Low molecular weight (meth) acrylate copolymer emulsions

Graft and block copolymers with a weight average molecular weight below 20,000, a hydroxy value more than 200 and acid value below 50 which copolymers comprise (a) 20-98% by weight of a water soluble macromonomer having a hydroxy value of at least 300, a weight average molecular weight below 6000, containing less than 10% by weight of an acid functional monomer and having a terminal unsaturated double bond; and (b) 2 to 80% of polymerized monomers selected from vinyl monomers, acrylate monomers, methacrylate monomers and mixtures thereof, form stable coating compositions that are compatible with polyisocyanate crosslinking agents and provide improved properties and appearance in automotive finishes.

BACKGROUND OF THE INVENTION
 This invention relates to an aqueous (meth)acrylate copolymer composition
 and process for preparation of such composition and the use of such
 composition in water-borne coatings with improved properties. In
 particular, this invention is directed to a (meth)acrylate copolymer
 composition with a hydroxy value above 200, acid value below 50 and weight
 average molecular weight less than 20,000 comprising a block or graft
 copolymer prepared directly in water, solvent or water/solvent blend in
 the presence of a water-soluble macromonomer substantially free of any
 acid groups so that the final emulsion has a high solids content at low
 viscosity. This invention also involves coating compositions based on such
 aqueous copolymer compositions.
 Automobiles and trucks receive exterior finishes for several reasons.
 First, such finishes provide barrier protection against corrosion. Second,
 consumers prefer an exterior having an attractive aesthetic finish,
 including high gloss and excellent DOI (distinctness of image). A typical
 automobile steel panel or substrate has several layers of finishes or
 coatings. The substrate is typically first coated with an inorganic
 rust-proofing zinc or iron phosphate layer over which is provided a primer
 which can be an electrocoated primer or a repair primer. Optionally, a
 primer surfacer can be applied to provide for better appearance and/or
 improved adhesion. A pigmented basecoat or colorcoat is next applied over
 the primer. A typical basecoat or colorcoat comprises a pigment, which may
 include metallic flakes in the case of a metallic finish. In order to
 protect and preserve the aesthetic qualities of the finish on the vehicle,
 it is well known to provide a clear (unpigmented) topcoat over the colored
 (pigmented) basecoat, so that the basecoat remains unaffected even on
 prolonged exposure to the environment or weathering. Coating compositions
 comprise one or more film-forming copolymers and for topcoats acrylic
 copolymer are preferred. Most commonly, acrylic polymers are linear in
 structure and cure upon application by reacting with crosslinking agents.
 The use of non-linear copolymers for coating compositions has also been
 disclosed, but the use of such polymers, however, have so far found only
 limited use in the automotive finishes area. See U.S. Pat. No. 5,010,140.
 The evolution of environmental regulations has led to the need for products
 with lower volatile organic content (VOC). However, it is far from trivial
 to develop aqueous products with desirable properties for automotive
 finishes. As mentioned above, such finishes must be high performance in
 terms of aesthetic qualities and durability. Water dispersible polymers
 are well known in the art and have been used to form waterbased coating
 compositions, pigment dispersions, adhesives and the like.
 The use of cobalt chelates in the preparation of macromonomers for aqueous
 copolymer dispersions is limited so far to graft copolymers in which
 either graft or backbone contain ionizable groups in the form of acid or
 amine. Most of the applications also teach the preparation of such graft
 copolymers first in a solvent before inverting into a water dispersion.
 SUMMARY OF THE INVENTION
 We have now found that aqueous graft or block copolymers can be prepared in
 water, solvent, or a water/solvent blend using a water-soluble
 macromonomer substantially free of any acid groups to produce coating
 compositions with a high solids content at a low viscosity. Dispersions
 and solutions containing such copolymers show excellent compatibility with
 polyisocyanates in water-borne two-package formulations.
 The aqueous graft or block copolymers comprise 20 to 98%, preferably 50 to
 85%, of a water-soluble macromonomer with a weight average molecular
 weight of below 6000, preferably below 2,000, a hydroxyl value of more
 than 300, preferably more than 400, and containing less than 10% of an
 acid functional and/or amine functional unsaturated monomer. The remaining
 2-80%, preferably 15-50%, of the copolymer comprises a backbone (in the
 graft copolymer embodiment) or a B block (in the block copolymer
 embodiment). The graft or block copolymer has a weight average molecular
 weight of less than 20,000, preferably between 1500 and 8000, a hydroxy
 value of at least 200, an acid value of less than 50 and preferably no
 more than 5% of an ionizable monomer.
 The water-soluble macromonomer is preferably prepared using a free radical
 initiator in water or solvent or blend with a Co(II) or Co(III) chelate
 chain transfer agent.
 DETAILED DESCRIPTION
 The backbone or the B block, as the case may be, of the graft or block
 copolymer has a lower OH value relative to the side chains (or the A
 block) and can contain polymerized ethylenically unsaturated acid or amine
 monomers or salts thereof. The backbone or B block can contain polymerized
 monomers which are preferably methacrylates, but which can contain up to
 500% of acrylates or vinyl aromatics. Such monomers can comprise
 alkylmethacrylates and acrylates, cycloaliphatic methacrylates and
 acrylates and aryl methacrylates and acrylates as listed hereinafter. It
 can contain up to 50% by weight based on the weight of the copolymer, of
 polymerized ethylenially unsaturated non-hydrophobic monomers which may
 contain reactive functional groups. Other vinyl monomers can be
 incorporated into the backbone or B block, e.g., ethylenically unsaturated
 sulfonic, sulfinic, phosphoric or phosphonic acid and esters thereof also
 can be used such as styrene sulfonic acid, acrylamido methyl propane
 sulfonic acid, vinyl phosphonic acid and its esters and the like.
 In one embodiment, the graft copolymer emulsion contains 50 to 85% of poly
 2-hydroxy ethyl methacrylate macromonomer and 15 to 50% of backbone (or B
 block) monomers essentially being polymethacrylates. Examples of
 methacrylate monomers are alkyl methacrylates such as methyl methacrylate,
 ethyl methacrylate, n-butyl methacrylate 2-ethyl hexyl methacrylate, nonyl
 methacrylate, lauryl methacrylate, stearyl methacrylate and the like.
 Examples of other methacrylates can be used such as trimethyl cyclohexyl
 methacrylate, isobornyl methacrylate, benzyl methacrylate and the like.
 Ethylenically unsaturated monomers containing hydroxy functionality include
 hydroxy alkyl acrylates and hydroxy alkyl methacrylates, wherein the alkyl
 has 1 to 12 carbon atoms. Suitable monomers include 2-hydroxy ethyl
 acrylate, 2-hydroxy propyl acrylate, 4-hydroxy butyl acrylate, 2-hydroxy
 ethyl methacrylate, 2-hydroxy propyl methacrylate, 4-hydroxy butyl
 methacrylate, and the like, and mixtures thereof. Reactive functionality
 can also be obtained from monomer precursors, for example, the epoxy group
 of a glycidyl methacrylate unit in a polymer. Such an epoxy group can be
 converted, in a post condensation reaction with water or a small amount of
 acid, to a hydroxy group, or with ammonia and/or a primary amine to give a
 hydroxy amine. Suitable other olefinically unsaturated comonomers include:
 acrylamide and methacrylamide and derivatives as alkoxy methyl
 (meth)acrylamide monomers, such as N-isobutoxymethyl methacrylamide and
 N-methylol methacrylamide; maleic, itaconic and maleic anhydride and its
 half and diesters; vinyl aromatics such as styrene and vinyltoluene;
 polyethylene glycol monoacrylates and monomethacrylates; aminofunctional
 (meth)acrylates as, e.g., diethylaminoethylmethacrylate and
 t-butylaminoethylmethacrylate; glycidyl functional (meth)acrylates as
 glycidylmethacrylate. Other functional monomers as acrylonitrile,
 acrolein, allyl methacrylate, aceto acetoxyethyl methacrylate, methylacryl
 amidoglycolate methylether, ethylene ureaethyl methacrylate,
 2-acrylamide-2methyl propanesulfonic acid, trialkoxy silyl propyl
 methacrylate, reaction products of mono epoxyesters or monoepoxy ethers
 with alpha-beta unsaturated acids and reaction products of glycidyl
 (meth)acrylate with mono functional acids up to 22 carbon atoms can be
 used. The above monomers also can be used in the backbone or B block of
 the copolymer. They also can be used in the macromonomer provided the
 macromonomer is soluble in water and does contain less than 10% of an acid
 functional monomer.
 To ensure that the resulting macromonomer only has one terminal
 ethylenically unsaturated group which will polymerize with the remaining
 monomers to form the graft or block copolymer, the macromonomer is
 polymerized by using a catalytic chain transfer agent. Typically, in the
 first step of the process for preparing the macromonomer, the monomers are
 blended with an inert organic solvent which is water miscible or water
 dispersible and a cobalt chain transfer agent and heated usually to the
 reflux temperature of the reaction mixture. In subsequent steps,
 additional monomers and cobalt catalyst and conventional polymerization
 catalyst are added and polymerization is continued until a macromonomer is
 formed of the desired molecular weight. Preferred cobalt chain transfer
 agents or catalysts are described in U.S. Pat. No. 4,680,352, U.S. Pat.
 No. 4,722,984 and WO 87/03605. Most preferred are pentacyanocobaltate (II
 or III), diaquabis(borondifluorodimethyl glyoximato) cobaltate (II or III)
 and diaquabis (borondifluorophenyl glyoximato) cobaltate (II or III).
 Typically these chain transfer agents are used at concentrations of about
 2 to 5000 ppm based on the monomers used.
 The macromonomer is preferably formed in a solvent or solvent blend using a
 free radical initiator and a Co (II or III) chelate chain transfer agent,
 although it can be formed in aqueous solution or emulsion when using, for
 example, diaquabis (borondifluorodimethyl-glyoximato) cobaltate (II or
 III). Azo initiators (0.5 to 5% weight on monomer) can be used in the
 synthesis of the macromonomers in the presence of 2 to 5000 ppm (on total
 monomer) of Co (II) chelate in the temperature range between 70 to
 140.degree. C., more preferably azo type initiators as, e.g., 2,2'-azobis
 (2,4-dimethylpentanenitrile), 2,2'-azobis(2-methylpropanenitrile),
 2,2'-azobis (2-methylbutanenitrile), 1,1'-azo(cyclohexane carbonitrile)
 and 4,4'-azobis (4-cyanopentanoic) acid. Typical solvents that can be used
 to form the macromonomer copolymer are aromatics, aliphatics, ketones such
 as methyl ethyl ketone, isobutyl ketone, ethyl amyl ketone, acetone,
 alcohols such as methanol, ethanol, n-butyanol, isopropanol, esters such
 as ethyl acetate, glycols such as ethylene glycol, propylene glycol,
 ethers such as tetrahydrofuran, ethylene glycol mono butyl ether and the
 like, and as mentioned above, water and mixtures thereof with
 water-miscible solvents.
 The graft or block copolymer is formed directly in water, solvent or in a
 water/solvent mixture by copolymerizing the rest of the monomer blend in
 the presence of the macromonomer which is soluble in water.
 Any of the aforementioned azo type catalysts can be used as can other
 suitable catalysts such as peroxides and hydroperoxides. Typical of such
 catalysts are di-tertiary butyl peroxide, di-cumyl peroxide, tertiary amyl
 peroxide, cumene hydroperoxide, di(n-propyl) peroxydicarbonate, peresters
 such as amyl peroxyacetate and the like. Polymerization is continued
 usually at the reflux temperature of the reaction mixture until a graft or
 block copolymer is formed of the desired molecular weight. Water-soluble
 free radical initiators can be used, suitable in the temperature range of
 20 to 98.degree. C., e.g., peroxides such as ammonium persulfate, or redox
 initiators such as t-butyl hydroperoxide/ascorbic acid. On copolymerizing
 the monomers with the macromonomer, chain transfer agents other than the
 cobalt chelates can be used as, e.g., mercaptans, mercaptoethanol,
 t-dodecylmercaptan, n-dodecylmercaptan. These binder systems are utilized
 to produce waterborne coatings by blending with other suitable components
 in accordance with normal paint formulation techniques. The graft
 copolymers of the present invention are useful as film-forming vehicles in
 the preparation of waterborne coating compositions such as, for example,
 clearcoat or basecoat compositions useful in automotive applications. The
 resultant coating compositions have low volatile organic content,
 preferably to a maximum of 3.50 pounds/gallon.
 The aqueous graft and block copolymers of this invention show remarkable
 compatibility with polyisocyanate crosslinking agents. Examples of
 polyisocyanates include biuret; cyclotrimers of hexamethylene
 diisocyanate; isophorone diisocyanate; 3,5,5'-trimethyl hexamethylene
 diisocyanate and isomers thereof; 4,4'-dicyclohexylmethane diisocyanate
 (available from Bayer AG as "Desmodur W"); toluene diisocyanate;
 4,4'-diphenylmethane diisocyanate; and tetramethyl xylylene diisocyanate.
 Further examples include reaction products of polyols (e.g., trimethylol
 propane) with an excess of a diisocyanate to form isocyanate functional
 polyurethanes. Optionally, polyisocyanate functional polyester-urethanes
 or acrylic-urethanes may also be used to advantage. Optionally, one can
 use a polyisocyanate functional acrylics derived from 2-isocyanato ethyl
 methacrylate or (benzene, 1-(1-isocyanato-1-methyl ethyl)-4(1-methyl
 ethenyl)) (commercially available from Cytec as "M-TMI") by polymerization
 with other unsaturated compounds.
 The isocyanates can be modified with hydrophilic groups to ease
 incorporation into water. Examples of hydrophilic groups include
 polyethyleneoxide. Preferably, hydrophobic polyisocyanates are used.
 Surprisingly, the aqueous copolymers of the present invention, when cured
 with an polyisocyanate showed film-forming properties, hardness,
 appearance and resistance to chemicals and solvents equal to that of a
 solvent based 2 package finish system.
 Cure promoting catalyst can be used in the coating compositions of this
 invention, as is typical in the art when isocyanate crosslinking or curing
 agents are employed. Preferred catalysts are organometallics, suitably
 dibutyl tin dilaurate, zinc octoate, dibutyl tin diacetate zinc
 naphthenate, vanadium acetyl acetonate or zirconium acetyl acetonate, in
 an effective curing amount, typically from about 0.1 to 2% by weight of
 binder. Such catalysts are optional, for example, elevated temperature
 and/or time may suffice to cure the composition.
 Other film-forming binders, such as acrylics, polyesters, polyurethanes,
 polyethers, polyamides and others may be present in the final coating
 composition. In addition, a composition according to the present invention
 can contain a variety of other optional ingredients, including pigments,
 pearlescent flakes, fillers, plasticizers, antioxidants, surfactants and
 flow control agents. To improve weatherability of a finish produced by the
 present coating composition, an ultraviolet light stabilizer or a
 combination of ultraviolet light stabilizers can be added in the amount of
 about 0.1 to 5% by weight, based on the weight of the binder. Such
 stabilizers include ultraviolet light absorbers, screeners, quenchers, and
 specific hindered amine light stabilizers. Also, an antioxidant can be
 added, in the about 0.1 to 5% by weight, based on the weight of the
 binder. Typical ultraviolet light stabilizers that are useful include
 benzophenones, triazines, benzoates, hindered amines and mixtures thereof.
 Specific examples of ultraviolet stabilizers are disclosed in U.S. Pat.
 No. 4,591,533. The composition can also include conventional formulation
 additives such as flow control agents, for example, Resiflow S
 (polybutylacrylate), BYK 320 and 325 (high molecular weight
 polyacrylates); rheology control agents, such as fumed silica, and the
 like. When the present composition is used as a clearcoat (topcoat) over a
 pigmented colorcoat (basecoat) to provide a colorcoat/clearcoat finish,
 small amounts of pigment can be added to the clear coat to provide special
 color or aesthetic effects such as tinting. The present composition can be
 pigmented and used as the colorcoat, monocoat, primer, or primer surfacer.
 The composition has excellent adhesion to a variety of metallic or
 non-metallic substrates, such as previously painted substrates, cold
 rolled steel, phosphatized steel and steel coated with conventional
 primers by electrodeposition.
 The present composition can be used to coat plastic substrates such as
 polyester reinforced fiberglass, reaction injection-molded urethanes and
 partially crystalline polyamides. When the present coating composition is
 used as a basecoat, typical pigments that can be added to the composition
 include the following: metallic oxides such as titanium dioxide, zinc
 oxide, iron oxides of various colors, carbon black, filler pigments such
 as talc, china clay, barytes, carbonates, silicates and a wide variety of
 organic colored pigments such as quinacridones, copper phthalocyanines,
 perylenes, azo pigments, indanthrone blues, carbazoles such as carbazole
 violet, isoindolinones, isoindolones, thioindigo reds, benzimidazolinones,
 metallic flake pigments such as aluminum flake and the like. The pigments
 can be introduced into the coating composition by first forming a mill
 base or pigment dispersion with any of the aforementioned polymers used in
 the coating composition or with another compatible polymer or dispersant
 by conventional techniques, such as high speed mixing, sand grinding, ball
 mill, attritor grinding or two roll milling. The mill base is then blended
 with the other constituents used in the coating composition. The coating
 composition can be applied by conventional techniques such as spraying,
 electrostatic spraying, dipping, brushing, flowcoating and the like. The
 preferred techniques are spraying and electrostatic spraying. The present
 composition can be used as an ambient cure, especially for refinish, or at
 elevated temperature. In OEM application, the composition is typically
 baked at 100 to 150.degree. C. for about 15 to 30 minutes to form a
 coating about 0.1 to 3.0 mils thick. When the composition is used as a
 clearcoat, it is applied over the colorcoat which can be dried to a
 tack-free state and cured or preferably flash dried for a short period
 before the clearcoat is applied. The colorcoat/clearcoat finish is then
 baked to provide a dried and cured finish. It is customary to apply a
 clear topcoat over a basecoat by means of a "wet-on-wet" application,
 i.e., the topcoat is applied to the basecoat without curing or completely
 drying the basecoat. The coated substrate is then heated for a
 predetermined time period to allow simultaneous curing of the base and
 clear coats.

The following examples illustrate the invention. All parts and percentages
 are on a weight basis unless otherwise indicated. All molecular weights
 disclosed herein are determined by gel permeation chromatography using a
 polystyrene standard.
 EXAMPLES
 Example 1
 Macromonomer
 This example illustrates the use of a Co (II) chelate in the synthesis of
 following macromonomers. The chelate BF.sub.2 bridged Co (II)
 (1,2-diphenyl-1,2-dioxoiminomethane) (H.sub.2 O) chelate is as described
 in example 44B of EP 0199436. Mixture 1 (below) was heated at reflux
 (.+-.100.degree. C.) in a reaction vessel that was kept under nitrogen.
 Mixture 2 was added over 4 hours followed immediately by mixture 3 for
 rinsing. Afterwards, the reaction mixture was held for another 30 minutes
 at reflux. During the process, the temperature was held at about
 100.degree. C. reflux.

Mixture 1
 Methyl ethyl ketone 9.5
 Co (II) chelate 0.01
 2-hydroxyethyl methacrylate 14
 Mixture 2
 2-hydroxyethyl methacrylate 56
 VAZO .RTM. 52 initiator (DuPont) 0.56
 Methyl ethyl ketone 18.93
 Mixture 3
 Methyl ethyl ketone 1
 Test Results:
 Solids 65.6
 Viscosity B
 (Gardner-Holdt)
 Acid value 0.8
 Mn 990
 Mw 1340
 This macromonomer is completely soluble in water.
 Example 2
 Macromonomer
 This example illustrates the use of Co (III) chelate in the synthesis of
 the macromonomer. The chelate is bis [1,2-dimethylethane-1,2-dione
 dioximato (difluoroborato)] (isopropyl) cobalt III hydrate. The procedure
 and percentage of Example 1 were used with the following changes.

Mixture 1
 Deionized water 16.95
 Co (III) chelate 0.05
 2-hydroxyethyl methacrylate 14
 Mixture 2
 2-hydroxyethyl methacrylate 56
 Deionized water 10.45
 (4,4'-azobis-)4-cyano pentanecarboxylic acid 0.3
 Dimethylethanolamine 0.25
 Mixture 3
 Deionized water 2
 Test Results:
 Solids 39.5%
 Viscosity less than A
 pH 6.4
 Mn 940
 Mw 1280
 This macromonomer is completely soluble in water.
 Examples 3 to 6
 Graft and Block Copolymer Emulsions
 These examples illustrate the invention in which low molecular weight graft
 copolymer emulsions are prepared directly in a water/solvent blend in
 which the solvent is stripped-off afterwards. The graft copolymer
 emulsions are formed in the presence of a macromonomer which is water
 soluble and basically free of any acid groups. Mixture 1 (below) was
 heated at reflux in a reaction vessel. Mixture 2 and Mixture 3 were added
 simultaneously over a 4 hour period following by the addition of Mixture 4
 as a rinsing step. The contents were kept 1 hour at reflux followed by
 addition of Mixture 5 over 20 minutes. Mixture 6 was added as a rinsing
 step and the reactor contents refluxed for another hour. Next, Mixture 7
 was added and the organic volatiles were stripped off.