Acrylic Polyester High Solids Coatings

This invention relates to high solids thermosetting protective coatings and particularly to low temperature cure, high solids coatings based on the combination of certain low molecular weight acrylic polymers and certain low molecular weight polyester polymers. The acrylic and polyester polymers are adapted to be heat cured and cross-linked with an amine derivative such as melamine. High solids coatings are non-aqueous coatings containing minor amounts or organic solvents and are particularly useful as coatings on appliances, aluminum extrusions, general metal surfaces, plastic and wood substrates.

Acrylic polymers are known to generally provide useful coatings exhibiting good film properties. These polymers can contain reactive functional groups which are coreactive with other polymers or resins to provide thermosetting binder systems. Such acrylic binders can be produced by solution or bulk polymerization of ethylenically unsaturated monomers including acrylic monomers. Solvent can be added in minor amounts to render the acrylic polymer fluent. Moderately high molecular weight acrylic polymers or polyesters polymers have been suggested to provide high solids polymeric compositions such as disclosed in U.S. Pat. No. 4,374,164. Acrylic polymers have been combined with a combination of polyester polyols as suggested in U.S. Pat. No. 4,397,989 or combined with an epoxy polymer as shown in U.S. Pat. No. 4,369,283.

It now has been found that certain low molecular weight acrylic solution polymers having a number average molecular weight below about 2000 combined with certain low molecular weight polyester polymers having a number average molecular weight below about 2000, provide an exceptionally versatile high solids, very low volatile organic solvent (VOC), thermosetting composition coreactive with a compatible fluent amine derivative cross-linking component. The amine derivative preferably is an etherified aminoplast or glycoluril. The acrylic polymer and the polyester polymer are mutually compatible and both contain functional hydroxyl groups which advantageously are adapted to coreact and cross-link with amine derivative to produce a cured paint film comprising an interpenetrating network of cross-linked polyester and acrylic polymer chains. The cured film of this invention exhibits superior all around film integrity properties including extraordinary flexibility and hardness, excellent adhesion, toughness, and impact resistance along with substantially improved salt spray and detergent resistance. The acrylic polymer further includes carboxyl functinality which provides some coreactivity or wetting of pigments such as titaium dioxide to provide an excellent grinding vehicle whereby an acrylic pigment grind composition can be subsequently combined with polyester polymer and aminoplast resin to provide a compatible pigmented composition. These and other advantages of this invention will become more apparent by referring to the detailed description of the invention and the illustrative examples hereinafter.

Briefly, the invention comprises a non-aqueous high solids polymeric composition based on the combination of low molecular weight acrylic polymer and low molecular weight polyester polymer wherein both polymers contain reactive hydroxyl groups adapted to be coreactive and become cross-linked with an etherified amine derivative coereactive cross-linking component. Preferably, the composition comprises by weight between 1.5% and 40% acrylic polymer, 1.5% and 78.5% polyester polymer, and 10% and 58.5% amine derivative crosslinking resin, wherein the foregoing totals 100%. The composition can be efficiently heat cured at low temperatures in the presence of sulfonic acid curing catalysts.

The high solids coating composition of this invention is based on the combination of low molecular weight polymers including acrylic polymer, polyester polymer, and coreactive cross-linking amine resin such as melamine or glycoluril.

Referring first to the acrylic polymer, the acrylic polymer comprises organic solvent solution or bulk copolymerized ethylenically unsaturated monomers, including acrylic monomers, to produce a non-aqueous low molecular weight acrylic polymer containing considerable amounts of reactive primary hydroxyl groups and lesser amounts of carboxyl groups. The molecular weight of the acrylic polymer is below 2000, broadly between 500 and 2000, and preferably between 1000 and 1500 based on number average molecular weights. Molecular weights below 500 cause a curing problem and soft films whereas above 2000 causes viscosity problems. Number average molecular weights are typically measured by GPC according to ASTM methods such as D3016-72; D3536-76; D3593-80; or D3016-78. The acrylic polymers are liquid at room temperature and generally have a Tg between -20.degree. C. and +20.degree. C. as calculated by the Fox equation based on the ratio of specific monomers. The acrylic polymer has a low acid number below about 30, broadly between 5 and 30, and preferably between 12 and 20. The hydroxyl number of the acrylic polymer is high and broadly between 50 and 300 and preferably between 100 and 175. The acrylic polymer can be produced by bulk polymerization of ethylenically unsaturated monomers activated by peroxide or azo or other free radical initiator at polymerization temperatures typically between 70.degree. C. to 170.degree. C. and preferably between 120.degree. C. to 150.degree. C. Typically 0.5 to 2% peroxide initiator is utilized based on the weight of the monomers. Typical polymerization initiators can include for instance benzoyl peroxide, t-butyl hydroperoxide, tertiary butyl perbenzoate, cumene peroxide and similar peroxide polymerization catalysts which are preferred; azo initiators such as azo bisisobutyronitrile and dimethylazobis-isobutyrate; as well as alkali metal persulfate or ammonium persulfates. Mercaptans are ordinarily added to provide copolymers of low molecular weight. Mercaptans useful in controlling molecular weight and primarily to produce low molecular weight polymers include for example 2-mercaptoethanol; 1-mercapto-2-propanol; and 2-hydroxyethyl-3-mercapto propionate. Mercaptans are added at levels around 4% based on monomer solids and broadly between about 2% and 6%. Preferably the monomers containing mercaptans are completely copolymerized before concluding the copolymerization of all monomers to preclude free levels of mercaptans in the polymer. The narrow range molecular weight is very important in this invention to provide compatibility with the polyester and amine derivative cross-linking polymer as well as maintaining sufficiently low viscosity for high solids coatings.

Copolymerizable ethylenically unsaturated monomers useful in producing the acrylic polymer are monomers containing carbon-to-carbon unsaturation and include vinyl monomers, acrylic monomers, allylic monomers, acrylamide monomers, and mono- and dicarboxylic unsaturated acids. Vinyl esters include vinyl acetate, vinyl propionate, vinyl butyrates, vinyl benzoates, vinyl isopropyl acetates and similar vinyl esters. Vinyl halides include vinyl chloride, vinyl fluoride, and vinylidene chloride. Vinyl aromatic hydrocarbons include styrene, methyl styrenes and similar lower alkyl styrenes, chlorostyrene, vinyl toluene, vinyl naphthalene, divinyl benzoate, and cyclohexene. Vinyl aliphatic hydrocarbon monomers include alpha olefines such as ethylene, propylene, isobutylene, and cyclohex as well as conjugated dienes such as 1,3 butadiene, methyl-2-butadiene, 1,3-piperylene, 2,3 dimethyl butadiene, isoprene, cyclopentadiene, and dicyclopentadiene. Vinyl alkyl ethers include methyl vinyl ether, isopropyl vinyl ether, n-butyl vinyl ether, and isobutyl vinyl ether. Acrylic monomers include monomers such as lower alkyl esters of acrylic or methacrylic acid having an alkyl ester portion containing between 1 to 12 carbon atoms as well as aromatic derivatives of acrylic and methacrylic acid. Useful acrylic monomer include, for example, acrylic and methacrylic acid, methyl acrylate and methacrylate, ethyl acrylate and methacrylate, butyl acrylate and methacrylate, propyl acrylate and methacrylate, 2-ethyl hexyl acrylate and methacrylate, cyclohexyl acrylate and methacrylate, decyl acrylate and methacrylate, isodecylacrylate and methacrylate, benzyl acrylate and methacrylate, and various reaction products such as butyl, phenyl an cresyl glycidyl ethers reacted with acrylic and methacrylic acids, hydroxyl alkyl acrylates and methacrylate such as hydroxyethyl and hydroxypropyl acrylates and methacrylates, as well as amino acrylates and methacrylates. Acrylic acids include acrylic and methacrylic acid, ethacrylic acid, alpha-chloracrylic acid, alpha-cycanoacrylic acid, crotonic acid, beta-acryloxy propionic acid, and beta-styrl acrylic acid. Olefinic unsaturated acids include fumaric acid, maleic acid or anhydride, haconic acid, citraconic acid, mesaconic acid, muconic acid, glutaconic acid, aconitic acid, hydrosorbic acid, sorbic acid, alpha-chlorsorbic acid, cinnamic acid, and hydromuconic acid as well as esters of such acids. Ethylenically unsaturated carboxylic acid amides and derivatives can be added in very minor amounts up to 5% and can include acrylamides or methacrylamides such as N-methylol acrylamide, N-ethanol acrylamide, N-propanol acrylamide, N-methylol methacrylamide, N-ethanol methacrylamide, and similar alkyl acrylamide or methacrylamide monomers containing methyl, ethyl, propyl, n-butyl or iso-butyl alkyl groups.

As noted before, functional monomers are included to provide both carboxyl and hydroxyl groups. Carboxy containing monomers include acrylic acid and lower alkyl substituted acrylic acids wherein the preferred carboxylic monomers are acrylic and methacrylic acids. Hydroxyl containing monomers are hydroxy containing ethylenically unaturated monomers including hydroxy alkyl acrylates such as 2-hydroxy ethyl acrylate and methacrylate, 2-hydroxypropyl acrylate and methacrylate, and similar hydroxy alkyl acrylates. Incorporating a minor amount of carboxyl acid functionality in the acrylic polymer was found to provide improved pigment dispersion properties and prevent pigment settling. The Acid No. level of the acrylic polymer is required to provide highly desirable wetting characteristics to wet the pigment when the acrylic polymer is used as a pigment grinding vehicle. The carboxyl groups provide pigment wetting and further maintain a stabilized pigment dispersion within the polymer and avoids undesirable settling. Preferred typical Acid No. can be between about 10 and 20 or moderately higher although high Acid Nos. can cause high viscosity or diminished film properties such as lower salt spray or detergent resistance. On a weight basis, the monomers comprise at least 5% acrylic monomer, between 0.5% and 5% carboxyl monomer, between 10% and 60% hydroxyl monomer, and the remainder being acrylic and/or other ethylenic monomers to provide an acrylic linear copolymer having an Acid No. between 5 and 30 and a hydroxyl No. between 50 and 300.

Referring next to the polyester component of the polymeric mixture, the polyester polymer is a low molecular weight linear polymer having a molecular weight between 250 and 2000 and preferably between 400 and 1000 based on number average. Linear aliphatic, non-ether type glycols are esterified with considerably lesser molar amounts of aromatic dicarboxylic acid and a linear saturated dicarboxylic acid having between 2 and 10 linear carbon atoms such as adipic, azelaic, succinic, glutaric, pimelic, suberic or sebacic acid to produce low molecular weight polyesters. Preferred and commercially available linear saturated dicarboxylic acids are adipic or azelaic acid. Aromatic dicarboxylic acids (anhydrides) include phthalic, isophthalic, terephthalic, and tetrahydrophthalic. Isophthalic is preferred for superior films exhibiting detergent, salt spray and corrosion resistance. Suitable glycols include non-ether linear aliphatic glycols having 2 to 8 carbon atoms such as 1,3 or 1,4 butylene glycol, 1,6 hexane diol, neopentyl glycol, propylene glycol and similar linear glycols. Preferred glycol is neopentyl glycol. The molar excess of the glycol over the lesser molar amounts of aromatic and linear saturated dicarboxylic acid is between about 10% and 80% and preferably between about 20% and 60%. On a molar basis, the preferred low molecular weight polyester polymer comprises between 0.1 and 0.5 moles of aromatic dicarboxylic acid and 0.5 and 0.1 moles of linear saturated dicarboxylic acid per 1 mole of glycol. Hence, the polyester contains considerable excess unreacted hydroxyl groups to provide a hydroxy polyester having a hydroxyl number between 115 and 285 and preferably between 175 and 240. The polyester contains free carboxyl groups preferably below 15 and typically between 5 and 10. Glycol can be esterified with minor amounts up to about 5% by weight of unsaturated dicarboxylic acids (anhydrides) including maleic, fumaric or itaconic acids; or monocarboxylic acids such as acetic, propyl-, and higher chain aliphatic acids up to about 8 carbon atoms.

The polyester can be produced by solvent or bulk polymerization although bulk polymerization is preferred. The raw materials can be charged in bulk and esterification polymerized at temperatures typically between 190.degree. C. to 240.degree. C. although moderately higher or lower temperatures can be utilized satisfactorily. An esterification catalyst can be used, typically at less than 1% levels based on charge, such as an organo tin compound.

The foregoing hydroxyl functional, low molecular weight acrylic and polyester polymers are further combined with a coreactive amine derivative cross-linking resin such as aminoplast or phenoplast resin. Aminoplast resins are preferred and further include glycolurils. Examples of useful aminoplast resins are the reaction products of ureas and melamines with aldehydes further etherified with an alcohol. Examples of aminoplast resin components are urea, ethylene urea, thiourea, melamine, benzoguanamine. Aldehydes useful in this invention are formaldehyde, acetaldehyde and propionaldehyde, although formaldehyde is clearly preferred. The aminoplast resins can be used in the alkylol form but, most preferably, are utilized in the ether form by etherifying with a monohydric alcohol containing from 1 to about 8 carbon atoms. In a melamine molecule, for instance, up to 3 of the 6 active hydrogens on the amine group can be advantageously substituted with an alkanol group having 1-8 carbon atoms. Higher levels such as 6 substitution can be used as in Cymel 300 which contains 6 substituted methylol groups. The alkanol groups stabilize the melamine or other amine derivative under oridinary temperatures, but enable heat reaction at higher temperatures. Preferred substitutions are between 2 and 4 substitutions to avoid popping or solvent entrapment with a fast curing film. Examples of aminoplast resins are methylol urea, dimethoxymethylol urea, butylated polymeric urea-formaldehyde resins, hexamethoxymethyl melamine, methylated polymeric melamine formaldehyde resin and butylated polymeric melamine formaldehyde resin. Glycoluril derivatives are disclosed in U.S. Pat. No. 4,064,191 and are also known as acetylendiureas. Glycolurils are derived by reacting two moles of urea with one mole of glyoxal to provide a complex ring structure where substitute constituents can be a hydrogen, or a lower alkyl radical, or can be methylolated partially or fully by reacting with 1 to 4 moles of formaldehyde to provide a methylol glycoluril. The preparation of various glycolurils is illustrated in U.S. Pat. No. 4,064,191 such as tetramethylol glycoluril, tetrabutoxymethyl glycoluril, partially methyolated glycoluril, tetramethoxymethyl glycoluril, and dimethoxydiethoxy glycoluril. Useful glycoluril derivatives include for example, mono- and dimethylether of dimethylol glycoluril, the trimethylether of tetramethylol glycoluril, the tetramethylether of tetramethylol glycoluril, tetrakisethoxymethyl glycoluril, tetrakisopropoxmethyl glycoluril, tetrakisbutoxymethyl glycoluril, tetrakisamyloxymethyl glycoluril, tetrakishexoxymethyl glycoluril and the like. Glycoluril derivatives can further include dimethylol dihydroxy ethylene urea which is disclosed in U.S. Pat. No. 4,540,735 and incorporated herein by reference. Less preferred amine derivatives are phenolplast resins, or phenol-aldehyde resins, which are the reaction products of phenols and aldehydes which contain reactive methylol groups. These compositions can be monomeric or polymeric in nature depending on the molar ratio of phenol to aldehyde used in the initial condensation reaction. Examples of phenols which can be used to make these phenolplast resins are phenol, o, m, or p-cresol, 2,4-xylenol, 3,4-xylenol, 2,5-xylenol, cardanol, p-tert-butylphenol, and the like. Aldehydes useful in this invention are formaldehyde, acetaldehyde and propionaldehyde. Particularly desirable phenolplast resins ae polymethylol phenols wherein the phenolic group is etherified with an alkyl, alcohol such as methyl, ethyl, propyl, etc. alcohol.

On a polymer solids weight basis, the blend of polymers comprises broadly between 1.5% and 40% acrylic polymer, between 1.5% and 78.5% polyester polymer, and between 10% and 58.5% amine derivative cross-linking resin. Preferred polymeric blend compositions comprise 15% to 30% acrylic polymer, 20% to 65% polyester polymer and 20% to 50% cross-linker such as aminoplast resin.

In use, the low molecular weight acrylic polymer containing major amounts of primary hydroxyl groups and considerably lesser amounts of unreacted or free carboxyl groups can be used as a grind vehicle for preparing a pigmented vehicle base. The carboxyl functionality on the acrylic polymer advantageously interacts with the normal basic pigment surfaces such as titanium dioxide. Other mineral pigments include carbon black, cadmiums, yellow, brown, and tan iron oxides as well as other oxide pigments and further include inert fillers such as talcs, clays, and fumed silicas. The acrylic polymer physically wets the titanium dioxide pigment surface for instance and further maintains the pigment in stable dispersion. Up to about six weight parts and typically between about 3.5 and 4 weight parts of pigment can be combined with one weight part of acrylic polymer depending on the adsorbtion properties of the pigment particles. Pigmented coatings typically comprise a pigment to binder ratio of about 1 to 1. The preformed acrylic polymer pigment grind can be combined with the low molecular weight polyester polymer and the amine derivative cross-linking resin.

Referring now to a further aspect of this invention, low molecular weight reinforcing resins selected from styrene-maleate copolymer, liquid epoxy resin, or a reaction product comprising an alkylene oxide adduct of a Bis-phenol can be added in lesser amounts to the basic polymer blend of acrylic polymer, polyester, and amine cross-linking resin to further provide specifically improved film properties such as detergent resistance, corrosion resistance, salt spray resistance and similar hard to achieve film integrity properties. All of the reinforcing resins have a number average molecular weight below about 3000 and preferably below 2500 to maintain compatibility with basic polymeric blend of low molecular weight acrylic and polyester polymers.

The styrene-maleic anhydride copolymers have a molecular weight less than about 3000 and preferably between 1500 and 2500 number average molecular weight. The most preferred sytrene-maleic anhydride copolymers are further reacted with an alcohol to provide a styrene-maleate half-ester. The ratio of styrene to maleic anhydride coreacted can be between 1 to 1 and 3 to 1 molar ratios of styrene to maleic anhydride. Preferably the resulting styrene-maleic anhydride copolymer is further reacted with a lower aliphatic alcohol on approximately a 1 to 1 molor ratio basis of maleic anhydride to aliphatic alcohol to provide a half-ester. Suitable alcohols include for instance methyl, ethyl, propyl, butyl, etc. alcohols. The resulting styrene-maleate half ester copolymers have carboxyl groups which impart adhesion properties as well as a coreactive cite for cross-linking with an aminoplast resin. The Acid No. can be between 100 and 170. On a weight basis, the styrene-maleate copolymer can be added to the basic polymeric blend of this invention broadly between 0 and 20 and preferably between 5 and 15 parts per 100 parts of basic polymer blend.

Referring next to the liquid epoxy reinforcing resin, the molecular weight thereof is below about 3000 and preferably between about 400 and 2500 number average molecular weight. Epoxy resins are polyglycidyl ethers of bisphenol-A synthesized by reacting bisphenol-A with epichlorohydrin where the molecular weight is controlled by controlling the amount of bisphenol-A available to react with available epoxy groups. As the mole ratio of bisphenol-A to epichlorhydrin increases, the average molecular weight of the epoxy resin increases. Low molecular weight liquid epoxy resins are particularly preferred as reinforcing resins in the basic polymeric blend of this invention. On a weight basis, broadly between 0 and 55 parts and preferably between 5 and 10 weight parts of epoxy resin can be added to 100 weight parts of basic polymeric blend.

In a particularly favorable aspect of this invention, the foregoing low molecular weight styrene-maleate copolymer can be combined with the foregoing low molecular weight epoxy resin to provide a highly desirable reinforcing resin in accordance with a preferred aspect of this invention. On a weight basis, about 15 to 40 parts of styrene-maleate copolymer can be combined with about 10 parts of epoxy resin; and the mixture thereof can be added to the basic polymeric blend broadly between 0 and 75 parts, and preferably between 2 and 16 parts, per 100 weight parts of basic polymer blend.

Referring next to the alkylene oxide adduct of a bis-phenol compound, this reinforcing compound has a low molecular weight below 1,000 and preferably between about 316 and 708. The alkylene oxide is an aliphatic alkyl derivative having up to about 16 carbon atoms although preferred oxides are lower alkyl oxides such as ethylene, propylene, and butylene oxides. Bis-phenol compounds include bisphenol-A, bisphenol-F and bis-sulfone or sulfides. Typically two or more moles of alkyl oxide are coreacted with one mole of bis-phenol compound. Preferred compositions are 2:1 molar reactions. On a weight basis, between 0 and 55 parts and preferably between 5 and 15 parts of alkylene oxide adduct of a bis-phenol compound can be added to 100 parts of the basic polymeric blend.

The merits of this invention based on the basic polymeric blend of low molecular weight acrylic polymer, low molecular weight polyester polymer, and cross-linking amine resin are further illustrated in the following examples.

Low molecular weight acrylic polymers were produced by copolymerizing monomers with 1% by weight dicumyl peroxide catalyst in solvent. The polymers comprise by weight 2.6% methacrylic acid and 20% 2-hydroxyl ethyl acrylate, 40% butyl acrylate, and 37.4% styrene. Based on 100 weight parts of monomer, 25 weight parts of methyl amyl ketone were added. A 2-mercaptoethanol was used at the level of 4% based on monomers to control molecular weight. Polymerization of the monomers was at approximately 145.degree. C. to 150.degree. C. until resulting acrylic polymer had a number average molecular weight of 1200, a weight average molecular weight of 1900.

Similar acrylic polymers were produced as indicated in the following Tables 1-8. Hydroxy numbers were held constant at 122 based on the inclusion of 2-hydroxy ethyl acrylate monomer. All molecular weights of the acrylic polymers were betwen 800 and 1200 number average molecular weight.

A molecular weight series of acrylic polymer was prepared by changing levels of chain transfer agent 2-mercaptoethanol.

A low molecular weight hydroxylated polyester was produced as follows:

The glycol was charged first to the reactor and heated to about 125.degree. C. to form a melt. Then the adipic and isophthalic acids were added and the mixture was upheated to about 190.degree. C. to remove water of esterification followed by gradual upheat to about 200.degree. C. to 210.degree. C. and held until the Acid No. was below 10 to product the polyester polymer. The finished polymer was then reduced with xylene and cooled to produce a fluid polyester resin with the following properties.

Acid No.=5

Hydroxyl No.=210 (calculated)

%NVM=86.9%

Color=1

Molecular Weight (No. Average)=466 (calculated)

Similar to Example III, low molecular weight hydroxylated polyesters were produced as follows:

The acrylic polymer in Example 1 was ground by ball mill grinding with titanium dioxide pigment on approximately a 1 to 4 weight basis of acrylic polymer to pigment as follows.

This preformed grind was reduced further with additional components including polyester polymer and melamine in accordance with this invention as follows.

The foregoing composition provided an excellent low temperature cure coating and was applied as a 0.8 mil film to cold rolled steel and cured for 30 minutes at 225.degree. F. Physical properties of the cured paint film were as follows.

60.degree. gloss: 96

Impact resistant: 160 pounds direct, 160 pounds reverse

Film pencil hardness: 2H

In a manner similar to Example V, a high solids coating particularly suitable for an appliance coating was prepared as follows.

The foregoing preformed grind was reduced with additional components including polyester polymer and melamine in accordance with this invention as follows.

The foregoing composition provided an excellent applicance coating and was applied as 1.2 mil film to cold rolled steel and cured for 20 minutes at 350.degree. F. Physical properties of the cured paint film were as follows:

In a manner similar to Example V, a high solids coating particularly suitable for aluminum extrusions was prepared as follows.

The foregoing grind was reduced further with additional components including polyester polymer and melamine in accordance with this invention as follows:

The foregoing composition provided an excellent aluminum coating and was applied as about 1 mil film thickness in an aluminum extrusion substrate and cured at 20 minutes 360.degree. F. Physical properties of the cured film are as follows:

60.degree. gloss: 50

Film pencil hardness: 4H

Impact resistance: 60+ (aluminum substrate)

Detergent resistance: above 24 hours

Humidity resistance: above 1000 hours

Salt spray resistance: above 1000 hours