Acrylic Polymers, their Production and Coating Compositions containing them This invention relates to synthetic polymers containing hydroxyliσ groups, to their manufacture and to solvent-based compositions that contain a resinous binder dissolved in an organic solvent. The invention particularly relates to such compositions that can be formulated for use as solvent-based inks or formulated for use as reprographic coating solutions. Synthetic polymers that contain hydroxylic groups, including copolymers of monomer mixtures that contain hydroxyalkyl- and alkyl- (meth) acrylates, are known but existing polymers have not proved satisfactory for use as an essential binder component in such inks and solutions. In such inks and solutions, the resinous binder has, in recent years, usually included cellulose acetate propionate (CAP) as part or all of the binder. CAP has come to be widely adopted as an essential part of such compositions because it imparts optimum performance characteristics to the compositions, such as low flammability, fast drying, high heat resistance, lack of colour and odour, good permeability to gases (especially important in reprographic coatings) and is reactive (through the hydroxyl groups) to give cross linking or to promote bonding to substrates.

In some compositions CAP is used as the sole binder but frequently it is used in combination with 0.2 to 3 parts (per part CAP) of a relatively hydrophobic acrylic polymer. Often the polymer is formed substantially entirely of alkyl (meth) aerylate monomer or monomers but in some instances inks are made using an acrylic binder that includes a small amount of carboxylic groups in the acrylic polymer. However these carboxylic products are unsatisfactory in reprographic coatings because they would react with ammonia.

It would be desirable to be able to replace some or all of the CAP both because this may permit the attainment

of an improvement in properties that is not available when CAP is uε d and because it broadens the range of materials that can be used for making the compositions. In particular it would reduce the dependence on the availability and the quality of CAP. Prior to the present invention, no synthetic polymer has been proposed that is a satisfactory replacement for part or all of the CAP in such compositions.

In the invention, a copolymer which is, inter alia, suitable for use as a replacement for CAP is a copolymer having Tg at least 50°C and which is formed from 20 to 60% by weight hydroxy alkyl (meth) acrylate and 40 to 80% by weight hydrophobic ethylenically unsaturated monomer.

One aspect of the invention includes novel polymers wherein the polymer is in a solid particulate form (preferably a bead form) and has molecular weight below 150,000.

In another aspect of the invention, a composition suitable for use as a solvent-based ink or as a reprographic coating solution comprises a solution in organic solvent of a resinous binder which comprises hydroxylie polymer, and is characterised in that at least 20% by weight of the hydroxylic polymer is a copolymer having Tg of at least 50°C and which is formed from 20 to 60% by weight hydroxy alkyl (meth) acrylate and 40 to 80% by weight hydrophobic ethylenically unsaturated monomer.

According to another aspect of the invention, dry beads of the copolymer are formed by oil-in-water suspension polymerisation in the presence of a chain terminator to produce beads which are separated from the suspension and dried, and in this process the chain terminator is water soluble and migrates into the beads during polymerisation.

The glass transition temperature (Tg) is measured on the dry polymer using a Setaram DSC92 differential scanning calorimeter at a heating rate of 10 degree C/minute. Tg must be relatively high in order for the polymer to perform

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satisfactorily in the solvent-based compositions and generally it is at least 60°C and frequently at least 70 or 75°C and preferably at least 90 ^β C. However it can be much lower than the Tg for conventional CAP (often 140°C or more) and in the invention Tg is generally below 130°C, usually below 120°C and frequently below 110°C. Tg of from 100 to 110°C is generally preferred. The particular value of Tg in any particular polymer is controlled by the choice of monomers, and the optimum Tg in any particular composition will depend on the other components in that composition.

At least 40% (preferably at least 50% and often at least 60%) by weight of the monomers from which the polymer is made must be hydrophobic ethylenically unsaturated monomers, that is to say the monomer or monomer blend providing the 40 to 80% hydrophobic monomer must be less hydrophilic than the hydroxy alkyl (meth) acrylate monomer. When, as is preferred, the polymer is made by oil-in-water polymerisation, it is necessary that the more hydrophilic monomer (the hydroxy alkyl monomer) is preferentially soluble in the hydrophobic monomer, so as to prevent it dissolving into the aqueous phase.

Generally 40 to 80% (preferably at least 50% and often at least 60%) by weight of the monomers are provided by alkyl (meth) acrylate and generally such monomer constitutes most or all of the hydrophobic monomer. However other hydrophobic monomers can be included, typically in amounts up to 40% by weight, usually not more than 20% by weight, of total monomers, such other hydrophobic monomers generally being selected from, for instance, styrene, alkyl styrenes, vinyl acetate, vinyl halides, vinylidene halides and acrylonitrile.

The alkyl group of the alkyl (meth) acrylate monomer should generally be short chain alkyl, namely C _t . ₆ usually C _t . ₄ alkyl as this promotes the provision of high Tg.

Typically it is methyl or ethyl. Preferably the monomer is a methaerylate rather than acrylate and the hydrophobic

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monomer preferably consists wholly or mainly of methyl methaerylate.

The hydroxy alkyl (meth) acrylate can contain a larger number of carbon atoms in the hydroxy alkyl group, and the hydroxy alkyl group can be interrupted by oxygen atoms (e.g., polyethoxy and/or polypropoxy and/or polybutoxy) provided this does not render the monomer too soluble in water for the synthesis method and/or for the intended use. Generally the hydroxy alkyl group contains less than eight carbon atoms, usually not more than four carbon atoms. The monomer is preferably a methaerylate rather than an acrylate and preferred monomer is hydroxy butyl methaerylate or, more preferably, hydroxy ethyl methaerylate or hydroxy propyl methaerylate (or a mixture) . Preferably the monomers from which the polymer is prepared are free of carboxylic, glycidic or other hydrophilic monomer that might cross link with the hydroxylic monomer although very low amounts may be tolerable. Generally the hydroxyalkyl ester is substantially the only hydrophilic monomer.

Preferred polymers are formed from 20 to 40% by weight hydroxy-ethyl or -propyl methaerylate and 60 to 80% by weight methyl methaerylate. Typically they have Tg 65 to 125°C, preferably 90 to 110°C, especially when being used for solvent inks although lower Tg values are suitable for reprographic coatings.

In the invention, the acrylic polymers generally have a molecular weight above about 8,000 or 10,000 and preferably above about 15,000. It is usually unnecessary for the molecular weight to be above about 100,000 or, at the most, above about 150,000, and is usually below about 70,000.

The molecular weight of the polymer is generally higher than the molecular weight of the CAP that it is suitable for replacing. For instance common CAP products fall into two classes termed CAP -0.2 (typically having a molecular weight of around 15,000) and CAP -0.5 (typically

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having a molecular weight of around 25,000) . Acrylic polymers suitable to replace CAP -0.2 often have molecular weights in the range 12,000 to 30,000, often 15,000 to 20,000. Acrylic polymers suitable to replace CAP -0.5 typically have molecular weights in the range 25,000 to 50,000, Often 30,000 to 40,000.

The molecular weights are number average molecular weights determined by gel permeation chromatography measured on a 200μl sample of 0.30% active concentration in tetrahydrofuran solvent at a flow rate of l.Oml/min through a column of PL gel (10 +10 +500)A+ precolumn using refractive index detection and polymethyl methaerylate standards.

The polymer is preferably made as solid beads by oil- in-water suspension polymerisation. Preferably the molecular weight distribution of the acrylic resins is lower than is achieved by conventional oil-in-water suspension polymerisation. This narrow distribution is best obtained by including in the suspension a chain terminator that is water soluble and that is present in an amount such that the polymerisation goes to the desired average molecular weight. The preferred water soluble chain terminator is a mercaptan, for instance 2-mercapto ethanol or thioglycollic acid. Mercaptans will add across the double bond under alkaline polymerisation conditions and so preferably the polymerisation is conducted at an acidic pH, preferably below pH 5, e.g., pH 2 to 4.

The aqueous phase of the suspension polymerisation system will normally include a dispersion stabiliser such as a water soluble polymer in order to promote stability. Conventional initiators may be used.

After polymerisation is completed, the beads may be recovered and dried. Typically the particle size is >80% by weight between 100 and lOOOμm, and preferably at least 95% by weight are above lOOμm.

Instead of making the desired polymers by oil-in-water suspension (generally bead) polymerisation, it is also

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possible (but less suitable) to make them by other polymerisation techniques that are suitable for making water insoluble polymers. Examples are oil-in-water emulsion polymerisation, precipitation polymerisation or solution polymerisation, provided any solvents present in such processes do not act as chain transfer agents that depress the molecular weight of the polymer unsatisfactorily.

The polymerisation is then preferably conducted so as to give a non-aqueous polymer, either as a solution in a solvent (that can then be evaporated to give a solid product) or a precipitate or spray dried product. For instance it is possible to conduct the oil-in-water polymerisation as an emulsion polymerisation, preferably followed by separation of the emulsion particles from the water in conventional manner such as spray drying.

The novel polymers of the invention can be used as the only hydroxylic resinous binder polymer in the compositions or can be used as part only of the hydroxylic resinous binder. Thus in some instances it is preferred that the defined polymer constitutes, for instance, 25 to 50% by weight of the hydroxylic resinous binder although in other instances larger amounts are present. When the defined hydroxylic acrylic polymer does not constitute the entire hydroxylic resinous binder, any remaining hydroxylic resinous binder is preferably CAP, typically having molecular weight below 30,000, for instance being CAP 0.2 or, preferably, CAP 0.5.

Although the hydroxylic polymer or polymers can provide the entire resinous binder, it is often desirable to include some non-hydroxylic binder, often in amounts of up to 70%, but usually more than 30%, by weight of total binder. These non-hydroxylic binders can be any of the conventional binders that are used with CAP, typically being homopolymers or copolymers of alkyl (meth) acrylates.

Solvent based inks according to the invention are of particular value for printing on films and generally have

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Tg above 65°C but below 120°C, for instance 90 to 110°C. 30 to 100% of the binder is preferably hydroxylic polymer and 0 to 70%, usually 30 to 70%, is non-hydroxylic acrylic polymer, typically a copolymer of ethyl acrylate with ethyl methaerylate. At least 25%, usually at least 50%, of the hydroxylic polymer is the defined hydroxylic acrylic copolymer, and if this does not provide 100% of the hydroxylic polymer the balance is usually CAP.

The inks contain conventional pigments, for instance titanium dioxide and/or other pigments, and may contain additives to promote adhesion to chosen films, for instance polyethylene imine, alkoxy titanates, silanes, zirconium complexes or titanium acetyl acetonate compounds. The compositions may also include plasticisers, for instance dibutyl phthalate or other phthalate plasticisers. The solvent for the composition is often an alkanol, ketone or ester, for instance a mixture of ethanol and ethyl acetate.

Typical amounts are 5 to 15% of the defined hydroxylic polymer, 0 to 10% other hydroxylic polymer such as CAP, 0 to 15% other binder polymer, 10 to 40% pigment, 0 to 5% plasticiser, 0 to 5% bonding additive and 30 to 75% solvent.

In reprographic compositions, 30 to 100%, frequently 50 to 100% and often 100%, of the binder can be hydroxylic polymer, and if other binder is present it is typically an alkyl (meth) acrylate polymer. Although the defined hydroxylic acrylic polymers of the invention can be used as the sole hydroxylic polymer, the hydroxylic polymer is preferably a blend of 20 to 70%, often 25 to 55% of the defined hydroxylic acrylic polymer, with the balance being CAP, generally CAP 0.5.

The composition generally includes diazo compound as an essential part of the reprographic composition and frequently also includes an acidic metal salt such as zinc chloride. Typical solvents are alcohols, ketones and esters. The composition is preferably clear, in order

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that it can be coated on a clear polyester or other sheet, without rendering it opaque.

Suitable proportions are 1.5 to 20% binder of which at least one fifth should be hydroxylic acrylic polymer of the invention, with hydroxylic polymer preferably constituting at least half of the binder, 0.2 to 5% zinc chloride, 0.2 to 5% diazo compound, 0.1 to 2% sulpho salicilic acid, 0.1 to 5% citric acid, 0.1 to 5% thio urea, 0.1 to 5% of coupling agents and 30 to 80% solvent, typically a blend of one or more of toluene, ester (e.g., ethyl acetate), alkanol (e.g., methanol) and ketone (e.g., acetone).

The following are examples of the invention. Example 1

A monomer phase is formed from 60 parts by weight methyl methaerylate, 40 parts by weight hydroxy propyl methaerylate and 1 part by weight AZDN initiator. An aqueous phase is formed from 200 parts by weight water containing 2 parts by weight medium molecular weight polyacrylic acid as dispersion stabiliser and 0.5 parts 2-mercapto ethanol as chain terminator. The aqueous phase is warmed to 60 to 70°C and the monomer phase is dispersed in the aqueous phase with mechanical agitation. Polymerisation is carried out at 60-70°C for 3-4 hours, at which time the beads are observed to be hard. The product (C) is then cooled and the beads are washed and dried at 50°C and recovered as a free flowing colourless bead product. Tg is 103°C. The molecular weight was 35,000, measured as above.

In a similar manner, the following monomer blends were copolymerised to give polymers having substantially the same molecular weight and having Tg values as set out below.

A B C D E F G Methyl Methaerylate 50 55 60 65 70 75 80 Hydroxypropyl Methaerylate 50 45 40 35 30 25 20

Tg (°C) 100102 103 105 107 109 110

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Example 2

A solvent-based ink was formulated from 10 parts hydroxylic polymer binder resin, 10 parts ethyl methacrylate-ethyl acrylate binder, 40 parts methanol, 40 parts ethyl acetate and 15 parts phthalocyanine blue RS, followed by dilution with sufficient of a mixture of equal amounts methanol and ethyl acetate to give the desired coating viscosity.

A comparison composition was formulated using CAP-0.5 as the sole hydroxylic polymer.

Compositions according to the invention were formulated using either 5 parts CAP 0.5 and 5 parts of polymer E, or 10 parts polymers E or F alone. In every instance the compositions of the invention had adhesion, scratch resistance and brittleness properties at least as good as the comparative composition and the adhesion of a composition in which the hyroxylic polymer was 10 parts of polymer E was better than the adhesion of the comparative composition, and was also slightly better than the adhesion of the composition using 5 parts polymer E and 5 parts CAP-0.5.

In a further test, compositions were formed using 1% dibutyl phthalate as a plasticiser. Again the compositions of the invention were satisfactory and, in particular, gave a satisfactory blocking temperature when two layers were pressed together for two seconds.

When the binder includes hydroxylic or other polymers in addition to the polymer of the invention, it is necessary that the blend should be compatible so that the composition is clear. For this purpose, test compositions can be formulated from 20 parts of the proposed binder mix, 40 parts methanol and 40 parts ethyl acetate, and left to stand for a week to see if any partial or complete phase separation occurs. When this was done with blends of polymers A to G with CAP-0.5 and/or commercial copolymers of ethyl methaerylate and ethyl acrylate (sold under the trade names Paraloid B72 or Plexigum N40) it was found that

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some phase separation occurred with compositions A, B, C and D, there was practically no phase separation or loss of clarity with compositions E, F or G, but composition G was rather difficult to dissolve. Compositions E and F are therefore preferred. Example 3

A reprographic coating composition was formed from 16 parts binder resin, 77 parts solvent (methanol acetone mixture), 0.3 parts sulpho salicylic acid, 1.5 parts citric acid, 0.8 parts zinc chloride, 0.2 parts thio urea, 2.4 parts couplers and 1.8 parts diazo compound. Films were coated using a K-Bar number 20 and dried atlOOC in an oven for 2 minutes and then exposed and the quality of exposure was judged for cloudiness (indicating trapping of nitrogen in the film) and other performance properties. It was observed that substantially equivalent performance was achieved when the polymer consisted of CAP-0.5 and when it consisted of mixtures of this with up to 55% of C, but that higher amounts of polymer C gave slightly inferior results.