Media heat curing method

Abstract

 A process for treating a polymer-based magnetic imagIng medium is provided. The process includes the step of placing the medium between a pair of substantially planar, rigid members, subjecting the medium to a temperature of from about 80°C. to 110°C. for a first period of at least one-half hour, and subjecting the medium to a temperature of from about 110°C. to 190°C. for a second period of at least about one hour. Another aspect of the invention is a magnetic imaging medium which has been subjected to the above-described process.

Description

Background of the Invention

[0001] This invention pertains to an imaging system for producing a desired final magnetic image in a magnetic image-storing medium. More particularly, it pertains to a process for curing such a medium of the type formed with a solvent-based particle-resin magnetic-particle binder process which renders the medium particularly suitable for use in a magnetographic imaging system.

[0002] Magnetographic imaging systems for which the invention may be used have been disclosed in my U.S. Patent No. 4,414,554, issued November 8, 1983, and entitled MAGNETIC IMAGING APPARATUS, and in my prior-filed U.S. patent applications, which are identified as follows: MAGNETIC IMAGING APPARATUS, Serial No. 170,788, filed 7/21/80; MULTIPLE HEAD MAGNETIC RECORDING ARRAY, Serial No. 381,923, filed 5/23/82; DIFFERENTIAL-PERMEABILITY FIELD-CONCENTRATING MAGNETIC WRITING HEAD, Serial No. 381,922, filed 5/26/82; THIN-FILM, COMPLIANT, NON-PRESSURE-POINT-TELEGRAPHING, ELECTROMAGNETIC READ/WRITE STRUCTURE, Serial No. 472,866, filed 3/8/83; THIN-FILM MAGNETIC WRITING HEAD WITH ANTISATURATION BACK-GAP LAYER, Serial No. 473,361, filed 3/8/83. These inventions have made possible high-resolution, low-cost, high-speed magnetic imaging.

[0003] Disclosed in the above-identified patent and patent applications are several different types of thin-film imaging heads, and an array of such heads, which may be used to create and read an endless variety of images, such as letters in the alphabet. Each writing head is capable of producing a magnetic image. When a facial expanse of the magnetic image-storing medium is passed across a writing head, a corresponding magnetic image is created) in the medium. The image-containing facial expanse of the medium is then transported past a toner applicator having a layer of toner disposed thereon. The magnetic images in the medium attract the toner, thereby creating toner images. The toner images disposed on the facial expanse of the medium are then bombarded with an ion shower to provide the toner with a positive charge. A positive charge is similarly imparted to the paper to which the toner will eventually be transferred. This positive charging of the paper prevents any premature jumping of toner across to the paper. Once the paper is disposed against the toner-laden medium, a negative charge is imparted to the paper so that the oppositely charged toner will transfer from the medium to the paper. Once the toner is transferred to the paper, it is fused to it using a heat and/or pressure fusing operation.

[0004] During the magnetographic reproduction process, the charges imparted to the toner and the paper often tend to accumulate in the medium. Further charging in the form of triboelectric energy, also accumulates as a result of the friction between the head array structures and the medium. These various residual charges are undesirable because they can result in toner being transferred to the medium and then to the paper even in the absence of a magnetic imaging signal from the head arrays. Alternatively, residual charging of the medium by corona ionization can cause a portion of the toner, and resins in the magnetic media binder system, to form a scum, to be retained after the toner was to have been transferred to the paper. Because the imaging medium is substantially electrically non-conductive, these undesirable residual charges will normally not equalize or be easily bled off.

[0005] The main supporting substrate typically used to carry the imaging heads is a glassy amorphous material known under the trade designation "METGLAS". METGLAS is an extremely hard material which permits it to be disposed in pressure-biased contact with the magnetic imaging medium, with extremely low-wear consequences.

[0006] The magnetic image-storing medium normally used with this technology is of conventional design, being formed of a Mylar or similar polymer substrate coated with a solvent-based, polymer-resin binder slurry containing gamma ferric oxide particles. A resin is typically applied over the solvent-based slurry to harden the surface.

[0007] Conventional media formed in this manner includes the following products:

1. Minnesota Manufacturing and Mining Co. magnetic media Nos. 530 and 810 (7-mil substrate);

2. Verbatim Co. medium sold under the trademark "DATALIFE";

3. Spin Physics Co. medium sold under the trademark "ISOMAX"; and

4. Syncom 7-mil substrate.

[0008] When a harder material, such as METGLAS, is pressure-biased against such a medium while relative movement is occurring, it is likely that over a period of time,.with repeated tonings of the medium, the same medium will score or exhibit other signs of deterioration. Absent other preventive measures, and to limit deterioration of the medium, the degree of pressure applied by the head structures must be controlled. Under certain circumstances, this limitation on the pressure biasing may have an adverse effect upon the magnetic coupling between the head structures and the medium.

[0009] It has been known in the past that the wear resistance of Winchester-type hard discs can be improved by sputtering graphite onto the surface of the disc. The graphite apparently acts as a lubricant between the read/write head and the disc. One drawback with this procedure is that it requires something to be added to the disc, thereby increasing the cost and raising the possibility that other performance characteristics of the disc may adversely be affected.

[0010] Conventional imaging media, such as those listed above, are normally adequate in their magnetizations to permit relatively high quality reproduction of an image. However, because the magnetographic process is dependent upon precise transfer of magnetic images, an improvement in the magnetic retentivity or saturation induction of the medium will normally result in an improvement in the quality of the resulting reproduction.

[0011] It is therefore an object of the present invention to provide a treatment process for a magnetic image-storing medium of the type outlined which improves the performance of the medium during magnetographic reproduction operations. More specifically, the present invention has the following as its objects: (1) to develop a treatment process for a solvent-based, polymer-resin magnetic-particle binder magnetic image-storing medium which increases the resistance of the medium to abrasion and other physical deterioration that can result when'the medium is placed in pressure-biased contact with a recording head structure; (2) to increase the hardness of conventional magnetic image-storing media of the type indicated so that pressure-biasing of a recording structure against a medium does not have to be controlled to an extent which may result in less than adequate magnetic coupling between the head structure and the medium during read/write operations; (3) to improve the degree of magnetization which is possible with such magnetic image-storing medium so that the magnetic retentivity and induction saturation of the medium are enhanced; (4) to reduce, any residual attraction which a magnetic image-storing medium has to toner particles; (5) to provide a treatment process which renders the medium less susceptible to thermal expansion; and (6) to provide a method for improving the resistance to abrasion of a conventional magnetic image-storing medium, without causing brittleness or warpage.

Summary of the Invention

[0012] The present invention achieves the above objects by providing a process for treating a magnetic imaging medium of the type mentioned. The process includes the steps of (1) rigidifying the medium, as by placing it between a pair of substantially planar, rigid members, to inhibit configuration changes during treatment, (2) subjecting the medium to a first temperature (of from about 80°C. to 110^oC.) for a first period (of at least about one-half hour) to create controlled evaporating of solvents therein, and (3) subjecting the medium to a second higher temperature (of from about 110°C. to 190°C.) for a second period (of at least about one hour) to increase polymeric cross-linking and to produce carbonization. The first and second time periods may alternatively be defined in functional terms. Thus, the first time period should last until the solvents in the medium coating begin to vaporize. The second time period should last until the medium begins to stabilize because at least a substantial part of polymer cross-linking has been completed. The term "substantial part" as used herein means that amount of cross-linking which results in enough liberation of carbon to produce an appreciable improvement in surface conductivity.

[0013] Another aspect of the invention is a magnetic imaging medium which has been subjected to the above-described process.

[0014] Other objects, features and advantages of the present invention will become apparent upon reading the following detailed description in conjunction with the accompanying drawing.

Brief Description of the Drawing

[0015] Fig. 1 is a side elevation view of an oven and heat application apparatus which may be used to perform the preferied embodiment of the invention.

Detaile Description of the Preferred Method

[0016] As a result of the present invention, it has been determined that, by applying heat to a conventional solvent-based, polymer-resin magnetic-particle binder magnetic imaging medium, solvents are liberated during the heating process, and polymer cross-linking occurs which causes the liberation of deposits of free carbon. The accumulation of carbon on the surface of the medium improves wear properties of the medium and thereby increases the suitability of a conventional polymer medium for use in magnetographic image reproduction. The presence of free carbon, and the resultant densification of the solids in the binder system increases the overall electrical conductivity of the medium - thereby permitting undesirable residual surface charges to equalize.

[0017] The application of heat, as mentioned, also liberates a certain portion of the solvents in the solvent-based coating in the medium. This solvent liberation causes the density of the remaining magnetic material to be increased, thereby promoting higher magnetization through increased coupling of the particles, which condition improves the quality of a retained image through increased magnetic retentivity and higher saturation induction. These factors improve the coupling of magnetic fields from head structures to the medium, thereby increasing the quality of the resulting toner-characteristized image on a finally printed paper.

[0018] The application of heat to the imaging medium should normally occur in stages so that the rate at which solvents are evaporated from the surface can be controlled. Without such control, the surface of the cured medium fails to reach the improved performance characteristics, mentioned above, attainable when proper control is carefully applied. This stage-heating of the medium will be described in detail below.

[0019] To ensure a uniform application of heat to the medium, the medium is preferably disposed between a pair of plates within an oven, as depicted in Fig. 1. The oven has been schematically depicted, being generally identified with the numeral 10, with the pair of plates being shown at 12 and 14. The medium is disposed between the plates and is indicated at 16. The depicted medium 16 is somewhat smaller in dimensions than plates 12 and 14 and is of the type which will subsequently be formed into a continuous belt for mounting to a cylinder. The process may, however, be used with other media forms, such as magnetic recording discs used in memory applications. Such disposition rigidifies the medium, and inhibits undesirable configuration changes during treatment.

[0020] Plates 12 and 14 are preferably formed of glass, although other materials such as copper or aluminum may be used in certain applications. For example, it may sometimes be desirable to use copper, because copper typically has a greater tendency to draw oxides out of the medium than would be true with glass or another metal. In place of nonporous plates, it may alternatively be desirable to use a mesh screen or other porous material - to promote liberation of vapors from the medium while providing for an even application of heat. If a mesh screen is used, it should be sufficiently rigid to constrain the medium uniformly during the heat curing cycle, thus to prevent any curling of the edges due to stress, or bulging of the central portions.

[0021] Oven 10 is of conventional design and should be sufficiently large that plates 12 and 14 are disposed a sufficient distance away from the oven heat elements, shown at 18, so that the plates themselves tend to heat at an even rate. Plates 12 and 14 and medium 16 are supported within oven 10 by a conventional grid 20. The depicted, preferred oven 10 heats the plates and the medium in the presence of air, but it may be desirable for certain applications to perform the heat application process in a carbon dioxide environment, which can enhance the formation of carbon on the surfaces of the medium.

[0022] The so-called stage-curing heat application process may take place over a variety of temperatures and time periods. However, it is desirable, depending upon the chemistry of the particular medium being treated, that at least two heating stages be utilized. In the first stage the oven temperature is set to a first selected temperature, between 80°C. and 110°C. This temperature is held for at least about half an hour, or until an initial period of solvent vaporization occurs. In the second heating stage, the oven temperature is raised to a second, higher selected temperature, between about 110°C. and 190°C. During this period, carbon deposits accumulate on the surface of the medium as polymer cross-linking takes place. The temperature in the oven preferably should not exceed about 190°C. since resulting thermal stresses in the medium may cause undesirable final surface morphology. The temperature should be held at this higher level preferably for about one hour, which is generally the appropriate amount of time for appropriate cross-linking to occur.

[0023] The following are two preferred examples of processes which substantially fulfil the objects of this invention, when applied to Mylar or acetate magnetic imaging media coated with a solvent-based resinous slurry of gamma ferric oxide particles (a structure characterizing the commercially available media set forth earlier herein):

Example No. 1

[0024] The medium is placed in an oven between a pair of nominally three-quarter inch thick glass plates. The oven temperature is set at 90°C. for one hour and then raised to slightly above 110^oC. for two hours. The plates are then removed and the medium is permitted to cool at room temperature.

Example No. 2

[0025] The medium is placed in an oven between a pair of one-half inch thick aluminum plates. The oven temperature is set at 100°C. for one hour, and is then raised to 140° to 190°C. for ninety minutes. The plates with the medium disposed therebetween are then permitted to cool at room temperature.

[0026] Changes and modifications to the preferred method described herein will be apparent to those skilled in the art. Such changes and modifications can be made without departing from the spirit and scope of the present invention and without diminishing its attendant advantages. It is therefore intended that such changes and modifications be covered by the following claims.

Claims

1. A process for treating a magnetic imaging medium formed with a solvent-based, polymer-resin magnetic-particle binder comprising:

rigidifying the medium to restrain it against configurational changes during treatment,

heating the medium for a first selected time period at a first selected temperature to create controlled vaporization of solvents therein, and

thereafter heating the medium for a second selected time period at a second, higher, selected temperature to effect increased cross-linking of the polymer binder.

2. A process as claimed in claim 1 in which the second temperature and the second time period are sufficiently high and long respectively to produce carbonization and carbon deposits within the medium.

3. A process for treating a polymer magnetic imaging medium formed with a solvent-based, polymer-resin magnetic-particle binder comprising:

rigidifying the medium to restrain it against configurational changes during treatment,

heating the medium to a temperature of from 80°C. to 110°C. for a first period which is long enough to begin to vaporize solvents therein, and

thereafter heating the medium to a temperature of from ₁₁0⁰_C. to 190⁰C. for a second period which is long enough to cause at least a substantial amount of cross-polymerization to occur within the medium.

4. A process as claimed in claim 3 in which the second time period is sufficiently long to produce carbonization within the medium.

5. A process as claimed in any preceding claim wherein such first period is at least one-half hour long and such second period is at least one hour long.

6. A process for treating a polymer-based magnetic imaging medium comprising the following steps in the order recited,

positioning the medium between a pair of substantially planar, rigid members,

heating the medium to a temperature of from about 80°C. to 110 C. for a first period of at least about one-half hour, and

heating the medium so a temperature of from about 110°C. to 190⁰C. for a second period of at least about one hour.

7. A process as claimed in any preceding claim further comprising a final step of cooling the medium at room temperature.

8. A process as claimed in any preceding claim, wherein the step of positioning the medium comprises sandwiching the medium between the members in substantial contact therewith.

Drawing