Alkylated isoadipoguanamine-formaldehyde crosslinking resin and improved coating compositions produced therefrom
This application is a division of prior U.S. application Ser. No. 176,551, filed Aug. 8, 1980. 
    
    
     BACKGROUND OF THE INVENTION 
     Conventional nitrogenous thermosetting crosslinking resins, or aminoplasts, are widely used in the coatings industry. Urea, melamine, substituted guanamine, and related triazines have each been the subject of much effort in the search for improved crosslinking resins. Such resins are formed by reacting the selected amine or amide with formaldehyde to form methylol groups which may or may not be temporarily blocked against excessive condensation by etherification with one of the lower alcohols. As a class, these resins are characterized by their ability to form hard, durable films under the influence of heat and/or suitable catalysts and are considered to be relatively rapid-curing. 
     It is often necessary, when formulating aminoplast crosslinking resins, to sacrifice one desired property in order to attain or enhance another. For example, melamine-formaldehyde resins are noted for their excellent color retention, hardness, and resistance to hydrolytic, caustic, detergent, or salt spray breakdown. Although benzoguanamine-formaldehyde resins surpass the melamine resins in imparting flexibility and chemical resistance, they are often severely deficient in exterior durability. Urea-formaldehyde resins promote superior intercoat adhesion, but their color and gloss retention are poorer than that of melamine resins. 
     As the search for new and improved aminoplast resins continues, emphasis will be placed on the overall balance of properties possessed by these resins, as well as on commercial availability and cost. 
     SUMMARY OF THE INVENTION 
     There is provided by the present invention a thermosetting crosslinking resins consisting essentially of the reaction product of: 
     (1) an isoadipoguanamine mixture consisting essentially of: 
     (a) about 87-95 percent by weight, based on the weight of the isoadipoguanamine mixture, of 2-methylglutaroguanamine, 
     (b) about 4-12 percent by weight, based on the weight of the isoadipoguanamine mixture, of 2-ethylsuccinoguanamine, and 
     (c) about 1-2 percent by weight, based on the weight of the isoadipoguanamine mixture, of adipoguanamine; 
     (2) from 3-6 moles of formaldehyde per mole of said isoadipoguanamine mixture; and 
     (3) a stoichiometric excess of an aliphatic alcohol having 1-4 carbon atoms or a mixture of such alcohols. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The alkylated isoadipoguanamine-formaldehyde crosslinking resins of the present invention, when reacted with any of a variety of other polymeric systems having complementary functional reactivity, provide useful finishes for appliances, automobiles, steel furniture, or general industrial use. These finishes cure at commercially-acceptable temperatures, and are characterized by an unexpected combination of beneficial properties heretofore unobtainable in any given crosslinking resin, viz., hardness, flexibility, resistance to chemicals and outdoor weathering, and excellent adhesion. Furthermore, the resins of the present invention are most attractive from a commercial standpoint because the starting material for the isoadipoguanamine mixture can be derived from the by-product, or waste stream, of a commercial manufacturing process. 
     This commercial process, for the manufacture of adiponitrile, employs methods well known in the art and results in the formation of a mixed dinitrile by-product containing primarily 2-methylglutaronitrile and 2-ethylsuccinonitrile. The composition of this by-product may vary, but a typical composition is 82-88 percent by weight, based on the weight of the by-product, of 2-methylglutaronitrile, 10-15 percent by weight of 2-ethylsuccinonitrile, 2-5 percent by weight of adiponitrile, and 2-5 percent by weight of cresol. These nitriles can be converted to their corresponding guanamines by reaction with dicyanidiamide, again using methods well known in the art. 
     The mixture obtained from the aforementioned reaction contains primarily 2-methylglutaroguanamine, and small amounts of 2-ethylsuccinoguanamine and adipoguanamine. Typically, the composition of the mixture, hereinafter referred to as an isoadipoguanamine mixture, is 87-95 percent by weight, based on the weight of the mixture, of 2-methylglutaroguanamine, 4-12 percent by weight of 2-ethylsuccinoguanamine, and 1-2 percent by weight of adipoguanamine. The isoadipoguanamine mixture, then, can be represented by the formula ##STR1## where R is selected from the group consisting of isobutylene, sec-butylene, and n-butylene. 
     In order to form the crosslinking resin of the present invention, this isoadipoguanamine mixture is reacted with formaldehyde to form methylol groups. These methylol groups exhibit a strong tendency to condense and form ether linkages, leading to the progressive development of linear, branched, and ring structures. It is necessary to arrest this condensation, and the concomitant loss of water solubility and increase in viscosity, by blocking the reactivity of the methylol groups by etherification with one of the lower aliphatic alcohols. 
     Ordinarily, some condensation takes place concurrently with etherification, although it is possible to control reaction conditions so as to obtain a totally alkylated crosslinker. The amount of condensation and etherification which occurs is largely pH-dependent and can be varied depending upon the properties ultimately desired in the coating composition. Methods for preparing alkylated aminoplast resins are well known in the art, and the method for preparing the particular crosslinking resin of the present invention is more fully detailed in Example 1, infra. 
     From 3 to 6, preferably 3 to 4, moles of formaldehyde per mole of isoadipoguanamine mixture, and an excess of a C 1  -C 4  aliphatic alcohol are used in the reaction. Straight-chain alcohols, particularly n-butanol and methanol, are preferred. Mixtures of C 1  -C 4  aliphatic alcohols, e.g., of methanol and n-butanol, may also be used. 
     Desired end properties of the coating composition will, to some extent, influence the type and amount of alcohol to be employed in etherification. The present invention encompasses an entire family of alkylated isoadipoguanamine-formaldehyde resins, ranging from stable water-soluble methanol ethers to stable organo-soluble higher alkyl ethers, from which an appropriate crosslinker for a given system may be selected. Any conventional hydroxyl- or carboxyl-functional film-forming material, e.g., acrylic resins, polyester or alkyd resins, epoxy resins, or mixtures thereof, can be crosslinked with these alkylated isoadipoguanamineformaldehyde resins. 
     A typical coating composition will contain about 60-80 percent, preferably about 70 percent, by weight of the hydroxyl- or carboxyl-functional film-forming material and about 20-40 percent, preferably about 30 percent, by weight of the isoadipoguanamine crosslinker. These percentages are based on the total weight of the binder constituents, i.e., of film-forming material plus crosslinker. 
     Preferred film-forming materials include hydroxyl-functional polyesters, hydroxyl- or carboxyl-functional acrylic polymers, and epoxy resins derived from epichlorohydrin and bisphenol-A. High solids coating compositions, wherein the solvent for the binder is present in an amount of no more than 100 percent by weight of the binder, can be prepared using the crosslinker of this invention and may, because of environmental considerations, become an extremely important application of this invention. 
     A highly preferred high solids polyester coating composition employs the alkylated isoadipoquanamine-formaldehyde resin to crosslink a film-forming material which consists essentially of: 
     (1) a polyester polyol that is the reaction product of 
     (a) neopentyl glycol and at least one other hindered diol containing two methylol groups wherein each methylol group is attached directly to a cycloaliphatic or aromatic structure or to a tertiary carbon atom, the molar ratio of neopentyl glycol to hindered diol being 2:1 to 6:1, and 
     (b) a mixture of an aromatic and an aliphatic dicarboxylic acid wherein the molar ratio of aromatic acid to aliphatic acid is from 2:1 to 10:1, 
     wherein the molar ratio of (a) to (b) is from 1.3:1 to 1.9:1 and wherein the polyol has a hydroxyl content of about 3.0-10.0% by weight; and 
     (2) 0-50% by weight, based on the weight of (1), of an epichlorohydrin-bisphenol-A epoxy resin, or the esterification product of said resin with a monocarboxylic acid, or mixtures of these. 
     Coating compositions containing the crosslinker of this invention may be pigmented. Useful pigments are, for example, metal oxides, metal hydroxides, sulfides, sulfates, carbonates, silica, metal flakes, and organic dyes. Ordinarily, pigments are present in the amount of about 0.1-60 percent by weight, based on the total weight of the coating composition. 
     The coating compositions may further contain, as a durability enhancer, an ultraviolet light stabilizer, an antioxidant, or both. The ultraviolet light stabilizer can be present in an amount of 1-20% by weight, based on the weight of the binder constituents; the antioxidant can be present in an amount of 0.1-5% by weight, based on the weight of the binder constituents. Typical ultraviolet light stabilizers include benzophenones, triazoles, triazines, benzotriazoles, benzoates, lower alkyl thiomethylene-containing phenols, substituted benzenes, organophosphorous sulfides, substituted methylene malonitriles, hindered amines, and nickel compounds. A combination of a hindered amine light stabilizer and 2-(o-hydroxyphenyl)benzotriazole is particularly recommended. Typical antioxidants include tetrakis alkylene (dialkylhydroxyaryl)alkyl ester alkanes, of which a favorite is tetrakis methylene-3-(3&#39;,5&#39;-dibutyl-4&#39;-hydroxyphenyl)propionate methane. 
     Coating compositions containing an isoadipoguanamine crosslinker can be applied to a variety of substrates by any of the conventional application methods such as spraying, dipping, brushing, or flow coating. Substrates that can be advantageously coated with the present composition are, for example, metal, steel, wood, glass, or plastics such as polypropylene, polystyrene, copolymers of styrene, and the like. 
     The coating composition can be cured by baking at 105°-210° C. for 10 to 60 minutes. The baking temperature will vary, depending upon, inter alia, the particular alcohol used in etherification. Ethers of higher alcohols, being more stable, increase the threshold temperature at which the methylol group becomes unblocked to exercise its potential reactivity. Thus, coating compositions containing butylated isoadipoguanamine-formaldehyde resins will require baking at a higher temperature than will their methylated counterparts. 
     It is also possible to apply coating compositions containing an isoadipoguanamine crosslinker as a two-coat system wherein a first, pigmented coat is applied to the substrate as previously described, allowed to cure to a point where it is tack-free, and then overlaid with a second, unpigmented coat. This can impart added gloss to the finish, and is especially desirable when the compositions are to be used for automotive finishes. 
    
    
     The present invention will be more fully understood from the following illustrative examples, wherein all quantities, percentages, and ratios are on a weight basis unless otherwise indicated. 
     EXAMPLE 1 
     
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Preparation of Butylated                                                  
Isoadipoguanamine-Formaldehyde Resin                                      
                      Parts by Weight                                     
______________________________________                                    
Formaldehyde solution (50% in water)                                      
                      332.3                                               
n-Butyl alcohol       597.3                                               
Caustic sodium hydroxide solution                                         
                      4.8                                                 
(4.0% in water)                                                           
Formic acid solution (4.6% in water)                                      
                      8.3                                                 
Monosodium phosphate monohydrate                                          
                      0.4                                                 
Isoadipoguanamine mixture (93%                                            
                      261.3                                               
2-methylglutaroguanamine, 6%                                              
2-ethylsuccinoguanamine, and                                              
1% adipoguanamine)                                                        
Total                 1204.4                                              
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     The formaldehyde solution is charged into a standard stainless steel direct-agitated kettle equipped with a vapor riser and parallel downflow condenser. The n-butyl alcohol and sodium hydroxide solution are added, and the batch is thoroughly mixed. The batch is then adjusted to a pH of 5.1 to 5.5 with the formic acid solution. Both the monosodium phosphate and the isoadipoguanamine mixture are added, in turn, while the mixing continues. The batch is heated to 80° C. for 1 to 4 hours, until complete solution is effected, and then is heated to reflux. The batch is held on total reflux for 30 minutes, after which time water take-off is begun. The water take-off schedule is as follows: after 1 hour, 4.37%; after 2 hours, 8.74%; after 3 hours, 13.11%; after 4 hours, 16.70%; between 41/4 and 5 hours, 17.55%. At least 17.55% of the water must be taken off, even if a longer distillation time is required. The end temperature is 112°-113° C. Next, 11.90% of n-butyl alcohol is taken off in 50 to 60 minutes. The batch is pumped out and filtered. After being cooled to 70° C. or below, the batch is thinned with n-butyl alcohol (approximately 13.9 parts) to 66% solids. 
     The resin thus prepared can be crosslinked with various other polymeric systems having complementary functional reactivity to form hard, durable, chemical-resistant finishes. 
     EXAMPLE 2 
     
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Preparation of Alkyd Resin Coating                                        
Composition with Butylated                                                
Isoadipoguanamine-Formaldehyde Resin                                      
                       Parts by Weight                                    
______________________________________                                    
Portion 1                                                                 
Dehydrated castor oil alkyd resin                                         
                       27.7                                               
(56% solids solution of dehydrated                                        
castor oil, glycerol phthalate, and                                       
glycerine in a solvent of V.M.&amp;P.                                         
naphtha, anhydrous alcohol, and                                           
toluene)                                                                  
Wax dispersion (12% low molecular                                         
                       70.4                                               
weight polyethylene in solution of                                        
dehydrated castor oil alkyd in                                            
butanol and hydrocarbons)                                                 
Silicone solution (2% solution of                                         
                       3.0                                                
linear and cyclic methyl siloxanes                                        
in xylol)                                                                 
Inhibitor solution (10% solution of                                       
                       4.0                                                
mixture of guaiacol and o-substituted                                     
phenols in aliphatic hydrocarbons)                                        
Butylated isoadipoguanamine-formaldehyde                                  
                       131.3                                              
resin (as prepared in Example 1)                                          
Wax dispersion (Candelilla wax in solu-                                   
                       10.1                                               
tion of butylated urea-formaldehyde                                       
resin in excess butanol)                                                  
Flatting dispersion (34% diatomatious                                     
                       272.6                                              
earth dispersion in solution of soy-                                      
bean oil alkyd in hydrocarbons)                                           
Portion 2                                                                 
Pigment dispersion (6% high-color                                         
                       264.7                                              
furnace-black alkyd resin dispersion                                      
in butylated urea-formaldehyde resin                                      
solution in butanol and hydrocarbons)                                     
Portion 3                                                                 
Epoxy resin solution (liquid condensa-                                    
                       41.1                                               
tion polymer of epichlorohydrin and                                       
bisphenol-A and 2,2&#39;-bis(n-hydroxy-                                       
phenyl)propane in a solvent of toluene                                    
and n-butyl alcohol)                                                      
Portion 4                                                                 
Hydrocarbon solvent (slow evaporation                                     
high solvency hydrocarbon with a                                          
distillation range of 182-219°)                                    
                       10.1                                               
Total                  835.0                                              
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     The components of Portion 1 are added in order, with mixing, to a stainless steel vessel and mixed for 15 minutes. Portion 2 is added and the mixing is continued for 10 more minutes, after which Portion 3 is added, followed by another 10 minutes of mixing. Finally, Portion 4 is mixed in, and the solution is adjusted, if necessary, to the desired viscosity. 
     The resultant coating composition is sprayed onto &#34;Bonderite 1000&#34; panels, cold rolled steel panels which have been treated with iron phosphate. The coated panels are baked for 10 minutes at 350° F. 
     The coated panels thus prepared are compared to panels coated with compositions containing either a butylated melamine-formaldehyde crosslinker or a butylated benzoguanamine-formaldehyde crosslinker. The melamine and benzoguanamine crosslinking resins are prepared in the same manner as the isoadipoguanamine crosslinker, and are then substituted for the isoadipoguanamine crosslinker in the alkyd resin of this example. Standard bump and bend tests are conducted, and reveal the coatings containing the isoadipoguanamine crosslinker to be slightly superior to those containing melamine or benzoguanamine crosslinkers. The methyl ethyl ketone rub test shows that the isoadipoguanamine-containing coating cures more rapidly than the benzoguanamine-containing coating, and at least as rapidly as the melamine-containing coating. The gloss and color retention of the isoadipoguanamine panel, upon overbake, is equivalent to that of the melamine panel, and better than that of the benzoguanamine panel. In addition, the isoadipoguanamine panel exhibits excellent hardness, water resistance, solvent resistance, and scratch and mar resistance. 
     It should also be possible to produce high-quality finishes that will comply with proposed solvent emission regulations by incorporating the crosslinking resin of the present invention into coating compositions that can be applied at relatively high solids levels, as illustrated by the following example. 
     EXAMPLE 3 
     
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Preparation of High-Solids Polyester                                      
Coating Composition with Butylated                                        
Isoadipoguanamine-Formaldehyde                                            
Crosslinker                                                               
      Preparation of Polyester                                            
I.    Polyol Solution     Parts by Weight                                 
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Portion 1                                                                 
Monoester of neopentyl glycol                                             
                      65.3                                                
and hydroxypivalic acid                                                   
Neopentyl glycol      133.1                                               
Phthalic anhydride    59.2                                                
Isophthalic acid      66.4                                                
Adipic acid           29.2                                                
Dibutyl tin oxide     0.3                                                 
Xylene                3.2                                                 
Portion 2                                                                 
Xylene                7.8                                                 
Portion 3                                                                 
2-Ethyl hexanol       20.3                                                
Total                 384.8                                               
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     Charge Portion 1 into a reaction vessel equipped with an agitator and vapor condenser, and heat to reflux, approximately 210° C. Maintain this temperature until the reaction is completed. Monitor the flow of the water of esterification from the condenser and conduct intermittent sampling to determine when the acid number reaches 5, signifying completion of the reaction. Add Portion 2, allow the mixture to cool to about 125° C., and add Portion 3. Agitate and filter the mixture. 
     
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II.   Preparation of Mill Base                                            
                          Parts by Weight                                 
______________________________________                                    
Polyester polyol solution                                                 
                      34.5                                                
(prepared above)                                                          
Amyl acetate          18.0                                                
Pigment dispersion (copolymer of                                          
                      3.0                                                 
methyl methacrylate and 2-ethyl-                                          
hexyl acrylate in a solvent of                                            
toluene, methyl isobutyl ketone,                                          
and methyl ethyl ketone)                                                  
TiO.sub.2 white pigment                                                   
                      100.0                                               
Total                 155.5                                               
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     Combine the above constituents in a mixing vessel and mix for about 1 hour. Then charge the mixture into a sand mill and grind at a temperature of about 35° C. 
     
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       Preparation of the                                                 
III.   Coating Composition                                                
                          Parts by Weight                                 
______________________________________                                    
Portion 1                                                                 
Polyester polyol solution                                                 
                      40.2                                                
(prepared above)                                                          
Epichlorohydrin-bisphenol-A                                               
                      8.3                                                 
(60% solution in methyl ethyl                                             
ketone and xylene)                                                        
Mill base (prepared above)                                                
                      155.5                                               
Ethyl acetate         8.2                                                 
Portion 2                                                                 
Dinonylnaphthalene disulfonic                                             
                      2.0                                                 
acid (40% in isobutanol)                                                  
Portion 3                                                                 
Butylated isoadipoguanamine-                                              
formaldehyde resin (as                                                    
prepared in Example 1)                                                    
                      30.0                                                
Total                 244.2                                               
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     Charge Portion 1 into a stainless steel vessel and mix for 15 minutes. Add Portion 2 and continue the mixing for an additional 5 minutes. Then add and thoroughly mix in Portion 3. 
     Spray the coating composition thus prepared onto the desired substrate and bake for 30 minutes at 135° C.