Thermoset acrylic coatings having improved gloss retention
BACKGROUND OF THE INVENTION 
     The present invention pertains to acrylic polymers containing hydroxybutyl acrylate and to coatings prepared therefrom. 
     Thermoset acrylic coatings have been employed in the automobile industry for top coatings on automobiles. While these coatings are very durable and withstand extreme variations in temperature and moisture environments, there is a need to improve their weatherability in terms of gloss retention. 
     The present invention provides a method for improving the gloss retention of thermoset acrylic coatings by replacing at least a portion of the hydroxy functional component with hydroxybutyl acrylate or hydroxybutyl methacrylate or a combination thereof. 
     SUMMARY OF THE INVENTION 
     One aspect of the present invention pertains to an improvement in a curable polymer resulting from polymerizing a composition comprising 
     (I) from about 40 to about 80, preferably from about 65 to about 75 percent by weight based on the combined weight of components (I) and (II) of a monomer composition consisting essentially of 
     (A) from about 0 to about 40, preferably from about 0 to about 14, percent by weight of at least one polymerizable monoethylenically unsaturated ester of a saturated aliphatic or aromatic monocarboxylic acid; 
     (B) from about 30 to about 60, preferably from about 40 to about 54, percent by weight of at least one polymerizable alkyl or aryl ester of a monoethylenically unsaturated monocarboxylic acid; 
     (C) from about 10 to about 40, preferably from about 25 to about 39, percent by weight of at least one polymerizable hydroxyalkyl ester of a monoethylenically unsaturated monocarboxylic acid; 
     (D) from about zero to about 3, preferably from about 1 to about 2, percent by weight of at least one polymerizable monoethylenically unsaturated monocarboxylic acid; 
     (E) from about zero to about 15, preferably from about zero to about 14, percent by weight of acrylonitrile, methacrylonitrile or mixture thereof; 
     (F) from about zero to about 20, preferably from about 0 to about 14, percent by weight of at least one polymerizable alkyl or aryl ester of a monoethylenically unsaturated dicarboxylic acid; and 
     (G) from about zero to about 50, preferably from about 20 to about 34, percent by weight of at least one polymerizable monoethylenically unsaturated aromatic monomer; and 
     (II) from about 20 to about 60, preferably from about 25 to about 35 percent by weight based on the combined weight of components (I) and (II) of a suitable solvent or mixture of solvents; and 
     wherein the improvement resides in employing 2-hydroxybutyl acrylate or 2-hydroxybutyl methacrylate or a combination thereof as at least a portion of component (C) in an amount such that at least about 75, preferably about 100, equivalent percent of the hydroxyl groups contained in component (C) is derived from hydroxybutyl acrylate, hydroxybutyl methacrylate or a combination thereof. 
     Another aspect of the present invention pertains to an acrylic coating composition comprising (A) one or more of the aforementioned curable polymer compositions and (B) a curing quantity of one or more curing agents for component (A). 
     Another aspect of the present invention pertains to an article coated with one or more layers of the aforementioned acrylic coating compositions. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The acrylic polymers of the present invention can be prepared by polymerizing a mixture of the monomers in the presence of a suitable catalyst or mixture of catalysts and in the presence of a suitable solvent or mixture of solvents. The temperature can be from about 70° C. to about 165° C., preferably from about 125° C. to about 150° C. 
     For the purposes of this application, the terms &#34;aromatic&#34; and &#34;aryl&#34; shall refer to &#34;the six-carbon ring characteristic of the molecules of all organic compounds of benzene and related series, or the condensed six-carbon rings of naphthalene, anthracene, phenanthrene, etc.&#34; as defined by the CONDENSED CHEMICAL DICTIONARY, Reinhold Publishing Corp., page 108 for &#34;aromatic nucleus&#34; and page 112 for &#34;aryl&#34;. 
     Suitable polymerizable monoethylenically unsaturated monocarboxylic acids which can be employed herein include, for example, acrylic acid, methacrylic acid, crotonic acid, vinyl benzoic acid, mixtures thereof and the like. 
     Suitable polymerizable alkyl or aryl esters of monounsaturated monocarboxylic acids which can be employed herein include, for example, ethyl acrylate, ethyl methacrylate, propyl acrylate, propyl methacrylate, butyl acrylate, butyl methacrylate, amyl acrylate, amyl methacrylate, hexyl acrylate, hexyl methacrylate, 2-ethylhexyl acrylate, 2-ethylhexyl methacrylate, mixtures thereof and the like. 
     Suitable polymerizable monoethylenically unsaturated esters of saturated aliphatic or aromatic monocarboxylic acids which can be employed herein include, for example, vinyl acetate, vinyl propionate, vinyl benzoate, allyl acetate, allyl propionate, allyl benzoate, mixtures thereof and the like. 
     Suitable hydroxyalkyl esters of monoethylenically unsaturated monocarboxylic acids which can be employed herein include, for example, hydroxyethyl acrylate, hydroxypropyl acrylate, hydroxyethyl methacrylate, hydroxypropyl methacrylate, 2-hydroxybutyl acrylate, 4-hydroxybutyl acrylate, 2-hydroxybutyl methacrylate, mixtures thereof and the like. 
     Suitable polymerizable alkyl or aryl esters of monoethylenically unsaturated dicarboxylic acids which can be employed herein include, for example, diethyl maleate, dibutyl maleate, diethyl fumarate, dibutyl fumarate, di(2-ethylhexyl)maleate, di(2-ethylhexyl)fumarate, mixtures thereof and the like. 
     Suitable polymerizable monoethylenically unsaturated aromatic monomers which can be employed herein include, for example, styrene, α-methyl styrene, t-butyl styrene, chlorostyrene, vinyl toluene, mixtures thereof and the like. 
     Suitable solvents which can be employed herein include, for example, aliphatic esters, alcohols, ketones, aromatic hydrocarbons, C 8  to C 12  aliphatic or cycloaliphatic hydrocarbons, hydroxyethers, glycol ethers, alkoxycarboxylic esters, mixtures thereof and the like. Particularly suitable solvents include, for example, ethyl acetate, propyl acetate, butyl acetate, amyl acetate, octyl acetate, toluene, xylene, methyl ethyl ketone, methyl propyl ketone, methyl butyl ketone, methyl amyl ketone, 1-methoxypropyl-2-acetate, n-butanol, 1-methoxypropyl-2-propionate, 1-methoxypropyl-2-butyrate, 1-methoxy-2-propanol, 2-methoxy-1-propanol, 2-butyoxyethanol, 2-ethoxyethanol, octane, nonane, decane, undecane, dodecane, mixtures thereof and the like. 
     Suitable polymerization catalysts or initiators which can be employed herein include, for example, organic peroxide and organic azo compounds such as, for example, dicumyl peroxide, t-butyl peracetate, benzoyl peroxide, di-t-butyl peroxide, tertbutylazo(isovaleronitrile) ethyl 3,3-di(t-amylperoxy)butyrate, 2,4-pentanedione peroxide, 1,1&#39;-azobis(cyclohexane carbonitrile), mixtures thereof and the like. 
     Suitable curing agents which can be employed herein include, for example, melamine-formaldehyde resins, particularly the methylated form, hexamethoxymethyl melamine, or the mixed methylated/butylated forms; urea-formaldehyde resins; or poly(isocyanate) compounds, such as isophorone diisocyanate as the free isocyanate or in the blocked form, mixtures thereof and the like. Particularly suitable curing agents include, for example, hexamethoxymethyl melamine. 
     The coatings can be cured by heating at temperatures of from about 70° C. to about 150° C., preferably from about 100° C. to about 125° C. in the presence of a suitable catalyst. 
     The coating compositions of the present invention can be applied to any suitable substrate which can withstand the curing conditions. They are particularly suitable for coating metals, previously coated metals, composite structures such as polyester-fiber glass, epoxy resin-fiber glass, extruded plastic objects having softening points above the bake temperatures and the like. 
     The coating compositions of the present invention can also contain, if desired, pigments, fillers, dyes, flow and leveling agents, flatting agents, curing catalysts, and the like in any combination. 
    
    
     The following examples are illustrative of the invention but are not to be construed as to limiting the scope thereof in any manner. 
     EXAMPLE 1 
     A. Preparation of Polymer 
     In a conventional glass polymerization vessel was charged 257 g of 2-ethoxyethyl acetate solvent. After heating to reflux temperature (145° C.) with stirring, a mixture consisting of 87.6 g of styrene, 87.6 g of methyl methacrylate, 265 g of butyl acrylate, 142.5 g (1 hydroxy equivalent) of hydroxybutyl acrylate, 29.4 g of acrylic acid and 18 g of tertbutylazo(isovaleronitrile) was added dropwise over a period of 6.75 hours (24,300 s) at 140° C. to 150° C. The mixture was maintained at 150° C. for 5.25 hours (18,900 s) more and then cooled and stored in a suitable container. 
     The above procedure was repeated substituting one at a time the following hydroxyalkyl acrylate or methacrylate monomers for the hydroxybutyl acrylate. 
     116.4 g (1 hydroxyl equiv.) of 2-hydroxyethyl acrylate 
     130.5 g (1 hydroxy equiv.) of 2-hydroxypropyl acrylate 
     130.5 g (1 hydroxy equiv.) of 2-hydroxyethyl methacrylate 
     142.5 g (1 hydroxy equiv.) of 2-hydroxypropyl methacrylate 
     The properties of the resulting copolymers are given in the following Table I. 
     
                       TABLE I                                                     
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Hydroxy                                                                   
Alkyl   Appear-   Viscosity                                               
                           Non-Vol.                                       
                                  Molecular Wt.                           
Monomer ance      cp/Pa · s                                      
                           %      Mw.sup.6                                
                                        Mn.sup.7                          
______________________________________                                    
2-HEA.sup.1 *                                                             
        clear,    5080/5.08                                               
                           70.0   9800  2400                              
        yellow                                                            
2-HPA.sup.2 *                                                             
        clear,    3560/3.56                                               
                           69.2   6100  2200                              
        yellow                                                            
2-HBA.sup.3                                                               
        clear,    2860/2.86                                               
                           69.1   5700  2200                              
        yellow                                                            
2-HEMA.sup.4 *                                                            
        faintly   5050/5.05                                               
                           70.1   5630  2180                              
        turbid                                                            
2-HPMA.sup.5 *                                                            
        faintly   5050/5.05                                               
                           69.8   3880  1540                              
        turbid                                                            
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 *Not an example of the present                                           
 .sup.1 2hydroxyethyl acrylate                                            
 .sup.2 2hydroxypropyl acrylate                                           
 .sup.3 2hydroxybutyl acrylate                                            
 .sup.4 2hydroxyethyl methacrylate                                        
 .sup.5 2hydroxypropyl methacrylate                                       
 .sup.6 Mw = weight average molecular                                     
 .sup.7 Mn = number average molecular weight                              
 
    
     B. Preparation of White Pigmented Baking Enamel 
     The copolymer prepared in A above was formulated into a white baking enamel by blending in a one pint (237 ml) paint can be means of a high-shear mechanical disperser, known as a Cowels Dissolver, the following components. 
     
         ______________________________________                                    
ENAMEL RECIPE                                                             
______________________________________                                    
Pigment Grind                                                             
copolymer solution from A above                                           
                        146 g                                             
Cymel ® 325*, curing agent                                            
                        42.6 g                                            
n-butanol, solvent      32.1 g                                            
methyl amyl ketone, solvent                                               
                        17.4 g                                            
Dow-Corning Paint additive #57                                            
                        0.16 g                                            
20% p-toluenesulfonic acid in i-propanol                                  
                        0.11 g                                            
TiPure R-902 TiO.sub.2, pigment                                           
                        111 g                                             
______________________________________                                    
 *Cymel ® 325 is hexamethoxy methyl melamine commercially available   
 from American Cyanamid Co.                                               
Diluent                                                                   
methyl amyl ketone to provide the coating                                 
                   with a viscosity of 120                                
                   cp (0.12 Pa · s)                              
 
    
     C. Coating Substrate and Baking Enamel 
     The five enamels prepared as above were air-sprayed onto primed 24-gauge (0.7 mm) Bonderite 40 panels to give dry-film thicknesses of 1.5-2.0 mils (0.0381-0.0508 mm) after baking 20 minutes (1200 s) at 150° C. These coated panels were then exposed outdoors on a rack facing south and inclined 45° to the vertical, in Freeport, Tex. The panels were tested after twenty months&#39; (51,840,000 s) exposure for gloss retention and after thirty months&#39; (77,760,000 s) exposure for gloss retention and chalking. The results are given in the following Tables II and III. The thirty month (77,760,000 s) evaluation was more detailed than the twenty month (51,840,000 s) evaluation. The gloss measurements were taken as is, after rinsing with deionized water, and after washing with deionized water and a soft cotton cloth. Gloss measurements were made at both 60° and 20° C. The coatings were tested for chalking before rinsing and the forward impact resistance was measured for comparison to the original values. 
     
                       TABLE II                                                    
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TWENTY MONTH EXPOSURE                                                     
Hydroxy                                                                   
Alkyl     60° Gloss Gloss Retention                                
Monomer   Initial   After 20 Mos.                                         
                               % of Original                              
______________________________________                                    
2-HEA.sup.1 *                                                             
          91        41         45                                         
2-HPA.sup.2 *                                                             
          93        38         41                                         
2-HBA.sup.3                                                               
          91        63         69                                         
2-HEMA.sup.4 *                                                            
          92        61         66                                         
2-HPMA.sup.5 *                                                            
          92        58         63                                         
______________________________________                                    
 *Not an example of the present                                           
 .sup.1 2hydroxyethyl acrylate                                            
 .sup.2 2hydroxypropyl acrylate                                           
 .sup.3 2hydroxybutyl acrylate                                            
 .sup.4 2hydroxyethyl methacrylate                                        
 .sup.5 2hydroxypropyl methacrylate                                       
 
    
     
                                           TABLE III                               
__________________________________________________________________________
THIRTY MONTH EXPOSURE                                                     
Property  2-HEA.sup.1 *                                                   
                2-HPA.sup.2 *                                             
                     2-HBA.sup.3                                          
                           2-HEMA.sup.4 *                                 
                                 2-HPMA.sup.5                             
__________________________________________________________________________
Original 60°                                                       
          88    92   94    92    92                                       
Gloss, %                                                                  
Gloss Retention, %                                                        
60°                                                                
Unrinsed  20    20   31    31    22                                       
Rinsed    20    20   33    34    29                                       
Washed    19    15   24    29    26                                       
20°                                                                
Rinsed     3     4    6    12    10                                       
Washed     2     3    3     9     7                                       
Chalking  slight                                                          
                slight                                                    
                     slight                                               
                           none  very slight                              
Forward Impact                                                            
Resistance                                                                
Original, in-lb/J                                                         
          16/1.81                                                         
                12/1.36                                                   
                     14/1.58                                              
                           16/1.81                                        
Final, in-lb/J                                                            
           6/0.68                                                         
                 7/0.79                                                   
                      5/0.56                                              
                            8/0.90                                        
                                  7/0.79                                  
__________________________________________________________________________
 *Not an example of the present                                           
 .sup.1 2hydroxyethyl acrylate                                            
 .sup.2 2hydroxypropyl acrylate                                           
 .sup.3 2hydroxybutyl acrylate                                            
 .sup.4 2hydroxyethyl methacrylate                                        
 .sup.5 2hydroxypropyl methacrylate