Curable powder coating composition comprising a mixture of an alkoxylated polyamide-aldehyde resin and a polyester resin
This is a continuation of prior copending application Ser. No. 419,463, filed Nov. 27, 1973, which is now abandoned. .Iaddend. 
    
    
     This invention relates to a curable coating composition which is suitable for application by a fusion process and is particularly concerned with homogeneous, pulverulent coating compositions suitable for application in powder spray applications and fluidized bed-coating processes. 
     Generally curable coating compositions are in the form of a solution of a synthetic resin in an organic solvent serving as a carrier. This solution is applied to the base surface, which may be metal, wood, paper or textile, and then the solvent carrier is evaporated and a synthetic resin film remains, which then may be subjected to a heat-hardening treatment. 
     The use of organic solvents as a carrier for the synthetic resin has the disadvantage that in most cases the solvents are inflammable or even may give rise to explosions. Moreover some organic solvents are physiologically unacceptable, as they endanger the health of operators. In addition, much energy is required for the evaporation of the organic solvent and for the conditioning of the air which must carry away the solvent vapors from the oven in which the coated objects are baked. 
     For these reasons it has been proposed to replace the organic solvents wholly or partially by water. The use of water as a carrier for the synthetic resin, however, dictates the use of certain types of synthetic resins, which might not impart the required technological properties, such as flow, levelling and gloss to the finished coatings. 
     In view of the disadvantages of the preceding methods, it has also been proposed to apply high-melting thermoplastic, noncurable coating compositions in the form of a homogeneous powder. This powder is mostly applied by the method known as the fluidized bed method. This method is essentially a dipping proess wherein the article to be coated is preheated to a temperature above the melting point of the coating composition and then introduced into a fluidized mass of coating composition powder particles. 
     Powdered thermosetting, curable coating compositions may be applied to the substrate in the same way, requiring however, two heat treatments, first the preheating of the object to be coated and subsequently the curing treatment of the coated object. Therefore, the electrostatic powder spray application method has been developed, in which only one heat treatment is required, namely the curing treatment, and this method has the additional advantage that thinner layers (less than 100 microns) can be applied. Due to the relatively high viscosity of the molten powder .[.even in one treatment.]., coatings are provided .Iadd.even in one treatment .Iaddend.which are substantially thicker than can be obtained by the conventional coating methods using solutions of the coating material. Sharp edges or bends are also much better coated by the powder-coating processes. 
     .[.It has been proposed in German patent specification 1,015,165 to prepare solvent-based, curable coating compositions, especially enamels and stoving lacquers, consisting of 20-60 percent by weight of alkoxylated melamine-formaldehyde or urea-formaldehyde resin and 40-80 percent by weight of a linear polyester resin. This polyester resin is prepared from a saturated or unsaturated, aliphatic or aromatic dicarboxylic acid and an aromatic dihydric alcohol having the general formula:.]. ##STR1## .[.in which A is a 2-alkylidene radical having three or four carbon atoms, R is an alkylene radical having two or three carbon atoms and m and n are always at least one and the average sum of m and n is not greater than three. Small amounts of other dihydric alcohols may be admixed with the aromatic diol..]. 
     .[.According to column 2, lines 28 to 39 of this specification it is preferred to use equimolar amounts of acid and alcohol with preparation of the polyester, whereas in some instances 1.2 moles of alcohol per mole of acid is used. Only in example 5 is the preparation of the polyester quantitatively described, viz. from 1.15 moles of propoxylated bisphenol A (2.2 moles of propylene oxide per mole of bisphenol A) and 1 mole of fumaric acid, 85 parts of a 50 percent by weight solution of this polyester resin are then mixed with 15 parts of a 50 percent by weight solution of butoxylated melamine-formaldehyde resin. The resin composition thus obtained, however, is unsuitable for application in the powder-coating method, even if solvent-free..]. 
     .[.None of the polyester resins which are only qualitatively described in the examples, upon combination with the alkoxylated melamine-formaldehyde or urea-formaldehyde resins in the specified weight ratios give a resin composition, suitable for application by a fusion process..]. 
     Various pulverant thermosetting coating compositions intended for use in the powder-coating method have been suggested heretofore. As such may be mentioned epoxy resins. Also polyester resins have been proposed for use in the powder-coating process. None of these thermosetting resin-coating compositions, however, have been completely satisfactory and hence there is still the need of a coating composition suitable for use in electrostatic powder spray-coating processes, which provides a uniform coating having satisfactory chemical and mechanical properties. 
     It is an object of the present invention to provide curable coating compositions which are suitable for application by a fusion process and particularly to provide homogeneous, pulverulent curable coating .[.composition.]. .Iadd.compositions .Iaddend.which are especially suitable for use in powder spray-coating processes and which upon application provide uniform coatings having good mechanical properties. 
     Accordingly the present invention provides a curable powder-coating composition, comprising a mixture of 3 percent to 15 percent by weight of an alkoxylated polyamine-aldehyde resin and 85 percent to 97 percent by weight of a polyester resin having an acid number below 20 and a softening point falling within the range between 60° C. and 130° C., of which the acid component comprises a dibasic, aromatic carboxylic acid and of which the alcohol component comprises: 
     a. from 0 to 100 mole percent (based on the total alcohol component of the polyester) of an alcohol having the general formula: ##STR2##  in which R 1  and R 2  represent alkylene groups having from two to four carbon atoms, R 3  represents an alkylidene group having three or four carbon atoms or a cycloalkylidene group having six carbon atoms, or a carbonyl group, or a sulfone group, x and y are numerals each being at least one, the sum of x and y being not greater than six, R 4  and R 5  represent a hydrogen atom or alkyl radical having from one to six carbon atoms: 
     b. from 0 to 40 mole percent (based on the total alcohol component of the polyester) of a glycidyl ester having the general formula: ##STR3##  in which R is a straight or branched chain, saturated or unsaturated alkyl group having from four to 20 carbon atoms, or a substituted or unsubstituted benzene nucleus, and 
     c. from  .[.5.]. .Iadd.0 .Iaddend.to 100 mole percent (based on the total alcohol component of the polyester) of at least one dihydric, aliphatic alcohol, the sum of the alcohol components (a), (b) and (c) being 100 mole percent, said curable coating composition having a good powder stability, as hereinbefore defined. 
     As the dibasic, aromatic carboxylic acid in the acid component of the polyester may be used: ortho-phthalic acid, terephthalic acid isophthalic acid, esters of these acids with lower aliphatic, monohydric alcohols .[.such as methanol, or ethanol, and halogen-substituted or hydrogenated derivatives of these acids.]. such as methanol, or ethanol, and halogen-substituted or hydrogenated derivatives of these acids, such as hexachlorophthalic acid, tetrahydrophthalic acid, endomethylene-tetrahydrophthalic acid or hexachloro-endomethylene-orthophthalic anhydride. Also the esters of the halogen-substituted or hydrogenated derivatives of their acids with lower aliphatic, monohydric alcohols such as methanol, or ethanol or the anhydrides of these derivatives may be used. .Iadd.Another useful derivative of dibasic aromatic carboxylic acid is hexachloroendomethylene tetrahydrothalic acid. .Iaddend.The dibasic, aromatic acids may be used either singly or in combination with one another. The preferred dibasic aromatic acid is terephthalic acid. It has in some cases been found advantageous to use up to 10 mole percent (based on the total acid component) of a polybasic, aliphatic carboxylic acid, such as adipic acid, or of .[.a.]. .Iadd.an at least .Iaddend.tribasic, aromatic carboxylic acid, such as trimellitic acid or pyromellitic acid. 
     Examples of the diols forming component (a) of the polyester resin .Iadd.in which R 1  and R 2  represent alkylene groups having from two to three carbon atoms .Iaddend.include, 2,2-di-(4-beta-hydroxyethoxyphenyl)-propane; 2,2-di-(4-beta-hydroxyethoxyphenyl)-butane; 2,2-di-(4-hydroxypropoxyphenyl)-propane; 2,2-di-(3-methyl-4beta-hydroxyethoxyphenyl)-propane; the polyoxyethylene or polyoxypropylene ether of 4,4-isopropylidene diphenol, wherein both phenolic groups are oxyethylated or oxypropylated and the average number of oxyethylene or oxypropylene groups is from 2 to 6; di(4-beta-hydroxyethoxy-phenyl)-ketone and di-(4-beta-hydroxypropoxyphenyl)-ketone. 
     The preferred diols are 2,2-di-(4-beta-hydroxyethoxy-phenyl-)propane and 2,2-di-(4-hydroxypropoxy-phenyl-)propane. 
     The alcohol component (b) of the polyester comprises glycidyl esters and those that can be used include, lauric acid glycidyl ester, benzoic acid glycidyl ester and glycidyl esters of saturated aliphatic monocarboxylic acids having tertiary or quaternary carbon atoms in alpha-position to the carboxyl groups (available under the trade name of &#34;Versatic Acids&#34;). These latter glycidyl esters are preferred. 
     The alcohol component (c) of the polyester comprises dihydric aliphatic alcohols such as 2,2-dimethylpropanediol-1,3, propylene glycol, ethylene glycol. The dihydric aliphatic alcohols may be used either singly or in combination with one another. 
     It has in some instances been found advantageous to use up to 10 mole percent (based on the total alcohol component) of aliphatic trihydric or polyhydric alcohols, such as trimethylol propane and pentaerythritol. In all cases the sum of the alcohol components (a), (b) and (c) is 100 mole percent. 
     The polyester resins are prepared by the general techniques employed in the preparation of polyester resins. They can be conveniently made by heating all the reactants, while stirring, up to a temperature of 240° C., whilst an inert gas (e.g. nitrogen) is continuously passed through the reaction mixture to remove the water formed during the esterification reaction. At the end of the esterification reaction, vacuum is applied of about 100 mm. Hg and the ester is polymerized and glycol is distilled off. 
     The polyester resin which has an acid number below 20 and a softening point falling within the range between 60° and 130° C. (modified ball and ring test) is mixed with an alkoxylated polyamine-aldehyde resin in a weight ratio of polyester resin to polyamine-aldehyde resin between 85:15 and 97:3 and preferably between 90:10 and 95:5. The resin mixture obtained will generally have a .[.melting.]. .Iadd.softening .Iaddend.point between about 60° and about 120° C. (modified ball and ring test), whilst by the application of the preferred conditions the .[.melting.]. .Iadd.softening .Iaddend.point will be between about 80° and about 110° C. (modified ball and ring test). In all cases, however, the resin mixture obtained has a good powder stability as hereafter defined. 
     The alkoxylated polyamine-aldehyde resin is in general a thermosetting aminoplast condensate, which is the alkoxylated condensation product of an aldehyde, preferably formaldehyde, with a polyamine compound, such as urea, aminotriazines, such as melamine, or substituted aminotriazines, such as benzoguanamine. 
     The alkoxylation may be carried out with aliphatic alcohols having from one to six carbon atoms, mixtures of alcohols may also be used. The manufacture of these alkoxylated polyamine-aldehyde resins is well known in the art. 
     In preparing the preferred homgeneous, pulverant coating compositions according to the present invention, the polyester resin and the methoxylated polyamine-aldehyde resin are mixed at a temperature just about their .[.melting.]. .Iadd.softening .Iaddend.points for example, in an extruder, cooled to solidify the mixture and subsequently crushed and ground in a suitable grinding device such as a pin disc mill to a free-flowing powder having a particle-size range of approximately 20-150 microns. The free-flowing powder so obtained may be subjected to a screening treatment to adjust the desired average particle size, for the specific type of application to be employed. 
     The homogeneous, pulverulent coating compositions obtained may also include various well-known functional modifiers such as grinding aids, curing agents or accelerators, flow control agents, surface-active agents, heat-stable organic or inorganic pigments, inert fillers, inhibitors, abrasives and plasticizers in their usual effective proportions. 
     Also small amounts of other resins or other organic compounds to improve the film-forming properties may be incorporated. It will be understood that the amount of these substances must be such that the physical properties of the powders and cured coating are not impaired. 
     After application of the coating compositions to the substrate, the coatings are cured at a temperature from 120° to  .[.250° C..]. .Iadd.220° C. .Iaddend.for a period of 10 to 60 minutes. The curing treatment is preferably carried out for 20 to 40 minutes at 150° to 200° C. 
    
    
     The invention will be further illustrated by the following examples. In these examples and in the appended claims all softening points were determined by a modified version of the ball and ring method according to A.S.T.M. Specification Number E 28-58 T. The method is modified in that as the first temperature is noted the temperature at which the ball begins to move and as the last temperature, the temperature at which the ball strikes the bottom plate. In view of the application as powder coating, it has been found advantageous for the judgment of the characteristics of the polyester resin and of the complete coating composition to use these figures, rather than those set out in the A.S.T.M. Specification mentioned above. 
     EXAMPLE 1 
     Into a suitable reaction vessel equipped with thermometer, stirrer, reflux condenser and inert gas inlet and outlet tubes, are introduced 309.6 g. (0.9 g. mol) of the diether of propylene glycol and bisphenol A.Iadd., .Iaddend.50 g. (0.2 g. mol) of Cardura-E (Registered Trade Mark; glycidyl ester of branched chain monocarboxylic acids having from nine to 11 carbon atoms) and 166 g. (1.0 g. mol) of terephthalic acid. The contents of the vessel are heated to a temperature of about 220° to 260° C. and held at that temperature while stirring, whereby the water of the esterification reaction is removed from the reaction mass by bubbling nitrogen gas through it. The heating is continued until an acid number of 15 is reached. The polyester resin obtained had a softening point of 85° -110° C. 
     EXAMPLES 2-7 
     In the same apparatus as described in example 1 a number of polyester resins were prepared, using the ingredients as summarized in table 1. The acid numbers (AN) and softening points (SP) in ° C. of the polyester resins obtained are also indicated in table I. 
     
                                           TABLE I                                 
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                Moles       Moles                                         
Example                                                                   
       Alcohol  alcohol                                                   
                      Acid  acid                                          
                                AN SP.° C.                         
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2      DEGBA.sup.1                                                        
                1.1   TPA.sup.2                                           
                            1.0 6.4                                       
                                   84-103                                 
       DEGBA    0.9                                                       
3                     TPA   1.0 7  78-96                                  
       Cardura E.sup.3                                                    
                0.2                                                       
       DEGBA    0.9                                                       
4                     HET-acid.sup.4                                      
                            1.0 7  88-108                                 
       Cardura E                                                          
                0.2                                                       
       DEGBA    0.7                                                       
5                     IPA.sup.5                                           
                            1.0 6  70-86                                  
       Cardura E                                                          
                0.4                                                       
       DEGBA    0.70                                                      
6      Cardura E                                                          
                0.40  TPA   1.0 0.88                                      
                                   67-88.5                                
       TMP.sup.6                                                          
                0.10                                                      
       DEGBA    0.90                                                      
7                     TPA   1.0 8  92-107                                 
       Glycidyl benzoate                                                  
                0.20                                                      
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 .sup.1 DEGBA = Diether of ethylene glycol and bisphenol A.               
 .sup.2 TPA = Terephthalic acid.                                          
 .sup.3 Cardura E (Registered Trademark) = Glycidyl ester of branched chai
 monocarboxylic acids having about 9 to 11 carbon atoms.                  
 .sup.4 HET-acid = Hexachloroendomethylenetetrahydrophthalic acid.        
 .sup. 5 IPA = Isophthalic acid.                                          
 .sup.6 TMP = Trimethylolpropane.                                         
 
    
     EXAMPLES 8-11 
     With the aid of the polyester resin prepared in example 3, a number of coating compositions were prepared by mixing this polyester resin with hexamethoxymethylmelamine (HMMM) resin in varying amounts (table II). 
     The &#34;powder stability&#34; as indicated throughout this application, in the examples and the appended claims was determined as follows: 
     20 g. of the unpigmented, pulverulent composition (of which 80-90 percent had a particle size between 70 and 100 μ) was heated in a glass beaker at 40° C. for 2×24 hours. If the resin particles were not sintered together, the powder stability was rated as good (G). If the particles were sintered together the powder stability was rated as bad (B). 
     The &#34;flexibility&#34; was determined on the basis of specimens of 1 mm. thickness mild steel panels, upon which an amount of the pulverulent resin mixture was applied by the electrostatic spray-coating method, such that on subsequent curing at 180° C. for 30 minutes a uniform coating having a thickness of about 50 m μ was obtained. 
     Flexibility was rated as passing (P) or failing (F), depending upon whether or not the steel panel with the coating could be bent 180° over a 3/4-inch diameter mandrel without impairing the coating. If the film cracked or broke, it was rated as failing. 
     The `shock resistance` indicated was determined with the &#34;Erichsen Schlagpruefgeraet&#34; according to German Specification DIN 53,156 with the specimens as indicated under &#34;flexibility.&#34; 
     
                       Table II                                                    
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Example        8        9        10    11                                 
______________________________________                                    
Polyester resin (wt. %)                                                   
               97       94       90    85                                 
HMMM resin (wt.%)                                                         
               3        6        10    15                                 
.[.Melting.]..Iadd.Softening .Iaddend.point                               
° C.    76°-103°                                     
                        74°-104°                            
                                 70°-93°                    
                                       65°-91°              
Powder stability                                                          
               G        G        G     B                                  
Flexibility    F        P        P     P                                  
Shock resistance in mm.                                                   
               2-3      6        &gt;6    &gt;6                                 
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     EXAMPLES 12-18 
     In the same apparatus as described in example 1 a number of polyester resins were prepared, using the ingredients and having the properties as summarized in Table III. 
     
                       Table III                                                   
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                Moles          Moles                                      
Ex.  Alcohol    Alcohol  Acid  Acid  AN   SP° C.                   
______________________________________                                    
12   DEGRA      0.675    TPA   1     6.6  83-108                          
     Cardura E  0.200                                                     
     Neopentyl                                                            
     glycol     0.225                                                     
13   DEGBA      0.55     TPA   1     10.2 86-104                          
     Ethylene                                                             
     glycol     0.55                                                      
14   Cardura E  0.2      TPA   1     7.7  84-98                           
     Neopentyl                                                            
     glycol     0.3                                                       
     Ethylene                                                             
     glycol     0.5                                                       
     Hexanediol                                                           
     1.6        0.1                                                       
15   DEGBA      0.06     TPA   1     4.7  87-101                          
     Cardura E  0.17                                                      
     Neopentyl                                                            
     glycol     0.94                                                      
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     With aid of these polyester resins a number of coating compositions were prepared by mixing them with hexamethoxymethylmelamine (HMMM) resin (table IV). 
     
                       TABLE IV                                                    
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     Polyester                                                            
              Polyester                                                   
                       HMMM   Powder       Shock                          
     from     wt.      wt.    stabil-                                     
                                     Flexi-                               
                                           resist-                        
Ex.  example- percent  percent                                            
                              ity    bility                               
                                           tance                          
______________________________________                                    
16   12       90       10     G      P.sup.1                              
                                           .sup.1 6                       
17   13       88       12     G      P.sup.2                              
                                           .sup.2 5                       
18   14       94       6      G      P.sup.3                              
                                           .sup.3 6                       
19   15       90       10     G      P.sup.4                              
                                           .sup.4 2--3                    
______________________________________                                    
 .sup.1 Coating thickness, 25 mμ.                                      
 .sup.2 Coating thickness, 100 mμ.                                     
 .sup.3 Coating thickness, 60 mμ.                                      
 .sup.4 Coating thickness, 50-60 mμ.