RADIATION CURABLE OLIGOMERS AND MAGNETIC RECORDING MEDIA PREPARED THEREFROM

Abstract: A novel class of radiation curable oligomers having formula (I) wherein: each R is independently an organic moiety comprising at least one radiation curable moiety; each n is independently at least 1; each W is independently an organic moiety having a valence of n+1; X is oxygen or -N(R DEG )-, wherein R DEG is H, a straight-chain, branched, or cyclic alkyl group of 1 to 10 carbon atoms; or a divalent organic moiety bridging the two nitrogens of the dinucleophile; and Z is a divalent organic moiety. In another aspect, the present invention concerns a process for making the radiation curable oligomers described above. In another aspect, the present invention concerns a magnetic recording medium incorporating the radiation curable oligomers described above.

Description: RADIATION CURABLE OLIGOMERS AND MAGNETIC RECORDING MEDIA PREPARED THEREFROM FIELD OF THE INVENTION The present invention relates to a novel class of radiation curable oligomers containing a plurality of urethane and/or urea moieties and a plurality of radiation crosslinkable moieties. The present invention also relates to magnetic recording media incorporating such oligomers. BACKGROUND OF THE INVENTION Magnetic recording media generally comprise a magnetizable layer coated on at least one side of a nonmagnetizable support. For particulate magnetic recording media, the magnetizable layer comprises a magnetic pigment dispersed in a polymeric binder. The magnetizable layer may also include other components such as lubricants; abrasives; thermal stabilizers; antioxidants; dispersants; wetting agents; antistatic agents; fungicides; bacteriocides; surfactants; coating aids; nonmagnetic pigments; and the like. Some forms of magnetic recording media, such as flexible magnetic recording tape, also have a backside coating applied to the other side of the nonmagnetizable substrate in order to improve the durability, conductivity, and tracking characteristics of the media. The backside coating typically comprises a polymeric binder, but may also include other components such as lubricants; abrasives; thermal stabilizers; antioxidants; dispersants; wetting agents; antistatic agents; fungicides; bacteriocides; surfactants; coating aids; nonmagnetic pigments; and the like. The magnetizable layer and the backside coating, if any, of a majority of conventional magnetic recording media are derived from materials which require curing in order to provide magnetic recording media with appropriate physical and mechanical properties. To prepare such magnetic recording media, the uncured components of the magnetizable layer or the backside coating, as appropriate, are dissolved in a suitable solvent and milled to provide a homogeneous dispersion. The resulting dispersion is then coated onto the nonmagnetizable substrate, after which the coating is dried, calendered if desired, and then cured. Curing can be achieved in a variety of ways. According to one approach, the polymeric binder of the magnetizable layer or the backside coating is derived from hydroxy functional polymers which rely upon a chemical reaction between the hydroxy functionality and an isocyanate crosslinking agent to achieve curing. The isocyanate crosslinking agent is typically added to the dispersion just prior to the time that the dispersion is coated onto the substrate. This approach, however, has a number of drawbacks. For example, the coating will have poor green strength until the cure reaction has progressed sufficiently. As a result, the coating will be susceptible to damage during subsequent processing unless an inconvenient and expensive time delay is incorporated into the manufacturing process. Moreover, after the isocyanate crosslinking agent is added to the dispersion, the viscosity of the solution begins to gradually increase as crosslinking reactions take place. After a certain period of time, the viscosity of the dispersion becomes sufficiently high such that it is then extremely difficult to filter and coat the dispersion onto the nonmagnetizable support. Radiation curable dispersions have been used as an alternative to isocyanate curable formulations. For radiation curable dispersions, the dispersion is coated onto the substrate, dried, calendered if desired, and then irradiated with ionizing radiation to achieve curing. Radiation curable dispersions are capable of providing, fast, repeatable, controlled crosslinking, thereby eliminating the inconvenient and expensive delays associated with isocyanate curable formulations. An interpenetrating polymer network, i.e., "IPN", is a blend of two polymer components in which each polymer component is crosslinked with itself, but there is very little crosslinking between components. The two components are held together by permanent entanglements between the components. A semi- interpenetrating polymer network, i.e., "semi-IPN", is a blend in which an uncrosslinked polymer is entangled in a crosslinked polymer matrix. IPN's and semi-IPN's have been described in R.A. Dickie et al., editors, Cross-linked Polymers: Chemistry. Properties, and Applications. American Chem. Society, pages 244-268 and 311-323 (1988) . Magnetic recording media incorporating semi-IPN compositions have been described in the art. For example, L.B. Lueck, Radiat. Phys. Chem., Vol. 25, Nos. 4-6, pp. 581-586 (1985) discusses radiation cured magnetic media. At page 583, Lueck proposes that "[t]he successful EB-curable magnetic coating combines well-known thermoplastic polymeric film-formers with radiation sensitive components in such a way that very little true 'cross-linking' in the real sense of the term takes place at all between the two resinous components." Karle et al., U.S. Pat. No. 4,916,021 describes magnetic recording media in which the binder is a semi-interpenetrating network derived from a radiation curable material and a material which is not cured by radiation. J. Seto, Radiat. Phys. Chem., Vol. 25, Nos. 4-6, pp. 567-579 (1985) , describes magnetic compositions formed by electron beam curing blends of acrylic oligomers and linear polymers. Imanaka et al., U.S. Pat. No. 4,348,456 describes (meth)acrylate functional oligomers comprising a pair of isocyanurate rings bonded together via a linking group such as -OR 20 -, wherein R 20 is a saturated hydrocarbon group. SUMMARY OF THE INVENTION In one aspect, the present invention concerns a novel class of radiation curable oligomers having the formula: 0 0 0 0 (ROC IIN) n --W-NO II -X— Z-X— C IIN-W-(NC IIOR) n H H H H wherein: each R is independently an organic moiety comprising at least one radiation curable moiety; each n is independently at least 1; each is independently an organic moiety having a valence of n+1; X is oxygen or -N(H)-; and Z is a divalent organic moiety. In another aspect, the present invention concerns a process for making the radiation curable oligomers described above. In a first step, a multifunctional isocyanate, or mixture of multifunctional isocyanates, of the formula W—(NC0) n+1 is reacted with a dinucleophile of the formula H—X—Z—X—H, such as a diamine or a diol, wherein the molar ratio of the multifunctional isocyanate(s) to the dinucleophile is about 2:1. The reaction product of this first step is an oligomer precursor of the formula o o ( OCN ) n -W-NO II-X— Z— X-C IIN-W- ( NCO ) n H H In a second step, the oligomer precursor is reacted with at least one alcohol of the formula R-OH in amounts such that the molar ratio of the alcohol to the oligomer precursor is at least 2n:l. The reaction product of this second step is a radiation curable oligomer of the present invention as is described above. In these formulae, n, , X, R, and Z are as defined above. In another aspect, the present invention concerns a magnetic recording medium incorporating the radiation curable oligomers described above. Magnetic recording media of the present invention comprise a magnetic layer provided on a nonmagnetizable support. The magnetic layer comprises a magnetic pigment dispersed in a polymeric binder. The polymeric binder is an irradiated blend of components comprising the radiation curable oligomer described above and a secondary polymer component. Preferably, the secondary polymer component comprises a polymer that is miscible with the radiation curable oligomer. The term "radiation curable moiety" means a moiety that undergoes a crosslinking reaction upon irradiation. An example of a radiation curable moiety is a carbon-carbon double bond such as is found in (meth)acrylate materials. The term "( eth)aerylate" includes acrylates and methacrylates. The term "miscible" with respect to the radiation curable oligomer and the polymer(s) of the secondary polymer component means that a blend of these materials shows a single glass transition temperature. For purposes of the present invention, glass transition temperature is determined using DSC techniques. DESCRIPTION OF THE PREFERRED EMBODIMENTS Magnetic recording media of the present invention comprise a magnetic layer provided on a nonmagnetizable support. The particular nonmagnetizable support of the present invention is not critical and may be any suitable support known in the art. Examples of suitable support materials include, for example, 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, polysulphone, aramid film, polyethylene 2,6-naphthalate film, fluorinated polymer, liquid crystal polyesters, polyamide; metals such as aluminum, or copper; paper; or any other suitable material. The components of the magnetic layer comprise a magnetic pigment dispersed in a polymeric binder. The type of magnetic pigment used in the present invention is not critical and may include any suitable magnetic pigment known in the art including iron oxides such as gamma Fe 2 0 3 and Fe 3 0 4 ; cobalt-modified iron oxides; chromium dioxide, hexagonal magnetic ferrites such as BaCo χ Ti x Fe 12 _ 2X 0 19 and the like; and metallic pigments such as Fe and- the like. The magnetic layer of the present invention generally contains from about 50 to 90, preferably about 65 to 90, and more preferably about 70 to 85 percent by weight of magnetic pigment. The percent by weight of magnetic pigment is based on the total weight of the magnetic layer. The polymeric binder of the present invention is an irradiated blend of components comprising a novel radiation curable oligomer and a secondary polymer component. The novel radiation curable oligomers of the present invention may be prepared according to a two-step reaction scheme. In a first step, a multifunctional isocyanate, or mixture of multifunctional isocyanates, of the formula W—(NCO) n+1 is reacted with a dinucleophile selected from the group consisting of a diol and a diamine. In the practice of the present invention, n is at least l. Preferably n is in the range from 1 to 4. More preferably, n is 1 or 2. Most preferably, n is 2. The multifunctional isocyanate(s) and the dinucleophile are used in amounts such that the molar ratio of the multifunctional isocyanate(s) to the dinucleophile is about 2:1. In the practice of the present invention, W is an organic moiety having a valence of n + 1. The nature of the moiety W is not critical in the practice of the present invention, so long as W is substantially unreactive to isocyanate groups, a ine groups, and OH groups under the reaction conditions employed to react the dinucleophile with the multifunctional isocyanate(s) . It is also preferred that W is stable upon exposure to ionizing radiation. "Stable" means that the moiety W undergoes substantially no scission or crosslinking reactions when exposed to radiation. Examples of structures suitable for W include straight chain, branched chain, or cyclic alkylene, arylene, aralkylene, alkoxy, or acyloxy moieties, and the like. As used hereinafter, the term "multifunctional isocyanate" shall mean a multifunctional isocyanate, or mixture of such isocyanates, as described above. Examples of preferred multifunctional isocyanates include a cyclic triisocyanate of the formula and a branched tri isocyanate of the formula wherein each of R 2 and R 3 is a divalent, organic linking group. The nature of the linking groups R 2 and R 3 is not critical in the practice of the present invention, so long as each of R 2 and R 3 is substantially unreactive to isocyanate groups, amine groups, and OH groups under the reaction conditions employed to react the dinucleophile with the multifunctional isocyanate. It is also preferred that the linking group is stable upon exposure to ionizing radiation. "Stable" means that the linking group undergoes substantially no scission or crosslinking reactions when exposed to radiation. Examples of structures suitable for R 2 and R 3 include straight chain, branched chain, or cyclic alkylene, arylene, aralkylene, alkoxy, acyloxy, and the like. R 2 is preferably alkylene. A specific example of such a cyclic triisocyanate for which R 2 is -(CH 2 ) 6 - is commercially available as Desmodur N3300 from Miles, Inc. A specific example of a branched triisocyanate for which R 3 is -(CH 2 ) 6 - is commercially available as Desmodur N100 from Miles, Inc. A wide variety of diols and/or diamines may be used as the dinucleophile. Examples of diols and diamines suitable in the practice of the present invention may be represented by the formula H-X—Z-X-H wherein each X is independently selected from -O- and -N(R°)-, wherein R° is H; a straight-chain, branched, or cyclic alkyl group of 1 to 10, preferably 1 to 2 carbon atoms; or a divalent organic moiety bridging the two nitrogens of the diamine, e.g., piperazine. Z is a divalent, organic linking group. For example, X is oxygen when the corresponding hydrogen active moiety of the dinucleophile is -OH, and X is -N(H)- when the corresponding hydrogen active moiety of the dinucleophile is -NH 2 . The nature of the linking group Z is not critical in the practice of the present invention, so long as Z contains no NCO or OH moieties. It is also desirable that Z is substantially unreactive to isocyanate groups, amine groups, and OH groups under the reaction conditions employed to react the dinucleophile with the multifunctional isocyanate. It is also preferred that Z is stable upon exposure to ionizing radiation. "Stable" means that the linking group undergoes substantially no scission or crosslinking reactions when exposed to radiation. Examples of structures suitable for Z include straight chain, branched chain, or cyclic alkylene, arylene, aralkylene, alkoxy, or acyloxy moieties, and the like. For example, suitable polydiols and diamines include polyester, polycaprolactone, polycarbonate, polydimethylsiloxane, polyether, and polyolefin diols and diamines, and the like. Preferred diols have a molecular weight in the range from about 100 to about 3000. One class of preferred dinucleophiles include diols in which Z is an alkylene moiety. A particularly preferred alkenyl diol is 1,6-hexane diol. Another preferred class of dinucleophiles include polyether diols, and more preferably polyether diols having a molecular weight of in the range from about 1000 to 6000, preferably 1000 to 3000. Representative polyether diols are essentially hydroxy- containing compounds having ether linkages. Examples of polyether diols include hydroxy-terminated poly(propylene oxide) , hydroxy-terminated poly(tetramethylene oxide) , hydroxy-terminated poly(pentamethylene oxide) , hydroxy-terminated poly(hexamethylene oxide) , hydroxy-terminated poly(ethylene oxide) , hydroxy-terminated poly(1,2-propylene oxide) , hydroxy-terminated poly(1,2-butylene oxide), tetrahydrofuran, ethylene oxide copolyethers, and the like. Particularly preferred polyether diols have the formula HO-(CH 2 CH 2 0) p -H, wherein the subscript p has an average value in the range from 20 to 25. When the multifunctional isocyanate is reacted with the dinucleophile at a molar ratio of 2:1, the reaction product is an oligomer precursor of the formula O O (OCN) n -W-NC II-X-Z-X-C IIN-W-(NCO) n H H wherein W, X, Z, and n are as defined above. In effect, W is the residue remaining after removal of NCO groups from the multifunctional isocyanate. For example, when the multifunctional isocyanate is the cyclic triisocyanate described above, W is given by Si ilarly, when the multifunctional isocyanate is the branched triisocyanate described above, W is given by Similarly, when the multifunctional isocyanate is a mixture of the cyclic and branched isocyanates described above, W may be or In effect, Z is the residue remaining after removal of the hydrogen active moieties from the dinucleophile. For example, when the dinucleophile is HO-(CH 2 ) 6 -OH, Z is -(CH 2 ) 6 -. In a second reaction step, the oligomer precursor is reacted with at least one alcohol of the formula R-OH, wherein R is an organic moiety comprising at least one radiation curable moiety. The alcohol, R-OH, is used in an amount such that there is a stoichiometric excess of OH groups from the alcohol relative to the number of NCO groups from the oligomer precursor. "Stoichiometric excess" means that the molar ratio of the (meth)acrylate functional alcohol to the oligomer precursor is greater than 2n:l, wherein n is as defined above. Using such an excess ensures that substantially all of the NCO groups from the oligomer precursor are converted into urethane linkages in the reaction with the alcohol. In the practice of the present invention, it is preferred to use no more than a 5% stoichiometric excess of the alcohol relative to the oligomer precursor. Preferably, the alcohol of the present invention is a (meth)acrylate functional alcohol comprising a single OH group and one or more (meth)acrylate groups. Representative (meth)acrylate functional alcohols of the present invention may be selected from the group consisting of O II HO-R 5 -OC-C=CH 2 R and O II HO—R 7 -C—[CHzOCC-CH *, Re wherein: each of R 5 and R 7 is independently a divalent aliphatic moiety of 1 to 24 carbon atoms; and each of R 6 and R 7 is independently -H or -CH 3 . The use of hydroxyethyl (meth)acrylate and pentaerythritol tri(meth)acrylate have been found to be particularly suitable for use as the (meth)acrylate functional alcohol in the practice of the present invention. Hydroxyethyl (meth)acrylate has a structure according to formula (7) in which R 5 is -CH 2 CH 2 -, and R 6 is -H or -CH 3 . Pentaerythritol tri(meth)acrylate has a structure according to formula (8) in which R 7 is -CH 2 - and R 8 is hydrogen or methyl. When the oligomer precursor is reacted with a stoichiometric excess of the (meth)acrylate functional alcohol, the reaction product is a radiation curable oligomer of the formula wherein n, W, X, R, and Z are as defined above. When prepared according to the two-step reaction scheme described above, R is the residue remaining after removal of the -OH moiety from the alcohol R-OH. For example, when the alcohol R-OH is hydroxyethyl (meth)acrylate, R is O II — CHzCHzOC-C-^CHz Re wherein R 6 may be -H or -CH 3 . Similarly, when the alcohol R-OH is pentaerythritol triacrylate, R is wherein R 8 may be -H or -CH 3 . Particularly preferred radiation curable oligomers of the present invention are selected from the group consisting of and wherein each subscript p in these formulae independently has an average value in the range from about 20 to about 25. The secondary polymer component of the present invention may be any polymer, or combination of polymers, known in the art to be suitable as a binder material for magnetic recording media. In preferred embodiments, the secondary polymer component is miscible with the radiation curable oligomer. Examples of polymers suitable for use as the secondary polymer component include thermoplastic or thermosetting polyurethanes, polyureas, nitrocellulose polymers, vinyl chloride copolymers, phenoxy resins, combinations of such polymers, and the like. As an option, the secondary polymer component may contain one or more pendant functional groups to enhance the performance of the magnetic recording medium. For example, the secondary polymer component may contain carbon-carbon double bonds and/or hydroxy groups to facilitate crosslinking of the secondary polymer component if desired. As other examples of pendant functional groups, the secondary polymer component may contain pendant dispersing moieties such as -S0 3 M; quaternary ammonium moieties; -COOM; 0 0 -P l(OM) 2 ; -OP »(OM) 2 ; and the li .ke wherei .n M is H ♦, Na ♦, K + , Li + , NH 4 + , and the like to facilitate dispersion of the magnetic pigment in the polymeric binder. In those instances where the secondary polymer component comprises a vinyl chloride copolymer, the vinyl chloride copolymer may have pendant epoxy groups to help prevent degradation of such copolymers due to outgassing of HC1. In one preferred embodiment of the present invention, the secondary polymer component comprises at least one hydroxy functional polymer and an isocyanate cross-linking agent, wherein the hydroxy functional polymer has no radiation crosslinkable moieties. When the radiation curable oligomers of the present invention are blended with such a hydroxy functional polymer and an isocyanate crosslinking agent, the weight percent of the oligomer is desirably in the range from 10 to 90 percent, preferably 15 to 70 percent, and more preferably 20 to 50 percent based on the total weight of the oligomer and the hydroxy functional polymer. The isocyanate crosslinking agent, if any, is a polyfunctional isocyanate having an average functionality of at least 2 isocyanate groups per molecule. Examples of specific polyfunctional isocyanate useful as the isocyanate crosslinking agent in the practice of the present invention include materials commercially available as Mondur CB-601 and CB-701 from Miles, Inc. The isocyanate crosslinking agent is preferably used in an amount such that the molar ratio of NCO groups from the isocyanate crosslinking agent to the total number of hydroxy groups from the hydroxy functional polymer is greater than 0. Preferably, the molar ratio of the NCO groups from the isocyanate crosslinking agent to the total number of hydroxy groups from the hydroxy functional polymer is in the range from 0.3 to 3.0, more preferably 1.2 to 1.8, and most preferably is about 1.5. As soon as the isocyanate crosslinking agent is blended with the hydroxy functional polymer(s) of the secondary polymer component, the NCO groups of the isocyanate crosslinking agent will begin to react with the hydroxy groups of the hydroxy functional polymer. Preferably, a catalyst, e.g., dibutyltin dilaurate, may also be added in suitable catalytic amounts in order to facilitate this crosslinking reaction. Generally, using from 0.02 to 0.2 parts by weight of catalyst per 100 parts by weight of magnetic pigment has been found to be suitable in the practice of the present invention. When radiation curable oligomers of the present invention are blended with at least one hydroxy functional polymer having no radiation curable moieties and an isocyanate crosslinking agent, it is believed that irradiation of the blend provides an interpenetrating polymer network in which the oligomer crosslinks with itself, and the hydroxy functional polymer is crosslinked with itself via the isocyanate crosslinking agent. Very little, if any, crosslinking takes place between the oligomer and the hydroxy functional polymer. When the magnetic layer of a magnetic recording tape includes isocyanate-cured binder materials, our experiments have shown that incorporating the radiation curable oligomer into the magnetic layer reduces the cupping of the tape. Cupping is the curling of an initially flat tape in the crossweb direction. When a magnetic recording tape is cupped, higher tension must be applied to the tape to ensure adequate head/media contact. Higher tension, however, can reduce the durability of the tape. In another preferred embodiment of the present invention, the secondary polymer component is a thermoplastic polymer which does not crosslink with itself or with the radiation curable oligomer. Suitable thermoplastic materials include polymers having no radiation crosslinkable moieties and, preferably no chemically crosslinkable moieties. As an option, however, thermoplastic polymers of the present invention may include chemically crosslinkable moieties so long as substantially no chemical crosslinking agent is present. For example, the thermoplastic polymer may be a hydroxy functional polymer so long as substantially no isocyanate crosslinking agent or other free isocyanate moieties available for crosslinking are present. Generally, the weight percent of the radiation curable oligomer is desirably in the range from 10 to 90 percent, preferably 30 to 70 percent, more preferably 30 to 50 percent based on the total weight of the oligomer and the thermoplastic polymer. When the radiation curable oligomer is blended with a suitable thermoplastic material, it is believed that irradiation of the blend provides a semi-IPN composition in which the radiation curable oligomer crosslinks only with itself to form a 3-dimensional, crosslinked matrix. The thermoplastic material undergoes no crosslinking reactions either with itself or with the oligomers. Instead, the thermoplastic material becomes entangled in the crosslinked matrix. In another preferred embodiment of the present invention, the radiation curable oligomer is blended with a secondary polymer component comprising at least one radiation curable polymer containing a plurality of radiation curable moieties and, optionally, a thermoplastic material as described above. In this embodiment, the radiation curable oligomer is used in an amount effective to induce crosslinking of the radiation curable polymer when the blend is irradiated. Generally, using 25 to 50 parts by weight of the radiation curable oligomer based on 100 parts by weight of the radiation curable polymer would be suitable in the practice of the present invention. When the radiation curable oligomer is blended with a secondary polymer component comprising a radiation curable polymer and a suitable thermoplastic material, it is believed that irradiation of the blend provides a semi-IPN composition in which the radiation curable oligomer and the radiation curable polymer crosslink only with themselves to form a 3-dimensional, crosslinked matrix. The thermoplastic material undergoes no crosslinking reactions, but rather becomes entangled in the crosslinked matrix formed by the radiation curable oligomer and the radiation curable polymer. Semi-IPN compositions of the present invention have better resilience and better toughness than irradiated compositions of only the radiation curable oligomer alone or only the secondary polymer component alone. Our investigations have also shown that preferred semi-IPN compositions of the present invention have better toughness than semi-IPN compositions based on previously known radiation curable oligomers. Advantageously, the radiation curable oligomers of the present invention are miscible with a wide variety of polymer materials, including both acidic and polymeric materials. It is believed that the excellent miscibility characteristics of the oligomers is attributable to the urethane and/or urea content of the oligomers. Inasmuch as urethane and urea moieties are both donor and acceptor groups for hydrogen bonding, it is believed that such moieties enhance the intermolecular interactions between the oligomers and other polymers. The radiation curable oligomers are particularly miscible with polyurethanes and polyureas since the oligomers are polyurethanes and/or polyureas themselves. In addition to the radiation curable oligomer, the secondary polymer component, and the magnetic pigment, the magnetic layer of the present invention may also comprise one or more conventional additives such as lubricants; abrasives; thermal stabilizers; antioxidants; dispersants; wetting agents; antistatic agents; fungicides; bactericides; surfactants; coating aids; nonmagnetic pigments; and the like in accordance with practices known in the art. For magnetic recording media of the present invention containing higher surface area pigments, e.g., pigments having a surface area of 25 m 2 /g to 70 m 2 /g such as cobalt-doped Fe 2 0 3 , barium ferrite, and metal particle pigments, a preferred class of dispersants comprises novel monomeric, oligomeric, or polymeric dispersants comprising at least one dispersing moiety and at least one radiation curable moiety selected from Such dispersants shall be referred to hereinafter as "α-methylstyrene functionalized" dispersants. Examples of suitable dispersing moieties include -S0 3 M; 0 0 quaternary ammonium -C00M; -P »(0M) 2 ; -0P II(0M) 2 ; and the like, wherein M is H + Na + , K + , Li + , NH 4 +, and the like. Advantageously these α-methylstyrene functionalized dispersants are capable of crosslinking with the other radiation curable binder materials when exposed to ionizing radiation in the presence of the radiation curable oligomer. One example of a preferred α-methylstyrene functionalized dispersant may be prepared by reacting an α-methylstyrene functionalized isocyanate selected from hereinafter referred to as "meta-TMI" or NCO I CH X CH hereinafter referred to as "para-TMI", with a phosphorylated polyoxyalkyl polyol, exemplified by the following formula: such that the NCO groups of the isocyanate react with the hydroxy groups of the polyol. In this formula, m is an integer from 1 to 5. Phosphorylated polyoxyalkyl polyols are described in U.S. Pat. No. 4,889,895. All or only a portion of the hydroxy groups of the phosphorylated polyoxyalkyl polyol may be reacted with NCO groups of the isocyanate. Preferably, the isocyanate is reacted with the polyol in an amount such that the ratio of NCO groups to OH groups is about 0.6. Another example of a preferred difunctional dispersant may be prepared by reacting the α-methylstyrene functionalized isocyanate with a polyoxyalkylated quaternary ammonium polyol such as the Emcol brand dispersing agents commercially available from Witco Chemical, New York, New York. The Emcol materials are exemplified by the formula In the above formula, " A is a monovalent, anionic counterion. " A typically is phosphate, acetate, or chloride. All or only a portion of the hydroxy groups of such polyols may be reacted with the NCO groups of the isocyanate. It is noted that the Emcol materials typically contain other free polyols. Preferably, therefore, the isocyanate is reacted with such polyols in an amount such that the ratio of NCO groups to OH groups of the polyoxyalkylated quaternary ammonium polyol is about 2.2:1. In this way, all of the hydroxy groups of the other polyols and the polyoxyalkylated quaternary ammonium polyol are reacted with the isocyanate. According to a preferred method of preparing magnetic recording media of the present invention, the magnetic pigment, the secondary polymer component (except for (meth)acrylate functional materials and the isocyanate crosslinking agent if any are used) and a suitable solvent are milled in a first step to form a magnetic dispersion. Optionally, either all or a portion of any conventional additives, if any of these are used, may also be milled in this first step. In this first step, using about 60 percent by weight of solvent based on the total weight of the magnetic pigment, the secondary polymer component, and any other additives has been found to be suitable in the practice of the present invention. Next, in a second step, the radiation curable oligomer, the (meth)acrylate functional materials of the secondary polymer component if any, the isocyanate crosslinking agent if any, and additional solvent are blended into the magnetic dispersion just prior to coating the dispersion onto a nonmagnetizable support. Optionally, either all or a portion of any conventional additives, if any of these are used, may be added to the dispersion during this second step as well as during the first step. In this second step, it is preferred to add a sufficient amount of solvent such that the resulting dispersion contains 25 to 40 percent by weight of solids based on the total weight of the dispersion. Examples of suitable solvents for preparing the magnetic dispersion may include ketones such as acetone, methyl ethyl ketone ("MEK") , methyl isobutyl ketone, or cyclohexanone; alcohols such as methanol, ethanol, propanol, or butanol; esters such as methyl acetate, ethyl acetate, butyl acetate, ethyl lactate, or glycol diacetate; tetrahydrofuran; glycol ethers such as ethylene glycol dimethyl ether, or ethylene glycol monoethyl ether; dioxane or the like; aromatic hydrocarbons such as benzene, toluene, or xylene; aliphatic hydrocarbons such as hexane or heptane; nitropropane or the like; and mixtures thereof. A particularly preferred solvent is a solvent blend containing 50 parts by weight of toluene and 50 parts by weight of methyl ethyl ketone. Use of such a solvent blend tends to provide a magnetic layer with very little solvent retention. Low solvent retention is desirable, particularly when the solvent interferes with hydrogen bonding between the radiation curable oligomer and the secondary polymer component. Such interference can reduce the toughness of the resulting polymeric binder. After blending the radiation curable oligomer, additional solvent, and other ingredients, if any, into the magnetic dispersion, the magnetic dispersion is then coated onto the nonmagnetizable substrate. The dispersion may be applied to the nonmagnetizable substrate using any conventional coating technique, such as gravure coating techniques or knife coating techniques. The coated substrate may then be passed through a magnetic field to orient the magnetic pigment, after which the coating is dried, calendered if desired, and then irradiated with ionizing radiation to cure the radiation curable binder components. Irradiation, i.e., curing of the radiation curable materials, may be achieved using any type of ionizing radiation, e.g., electron beam radiation or ultraviolet radiation, in accordance with practices known in the art. Preferably, curing is achieved with an amount of electron beam radiation in the range from l to 20 Mrads, preferably 4 to 12 Mrads, and more preferably 5 to 9 Mrads of electron beam radiation having an energy in the range from 100 to 400 kev, preferably 200 to 250 keV. Although electron beam irradiation can occur under ambient conditions or in an inert atmosphere, it is preferred to use an inert atmosphere as a safety measure in order to keep ozone levels to a minimum and to increase the efficiency of curing. "Inert atmosphere" means an atmosphere comprising flue gas, nitrogen, or a noble gas and having an oxygen content of less than 500 parts per million ("ppm") . A preferred inert atmosphere is a nitrogen atmosphere having an oxygen content of less than 75 parts per million. The present invention will now be described with reference to the following examples. Example 1 Preparation of 1,6-Hexane Diol Based Methacrylate Functional Oligomer 58.5 g polyisocyanate (Desmodur N3300, Miles, Inc., NCO equivalent weight 195) and 5.9 g 1,6-hexane diol (Aldrich) were dissolved in 350 g of toluene in a one liter resin flask equipped with a mechanical stirrer, a reflux condenser fitted with a drying tube filled with anhydrous calcium sulfate, an additional funnel, and a glass thermowell containing a thermometer with a Thermo-O-Watch sensor/controller connected to two 250 watt infra-red lamp heaters. The diol did not completely dissolve. Dibutyl tin dilaurate (Aldrich, 2 drops) was added and the mixture was heated with stirring to 50°C. The diol dissolved on heating. After five hours at 50°C, 26 g of hydroxyethyl methacrylate (Rohm and Haas, Rocryl 400) was added. After one-half hour at 50°C another 2 drops of dibutyl tin dilaurate was added. After 2 hours the temperature of the mixture was increased to 60°C and held there for 5 hours. On cooling, the solution become cloudy but cleared on the addition of 30 g of a 50:50 by weight blend of methyl ethyl ketone and toluene. Example 2 Preparation of Polyethylene Oxide Based Methacrylate Functional Oligomer To a five liter resin flask equipped with a) a paddle stirrer, b) a reflux condenser fitted with a drying tube filled with anhydrous calcium sulfate, c) a dropping funnel and d) a glass thermowell containing a thermometer with a Therm-O-Watch sensor/controller connected to two 250 watt infra-red lamp heaters was added 300 g polyethylene oxide (Carbowax 1000, OH equivalent weight 500, Union Carbide), 3228 g toluene, 351 g trifunctional isocyanate (Desmodur N3300, Miles, Inc., NCO equivalent weight 195) and 15 drops of dibutyl tin dilaurate (Aldrich) . The solution was heated with stirring to 60°C for two hours. A 156 g portion of hydroxyethyl methacrylate (Rohm and Haas, Rocryl 400) was added through the dropping funnel and the mixture was kept at 60°C overnight with mild stirring. A portion of the solvent was removed by evaporation on a rotary evaporator until the total weight of solution was 1370 g (59% solids) . Example 3 Preparation of Polytetra ethylene Oxide Based Acrylate Functional Oligomer 28.6 g of polytetramethylene oxide diol (DuPont Terethane 2900) (0.02 eq hydroxyl) was dissolved in 150 g methylethyl ketone ("MEK") . The solution was heated to distill out MEK and azeotrope out traces of water that may have been present in the diol. Fresh MEK was added at various times to make up the solvent volume removed. A total of 270 g fresh MEK was added over the course of the distillation and 181 g MEK distillate was collected. 12.6 g triisocyanate (Miles Inc.'s Desmodur N3300) (0.06 eq NCO) in 25.3 g MEK was added followed by one drop dibutyl tin dilaurate catalyst (Aldrich Chemical Company) . The mixture was held at 65°C for two hours. Percent solids were determined from loss on drying and NCO equivalent weight was determined by treating an aliquot with excess standardized di-n-butyl amine in toluene and back-titrating the unreacted amine with standard hydrochloric acid. To an 82.4 g portion of the solution containing 0.2 eq NCO (determined from the above tests) was added 2.39 g hydroxyethyl acrylate (97%) (0.02 eq hydroxyl). The mixture was held at 70°C for five hours, after which no isocyanate peak remained in the infra red spectrum around 2270 cm "1 . Example 4 A solution of 12.5 grams (0.0213 mole) of a trifunctional isocyanate (Desmodur N3300 from Miles, Inc.), 1.26 grams (0.0107 mole) 1,6-hexanediol, and two drops of dibutyltin dilaurate was prepared in methyl ethyl ketone (15% solids) in a sealed jar and heated at 60°C for 1 hour. Then 0.043 moles of hydroxyethyl methacrylate was added and heating was continued at 60°C until no isocyanate peak was present in the IR spectrum (about 3 hours) . Example 5 A solution of 12.5 grams (0.0213 mole) of a trifunctional isocyanate (Desmodur N3300 from Miles, Inc.), 1.26 grams (0.0107 mole) 1,6-hexanediol, and two drops of dibutyltin dilaurate was prepared in methyl ethyl ketone (15% solids) in a sealed jar and heated at 60°C for 1 hour. Then 0.043 moles of hydroxyethyl acrylate was added and heating was continued at 60°C until no isocyanate peak was present in the IR spectrum (about 3 hours) . Example 6 A solution of 12.5 grams (0.0213 mole) of a trifunctional isocyanate (Desmodur N3300 from Miles, Inc.), 1.26 grams (0.0107 mole) 1,6-hexanediol, and two drops of dibutyltin dilaurate was prepared in methyl ethyl ketone (15% solids) in a sealed jar and heated at 60°C for 1 hour. Then 0.043 moles of pentaerythritol triacrylate was added and heating was continued at 60°C until no isocyanate peak was present in the IR spectrum (about 3 hours) . Example 7 A solution of 12.5 grams (0.0213 mole) of a trifunctional isocyanate (Desmodur N100 from Miles, Inc.), 1.26 grams (0.0107 mole) 1,6-hexanediol, and two drops of dibutyltin dilaurate was prepared in methyl ethyl ketone (15% solids) in a sealed jar and heated at 60°C for 1 hour. Then 0.043 moles of hydroxyethyl methacrylate was added and heating was continued at 60°C until no isocyanate peak was present in the IR spectrum (about 3 hours) . Example 8 A solution of 12.5 grams (0.0213 mole) of a trifunctional isocyanate (Desmodur N100 from Miles, Inc.), 1.26 grams (0.0107 mole) 1,6-hexanediol, and two drops of dibutyltin dilaurate was prepared in methyl ethyl ketone (15% solids) in a sealed jar and heated at 60°C for 1 hour. Then 0.043 moles of hydroxyethyl acrylate was added and heating was continued at 60°C until no isocyanate peak was present in the IR spectrum (about 3 hours) . Example 9 A solution of 12.5 grams (0.0213 mole) of a trifunctional isocyanate (Desmodur N100 from Miles, Inc.), 1.26 grams (0.0107 mole) 1,6-hexanediol, and two drops of dibutyltin dilaurate was prepared in methyl ethyl ketone (15% solids) in a sealed jar and heated at 60°C for 1 hour. Then 0.043 moles of pentaerythritol triacrylate was added and heating was continued at 60° until no isocyanate peak was present in the IR spectrum (about 3 hours) . Example 10 Preparation of α-Methylstyrene Functionalized Dispersant 133.33 parts by weight of a 75% solution of a phosphorylated polyoxyalkyl diol (0.238 equivalents OH) in toluene, 0.03 parts by weight BHT, and 0.28 parts by weight dibutyltindilaurate were dissolved in 38.37 parts by weight toluene. Next, 28.70 parts by weight meta-TMI (0.143 equivalents NCO) was slowly added at room temperature. The resulting reaction mixture was stirred for 30 minutes. After 30 minutes, stirring was stopped, and the reaction was allowed to proceed. The reaction was complete after 2 days when an IR absorption for NCO at 2250 cm" 1 could no longer be detected. Six hours after the meta-TMI had been added, a 10,000 gram batch showed a maximum exotherm of 37°C. Example 11 Preparation of Radiation Curable Polymer A radiation curable polymer sample was prepared under ambient conditions by reacting 100 parts by weight of a hydroxy functional polymer (PKHH UCAR phenoxy resin commercially available from Union Carbide Corporation) with 42.46 parts by weight of meta-TMI. The hydroxy-functional polymer was first dissolved in 233 parts by weight methyl ethyl ketone. Next, 200 ppm (based on total weight of the hydroxy functional polymer and the meta-TMI) of BHT gellation inhibitor and 0.15 weight percent (based on the total weight of the hydroxy functional polymer and the meta-TMI) of dibutyltindilaurate catalyst were added to the solution with mixing. The meta-TMI was then slowly added with mixing. After all of the meta-TMI had been added, the reaction between the hydroxy-functional polymer and the meta-TMI was monitored by measuring the IR absorption peak of the NCO group (2250 cm" 1 ) from the meta-TMI. The reaction was deemed to be complete when an IR absorption peak for the NCO group could no longer be detected. Based upon a 15,000 gram batch, a maximum exotherm occurred about 1 hour after the meta-TMI had been added to the reaction mixture. After the maximum exotherm temperature was reached, the reaction mixture was heated to maintain the reaction mixture at 120°F. After the reaction was complete, the reaction mixture was cooled to room temperature. Optionally at this time, additional methyl ethyl ketone may be added to the reaction mixture in order to reduce the weight percent solids to about 33% solids. Adding additional solvent in this manner lowers the viscosity of the reaction mixture, thereby facilitating subsequent processing. Example 12A Preparation of Backside Dispersion 0.80 lbs (0.36 kg) of a 75% by weight solution of the dispersant of Example 10 in toluene, 7.8 lbs (3.53 kg) of a 35% by weight solution of a radiation curable polymer (prepared in accordance with Example 11) in methyl ethyl ketone, and then 12.10 lbs (5.49 kg) of a 15% by weight solution of Estane 5703 polyurethane (B.F. Goodrich) in a solvent (85% by weight methyl ethyl ketone, 15% by weight toluene) were sequentially added to 8.6 lbs (3.9 kg) of methyl ethyl ketone. The resulting mixture was mixed with an enclosed high shear mechanical mixer for 10 minutes. Next, 3.30 lbs (1.50 kg) of carbon black was slowly added to the mixture. After this, the mixture was mixed in the high shear mechanical mixer for 1 hour. Next, 7.34 lbs (3.33 kg) of Ti0 2 was slowly added to the mixture. After this, the mixture was mixed with the high shear mechanical mixer for 1 hour. The mixture was then milled pass to pass until smooth in a sandmill. Just prior to coating the resulting backside dispersion onto a substrate, 1.95 lbs (0.88 kg) of Ebecryl 220 brand oligomer and 0.087 lbs (40 grams) of myristic acid were blended into the dispersion. The dispersion was also diluted to 20% solids with methyl ethyl ketone and cyclohexanone, wherein the cyclohexanone was used in an amount such that the total amount of solvent in the dispersion was comprised of 18% by weight of cyclohexanone. Example 12B A backside dispersion was prepared in accordance with Example 12A, except that no myristic acid was blended into the dispersion. Example 12C A backside dispersion was prepared in accordance with Example 12B, except that the dispersion was diluted to 20% solids with MEK and toluene, wherein the toluene was used in an amount such that the total amount of solvent in the dispersion contained 20 weight percent toluene. Example 13 6 grams of propyl gallate, 6 grams of Irgafos 168 (process stabilizer sold by Ciba-Geigy Corporation) , 467 grams of a 75% solution of the dispersant of Example 10 in toluene, 25 grams Emcol Phosphate, 3.06 lbs (1.39 kg) of a 30% solution of a polyester polyurethane in 80/20 (by weight) methyl ethyl ketone/cyclohexanone were sequentially added to 6.74 lbs (3.06 kg) of methyl ethyl ketone. The resulting solution was mixed in an enclosed high shear mixer for 10 minutes. The mixing apparatus was then purged with N 2 gas. Next, 11.0 lbs (5.0 kg) of Fe metal particle magnetic pigment followed by 0.88 lbs (400 grams) of alumina were slowly added sequentially to the solution. The mixture was then mixed under the N 2 atmosphere in the high shear mixer for an additional 2 hours. Next, 11.60 lbs (5.26 kg) of methyl ethyl ketone was added to the mixture. The mixture was then mixed with the high shear mixer for an additional 1 hour. After this, the mixture was milled pass to pass in a sandmill until smooth using ceramic media. Just prior to coating the resulting magnetic dispersion onto a substrate, 1.66 kg of a 25% solution of the acrylate oligomer of Example 5 in 46/54 (by weight) methyl ethyl ketone/toluene was added to the dispersion. Then a solution of 100 grams myristic acid and 50 grams butyl stearate dissolved in 0.59 kg methyl ethyl ketone and 2.74 kg cyclohexanone was blended into the dispersion. Finally, 1.66 kg of MEK was added to the dispersion. The backside dispersion of Example 12A was coated onto one side of a thin gauge, pre-primed polyester substrate. The coated substrate was then dried at 140°F. Next, the magnetic dispersion was coated onto the other side of the pre-primed polyester substrate. The coated substrate was then passed through a magnetic field (3000 gauss) to orient the magnetic pigment. After this, the coated substrate was again dried at 140°F, and then the magnetic and backside coating were calendered. The backside coating and the magnetic coating were then cured with 8 megarads of electron beam radiation in a N 2 atmosphere containing no more than 50 ppm 0 2 . The resulting magnetic medium showed a squareness of 0.791, a coercivity of 1461, and a remanence of 2659 gauss. All bulk magnetic measurements were made with a vibrating sample magnetometer ("VSM") at 12.3 KOe. Example 14 6 grams of propyl gallate, 6 grams of Irgafos 168, 443 grams of a 79% solution of the dispersant of Example 10 in toluene, 25 grams Emcol Phosphate, 3.34 lbs (1.39 kg) of a 30% solution of a polyester polyurethane in 80/20 (by weight) methyl ethyl ketone/cyclohexanone were sequentially added to 6.56 lbs (2.97 kg) of methyl ethyl ketone. The resulting solution was mixed in an enclosed high shear mixer for 10 minutes. The mixing apparatus was then purged with N 2 gas. Next, 11.0 lbs (5.0 kg) of Fe metal particle magnetic pigment followed by 0.88 lbs (400 grams) of alumina were slowly added sequentially to the solution. The mixture was then mixed under the N 2 atmosphere in the high shear mixer for an additional 2 hours. Next, 11.46 lbs (5.20 kg) of methyl ethyl ketone was added to the mixture. The mixture was then mixed with the high shear mixer for an additional 1 hour. After this, the mixture was milled pass to pass in a sandmill until smooth using ceramic media. Just prior to coating the resulting magnetic dispersion onto a substrate, 1.89 kg of a 24% solution of the acrylate oligomer of Example 8 in methyl ethyl ketone was added to the dispersion. Then, a solution of 50 grams myristic acid and 25 grams butyl stearate dissolved in 0.75 kg methyl ethyl ketone and 2.72 kg cyclohexanone was blended into the dispersion. The backside dispersion of Example 12B was coated onto one side of a thin gauge, pre-primed polyester substrate. The coated substrate was then dried at 140°F. Next, the magnetic dispersion was coated onto the other side of the pre-primed polyester substrate. The coated substrate was then passed through a magnetic field (2500 gauss) to orient the magnetic pigment. After this, the coated substrate was again dried at 140°F, and then the magnetic and backside coating were calendered. The backside coating and the magnetic coating were then cured with 8 megarads of electron beam radiation in a N 2 atmosphere containing no more than 50 ppm 0 2 . The resulting magnetic medium showed a squareness of 0.752, a coercivity of 1427, and a remanence of 2704 gauss. All bulk magnetic measurements were made with a vibrating sample magnetometer ("VSM") at 12.3 KOe. Example 15 6 grams of propyl gallate, 6 grams of Irgafos 168, 300 grams of a 50% solution of Rhodafac BG510 dispersant (Rhone Poulenc) in methyl ethyl ketone, 4.64 lbs (2.10 kg) of a 25% solution of MR-120 brand vinyl chloride copolymer (with pendant OH, -S0 3 Na, and epoxy moieties sold by Nippon Zeon) in methyl ethyl ketone were sequentially added to 5.12 lbs (2.32 kg) of methyl ethyl ketone. The resulting solution was mixed in an enclosed high shear mixer for 10 minutes. The mixing apparatus was then purged with N 2 gas. Next, 11.0 lbs (5.0 kg) of Fe metal particle magnetic pigment followed by 0.88 lbs (400 grams) of alumina were slowly added sequentially to the solution. The mixture was then mixed under the N 2 atmosphere in the high shear mixer for an additional 2 hours. Next, 8.25 lbs (3.74 kg) of methyl ethyl ketone and 2.90 lbs (1.31 kg) of toluene were added to the mixture. The mixture was then mixed with the high shear mixer for an additional 1 hour. After this, the mixture was milled pass to pass in a sandmill until smooth using ceramic media. 4.79 lbs (2.17 kg) of methyl ethyl ketone was added to the dispersion during milling to lower the dispersion viscosity for effective milling. Just prior to coating the resulting magnetic dispersion onto a substrate, 1.60 kg of a 32.7% solution of the acrylate oligomer of Example 5 in methyl ethyl ketone, and a solution of 100 grams myristic acid and 50 grams butyl stearate in 567 grams toluene, were blended into the dispersion. The backside dispersion of Example 12C was coated onto one side of a thin gauge, preprimed polyester substrate. The coated substrate as then dried at 140°F. Next, the magnetic dispersion was coated onto the other side of the pre-primed polyester substrate. The coated substrate was then passed through a magnetic field (3000 gauss) to orient the magnetic pigment. After this, the coated substrate was again dried at 140°F, and then the magnetic and backside coating were calendered. The backside coating and the magnetic coating were then cured with 8 megarads of electron beam radiation in a N 2 atmosphere containing no more than 50 ppm 0 . The resulting magnetic medium showed a squareness of 0.723 and a coercivity of 1444 and a remanence of 2304 gauss. All bulk magnetic measurements were made with a vibrating sample magnetometer ("VSM") at 12.3 KOe. Example 16 6 grams of propyl gallate, 6 grams of Irgafos 168, 200 grams of a 50% solution of Rhodafac BG510 dispersant (Rhone Poulenc) in methyl ethyl ketone, and 4.28 (1.94 kg) of a 35% solution of UR8200 (Toyobo) sulfonated polyurethane in 50/50 (by weight) methyl ethyl ketone/toluene were sequentially added to 5.12 lbs (2.32 kg) of methyl ethyl ketone. The resulting solution was mixed in an enclosed high shear mixer for 10 minutes. The mixing apparatus was then purged with N 2 gas. Next, 11.0 lbs (5.0 kg) of Fe metal particle magnetic pigment followed by 0.55 lbs (250 grams) of alumina were slowly added sequentially to the solution. The mixture was then mixed under the N 2 atmosphere in the high shear mixer for an additional 2 hours. Next, 7.45 lbs (3.38 kg) of methyl ethyl ketone and 4.47 lbs (2.03 kg) of toluene were added to the mixture. The mixture was then mixed with the high shear mixer for an additional 1 hour. Before milling, an additional 0.91 kg of 35% solution of UR8200 sulfonated polyurethane in 50/50 (by weight) methyl ethyl ketone/toluene was added to the dispersion with mixing. After this, the mixture was milled pass to pass in a sandmill until smooth using ceramic media. Just prior to coating the resulting magnetic dispersion onto a substrate, 3.2 kg methyl ethyl ketone and 1.4 kg of toluene was added to the dispersion with mixing. Next 1.82 kg of a 45% solution of the acrylate oligomer of Example 5 in methyl ethyl ketone, and 50 grams butyl stearate were added to the dispersion with mixing. The backside dispersion of Example 12C was coated onto one side of a thin gauge, preprimed polyester substrate. The coated substrate was then dried at 140°F. Next, the magnetic dispersion was coated onto the other side of the pre-primed polyester substrate. The coated substrate was then passed through 2 magnetic fields to orient the magnetic pigment. The first magnetic field (2000 gauss) was located just after the coating head, and the second magnetic field (3400 gauss) was coated 47" into the drying oven. After this, the coated substrate was again dried at 140°F, and then the magnetic and backside coating were calendered. The backside coating and the magnetic coating were then cured with 8 megarads of electron beam radiation in a N 2 atmosphere containing no more than 50 ppm 0 2 . The resulting magnetic medium showed a squareness,of 0.662 and a coercivity of 1498 and a remanence of 1804 gauss. All bulk magnetic measurements were made with a vibrating sample magnetometer ("VSM") at 12.3 KOe. Comparison Sample A A magnetic recording medium was prepared according to the procedure of Example 15, except that Ebecryl 220 (Trademark) radiation curable oligomer commercially available from Radcure Specialties, Inc. was used instead of the radiation curable oligomer of the present invention. Comparison Sample B A magnetic recording medium was prepared according to the procedure of Example 16, except that Ebecryl 220 (Trademark) radiation curable oligomer was used instead of the radiation curable oligomer of the present invention. Example 17 The DMTA modulus at 20°C and 50°C (units are in GPa) , the percent strain to microfracture (± 0.03), and slit edge quality of each of Samples 15 and 16 and Comparison Samples A and B were determined as follows: DMTA modulus was measured at 1 N (newton) , 10 Hz, using the DMTA device commercially available from Polymer Laboratories, Ltd. Percent strain to microfracture was measured using a Miniature Materials Tester by Polymer Laboratories, Ltd. Slit edge quality of the coatings was visually observed at a magnification of 500X. Other embodiments of this invention will be apparent to- those skilled in the art from a consideration of this specification or from practice of the invention disclosed herein. Various omissions, modifications, and changes to the principles described herein may be made by one skilled in the art without departing from the true scope and spirit of the invention which is indicated by the following claims.

Claims: WHAT IS CLAIMED IS: 1. A process of making a radiation curable oligomer, comprising the steps of: (a) reacting at least one multifunctional isocyanate of the formula W-(NC0) n+1 with a dinucleophile of the formula H-X-Z-X-H, wherein the molar ratio of the multifunctional isocyanate to the dinucleophile is about 2:1 such that an oligomer precursor of the formula 0 O II II (OCN) n -W-NC-X—Z—X-CN-W-(NCO) n H H is formed, wherein: each n is independently at least 1; each W is independently an organic moiety having a valence of n + 1; X is oxygen or -N(R°)-, wherein R° is H; a straight-chain, branched, or cyclic alkyl group of 1 to 10 carbon atoms; or a divalent organic moiety bridging the two nitrogens of the dinucleophile; and Z is a divalent organic moiety; and (b) reacting the oligomer precursor with at least one alcohol of the formula R-OH in amounts such that the molar ratio of the alcohol to the oligomer precursor is at least 2n:l, whereby a radiation curable oligomer of the formula O O O O (R0C IIN) n -W-^C II—X—Z— X-C IIN-W- (NC IIOR) n H H H H is formed, wherein each R is independently an organic moiety comprising at least one radiation curable moiety. 2. A radiation curable oligomer of the formula R) n wherein: each R is independently an organic moiety comprising at least one radiation curable moiety; each n is independently at least 1; each W is independently an organic moiety having a valence of n+1; X is oxygen or -N(R°)-, wherein R° is H; a straight-chain, branched, or cyclic alkyl group of 1 to 10 carbon atoms; or a divalent organic moiety bridging the two nitrogens of the dinucleophile; and Z is a divalent organic moiety. 3. The radiation curable oligomer of claim 2, wherein the radiation curable oligomer has the formula 4. The radiation curable oligomer of claim 2, wherein the radiation curable oligomer has the formula wherein the subscript p has an average value in the range from about 20 to about 25. 5. The radiation curable oligomer of claim 2, wherein the radiation curable oligomer has the formula 6. The radiation curable oligomer of claim 2, wherein the radiation curable oligomer has the formula wherein the subscript p has an average value in the range from about 20 to about 25. 7. A magnetic recording medium, comprising a nonmagnetizable support, and a magnetic layer provided on the nonmagnetizable support, wherein the magnetic layer comprises a magnetic pigment dispersed in a polymeric binder, said polymeric binder being an irradiated blend of components comprising: (a) a radiation curable oligomer of the formula wherein: each R is independently an organic moiety comprising at least one radiation curable moiety; each n is independently at least 1; each W is independently an organic moiety having a valence of n+l; X is oxygen or -N(R 0 )-, wherein R° is H; a straight-chain, branched, or cyclic alkyl group of 1 to 10 carbon atoms; or a divalent organic moiety bridging the two nitrogens of the dinucleophile; and Z is a divalent organic moiety; and (b) a secondary polymer component. 8. The magnetic recording medium of claim 7, wherein the radiation curable oligomer has the formula 9. The magnetic recording medium of claim 7, wherein the radiation curable oligomer has the formula wherein the subscript p has an average value in the range from about 20 to about 25. 10. The magnetic recording medium of claim 7, wherein the radiation curable oligomer has the formula 11. The magnetic recording medium of claim 7, wherein the radiation curable oligomer has the formula wherein the subscript p has an average value in the range from about 20 to about 25.

Inventors: EIAN GILBERT L, GORRES BYRON T, PATEL SUMAN K, ROTTO NELSON T, EIAN, GILBERT, L., GORRES, BYRON, T., PATEL, SUMAN, K., ROTTO, NELSON, T.
Applicants: MINNESOTA MINING & MFG, MINNESOTA MINING AND MANUFACTURING COMPANY
Application year: 1993
Main classification: C08G18
IPC codes: C08F290, C08F299, C08G18, C09D4, C09D175, G11B5