Stretched surface recording disk (SD) 10 comprises an annular support 12 having raised annular ridges 42 and 28 at its inside and outside diameters 22 and 30 with a base portion 24 in between the two ridges, and an annular recording medium film 14 held in radial tension and stretched across the base portion by adhering the film to attachment surfaces 48 and 36 near the inside and outside diameters respectively. The SD is annealed twice during manufacture to relieve stress in the stretched film 14. This annealing results in SD essentially free from track anisotropy and stress distribution anisotropy in the plane of the disk. As a result, data tracks are more stable than in previous SD, track density can be greater, and the SD is generally more reliable.
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1. A process for annealing a stretched surface recording disk comprising a rigid annular support, having at least two concentric raised annular reference surfaces and an annular polymeric film, having a recording layer, attached to the support and stretched between and supported by the raised annular reference surfaces, said process comprising:
A. annealing a polymeric film which has been fixed in radial tension, at a temperature above its glass transition temperature for a time sufficient to obtain a substantial absence of anisotropic stress distribution in the film; B. bonding the annealed film from step A to the annular support at or near the inside and outside diameters of the support; and C. annealing the product of step B, at a temperature above the glass transition temperature of the polymeric film but below the temperature which would cause warping of the annular support, for a time sufficient to obtain a substantial absence of anisotropic stress distribution in the polymeric film.
8. A process for annealing a stretched surface recording disk comprising a rigid annular support, having at least two concentric raised annular reference surfaces and an annular polymeric film, having a recording layer, attached to the support and stretched between and supported by the raised annular reference surfaces, said process comprising:
A. annealing a polymeric film which has been fixed in radial tension, at a temperature above its glass transition temperature for a time sufficient to obtain a substantial absence of anisotropic stree stress distribution in the film; B. bonding the annealed film from step A to the annular support at or near the inside and outside diameters of the support; and C. annealing the product of step B in an oven by placing the product of step B within a temperature dampening chamber within an oven, said temperature dampening chamber consisting essentially of a container larger than the annular support and a holding means for holding the product of step B, and maintaining temperature above the glass transition temperature of the polymeric film but below the temperature which would cause warping of the annular support, for a time sufficient to obtain a substantial absence of anisotropic stress distribution in the polymeric film.
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9. A process for producing a magnetic disc comprising (1) bonding a recording medium film disc comprising a biaxially oriented flexible polyethylene terephthalate film substrate having a magnetic layer coated thereon to at least one side of an annular support wherein a gap is formed between the back surface of the recording medium film and the annular support and, then, (2) annealing the magnetic disc at a temperature in the range of 66°C to 93°C for 4 minutes to 36 hours. 10. The process of
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This a division of application Ser. No. 711,825, filed Mar. 14, 1985, now U.S. Pat. No. 4,623,570.
This invention relates to magnetic recording media, particularly flexible magnetic recording sheets stretched across the major surface of a disk-shaped support to provide a planar recording surface. The invention further relates to the preparation of double-sided disks with flexible magnetic recording film stretched across both sides of the plastic disk-shaped support.
Stretched surface recording disks (SD) generally consist of a rigid, circular support and a thin polymer film, having a recording layer, suitably attached to the periphery of the support. The development of SD has progressed over the years from a rather simple configuration described in U.S. Pat. No. 3,373,413 (Treseder) in which a film was stretched and clamped between two circular rings, to a dish-shaped support to which a stretched film was attached at the periphery as illustrated by U.S. Pat. No. 3,509,274. Subsequently, U.S. Pat. No. 3,537,083 introduced the concept of bonding the film at the center of the support in addition to the support periphery, and U.S. Pat. No. 3,599,226 described an SD which included two stretched film surfaces, one on either side of the support, which were attached at the periphery and near the center of the support.
SD are superior to floppy disks in that a pressure-pad is not required, and the recording surface has substantially the same overall dimensional stability as the relatively massive base to which it is attached. In addition, the SD provides a flat recording surface which may be deformed slightly to conform to a transducer head and irregularities in the surface of the head.
Although the patents described above have illustrated the general configuration and certain desirable features of SD, practical problems remain as impediments to the mass production and general acceptance of SD. These include the problems of track stability and stress relaxation of the stretched film.
Track dimensional stability is an important parameter in the design of recording media and drive systems. A change in track shape can cause a serious loss in signal-to-noise ratio or the loss of prerecorded data. It has been determined that circular data tracks originally recorded on SD became elliptical or distorted as the SD were exposed to variations in environmental conditions. This track movement was not completely reversible, i.e., the original circular configuration was not obtained when the SD were returned to original environmental conditions. Thus track density is limited by the dimensional instability or anisotropy of the tracks.
It has been found that the tension or stress present in the stretched recording sheet decreases over time even without changing environmental conditions. This is referred to as stress relaxation and can also distort the data tracks.
A means has been found for stabilizing the data tracks of an SD and for alleviating the problem of stress relaxation of the stretched recording medium film over time.
The invention is summarized as a stable stretched surface recording disk comprising:
A. an annular support having:
1. at least one base portion between its inside and outside diameters;
2. an outer raised annular ridge attached to and projecting from the base portion;
3. an inner raised annular ridge attached to and projecting from the base portion and having a diameter less than that of the outer raised annular ridge; and
B. an annular recording medium film held in radial tension across and out of contact with the base portion, being supported by the inner and outer raised annular ridges and adhered to the support, in which there is a substantial absence of anisotropic stress distribution in the annular recording medium film at constant conditions of temperature and humidity within the range of 10° to 60°C and 8 to 80% relative humidity.
The term "radial tension" as used herein means tension stress pulling along all radii of the annulus of the recording medium film. This type of stress is produced by stretching the film over a circular ring or hoop. It should be noted that, while radial stress is of concern because it can cause track anisotropy, there may be other stresses in the film such as circumferential stress.
The temperature to which the SD is exposed should remain at least 10°C below the glass transition temperature of the recording medium film.
The inventive SD may also be described as being essentially without track anisotropy within the same temperature and humidity ranges stated above. As used herein, the absence of track anisotropy means having no more track anisotropy than a rigid aluminum disk, such as a Winchester disk having a magnetic layer comprising an oxide coating, of the same diameter would have under the same conditions. Track anisotropy is evidenced when data tracks deviate from their original circular shape. Aluminum recording disks are considered isotropic for purposes of this invention, and they have been found to have a track anisotropy of no more than about ±15 microinches (±0.4 micrometers).
These stable disks are made by a process which anneals the recording medium film. In general, the stretched film is subjected to an elevated temperature above its glass transition temperature, but below the glass transition temperature of the annular support to prevent warping. The following steps can be used to manufacture
1. Provide a polymeric sheet, which will be the substrate of the annular recording medium film, fixed in radial tension.
2. Anneal the polymeric sheet from step 1 at a temperature above its glass transition temperature for a sufficient time to obtain a substantially isotropic and stable stretched disk.
3. Bond the annealed sheet from step 2 to the annular support at or near both its inside and outside diameters to obtain an SD. It is very desirable for the bonds between the annular support and the annealed sheet to form a smooth seal free from bumps and loose spots. This step may involve additional stretching of the film, due to the geometry of the support.
4. Anneal the SD from step 3 at a temperature above the glass transition temperature of the annular recording medium film but below the glass transition temperature of the annular support for a sufficient time to obtain a substantially isotropic and stable SD.
Normally, the annealed material from steps 2 and 4 is allowed to cool to room temperature before further processing.
By reducing track anisotropy, more data can be stored on an SD of a given size by recording narrower data tracks, closer together than was previously feasible
FIG. 1 is a perspective, cut-away view of an SD of this invention.
FIG. 2 is a cross-sectional view of the SD of FIG. 1.
FIG. 3 is an exploded view of a stretching apparatus used for placing the recording medium film in radial tension before it is adhered to the support.
FIG. 4 is an elevation view of the stretching apparatus of FIG. 3.
FIG. 5 is an exploded view of a stretching fixture by means of which the components of the SD are assembled.
FIG. 6 is a sectional view of the stretching fixture of FIG. 5.
FIG. 7 is a graph of track anisotropy for two SD, one of this invention indicated by a solid line, and the other being a control sample indicated by a dashed line.
FIG. 8 is a graph of penetration stiffness of two SD vs. time in days.
Track anisotropy of the annealed SD of this invention, measured by the above-described technique, is essentially the same as aluminum rigid disks. Measured SD distortion over changes in temperature and humidity within the ANSI specification has been less than the noise level (10-15 microinches or 0.25-0.4 micrometers) in the measuring system.
The invention will be further clarified by the following example.
Two SD were made using the following components: recording medium film of polyester having a pigment/binder type magnetic layer, injection molded polyether imide annular support; and adhesive made of a mixture of hydantoin hexacryalte and N,N-dimethylacrylamide. They were made by processes which were the same in all respects, except that SD1 (solid line in FIG. 7) was treated by the annealing steps described earlier (both annealing of the stretched polymeric sheet and of the bonded SD), while SD2 (dashed line) was not annealed. Each SD was held at 15°C and 20% relative humidity for two hours after which the concentric tracks were recorded at about the middle of the data band. The environment was changed to 57°C and 20% relative humidity for 8 hours; then the temperature was returned to 15° at 20% relative humidity and held at that condition for two hours. The SD were then tested for irreversible track anisotropy and the results were plotted on polar coordinates and are shown in the graph of FIG. 7. The numbers -40 to 40 indicate deviation from the circular datum track in microinches. The solid curve for the inventive SD1 shows essentially no track anisotropy (about 0-15 microinches, 0.4 micrometers peak-to-peak); whereas, the dashed curve of the control sample SD2 exhibits marked anisotropy (about 72-102 microinches, 2-3 micrometers, peak-to-peak).
The annular, stretched recording medium film should also maintain a relatively constant radial tension over time. Any relaxation in the film can affect track stability and the relationship between a record or read head or transducer and the medium. The extent of any stress relaxation can be detected by measuring penetration stiffness of aged SD. If penetration changes unevenly around the SD with the passage of time, the stress distribution around the disk has become anisotropic.
The horizontal solid line in FIG. 8 represents the constant penetration stiffness and the absence of stress relaxation in one of the inventive SD held over 600 days (at 66°C and 17% relative humidity). Penetration stiffness, and thus radial tension, did not decrease appreciably (no more than 5%) over the period of the test. The dashed, slanted line shows the decline in penetration stiffness of an SD which had not been annealed, over 100 days under the same conditions. Significant stress relaxation had occurred after 10 days.
Tests of track position repeatability (i.e., constant location of a track after repeated traversals by a head) using an interferometer have shown that the inventive SD have track repeatability similar to that of rigid aluminum disks with an oxide magnetic layer.
Other embodiments of this invention will be apparent to those skilled in the art from a consideration of this specification or practice of the invention disclosed herein. Various omissions, modifications and changes to the principles described herein may be made by one skilled in the art without departing from the true scope and spirit of the invention which is indicated by the following claims.
Nelson, Charles E., Gerfast, Sten R., Alexander, Jerry L., Narayan, Sankar B.
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| Executed on | Assignor | Assignee | Conveyance | Frame | Reel | Doc |
| Nov 02 1988 | Minnesota Mining and Manufacturing Company | (assignment on the face of the patent) | / | |||
| May 26 1989 | ALEXANDER, JERRY L | MINNESOTA MINING AND MANUFACTURING COMPANY, A CORP OF DE | ASSIGNMENT OF ASSIGNORS INTEREST | 005077 | /0949 | |
| May 26 1989 | NARAYAN, SANKAR B | MINNESOTA MINING AND MANUFACTURING COMPANY, A CORP OF DE | ASSIGNMENT OF ASSIGNORS INTEREST | 005077 | /0949 | |
| May 26 1989 | GERFAST, STEN R | MINNESOTA MINING AND MANUFACTURING COMPANY, A CORP OF DE | ASSIGNMENT OF ASSIGNORS INTEREST | 005077 | /0949 | |
| May 26 1989 | NELSON, CHARLES E | MINNESOTA MINING AND MANUFACTURING COMPANY, A CORP OF DE | ASSIGNMENT OF ASSIGNORS INTEREST | 005077 | /0949 |
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