Curable coating composition
BACKGROUND OF THE INVENTION 
     This invention relates to reduced and ambient temperature curable paint compositions containing amino cross-linking agents and particularly to such compositions containing fully alkylated, low imino aminoplast cross-linking agents. 
     Changing market preferences and increasing manufacturing concerns have inspired the automotive refinish industry to seek alternative coatings curing chemistries to isocyanate based coatings for reduced and ambient temperature curing systems. 
     Thermoplastic coating systems are well established for reduced and ambient temperature applications. However these coatings, typically high molecular weight polymers which form films through solvent evaporation, usually require additional processing to achieve the desired appearance, for example, in topcoats, and/or lack the desired physical properties, such as chemical resistance or exterior durability. In comparison, thermosetting coating systems, typically lower molecular weight polymers that build molecular weight during film formation through cross-linking of polymer chains, generally require less processing after application and are superior in physical properties relative to thermoplastic coating systems. 
     A number of isocyanate-free, reduced and ambient temperature curing, thermosetting coating systems are well established, however, these systems are usually lacking in some aspect of coatings performance, such as: speed of coating cure at reduced temperatures; humidity of chemical resistance; initial color or color change upon exterior exposure, a property important in topcoats; substrate adhesion; etc. 
     This invention relates to a curable coating compositions containing a film forming polymer cure with an aminoplast cross-linking agent and particularly to such compositions containing a fully alkylated aminoplast of a low imino content. 
     Often, known coating systems based on conventional thermosetting aminoplast resins require elevated temperatures of at least 82 C. for adequate curing. It would be desirable to provide a coating method in which relatively low temperatures, for example, below about 71 C. and preferably ambient temperatures, could be utilized. Previous attempts to develop such coating systems resulted in systems which had the disadvantages of being too time consuming and/or energy intensive or resulted in cured films which were deficient in various combinations of physical properties. 
     In accordance with the present invention, a coating composition and coating system have been developed which provides a superior rate of cure at reduced temperatures and results in coated products in which the films exhibit an excellent combination of properties for an intended application. 
     SUMMARY OF THE PRESENT INVENTION 
     The present invention is therefore directed to a reduced and ambient temperature cure coating composition containing a functional film forming resin and a fully alkylated, low imino aminoplast cross-linking agent. More particularly, the invention comprises a coating composition of: 
     a. a film forming resin(s) with pendant reactive functional groups, generally hydroxyl. 
     b. a substantially fully alkylated and low imino aminoplast as indicated by a lower Fourier transform infra-red spectroscopic absorption value in the region of 3250-3650 cm -1  relative to a more conventional aminoplast such as Cymel 303 (American Cyanamid Corp.) (refer to FIG. 1) with comprising nuclei selected from the group consisting of melamine, acetoguanamine, adipoguanamine, and benzoguanamine and attached thereto alkyloxymethyl groups in the range of about 2n-2 to about 2n per nucleus, where n is the number of amino groups per nucleus. 
     Said curable coating composition is especially useful in, but not restricted to, automotive refinish applications. 
    
    
     DESCRIPTION OF THE DRAWING 
     FIG. 1 shows infrared spectral data comparing a conventional melamine resin with several (A--C) low imino resins of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The reduced temperature coating composition of the present invention contains an essential ingredient a fully alkylated, low imino aminoplast cross-linking agent. More specifically, the invention comprises a coating composition of: 
     a. a principle film forming resin(s) with pendant functional groups which are reactive with an aminoplast resin and, 
     b. a substantially fully alkylated, low imino aminoplast as indicated by a lower Fourier transform infra-red spectroscopic absorption value in the region of 3250-3650 cm -1  relative to a more conventional aminoplast such as Cymel 303 (American Cyanamid Corp.) (refer to FIG. 1) with comprising nuclei selected from the group consisting of melamine, acetoguanamine, adipoguanamine, or benzoguanamine and attached thereto alkyloxymethyl groups in the range of about 2n-2 to 2n per nucleus where n is the number of amino groups per nucleus. 
     The principle film forming resin or resins used in the present invention may be any type of available thermosetting resins with hydroxy, carboxyl, amide, acetoacetoxy, or mercaptan functional groups or a combination thereof. The preferred functional group is hydroxyl. Suitable resin types include acrylic, polyester (including alkyd), epoxy, polyurethane, etc., or hybrids thereof. 
     The term acrylic refers in the present invention to the resins typically prepared by the polymerization of ethylenically unsaturated monomers. Suitable ethylenically unsaturated monomers include acrylic and methacrylic acid and their alkyl esters such as methyl, ethyl, propyl, butyl, 2-ethyl hexyl, decyl, lauryl, stearyl, isobornyl, and the functional esters such as hydroxyethyl, hydroxypropyl, hydroxybutyl, acrylamide, acetoacetoxyethyl, glycidyl, diacrylates such as 1,4 butanedioldiacrylate, ethylene glycol dimethacrylate, 1,6 hexanediol diacrylate, ethylenically unsaturated aromatic hydrocarbons such as styrene, alpha methyl styrene, vinyl toluene, vinyl chloride, acrylonitrile, butadiene, divinylbenzene, and others such as maleic acid or anhydride, fumaric acid, allyl alcohol, crotonic acid, etc. 
     The term epoxy refers in the present invention to those resins constructed through an oxirane functional group reacted with a phenolic or carboxylic acid group. Typically, these resins are a result of stoichiometric blend of epichlorhydrin with phenolics such as bisphenol A or novalac resins, and derivatives thereof, or oxirane functional resins, such as acrylic resins prepared with glycidyl acrylate or methacrylate, and derivatives thereof. 
     The term polyester refers in the present invention to those types generally used in baking, and ambient cure applications and includes alkyd resins. Typically, these resins are a result of a stoichiometric blend of a polyhydric alcohol with a polybasic acid. Often, monohydric alcohols or monobasic acids may be blended into the polyester for molecular weight control. These resins may or may not be modified with a saturated or unsaturated fatty acids. Examples of polyhydric alcohols include ethylene glycol, propylene glycol, neopentyl glycol, cyclohexyl dimethanol, diethylene glycol, dipropylene glycol, hydrogenated bisphenol A, 1,6 hexanediol, glycerol, trimethylol propane, pentaerythritol, sorbitol, etc. Examples of polybasic acids include maleic acid, fumaric acid, succinic acid, glutaric acid, adipic acid, phthalic acid, isophthalic acid, tetrahydrophthalic acid, itaconic acid, trimellitic acid, and the anhydrides of the above where they exist. Examples of fatty acids include those derived from the oils of linseed, soya, castor, coconut, tall, safflower, etc. Optionally, any of the aforementioned oils may be &#34;broken&#34; or prereacted with any of the aforementioned polyols to be later incorporated into the polyester resin. 
     The term polyurethane refers in the present invention to those types generally used in baking and ambient cure applications. Typically, these resins are a result of a stoichiometric blend of polyhydric alcohol with a polyisocyanate. Often, monohydric alcohols or monoisocyanates may be blended to control the molecular weight. Examples of polyhydric alcohols include those previously listed. Examples of polyisocyanates include toluene diisocyanate, diphenylmethane diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate, dicyclohexyl methane diisocyanate, biurets of and isocyanurates of the aforementioned where they exist, etc. 
     The aminoplast resin is comprised of a nucleus or nuclei of acetoguanamine, benzoguanamine, adipoguanamine, or melamine, with the preferred being melamine. The aminoplast resin is considered to be fully alkylated, or substantially completely methylolated and subsequently substantially fully etherified with alcohol, with the number of alkyloxymethyl groups ranging from 2n-2 to 2n where n is the number of amino groups on the triazine ring. The preferred degree of polymerization of this aminoplast is from 1 to 3. The important aspect of this aminoplast is that besides the high degree of alkylation, it has a reduced number of imino groups per nucleus relative to other commercially available aminoplast cross-linking agents typically used in higher temperature curable compositions such as Cymel 303 (American Cyanamid Corp.). This high degree of alkylation and low imino content is indicated by a lower Fourier transform infra-red spectroscopic absorption value in the region of 3250-3650 cm -1  relative to a more conventional aminoplast such as Cymel 303 (American Cyanamid Corp.) (refer to FIG. 1). It is believed that this high degree of alkylation together with a lower imino content is responsible for the observed reactivity at reduced temperatures. This reactivity is manifested by the superior film properties of the formulated coating at reduced temperatures. 
     The term imino refers to those substituted amine groups radiating from the triazine nucleus. The preparation of an amino resin involves the methylolation and subsequent alcohol etherification of these same imino groups. The imino groups are those which are not completely reacted and are schematically represented as: ##STR1## where R=a carbon from the triazine nucleus. 
     R&#39;=an alkyloxymethyl group such as: 
     -CH2-O-CH3, 
     -CH2-O-C4H5, 
     -CH2-O-C8H17, etc. 
     The term reduced temperatures refers to in the present invention at or below 71 degrees C. and preferably ambient temperatures. 
     The preferred aminoplast resin is supplied by Monsanto Corporation under the designation Resimene RF-4518 (refer to FIG. 1). It is reported to be a fully alkylated melamine with a mixture of methyloxy and 2-ethylhexyloxy ethers. Similar fully alkylated, low imino aminoplasts with other ethers, such as butyloxy and isobutyloxy, have exhibited similar properties and can be used according to this invention. 
     The components of the coating composition may be combined in various amounts to provide a curable composition. A curable coating is one which forms a cross-linked film that achieves certain acceptable physical and appearance requirements for an intended application. The amount of aminoplast resin is selected to provide a sufficient concentration of alkoxymethyl groups to provide an adequate degree of cross-linking by reaction with the functional groups on the principle film forming resin. Advantageously, the concentration of alkoxymethyl groups is in the range of about 1 to about 6 per functional group on the principle film forming resin. Coating viscosity and non-volatile requirements depend on the application. 
     The coating composition of the invention may be colored with a pigment usually employed for coloring of such coating compositions such as organic, inorganic, aluminium flake, mica flake, etc. 
     The coating composition of the invention may also have incorporated therein other additives such as wetting agents, conditioning agents, rheology control agents, ultra violet light stabilizers, plasticizers, antioxidants, fungicides, etc. 
     In addition to the foregoing components, the coating compositions employed may contain lower molecular weight reactive diluents. These reactive diluents are generally added to improve non-volatile content. Examples of suitable reactive diluents are the aforementioned polyhydric alcohols and other functional monomeric or oligomeric materials. 
     In order to achieve sufficient cure at lower temperatures in a reasonable length of time, it may be necessary to include a catalyst in the coating composition. Strong acid catalysts are generally preferred. Examples of suitable catalysts are para toluene sulfonic acid, methane sulfonic acid, butyl acid phosphate, phenyl acid phosphate, hydrochloric acid, dodecylbenzene sulfonic acid, dinonylnaphthalene sulfonic acid along with other organic and mineral acids having at least one active hydrogen per molecule. Metal salts, such as zinc nitrate, also have exhibited some catalytic effect. 
     The application of the coating composition of the invention may be executed by a conventional method. That is, the composition may be applied by brushing, roller coating, calendering, dip coating, flow coated, air atomized, cetrifugally atomized, electrostatically applied, etc. 
     The coating composition of the present invention may be applied to a variety of materials such as glass, metal, stone, wood, plastics, and previously coated materials. 
     The coating composition of the present invention may be at 100% non-volatile content, solvent reducible, or water reducible. In general practice, resins are made water reducible by salting pendant carboxylic acid or amine groups on the polymer chain with a volatile amine or acid, respectively. 
     EXAMPLES 
     The following examples are intended to illustrate the invention: 
     
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Example #1: Silver Topcoat                                                
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A silver topcoat was prepared as follows:                                 
Component A:         Parts by volume                                      
MS-6 solvent blend   100.0                                                
MB-081 aluminium mill base                                                
                     600.0                                                
MB-952 titanium dioxide mill base                                         
                      30.0                                                
MB-431 carbon black mill base                                             
                      5.0                                                 
MB-055 clear base    265.0                                                
Total                1000.0                                               
All of the above mentioned bases and solvent blends are sold              
commercially as the &#34;Miracryl-II&#34; line by BASF                            
Corporation, Inmont division.                                             
Component B:                                                              
p-toluene sulfonic acid solution                                          
(40% n.v. in 2-propanol)                                                  
Component C:         Parts by weight                                      
RF-4518               10.41                                               
MS-6 solvent blend    35.26                                               
Total                 45.67                                               
Component C was added to the mixture of Component B and                   
Component A, according to the following ratio:                            
                     Parts by weight                                      
Component A          100.00                                               
Component B           3.17                                                
Component C           45.67                                               
Total                148.84                                               
Blended well, and applied via air atomization over primed                 
cold rolled steel. The coating was allowed to cure at                     
ambient temperature overnight. The dry film thickness was                 
approximately 2.5 mils. The cured film exhibited good                     
substrate adhesion, chemical and water resistance, and                    
hardness.                                                                 
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Example #2: Primer Surfacer                                               
                   Parts by weight                                        
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A primer surfacer was prepared according to the following:                
Mill base:                                                                
Polyester resin solution                                                  
                     19.02                                                
(90% n.v. in propylene glycol-                                            
monoether acetate with a hydroxyl                                         
number of 175)                                                            
Nitrocellulose solution                                                   
                     20.35                                                
(23.5% n.v. in a complex mixture of                                       
alcohols, esters, and ketones)                                            
toluene              3.30                                                 
isobutyl acetate     7.79                                                 
butyl acetate        13.36                                                
methyl ethyl ketone  4.98                                                 
montmorillonite clay 0.82                                                 
anti-settling agent  0.58                                                 
fumed silica         1.07                                                 
magnesium silicate   24.24                                                
carbon black         2.90                                                 
titanium dioxide     0.20                                                 
red iron oxide       1.23                                                 
citric acid          0.16                                                 
Total                100.00                                               
The constituents were charged to a mill and dispersed until               
an average particle size of 35 microns was obtained                       
Component A:                                                              
Mill base            69.62                                                
p-toluene sulfonic acid                                                   
                     2.07                                                 
(40% solution in 2-propanol)                                              
Total                71.69                                                
Component B:                                                              
RF-4518              10.90                                                
butyl acetate        17.41                                                
Total                28.31                                                
Component B was added to Component A according to the                     
following ratio:                                                          
Component A          71.69                                                
Component B          28.31                                                
Total                100.00                                               
Blended well, applied by air atomization over abraded cold                
rolled steel and cured at ambient temperature overnight. The              
dry film thickness was approximately 2.8 mils. The cured                  
film exhibited good sandability, and adhesion to the                      
substrate and as a substrate.                                             
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Example #3: Clear Acrylic Coating                                         
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An acrylic clearcoat was prepared according to the following              
formula:                                                                  
Component A:           Parts by weight                                    
acrylic resin solution 40.30                                              
(55% n.v. in high flash aromatic                                          
naphtha 100 and xylene with a hydroxyl                                    
number of 128)                                                            
U.V. light stabilizer  1.20                                               
ethanol                19.80                                              
methyl isobutyl ketone 21.00                                              
butyl acetate          2.60                                               
p-toluene sulfonic acid                                                   
                       0.40                                               
(40% n.v. in 2-propanol)                                                  
Total                  85.30                                              
Component B:                                                              
RF-4518                                                                   
Component B was added to Component A in according to the                  
following ratio:                                                          
                       Parts by weight                                    
Component A            85.30                                              
Component B            12.60                                              
Total                  100.00                                             
blended well, and applied via air atomization over a dry                  
lacquer color basecoat. The film was then allowed to cure at              
ambient temperature overnight. The cured film was tested                  
for Konig hardness and water resistance by allowing a spot                
of water on the film to evaporate while in a 120 F. oven. The             
one week aged film was tested for humidity resistance by                  
placing the film in a cabinet with 100% humidity at 100 F. for            
one week. The following results were obtained:                            
Gloss (20)        90                                                      
Konig Hardness (sec)                                                      
                  14.2                                                    
Water Resistance  good                                                    
Humidity Resistance                                                       
                  excellent                                               
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Example #4: Polyester Clearcoat                                           
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A polyester clearcoat was prepared according to the                       
following                                                                 
Component A:         Parts by weight                                      
polyester resin solution                                                  
                     31.90                                                
(60% n.v. in xylene with a                                                
hydroxyl number of 115)                                                   
cellulose acetate butyrate solution.                                      
                     25.40                                                
(30% n.v. in butyl acetate)                                               
ethanol              10.50                                                
methyl isobutyl ketone                                                    
                     8.90                                                 
2-propanol           3.30                                                 
methyl ethyl ketone  4.90                                                 
Exxate 600 solvent   7.00                                                 
p-Toluene sulfonic acid                                                   
                     0.70                                                 
Total                92.60                                                
Component B:                                                              
RF-4518                                                                   
Component B was added to Component A in the following ratio:              
                     Parts by weight                                      
Component A          92.60                                                
Component B          7.40                                                 
Total                100.00                                               
Blended well and applied via air atomization over a dry                   
lacquer color basecoat. The film was allowed to cure at                   
room temperature overnight. The dry film thickness was                    
approximately 2.0 mils. The film was tested for Konig                     
hardness and water resistance by allowing a spot of water on              
the film to evaporate while in a 120 F. oven. The one week                
aged film was tested for humidity resistance by placing the               
film in a cabinet with 100% humidity at 100 F. for one week.              
The following test results were obtained:                                 
Gloss (20)       76                                                       
Konig Hardness (sec)                                                      
                 12.8                                                     
Water Resistance excel.                                                   
Humidity Resistance                                                       
                 excel.                                                   
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Example #5: Polyurethane Clearcoat                                        
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A polyurethane clearcoat was prepared as follows:                         
Part A                 Parts by weight                                    
Polyurethane resin solution                                               
                       40.40                                              
(64% n.v. in xylene and methyl                                            
isobutyl ketone with a hydroxyl number                                    
of 107)                                                                   
ethanol                10.60                                              
methyl isobutyl ketone 15.80                                              
butyl acetate          9.00                                               
isopropanol            3.30                                               
methyl ethyl ketone    5.00                                               
Exxate 600             7.00                                               
p-toluene sulfonic acid                                                   
                       0.70                                               
Total                  91.80                                              
Part B:                                                                   
RF-4518                                                                   
Component B was added to Component A in the following ratio:              
Component A            91.80                                              
Component B            8.20                                               
Total                  100.00                                             
Blended well and applied via air atomization over a dry                   
lacquer color basecoat. The film was allowed to cure at                   
ambient temperature overnight. The dry film thickness was                 
approximately 2.0 mils. The cured film was tested for Konig               
hardness and water resistance by allowing a spot of water on              
the film to evaporate while in a 120 F. oven. The one week                
aged film was tested for humidity resistance by placing the               
film in a cabinet with 100% humidity at 100 F. for one week.              
The following results were obtained:                                      
Gloss (20)        87                                                      
Konig Hardness (sec)                                                      
                  7.1                                                     
Water Resistance  excel.                                                  
Humidity Resistance                                                       
                  excel.                                                  
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Example #6: Comparison of a fully alkylated, low imino                    
amino resin versus a conventional amino resin in a clearcoat              
(refer to FIG. 1).                                                        
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Clear Base:           Parts by weight                                     
acrylic resin solution                                                    
                      28.80                                               
(55% n.v. in xylene and high flash                                        
aromatic naphtha with a hydroxyl                                          
number of 128)                                                            
polyester resin solution                                                  
                      2.20                                                
(80% n.v. in toluene and xylene                                           
with a hydroxyl number of 200)                                            
cellulose acetate butyrate solution                                       
                      23.10                                               
(30% n.v. in butyl acetate)                                               
U.V. light stabilizer 1.00                                                
methyl isobutyl ketone                                                    
                      26.30                                               
butyl acetate         7.80                                                
p-toluene sulfonic acid                                                   
                      0.40                                                
Total                 89.60                                               
The two clearcoats were prepared by blending the following:               
Formulation A:        Parts by weight                                     
clear base            89.60                                               
RF-4518               10.40                                               
(a low imino melamine resin                                               
supplied by Monsanto Corp.)                                               
Total                 100.00                                              
Formulation B:        Parts by weight                                     
clear base            89.60                                               
Cymel 303             10.40                                               
(a conventional melamine resin                                            
supplied by American Cyanamid Corp.)                                      
Total                 100.00                                              
and applying via air atomization over a dry lacquer color                 
basecoat. The films were allowed to cure at ambient                       
temperature overnight. The dry film thicknesses were                      
approximately 2.0 mils. The cured films were tested for                   
Konig hardness and water resistance by allowing a spot of                 
water on the film to evaporate while in a 120 F. oven. The                
one week aged films were tested for humidity resistance by                
placing the films in a cabinet with 100% humidity at 100 F.               
for one week. The following results were obtained:                        
                Formulation A                                             
                             Formulation B                                
Gloss (20)      85           87                                           
Konig Hardness (sec)                                                      
                19.9         14.9                                         
Water Resistance                                                          
                good         poor-whitening                               
Humidity Resistance                                                       
                excel.       poor-dulling                                 
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Example #7: Clearcoats prepared with four fully alkylated,                
low imino amino resins (refer to FIG. 1).                                 
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The four clearcoats were prepared by blended the following:               
Clear Base:             Parts by weight                                   
acrylic resin solutin   28.80                                             
(55% n.v. in xylene and high flash                                        
aromatic naphtha with a hydroxyl                                          
number of 128)                                                            
polyester resin solution                                                  
                        2.10                                              
(80% n.v. in toluene and xylene                                           
with a hydroxyl number of 200)                                            
cellulose acetate butyrate solution                                       
                        23.50                                             
(30% n.v. in butyl acetate)                                               
U.V. light stabilizer   1.10                                              
ethanol                 15.30                                             
methyl isobutyl ketone  16.20                                             
p-toluene sulfonic acid solution                                          
                        1.80                                              
(40% n.v. in 2-propanol)                                                  
Total                   89.40                                             
Formulation A:          Parts by weight                                   
clear base              89.40                                             
fully alkylated, low imino melamine resin                                 
                        10.60                                             
(substituents composed of                                                 
n-butyloxymethyl and i-butyloxymethyl                                     
groups)                                                                   
Total                   100.00                                            
Formulation B:          Parts by weight                                   
clear base              89.40                                             
fully alkylated, low imino melamine resin                                 
                        10.60                                             
(substituents composed of                                                 
methyloxymethyl and                                                       
n-butyloxymethyl groups)                                                  
Total                   100.00                                            
Formulation C:          Parts by weight                                   
clear base              89.40                                             
fully alkylated, low imino melamine resin                                 
                        10.60                                             
(substituents composed of                                                 
methyloxymethyl and                                                       
n-butyloxymethyl groups)                                                  
Total                   100.00                                            
Formulation D:          Parts by weight                                   
clear base              89.40                                             
fully alkylated, low imino melamine resin                                 
                        10.60                                             
(substituents composed of                                                 
methyloxymethyl and                                                       
2-ethyloxymethyl groups)                                                  
Total                   100.00                                            
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and applying via air atomization over a dry lacquer color                 
basecoat. The films were allowed to cure at ambient                       
temperature overnight. The dry film thicknesses were 2.0                  
mils. The cured films were tested for Konig hardness and                  
water resistance by allowing a spot of water on the film to               
evaporate while in a 120 F. oven. The two week aged films                 
were tested for humidity resistance by placing them in a                  
cabinet with 100% humidity at 100 F. for one week. The                    
following results were obtained:                                          
Formulation:    A       B       C     D                                   
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Gloss (20)      78      87      85    85                                  
Konig Hardness (sec)                                                      
                22.7    21.3    21.3  25.6                                
Water Resistance                                                          
                excl.   excl.   good  excl.                               
Humidity Resistance                                                       
                excl.   excl.   excl. excl.                               
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