magnetic tape,
i.e., [maximum of absolute value of (reflectance-average
reflectance)].times.100/(average reflectance), is 10% or lower.
Further, the error rate is decreased, and the S/N of the servo
signal is increased, by cleaning the backcoat layer of the magnetic
tape, i.e., by spraying solid CO.sub.2 onto the backcoat layer or
by allowing a raised fabric to contact with the surface of the
backcoat layer. Therefore, these treatments are effective to remove
the burnt residues adhered to the pits of the backcoat layer.
* * * * *