WO1997023869A1

WO1997023869A1 - Magnetic recording medium having polymeric radiation cross-linking agent

Info

Publication number: WO1997023869A1
Application number: PCT/US1996/020124
Authority: WO
Inventors: Ravindra L. Arudi; Ramesh C. Kumar
Original assignee: Imation Corp.
Priority date: 1995-12-21
Filing date: 1996-12-17
Publication date: 1997-07-03

Abstract

The present invention provides magnetic recording media comprising a magnetic layer and optionally a backside coating provided on a substrate. At least one of the magnetic layer and the optional backside coating comprises a polymeric binder, wherein the polymeric binder is an electron beam radiation cross-linked matrix of ingredients comprising a binder polymer component and a polymeric radiation cross-linking agent. The polymeric radiation cross-linking agent comprises a plurality of (meth)acrylate-functional chain segments of formula (I), wherein B1 is a segment of the polymer backbone, X1 is a single bond or a divalent organic linking group, n has a value in the range from 2 to 6, and R9 is H or -CH¿3?. The polymeric radiation cross-linking agent has a molecular weight of at least 4000 and has sufficiently high molecular flexibility to effectively cross-link the polymeric binder upon exposure of the polymeric radiation cross-linking agent to electron beam radiation.

Description

MAGNETIC RECORDING MEDIUM HAVING POLYMERIC RADIATION CROSSLINKING AGENT

FIELD OF THE INVENTION

The present invention relates to magnetic recording media, and more particularly to magnetic recording media in which the polymeric binder of at least one of the magnetic layer and the optional backside coating is a radiation crosslinked matrix of a binder polymer component and a polymeric radiation crosslinking agent. The present invention also relates to a process of making such magnetic recording media.

BACKGROUND OF THE INVENTION

Magnetic recording media generally comprise a magnetic layer coated on at least one side ofa nonmagnetizable substrate. For particulate magnetic recording media, the magnetic layer comprises a magnetic pigment dispersed in a polymeric binder. The magnetic 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 ofthe nonmagnetizable substrate in order to improve the durability, conductivity, and tracking characteristics ofthe 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 magnetic layer and the backside coating, if any, ofa majority of conventional magnetic recording media are derived from components which require crosslinking in order to provide magnetic recording media with appropriate physical and mechanical properties. To prepare such magnetic recording media, the uncrosslinked components of the magnetic layer or the backside coating, as appropriate, are mixed with a suitable solvent and milled to provide a homogeneous dispersion. The resulting dispersion is then coated onto the substrate, after which the magnetic pigment, if any, is oriented or randomized as desired. The coating is then dried, calendered if desired, and finally crosslinked. Crosslinking can be achieved in a variety of ways. According to one approach, the polymeric binder ofthe magnetic layer or the backside coating is derived from hydroxyl-functional polymers which rely upon a chemical reaction between the hydroxy functionality and an isocyanate crosslinking agent to achieve crosslinking. 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 crosslinking 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 incoφorated into the manufacturing process. Moreover, after the isocyanate crosslinking agent is added to the dispersion, the viscosity ofthe solution begins to gradually increase as crosslinking reactions take place. After a certain period of time, the viscosity ofthe dispersion becomes high enough that it is extremely difficult to filter and coat the dispersion onto the nonmagnetizable substrate. Radiation crosslinkable dispersions have been used as an alternative to isocyanate crosslinkable formulations. Radiation crosslinkable dispersions are capable of providing fast, repeatable, controlled crosslinking, thereby eliminating the inconvenient and expensive delays associated with isocyanate crosslinkable formulations. To use a radiation crosslinkable dispersion, the dispersion is coated onto the substrate, after which the magnetic pigment, if any, may be oriented or randomized as desired. The coating is then dried, calendered if desired, and finally irradiated with ionizing radiation to achieve crosslinking. In this approach, the polymeric binder ofthe magnetic layer or the backside coating contains one or more binder polymers having radiation crosslinkable moieties which respond to the ionizing radiation by forming free radicals capable of crosslinking. The polymeric binder may also include one or more binder polymers which have no radiation crosslinkable moieties. -3-

Traditionally, radiation crosslinkable dispersion formulations for magnetic recording media have employed radiation crosslinkable binder polymers containing highly reactive moieties such as acrylates, methacrylates, methacrylamides, acrylamides, and the like to achieve crosslinking. Unfortunately, magnetic dispersions prepared from such materials tend to undergo unwanted, premature crosslinking reactions under ambient conditions to form insoluble gels, especially during dispersion milling. This problem makes it extremely difficult to manufacture magnetic recording media that are derived from dispersions containing binder polymers with these types of moieties. A radiation crosslinking agent may be used in the dispersion in addition to radiation crosslinkable binder polymers and non-radiation crosslinkable binder polymers to form high concentrations of chain propagating free radicals when irradiated, with the intention of increasing the rate ofthe crosslinking reaction and the overall level of crosslinking. Such a crosslinking agent generally comprises a plurality of highly reactive functional groups such as (meth)acrylates or (melh)acrylamides. The crosslinking agent is added to the dispersion just before coating onto the substrate, so that crosslinking is not induced during dispersion milling. A crosslinking agent may be monomeric, oligomeric or polymeric. Commercially available crosslinking agents for radiation crosslinkable binder polymers are generally low molecular weight ( < 1000) monomeric or oligomeric multifunctional (meth)acrylates. Despite a high degree of reactivity, such crosslinking agents have been observed to contain low molecular weight species which may plasticize the coating, leading to reduced green strength and increased running torque and friction between the magnetic recording medium and the head Moreover, monomeric and oligomeric crosslinking agents are generally considered to be toxic and thus present a health hazard to people handling them.

SUMMARY OF THE INVENTION

The present invention provides radiation crosslinked magnetic recording media which are prepared from radiation crosslinkable formulations which do not employ hazardous monomeric or oligomeric crosslinking agents. These media exhibit improved durability and friction characteristics, and are expected to have higher green strength than media which do not employ such radiation crosslinkable formulations. Formulations incoφorating the preferred radiation crosslinkable binder polym sr of this invention do not crosslink prematurely during milling. In one aspect, the advantages ofthe present invention are achieved by a magnetic recording medium comprising a magnetic layer provided on a substrate. The magnetic layer is prepared from components which comprise a magnetic pigment dispersed in a polymeric binder. The polymeric binder is an electron beam radiation crosslinked matrix of ingredients comprising a binder polymer component and a polymeric radiation crosslinking agent. The polymeric radiation crosslinking agent comprises a plurality of (meth)acrylate-functional chain segments ofthe formula

wherein B¹ is a segment ofthe polymer backbone, X¹ is a single bond or a divalent organic linking group, n has a value in the range from 2 to 6 and R* is H or -CH₃. The polymeric radiation crosslinking agent has a molecular weight of at least 4000 and has sufficiently high molecular flexibility to effectively crosslink the polymeric binder upon exposure ofthe polymeric radiation crosslinking agent to electron beam radiation. The magnetic layer is formed by the steps of: a) dispersing the magnetic pigment in the binder polymer component to form a magnetic dispersion; b) blending the polymeric radiation crosslinking agent into the magnetic dispersion of step a) in an amount effective to induce a crosslinking reaction when the polymeric binder is exposed to electron beam radiation; c) after blending the polymeric radiation crosslinking agent into the magnetic dispersion, coating the magnetic dispersion onto the substrate to form a magnetic coating; and d) irradiating the coated substrate with an effective amount of electron beam radiation to substantially crosslink the magnetic coating.

As used herein, "molecular flexibility" means the ability of the polymeric radiation crosslinking agent molecules to bend, twist and move about in the magnetic coating. Molecular flexibility is a factor in determining how well the polymeric radiation crosslinking agent of this invention will perform as a crosslinking agent. A person skilled in the art can choose parameters such as molecular weight, glass transition temperature (T_g), and polymer backbone chemistry of the polymeric radiation crosslinking agent to optimize its molecular flexibility. "Effectively crosslink" means that the polymeric binder, after exposure to a dose of electron beam radiation which is suitable for production of magnetic recording media, has been crosslinked to a level such that resulting magnetic recording medium performs acceptably in standard performance tests for properties such as durability and friction.

Preferably, the binder polymer component ofthe polymeric binder comprises at least 30% by weight ofa radiation crosslinkable binder polymer, the radiation crosslinkable binder polymer comprising a plurality of pendant radiation crosslinkable moieties, wherein substantially none of the pendant radiation crosslinkable moieties are (meth)acrylate or (meth)acrylamide moieties. More preferably, the binder polymer component also contains 70% by weight or less of a non-radiation crosslinkable binder polymer component.

The polymeric radiation crosslinking agent is preferably present in an amount in the range from 2 to 20 parts by weight based on 100 parts by weight ofthe magnetic layer. In a preferred embodiment, the (meth)acrylate-functional chain segments ofthe polymeric radiation crosslinking agent preferably have the formula

B¹— X^L-OCNH-CH₂CH₂— O— CC=CH₂ In a particularly preferred embodiment, the polymeric radiation crosslinking agent having the preferred (meth)acrylate-fύnctional groups described above is a vinyl copolymer having a glass transition temperature (T_g) in the range from 40 to 80°C and a molecular weight in the range from 10,000 to 50,000. This vinyl copolymer comprises a plurality of pendant OH groups, a plurality of pendant nitrile groups, and a plurality of pendant fluorine-containing groups, in addition to the (meth)acrylate-functional groups. According to one method, this vinyl copolymer may be formed by reacting a (meth)acrylate-functional isocyanate known as isocyanatoethylmethacrylate ("IEM") with a primary vinyl copolymer having pendant Oi l groups, pendant nitrile groups, and pendant fluorine-containing groups in a manner such that the NCO groups on the isocyanate react with a plurality of OH groups on the primary vinyl copolymer according to the generalized reaction scheme:

wherein R^N is a pendant nitrile group and R^F is a pendant fluorine-containing group. The IEM is selected for its high reactivity toward all types of OH groups. In addition, the polyurethane linkage formed between the isocyanate group of the IEM and the OH group on the vinyl copolymer has been observed to promote compatibility ofthe polymeric radiation crosslinking agent with polyurethane binders commonly used in magnetic recording media. In another aspect, the present invention concerns a magnetic recording medium comprising a substrate having first and second major surfaces. A magnetic layer is provided on the first major surface, and a backside coating is provided on the second major surface. The backside coating is prepared from components which include at least one nonmagnetic pigment dispersed in a polymeric binder. The polymeric binder is an electron beam radiation crosslinked matrix of ingredients comprising the binder polymer component described above and the polymeric radiation crosslinking agent described above. In this aspect, the backside coating is formed by the steps of: a) dispersing the nonmagnetic pigment in the binder polymer component to form a backside dispersion; b) blending the polymeric radiation crosslinking agent into the dispersion of step a) in an amount effective to induce a crosslinking reaction when the polymeric binder is exposed to electron beam radiation; c) after blending the polymeric radiation crosslinking agent into the dispersion, coating the dispersion onto the substrate to form a backside coating; and d) irradiating the coated substrate with an effective amount of electron beam radition to substantially crosslink the backside coating.

In a particularly preferred embodiment, both the magnetic layer and the backside coating comprise a polymeric binder including a binder polymer component and a polymeric radiation crosslinking agent of this invention.

In another aspect, the present invention concerns a process of making a magnetic recording medium. In a first step, ingredients comprising a magnetic pigment, the binder polymer component described above, and a solvent are milled to form a magnetic dispersion. After the milling step, the polymeric radiation crosslinking agent described above is blended into the dispersion, in an amount effective to induce a crosslinking reaction in the polymeric binder when the polymeric binder is exposed to electron beam radiation. Unlike the binder polymer component, the polymeric radiation crosslinking agent is added after milling is complete so that the highly reactive (meth)acrylate moieties ofthe polymeric radiation crosslinking agent do not form free radicals and begin the crosslinking reaction prematurely. The magnetic dispersion is then coated onto a substrate. Optionally, the coated substrate may be passed through a magnetic field in order to orient or randomize the magnetic orientation of the magnetic pigment. The coated substrate is then dried to form a dried magnetic coating on the substrate. The dried magnetic coating optionally may be calendered, if desired, and then is irradiated with an effective amount of electron beam radiation to substantially crosslink the magnetic coating.

Our investigations have shown that pigment-induced autoxidation of he magnetic dispersion ingredients is one ofthe causes of he premature crosslinking and gellation problem associated with magnetic pigment dispersions prepared from acrylate-, methacrylate-, methacrylamide- and acrylamide-functional binder materials. High surface area pigments such as cobalt-modified iron oxides, barium ferrite, and metal particle pigments, in particular, have a greater tendency to induce autoxidation relative to other kinds of magnetic pigments. Such autoxidation tends to generate free radicals which, in the presence of highly reactive acrylate-, methacrylate-, methacrylamide- and acrylamide-functional binder materials, initiate premature crosslinking and gelladon. This problem is aggravated by the ketone solvents generally used to prepare magnetic pigment dispersions as well as by the energy released during dispersion milling.

In making the magnetic recording media of this invention, premature crosslinking and gellation may be reduced by using a radiation crosslinkable bin er polymer having substantially no pendant (meth)acrylate or (meth)acrylamide moieties as part of the binder polymer component. This radiation crosslinkable binder polymer instead includes pendant radiation crosslinkable moieties such as allyloxy and α-methylstyrene groups, which have carbon-carbon double bonds which are much less reactive than the carbon-carbon double bonds of acrylates, methacrylates, methacrylamides, and acrylamides. In the absence of a radiation crosslinking agent, these groups remain substantially dormant, or unreactive, during milling, especially when a small amount (less than about 0.1 % by weight) ofa gellation inhibitor (e.g., a combination ofa peroxide decomposer such as tetramethyl thiuram disulfide and a radical scavenger such as propylgellate) is also present.

In the presence ofa radiation crosslinking agent such as the polymeric radiation crosslinking agent of this invention, however, the pendant radiation crosslinkable moieties ofthe binder polymer are activated and readily crosslink when exposed to electron beam radiation to provide a crosslinked polymer network. In the practice ofthe present invention, the polymeric radiation crosslinking agent is not combined with the other components of the magnetic layer until just before the:ie components are to be coated onto the substrate. In this way, premature crosslinking and gellation are minimized during the milling step but the magnetic layer can be easily crosslinked by electron beam irradiation after coating.

We believe that polymeric radiation crosslinking agents having relatively high molecular flexibility have greater mobility in the magnetic layer or backside coating during the crosslinking reaction and thus induce crosslinking more effectively than polymeric radiation crosslinking agents having lower molecular flexibility. Our investigations have shown that magnetic recording media prepared with the polymeric radiation crosslinking agents of this invention show greater durability and reduced friction characteristics as compared to media prepared with well known monomeric and oligomeric crosslinking agents. In addition, we believe that the polymeric radiation crosslinking agents of this invention have sufficiently high molecular weight to provide adequate green strength to freshly coated magnetic media. "Green strength" is defined as the adhesive and cohesive strength ofa magnetic layer or a backside coating after drying and before crosslinking. The polymeric radiation crosslinking agents of this invention also do not exhibit the potentially toxic properties associated with lower molecular weight monomeric and oligomeric crosstinking agents.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The particular substrate ofthe present invention is not critical and may be obtained from any suitable substrate material known in the art. Examples of suitable substrate 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 ketone, polyetherimide, polysulphone, aramid film, polyethylene 2,6-naphthalate film, fluorinated polymer, liquid crystal polyesters, polyamide, or polyhydric acid; metals such as aluminum, or copper; paper; or any other suitable material. A magnetic layer is provided on the substrate. 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 gamrαa-Fe O and Fe 0₄; cobalt-modified iron oxides; chromium dioxide, substituted and unsubstituted hexagonal platelet-shaped ferrites such as BaCo_xTi_xFeι₂.i(Oi9 and the like; and metal particles such as Fe and the like. The magnetic layer ofthe present invention generally comprises from about 50 to 90, preferably about 65 to 90, and more preferably about 70 to 85 parts by weight of magnetic pigment and about 10 to 50 parts by weight ofthe polymeric binder.

The polymeric binder ofthe present invention contains a electron beam radiation crosslinked matrix of ingredients comprising a binder polymer component and a polymeric radiation crosslinking agent. The binder polymer component may comprise one or more of any polymer which is suitable for use as a binder in magnetic recording media. Examples of suitable binder polymers include polyurethanes, alkyd resins, acrylic polymers, polyesters, epoxy resins, cellulosic resins, vinyl copolymers, and the like. It is preferred, but not required that at least one ofthe polymers comprising the binder polymer component be radiation crosslinkable in order to provide maximum durability to the magnetic layer.

In a preferred embodiment, the binder polymer component includes at least 30% by weight ofa radiation crosslinkable binder polymer which includes a plurality of pendant radiation crosslinkable moieties, wherein substantially none ofthe pendant radiation crosslinkable moieties are (meth)acrylate or (meth)acrylamide moieties. Traditionally, radiation crosslinkable dispersion formulations have most commonly relied on the reactivity of such moieties to achieve radiation-induced crosslinking. Unfortunately, magnetic dispersions prepared from such materials tend to undergo unwanted crosslinking reactions under ambient conditions to form gels, particularly during dispersion milling or when the magnetic pigment is a metal particle pigment or a high surface area oxide.

The preferred radiation crosslinkable binder polymer avoids these disadvantages by using relatively unreactive radiation crosslinkable moieties. The radiation crosslinkable binder polymer ofthe present invention preferably comprises a plurality of chain segments ofthe formula -f-RH- X² M> wherein R¹ is a segment ofthe radiation crosslinkable binder polymer backbone, X² is a single bond or a divalent linking group, and M¹ is a relatively unreactive pendant radiation crosslinkable moiety. M¹ may comprise suitable pendant radiation crosslinkable moieties such as allyloxy moieties (-0-CH₂-CH=CH₂) and α-methylstyrene moieties of the formula

In the absence ofa crosslinking agent, such allyloxy and α-methylstyrene moieties are more stable in the milling process than (meth)acrylate and (meth)acrylamide groups while still being sufficiently reactive in the presence of a (meth)acrylate-functional and/or a (meth)acryiamide-functional radiation crosslinking agent to be able to undergo crosslinking reactions. Most preferably, M¹ comprises an α-methylstyrene moiety. The radiation crosslinkable binder polymer is preferably present in the range from about 5 to 40 parts by weight based on 100 parts by weight magnetic pigment in the polymeric binder

According to one possible approach, the preferred radiation crosslinkable binder polymer may be prepared by reacting a suitable OH-functional polymer with an appropriately functionalized isocyanate according to the following generalized reaction scheme:

-

wherein Rl is a segment ofthe OH-functional polymer backbone, X9 is a single bond or a divalent linking group, and Ml is a relatively unreactive pendant radiation crosslinkable moiety as described above. Techniques for incoφorating allyloxy and α-methylstyrene moieties into OH-functional polymers using this reaction scheme have been described in U.S. Patent Numbers 5,380,905 and 5,510,187.

In a preferred reaction, a radiation crosslinkable binder polymer of this invention having α-methylstryene radiation crosslinkable moieties is made by reacting an OH-functional polymer with an α-methylstyrene-functional isocyanate having the formula

hereinafter referred to as "TMI".

TMI has been described in Dexter et al., "M-TMI, A Novel Unsaturated Aliphatic Isocyanate," Journal of Coatings Technology, Vol. 58, No. 737, pp. 43-47 (June 1986), U.S. Pat. No. 4,853,478; U.S. Pat. No. 4,839,230; U.S. Pat. No. A.788,303; and U.S. Pat. No. 4,617,349. The reaction occurs according to the following reaction scheme:

wherein

represents the OH-functional chain segment ofthe OH-functional polymer described above, R¹ is a segment ofthe polymer backbone, and X⁹ is a single bond or a divalent linking group. If X⁹is a divalent linking group, the linking group is preferably stable upon exposure to ionizing radiation, e.g., ultraviolet or electron beam radiation. "Stable" means that the linking group does not undergo any scission or crosslinking reactions when exposed to radiation All or only a portion ofthe OH groups ofthe OH-functional polymer may be reacted with the TMI. Preferably the OH-functional polymer is reacted with an amount of TMI such that there is a molar excess of OH groups on the OH-functional polymer relative to NCO groups on the TMI. It is preferred that there is a sufficient excess of OH groups relative to NCO groups such that 10% to 90%, preferably 50% to 80%, and more preferably 80% ofthe OH groups are reacted with TMI. Generally, reacting a greater percentage ofthe OH groups from the OH-functional polymer with TMI increases the crosslink density and durability ofthe resulting polymeric binder. According to one process, the OH-functional polymer is reacted with TMI under ambient conditions (i.e., at room temperature and atmospheric pressure) in a suitable solvent. Examples of suitable solvents include ketones such as acetone, methyl ethyl ketone ("MEK"), methyl isobutyl ketone, or cyclohexanone; esters such as methyl acetate, ethyl acetate, butyl acetate, ethyl lactate, or glycol diacetate; tetrahydrofuran; dioxane or the like; and mixtures thereof.

The amount of solvent used is not critical as long as enough solvent is used such that substantially all ofthe OH-functional polymer and the TMI dissolve in the solvent. Generally, using 70% by weight of solvent based on the total weight ofthe solvent, the TMI, and the OH-functional polymer has been found to be suitable in the practice ofthe present invention. A catalyst, such as dibutyltindilaurate may be added to the solution to accelerate the reaction ofthe OH-functional polymer with the TMI. Optionally, a gellation inhibitor may be added to the solution, although the use ofa gellation inhibitor is not required. The gellation inhibitor, if used, may be any suitable gellation inhibitor known in the art, such as phenothiazine and butylated hyd oxytoluene

("BHT"). The reaction mixture may be stirred slowly as the reaction takes place.

The progress ofthe reaction between the OH-functional polymer and the TMI may be monitored by measuring the IR absorption ofthe NCO group from the TMI. The reaction is deemed to be complete when an IR absoφtion for the NCO group ofthe TMI can no longer be detected. When the reaction is carried out under ambient conditions, the reaction is typically completed after 3 to 4 days.

A wide variety of OH-functional polymers can be functionalized with relatively unreactive radiation crosslinkable moieties according to the above-described reaction scheme in order to provide the radiation crosslinkable binder polymer ofthe present invention. In fact, any ofthe conventional polymers used as a binder in magnetic recording media would be suitable, provided the polymer is OH-functional Examples of suitable polymers include polyurethanes, alkyd resins, acrylic polymers, polyesters, epoxy resins, cellulosic resins, vinyl copolymers, and the like. Self-wetting OH- functional polymers comprising polar wetting groups such as sulfonate groups, carboxyl groups, amine groups, quaternary ammonium groups, phosphorus-containing groups, and the like, would also be suitable for use as the OH-functional polymer in :he practice of the present invention. Mixtures ofthe above-described polymers may also be used. Preferred OH-functional polymers for making the radiation crosslinkable binder polymer contain substantially no pendant acrylate, methacrylate, methacrylamide, or acrylamide moieties and have a hydroxy equivalent weight of from 100 to 10,000, preferably from 200 to 1 ,000, and more preferably from 200 to 500.

In addition to having relatively unreactive pendant radiation crosslinkable moieties as described above, the radiation crosslinkable binder polymer ofthe present invention optionally may include one or more other pendant functional groups in order to enhance the performance ofthe magnetic recording medium. For example, the radiation crosslinkable binder polymer may include a plurality of pendant OH groups to facilitate dispersion ofthe magnetic pigment and/or to facilitate crosslinking ofthe magnetic layer with isocyanate crosslinking agents in the event that a dual approach using both radiation crosslinking and chemical reaction crosslinking is desired. The radiation crosslinkable binder polymer may also include a plurality of pendant nitrile groups to improve toughness ofthe magnetic layer in which the radiation crosslinkable binder polymer is used. In addition, we believe that pendant nitrile groups and pendant OH groups present on the radiation crosslinkable binder polymer may promote the compatibility ofthe radiation crosslinkable binder polymer with polyurethanes typically used in the polymeric binder. The radiation crosslinkable binder polymer may also include at least one pendant dispersing group and/or a plurality of pendant fluorine- containing groups. As used throughout this specification, the term "dispersing group" means that a group is capable of dispersing the magnetic pigment. Such dispersing groups are well known, and include examples such as quaternary ammonium moieties (e.g., -N(CH₃)₃ ⁺Cr as one example), amines (e.g., -N(CH₃)₂ as one example), heterocyclic moieties as described in U.S. Pat. No. 5,081,213, sulfobetaines (e.g.,

-N⁴(CH₃)₂(CH₂CH₂CH₂SO₃ ^')), salts or acids based on sulfate (e.g., -OSO₃Na as one example), salts or acids based on sulfonate (e.g., -SO Na as one example), salts or acids based on phosphate (e.g., -OPO(OH)₂ as one example), salts or acids based on phosphonate (e.g., -PO(OH)₂ as one example), salts or acids based on carboxyl (e.g., -COONa as one example), mixtures thereof, and the like. The dispersing group is preferably a quaternary ammonium moiety. A radiation crosslinkable binder polymer having pendant fluorine-containing groups may impart properties of lubricity, release and water repellency to a radiation crosslinked magnetic layer or backside coating containing such a radiation crosslinkable binder polymer.

In a particularly preferred embodiment, the radiation crosslinkable binder polymer ofthe present invention is a nonhalogenated functionalized vinyl copolymer having a glass transition temperature (T_g) > 50°C and which includes a plurality of pendant nitrile groups, OH groups, dispersing groups and relatively unreactive radiation crosslinkable moieties as described above. Most preferably, such a nonhalogenated functionalized vinyl copolymer is obtained by reacting a functionalized isocyanate having suitable pendant radiation crosslinkable moieties with a primary vinyl copolymer having pendant OH groups, nitrile groups, and dispersing groups. In this reaction, Ihe pendant Ol 1 groups ofthe vinyl copolymer are in molar excess relative to the NCO groups of the functionalized isocyanate such that unreacted pendant OH groups remain after all the NCO groups have reacted. This reaction may be represented schematically as follows:

wherein R^N is a pendant nitrile group, R^β is a pendant dispersing group, and M¹ is a relatively unreactive pendant radiation crosslinkable moiety.

According to one aspect of this particularly preferred embodiment, the primary vinyl copolymer described above is a copolymer of vinyl monomers comprising one or more nitrile-functional vinyl monomers, one or more OH-functional vinyl monomers, one or more vinyl monomers bearing a dispersing group, and one or more copolymerizable vinyl monomers. Examples of nitrile-functional vinyl monomers include (meth)acrylonitrile, β-cyanoethyl-(meth)acrylate, 2-cyanoethoxyethyl (meth)acrylate, p-cyanostyrene, p-(cyanomethyl)styrene, and the like. Preferably, the nitrile-functional vinyl monomer is (meth)acrylonitrile, and more preferably ac rylonitrile. Examples of suitable OH-functional vinyl monomers include an ester of an α,β-unsaturated carboxylic acid with a diol, e.g., 2-hydroxyethyl (meth)acrylate, or 2-hydroxypropyl (meth)acrylate; l,3-dihydroxypropyl-2-(meth)acrylate; 2,3-dihydroxy- propyl-l-(meth)acrylate; an adduct of an α,β-unsatu rated carboxylic acid with caprolactone; an alkanol vinyl ether such as 2-hydroxyethyl vinyl ether; 4-vinylbenzyl alcohol; allyl alcohol; p-methylol styrene; or the like Preferably, the OH-functional vinyl monomer is selected from the class of hydroxyalkyl (meth)acrylates. Most preferably, the OH-functional vinyl monomer is selected from 2-hydroxyethyl methacrylate and 2-hydroxypropyl acrylate. The presence ofthe OH-functional vinyl monomer tends to promote the solubility of other monomers present during copolymerization of he vinyl copolymer. In particular, when a vinyl copolymer is produced without an OH-functional vinyl monomer and a vinyl monomer bearing a dispersing group as described below is present during copolymerization, the vinyl copolymer has been observed to be inhomogeneous and phase separable in common organic solvents. The vinyl monomer bearing a dispersing group, particularly methacryloyloxyethyl trimethyl ammonium chloride, is believed to be insoluble unless a sufficient amount of OH-functional vinyl monomer is present. In addition, the presence ofthe OH-functional vinyl monomer enables a larger amount ofthe nitrile-functional vinyl monomer described above to be incoφorated into the vinyl copolymer.

Examples of suitable vinyl monomers bearing a dispersing group include (meth)acryloyloxyethyl trimethyl ammonium chloride, (meth)acryloyloxyethyl acid phosphate, diphenyl 2-(meth)acryloyloxyethyl phosphate, (meth)acrylamidopropyl trimethylammonium chloride, (meth)acryloyloxypropyl dimethylbenzylammonium chloride, vinylbenzyl trimethylammonium chloride, 2-hydroxy-3-allyloxypropyl trimethylammonium chloride, (meth)acrylamidopropyl sodium sulfonate, sodium styrene sulfonate, styrene sulfonic acid, (meth)acrylic acid, maleic acid, fumaric acid, maleic anhydride, vinyl sulfonic acid, 2-(meth)acrylamide-2-methyl-l-propanesulfonic acid, dimethylaminoethyl (meth)acrylate, maleic anhydride, N-(3-sulfopropyl)-N- (meth)acryloyloxyethyl-N,N-dimethyIammonium betaine, 2-[(meth)acryloyloxy]ethyl trimethylammonium methosulfate, N-(3-sulfopropyl)-N-(meth)acrylamidopropyl-N, N- dimethylammonium betaine, vinylbenzyl trimethylammonium chloride, mixtures thereof, and the like.

The term "copolymerizable" with respect to the copolymerizable vinyl monomer means that the monomer has a vinyl moiety for undergoing copolymerization with other vinyl monomers but bears no dispersing group, no fluorine-containing group, no nitrile group, and no OH group. Representative examples of suitable copolymerizable vinyl monomers include styrene; alkylated styrenes; alkoxy styrenes; vinyl naphthalene; alkylated vinyl naphthalenes; alkoxy vinyl naphthalenes; (meth)acrylamides; N-vinyl pyrrolidone; linear, branched, or alicyclic alkyl esters of (meth)acrylic acid wherein the alkyl groups contain from 1 to 20, preferably 1-8, carbon atoms, such as methyl

(meth)acrylate, n-butyl (met h)acr late, t-butyl (meth)acrylate, ethyl (meth)acrylate, isopropyl (meth)acrylate, and 2-ethylhexyI (meth)acrylate; vinyl esters of alkanoic acids wherein the alkyl moiety ofthe alkanoic acids contain 2 to 20, preferably 2 to 4, carbon atoms and may be linear, branched, or alicyclic; isobornyl (meth)acrylate; glycidyl (meth)acrylate vinyl acetate; allyl (meth)acrylate, and the like.

In preferred aspects of this embodiment, the monomers ofthe primary vinyl copolymer comprise 10 to 30 parts by weight ofthe nitrile-functional vinyl monomer, 2 to 30 parts by weight ofthe OH-functional vinyl monomer, 0.1 to 5 parts ^• by weight ofthe vinyl monomer bearing a dispersing group, and 30 to 80 pads by weight ofthe copolymerizable vinyl monomer.

As an option, the monomers ofthe primary vinyl copolymer may further comprise one or more vinyl monomers bearing a fluorine-containing group such that pendant fluorine-containing groups are incoφorated into the primary vinyl copolymer. Such a primary vinyl copolymer can be reacted with a functionalized isocyanate as described above to produce a radiation crosslinkable binder polymer having pendant fluorine-containing groups. The vinyl monomer bearing a fluorine-containing group is preferably present in the range from about 5 to about 20 parts by weight. In the practice ofthe present invention, such monomers generally comprise at least one fluorine- containing moiety and at least one ethylenically unsaturated polymerizable group. Such monomers are described in U.S. Patent Nos. 4,582,882 and 4,761,459. Prefeιτed examples of such materials may be represented by the formula -19-

wherein R³ is hydrogen or CH₃; X* is a single bond or a divalent organic linking group; and Rr is a fluorinated moiety.

One exemplary class of compounds according to Formula (1) includes (meth)acrylate acid esters of fluorine-containing alcohols. Such alcohols include (a) 1 , 1-dihydrofluoroalkanols such as

CF₃(CF₂)_x(CH₂)_yOH (2) wherein x is an integer from 0 to 20, and y is an integer from 1 to 10; and

HCF₂(CF₂)_x(CH₂)_yOH (3) wherein x is an integer from 0 to 20, and y is an integer from 1 to 10;

(b) Fluoroalkylsulfonamido alcohols such as

wherein x is an integer from 0 to 20, R¹ is H or an alkyl, cycloalkyl, or arylalkyl of 1 to

20 carbon atoms, and R² is an alkylene group of 1 to 20 carbon atoms; (c) Periluorocyclodihydroalkyl alcohols such as

wherein z is an integer from 0 to 7, and y is an integer of 1 to 10;

(d) Fluoroether alcohols such as wherein q is 2 to 20 and greater than x, x is 0 to 20, y is 1 to 10, and the perfluoroalkoxy moieties -CF_CF₂0- and -CF₂0- may be either arranged in blocks or randomly distributed along the backbone ofthe material; and

CF₃(CF₂)_rO— (CFCF₂O)p-(CH₂)_sOH

CF₃ <⁷>

wherein p is 1 or more, preferably 1 to 6, s is 1 or more, preferably 1 to 3, and r is 1 to 6. In one embodiment, the vinyl monomer bearing a fluorine-containing group according to Formula (1) has the formula

wherein n has a value in the range from 6 to 10, and is preferably 8; R⁴ is -CHi or -C₂Hj, and is preferably -C₂Hj; X⁷ is an organic divalent linking group and is preferably -CH₂-; and R⁵ is hydrogen or -CH₃.

Specific examples of particularly preferred materials according to Formula (1) are the materials selected from the group consisting of

O C₇F,₅CH₂OCCH=CH₂ i W

O

II C₇F,5CH₂OCC=CH₂ (9)

CH₃

O cyclo— C₆F_uCH₂OCCH=CH₂ ; (¹⁰> O cyclo— C₆F, ,CH₂OCC=CH₂ and ( 1 1) CH₃

O O

C„F_2n+r— S-N-CHbCHjOCC-CHj ^{(l la)}

O C₂H_S CH₃

wherein n has an average value from about 7 to about 8. Another exemplary class of compounds containing at least one fluorine- containing moiety and at least one ethylenically unsaturated moiety includes pcrfluoroalkyl group-containing urethane monomers represented by the following general formula:

O O

R_j — X⁸— O— -NH— Z¹— NH-C-W (¹²)

wherein:

Rr is a perfluoroalkyl group comprising from about 4 to about 20 carbon atoms; X' is a divalent organic linking group and is preferably selected from the group consisting of -CH₂CH(A)C_jH_2j-, -C_kH_2k-, and -SO₂N(R⁶)C_nιH_2ιn,ι; wherein A is selected from the group consisting of hydrogen, lower alkyl of 1 to 5 carbon atoms, hydroxyl, lower alkoxy of 2 to 6 carbon atoms, and carbonyloxy; j is 0 to 4, each of k and m is independently I to 4, and R* is selected from the group consisting of hydrogen and a lower alkyl group of 1 to 4 carbon atoms;

Z¹ is a divalent organic linking group; and

W is a monovalent organic group containing an ethylenically unsaturated group

Examples of divalent organic groups represented by Z¹ in the perfluoroalkyl group-containing urethane monomer ofthe Formula (12) are those selected from the group consisting of

10

15

30 (19)

-(CH₂)₆— Examplcs of monovalent organic groups represented by W in the perfluoroalkyl group containing urethane monomer ofthe Formula (12) include but are not limited to those selected from the group consisting of

O

CH₂ - (20) tCC-L- -R⁹ -Z²—

wherein R⁷ is selected from the group consisting of hydrogen, methyl, ethyl, cyano, and carboxymethyl; L is selected from the group consisting of -O- and

-Ni l-; R* is an alkylene group comprising 1 to 12 carbon atoms; and Z² is selected from the group consisting of -0-, -NH-, and

The perfluoroalkyl group-containing urethane monomer ofthe Formula (12) can be prepared by the processes described in U.S. Pat. Nos. 3,398,182 and

3,484,281. More specifically, one mole of fluorine-containing alcohol (RfXOH) and one mole of diisocyanate compound (OCN-Y-NCO) are mixed and heated in the presence or absence of a catalyst such as triethylamine. After this first stage ofthe reaction, one moie of ethylenically unsaturated alcohol, amine, carboxylic acid, or the like is added to the reaction product, RfX⁸OC(0)NHZ'NCO ofthe first stage, thereby giving a fluorine- containing compound with ethylenic unsaturation wherein Rr, X⁸, and Z¹ are as defined for Formula (12).

Representative examples of useful ethylenically unsaturated alcohols, amines, carboxylic acids, or the like, for preparing perfluoroalkyl group-containing urethane monomer include the alcohols shown in Formulas (2) through (7) above.

In an alternative aspect of he particularly preferred embodiment, the primary vinyl copolymer may be a copolymer of vinyl monomers comprising one or more nitrile-functional vinyl monomers, one or more OH-functional vinyl monomers, one or more copolymerizable vinyl monomers, and optionally one or more vinyl monomers bearing a fluorine-containing group. The pendant dispersing groups are then attached to the primary vinyl copolymer in a separate reaction after copolymerization. This reaction preferably comprises reacting a difunctional material comprising an NCO group and a dispersing group with all or a portion ofthe OH groups on the primary vinyl copolymer to provide a vinyl copolymer having corresponding pendant dispersing groups. (See, e.g., U.S. Pat. No. 5,503,938, which describes reacting a monomer comprising NCO and quaternary ammonium moieties with hydroxyl-functional polymers to provide such polymers with pendant quaternary ammonium functionality). This reaction may be conducted at the same time that the primary vinyl copolymer is reacted with the functionalized isocyanate to produce the nonhalogenated functionalized vinyl copolymer comprising the radiation crosslinkable binder polymer of this invention.

The primary vinyl copolymer having pendant OH groups, nitrile groups, dispersing groups and/or fluorine-containing groups as described above may be prepared from vinyl monomers by free-radical polymerization methods known in the art, including but not limited to bulk, solution, emulsion and suspension polymerization methods. For example, according to the solution polymerization method, the vinyl copolymer is prepared by dissolving the desired monomers in an appropriate solvent, adding a chain-transfer agent, a free-radical polymerization initiator, and other additives known in the art, sealing the solution in an inert atmosphere such as nitrogen or argon, and then agitating the mixture at a temperature sufficient to activate the initiator.

Solvents useful in such polymerization can vary according to solubility ofthe monomers and additives. Typical solvents include ketones such as acetone, methyl ethyl ketone, 3-pentanone, methyl isobutyl ketone, diisobutyl ketone, and cyclohexanone; esters such as ethyl acetate, butyl acetate, isobutyl acetate, isopropyl acetate, and the like; aromatic hydrocarbons such as benzene, toluene, xylenes, cresol, and the like; ethers such as dϋsopropyl ether, diisobutyl ether, tetrahydrofuran, tetrahydropyran, and dioxane; and aprotic solvents such as dimethylformamide, dimethylsulfoxide and the like, and mixtures thereof. Alcohols such as methanol, ethanol, propanol, n-butanol, isopropanol, isobutanol, cyclohexanol and methyl cyclohexanol are not preferred in cases where isocyanates are used for crosslinking of the magnetic layer. The preferred solvent for preparation ofthe vinyl copolymers ofthe present invention is methyl ethyl ketone (MEK) because it is also the preferred medium in which the magnetic dispersions, described below, are prepared due to the ready solubility therein of polyurethane-vinyl copolymer blends. Chain transfer agents suitable for solution polymerization include but are not limited to alcohols, mcrcaptans, certain halogenated small molecules, and mixtures thereof. Preferably, the chain transfer agent is chosen from the group consisting of carbon tetrabromide, isooclylthioglycolatc, mercaptosuccinic acid, mercaptopropane diol, dodecyl mercaptan, ethanol and carbon tetrachloride. Most preferably, the chain transfer agent is mercaptopropane diol.

Free-radical polymerization initiators suitable for solution polymerization include those that are soluble in the reaction solvent and that are thermally activated, including but not limited to azo compounds, peroxides, and mixtures thereof. Useful peroxide initiators include those chosen from the group consisting of benzoyl peroxide, lauroyl peroxide, di-t-butyl peroxide and the like, and mixtures thereof. Useful azo compound initiators include those chosen from the group consisting of 2,2'-azobis(2- mcthylbutyronitrile); 2,2'-azobis(isobutyronitrile); and 2,2'-azobis(2,4- dimethylpentanenitrile); each of which is commercially available as VAZO 67, VAZO 64, and VAZO 52, respectively, from E. I. Du Pont de Nemours and Co. The preferred thermal polymerization initiator is the VAZO 64 brand initiator because of its ease of use and its half-life characteristics (e.g., at 64°C, half-life is 10 hours).

Primary vinyl copolymers suitable for use in the present invention may also be prepared by emulsion polymerization methods. Typically, an emulsion comprising vinyl monomers, a chain-transfer agent and a water-soluble oxidation - reduction ("redox") -type initiator system is prepared in an inert atmosphere, then heated carefully until a reaction exotherm occurs. The reaction mixture is stirred and cooled and the resulting latex is collected. Optionally, an ionic or nonionic surfactant may be added to the reaction mixture. Redox free-radical initiators useful in the invention include but are not limited to those chosen from the group consisting of tertiary amines with organic peroxides (exemplified by the N, N-diethylaniline-benzoyl peroxide pair); organic halides with transition metal complexes (exemplified by the carbon tetrachloride - molybdenum hexacarbonyl pair); inorganic oxidation - reduction systems (exemplified by the potassium persulfate - sodium metabisulfite pair); and organic - inorganic systems (exemplified by the 2-mercaptoethanol - Fe⁺³ pair). Inorganic redox initiators are preferred for the copolymers ofthe invention because of their ease of handling and useful reaction temperature range.

These polymerization methods are described in Applicant's copending application , U.S. Serial Number 08/404,234 filed 3/15/95 under the title " Magnetic Recording Medium Incoφorating Fluorine-Containing, Solvent-soluble Vinyl Copolymer Having No Vinyl Chloride or Vinylidene Chloride Components" in the names of Ravindra L. Arudi and Ramesh Kumar and having Attorney's Docket Number

49018USA1 A. Specific methods are also described in the examples which follow.

In any embodiment ofthe invention, the binder polymer component of the polymeric binder may include more than one radiation crosslinkable binder polymer having a plurality of radiation crosslinkable moieties, substantially none of which are (meth)acrylate and or (meth)acrylamide moieties. For example, the binder polymer component may have a first radiation crosslinkable binder polymer having T, >50°C and a second radiation crosslinkable binder polymer having T_g < 25°C, both of which contain substantially no radiation crosslinkable moieties which are (meth)acrylate and/or (meth)acrylamide moieties. In preferred aspects of such an embodiment, the first radiation crosslinkable binder polymer having T«>50⁰C is a member ofthe class of nonhalogenated unctionalized vinyl copolymer described above, and the second radiation crosslinkable binder polymer having T_g<25°C is a polyurethane polymer for increasing flexibility and toughness ofthe magnetic layer. In this embodiment, the first radiation crosslinkable binder polymer preferably comprises 30 to 50 parts by weight based on 1 0 parts by weight ofthe magnetic pigment in the polymeric binder, while the second radiation crosslinkable binder polymer preferably comprises 20 to 40 parts by weight based on 100 parts by weight ofthe magnetic pigment in the polymeric binder. In addition to one or more radiation crosslinkable binder polymers, the binder polymer component may further comprise a secondary binder polymer component comprising one or more polymers which have no radiation crosslinkable functionality. Such a non-radiation crosslinkable binder polymer component preferably comprises 70% by weight or less o the binder polymer component. The non-radiation crosslinkable binder polymer component may nevertheless have other pendant functionalities such as the nitrile groups, OH groups, dispersing groups, and fluorine- containing groups described above, or other functional groups which provide desirable properties to the magnetic recording medium. The non-radiation crosslinkable binder polymer component may include one or more polymers selected from a wide variety of polymers suitable for use as binders in magnetic recording media, such as polyesters, polyurethanes, vinyl copolymers, epoxy resins, cellulosic resins, acrylic polymers, and alkyd resins. An example ofa suitable non-radiation crosslinkable binder polymer component is Estane 5703 polyester polyurethane from The B.F. Goodrich Company.

The weight ratio ofthe radiation crosslinkable binder polymer to the non-radiation crosslinkable binder polymer component, if used, is in the range from 0.3 to 3, more preferably 0.5 to 2.

The polymeric radiation crosslinking agent ofthe present invention initiates and increases the rate ofthe crosslinking reaction in the polymeric binder. The polymeric radiation crosslinking agent comprises a plurality of (meth)acrylate-functiona! chain segments ofthe formula

wherein B¹ is a segment of the polymeric radiation crosslinking agent backbone, X¹ is a single bond or a divalent organic linking group, n has a value in the range from 2 to 6, and R⁹ is H or -CH₃.

The polymeric radiation crosslinking agent has a number average molecular weight of at least 4,000 and has sufficiently high molecular flexibility to effectively crosslink the polymeric binder upon exposure ofthe polymeric radiation crosslinking agent to electron beam radiation. For the purposes of this invention, molecular weight is measured by gas permeation chromatography (GPC). If the polymeric radiation crosslinking agent has a molecular weight which is too low, a magnetic coating made with the polymeric radiation crosslinking agent may have poor green strength and become tacky, and the polymeric radiation crosslinking agent may plasticize the coating. We believe that relatively high molecular flexibility of the polymeric radiation crosslinking agent will help the polymeric radiation crosslinking agent molecules to move about in the coating, particularly when the coating is being irradiated. We believe this mobility is necessary for the polymeric radiation crosslinking agent to effectively perform its crosslinking function. In other words, a polymeric radiation crosslinking agent which is too "stiff' will not be able to distribute itself among the polymeric binder molecules in order to quickly and uniformly initiate the crosslinking reaction. The crosslinking efficiencies of various polymeric radiation crosslinking agents may be evaluated and compared in one way by preparing magnetic or backside coatings with the polymeric radiation crosslinking agents, irradiating the coatings with electron beam radiation, and determining the level of crosslinking in each coating through solvent extraction and GPC analysis ofthe extractables. In another approach, samples cf magnetic recording media can be prepared according to the method of this invention and evaluated using conventional performance tests for durability and friction. A person skilled in the art can choose the parameters ofthe polymeric radiation crosslinking agent of this invention to optimize its molecular flexibility. These parameters typically include molecular weight and glass transition temperature (T_g), although the specific values will depend upon the type of polymer backbone chosen for the polymeric radiation crosslinking agent. In general, lower molecular weight and lower T_g will provide increased molecular flexibility. For the puφoses of this invention, T_g is measured by differential scanning calorimetry (DSC).

Preferably, the (meth)acrylate-functional chain segments ofthe polymeric radiation crosslinking agent have the formula

In a preferred approach, this polymeric radiation crosslinking agent is formed by reacting a (meth)acrylate-functional isocyanate ofthe formula

known as isocyanatoethylmethylmethacrylate ("IEM) with a suitable OH-functional polymer in a manner such that the NCO groups ofthe isocyanate react with a plurality ofthe OH groups on the OH-functional polymer according to the reaction scheme

wherein B¹ is a segment ofthe backbone ofthe OH-functional polymer and X¹ is a divalent organic linking group or a single bond.

In this reaction, there is preferably a molar excess of OH groups on the OH-functional polymer relative to the NCO groups on the isocyanate in order to drive substantially all ofthe NCO groups to react with OH groups and avoid undesirable side products. The molar ratio of OH groups to NCO groups is preferably in the range from greater than 1: 1 to 10:1, more preferably greater than 1 : 1 to 2: 1, and most preferably 1.1 : 1 to 1.7:1

The reaction is preferably conducted under ambient conditions (i.e., at room temperature and atmospheric pressure) in a suitable solvent. Examples of suitable solvents include ketones such as acetone, methyl ethyl ketone ("MEK"), methyl isobutyl ketone, or cyclohexanone; esters such as methyl acetate, ethyl acetate, butyl acetate, ethyl lactate, or glycol diacetate; tetrahydrofuran; dioxane or the like; and mixtures thereof. The amount of solvent used in this method is not critical as long as enough solvent is used such that substantially all ofthe OH-functional polymer and the IEM dissolve in the solvent. Generally, using 70% by weight of solvent based on the total weight ofthe solvent, the IEM, and the OH-functional polymer has been found to bc suitable in the practice of the present invention. A catalyst such as dibutyltindilaurate may be added to the solution to accelerate the reaction ofthe OH-functional polymer with the IEM. Optionally, a gellation inhibitor may be added to the solution. Examples of suitable gellation inhibitors, if used, include phenothiazine and butylated hydroxytoluene ("BHT"). The reaction mixture may be stirred slowly as the reaction takes place.

The progress ofthe reaction between the OH-functional polymer and the IEM may be monitored by measuring the IR absorption of the NCO group from Ihe IEM. The reaction is deemed to be complete when an IR absoφtion for the NCO group o the IEM can no longer be detected. When the reaction is carried out under ambient conditions, the reaction is typically completed after 3 to 4 days.

The OH-functional polymer for use in making the polymeric radiation crosslinking agent may be any OH-functional polymer suitable for use in magnetic recording media, such as OH-functional polyurethanes, alkyd resins, acrylic polymers, polyesters, epoxy resins, cellulose resins, vinyl copolymers, and the like. The OH- functional polymer chosen for making the polymeric radiation crosslinking agent should be compatible with any OH-functional polymer used to make the radiation crosslinkable binder polymer described previously. It not necessary for these OH-functional polymers to be the same. In addition to the pendant (meth)acrylate-functional groups, the polymeric radiation crosslinking agent of this invention may optionally include one or more other pendant functional groups in order to enhance the performance ofthe magnetic recording medium. For example, the polymeric radiation crosslinking agent may include a plurality of pendant OH groups and a plurality of pendant nitrile groups. The polymeric radiation crosslinking agent may additionally include a plurality of pendant fluorine-containing groups. The nature and function of these pendant groups were described previously with respect to the radiation crosslinkable binder polymer of this invention.

In one embodiment, the polymeric radiation crosslinking agent of this invention is a vinyl copolymer having a T_g in the range from 40 to 80°C and a molecular weight in the range from 10,000 to 50,000, the vinyl copolymer including a plurality of pcndant OH groups and nitrile groups in addition to the preferred (meth)acrylate- functional moieties described above.

In another embodiment, the polymeric radiation crosslinking agent is a vinyl copolymer having a T_gin the range from 40 to 80°C and a molecular weight in the range from 10,000 to 50,000, the vinyl copolymer including a plurality of pendant nitrile groups, OH groups and fluorine-containing groups in addition to the prefeπed ( met h)acry late-functional moieties described above.

Most preferably, such vinyl copolymers are obtained by reacting a methacrylate-functional isocyanate known as isocyanatoethylmethacrylate ("IEM) with a primary vinyl copolymer having pendant nitrile groups, OH groups, and, optionally, fluorine-containing groups. In this reaction, the pendant OH groups ofthe primary vinyl copolymer are in molar excess relative to the NCO groups of the isocyanate such that unreacted pendant OH groups remain after all the NCO groups have reacted. This reaction can be represented schematically as follows:

wherein R^N is a pendant nitrile group and R^F is a pendant fluorine-containing group. According to one approach, the primary vinyl copolymer suitable for making the polymeric radiation crosslinking agent ofthe present invention is a copolymer of vinyl monomers comprising one or more nitrile-functional vinyl monomers, one or more OH-functional vinyl monomers, and one or more copolymerizable vinyl monomers. The vinyl monomers may optionally further comprise one or more vinyl monomers bearing a fluorine-containing group. Representative examples of these monomers were described previously with respect to the primary vinyl copolymer used for making the radiation crosslinkable binder polymer. Preferably, the vinyl monomers comprise 35 to 105 parts by weight ofthe nitrile-functional vinyl monomer, 30 to 90 parts by weight ofthe OH-functional vinyl monomer, 45 to 130 parts by weight ofthe copolymerizable vinyl monomer, and, if used, 10 to 30 parts by weight of the vinyl monomer bearing a fluorine-containing group. One example of this embodiment is a copolymer of 90 parts by weight styrene, 75 parts by weight acrylonitrile, 65 parts by weight 2-hydroxy ethyl methacrylate and 20 parts by weight of N-ethyl perfluorosulfonamido ethyl methacrylate. This primary vinyl copolymer is also suitable for making the radiation crosslinkable binder polymer of this invention. Suitable polymerization techniques for preparing the primary vinyl copolymer used for making the polymeric radiation crosslinking agent are the same as those described previously with respect to the primary vinyl copolymer used for making the radiation crosslinkable binder polymer of this invention. Preparation of the primary vinyl copolymer is also described in the examples which follow. The polymeric radiation crosslinking agent of this invention is present in the polymeric binder in an amount effective to induce and promote a crosslinking reaction in the polymeric binder when the polymeric binder is exposed to electron beam radiation. If the binder polymer component contains a radiation crosslinkable binder polymer, the pendant radiation crosslinkable moieties ofthe radiation crosslinkable binder polymer will be crosslinked in the reaction. If the binder polymer component contains only non-radiation crosslinkable polymers, the polymeric radiation crosslinking agent itself will form a crosslinked polymer network around the binder polymer component. The degree of crosslinking which is present after irradiation can be evaluated by a test such as solvent extraction followed by GPC analysis of the extractables. Using from 5 to 50 parts by weight, preferably 2 to 20 parts by weight of the polymeric radiation crosslinking agent based upon 100 parts by weight of tlie magnetic layer is suitable in the practice ofthe present invention.

It is preferred that the components of the magnetic layer include at least one dispersant to facilitate the dispersion ofthe magnetic pigment in the polymeric binder. A variety of dispersants may be used in the practice of the present invention, and the particular choice of dispersant will depend, in part, upon the type of magnelic and nonmagnetic pigments that are used. For example, in the case of γ-Fe₂0₃ magnetic pigment or carbon black, amine-functional, polymeric dispersants (such as the Disperbyk brand dispersants commercially available from BYK-Chemie USA) have been found to be suitable in the practice ofthe present invention. In the case of higher surface area pigments, e.g., pigments having a surface area of 25 m²/g to 70 m²/g such as cobalt-doped Fe₂0₃, barium ferrite, and metal particle pigments, a preferred class of dispersants comprises monomeric, oligomeric, or polymeric dispersants comprising at least one dispersing moiety and at least one radiation crosslinkable α-methylstyrene moiety. Such dispersants are described in U.S. Patent Number 5,380,905.

In addition to the polymeric binder comprising the binder polymer component and the polymeric radiation crosslinking agent; the dispersant; and the magnetic pigment; the magnetic layer ofthe present invention may also comprise one or more conventional additives such as lubricants; abrasives; thermal stabilizers; antioxidants; antistatic agents; fungicides; bacteriocides; surfactants; coating aids; nonmagnetic pigments, and the like in accordance with practices known in the art. In an alternative embodiment ofthe present invention, the magnetic recording medium comprises a substrate having first and second major surfaces. A magnetic layer is provided on the first major surface, and a backside coating is provided on the second major surface. The backside coating is prepared from components which comprise at least one nonmagnetic pigment dispersed in a polymeric binder. The polymeric binder is an electron beam radiation crosslinked matrix of ingredients comprising the polymeric radiation crosslinking agent and the binder polymer component as described previously with respect to the magnetic layer. In this embodiment, the backside coating is formed by the steps of: a) dispersing the nonmagnetic pigment in the binder polymer component to form a backside dispersion; b) blending the polymeric radiation crosslinking agent into the dispersion of step a) in an amount effective to induce a crosslinking reaction when the polymeric binder is exposed to electron beam radiation; c) after blending the polymeric radiation crosslinking agent into the dispersion, coating the dispersion onto the first major surface of the substrate to form a backside coating; and d) irradiating the coated substrate with an effective amount of electron beam radiation to substantially crosslink the backside coating.

Since high surface area nonmagnetic pigments such as titanium dioxide and carbon black that are commonly used in backside coatings can promote premature crosslinking and gellation of the backside dispersion, the polymeric radiation crosslinking agent is added after milling of he backside dispersion has been completed. The polymeric radiation crosslinking agent is preferably present in an amount in the range from 2 to 20 parts by weight based on 100 parts by weight ofthe backside coating.

In addition to the nonmagnetic pigment and the polymeric binder comprising the binder polymer component and the polymeric radiation crosslinking agent, backside coatings 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; bacteriocides; surfactants; coating aids; and the like in accordance with practices known in the art.

In a particularly preferred embodiment of the present invention, both the magnetic layer and the backside coating include a polymeric binder which is an electron beam radiation crosslinked matrix of ingredients comprising a polymeric radiation crosslinking agent and a binder polymer component as described previously. The polymeric binders of the magnetic layer and the backside coating may contain the same polymeric radiation crosslinking agent and the same binder polymer component, or the polymeric radiation crosslinking agent and binder polymer component may be different.

In this embodiment, the magnetic recording medium is prepared by forming the backside coating and the magnetic layer on a substrate according to the steps described previously.

The magnetic recording media ofthe present invention are prepared according to the following method: The magnetic layer is prepared by milling the magnetic pigment, the binder polymer component described previously, and a suitable solvent in a first step to form a homogeneous magnetic dispersion. Optionally, all or a portion ofthe dispersant and any conventional additives, if any of these are used, may also be milled in this first step. In this first step, using 30 to 75, and more preferably 45 to 65, percent by weight of solvent based on the total weight ofthe magnetic pigment, the binder polymer component including any radiation crosslinkable binder polymer, any non-radiation crosslinkable binder polymer component, and any other additives has been found to be suitable in the practice of the present invention.

In a second step, the polymeric radiation crosslinking agent described previously and additional solvent are blended with the magnetic dispersion just prior to coating the dispersion onto the substrate. The polymeric radiation crosslinking agent is added in an amount effective to induce a crosslinking reaction when the binder polymer component and the polymeric radiation crosslinking agent are exposed to electron beam radiation. The polymeric radiation crosslinking agent is added after the dispersion has been milled to avoid premature crosslinking of the binder polymer component or the polymeric radiation crosslinking agent itself to form insoluble gels. 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 is comprised of 45 to 75 percent by weight ofthe solvent based on the total weight ofthe 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.

After blending the polymeric radiation crosslinking agent, additional solvent, and other ingredients, if any, into the magnetic dispersion, the magnetic dispersion is then coated onto the substrate. The dispersion may be applied to the substrate using any conventional coating technique, such as gravure coating techniques or knife coating techniques. The coated substrate may then be optionally passed through a magnetic field to orient or randomize the magnetic pigment as desired, after which the coating is dried, calendered if desired, and then crosslinked with electron beam radiation. Crosslinking is achieved using electron beam radiation in accordance with practices known in the art. Preferably, crosslinking is achieved with an amount of electron beam radiation in the range from 1 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. The amount and energy level of radiation may be adjusted in accordance with practices known in the art to achieve sufficient crosslinking ofthe magnetic layer. The level of crosslinking in the magnetic layer after irradiation may be evaluated using a test such as solvent extraction o the coating followed by GPC analysis ofthe solvent extractables.

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 crosslinking. "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.

If the magnetic recording medium includes a backside coating having a polymeric radiation crosslinking agent of this invention, the backside coating is prepared in a manner similar to the method described above for the magnetic layer. Preparation steps include milling a dispersion in solvent, adding a polymeric radiation crosslinking agent after milling, coating, drying, and irradiation. Advantageously, if both the magnetic layer and the backside coating contain a polymeric radiation crosslinking agent of this invention, crosslinking of both coatings may be accomplished in a single pass through an electron beam apparatus.

The present invention will now be further described with regard i:o the following examples. EXAMPLE 1 Five samples in solution of primary vinyl copolymers having pendant OH groups and pendant nitrile groups were prepared from the following ingredients:

Ingredient parts by weight of solids

IA IB IC ID IE

1 Styrene ("ST") 121 121 121 121 121

2-hydroxy ethyl 57.2 57.2 57.2 57.2 57.2 | methacrylate ("HEMA")

Acrylonitrile ("AN") 41.8 41.8 41.8 41.8 41.8

3 -mercapto 1,2-propane 0 0.4 1.1 2.2 4.4 diol ("MPD")

VAZO™ 64 thermal 0.66 2.2 2.2 0.66 3.3 initiator (E.I. Dupont de Nemours and Co.) (2,2'- a obisisobutyronitrile)

For each sample, the ingredients were charged to a glass bottle, along with sufficient MEK solvent to produce an admixture determined to be at 40% solids. The admixture was purged with N₂ gas for 5 minutes at the rate of 1 liter per minute after which the bottle was sealed and tumbled in a constant temperature bath at 65^'C for 60 hours. The reaction product in each case was a viscous, clear, and slightly yellow solution containing a copolymer ofthe above-listed monomers ST, HEMA, and AN.

The value of % solids for each sample indicated a nearly quantitative conversion of monomers to copolymer.

The number average molecular weight of each sample was determined by gas permeation chromatography (GPC). As can be seen in the table below, molecular weight decreases as a function of increasing the level of MPD used to prepare the sample. Each ofthe primary vinyl copolymer solutions had the same composition of styrene, acrylonitrile and hydroxyethylmethacrylate, with a hydroxy equivalent weight of about 500.

EXAMPLE 2 Five polymeric radiation crosslinking agents ofthe present invention in solution were prepared by the further reaction ofthe primary vinyl copolymer solutions identified as Samples 1 A IB, IC and ID prepared in Example 1 with isocyanatoethyl methacrylate ("IEM") so that 70% of hydroxyl groups on each copolymer were converted to methyl methacrylate groups. IEM has the formula

The NCO group ofthe IEM molecule reacts with hydroxyl groups on the copolymer according to the following generalized reaction scheme:

The polymeric radiation crosslinking agents, identified as samples 2A - 2E, were prepared according to the following formulations.

Ingredient parts by weight of solids

2A 2B 2C 2D 2E

Sample 1 A copolymer 120 — — — — (40% in MEK)

Sample IB copolymer ~ 120 — — - (40% in MEK)

Sample 1 C copolymer ~ ~ 120 — — (40% in MEK)

Sample ID copolymer — — — 120 — (40% in MEK)

Sample IE copolymer — — — — 120 (40% in MEK)

Isocyanatoethyl 26 26 26 26 26 methacrylate ("IEM")

Butylated hydroxy 0.03 0.03 0.03 0.03 0.03 toluene ("BHT") stabilizer

Dibutyltindilaurate 0.23 0.23 0.23 0.23 0.23 catalyst

For each sample, the ingredients were placed in ajar along with sufficient MEK solvent to result in an admixture at about 40% solids. The jar was shaken well to combine the ingredients, sealed and allowed to stand undisturbed at room temperature for 48 hours. The reaction was judged to be complete by observing with infrared spectroscopy the disappearance ofthe NCO peak at 2270 cm^*'.

EXAMPLE 3 Five magnetic dispersions were prepared using the five polymeric radiation crosslinking agent solutions prepared in Example 2. The dispersions were prepared according to the following formulations and procedure. First a stock dispersion was prepared. parts by weight of solids

Stock Dispersion Ingredient

Charge A

Titan MRD γ-Fe 0₃ Magnetic Pigment 100

Vulcan XC-72 Carbon Black (Cabot Corp ) 8.25

DISPERBYK- 160 polymer (29% by weight solution ofa 6 polyurethane polymer having pendant tertiary amine wetting groups (BYK-Chemie USA)

Binder prepared according to U.S. Pat. No. 5,380,905, Example 15.17 1, Sample 2.

ESTANE 5703 polyurethane polymer (The B.F Goodrich 3.46 Company)

Propyl Gallate 0.08

Tetramethyl Thiuram Disulfide 0.08

Charge A was mixed together with sufficient MEK solvent to give an admixture at approximately 39% solids. The admixture was then milled in 4 liter horizontal sand mill (Netzch) until the dispersion was smooth. To 1647 grams (g) of the resulting dispersion was added 4 g of myristic acid and 21 g of isocetylstearate, followed by mixing for 15 minutes to produce the stock dispersion. Each of five final magnetic dispersions (3 A - 3E) were then prepared by adding 12.75 g of one ofthe polymeric radiation crosslinking agents prepared in Example 2 (40% solids in MEK) to 200 g ofthe stock dispersion, along with sufficient MEK solvent to bring the solids down to 30%. Each ofthe dispersions was then filtered through a 15 micron disc filter. The polymeric radiation crosslinking agent contained in each dispersion is shown in the table below.

Magnetic Dispersion Number Polymeric Radiation Crosslinking Agent Sample Number

3A 2A

3B 2B

3C 2C

3D 2D

3E 2E

Each dispersion was then applied to a 76.2 mm (3 mil), biaxially-oriented polyethylene terephthalate (PET) film substrate and allowed to dry at room temperature for 48 hours. The dried coatings were then crosslinked with a 9 Mrad dose of a 225 KeV electron beam (e-beam) curing apparatus from Energy Sciences, Inc. The coatings were then tested for crosslinking level by extracting the coating with DMSO solvent and analyzing the extracted material using ultraviolet (UV) spectroscopy for absorbance at 280 nm wavelength. The % drop in absorbance, ΔA, corresponding to the level of crosslinking, is given by the formula (A-A_f)/A; x 100, where A, is the absorbance at 280 nm before crosslinking and A is the absorbance at 280 nm after crosslinking. As shown in the table below, the % drop in absorbance generally increased with decreasing molecular weight ofthe polymeric radiation crosslinking agent used in the dispersion, with the exception of Sample 3B.

These results indicate that a relatively lower molecular weight polymeric radiation crosslinking agent may induce crosslinking more efficiently, perhaps due to increased mobility ofthe crosslinking agent molecule. Sample 3B appears to be outside the general trend.

EXAMPLE A polymeric radiation crosslinking agent ofthe present invention was prepared in the following manner. A primary vinyl copolymer solution was first prepared by charging 90 g of styrene ("ST"), 75 g of acrylonitrile ("AN"), 65 g of 2-hydroxy ethyl methacrylate ("HEMA"), 20 g of N-ethyl perfluorosulfonamido ethyl methacrylate ("ElF"), 1.25 g of 2,2'-azobisisobutyronitrile VAZO™ thermal initiator (E.I. Dupont de Nemours and Co.), 0.75 g of 3-mercapto 1,2-propane diol ("MPD") and 375 g of MEK into a glass bottle. The bottle was shaken to mix the ingredients, and the resulting admixture was purged with N₂ gas at a rate of 1 l/min for 5 minutes after which thϊ bottle was sealed and tumbled in a constant temperature bath at 65^*C for 60 hours The resulting copolymer solution was a clear viscous material with essentially no monomer smell, indicating a complete copolymerization reaction. Several batches of copolymer solution were prepared for subsequent use.

2400 g ofthe primary vinyl copolymer solution was then combined with 232 g of isocyanatoethyl methacrylate ("IEM") as described in Example 2, 1.9 g of dibutyltin dilaurate catalyst, 0.24 g of BHT stabilizer and 272 g of MEK solvent in a glass bottle. The bottle was sealed, shaken thoroughly, and allowed to stand undisturbed for 48 hours at room temperature. The resulting polymeric radiation crosslinking agent solution contained no unreacted isocyanate groups from the IEM, as shown by the absence of an infrared absoφtion peak at 2200 - 2300 cm^"1. The solution was clear and yellow in appearance with relatively low viscosity.

EXAMPLE 5 A solution containing a radiation crosslinkable binder polymer ofthe present invention having pendant quaternary ammonium functionalities was prepared in the following manner. An amine isocyanate intermediate, hereinafter referred to as Compound I, was first prepared as follows. Isophorone diisocyanate (IPDI, 192.0 g) and dimethylethanolamine (DME, 85.5 g) were added to methyl ethyl ketone (MEK, 160.0 g) while stirring. The reaction flask was equipped with a water-cooled reflux condenser. In about 15 minutes the temperature rose to 60°C and stayed at 60-70°C for an hour. The reaction was allowed to go to completion for 16 hours, thereby minimizing the IPDI residual level which may cause gellation from crosslinking and chain extension in the subsequent quaternization step. The final solution of Compound I was slightly yellow and had a low viscosity.

Compound I was then converted to an isocyanate having ammonium methyl sulfate groups, hereinafter referred to as Compound II. To accomplish the conversion, a solution of dimethyl sulfate, (CH₃)₂S0₄ (114 g), in MEK (54 g) was added to the entire solution of Compound I slowly with stirring. The rate of addition was varied to keep the temperature at 45-55°C for 40 min. The reaction mixture was then allowed to stand for 16 hours to yield a slightly orange colored low viscosity solution of Compound II in MEK (65% solids). The orange color most likely came from dark brown oily contaminants in dimethylsulfate. The molar ratio of Compound I:(CH₃)₂S0₄ in this example was calculated for complete conversion ofthe amine to the quaternary ammonium group.

The primary vinyl copolymer solution prepared in Example 4 was then involved in a reaction with Compound II and an α-methylstyrene-functional isocyanate to form a solution ofa radiation crosslinkable binder polymer having pendant quaternary ammonium dispersing moieties and α-methylstyrene radiation crosslinkable moieties. 2400 g of the primary vinyl copolymer solution (39% solids in MEK) was charged to a glass jar along with 297 g of l-(l-isocyanato-l-methylethyl)-3-(l-methylethenyl)- benzene ("TMI"), 43 g of Compound II ("IDM") solution as prepared above (65% solids in MEK), 2 g of dibutyltindilaurate catalyst, 0.2 g of butylated hydroxy toluene

("Bl IT") stabilizer, and 425 g of MEK. The contents of the jar were shaken thoroughly and allowed to stand undisturbed for 48 hours at room temperature. After 48 hours, the reaction product was a clear, slightly yellow, viscous solution ofthe final copolymer. When analyzed by infrared spectroscopy, the solution showed no absoφtion peak at 2270 cm^"1, indicating a complete reaction ofthe isocyanate groups on the TMI and IDM molecules. EXAMPLE 6 Three magnetic dispersions, hereinafter referred to as Dispersions 6 6B and 6C, were prepared according to the following formulations. Dispersions 6A and 6B contained commercially available monomeric and monomcric/oligomeric crosslinking agents, while Dispersion 6C contained the polymeric radiation crosslinking agent prepared in Example 4.

Magnetic Dispersion Ingredient parts by weight solids

6A 6B 6C

Charge A

SMO III™ Co-doped γ -Iron oxide magnetic 20 20 20 pigment

Ketjenblack® conductive carbon black (Akzo 1.4 1.4 1.4 Chemie), about 10 nm

Radiation crosslinkable binder polymer of Example 5 2.42 2.42 2.42 (40% solids in MEK)

1 lydroxy-functional polyester polyurethane (30% 1.61 1.61 1.61 solids in MEK)

Charge B

HP-F Alumina (Reynolds Metal, Inc.) 2 2 2

Isocetyl stearate 0.9 0.9 0.9

Myristic Acid 0.3 0.3 0.3

SR-399 Radiation Crosslinker (Sartomer Co., Inc.) 1.8 — ... (100% solids)

Ebecryl-220 aromatic hexacrylate oligomer (Radcure ~ 1.8 ... Specialties, Inc.)

Polymeric radiation crosslinking agent of Example 4 ~ — 1.8 (40% solids in MEK)

For each dispersion, Charge A was mixed together with sufficient solvent (An 80:20 blend of MEK and cyclohexanone) to give an admixture at approximately

44% solids. The admixture was then milled in a horizontal sand mill for 12 passes, resulting in a smooth dispersion. The alumina was predispersed and premilled using phosphorylated polyoxyalkyl polyol (75% solids in toluene) (see U.S. Pat. No. 5,028,483 col. 5, lines 32-45) and EMCOL phosphate (Witco Coφ.) as dispersing agents with MEK as the solvent. The dispersing agents were added at 1 % (by weight) each based on weight of alumina. The resulting dispersion was approximately 7:5% solids. Charge B was then added to the dispersion, and the dispersion was thinned down to approximately 30% solvents using a 80:20 MEK: cyclohexanone solvent mixture. The dispersion was filtered.

Each dispersion was coated onto a 76.2 mm (3 mil), biaxially oriented polyethylene terephthalate (PET) film substrate using a rotogravure coating method, dried at 240^°F (116°C), and crosslinked in-line with a 225 KeV electron beam (E-beam) crosslinking apparatus from Energy Sciences, Inc. using a 10 Mrad dosage level. The coated web was then converted into 3.5" diameter diskettes which were evaluated for electromagnetic performance, error quality, durability and running torque. Electromagnetic performance and error quality were measured according to ANSI

X3 171. Durability was measured in two ways. First an accelerated test was run using a modified single-sided head drive at a speed and a head loading force which were higher than standard conditions. A long term test was then run using a standard two-sided drive under normal operating conditions. Media wear in both tests was graded visually. Running torque was measured both as a short term test and an extended test according to ANSI X3.171 using a CTC 5300 Torque Tester (Cyber Technics Coφ.) with heads loaded. The results are shown in the following table.

Test 6A 6B 6C

Electromagnetic Performance (ave. of 10)

2F Amplitude [% gold] 99 108 109

Resolution [% gold] 102 107 105

J Modulation [% gold] 5 4 5

Error Quality (ave. of 10)

Extra Pulse Threshold 14 13 13

Missing Pulse Threshold 70 69 !_ ⁷⁰

Durability

4 hour accelerated (ave. of 8) 100 75 38 [% passing]

Long term (ave. of 5) 3.6 7.7 12.7

[no passes in millions at 50% failure]

Running Torque [g-cm]

Short term (ave. of 6) t=0 35.6 42.8 34.0 t=15 min 40.4 43.9 39.3 t= 120 min 45.4 46.9 42.8

Extended (ave. of 6) t=0 33.0 31.8 34.0 t=2 weeks 34.3 33.7 33.2 |

Diskettes made with the various dispersion showed no significant differences between them in electromagnetic performance, error quality and extended running torque. The diskette made with Dispersion 6C (having the polymeric radiation crosslinking agent of this invention) performed better in the areas of long term durability and short term running torque.

EXAMPLE 7 Two magnetic dispersions, hereinafter referred to as Dispersions 7A and 7B, were prepared according to the following formulations. Dispersion 7A contained the polymeric radiation crosslinking agent of this invention prepared in Example 4, while Dispersion 7B contained a commercially available monomeric/oligomeric crosslinking agent.

To prepare each magnetic dispersion, Charge A was mixed together in a double planetary mixer under a nitrogen atmosphere, along with sufficient MEK solvent to give an admixture at approximately 40% solids. The admixture was then milled in a 4 liter horizontal sand mill (Netzch) until a smooth dispersion was obtained. Charge B was added to the dispersion, and the dispersion was thinned down using 92:8 MEK. toluene solvent to an appropriate solids level (38% for Dispersion 7A and 32% for Dispersion 7B).

Each dispersion was then coated onto a 0.36 mil biaxially-oriented polyethylene terephthalate web substrate using a gravure coating method, dried at 140°F (60°C), and crosslinked in-line with a 200 keV e-beam crosslinking apparatus from Energy Sciences, Inc. using an 8 Mrad dosage level. The coated webs were then converted to 0.31 in (8 mm) computer tape.

The tapes were tested for surface roughness using a Wyco interferometer, magnetic coating to ferrite head friction using a laboratory tester from Intcrnational Business Machines (IBM) and dropout quality using a Sony Hi-8 videodeck modified for sine wave input and output with a Sony Hi-8 8 mm metal particle tape as a reference. Results are shown in the table below.

The tape made with Dispersion 7A (having the polymeric radiation crosslinking agent of this invention) exhibited acceptable friction levels while the tape made with Dispersion 7B (having the monomeric/oligomeric crosslinking agent) showed extremely high friction, to the point of sticking to the ferrite head material.

Other embodiments of this invention will be apparent to those skilled in the art upon 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 magnetic recording medium comprising a magnetic layer provided on a substrate, wherein the magnetic layer comprises a magnetic pigment dispersed in a polymeric binder, wherein the polymeric binder is an electron beam radiation crosslinked matrix of ingredients, the ingredients comprising: a binder polymer component; and a polymeric radiation crosslinking agent comprising a plurality of (meth)acrylate-functional chain segments ofthe formula

B— X-O ?CNH— CπHjn- O— ? CC=CH₂

wherein:

B is a segment ofthe polymeric radiation crosslinking agent backbone; X is a single bond or a divalent organic linking group; n has a value in the range from 2 to 6; and R is H or -CH₃, wherein the polymeric radiation crosslinking agent has a molecular weight of at least 4000 and has sufficiently high molecular flexibility to effectively crosslink the polymeric binder upon exposure ofthe polymeric radiation crosslinking agent to electron beam radiation, wherein the magnetic layer is formed by the steps of: a) dispersing the magnetic pigment in the binder polymer component to form a magnetic dispersion; b) blending the polymeric radiation crosslinking agent into the magnetic dispersion of step a) in an amount effective to induce a crosslinking reaction when the polymeric binder is exposed to electron beam radiation; c) after blending the polymeric radiation crosslinking agent into the magnetic dispersion, coating the magnetic dispersion onto the substrate to form a magnetic coating; and d) irradiating the coated substrate with an effective amount of electron beam radiation to substantially crosslink the magnetic coating.

2. A magnetic recording medium comprising a substrate having first and second major surfaces, a magnetic layer provided on the first major surface of the substrate, and a backside coating provided on the second major surface ofthe substrate, wherein the backside coating comprises at least one nonmagnetic pigment dispersed in a polymeric binder, wherein the polymeric binder is an electron beam radiation crosslinked matrix of ingredients, the ingredients comprising: a binder polymer component; and a polymeric radiation crosslinking agent comprising a plurality of (meth)acrylate-functional chain segments ofthe formula

wherein

B is a segment ofthe polymer backbone; X is a single bond or a divalent organic linking group; n has a value in the range from 2 to 6; and

R is H or -CH3, wherein the polymeric radiation crosslinking agent has a molecular weight of at least 4000 and has sufficiently high molecular flexibility to effectively crosslink the polymeric binder upon exposure ofthe polymeric radiation crosslinking agent to electron beam radiation, wherein the backside coating is formed by the steps of: a) dispersing the nonmagnetic pigment in the binder polymer component to form a backside dispersion; b) blending the polymeric radiation crosslinking agent into the dispersion of step a) in an amount effective to induce a crosslinking reaction when the polymer binder is exposed to electron beam radiation; c) after blending the polymeric radiation crosslinking agent into the dispersion, coating the dispersion onto the first major surface ofthe substrate to form a backside coating; and d) irradiating the coated substrate with an effective amount of electron beam radiation to substantially crosslink the backside coating.

3. The magnetic recording medium of claim 1 or 2, wherein the binder polymer component comprises at least 30% by weight ofa radiation crosslinkable binder polymer, the radiation crosslinkable binder polymer comprising a plurality of pendant radiation crosslinkable moieties, wherein substantially no pendant radiation crosslinkable moieties ofthe radiation crosslinkable binder polymer are (meth)acrylate or (meth)acrylamide moieties.

4. The magnetic recording medium of claim 3, wherein the binder polymer component further comprises 70% by weight or less ofa non-radiation crosslinkable binder polymer component.

5. The magnetic recording medium of claim 3, wherein the radiation crosslinkable binder polymer comprises a plurality of chain segments ofthe formula

wherein:

Rl is a segment ofthe radiation crosslinkable binder polymer backbone; χ2 is a single bond or a divalent linking group; and

M¹ is a radiation crosslinkable moiety selected from the group consisting of α-methylstyrene-functional radiation crosslinkable moiety ofthe formula.

allyloxy radiation crosslinkable moiety ofthe formula:

-O-CH₂-CH=CH₂; and combinations thereof.

6. The magnetic recording medium of claim 3, wherein the radiation crosslinkable binder polymer is a first radiation crosslinkable binder polymer having a glass transition temperature (Tg) > 50°C, and the binder polymer component further comprises a second radiation crosslinkable binder polymer having T_g < 25°C, wherein the second radiation crosslinkable binder polymer comprises a plurality of pendant radiation crosslinkable moieties, wherein substantially no pendant radiation crosslinkable moieties ofthe second radiation crosslinkable binder polymer are (meth)acrylate or (meth)acrylamide moieties.

7. The magnetic recording medium of claim 6, wherein the first radiation crosslinkable binder polymer is a nonhalogenated functionalized vinyl copolymer, wherein in addition to the radiation crosslinkable moieties, the vinyl copolymer further comprises a plurality of pendant OH groups, a plurality of pendant nitrile groups, and a plurality of pendant dispersing groups; and the second radiation crosslinkable binder polymer is a polyurethane polymer.

8. The magnetic recording medium of claim 1 or 2 wherein the polymeric radiation crosslinking agent is present in an amount in the range from 2 to 20 parts by weight based on 100 parts by weight ofthe magnetic layer.

9. The magnetic recording medium of claim 1 or 2 wherein the polymeric radiation crosslinking agent is formed by reacting a methacrylate- functional isocyanate ofthe formula

with an OH-functional polymer in a manner such that the NCO groups on the isocyanate react with a plurality ofthe OH groups on the OH-functional polymer according to the reaction scheme

wherein

B is a segment ofthe backbone ofthe OH-functional polymer; and

X is a divalent organic linking group or a single bond.

10. The magnetic recording medium of claim 9 wherein there is a molar excess of OH groups on the OH-functional polymer relative to NCO groups on the isocyanate.

11. The magnetic recording medium of claim 1 or 2 wherein the polymeric radiation crosslinking agent is a vinyl copolymer having a glass transition temperature (T_g) in the range from 40 to 80°C and a molecular weight in the range from 10,000 to 50,000, wherein, in addition to the (meth)acrylate-functional moieties, the vinyl copolymer comprises a plurality of pendant OH groups, a plurality of pendant nitrile groups, and optionally, a plurality of pendant fluorine- containing groups.

12. The magnetic recording medium of claim 11 , wherein the vinyl copolymer is formed by reacting a (meth)acrylate-functional isocyanate with a primary vinyl copolymer having pendant OH groups, pendant nitrile groups, and pendant fluorine-containing groups in a manner such that the NCO groups on the isocyanate react with a plurality ofthe OH groups on the primary vinyl copolymer according to the generalized reaction scheme: wherei

RN is a pendant nitrile group; and Rp is a pendant fluorine-containing group.

13. The magnetic recording medium of claim 12, wherein the primary vinyl copolymer is a copolymer of vinyl monomers, the vinyl monomers comprising: a) 35 to 105 parts by weight of one or more nitrile-functional vinyl monomers; b) 30 to 90 parts by weight of one or more OH-functional vinyl monomers; c) 45 to 130 parts by weight of one or more copolymerizable vinyl monomers; and d) 10 to 30 parts by weight of one or more vinyl monomers bearing a fluorine-containing group.

14. The magnetic recording medium of claim 2, wherein the magnetic layer comprises a magnetic pigment dispersed in a second polymeric binder, wherein the second polymeric binder is an electron beam radiation crosslinked matrix of ingredients, the ingredients comprising: a second binder polymer component; and a second polymeric radiation crosslinking agent comprising a plurality of

(meth)acrylate-functional chain segments ofthe formula

wherein: βl is a segment ofthe polymer backbone;

X¹ is a single bond or a divalent organic linking group; n has a value in the range from 2 to 6; and

R2 is H or -CH₃, wherein the second polymeric radiation crosslinking agent has a molecular weight of at least 4000 and has sufficiently high molecular flexibility to effectively crosslink the polymeric binder upon exposure ofthe polymeric radiation crosslinking agent to electron beam radiation, wherein the magnetic layer is formed by the steps of: e) dispersing the magnetic pigment in the second binder polymer component to form a magnetic dispersion; f) blending the second polymeric radiation crosslinking agent into the magnetic dispersion of step e) in an amount effective to induce a crosslinking reaction when the second polymeric binder is exposed to electron beam radiation; g) after blending the second polymeric radiation crosslinking agent into the magnetic dispersion, coating the magnetic dispersion onto the second major surface ofthe substrate to form a magnetic coating; and h) irradiating the coated substrate with an effective amount of electron beam radiation to substantially crosslink the magnetic coating.

15. A process of making a magnetic recording medium, comprising the steps of a) dispersing a magnetic pigment in a binder polymer component and a solvent to form a magnetic dispersion; b) blending a polymeric radiation crosslinking agent into the magnetic dispersion of step a) in an amount effective to induce a crosslinking reaction when the polymeric radiation crosslinking agent is exposed to electron beam radiation, wherein the polymeric radiation crosslinking agent comprises a plurality of (rneth)acrylate-functional chain segments ofthe formula

T B— X— O ?CNH— C_nH₂ -O 1C?C=CH₂

wherein: B is a segment ofthe polymer backbone;

X is a single bond or a divalent organic linking group n has a value in the range from 2 to 6; and R is H or CH₃; c) after blending the polymeric radiation crosslinking agent into the magnetic dispersion, coating the magnetic dispersion onto a substrate; d) optionally passing the coated substrate through a magnetic field to orient or randomize the magnetic orientation ofthe magnetic pigment; e) drying the coated substrate to form a dried magnetic coating; and f) irradiating the coated substrate with an effective amount of electron beam radiation to substantially crosslink the magnetic coating, whereby a magnetic recording medium comprising a magnetic layer provided on the substrate is formed; wherein the polymeric radiation crosslinking agent has a molecular weight of at least 4,000 and has sufficiently high molecular flexibility to effectively crosslink the magnetic coating upon exposure ofthe polymeric radiation crosslinking agent to electron beam radiation.

16. The process of claim 15, wherein the binder polymer component comprises at least 30% by weight ofa radiation crosslinkable binder polymer, the radiation crosslinkable binder polymer comprising a plurality of pendant radiation crosslinkable moieties, wherein substantially no pendant radiation crosslinkable moieties ofthe radiation crosslinkable binder polymer are (meth)acrylate or (meth)acrylamide moieties.

17. The process of claim 16, wherein the binder polymer component further comprises 70% by weight or less of a non-radiation crosslinkable binder polymer component.

18. The process of claim 16, wherein the radiation crosslinkable binder polymer comprises a plurality of chain segments ofthe formula

wherein: R s a segment ofthe radiation crosslinkable binder polymer backbone;

X^ is a single bond or a divalent linking group; and M s a pendant radiation crosslinkable moiety selected from the group consisting of α-methylstyrene functional radiation crosslinkable moiety ofthe formula:

allyloxy radiation crosslinkable moiety ofthe formula:

-O-CH₂-CH=CH₂;

and combinations thereof.

19. The process of claim 15, wherein the polymeric radiation crosslinking agent is used in an amount in the range from 2 to 20 parts by weight based on 100 parts by weight ofthe magnetic layer.

20. The process of claim 15, wherein the polymeric radiation crosslinking agent is formed by reacting a methacrylate-functional isocyanate ofthe formula

with an OH-functional polymer in a manner such that the NCO groups on the isocyanate react with a plurality of OH groups on the OH-functional polymer according to the following reaction scheme:

wherein:

B is a segment ofthe backbone o the OH-functional polymer; and X is a divalent organic linking group or a single bond.

21. The process of claim 20, wherein there is a molar excess of OH groups on the OH-functional polymer relative to NCO groups on the isocyanate.

22. The process of claim 15, wherein the polymeric radiation crosslinking agent is a vinyl copolymer having a glass transition temperature (Tg) in the range from 40 to 80°C and a molecular weight in the range from 10,000 to 50,000 wherein, in addition to the (meth)acrylate-functional moieties, the vinyl copolymer comprises a plurality of pendant OH groups, a plurality of pendant nitrile groups, and a plurality of pendant fluorine-containing groups.

23. The process of claim 22, wherein the vinyl copolymer is formed by reacting a (meth)acrylate-functional isocyanate with a primary vinyl copolymer having pendant OH groups, pendant nitrile groups, and pendant fluorine-containing groups in a manner such that the NCO groups on the isocyanate react with a plurality ofthe OH groups on the primary vinyl copolymer according to the generalized reaction scheme:

wherein: RN is a pendant nitrile group; and

Rp is a pendant fluorine-containing group.

24. The process of claim 23, wherein the primary vinyl copolymer is a copolymer of vinyl monomers, the vinyl monomers comprising: a) 35 to 105 parts by weight of one or more nitrile-functional vinyl monomers; b) 30 to 90 parts by weight of one or more OH-functional vinyl monomers; c) 45 to 130 parts by weight of one or more copolymerizable vinyl monomers; and d) 10 to 30 parts by weight of one or more vinyl monomers bearing a fluorine-containing group.

25. The process of claim 24 wherein the vinyl monomer bearing a fluorine-containing group has the formula

wherein:

Rf is a fluorine-containing moiety;

X^ is a single bond or an organic divalent linking group; and R3 is hydrogen or CH3, or the formula

wherein: n has a value in the range from 6 to 10;

R⁴ is -CH₃ or -C₂H₅;

X^ is an organic divalent linking group; and R5 is hydrogen or CH3.