GB2174620A - Vapour phase curing of magnetic recording medium - Google Patents

Abstract

A method of completing the curing process for a magnetic recording medium having a polymeric binder system and a polyisocyanate curing agent is disclosed. In this process, the medium is exposed to a volatile vapor phase catalyst to accelerate and complete the cure.

Description

SPECIFICATION Improved magnetic recording medium curing process Technical Field The present invention relates to the manufacture of magnetic recording media. In particuiar, the invention concerns an improvement in curing the magnetic coat of magnetic recording media by exposure to a volatile vapor phase catalyst.

Background Art Particulate magnetic recording media, in general, include a thin magnetic layer supported by a non-magnetic substrate, with the magnetic layer containing a suitable particulate magnetic pigment dispersed and retained in a matrix of organic, polymeric binder. The magnetic layer is applied to the non-magnetic substrate in the form of a dispersion in a solvent, the dispersion including the particulate magnetic pigment, a binder, a curing agent, and, generally, minor amounts of one or more conventional additives.Examples of such examples are: dispersants that aid in the deagglomeration and dispersal of the magnetic particles, conductive pigments that reduce the electrical resistivity of the media, lubricants that minimize head-to-media friction, and abrasives that prevent debris obtained during use of the media from accumulating on the record/ reproduce heads and clogging them.

In a typical process of manufacturing magnetic recording media, the solvent-based magnetic dispersion is prepared, thoroughly mixed, and readied for coating on the non-magnetic substrate, which is in the form of a long web. The solvent-based magnetic dispersion is applied to the web by a suitable coater to form a magnetic layer thereon of desired thickness. After coating, the solvent is removed by evaporation in a drying oven, leaving a flexible, but solid magnetic layer. The coated web is then calendered-i.e., the surface is smoothed, typically by squeezing through rollers. After calendering, the coated web is further processed according to its final form-if to be tape, it is slit to the proper width, cleaned, and wound on spools; if to be disc or magnetic card, it is punched or cut to the proper size and shape, and cleaned.The finished media are then subjected to quality assurance testing, packaged and stored.

During the manufacturing process, the binder is cured to bind the particulate magnetic pigment within the flexible magnetic layer so that it is resistant to mechanical abrasion. Many present-day magnetic media incorporate a polymeric binder, which chemically reacts with a curing agent, whereby the binder is cross-linked to form a matrix that binds the magnetic particles within the magnetic layer. This chemical curing process begins as soon as the magnetic dispersion containing the binder and curing agent is prepared and mixed, and proceeds at a concentrationdependent rate-i.e., as the unreacted curing agent is depleted, the reaction slows down-and continues until the curing agent is consumed. This concentration-dependent rate of the process is exponential in nature, resulting in a protracted time frame for the completion of curing.

In most media manufacturing processes relying on chemical curing, a period of several weeks may be required to complete the curing, it steps are not taken to accelerate the curing process.

The curing process should not, however, be allowed to proceed too far towards completion before the coated web is calendered; otherwise, the desired smooth surface will not be achieved through calendering. Typically, the final stage of curing occurs following calendering while the medium is stored. Because the curing agent is reactive with water and the process of curing is influenced by the conditions of the environment under which the process occurs, control of moisture as well as other storage environment conditions is required for reproducible curing.

Thus, unless special precautions are taken during storage over the final stage of curing to assure control of moisture, maintenance of constant temperature, and, in general, to assure reproducible curing conditions, an inconsistency in the composition of the manufactured magnetic recording media results. It would be advantageous to accelerate the final stage of the curing so that it would be complete at the end of the media manufacturing process before the media are stored.

Such disadvantages of chemical curing have long been recognized. One proposed solution rejects chemical curing altogether and substitutes the use of an electron beam or other forms of radiation to catalyze the magnetic particle binding material forming the cross-linking reaction.

Radiation curing suffers from several disadvantages, however: the equipment required is extremely costly, and the cost is further increased because the conduct of the radiation curing process requires shielding and a more complex physical plant. The necessity for relatively elaborate and expensive equipment and environment control makes this approach not conveniently adaptable to a multi-stage curing process that allows for pre-calendering partial curing and post-calendering final curing. Also, different, special binders must be used, which are susceptible to electron beam or other radiation interaction. In short, a solution which could be superipmosed on presently used chemical curing processes to bring curing to rapid completion under controlled conditions would be preferred.

Various solutions have been proposed to accelerate the chemical curing process and avoid the aforementioned disadvantages characterizing that process. One such solution has been the acceleration of the completion of chemical curing following calendering by the application of heat to the coated and calendered web. While permitting the use of conventional binders, this process is also costly. The cost problem is significant: the required space and equipment are expensive, as is the time required to heat and cool the media being cured. An even more significant drawback in the use of heat is its tendency to cause dimensional distortions in the resulting product, which can be more detrimental than the problems heat was designed to correct.

A process called "kiss coating" is another approach that has been proposed for improving the curing of present-day chemical curing agent based magnetic recording media formulations. For example, U.S. Patent No. 3,366,505 discloses a kiss-coating system for catalyzing curing completion of magnetic media. In this approach, the cross-linking agent in the magnetic dispersion originally coated on the non-magnetic substrate is exposed to one or more catalysts in a second, liquid-based, coating step. A typical polyisocyanate curing agent-based system is used and curing is brought to completion through catalysts applied in the kiss-coating step. While this process may achieve a complete cure, it requires a second processing step-in effect, a second coating line, with its attendant inconvenience, expense, and opportunity for error.

Another solution for overcoming the problems characterizing the conventional curing process has been employment of a blocked catalyst, which requires the application of heat to be unblocked and become active to catalyze the curing reaction. Thus far, however, blocked catalysts have proven unsatisfactory. Some blocked catalysts require excessively long heating times, i.e., more than that needed for drying the magnetic layer, to become unblocked to the extent necessary to achieve the desired cure. Other catatlysts are susceptible to partial unblocking at ambient temperature, and hence may include premature initiation of the curing process that reduced the pot life of magnetic dispersion mixtures.

There is no known process of catalyst which is successful in assuring complete, consistent curing of magnetic recording media, is integratable with common chemical cure magnetic recording media manufacturing procedures, and allows present chemical curing agent based magnetic media formulations to be used. A preferred catalyzing process for accelerating the complete cure of magnetic recording media is one that avoids having to introduce the cure accelerating catalyst into the magnetic dispersion before it is coated on the substrate, and that does not require the application of heat to activate catalyzation of the acceierated curing ractions. The present invention does succeed in these respects by employing vapor-phase catalysts to effect complete curing of a binder system, including a mixture of a polymer and a polyisocyanate curing agent.

The use of catalytic vapors to accelerate the reaction of isocyanates with other organic binder precursors in a coated mixture has been reported in U.S. Patents 2,967,117, 4,366,193, and 4,396,647; in Toso Gijutsu (1983) 22:87-99; and in Chemical Week, 12 January 1983, p. 52.

In the referenced vapor-curing processes, which have been exploited in appliance, automotive, and general paint applications, polyhydroxy compounds are described as coreactants with polyisocyanates. These polyhydroxy substances include low-molecular-weight aliphatic derivatives, that is, monomers, oligomers, prepolymers, or generally products which are incompletely polymerized and have the consistency of waxes and oils. However, none of the references describes how the vapor-curing processes and compounds could be adapted to the formation of the flexible, smooth magnetic layers needed in magnetic recording media. Furthermore, none of the references describe a multi-stage curing process that allows for pre-calendering partial curing and post-calendering final curing.However, monomeric, oligomeric, and prepolymeric polyhydroxy compounds, such as those described in detail in the above references and often used in various surface-coating applications, are not suitable binder components for flexible magnetic media coatings because they are either too tacky to permit spooling of magnetic recording media webs after coating, or their mechanical properties are inadequate to enable the coated web to be calendered and otherwise processed properly before curing is completed. Other polyhydroxy derivatives that have been used in vapor-curing processes contain only aromatic hydroxyl functionality, i.e. they are phenolic in nature, and the polyurethanes produced from their reaction with polyisocyanates are not suitable as magnetic recording media binders.

Disclosure of the Invention The conventional chemical curing process for magnetic recording media employing a polyisocyanate curing agent can now, by the method of the present invention, be rapidly brought to completion at ambient temperature and under controlled conditions, assuring uniform results in the manufactured magnetic recording media. Moreover, this can be accomplished without economically unacceptable modifications of existing formulations and manufacturing protocols, and can be controlled so that the magnetic layer is partially cured to a malleable state suitable for the calendering process and then the curing quickly completed to bring the magnetic layer to the desired toughness. This improvement in the process of manufacture, and in the magnetic recording media so made, is a result of the utilization of a vapor-phase catalyst to complete the final stages of the reactions which constitute the curing process, at which time the polyisocyan ate is entirely converted and all significant polyisocyanate reactions end. The employment of a vapor-phase catalyst permits a convenient means to complete this process, and is most advantageously carried out immediately after calendering the web of magnetic recording media under prescribed conditions, thus resulting in reliable high quality media.

In one aspect, the invention relates to a process useful in manufacturing magnetic recording media. The manufacture to which the process applies involves coating a substrate with a layer of solvent-based magnetic dispersion. The dispersion contains a magnetic pigment in admixtures with a polymeric binder and a polyisocyanate curing agent. If desired, other addtives of the kind previously discussed herein may be included in the dispersion. The coated support is calendered and then exposed, according to the process of the invention, to a volatile, preferably basic, catalyst, which brings the curing process to completion. In another aspect, the invention relates to the magnetic recording media produced by the aforedescribed process.

Modes of Carrying Out the Invention A. Definitions "Curing" is a process by which small molecules are converted to larger molecules through chain extension, normally involving the formations of a three-dimensional, or "cross-linked", network. Magnetic recording media coating formulations, according to the present invention, contain at the outset polymers that are used as binders for the magnetic pigments and which are ultimately converted via the curing process to a three-dimensional network of polymer chains having the desired mechanical properties. The end result of such curing is magnetic recording media whose magnetic layer coatings are durable and resilient to mechanical abrasion.

The "binders" used in the magnetic dispersion of the present invention are preformed polymeric materials. There is a variety of suitable binders, including, for example, polyurethane, phenoxy resins, polyesters, poly(vinyl acetate/vinyl chloride/vinyl alcohol), cellulose nitrate, polyvinyl butyral, poly(vinylidene chloride/acrylonitrile) and poly(butadiene/acrylonitriles).

"Magnetic medium" and "magnetic recording medium" include both the finished medium and the medium at its various stages of manufacture, once the magnetic coating has been applied to an underlying substrate. Thus, these terms refer to the coated substrate, whether or not the particles are oriented, or the solvent has been evaporated, and whether or not the coated substrate has been calendered, slit, or otherwise configured and packaged.

"Web" refers to a substrate with or without a magnetic layer coated thereon at those stages during manufacture which precede final processing into defined form, such as tape, disc, card, and the like.

"Polyisocyanates curing agent" refers to a relatively small (i.e., compared to the major polymeric constituent of the binder) molecule containing at least two isocyanate groups. The principal feature in isocyanates that are employed to advantage in curing is their reactivity with active hydrogen atoms. Isocyanates react very rapidly with water, primary and secondary amines, and primary alcohols. Secondary and tertiary alcohols react more slowly. Other, less reactive, materials include ureas and urethanes containing N-H groups. Isocyanates may also react with themselves to form dimers and trimers. More than one of these reactions may occur in a given curing process in a coating depending on the nature of the binders used.Typical curing agents include Mondur CB (the reaction product of toluene diisocyanate (TDI) and trimethylolpropane), Mondur HC (the reaction product of TCI and hexamethylene diisocyanate), Desmodur N (the seif- condensation product of hexamethylene diisocyanate), Desmodur IL (the self-condensation product of TDI), and PAPI (a trimer of methylene-bridged phenylisocyanate residues).

"Binder systems" refers to the combinations of binder (i.e., polymer) and curing agent (i.e., polyisocyanate cross-iinker).

B. Detailed Description In a preferred embodiment, the volatile curing process characterizing the present invention is applied to a web having a magnetic coat immediately after calendering. As the magnetic coat on the web should remain relatively malleable for calendering, completion of curing of the binder system cannot be permitted to take place before calendering; however, partial curing of the binder system is permitted before calendering, which depletes the concentration of reactants in the magnetic coat.

The manufacturing process for magnetic recording media is conducted, in general, as follows: a coating mixture which may contain approximately 75% by solid weight of magnetic particles, approximately 20% by weight of polymeric binder, and approximately 2% by weight of curing agent (i.e., a polyisocyanate), is mixed, shortly before use, in a suitable solvent to form a dispersion. The amounts and percentages specified for dispersion ingredients are merely iilustra- tive and not limiting; the effectiveness of the invention process depends only on the presence of suitable amounts of curing agent. The dispersion may further contain, if desired, other additives, such as dispersants, lubricants, conductive pigments, and abrasives, as is understood in the art.

The curing process begins as soon as the curing agent and binder are added to the coating mixture and continuous during further processing. For proper calendering, it is desired the magnetic layer be partially cured prior to calendering so that calendering produces the desired compactness and smoothness in the magnetic layer. Thus, in accordance with the present invention, curing during the initial steps of-the magnetic recording media manufacturing process is acceptable and used to advantage. However, such partial curing is not allowed to proceed beyond that which prevents proper calendering and, in accordance with the present invention, the curing process is accelerated after calendering to completion by subjecting the magnetic coated web to a vapor-phase catalyst.

After the dispersion is prepared, mixed thoroughly, and readied for coating, the resulting dispersion is applied in a coating line to a substrate, typically a non-magnetic flexible base film, to form a magnetic coated web. For some magnetic recording media, the coated web may be passed through a magnetic field to orient the magnetic particles while the curing process continues. Thereafter, the solvent is evaporated from the coating mixture, resulting in a nontacky, paritially cured web, which is smoothed by calendering.

The calendered web contains a residue of unreacted curing agent. At normal processing temperature, which are on the order of room temperature, completion of the curing process that consumes the remaining, unreacted curing agent takes several days or weeks. The precise nature of the cure will depend on the conditions experienced by the coated web. Without further processing steps relating to curing, the web would ordinarily be wound onto spools and stored for several days. Then the web would be slit or cut into the desired recording media form, cleaned, appropriately packaged, and stored again before use.The conditions of storage, cutting or slitting, cleaning, and packaging, unless carefully controlled, will result in non-uniform magnetic recording media due to variations in the rate of curing completion, as well as to the effect of varying amounts of environmental contaminants, most commonly water vapor.

In the process of the present invention, after calendering and before storage or any further processing, the web is exposed to vapors of a volatile catalyst for several minutes at room temperature (higher temperatures are permissible, but not necessary). This exposure completes the curing process in a limited time, and the web is rendered resistant to further modification by its environment.

The exposure can take place by passing the web through a chamber containing the vapor, or spools of web can simply be placed in such chamber before storage. The chamber containing the vaporized catalyst will optimally be kept at approximately ambient temperature, and the vapors will comprise approximately 1-15%, preferably around 5% of the total atmosphere of the chamber. The chamber atmosphere may contain other gases, as well, that are inert to the curing process, including air freed from paticulate contaminants, for example, by filtering, or other inert atmospheres; air is generally the most convenient.

Here complex procedures, involving, for example, increased pressures and concentrations of catalyst, could, of course, be used, but are not particularly advantageous.

In one configuration of the chamber, an enclosure is provided with a liquid source of the volatile catalyst. Preferably, catalyst containers of large, exposed surface area are located at the bottom of the chamber, and the web is passed through the chamber above the containers. The chamber is kept at atmospheric pressure by a pressure-equalizing conduit to the exterior atmosphere, and a consistent level of catalyst is automatically maintained in the chamber atmosphere by the vapor pressure of the liquid.

Suitable volatile catalysts include any material which is capable of forming a vapor at the temperature of the chamber and of catalyzing the curing reaction of the isocyanate. A variety of catalysts for this chemistry are known, such as metal cations and chelates thereof, carboxylic acids, and tertiary amine bases. However, most of these are not volatile at room temperature and, therefore, require conducting the curing process of the present invention at elevated temperatures. Volatile catalysts workable at room temperature are generally amines. Thus, in general suitable for catalysts are tertiary amines of low molecular weight, in particular, preferably trimethylamine as well as triethylamine, diethyl methylamine, and ethyl dimethylamine.

In the foregoing description of the curing process of the present invention and in the following illustrative example of the process, certain ranges and specific examples of process operating parameters, consitutents, and constituent concentrations are given. Those are not intended to define the only workable conditions for the practice of the curing process of the present invention. A variety of workable conditions are possible and may be selected to accomplish the complete cure of magnetic recording media. The selection of the conditions is governed only by the requirement that the magnetic recording media be completely cured at the conclusion of the curing process of the present invention.

C. Example The following example is intended to illustrate the curing process of the present invention but not to limit its scope.

Identical samples of a web having a magnetic layer with a binder system containing a polyurethane binder polymer and a Mondur CB polyisocyanate curing Agent were prepared as follows: INGREDIENT PARTS BY WT.

Polyurethane (Eatane 5701- , 8e B.F. aoodrlch Co.) Soya Lecithin (Yelkin TTS) 6.9 Gatac RE-610 (OAF Co) 2.6 Conductive Carbon Black 1.Z Cobalt.doped y-Ferric Oxide ISO Butyl stearate 6. 4 Tetrchydroturan 320 Cyclohexanone 180 nondut CB-75 (Mobay Inc) 1.O The polyisocyanate curing agent was added after milling the other ingredients in a sand mill until the desired degree of dispersion was achieved.The mixture was then coated onto a thin film of polyethylene terephthalate using a reverse-roll coater, and the solvents were removed in a drying oven for 45 seconds at 90-100 C.

The samples were allowed to stand in atmospheres at 23 C and 40% relative humidity after coating and drying to remove organic solvents. Samples in one of the atmospheres were treated with vapors of triethylamine by placing the samples in a closed container adjacent a reservoir containing triethylamine. Samples in the other atmosphere served as the control, and received no vapor curing treatment. The cure was monitored in two ways: by counting the number of wipes, using a "Q-tip" applicator moistened with methyl ethyl ketone (MEK), required to remove the magnetic layer from the web, and by determination of the percentage of isocyanate (%NCO) converted using IR transmission spectrometry (Nicolet FR-IR). The results are shown in the following table.

Tfme (tir) MEK wires NCO c0nvrted Control 0 1-2 0 4.5 2 23.0 2 60 Treated 0.25 {-5 100 The tabular results clearly show the catalysis of isocyanate conversion by the vapor treatment.

Claims (12)

1. A process for curing to completion a magnetic coat on a web comprising: treating said magnetic coat on said web with a volative vapor phase catalyst.

2. A process according to claim 1 wherein the web is coated with a dispersion including magnetic particles, a polymeric binder, and a polyisocyanate curing agent.

3. A process according to claim 2 wherein the volatile vapor phase catalyst is an amine.

4. A process according to claim 3 wherein the volatile vapor phase catalyst is a tertiary amine.

5. A process according to claim 4 wherein the volatile vapor phase catalyst is triethylamine, ethyl dimethylamine, diethyl methylamine, or trimethylamine.

6. A process according to any foregoing claim wherein prior to treating the magnetic coat with the volatile vapor phase catalyst, the magnetic coat is partially cured and the partially cured magnetic coat is calendered.

7. A magnetic medium when prepared by a process including a process according to any foregoing claim.

8. A process for the manufacture of a magnetic recording medium comprising applying to a non-magnetic substrate a coat of a particulate magnetic dispersion in a curable binder system, allowing partial curing of the binder system, calendering the coat and thereafter accelerating curing of the binder system by means of a vapor phase catalyst.

9. A process according to claim 8 in which the binder system comprises a polymeric binder and a polyisocyanate curing agent.

10. A process according to claim 8 in which the catalyst comprises a tertiary amine of low molecular weight.

11. A magnetic medium prepared by a process according to any one of claims 8 to 10.

12. The use of a vapor phase catalyst for accelerating curing of a magnetic recording medium comprising a binder system containing a dispersion of magnetic particles.