WO1997038419A1

WO1997038419A1 - Method of preparing a magnetic recording medium

Info

Publication number: WO1997038419A1
Application number: PCT/US1997/002058
Authority: WO
Inventors: Norman E. Rustad; Loren W. Hammond; Ralph P. Johnson; Ricky L. Carlson
Original assignee: Minnesota Mining And Manufacturing Company
Priority date: 1996-04-05
Filing date: 1997-02-13
Publication date: 1997-10-16

Abstract

A method of preparing a magnetic recording medium, the method comprising the step of providing a magnetic recording medium comprising a magnetic coating disposed on a substrate. The magnetic coating comprises a curable polymeric binder. The magnetic recording medium is wound into a stockroll while the magnetic coating is at a winding temperature that will accelerate cure of the magnetic coating. The magnetic coating is allowed to cure, optionally in an environment that maintains the magnetic coating at a temperature that accelerates cure. The magnetic recording medium is then slit.

Description

METHOD OF PREPARING A MAGNETIC RECORDING MEDIUM

FIELD OF THE INVENTION The present invention relates to a method of preparing magnetic recording media, and specifically, to a method of preparing a magnetic recording medium wherein a coating disposed on a magnetic recording medium is at least partially cured before the magnetic recording medium is slit.

BACKGROUND

Magnetic recording media generally comprise a substrate coated with one or more coatings. The coatings typically include a magnetic coating coated on one side of the substrate, and optionally, a backside coating coated on the other side of the substrate. Both the magnetic and backside coatings generally include a pigment dispersed in a polymeric binder. The polymeric binder is made of a cured polymeric matrix derived from lower molecular weight reactive molecules.

Each coating of a magnetic recording medium is generally coated onto a substrate as a solution, followed by drying solvent from the solution to produce a dried backside or magnetic coating that contains reactive molecules that can later be cured. Typical reactive molecules include monomers, oligomers, polymers, and/or copolymers containing reactive groups such as hydroxyl groups and isocyanate groups, among others. The cure reaction preferably takes place at a rate that balances the need for a rapid cure against the need for an efficient production process. A rapid cure is preferred because this will provide a stronger magnetic coating during processing. On the other hand, the cure rate must be slow enough to prevent the viscosity of the coating solution from being so high that it causes difficulty within the coating apparatus during coating. After the magnetic and/or backside coatings are provided on the substrate, the magnetic recording medium is wound onto a core to produce what is known as a "stockroll." The magnetic recording medium is later slit to an appropriate width and converted into a final magnetic recording medium product.

The degree of cure of the coatings on the magnetic recording medium at the time of slitting can affect the physical properties of the slit magnetic recording medium. For instance, a partially or completely cured coating provided on the magnetic recording medium can add strength to the magnetic recording medium, resulting in an improved magnetic recording medium.

Specifically, a magnetic recording medium that is slit after a coating provided thereon has been partially or completely cured, will have improved slit edge quality over a magnetic recording medium having uncured coatings, or coatings that have been less completely cured. Poor slit edge quality can take the form of a torn edge or cracked coating near the edge of the medium. These defects can cause problems such as increased debris during use of the magnetic recording medium, potentially causing data dropout and signal loss at that edge.

There are a number of methods used to at least partially cure a coating on a magnetic recording medium prior to slitting. Cure of many coatings used in magnetic recording media can be facilitated by the presence of moisture. These coatings will cure slowly at room temperature, as long as moisture is present. Therefore, one method of allowing a coating to cure is by holding a stockroll of the coated magnetic recording medium for days or weeks at an ambient temperature (about 20 to 25°C), and at normal humidity levels, during which time a coating has an opportunity to cure. This method of allowing a coating to cure prior to slitting is undesirable from a manufacturing standpoint because the method requires an extra step in the production of the finished product. The additional step creates logistic complications during the manufacturing process by requiring movement of each stockroll before and after a curing period. Further, additional inventory of stockrolls must be transported, maintained, and monitored in a proper curing environment. Cure of magnetic and backside coatings can also be accelerated by elevating the temperature of the coating, for example by placing an entire stockroll of coated media into an oven. This, however, is also an undesirable method of curing a coating prior to slitting because it can create the same logistical problems described above, because it is expensive and inefficient, and because exposing a stockroll to heat can have damaging effects on the wound magnetic recording medium. Heating a stockroll in an oven is inefficient because the plastic substrate of a magnetic recording medium is a poor conductor of heat. Thus, it can take several hours or days for heat to reach the inner layers of a wound stockroll, and cure of the inner layers is very much delayed. Furthermore, the temperature gradient created within the stockroll during this type of heating process can cause contraction and expansion of the magnetic recording medium, creating stresses within the stockroll. The stresses on the wound magnetic recording medium can potentially cause damage to the magnetic recording medium such as surface defects, including embossing, blocking and picking of the coating or coatings.

What is needed but not provided by the prior art is a method of partially or completely curing a magnetic coating or a backside coating provided on a magnetic recording medium, to increase the strength of the magnetic recording medium prior to slitting, and thereby increase slit edge quality of the magnetic recording medium. Preferably the method would not cause delay or compUcation in the manufacturing process, or produce undesired stresses on or coating defects to the magnetic recording medium. SUMMARY OF THE INVENTION

The present invention provides a method of producing a magnetic recording medium that includes a coating provided on a major surface of a substrate. The coating can be a magnetic coating or a non-magnetic coating. Preferably, the magnetic recording medium includes a magnetic coating coated on a major surface of the substrate. More preferably, a non-magnetic backside coating is also provided on a second major surface of the substrate. The magnetic coating, the backside coating, or both, are heated before or while the magnetic recording medium is wound to form a stockroll. The magnetic recording medium is heated to a temperature that accelerates cure of one or more of the coatings. The coating or coatings are allowed to cure, and are at least partially cured at the time the magnetic recording medium is slit.

The method overcomes shortcomings of prior magnetic recording medium production techniques that provide an at least partially cured coating at the time of slitting. For example, the present method can reduce the time required between coating and slitting during which the magnetic or backside coatings can cure. By winding the magnetic recording medium while it is at a temperature that accelerates cure of a coating, the magnetic recording medium stockroll can be at a cure temperature as the stockroll is formed. Therefore an advantage of the present method is the elimination of the time needed for heat to flow into the wound stockroll to heat the magnetic recording medium, especially the inner layers, to a temperature that accelerates cure of the coating. The method also eliminates the stresses produced by a temperature gradient within the stockroll as it is heated in an oven. Overall, the method provides a more streamlined method of accelerating cure of coatings on a magnetic recording medium prior to slitting. The method provides a stronger magnetic recording medium at the time of slitting, which improves the physical properties of the slit magnetic recording medium. Specifically, a magnetic recording medium produced by this streamlined production method will exhibit improved slit edge quality, and will therefore produce less debris during use.

An aspect of the present invention is a method of preparing a magnetic recording medium. The method includes the step of providing a magnetic recording medium that includes a coating disposed on a substrate, the coating containing a curable polymeric binder. The coating is heated to a temperature that accelerates cure of the coating, before or while the magnetic recording medium is wound to form a stockroll. The coating is allowed to at least partially cure, optionally in an environment that maintains the coating at a temperature that accelerates cure. The magnetic recording medium is then slit.

A further aspect of the invention is a method of heating a moving magnetic recording medium. The method includes the steps of providing a moving magnetic recording medium comprising a coating disposed on a substrate, the coating containing a curable polymeric binder. Using a source of infrared radiation, the magnetic recording medium is heated to a temperature that accelerates cure of the coating. The moving magnetic recording medium, which is moving at least 100 meters per minute, is then wound to form a heated magnetic recording medium stockroll.

Yet a further aspect of the present invention is a magnetic recording medium prepared from the above methods.

As used herein, the term "magnetic recording medium" refers to a substrate coated on at least one side with either a magnetic coating or a non¬ magnetic coating. The term "stockroll" refers to a magnetic recording medium wound to form a roll.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows a schematic side view of a heating step of the present invention wherein a magnetic recording medium is heated as it is wound to produce a heated stockroll. In a step that is not shown, the stockroll can be slit after a coating provided on the magnetic recording medium is partially or completely cured, to provide a slit magnetic recording medium.

Figure 2 shows a schematic of a heating step of another embodiment of the present invention. In this embodiment, the magnetic recording medium is heated prior to formation of a stockroll, as the magnetic recording medium travels over a packroll.

DETAILED DESCRIPTION

In the method of the present invention, a magnetic recording medium is provided wherein the magnetic recording medium comprises a coating disposed on a major surface of a substrate. The coating comprises a curable polymeric binder. Preferably, the coating can be a magnetic coating comprising a curable polymeric binder and a magnetic pigment. Optionally and preferably, a backside coating can be disposed on a second major surface of the substrate. The backside coating can contain a curable polymeric binder and preferably, a non-magnetic pigment.

The substrate may be any suitable substrate material known in the magnetic recording media art. Examples of suitable substrate materials include films and film-like materials having opposed major surfaces, for example films comprising polymers or copolymers of the following materials: polyesters such as polyethylene terephthalate ("PET"); polyolefins such as polypropylene; cellulose derivatives such as cellulose triacetate or cellulose diacetate; polymers such as polycarbonate, polyvinyl chloride, polyimide, polyphenylene sulfide, polyacrylate, polyether sulphone, polyether ether ketone, polyetherimide, polysulfone, aramide film, polyethylene 2,6- naphthalate film, fluorinated polymer, liquid crystal polyesters, polyamide, or polyhydric acid; paper, mixtures of these materials, or any other suitable material.

The pigment may be either a magnetic or a nonmagnetic pigment, depending upon whether the coating is a magnetic coating or a backside coating, respectively. A magnetic pigment may comprise any of the magnetizable materials known in the art. These magnetizable materials are typically finely divided particles of magnetic pigments, including pigments of magnetic oxides such as gamma hematite (y-FeiOi), magnetite (FesO*), mixed crystals of (γ- FeiOs), and (Fe3θ ), Co-doped Fe2θj, Co-doped FesO Co-doped mixed crystals of Fe2θ3 and FesO , barium ferrite, Berthollide compounds; various kinds of acicular magnetic alloy powders such as Fe, Fe-Co, Co-Ni, Fe-Co-Ni, Co-Cr, CrO_, Fe-Co-B, Fe-Co-Cr-B, Fe-Co-V, Mn-Bi, Mn-Al etc.; nitrides of Fe, Fe-Co, Fe-CS-Ni, fine iron, etc., and mixtures of two or more of the above. Examples of nonmagnetic pigment useful in backside coatings can be any pigment known to be useful in producing backside coatings for magnetic recording media. Specific examples include carbon black, TiOi, a-FeiOs, AI2O3,

The curable polymeric binder can be any material that contains reactive ingredients capable of curing via a chemical reaction, to produce a reaction product having a higher molecular weight than the individual reactants. The cured reaction product is preferably capable of adhering a pigment to a substrate. The reactive ingredients can include any of a number of commonly known monomers, oligomers, polymers or copolymers, existing in the form of thermoplastic materials, thermosetting materials, relatively low molecular weight crosslinking agents, or mixtures of these ingredients.

Specific examples of thermoplastics useful in the curable polymeric binder include polymers or copolymers comprising constitutive units of vinyl chloride, vinyl acetate, vinyl alcohols, maleic acid, acrylic acid, acrylates, vinylidene chloride, acrylonitrile, methacrylic acid, methacrylates, styrene, butadiene, ethylene, vinyl butyrals, vinyl acetals, and/or vinyl ethers, polyurethanes, polyesters, and natural rubbers.

Examples of thermosetting resins and reactive resins useful in the curable polymeric binder include phenolic resins, epoxy resins, hardening-type polyurethane resins, urea resins, melamine resins, alkyd resins, acrylic reactive resins, formaldehyde resins, silicone resins, epoxypolyamide resins, mixtures of polyester resins and isocyanate prepolymers, mixtures of polyester polyols and polyisocyanates, and mixtures of polyurethanes and polyisocyanates. Preferred ingredients of the curable polymeric binder include combinations of a polyisocyanate-functional ingredient with one or more of a vinyl chloride copolymer, a vinyl chloride-vinyl acetate copolymer, a vinyl chloride-vinyl acetate-vinyl alcohol copolymer, a vinyl chloride-vinyl acetate- maleic anhydride copolymer, a polyurethane polymer or copolymer, or combinations thereof.

Examples of preferred polyurethane resins include polyester polyurethanes, polyether polyurethanes, polyether polyester polyurethanes, polycarbonate polyurethanes, polyester polycarbonate polyurethanes, and polycaprolactone polyurethanes. Optionally and preferably, one or more components of the binder can comprise one or more pendant groups, such as polar groups, that facilitates dispersion and or wetting of a pigment in the coating. Examples of useful polar groups include -COOM, -SO3M, -OSO3M, -P«0(OM)2, -0-P-0(OM)2, (wherein M is a hydrogen atom or an alkali metal), -OH, -NR2, -N⁺R3, (wherein R is a hydrocarbon residue), an epoxy group, and -SH or -CN groups.

Specific examples of preferred binder ingredients used in the practice of the present invention include those commercially available from Union Carbide Corp. under the trade designations VAGH, VYNN, VMCH, VAGF, VAGD, VROH, VYES, VYNC, VMCC, XYHL, XYSG, those commercially available from Nippon Zeon Co., under the trade designations MR105, MRI 10, MR100, and 400X100 A; those commercially available from Toyobo Co., Ltd. under the trade designations VYLON UR8200, UR8300, UR8600, UR5500, UR4300, RV530, and RV280; RJIOO, commercially available from Monsanto; phenoxy resins such as PKHH phenoxy resin (commercially available from Phenoxy Associates), and YP.50S (commercially available from Tohto Kasei); nitrocellulose resins such as RS 1/2 sec (commercially available from Aqualon); Cellulostic Esters such as CAB-553-0.4 and CAP-504-0.2 (commercially available from Eastman Chemical); and Estane 5703, 5701, and 5705 (commercially available from B.F. Goodrich).

Preferably, the curable polymeric binder comprises a multi-functional crosslinking agent that facilitates cure of the curable polymeric binder. Crosslinking agents that are preferred in the practice of the present invention include multi-functional isocyanate crosslinking agents, such as di- or tri- functional isocyanate molecules. Examples of multi-functional isocyanate crosslinking agents include tolylene diisocyanate, 4,4^>-diphenylmethane diisocyanate, hexamethylene diisocyanate, xylylene diisocyanate, naphthylene- 1,5-diisocyanate, o-tolyidine isocyanate, isophorone diisocyanate and triphenylmethane triisocyanate; products of these isocyanates and polyalcohols, and polyisocyanates to be formed by condensation of these isocyanates. Examples of commercially available multi-functional isocyanates include CORONATE L, CORONATE HL, CORONATE 2030, CORONATE 2031, MILLIONATE MR, and MILLIONATE MTL (commercially available from Nippon Polyurethane Co., Ltd.); TAKENATE D-110N (commercially available from Takeda Chemicals Industries Co., Ltd.); DESMODUR L, DESMODUR IL, DESMODUR N, DESMODUR HL, DESMODUR CB 601, DESMODUR CB701, AND DESMODUR CB75 (commercially available from Bayer Co., Ltd.). Other useful polyisocyanate crosslinking agents are those described in published International Patent Apphcation PCT/US94/06382 (International Publication Number WO 95/01636).

The relative amounts of magnetic pigment and curable polymeric binder present in a magnetic coating can depend on many factors, including the intended application of the magnetic recording medium. Typically, for a magnetic recording medium used in data cartridge applications, a magnetic coating can contain from about 70 to 85 parts per weight, more preferably from about 72 to 80 parts by weight magnetic pigment and from about 15 to 30, more preferably from 20 to 30 parts by weight reacted binder.

The relative amount of nonmagnetic pigment and curable polymeric binder present in a backside coating can depend on factors including the intended application of the magnetic recording medium and the desired frictional properties of the backside coating. Typically a backside coating may contain up to about 60 parts by weight, more preferably less than about 50 parts by weight nonmagnetic pigment, and from about 80 to 40, more preferably from 70 to 50 parts by weight polymeric binder.

In addition to a pigment and a curable polymeric binder, coatings provided on a magnetic recording medium may also contain other ingredients, including for example dispersants, lubricants, wetting agents, anti-static agents, fungicides, bactericides, surfactants, coating aids, non-magnetic pigments, etc., known in the magnetic recording art. Examples of lubricants include saturated and unsaturated fatty acids, fatty acid esters, higher fatty acid amides, higher alcohols, silicone oils, mineral oils, molybdenum disulfide, fluorinated polymers, such as perfluoro ethers, and the like. Non-magnetic pigments may also be included in a magnetic coating to act as a head cleaning agent, as an abrasive, as an antistatic agent, or for other purposes. Examples of useful non¬ magnetic pigments include silicon oxide, titanium oxide, aluminum oxide, chromium oxide, calcium carbonate, zinc oxide, talc, kaolin, silicon carbide, and carbon black.

The thickness of a magnetic coating can depend on such factors as the pigment loading and the intended application of the magnetic recording medium. In general a magnetic coating will have a coating thickness (taken after the coating has been dried) in the range from about 40 to 400 μinches (μ"), or about 1 to 10 μm (microns). For data cartridge applications, the dry coating thickness is preferably in the range from about 40 to 100 μ" (1 to 2.5 μm); for video applications, the dry coating thickness is preferably in the range from 80 to 200 μ" (2 to 5 μm); and for audio applications the dry coating thickness is preferably in the range from about 100 to 400 μ" (2.5 to 10 μm).

The thickness of a backside coating (taken after the coating has been dried) is typically in the range from about 10 to 100 μ" (0.25 to 2.5μm), preferably from about 30 to 80 μ" (0.75 to 2 μm), and more preferably about 50 μ" (1.25 μm).

A magnetic coating, a backside coating, or both, can be disposed onto a substrate by conventional techniques known in the magnetic recording media art. A summary of the principles of manufacturing magnetic tapes can be found in "The Complete Handbook of Magnetic Recording" - Chapter 13 entitled "Manufacture of Magnetic Tapes and Discs" by Finn Jorgensen (3rd ed., 1988). Generally, magnetic and/or nonmagnetic pigments are dispersed in a curable polymeric binder solution to produce either a magnetic coating dispersion or a backside coating dispersion, respectively. The magnetic or backside dispersion is then coated onto a major surface of a substrate and dried, to form a magnetic coating or a backside coating comprising a curable polymeric binder and a dispersed pigment.

The magnetic coating dispersion or backside coating dispersion of the present invention preferably includes a pigment and a curable polymeric binder comprising one or more reactive ingredients, and may also include any of the other ingredients described above to be useful in a magnetic or backside coating. Additionally, the magnetic or backside coating dispersions may contain coating aids such as wetting agents and/or dispersants, and may also contain a solvent that facilitates coating of the dispersion onto the substrate. Useful solvents include MEK (methylethylketone), toluene, cyclohexanone, THF (tetrahydrofuran), MIBK (methylisobutylketone), ethyl acetate, and mixtures thereof. The actual choice of solvent is largely governed by the particular solubility characteristics of the chosen binder components, but it should not be reactive with any other components of the magnetic recording medium, for example the substrate. Preferred solvents include toluene, cyclohexanone, tetrahydrofuran, methyl ethyl ketone and mixtures thereof.

The solvent may be present in a magnetic coating dispersion in any amount that will facilitate coating, for example from 50 to 90 parts by weight solvent per 100 parts by weight of a magnetic coating dispersion. "Parts by weight of the magnetic coating dispersion" includes the ingredients of the magnetic coating, plus any additional solvent. The solvent may be present in a backside coating dispersion in an amount useful to facilitate coating, for example from 70 to 90 parts by weight solvent per 100 parts by weight of a backside coating dispersion. "Parts by weight of a backside coating dispersion" includes the ingredients of the backside coating, plus any additional solvent. Any of a number of conventional procedures known in the magnetic recording media art may be used to apply the magnetic or backside coating dispersions onto a substrate to form a magnetic coating or a backside coating. For example, gravure coating systems, extrusion coating systems, knife coating systems, etc. may be used. Once applied to the substrate, a magnetic coating may be subjected to one or more after-treatments, such as magnetic orientation, prior to drying. The magnetic and/or backside coatings can then be dried by conventional methods, for instance by exposure to heated air, an infrared heater, microwave oven, etc. During the drying stage, the solvent contained in the coating dispersion is evaporated, leaving behind a coating comprising a curable polymeric binder. The dried magnetic and/or backside coatings may be subjected to further processing steps at appropriate stages of the production process including for example, aging to remove residual solvent, calendering to compact the binder and smooth the coated surface, burnishing, buffing, etc.

The magnetic recording medium is wound to form a stockroll which can later be slit to a desired width and converted into a finished magnetic recording media product. In the practice of the present invention a coating provided on the magnetic recording medium is at least partially cured prior to slitting the magnetic recording medium. A magnetic or backside coating is considered to "cure" when the reactive ingredients of the binder react to form a reaction product having a higher molecular weight than the individual reactive ingredients.

The slitting process generally tends to stretch and/or tear the edge of a magnetic recording medium, creating undesired physical imperfections at the slit edge of the magnetic recording medium, such as tearing of the substrate, or cracking of a coating. As a polymeric binder contained in magnetic or backside coating cures, the reaction between the reactive components of the binder causes the coating to strengthen (e.g., tensile strength of the coating increase). The increased strength of the coating provides added strength to the magnetic recording medium, which can prevent stretching and tearing of the magnetic recording medium during the slitting process. Reduced stretching and tearing during slitting results in an improved slit edge quality of the magnetic recording medium, i.e., reduced torn edges and reduced cracking of the coating. The improved slit edge quality can provide improved performance characteristics, such as reduced debris created during use.

The following description of embodiments of the present invention refers almost entirely to the cure of a magnetic coating provided on a magnetic recording medium. It is to be understood, however, that the present invention also contemplates the cure of a backside coating prior to slitting, to improve slit edge quality of a magnetic recording medium.

To accelerate cure of a magnetic coating (or a backside coating), the magnetic recording medium of the present invention is wound while the magnetic recording medium is at a winding temperature that accelerates cure of the coating, and a heated stockroll is produced. The winding temperature can be any temperature sufficient to accelerate cure of the coating, but should not be high enough to damage the magnetic recording medium. Preferably, the winding temperature will be at or above the "Optimal Cure Temperature" of the coating. The "Optimal Cure Temperature" of a coating refers to a curing temperature that will result in a cured coating having its maximum glass transition temperature (Tg). When the coating cures at or above its Optimal Cure Temperature, it has been observed that the coating can achieve its optimum (highest possible) glass transition temperature. If the coating cures at a temperature below its Optimal Cure Temperature, it has been observed that the coating does not cure to its highest possible glass transition temperature, but will cure to a sub-optimal glass transition temperature, generally equal to the curing temperature.

The Optimal Cure Temperature of a coating will depend on many factors, especially the particular chemistry of the coating. Each coating system (i.e., binder, pigments, etc.) could potentially have a different Optimal Cure Temperature. The Optimal Cure Temperature of a coating can be determined experimentally by measuring the glass transition temperature of identical coatings that have been cured at different temperatures. The maximum achieved glass transition temperature will be the Optimal Cure Temperature. Glass transition temperature of a coating can be determined by methods that are known in the magnetic recording media art, for instance by use of a Dynamic Mechanical Analyzer (DMA) such as the 981 Dynamic Mechanical Analyzer available from DuPont.

Although it is preferred to cure a coating at or above its Optimal Cure Temperature, it may not always be possible to do so because the upper limit of the winding temperature might be defined by other factors. For instance, the magnetic recording medium substrate might be thermally unstable above certain temperatures, and can tend to relax and lose their tensile strength above e.g., 60°C. For magnetic recording media that include these substrates, the winding temperature should not be so high that the substrate becomes thermally unstable. In these situations, the winding temperature can be below the Optimal Curing Temperature of the coating, while still being sufficiently high to accelerate cure of the coating.

For the preferred magnetic coatings described above, that cure due to the reaction of hydroxyl groups with isocyanate groups, the winding temperature is preferably at least about 40° C, and the heated magnetic recording medium stockroll is preferably at a temperature of at least about 40°C. More preferably the winding temperature is in the range from about 50 to 60°C, and the heated magnetic recording medium stockroll is at a temperature in the range from about 50 to 60°C. Lower or higher winding temperatures can be used, especially for magnetic coatings having different chemistries that will cure at different temperatures.

The magnetic coating can be heated to a winding temperature by any method capable of heating the magnetic recording medium to a winding temperature before or during winding of the magnetic recording medium into a stockroll. Useful methods might include exposing the magnetic recording medium to a heated atmosphere (i.e., an oven), or to electromagnetic radiation. Preferred heating methods involve exposing the magnetic recording medium to infrared radiation.

Figure 1 illustrates a preferred embodiment of the present invention, wherein magnetic recording medium 2 comprises magnetic coating 4 coated on one major surface of substrate 6. Magnetic recording medium 2 may optionally have a backside coating disposed on the other surface of substrate 6. In Figure 1, magnetic recording medium 2 is heated by heating apparatus 8, as magnetic recording medium 2 is wound to form stockroll 10. By heating magnetic recording medium 2, magnetic coating 4 is heated to a temperature that accelerates cure of the coating. Although Figure 1 illustrates heating of magnetic coating 4 at a point where magnetic recording medium 2 forms stockroll 10, magnetic coating 4 could also be heated at any point prior to the winding apparatus that would provide a stockroll having a temperature that accelerates cure of coating 4. Thus, heating apparatus 8 could be positioned as illustrated in Figure 1, or, heating apparatus 8 could be positioned as illustrated in Figure 2, and directed toward magnetic recording medium 2 before it forms stockroll 10. As yet another possibility, heating apparatus 8 could be positioned a further distance from the winding apparatus, as long as the magnetic recording medium is at a desired winding temperature once it reaches the winding apparatus.

In the practice of the present invention either side of the magnetic recording medium can be exposed to the heating apparatus in order to heat the magnetic recording medium to a winding temperature. In Figure 1 substrate 6 of magnetic recording medium 2 is exposed to heating apparatus 8, and magnetic layer 4 is heated through substrate 6. In Figure 2 on the other hand, magnetic coating 4 is exposed to heating apparatus 8, and is heated directly. Although only the magnetic coating needs to be heated in order to accelerate cure, the entire magnetic recording medium will typically absorb heat, thereby raising the temperature of the entire magnetic recording medium. It can be advantageous to heat the entire magnetic recording medium because the substrate can act as a heat sink, which can then maintain the temperature of the magnetic coating. The heating apparatus used in the practice of the present invention can be any heating apparatus capable of heating a magnetic recording medium, before or during formation of a stockroll, to produce a wound stockroll heated to a temperature that accelerates cure of a coating on the magnetic recording medium. The required heat output of the heating apparatus will depend on the speed of the magnetic recording medium (web speed) as the magnetic recording medium is transported and wound into a stockroll. Preferably, the method of the present invention can be implemented according to Figures 1 and 2, wherein a heating apparatus is installed at the end of a magnetic recording medium production line. According to this configuration, heating of the magnetic coating to a winding temperature, and winding of the magnetic recording medium into a stockroll, occur soon after the magnetic coating (and optional backside coating) has been coated onto the substrate and dried. Typical web speeds of a magnetic recording medium production line can be above 100 or 200 meters per minute (m/min), are preferably above 250 m min, and are most preferably above 300 or 400 m/min. Thus, it is preferred that the heating apparatus be capable of heating the magnetic recording medium to a winding temperature as the magnetic recording medium passes the heating apparatus at or above these web speeds.

The distance of the heating apparatus from the magnetic recording medium (as measured perpendicularly from the surface of the magnetic recording medium) can vary depending on the nature of the magnetic recording medium, the desired winding temperature, the type and intensity of the heating apparatus, etc. In general, the heating apparatus can be any distance from the magnetic recording medium that will result in the magnetic recording medium being heated to the desired winding temperature. At the above web speeds, and with the preferred heating apparatuses (described directly below) distances from the magnetic recording medium to the element of the heating apparatus in the range from about 1 to 6 inches (about 2.5 to 15 centimeters) have been found to be useful in heating the magnetic recording medium to desired winding temperatures. More preferably the distance from the magnetic recording medium to the heating apparatus can be in the range from about 3 to 5 inches (about 7.5 to 13 cm).

Heating apparatuses that have been found to be useful include "high efficiency" heating apparatuses such as those that emit infrared electromagnetic radiation. Preferred infrared heating apparatuses emit electromagnetic radiation of a wavelength in the range from about 1 about to 5 micron, more preferably from about 1.2 to 2.1 micron. One particularly preferred heating apparatus is a RADPLANE Series 80 infrared (IR) heater from Glenro Co., of Patterson NJ. This heating apparatus emits radiation of a wavelength of about 1.7 micron from tungsten elements enclosed by quartz emitter tubes that are partially surrounded by gold reflectors. The rating of the tubes is 1.5 Kilowatts (kW) at 220 Volts AC (VAC).

After the magnetic recording medium has been wound to produce a heated stockroll, the magnetic coating is allowed to at least partially cure prior to slitting the magnetic recording medium. The magnetic recording medium of the heated stockroll can preferably be maintained at a temperature that accelerates cure of the coating (i.e., a cure temperature). The cure temperature can be the same as or different (either higher or lower) than the winding temperature, and need not be constant throughout the cure period. As described above, it is preferred that the magnetic coating be cured at or above the Optimal Cure Temperature of the coating. On the other hand, the cure temperature can be limited by the stability of the magnetic recording medium, for example, the substrate. For the preferred coatings described above, comprising reactive hydroxyl groups and an isocyanate-functional crosslinker, the coating is preferably maintained at a cure temperature of at least about 40°C, more preferably, from about 50 to 60°C. Again, lower or higher temperatures can be used, especially for magnetic coatings having different chemistries. The magnetic recording medium of a heated stockroll can be maintained at a cure temperature by any useful method. In one embodiment of the present invention, the heated stockroll can be stored in an atmosphere that will keep the magnetic coating at a cure temperature. This can be accomplished, at least for a relatively short period of time such as for several hours, by holding the heated stockroll at ambient temperature (e.g., 20 to 25°C, or possibly even lower) and allowing the magnetic recording medium to cool naturally. Due to the typical low thermal conductivity of the magnetic recording medium (e.g., PET generally has a thermal conductivity of about 3.5 x 10^"4 cal cm/sec cm² °C), the amount of heat added to the magnetic recording medium during the heating and winding steps can take several hours to dissipate in an ambient temperature environment. For example a stockroll having a total diameter of 18" (45.7 cm), and comprising a magnetic recording medium wound onto a 6" (15.25cm) fiber core, can take in excess of twelve hours to cool from 65°C to an ambient temperature of about 25°C.

Alternatively, the magnetic recording medium of a heated stockroll can be maintained at a cure temperature by insulating the stockroll, for example with blankets, to prevent the flow of thermal energy from the stockroll. The insulating blanket can be comprised of any type of insulating material, such as a heat reflective aluminum space blanket, an insulating blanket, or a blanket that is heated by resistive heating mechanisms (an electric blanket). As another alternative, the temperature of the heated stockroll can be maintained at a cure temperature by placing the entire stockroll into a heated atmosphere, such as an oven. The oven can be heated to a temperature that will maintain the coating at a cure temperature, e.g., the oven can be maintained at a temperature in the range from about 40 to 60°C, preferably from about 50 to 60°C.

The coating can be allowed to cure to any extent that provides greater strength of the magnetic recording medium, and therefore results in an improved slit edge quality of the magnetic recording medium upon slitting. A useful extent of cure of a magnetic coating will depend on many factors, including the particular chemistry of the binder in a coating, the construction of the magnetic recording medium, for example the substrate, or other factors. In the practice of the present invention, and for magnetic coatings comprising the preferred polymeric binders described above, a magnetic coating can preferably be at least 50% cured, and more preferably at least 60% cured, at the time of slitting, as determined by the gel permeation chromatography method described in detail in Example 1 below.

The amount of time necessary to provide a coating with a useful extent of cure will depend on many factors, including the desired percent cure of the coating at slitting, the winding temperature, the cure temperature, and on the particular chemistry of the coating. For the preferred magnetic coatings described above, the time needed at a cure temperature of about 55°C, to provide a cure of at least about 60%, is in the range from about 2 to 4 hours. For magnetic coatings that cure relatively more rapidly, less time may be needed to reach the desired extent of cure. In such cases, there may be no need for external influences (e.g., blankets or ovens) to maintain the temperature of the heated stockroll at a cure temperature, and the cure step can be accomplished by holding the heated stockroll at ambient temperature for a relatively short period of time, e.g., less than 4 to 6 hours. (It may still be desirable to insulate the heated stockroll in order to prevent heat loss, which might create a temperature gradient within the stockroll, causing expansion and/or contraction of the stockroll.) If the heated magnetic recording medium stockroll is moved rapidly from the winding apparatus- to the slitting apparatus, the coating may have experienced sufficient cure to provide acceptable slit edge quality.

Extent of cure of a magnetic coating can be measured by a number of known techniques, for instance by comparing the amount of reactive ingredients in a coated but uncured magnetic or backside coating to the amount of reactive ingredient remaining after cure. One method of determining the concentration of a reactive ingredient in a coating is through the use of gel permeation chromatography (GPC). With GPC, a solvent such as THF can be used to extract reactive ingredients from a coating at room temperature. The amount of reactive ingredients present in a cured or partially cured coating is compared to the amount of reactive ingredients initially present in the coating, to determine an extent of cure. Other methods of monitoring the extent of cure of a coating include methods that extract aromatic reactive ingredients (with dimethyl sulfoxide, or DMSO) from a coating. The amount of aromatic reactive ingredient present in the extracted solution can be determined using a spectrophotometer. Still other known techniques can be used to monitor the amount of -NCO groups before and after cure.

The magnetic recording medium can be slit by methods known in the magnetic recording art, for instance by use of a shear type slitter. An example of a preferred slitting apparatus is a shear type slitter such as the Model 609, commercially available from the Dusenbery Co., of Randolph NJ.

The present invention will now be described in terms of the following non-limiting examples. Although the examples relate to the production of a magnetic recording medium comprising a single magnetic layer on one side, and a backside coating on the other side, it will be appreciated that the present invention may be useful in the production of magnetic recording media having other product configurations, for example magnetic recording media that do not contain a backside coating, magnetic recording media that comprise more than one magnetic layer, or magnetic recording media that comprise a magnetic coating disposed on top of a non-magnetic coating, etc.

EXAMPLES

PREPARATION OF COATING MIXTURES

Magnetic dispersions were prepared according to Table 1:

For Magnetic Dispersion 1 (MD-1), the ingredients of Table 1 were premixed with a Shar mixer for 8 hours and then dispersed in a sand mill for 8 hours to prepare a dispersion at 38% solids. To this dispersion was added, with mixing, 9.5 parts of a predispersed alumina solution (75% solids in THF), 42 parts cyclohexanone, and 29.9 parts THF, to product a 32.5% solids dispersion. For Magnetic Dispersion 2 (MD-2), the ingredients of Table 1 were premixed with a Shar mixer for 8 hours and then dispersed in a sand ill for 8 hours to prepare a 38% solids dispersion. To this dispersion was added, with mixing, 9.5 parts of predispersed alumina solution (75% solids in THF), 40.9 parts cyclohexanone, and 29.3 parts THF, making a 32.5% solids dispersion.

Table 2

Magnetic Coating Solution

MCI MC2 MC3

Magnetic Dispersion Used MDl MDl MD2 pans by weight CB601 crosslinking agent from Bayer 12.3

Parts by weight NR320 (crosslinking agent described 15.7 15.3 in U.S. Patent Application Serial No. 08/085,189; 60% in propyleneglycol methyl ether acetate)

Magnetic Coating Solutions 1 through 3 (MCI through MC3) were prepared according to Table 2. To prepare each magnetic coating solution, the identified magnetic dispersion was blended with the identified crosslinking agent. Each coating solution was then passed through HT40 filters (from Nippon Roki) before coating the magnetic coating solution onto a substrate. BACKSIDE DISPERSION

A carbon backside coating dispersion was prepared from the following ingredients:

Table 3

Ingredients in parts by weight

THF 390.4

Carbon 1 (Acetylene Carbon Black, Chevron Chem. Co.) 30.4

Carbon 2 (Thermax N-991, Cancarb, Ltd.) 7.6

TiOz (Ti-Pure R-960, DuPont) 9.5

Lecithin 1.5

Nitrocellulose (RS 1/2 sec, Aqualon) 14.7

ESTANE 5703P (Polyurethane from B F Goodrich) 22.0

The ingredients of Table 3 were premixed with a Shar mixer for 2 hours and then dispersed in a sand mill for 2 hours to prepare a dispersion at 18% solids. To this dispersion was added, with mixing, 2.2 parts of a predispersed alumina solution (75% solids in THF), 53.6 parts cyclohexanone, and 92.5 parts THF, making a 14% solids dispersion. At coating the dispersion was blended with 20.5 parts NR320, and passed through HT40 filters (Nippon Roki) before applying the coating solution to a substrate.

EXAMPLE 1 The backside dispersion was applied to a major surface of a 7 micron

(0.28 mil), biaxially oriented polyethylene terephthalate (PET) substrate using a rotogravure coating apparatus, and dried by passing the web through an oven heated to 140°F (60°C). In the same pass the magnetic coating solution MCI was applied to the opposite major surface of the substrate, magnetically oriented, passed through an oven at 80°C to drive off volatile materials, and immediately calendered at 214 kg/cm and 54°C to provide a very smooth magnetic recording surface. After calendering the magnetic recording medium was heated to 65 °C by passing it under a RADPLANE Series 80 infrared heating apparatus, from Glenro, Inc. of Patterson NJ, just prior to winding the magnetic recording medium into a stockroll. The heating apparatus included three short wavelength twin tube emitters comprising tungsten elements. The tungsten elements were located approximately 4.5" (11.4cm) from the magnetic recording medium, with the magnetic coating side of the medium facing the heating apparatus. When used in the method of the present invention, a silicone controlled rectifier was used to control the tube temperature via an optical feedback control loop. Forced air was blown over the emitter tubes to reduce excess heat in the system enclosure. The wound stockroll was kept warm by placing it in a circulating air oven at 50°C. After 4 hours the stockroll was removed from the oven and slit to 1/4 inch width (6.26 mm).

COMPARATIVE EXAMPLE 2

A magnetic recording medium was prepared according to the same method used for Example 1, except that the magnetic recording medium was not heated with the infrared heating apparatus just prior to winding, and the stockroll was maintained at ambient temperature (20°C) for four hours until slitting.

EXAMPLE 3 The backside dispersion was applied to a major surface 7 micron (0.28 mil), biaxially oriented polyethylene terephthalate (PET) substrate using a rotogravure coating apparatus and dried by passing the web through an oven heated to 60° C. In the same pass the magnetic coating solution MC2 was applied to the opposite major surface of the substrate, magnetically oriented, passed through an oven at 80° C to drive off volatile materials, and calendered at 214 kg/cm and 54 °C to provide a very smooth magnetic recording surface. After calendering, the web was heated to 55 °C by passing it under the infrared heating apparatus described in Example 1, just prior to winding into a stockroll. The wound stockroll was kept warm by placing the stockroll in an oven at 50°C. After 2 hours in the oven the stockroll was removed and slit to 1/4 inch width (6.26mm).

COMPARATIVE EXAMPLE 4 A magnetic recording medium was produced using the same method as used for Example 3, except that the magnetic recording medium was not heated with the infrared heating apparatus just prior to winding, and the stockroll was maintained for two hours at ambient temperature (20°C) until slitting.

EXAMPLE 5

The backside dispersion was applied to a major surface of a 7 micron (0.28 mil), biaxially oriented polyethylene terephthalate (PET) substrate using a rotogravure coating apparatus and dried by passing the web through an oven. In the same pass the magnetic coating solution MC3 was applied to the opposite major surface of the substrate, magnetically oriented, passed through an oven at 80°C to drive off volatile materials, and immediately calendered at 214 kg/cm and 54°C to provide a very smooth magnetic recording surface. After calendering, the magnetic recording medium was heated to 55°C by passing it under the infrared heating apparatus described in Example 1, just prior to winding into a stockroll. The wound stockroll was kept warm by placing it in an oven at 50°C. After 2 hours the stockroll was removed from the oven and slit to 1/4 inch width (6.26 mm).

COMPARATIVE EXAMPLE 6 A magnetic recording medium was prepared according to the same method used in Example 5 except that the magnetic recording medium was not heated using the infrared heating apparatus, and the stockroll was maintained for two hours at ambient temperature (20 °C) until it was slit. EXAMPLE 7

The backside dispersion was applied to a major surface of a 7 micron (0.28 mil), biaxially oriented polyethylene terephthalate (PET) substrate using a rotogravure coating apparatus and dried by passing the web through an oven. In the same pass the magnetic coating solution MCI was applied to the opposite major surface of the substrate, magnetically oriented, passed through an oven at 80°C to drive off volatile materials, and immediately calendered at 214 kg/cm and 54 °C to provide a very smooth magnetic recording surface. After calendering the magnetic recording medium was heated to 60°C by passing it under the infrared heating apparatus described in Example 1, just prior to winding into a stockroll. The wound stockroll was insulated with a thin sheet of aluminum space blanket and set at ambient temperature (20°C) until it was slit. Part of the stockroll was slit to 1/4 inch (6.26 mm) after seven hours (example 7a) and the remainder was slit after 24 hours (example 7b).

COMPARATIVE EXAMPLE 8

A magnetic recording medium was prepared according to the method of Example 7 except that the magnetic recording medium was not heated using the infrared heating apparatus just prior to winding, and the stockroll was maintained at ambient temperature (20°C). Part of the stockroll was slit to 1/4 inch (6.26 mm) after seven hours (example 8a) and the remainder was slit after 24 hours (example 8b).

EXAMPLE 9 The backside dispersion was applied to a major surface of a 7 micron

(0.28 mil), biaxially oriented polyethylene terephthalate (PET) substrate using a rotogravure coating apparatus and dried by passing the web through an oven. In the same pass the magnetic recording solution MCI was applied to the opposite major surface of the substrate, magnetically oriented, passed through an oven at 80°C to drive off volatile materials, and immediately calendered at 214 kg/cm and 54°C to provide a very smooth magnetic recording surface. After calendering the web was heated to 60°C by passing it under the infrared heating apparatus described in Example 1, just prior to winding into a stockroll. The wound stockroll was kept warm by placing it in an oven at 50°C. After seven hours the stockroll was removed from the oven and slit to 1/4 inch width (6.26 mm).

COMPARATIVE EXAMPLE 10 A magnetic recording medium was prepared according to the method of

Example 9 except that the magnetic recording medium was not heated using the infrared heating apparatus just prior to winding, and the coated stockroll was maintained for seven hours at ambient temperature (20°C) until it was slit.

Curing and Slit Edge Quality Comparison

Extent of cure of the sample magnetic recording coatings was determined by gel permeation chromatography analysis according to the following method. Extent of cure was measured over an identified period of time expressed in hours, by measuring samples having been cured for different amounts of time. Free or uncrossUnked polymer was solubilized from each coating sample using 20 milliUters (mis) of inhibited THF (toluene was added at 5 mls/4 L THF) for at least 16 hours. The extracting solution was decanted into a glass syringe fitted with a 5 micron Teflon filter to remove particulates. An aliquot of the filtered extracting solution was transferred into a 1-3 ml vial and placed into an appropriate carousel for analysis using a Waters LC/GPC chromatographic system. A five-column set was used to separate the selected polymer matrix of polyurethane and vinyl resins. The standard column set for GPC % cure included five TSK-HXL 5 μm particle size, 30 cm length columns. The toluene added to the preparation solvent was used to monitor the system performance and also as an internal standard to normaUze data within the cure set. Refractive index (RI) detectors and ultra-violet (UV) detectors were used in tandem to measure the separation results. The areas associated with the appropriate peak(s) were obtained and put into an automatically calculating Excel worksheet. Typically, areas were coUected for peaks associated with the polymer(s), oUgomer(s), low molecular weight components (lubricants and trimer(s)), as requested, and for the toluene internal reference. The percent cure was determined by comparing the polymer and oligomer extractables obtained after an aging period to those obtained in a sample extracted immediately after coating (at time To).

The sUt film was inspected under a 650X microscope to evaluate sUt edge quaUty of the magnetic recording media samples. Results are given in Table 4.

Table 4

The data of Table 4 show that magnetic recording media that, at about the time of winding to form a stockroU, were heated to a temperature that accelerated cure of a magnetic or backside coating, could be cured in as Uttle as 2 hours to a degree that provided improved sUt edge quality.

Claims

WHAT IS CLAIMED IS:

1. A method of preparing a magnetic recording medium, the method comprising the steps of: a) providing a magnetic recording medium comprising a coating disposed on a substrate, the coating comprising a curable polymeric binder, b) heating the magnetic recording medium to a winding temperature that accelerates cure of the coating; c) winding the magnetic recording medium into a heated stockroll; d) aUowing the coating to cure; and e) slitting the magnetic recording medium.

2. The method of claim 1, wherein the winding temperature is at least about 40° C.

3. The method of claim 1, wherein the winding temperature is equal to or greater than a temperature that will provide the maximum glass transition temperature of the coating.

4. The method of claim 1, wherein the magnetic recording medium is heated to a winding temperature using a heating apparatus that emits infrared radiation.

5. The method of claim 4, wherein the infrared radiation has a wavelength in the range from about 1 to 5 microns.

6. The method of claim 4, wherein the element of the heating apparatus is located a distance in the range from about 2.5 to 15 centimeters from the magnetic recording medium.

7. The method of claim 1, wherein during step d), the heated stockroU is maintained at a cure temperature that accelerates cure of the coating.

8. The method of claim 7, wherein the cure temperature is at least about 40°C.

9. The method of claim 7, wherein the heated stockroU is maintained at the cure temperature for a time ranging from about 2 to 4 hours.

10. The method of claim 7, wherein the cure temperature is equal to or greater than a temperature that will provide the maximum glass transition temperature of the coating.

11. The method of claim 1, wherein the coating is at least 50 percent cured at the time the magnetic recording medium is sUt, as determined by gel permeation chromatography.

12. A method of heating a moving magnetic recording medium, comprising the steps of: a) providing a moving magnetic recording medium comprising a coating comprising a curable polymeric binder, b) heating the moving magnetic recording medium with a heating apparatus, to a temperature that accelerates cure of the coating; and c) winding the magnetic recording medium to form a heated magnetic recording medium stockroU; wherein the magnetic recording medium moves past the heating apparatus at a speed of at least 100 meters per minute.

13. The method of claim 12, wherein the heating apparatus emits infrared radiation having a wavelength in the range from about 1 to 5 microns.

14. The method of claim 13, wherein the element of the heating apparatus is located a distance in the range from about 2.5 to 15 centimeters from the magnetic recording medium.

15. The method of claim 12, wherein the heated stockroU has a temperature of at least 40°C.

16. A magnetic recording medium prepared by the method of any of claims 1-15.