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DISCLOSURE OF THE INVENTION
This application is a continuation and claims the benefit of priority under 35 USC 120 of U.S. application Ser. No. 10/341,739, filed Jan. 14, 2003, now abandoned, U.S. appli- 5 cation Ser. No. 10/170,830, filed Jun. 13, 2002, now abandoned, and U.S. application Ser. No. 09/530,005, filed Jul.
21, 2000, now abandoned. In addition, this application claims the benefit of priority under 35 U.S.C. 119 of: 371 PCT/JP98/04767, filed Oct. 21, 1998, which claims the 10 benefit of Japanese Application Number 9-289885, filed Oct.
The present invention relates to magnetic tape having optical servo tracks. More particularly, it relates to magnetic tape having optical servo tracks on the side opposite to the magnetic recording side.
The recent expanding scale of the computer network and the importance of security for data management have been increasing the demand for magnetic tape having an 25 increased recording capacity for use as a medium for data backup. Approaches to high recording capacity are divided into improvement on recording density and extension of the tape length.
Since the tape length that can be put in a tape cartridge as 30 wound is the upper limit of the recording capacity, extension of the tape length for increasing the recording capacity cannot be achieved but by reducing the tape thickness. Therefore, an increase in recording capacity attained by this approach is of necessity limited. With respect to the method 35 of increasing a recording density, it is known that magnetic tape has a lower recording density than a hard disc drive. Serpentine type magnetic tape particularly has a low recording density, which is due to the low track density. On the other hand helical scan type magnetic tape is known to have 40 a higher track density than the serpentine type magnetic tape. This is because the magnetic tape of helical scan type uses a servo tracking system called automatic track finding (ATF).
A servo tracking system has also been adopted to serpen- 45 tine type magnetic tape to improve the track density. Methods that have been proposed as such a servo tracking system include an embedded servo system, in which servo signals are written on the same track as the data track on the magnetic recording surface, and a system in which a track 50 exclusive to servo signals is provided on the magnetic recording surface. Japanese Patent Publication No. 82626/ 95 proposes a tracking system particularly useful where the pitch of data tracks is as small as several tens of microns, in which a dedicated track for servo information is provided on 55 the magnetic recording surface, and a plurality of servo signal reproduction heads are used to read the servo signals for tracking. According to this technique, however, the number of servo signal reproduction heads must be increased as the number of tracks increases. In order to avoid 60 this, the servo track should be increased. Like this, conventional servo tracking systems use the same side of magnetic tape as used for data recording, which results in reduction of the data recording area. This problem is conspicuous in the servo tracking system of Japanese Patent Publn. No. 82626/ 65 95 when a track density is as high as about 30 tpmm (tracks per mm) or even more.
Accordingly, an object of the present invention is to provide magnetic tape which is capable of servo tracking without reducing the data area.
Another object of the present invention is to provide magnetic tape having an increased track density.
Still another object of the present invention is to provide magnetic tape having a high recording capacity.
As a result of extensive investigation, the inventors of the present invention have found that magnetic tape accomplishing the above objects can be obtained by incorporating specific fine particles into the backcoating layer of the magnetic tape and forming specific voids in the backcoating layer to make the backcoating layer capable of forming servo tracks.
Completed based on the above finding, the present invention accomplished the above objects by providing magnetic tape comprising a substrate, a magnetic layer provided on one side of the substrate and a backcoating layer provided on the other side of the substrate, wherein the backcoating layer comprises a binder and fine particles having been dispersed in the binder and being capable of irreversibly changing in color on oxidation reaction, and has a sufficient number of microvoids of sufficient size to supply sufficient oxygen to cause the oxidation reaction.
BRIEF DESCRIPTION OF THE DRAWINGS
Various other objects, features and attendant advantages of the present invention will be better understood from the following description and the accompanying drawings, in which like reference characters designate like parts and wherein:
FIG. 1 is a schematic view showing the structure of one embodiment of the magnetic tape according to the present invention.
FIG. 2 schematically illustrates a method for forming a color change pattern by irradiating a backcoating layer with light beams.
FIG. 3 is an enlarged partial view of the backcoating layer irradiated with light beams.
FIGS. 4(a), 4(b), 4(c), and 4(d) schematically illustrate a method for carrying out servo tracking by a push-pull method.
FIG. 5 schematically shows another color change pattern (corresponding to FIG. 3).
BEST MODE FOR CARRYING OUT THE
The magnetic tape of the present invention will be described with reference to the preferred embodiments thereof by referring to the accompanying drawings, in which FIG. 1 is a schematic view showing the structure of an embodiment of the magnetic tape according to the present invention, FIG. 2 schematically illustrates a method for forming a color change pattern by irradiating a backcoating layer with light beams, and FIG. 3 is an enlarged partial plane view of the backcoating layer irradiated with light beams.
Magnetic tape 1 of the embodiment shown in FIG. 1 comprises a substrate 2 having provided thereon an intermediate layer 3 and a magnetic layer 4 as a top layer adjoining the intermediate layer 3. The substrate 2 has on the other side a backcoating layer 5.
The magnetic tape 1 shown in FIG. 1 is used for a serpentine recording system. The magnetic layer 4 has a plurality of data tracks in parallel with the running direction of the magnetic tape 1. On use, a head unit having a predetermined number of magnetic heads is moved across 5 the magnetic tape 1, switching among data tracks, to record or reproduce data on the data track corresponding to each magnetic head. Servo tracking is carried out so that each magnetic head may be positioned on a right data track on switching among the tracks or during recording or repro- 10 duction.
The backcoating layer 5 is formed of a binder having dispersed therein fine particles that change its color irreversibly on being oxidized. Oxidation reaction of the fine particles can be induced by affording energy necessary for 15 the reaction. While the method of affording energy is not particularly limited, a method in which energy can be given only to a specific small area is preferably used. Such a method includes irradiation with a light beam, such as a laser beam. 20
The manner of irradiating the backcoating layer 5 with a light beam to oxidize the fine particles is explained by referring to FIG. 2.
As shown in FIG. 2, a plurality of laser beams 41 are emitted in parallel from the respective laser light sources 40 25 aligned at prescribed intervals across the width direction of the magnetic tape 1 and illuminate the backcoating layer 5 of the magnetic tape 1 running in direction A at a predetermined speed. The fine particles present in the parts irradiated with the laser beams 41 undergo oxidation reaction with 30 oxygen present in air and change in color. The irradiation conditions with the laser beams 41 are controlled so that the color change may occur over the whole thickness of the irradiated part of the backcoating layer 5. The color change provides a prescribed color change pattern 10 in the back- 35 coating layer 5. The color change pattern formed in this particular embodiment is comprised of a plurality of continuous lines of prescribed width along the longitudinal direction of the magnetic tape 1 as illustrated in FIG. 2. The width w of each line of the color change pattern 10 and the 40 degree of color change in the thickness direction of the backcoating layer 5 can be adjusted by controlling the beam diameter and output of the laser beams 41. In this embodiment, the beam diameter is preferably 0.25 to 30 urn, particularly 1 to 25 urn, and the output is preferably 0.02 to 45 2 W, particularly 0.02 to 0.5 W. The color change pattern 10 in FIG. 2 is magnified.
FIG. 3 is referred to for going into details of the color change pattern thus formed. The color change pattern 10 is comprised of straight lines having a prescribed width w, 50 arrayed in parallel to each other in the longitudinal direction of the magnetic tape 1 and spaced equally in the width direction of the magnetic tape 1. The color change pattern 10 is formed over the whole length of the magnetic tape 1. The color change pattern 10 is such that makes an optical 55 contrast so that servo tracking for the data tracks on the magnetic layer 4 may be carried out based on the optical information provided from the color change pattern 10. As stated above, while the data tracks on the magnetic layer 4 are also formed in parallel to the longitudinal direction of the 60 magnetic tape 1 similarly to the color change pattern 10, the relative positional relationship between the data tracks and the color change pattern 10 is not particularly limited.
The optical contrast made by the above-described color change pattern 10 includes a contrast of intensity of trans- 65 mitted light when light of prescribed wavelength is incident on the color change pattern 10 and a contrast of intensity of
reflected light when light of prescribed wavelength is incident on the color change pattern 10.
Where the contrast of transmitted light intensity is used for servo tracking, the intensity of transmitted light is detected to conduct servo tracking by an optical servo system, such as a push-pull method or a three-beam method. In using the contrast of reflected light intensity, the intensity of reflected light is detected to carry out servo tracking by the above-described servo system in the similar manner. The optical servo systems, such as a push-pull method and a three-beam method, are techniques commonly employed for achieving servo tracking in various optical discs.
Servo tracking based on the detected intensity of transmitted light by, for example, a push-pull method is described by referring to FIG. 4. As shown in FIG. 4(a), light is emitted from a light source 30, such as a semiconductor laser, which is placed to face the backcoating layer 5 of the magnetic tape running in the direction perpendicular to the surface of the paper, condensed through a lens 31 to a prescribed beam diameter, and enters the color change pattern 10 formed on the backcoating layer 5. The beam diameter is slightly smaller than the line width of the color change pattern 10. The intensity of the light having been transmitted through the color change pattern 10, the substrate 2 (not shown), the intermediate layer 3 (not shown), and the magnetic layer 4 (not shown), i.e., transmitted light is detected by a light detector 33. The transmitted light intensity is converted to electrical signals and sent to a servo tracking processor 34. The symmetry of the transmitted light beam intensity is processed in the servo tracking processor 34. If the beam intensity displays bilateral symmetry about the center line of the beam, it means that the beam 35 is incident on the center line of the color change pattern 10 as shown in FIG. 4(b). This state is an "on-track" state, that is, the magnetic head is properly positioned on an aimed data track of the magnetic layer. If the beam intensity lacks bilateral symmetry about the center line of the beam, it indicates that the beam 35 is deviating from the center line to either left or right as shown in FIG. 4(c) or (d). This state is an "off-track" state, that is, the magnetic head is not properly positioned on the data track of the magnetic layer. Then the servo tracking processor 34 gives a drive 35 of the magnetic head 36 instructions to move the magnetic head 36 to a proper position as shown in FIG. 4(a). As a result, the magnetic head 36 is properly positioned by the drive 35 to restore the "on-track" state.
As shown in FIG. 3, the line width w of the color change pattern 10 is preferably 0.25 to 50 urn. If the line width w is smaller than 0.25 urn, optical detection of the color change pattern may be disturbed because it is difficult with the state-of-the-art technique to condense the beam sufficiently. If the line width w exceeds 50 um, the density of the color change pattern 10 unfavorably decreases where the pattern is comprised of a large number of lines as illustrated in FIG. 3. Therefore, the above-described range is preferred. A still preferred line width w of the color change pattern 10 is 0.25 to 30 um, particularly 0.8 to 25 um. In the present invention, it is preferred to use transmitted light for servo tracking. In that case, it is preferred for the whole magnetic tape before color change (the magnetic layer, the intermediate layer, the substrate, and the backcoating layer as a whole) has a light transmission of 15 to 40% for servo tracking.
While depending on the number of the lines forming the color change pattern 10, it is preferred that the pitch p of the color change pattern 10, i.e., the pitch of the adjacent color change lines be not less than the width of the data track formed on the magnetic layer 4 and be an integral multiple of the width of the data track.
The color change pattern 10 may be arranged over the whole width of the magnetic tape 1 at prescribed intervals as shown in FIG. 3, or in part of the width of the magnetic tape 10. For example, a plurality of lines spaced at prescribed intervals may be arranged in the central portion or either one 5 of side portions of the tape in the width direction. Further, a plurality of lines spaced at prescribed intervals may be arranged in two or more portions of the magnetic tape 10 in the width direction. For example, two groups of lines (each group consists of at least one line, and the groups may 10 consist of the same or different number of lines) can be arranged on each side portion of the tape; two groups of lines (each group consists of at least one line, and the groups may consist of the same or different number of lines) can be arranged on the central portion and one of the side portions 15 of the tape; or three groups of lines (each group consists of at least one line, and the groups may consist of the same or different number of lines) can be arranged on the central portion and each side portion of the tape. In any case, the total number of the lines making up the color change pattern 20 10 is preferably a measure of the number of the data tracks of the magnetic layer 4.
The backcoating layer 5 has microvoids the number and the size of which are sufficient for supplying sufficient amount of oxygen to induce oxidation reaction of the 25 above-described fine particles. Oxygen is supplied through the microvoids to the whole thickness of the backcoating layer 5 thereby making the fine particles undergo sufficient oxidation reaction. As a result, there is formed a color change pattern 10 providing sufficient optical contrasts. The 30 microvoids may be either open pores exposed on the surface of the backcoating layer 5 or closed pores which exist inside the backcoating layer 5 and are not exposed on the surface. However, if there are too many closed pores, the amounts of various particles such as the above-described fine particles 35 and a binder per unit volume are reduced relatively, which tends to make the contrasts of the color change pattern insufficient or make the film strength of the backcoating layer 5 insufficient. Accordingly, it is preferred that the microvoids are open pores or most of the microvoids are 40 open pores. As long as the contrasts of the color change pattern and the film strength of the backcoating layer 5 retain sufficient levels, it is not at all problematical that the microvoids exist in a closed state.
Microvoids can be formed in the backcoating layer 5 by 45 controlling the weight ratio of the total amount of various particles hereinafter described (i.e., the total amount of all inorganic particles contained in the backcoating layer 5) to the total resinous content including a binder, a hardener, etc. (hereinafter referred to as P/B ratio). A preferred P/B ratio is 50 100/10 (=10) to 100/30 (=3.33), particularly 100/14 (=7.14) to 100/25 (=4). With the P/B ratio of the backcoating layer 5 being within this range, it is possible to form microvoids preferably having a diameter of 1 to 20 nm, particularly 2 to 15 nm, and a void volume (volumetric ratio of the micro- 55 voids in the volume of the backcoating layer 5) of 5 to 40% by volume, particularly 10 to 35% by volume.
The diameter and volume of the microvoids are measured by a nitrogen adsorption method according to the following procedure. 60
A high-precision automatic gas adsorption apparatus "BELSORP 36" manufactured by Nippon Bell K.K. is used as measuring equipment.
A piece measuring about 100 cm2 is taken out of a magnetic tape having only the backcoating layer side left on 65 the substrate (i.e., a magnetic tape from which the magnetic layer 4 and the intermediate layer 3 have been removed),
which was used as a sample of measurement. The sample is sealed into a sample tube. Nitrogen having a purity of 99.9999% and helium having a purity of 99.99999% are used as an adsorbing gas and a carrier gas, respectively.
The sample is allowed to stand at room temperature for 1 hour (reached degree of vacuum: 0.2 to 0.4 Pa) prior to the measurement, and then measurement is made at an adsorption temperature of 77 K. The measurement mode is an isothermal adsorption-desorption mode. The measuring range is from 0.00 to 0.99 in terms of relative pressure (P/P0), and the equilibrating time is 300 seconds for every relative pressure.
The distribution of the measured void diameters is calculated by a DH (Dollimore & Heal) method and smoothed. Prior to the measurement of the sample, measurement is made on graphite carbon available from NPL (National Physical Laboratory), an international standard sample (proof value: 11.1 m2/g; a=0.8 m2/g), to confirm that the precision and accuracy of measurement are within 2% and within 5%, respectively. No voids are present in the substrate.
The terminology "(void) diameter" as used herein means the void diameter at which the distribution curve obtained from the measurement of void diameter reaches the maximum peak (the highest frequency in the distribution curve).
The "void volume" is a value obtained by dividing the total volume of the microvoids calculated by the abovedescribed DH method by the volume of the backcoating layer (the product of the thickness and the area) and multiplying the quotient by 100.
The above-described fine particles are now described in detail.
Any fine particles that undergo irreversible color change on being oxidized can be used with no particular restriction. It is particularly preferred to use metal oxides for their readiness to discoloration and the color contrast produced by the discoloration. The metal oxides include, for example, FeO (1.34<x<1.5), TiO, SnO, MnO, and Cr203. It is particularly preferred to use FeOx for its satisfactory discoloration properties.
FeOx is iron oxide of magnetite type comprising divalent Fe and trivalent Fe. It is preferred for the FeOx to have a divalent Fe content of 5 to 24% by weight, especially 10 to 20% by weight, based on the total FeOx.
The fine particles preferably have a primary particle size of 1 to 200 nm, particularly 5 to 80 nm, from the viewpoint of the surface smoothness of the backcoating layer. For the consideration of the above-mentioned P/B ratio, it is preferred that the fine particles be present in an amount of 300 to 1200 parts by weight, particularly 350 to 1000 parts by wight, per 100 parts by weight of the binder. More specifically, where the amount of the fine particles is less than 300 parts by weight, the sensitivity to color change tends to be insufficient for obtaining optically sufficient contrasts. If it exceeds 1200 parts by weight, the coating film of the backcoating layer tends to have reduced strength. Therefore, the above-described range is preferred.
Any binders can be used with no restriction as long as applicable to magnetic tape. For example, thermoplastic resins, thermosetting resins, reactive resins, and mixtures thereof can be used. Specific examples are vinyl chloride copolymers or modified vinyl chloride copolymers, copolymers comprising acrylic acid, methacrylic acid or esters thereof acrylonitrile copolymers (rubbery resins), polyester resins, polyurethane resins, epoxy resins, cellulosic resins, and polyamide resins. These binders preferably have a number average molecular weight of 2,000 to 200,000. The