GB2318581A - Binders for magnetic media - Google Patents

Abstract

A graft copolymer useful as a binder for magnetic recording media comprises a polyurethane backbone having at least one aminomethylphosphonate diester group with the amine nitrogen atom in the polyurethane backbone, and having pendant to said backbone at least one polymeric segment comprising a homopolymer of a vinyl monomer or a copolymer of vinyl monomers.

Description

BINDERS FOR MAGNETIC MEDIA The invention relates to binder materials for magnetic recording media, and in particular to copolymers having a polyurethane backbone with pendant polymeric segments comprising a polymer or copolymer of vinyl monomers.

Magnetic recording media generally include a substrate bearing a binder dispersion layer comprising a pigment dispersed within a binder. Typically, the pigment is a magnetizable pigment comprising small, magnetizable particles. In some instances, the medium may be in the form of a composite having both back-coat and front-coat binder dispersion layers, although the pigment in the backcoat may or may not be a magnetizable pigment.

It has become desirable to have as high a loading of magnetizable pigment in the magnetic recording media as is reasonably possible. It is often preferred to have a binder dispersion comprising from 70% to 85k by weight magnetizable pigment relative to the binder with as many magnetizable particles per unit area or unit volume as possible. It is also preferred to have a binder dispersion in which the magnetizable pigment comprises a plurality of small particles having a relatively high specific surface area. Higher pigment loading has the potential to provide high density magnetic recording capable of storing more information.

However, problems remain in the art concerning magnetic recording media having a relatively high loading of magnetizable pigment. To begin with, magnetizable pigments tend to agglomerate, and they are difficult to properly and fully disperse within the binder. Wetting agents, or dispersants, are often employed to facilitate such dispersion. For higher pigment loading, i.e., the use of greater amounts by weight and number of magnetizable particles, greater amounts of such dispersants are required, which is not always desirable. There are a number of reasons for using as little dispersant as possible. Costs, for example, can be reduced by using less dispersant. Additionally, binder dispersions can be more readily and reproducibly prepared when less dispersant is used. Further, excess dispersant may have a tendency to bloom from a cured binder layer over time, leading to contamination of a recording head or the like, or causing a change in the physical or chemical characteristics of the media.

Another problem in the art is that the viscosity of a binder dispersion generally increases with higher loading of magnetizable pigment. If the dispersion is too viscous, it can be difficult to apply to the substrate, and good magnetic orientation of the pigment, i.e., a squareness ratio of 0.75 or more, can be hard to obtain. The squareness ratio (Br/Bm), is the ratio of the remnant saturation induction, or residual magnetization (Br), to the saturation induction, or saturation magnetization (Bm).

Squareness ratio is a measure of the effectiveness of the orientation of the magnetic particles. For randomlyorientated particles, the squareness ratio is 0.5, and for ideally and perfectly oriented particles, the ratio is equal to 1.0. Values for the squareness ratio of media exhibiting good performance normally fall around 0.75 to 0.85, with higher values being better.

To help alleviate these problems with high pigment loading, polymeric binders having intrinsic dispersing characteristics have been developed. Such polymers have functional moieties pendant from the polymer backbone that help disperse the magnetizable pigment. As a result of using these compositions, less dispersant is needed for dispersion of magnetizable pigment in the binder.

The incorporation in the binder of pendant functional groups having pigment-dispersing properties i.e., wetting groups, is described in numerous patents and publications.

The wetting groups most commonly described are acids (such as carboxylic, sulphonic, phosphoric and phosphonic acids) and the corresponding alkali metal or ammonium salts, phosphate or phosphonate esters, tertiary amines, nitriles, and quaternary ammonium salts. An aminomethylphosphonate diester wetting group may also be used and is described in JP63-0375071, JP02-129217 and UK Patent Appln. No.

9320711.6.

Magnetic recording materials are generally prepared by dispersing the magnetic pigment in a polyurethane resin, coating the dispersion on a nonmagnetic substrate and subjecting it to a curing process to form a crosslinked three-dimensional matrix. However, there are certain types of media constructions where high glass transition temperature (Tg) and abrasion resistance in the uncured state, and high final coating modulus are both desirable and necessary. For example, stiffer tapes may be needed for better handling in the recording and playback machine, or manufacturing methods may necessitate coatings with high initial strength that are resistant to damage during manufacture or processing. To this end, hard resins are frequently blended with the soft polyurethane before or during the pigment dispersion process. Suitable hard resins are high Tg materials which are miscible with the polyurethane component. The hard resin may comprise pigment wetting groups of the type described above. The most commonly used hard resins are polymers and copolymers of vinyl chloride, because these materials show good compatibility with the polyurethane component. However, alternatives are required due to long-term stability problems and environmental considerations. Hard resins derived from monomers such as styrene, acrylonitrile, acrylate and methacrylate esters are possible alternatives but generally show very poor compatibility with polyurethanes, which leads to phase separation and poor physical properties in the final coating.

In recent years, attempts have been made to solve the compatibility problem by chemically linking the hard resin to the polyurethane, e.g., by forming a graft copolymer in which hard resin chains are pendant to a polyurethane backbone. This may be achieved by carrying out vinyl polymerisations in the presence of polyurethanes having initiating or chain-transfer sites on the backbone, as described in EP-A-0465070. However, a more controlled method involves the use of macromonomers.

Macromonomers are linear polymers, usually of moderate molecular weight (e.g., 1000 - 10000), having a functional group at one end which enables them to participate in chain-forming reactions. An example of these macromonomers are the macromonomer diols, which have a diol functionality at one end of the polymer chain which enables the macromonomer to participate in reactions with diisocyanates to produce a polyurethane backbone.

EP-A-0483406 discloses the use of acrylic macromonomer diols in the synthesis of polyurethane for use as binders in magnetic recording media. In addition to the pendant acrylic segments, preferred embodiments have one or more wetting groups, selected from -COOM, -SO3M, -OSO3M, -PO3M, -OPO3M and tertiary amine (where M = H or a cation), attached to the polyurethane backbone. In the formulations described, the graft copolymer is blended with a vinyl chloride resin namely VAGH, and therefore the graft copolymer is not used as a one-component binder.

EP-A-0463806 also discloses the use of acrylic macromonomer diols in the synthesis of polyurethanes for use as binders in magnetic recording media. The resulting graft copolymers either have no wetting groups or have at least one -SO3M group pendant from the polyurethane backbone, where M is selected from H, NR4 (in which R is H or alkyl), Li, Na, or K. Although the embodiments having SO3M groups function well as one-component binders, the incorporation of these wetting groups adds at least one extra step to the synthesis, and restricts the range of materials that can be produced.

The wetting groups are incorporated by means of a sulphonated diol, which is prepared by transesterification of dimethyl sulphoisophthalate with a diol such as a polycaprolactone diol having a hydroxy equivalent weight in the range 200 - 2000. Not only does this add an extra step to the synthesis, but the result of the transesterification is generally a mixture of sulphonated and non-sulphonated diols, and the chain length of the sulphonated diol is relatively long, so that any variation in the wetting group content produces a significant change in the chain length of the polyurethane product. Furthermore, the wetting groups themselves are highly polar salts of sulphonic acids, and hence the solubility and viscosity properties of the polymer product are highly dependent on the wetting group content.

There is therefore a need for alternative materials having effective pigment-dispersing properties and capable of functioning as a one-component binder for magnetic pigments. In particular, there is a need for materials that can be synthesised easily from commercially-available materials, with physical properties that may be tailored to particular applications.

In one aspect, the invention provides a graft copolymer comprising a polyurethane backbone having at least one aminomethylphosphonate diester group, and having pendant to said backbone at least one polymeric segment comprising a homopolymer of a vinyl monomer or copolymer of vinyl monomers.

In a second aspect, the invention provides a recording element comprising a non-magnetic substrate coated on one or both sides with a dispersion of magnetic particles in a binder comprising a graft copolymer of the type defined above.

The graft copolymers of the invention may be used as binders for coated magnetic recording media, forming good dispersions of magnetic particles without the need for additional low molecular weight dispersing agents.

Furthermore, the coated recording media display good physical properties without the need for blending with additional hard resins, and so the copolymers of the invention function as one-component binders for magnetic media.

Unlike the one-component binders of the prior art, the copolymers of the invention are devoid of the ionic wetting groups, the aminomethylphosphonate diester groups being neutral covalent species. This gives compatibility with a wider range of organic solvents, and enables the use of more concentrated solutions and/or higher levels of incorporation of wetting groups (which in turn enables higher pigment loadings) without encountering the problems of poor solubility and/or high viscosity which may be found with the prior art binders. Another advantage of the copolymers of the invention over the prior art materials is the fact that they are available by simple, one-pot procedures from readily-available starting materials. The relative proportions of the different components may be varied over a wide range without the need to alter the reaction conditions significantly.

The polyurethane backbone of the graft copolymers of the invention is produced by the reaction of one or more polyisocyanates with one or more polyhydroxy compounds.

Polyisocyanates are compounds equipped with at least two isocyanate groups capable of reaction with hydroxy groups to form urethane linkages. Typical examples are diisocyanates such as diphenylmethane diisocyanate, isophorone diisocynate, hexamethylene diisocyanate, toluene diisocyanate and p-phenylene diisocyanate.

Similarly, polyhydroxy compounds (or polyols) are compounds possessing two or more hydroxy groups (preferably alcohol groups, especially primary alcohol groups, rather than phenols) capable of reacting with isocyanate groups to form urethane linkages. Typical examples include diols and triols such as polycaprolactone diols and triols, 1,6hexanediol, 1,4-butanediol, ethylene glycol, cyclohexanedimethanol and neopentyl glycol.

The pendant polymerised vinyl segments and the aminomethylphosphonate diester groups, which are essential features of the polymers of the invention, may be incorporated therein by means of suitably functionalised polyisocyanates and/or polyols. However, in practice it is easier to synthesise the relevant functionalised polyols than the corresponding functionalised polyisocyanates, and so the invention will be explained with reference to this mode of incorporation, but is not intended to be restricted thereto.

The graft copolymers of the invention possess one or more pendant vinyl polymeric segments. The polyurethane backbone typically possesses on average about 0.5 to about 2 pendant polymerised vinyl segments. The weight ratio of the polyurethane backbone to the pendant polymerised vinyl segment(s) ranges from about 99.5 : 0.5 to about 10 : 90, preferably from about 95 : 5 to about 20 : 80, most preferably from about 95 : 5 to about 30 : 70. The average molecular weight of each pendant polymerised vinyl segment typically ranges from about 1000 to 20,000, preferably about 5000 to about 15,000. The glass transition temperature (Tg) of the pendant polymerised vinyl segment(s) is typically in the range from about 200C to about 1200C, but is preferably at least 500C, e.g., in the range 50 - 1000C, in order to provide binder media with the appropriate level of stiffness.

The pendant polymerised vinyl segment(s) comprise polymerised vinyl monomers. Useful monomers include (but are not restricted to) styrene, halogenated sytrenes, alkylated styrenes, alkoxystyrenes, acrylonitrile, methacrylonitrile, acrylic acid, methacrylic acid, esters or amides of acrylic acid or methacrylic acid (including fluorinated derivatives thereof), vinyl chloride, vinylidene chloride, vinylidene fluoride, vinyl acetate etc. Mixtures of two or more monomers may be used.

Preferably, the monomers from which the pendant polymerised vinyl segment(s) are formed are selected from the group consisting of styrene, methyl methacrylate, and a mixture of styrene and acrylonitrile.

The pendant polymerised vinyl segment(s) are readily incorporated in the polymers of the invention through the use of macromonomer diols, comprising polymerised vinyl monomers and a diol functionality at one end of the chain.

The diol functionality is capable of reacting with diisocyanates (along with other diols) to form the polyurethane backbone with pendant polymerised vinyl segment(s) Macromonomer diols are a well known class of materials, and may be prepared, for example, by subjecting the appropriate vinyl monomer(s) to free-radical polymerisation in the presence of a chain transfer agent which possesses a diol functionality, such as 3-mercaptopropane-1,2-diol. The preparation of macromonomer diols is described in the literature, e.g., Chuyo et al Polvmer Bulletin, 8, 239 (1982). Several examples are commercially available, notably the materials sold under the trade names HA-6, HS-6 and HN-6 by Toagosei Chemical Industry Co. Ltd. These are diol-terminated polymers of molecular weight of approximately 6000 derived, respectively, from methyl methacrylate, styrene and a mixture of styrene and acrylonitrile, and are the preferred materials for use in the synthesis of the polymers of the present invention.

An alternative method of incorporating the pendant polymerised vinyl segment(s) in the polymers of the invention is to provide a polyurethane backbone with freeradical initiation or chain transfer sites, and to carry out free-radical polymerisation of the vinyl monomers in the presence of the polyurethane backbone, as described in EP-A-0465070. For example, 3-mercaptopropane-l,2-diol may be copolymerised with one or more diisocyanates to provide a polyurethane with pendant thiol groups. The thiol groups can act as chain transfer agents in the free-radical polymerisation of vinyl monomers, thus enabling the growth of pendant polymerised vinyl segment(s). However, this method is less convenient than the macromonomer method, and generally gives a mixture of the desired graft copolymer and vinyl homopolymer, and so is not preferred.

The graft copolymers of the invention possess at least one aminomethylphosphonate diester group, and generally have a phosphorus equivalent weight in the range 1000 20000, preferably in the range 2000 - 10000. The aminomethylphosphonate diester groups may be represented by the following formula:

in which R1 and R2 independently represent alkyl groups of up to 10, preferably up to 5, carbon atoms, cycloalkyl groups, aryl groups, or together comprise the carbon atoms necessary to complete a ring; R3 and R4 independently represent H or alkyl groups of up to 5 carbon atoms; and the nitrogen atom forms part of the polyurethane backbone.

Preferably, R3 = R4 = H, and R1 and R2 represent lower alkyl groups such as methyl, ethyl, propyl etc.

The aminomethylphosphonate diester groups of formula I may be readily incorporated in the graft copolymers of the invention by means of phosphonated diols of formula II:

in which Rl to R4 have the same meaning as before, and R5 and R6 independently represent divalent linking groups.

There is no particular restriction on the identity of Rs and R6, except that they should not comprise functionalities capable of reaction with isocyanates other than the terminal hydroxy groups shown. Suitable divalent linking groups include alkylene groups, arylene groups and poly(alkyleneoxy) groups, but alkylene groups of up to 6 carbon atoms such as C2H4, C3H6 etc., are preferred. In a preferred phosphonated diol of formula II, Rl = R2 = C2Hs, R3 = R4 = H, and Rs = R6 = C2H4. This compound is commercially available from Akzo under the tradename Fyrol-6.

In addition to units derived from macromonomer diols and phosphonated diols, the polyurethane backbone of the graft copolymers of the invention generally comprises units derived from one or more polyisocyanates such as diphenylmethane diisocyanate, isophorone diisocyanate, hexamethylene diisocyanate, toluene diisocyanate or pphenylene diisocyanate, and units derived from one or more additional polyols.

A variety of additional polyols may be employed in the synthesis of the polymers of the invention for the purposes of chain extension and tailoring of the physical properties of the final product. Typically, at least one hard segment diol, at least one soft segment diol and at least one triol will be employed.

Hard segment diols are relatively low molecular weight compounds (e.g.,less than about 120) in which the hydroxy groups are separated by a relatively inflexible spacer group. Examples include neopentyl glycol (2,2dimethylpropane-1,3-diol), ethylene glycol and cyclohexanedimethanol. Soft segment diols are relatively high molecular weight compounds (e.g., greater than about 180) in which the hydroxy groups are separated by a relatively flexible spacer group. Examples include polyether diols, polyester diols and polycarbonate diols, such as Tone rM 0210 and 0230, (polycaprolactone diols available from Union Carbide), Desmophenw 2020E (a polycarbonate diol available from Bayer), and RavecarbM 106, (a polycarbonate diol available from Enichem).

At least one polyol having more than two hydroxy groups (e.g., a triol) is generally included in the synthesis in order to ensure that the final product possesses free hydroxyl groups. These hydroxy groups enable crosslinking of the product when it is incorporated in coatings of magnetic recording media. Preferred triols have a molecular weight greater than 180, typically in the range 200 - 1000, and are exemplified by the polycaprolactone triols such as Tones 0305, which is available from Union Carbide and has a molecular weight of about 540 and hydroxy equivalent weight of about 180.

Preparation The copolymers of the invention may be prepared by normal methods for the synthesis of polyurethane resins.

In a typical procedure, the desired mixture of macromonomer diol(s), phosphonated diol(s), hard segment diol(s) and soft segment diol(s) is charged to a reaction vessel together with an aprotic organic solvent such as toluene or methyl ethyl ketone (MEK). The reagents should be free from moisture, and this is most readily achieved by azeotropic distillation of a portion of the solvent. A catalyst such as dibutyltin dilaurate is then added to the cooled mixture, followed by the desired polyisocyanate(s).

The polyisocyanate(s) are added in a quantity sufficient to consume all of the hydroxy groups present and hence provide a prepolymer with terminal isocyanate groups. After the completion of this reaction, a polyol is added in sufficient quantity to react with all the isocyanate groups present, and hence form the final product. Thereafter, the solution containing the product is diluted or concentrated to the desired W solids, or the product is isolated by precipitation in non-solvent, as required.

Thus, the copolymers of the invention are available by a straightforward, one-pot procedure. Furthermore, the level of incorporation of the phosphonate wetting group can be varied over a wide range with minimal effect on the other parameters. Because the chain length of the phosphonated diol is typically short (e.g., less than 10 atoms), variations in the quantity present in the monomer mix has relatively little effect on the chain length of the product polyurethane. Also, because the aminomethylphosphonate diester groups are neutral species, the solubility and viscosity properties of the product polymer do not vary markedly with wetting group content.

This is in contrast to comparable polymers of the prior art, such as those disclosed in EP-A-0463806 which cannot generally be produced by a one step process and have variable properties depending on the wetting group content as discussed above.

The graft copolymers of the invention find use as onecomponent binders for particulate magnetic recording media.

If desired, the copolymers may be used in conjunction with additional dispersing agents and/or hard resins, but it is a particular advantage of the present invention that such ingredients are not required, and their use is not preferred.

Binder compositions of the invention find use in magnetic recording media comprising a substrate and one or more coated layers of dispersed magnetic particles. The binder compositions may form part of the layer(s) of dispersed magnetic particles, or may form part of a nonmagnetic coated layer, such as a backcoat providing antistatic properties, or both. However, the presence of the pigment-wetting groups in the binder compositions makes them particularly suitable for use in magnetic particle dispersions.

Magnetic recording media of the present invention comprise a magnetic layer provided on a non-magnetisable substrate. When the media is in the form of a tape, a backside coating is optionally provided on the opposite side of the substrate. The particular non-magnetizable substrate of the present invention can be formed from any suitable substrate material known in the art. Examples of suitable substrate materials include, for example, polymers such as polyethylene terephthalate ("PET"), polymide, and polyethylene naphthenate ("PEN"), metals such as aluminium, or copper, paper, or any other suitable material.

The components of a magnetic layer comprise a magnetic pigment dispersed in a polymeric binder. Typically, the magnetic layer can contain 100 parts by weight of the magnetic pigment and 5 to 40 parts by weight of the polymeric binder. The type of magnetic pigment used in the present invention can include any suitable magnetic pigment known in the art including -Fe203, cobalt-doped -Fe203, Fe304, CrO2, barium ferrite, barium ferrite derivatives, metal particles, and the like.

In addition to the binder composition of the invention and the magnetic pigment, the magnetic layer of the present invention can also comprise one or more conventional additives such as lubricants; abrasives; crosslinking agents; head cleaning agents; thermal stabilisers; antioxidants; dispersants; wetting agents; antistatic agents; fungicides; bactericides; surfactants; coating aids; non-magnetic pigments; and the like in accordance with practices known in the art.

Backside coatings for magnetic recording media can be prepared by the incorporation of non-magnetic pigment dispersed in the polymeric binder system of the present invention. The backside coating can be prepared by coating an uncured polymeric binder system as described herein, onto a non-magnetic substrate. The system is then dried and cured using techniques within the skill of those in the art, to provide a tough, durable backside coating.

Non-magnetic pigments useful for the preparation of backside coatings include, for example, carbon black, Al203, TiO2 and the like. The amount of pigment can vary, but is preferably within the range from about 30 to 55 parts by weight and most preferably within the range from about 40 to 50 parts by weight, based on 100 parts (dried weight) backside coating.

In addition to the binder system of the invention and the non-magnetic pigment, the backside coating of the present invention may also comprise one of more conventional additives such as lubricants; abrasives; crosslinking agents; head cleaning agents; thermal stabilisers; antioxidants; dispersants; wetting agents; antistatic agents; fungicides; bactericides; surfactants; coating aids; and the like in accordance with practices known in the art.

As one example of a process for preparing a magnetic recording medium, the components of the magnetic layer or the backside coating are combined and mixed with a suitable solvent to form a substantially homogeneous dispersion.

The dispersion is then coated onto a non-magnetisable substrate, which can be primed or unprimed. The dispersion can be applied to the substrate using any conventional coating technique, such as gravure or knife coating techniques. The coated substrate can then be passed through a magnetic field to orient the magnetic pigment after which the coating is dried, calendered if desired, and then allowed to cure.

Curing can be accomplished in a variety of ways. As one approach, an isocyanate crosslinking agent can be added to the dispersion just before the dispersion is coated onto the substrate. As soon as the isocyanate crosslinking agent is added to the dispersion, the NCO groups of the isocyanate crosslinking agent will begin to react with the hydroxyl groups of the polymeric binder. Preferably, a catalyst, e.g. dibutyltin dilaurate, can also be added in suitable catalytic amounts in order to facilitate this crosslinking reaction. Generally, using from 0.02 to 0.2 parts by weight of catalyst per 100 parts by weight of magnetic pigment is preferred.

The isocyanate crosslinking agent, if any, is a polyfunctional isocyanate having an average functionality of at least 2 isocyanate groups per molecule. Examples of specific polyfunctional isocyanate useful as the isocyanate crosslinking agent in the practice of the present invention include materials commercially available as MONDUR CB-601, CB-75, CB-701, MONDUR-MRS from Miles, Inc.; DESMODUR L available from Bayer A.G.; CORONATE L available from Nippon Polyurethane Ind., Ltd.; and PAPI available from Union Carbide Corp.

The isocyanate crosslinking agent is preferably used in an amount that the molar ratio of NCO groups from the isocyanate crosslinking agent to the total number of hydroxy groups from the hydroxy functional polymer is greater than 0. Preferably, the molar ratio of the NCO groups from the isocyanate crosslinking agent to the total number of hydroxy groups from the hydroxy functional polymer is in the range from about 1 to 1, to about 2 to 1, or more preferably from about 1.2 to about 1.5 to 1.

As another approach, when one or more components of the polymeric binder contain radiation curable moieties, the dried coating can be irradiated to achieve curing of the radiation curable materials. Those skilled in the art, given the present teaching, will appreciate the manner in which irradiation can be achieved using any type of ionising radiation that is capable of penetrating the pigment, e.g., electron beam radiation. Preferably, radiation curing is achieved with an amount of electron beam radiation in the range from 1 to 20 Mrads, preferably 4 to 12 Brads, and more preferably 5 to 9 Mrads of electron beam radiation having an energy in the range from 100 to 400 keV, preferably 200 to 250 keV.

Although electron beam irradiation can occur under ambient conditions or in an inert atmosphere, it is preferred to use an inert atmosphere as a safety measure in order to keep ozone levels at a minimum and to increase the efficiency of curing. "Inert atmosphere" means an atmosphere comprising nitrogen or a noble gas and having an oxygen content of less than 500 parts per million ("ppm").

A preferred inert atmosphere is a nitrogen atmosphere having an oxygen content of less than 75 parts per million.

The invention is hereinafter described in more detail by way of example only.

Example 1 Synthesis of a Phosphonated hydroxy-functional Polourethane P-HPU comprising 25 wt% vinyl macromonomer 76.92 g of 40wt% solu (Toagosei, diol terminated styrene-acrylonitrile copolymer, Mn approx. 6,000) were placed in a 500 ml reactor flask with 100 ml of additional MEK. The solution was heated to reflux and 40 ml of MEK azeotrope distillate was collected.

The solution was allowed to cool to 400C and 0.1 ml of dibutyltin dilaurate (Aldrich) was added.

A solution of 26.09 g of MDI in 40 ml of MEK was added in a steady stream with vigorous stirring,, and a further 40 ml of MEK was added via the addition funnel to wash in any remaining traces of MDI. The solution was heated to reflux for 2 hours.

At reflux, a solution of 10 g of Tone 0305 (Union Carbide) polycaprolactone triol in 20 ml of MEK was added as rapidly as possible and without interruption. Reflux was maintained until i.r. analysis showed that no isocyanate remained unreacted (about 30 minutes).

The polymer had the following calculated properties: 25 wtW macromonomer, phosphorus equivalent weight 7005, and hydroxy equivalent weight 2260. After transfer to a storage jar, and cooling to room temperature, the polymer solution contained 36% solids (determined by sample evaporation).

Example 2 Synthesis of a P-HPU comprising 40 wt% vinyl macromonomer 45.89 g of 40 wt solution of Tone 0210 (Union Carbide) polycaprolactone diol in MEK (net 18.36 g of diol), 3.64 g of Fyrol-6 [Akzo, Diethyl N,N-bis(2hydroxyethyl)aminomethylphosphonate], 4.50 g of neopentyl glycol (2,2-dimethylpropane-1,3-diol), and 40.00 g of HN-6 (Toagosei, diol terminated styrene-acrylonitrile copolymer, Mn approx. 6,000) were placed in a 500 ml reactor flask with 80 ml of additional MEK. The solution was heated to reflux and 40 ml of MEK azeotrope distillate was collected.

The solution was allowed to cool at 400C and 0.1 ml of dibutyltin dilaurate (Aldrich) was added.

With vigorous stirring, a solution of 23.50 g of MDI in 35 ml of MEK was added in a steady stream, and a further 40 ml of MEK was added via the addition funnel to wash in any remaining traces of MDI. The solution was heated to reflux for 2 hours 45 minutes.

At reflux, a solution of 10 g of Tone 0305 (Union Carbide) polycaprolactone triol in 20 ml of MEK was added as rapidly as possible and without interruption. Reflux was maintained until i.r. analysis showed that no isocyanate remained unreacted (about 20 minutes).

The polymer had the following calculated properties: 40 wtt macromonomer, phosphorus equivalent weight 7005, and hydroxy equivalent weight 2506. After transfer to a storage jar, and cooling to room temperature, the polymer solution contained 38% solids (determined by sample evaporation).

Example 3 Synthesis of a P-HPU comprising 60 wtt vinyl macromonomer 4.52 g of 40 wtt solution of Tone 0210 (Union Carbide) polycaprolactone diol in MEK (net 1.81 g of diol), 3.64 g of Fyrol-6 [Akzo, Diethyl N,N-bis(2hydroxyethyl)aminomethylphosphonate], 4.50 g of neopentyl glycol (2,2-dimethylpropane-1,3-diol), and 60.00 g of HN-6 (Toagosei, diol terminated styrene-acrylonitrile copolymer, Mn approx. 6,000) were placed in a 500 ml reactor flask with 80 ml of additional MEK. The solution was heated to reflux and 40 ml of MEK azeotrope distillate was collected.

An additional 40 ml of fresh MEK was added to dilute the reaction mixture, which had become very viscous. The solution was allowed to cool at 400C and 0.1 ml of dibutyltin dilaurate (Aldrich) was added.

With vigorous stirring, a solution of 20.05 g of MDI in 30 ml of MEK was added in a steady stream, and a further 40 ml of MEK was added via the addition funnel to wash in any remaining traces of MDI. The solution was heated to reflux for 2 hours.

At reflux, a solution of 10 g of Tone 0305 (Union Carbide) polycaprolactone triol in 20 ml of MEK was added as rapidly as possible and without interruption. Reflux was maintained until i.r. analysis showed that no isocyanate remained unreacted (about 20 minutes).

The polymer had the following calculated properties: 60 wt% macromonomer, phosphorus equivalent weight 7005, and hydroxy equivalent weight 2939. After transfer to a storage jar, and cooling to room temperature, the polymer solution contained 43% solids (determined by sample evaporation).

Evaluation of P-HPU draft copolymers Dispersion Preparation Evaluation of the binders was by dispersion and milling of a standard formulation using ISK9966 iron oxide pigment (75 wt%) and the binder (25 wt%). The formulation comprised 35 wt% total solids in 60 wt butanone: 20 wtW cyclohexanone: 20 wt% toluene solvent mixture.

The binder solution was added gradually to the required balance of. solvents in the blender until the solution became viscous. A small amount of the pigment was then added, and slow addition of both binder and pigment was continued until the formulation was complete.

Dispersion was continued for about 3 hours before transferring to the sandmill with 0.8-1.0 mm spherical zirconium dioxide media.

Tables 1 and 2 contain viscosity, gloss (450) and bulk magnetic data from two separate dispersions, each milled for 9 hours. High shear viscosity was measured using an ICI cone and plate viscometer at a shear rate of 10,000 s-l.

In addition, dispersions were examined by optical microscopy.

Table 1 25% macromonomer P-HPU

Milling Dispersion Gloss Coating Squareness Time/h Viscosity/cps (450) Coercivity/Oe 0 20 0.5 908.6 0.612 1 20 28.5 973.2 0.759 2 18 34.7 963.8 0.758 3 18 38.7 956.4 0.760 4 20 39.5 968.3 0.775 5 20 39.3 969.0 0.772 6 20 42.7 968.2 0.769 7 20 43.3 966.8 0.790 8 20 46.0 966.7 0.771 9 18 46.5 968.7 0.782 Table 2 40% macromonomer P-HPU

Milling Dispersion Gloss Coating Squareness Time/h Viscosity/cps (450) Coercivity/Oe 0 30 1.4 909.2 0.679 1 30 45.3 942.1 0.768 2 30 55.5 948.9 0.769 3 29 59.9 950.2 0.780 4 28 60.1 950.5 0.760 5 28 61.0 950.3 0.765 6 27 62.7 950.3 0.773 7 27 64.8 951.1 0.769 8 26 65 3 --- --- 9 26 66.2 942.1 0.772 For each milled dispersion, good bulk magnetics (Coercivity > 950 and squareness between 0.77 and 0.79), and low ICI viscosities were observed, indicating that good quality dispersions can be formed using the graft copolymers of the invention as one-component binders.

Mechanical Properties Mechanical and thermal properties of each of the three experimental binders were evaluated by DMA (Dynamic Mechanical Analysis-Perkin Elmer DMA7). The P-HPU containing 60% SAN macromonomer was too brittle to form a self-supporting film, essential for this form of evaluation. Thus, 60% SAN loading appears to offer an upper limit of added macromonomer component for this particular polyurethane backbone as poor film forming properties result.

Knife-coatings on PET base made from both 25% and 40% SAN-containing polymers and crosslinked using Desmodur L75 at an activation index of approximately 0.75, were of good quality and could be easily removed from the PET base. Each film was evaluated over the temperature range -25 to 800C.

The 25% SAN-containing P-HPU exhibited a modulus of > 1.0 x 109 Pa at 250C, although the mechanical properties dropped off dramatically at 400C, presumably due to softening of the urethane component of the polymer when the temperature exceeded the urethane Tg. With only 25% SAN-macromonomer present, the polymer appears to behave like a semi-crosslinked urethane with a slightly higher modulus.

The 40k SAN-containing P-HPU possessed a modulus of approximately 9.0 x 108 Pa at temperatures as high as 700C, the polymer maintaining good mechanical properties until the SAN component Tg is reached at 75-800C. It is clear that that 40% loading of SAN-macromonomer into P-HPU dramatically changed its physical properties. The ability to produce a range of polymers with different physical and thermal properties by varying the macromonomer content provides a versatile route to novel binder systems. The above results only reflect the potential of such binder systems. As both coatings were cured using a low activation index ( < 0.75), improved moduli could be expected if higher activation indices were to be employed.

Claims (25)

1 A graft copolymer comprising a polyurethane backbone having at least one aminomethylphosphonate diester group, and having pendant to said backbone at least one polymeric segment comprising a homopolymer of a vinyl monomer or copolymer of vinyl monomers.

2 A graft copolymer according to Claim 1 wherein said polyurethane backbone comprises a copolymer of one or more polyisocyanate monomers and one or more polyhydroxy monomers.

3 A graft copolymer according to Claim 2 wherein said polyisocyanate is selected from diphenylmethane diisocyanate, isophorone diisocyanate, hexamethylene diisocyanate, toluene diisocyanate and p-phenylene diisocyanate.

4 A graft copolymer according to Claim 2 or Claim 3 wherein said polyhydroxy monomer is selected from polycaprolactone diols and triols, 1,6-hexanediol, 1,4-butanediol, ethylene glycol, cyclohexanedimethanol and neopentyl glycol.

5 A graft copolymer according to any preceding claim wherein the weight ratio of polyurethane backbone to the pendant polymerised vinyl segment ranges from 99.5 : 0.5 to 10 = 90

6 A graft copolymer according to Claim 5 wherein the weight ratio of the polyurethane backbone to the pendant polymerised vinyl segment ranges from 95 : 5 to 20 : 80.

7 A graft copolymer according to Claim 6 wherein the weight ratio of the polyurethane backbone to the pendant polymerised vinyl segment ranges from 95 : 5 to 30 : 70.

8 A graft copolymer according to any preceding claim wherein the weight average molecular weight of each pendant polymerised vinyl segment ranges from 1000 to 20,000.

9 A graft copolymer according to Claim 8 wherein the average molecular weight of each pendant polymerised vinyl segment ranges from 5000 to 15,000.

10 A graft copolymer according to any preceding claim wherein the glass transition temperature of the pendant polymerised vinyl segment is in the range from 50 to 1000C.

11 A graft copolymer according to any preceding claim wherein the pendant polymerised vinyl segment comprises vinyl monomer subunits selected from styrene, halogenated styrenes, alkyated styrenes, alkoxystyrenes, acrylonitrile, methacrylonitrile, acrylic acid, methacrylic acid, esters or amides of acrylic acid or methacrylic acid, vinyl chloride, vinylidene chloride, vinyl fluoride and vinyl acetate.

12 A graft copolymer according to Claim 11 wherein said vinyl monomers are selected from styrene, methyl methacrylate, and a mixture of styrene and acrylonitrile.

13 A graft copolymer according to any preceding claim wherein said aminomethylphosphonate diester group has the general formula

in which R1 and R2 independently represent alkyl groups of up to 10, preferably up to 5, carbon atoms, cycloalkyl groups, aryl groups, or together comprise the carbon atoms necessary to complete a ring; R3 and R4 independently represent H or alkyl groups of up to 5 carbon atoms; and the nitrogen atom forms part of the polyurethane backbone.

14 A graft copolymer according to Claim 13 wherein R3 = R4 = H, and R1 and R2 represent lower alkyl groups of up to 5 carbon atoms.

15 A graft copolymer according to Claim 2 in which said polyhydroxy monomers comprise at least one hard segment diol, at least one soft segment diol and at least one triol.

16 A graft copolymer according to Claim 15 wherein said hard segment diol is selected from neopentyl glycol, ethylene glycol and cyclohexanedimethanol and said soft segment diol is selected from polyether diols, polyester diols and polycarbonate diols and said triol is a polycaprolactone triol.

17 A magnetic recording element comprising a substrate and one or more coated layers of dispersed magnetic particles, and a binder composition comprising a graft copolymer according to any preceding claim.

18 A magnetic recording element according to Claim 17 wherein said magnetic particles are dispersed within a binder composition comprising the graft copolymer of Claims 1 to 16.

19 A magnetic recording element according to Claim 18 wherein the magnetic layer comprises a 100 parts by weight of magnetic pigment and 5 to 40 parts by weight of the graft copolymer.

20 A method of producing a graft copolymer according to any of Claims 1 to 16 which comprises mixing a macromonomer diol, comprising polymerised vinyl monomers having a diol functionality at one end of the chain, together with a phosphonated diol of formula II:

where R1-R4 are as defined. in Claim 13 and R5 and R6 independently represent divalent linking groups, and one or more additional polyols in an aprotic organic solvent and subsequently adding at least one polyisocyanate to produce the graft copolymer.

21 A method according to Claim 20 wherein a catalyst is added prior to addition of the polyisocyanate.

22 A method of producing a magnetic recording element comprising mixing a solution of the graft copolymer of any of Claims 1 to 16 with a magnetic pigment, coating said mixture onto a non-magnetisable substrate, passing said coating through a magnetic field to orientate the magnetic pigment and subsequently drying and curing the coating.

23 A method according to Claim 22 wherein said curing is carried out by addition of an isocyanate cross linking agent prior to coating of the substrate.

24 A method according to Claim 23 wherein a catalyst is added to facilitate the cross linking reaction.

25 A method according to Claim 24 wherein said catalyst comprises dibutyltin dilaurate.