Siliconized polyester thermosetting coating compositions
TECHNICAL FIELD 
     This invention relates to weather resistant thermosetting solution coating compositions containing a polysiloxane component which enhances resistance to weathering, especially as indicated by accelerated testing in an unshielded dew-cycle weatherometer. 
     BACKGROUND ART 
     Polysiloxane-containing thermosetting solution coating compositions are known in which the polysiloxane component is condensed with a reactive resin to provide a combination which retains its reactivity and which is combined with a curing agent, such as a melamine-formaldehyde condensate, so that the coating can be applied and thermally cured upon a metal substrate. In these known compositions, an excessive proportion of polysiloxane is needed to achieve the desired weather resistance. 
     It is also known to use a polysiloxane which is not reactive so that it stratifies and concentrates at the surface of the coating during the cure. However, these polysiloxanes volatilize and burn during the baking operation, forming silica particles which deposit in the oven. This is undesirable and wasteful of the expensive polysiloxane component. Also, inadequate compatibility in solution causes these systems to have poor package stability 
     In this invention, we minimize the problems encountered by the prior art by having the polysiloxane component in a nonvolatile combination which still permits controlled incompatibility to develop during cure to concentrate the polysiloxane at the surface and thereby improve the weather resistance which is obtainable, using a given proportion of polysiloxane. 
     DISCLOSURE OF INVENTION 
     In accordance with this invention, an organic solvent solution coating composition adapted to deposit a thermosetting weather-resistant coating, using a reduced proportion of organic polysiloxane, comprises three essential resin components, as follows: 1- an essentially nonfunctional organic solvent-soluble silicone resin reaction product of a highly branched, low molecular weight hydroxy functional polyester with an hydroxy or alkoxy-functional organic polysiloxane, the polysiloxane being present in an amount of from 8% to 20% of the weight of the three components, and the polyester and the polysiloxane being combined in approximately stoichiometric proportions (±5%) and reacted extensively to provide a substantially inert condensate; 2- a reactive organic solvent-soluble, essentially linear high molecular weight, hydroxy functional polyester constituting the balance of the three resin components; and 3- from 3% to 15%, based on the weight of the three components, of a heterocyclic aminoplast resin for curing the reactive polyester of component 2. Components 1 and 2 are compatible in the solution and at least partially incompatible in a film as the solvent evaporates so that components 2  and 3 can cure in a deposited film and adhere the coating to a metal substrate, and component 1 can concentrate at the surface to enhance the weather resistance when a deposited coating is baked. 
     The concentration of component 1 which contains the polysiloxane allows this expensive component to be more efficiently utilized, and the prereaction of the polysiloxane with the polyester reduces its volatility so that this component is not lost during the bake and siliceous deposits do not form in the ovens which are used. Also, the fact that polyesters are present in both components 1 and 2 allows the polysiloxane to be compatibly retained in the organic solvent solution. 
     Referring more particularly to the highly branched low molecular weight hydroxy functional polyesters which are prereacted with the polysiloxane to produce component 1, these are polyesters of dicarboxylic acids with dihydric alcohols, and the desired branching and low molecular weight is obtained by selecting at least 50% of the hydroxy functional reactant to be either a triol a tetrol or a mixture thereof. In particularly preferred practice from 70% to 85% of the hydroxy functionality comes from trimethylol propane, balance diol. To provide the desired hydroxy functionality, the hydroxy to carboxy ratio should be in the range of 1.4:1 to 210:1, most preferably 1.5:1 to 1.8:1. Examples of polyesters which can be used will be presented later. 
     The preferred polysiloxanes have a methoxy content of from 10% to 25% by weight. 
     Considering the essentially linear, high molecular weight reactive polyester of component 2, the desired linearity is obtained by using essentially only difunctional reactants (less than 10% by weight of reactants of higher functionality and less than 5% by weight of monofunctional reactants). The desired high molecular weight is obtained by having the hydroxy to carboxy ratio in the reactants which are polyesterified in the range of 1.03:1 to 1.30:1. The polyesterification is continued until the acid value of the polyester is less than 30. The hydroxy number of the polyester is from 80 to 140, preferably from 90 to 120. 
     The proportion of the siliconized polyester to the linear reactive polyester is regulated to provide the desired amount of polysiloxane in the final product which includes the heterocyclic aminoplast curing agent, component 3 herein. From 8% to 20% polysiloxane based on the total of all three resin components is particularly preferred. 
     Proportions as set forth above are important to this invention. The proportion of heterocyclic aminoplast resin, especially when melamine formaldehyde condensates which are preferred herein are used, is preferably from 10% to 12% of the weight of the three components. On this same basis, the polysiloxane containing 10% to 25% of methoxy functionality used to form component 1 preferably constitutes from 10% to 17%, and the reactive high molecular weight polyester of component 2 will constitute the balance of the three essential resin components. Other curing agents, like phenoplast and urea-based resins do not provide comparable weather resistance. Other heterocyclic amines, such as benzoguanamine, may be reacted with formaldehyde to form the heterocyclic aminoplast resins used in this invention. 
     The weight ratio of polyester to polysiloxane in component 1 may range from 1:3 to 3:1 and is not critical so long as the functionalities in these two materials are in balance and are consumed to provide a reaction product which is essentially nonfunctional. 
     The coating compositions of this invention are usually pigmented, and pigment choice is secondary and illustrated in the examples. The pigment to binder weight ratio is typically in the range of 1:2 to 1:6. 
     Any organic solvent which does not react with any of the three components of the binder may be used. For example, toluene, aromatic hydrocarbon mixtures and butanal can be used. The solvent selection is conventional and also illustrated in the examples. 
     The contribution provided by this invention will be apparent from a summary consideration of data obtained by accelerated testing in an unshielded dew-cycle weatherometer. Typical panels tested in this manner using prior construction coatings indicate that the color change produced by 500 hours testing increases as the proportion of the polysiloxane resin component is reduced. The higher the color change, the greater the number assigned to describe that change. On this basis, a typical prior composition containing 15% silicone resin gave a value of 8 and the system tested had only a moderate resistance to chalking. Increasing the silicone content of the system to 30% improved the color change to 6 and the chalk resistance became excellent. At 50% silicone content, the color change resistance was still better, being assigned a value of 3, and the chalk resistance remained excellent. In this invention, excellent chalk resistance and better resistance to color change, as indicated by a value of 2, is obtained when only 10% of the expensive silicone component is present in the composition which is tested. 
     The essentially nonfunctional organic solvent-soluble reaction product of the described reactive polyester with an hydroxy or alkoxy-functional organic polysiloxane should contain so little reactive functionality that the subsequent cure of the applied coating composition will not cause significant combination with either the reactive polyester of high molecular weight or the aminoplast resin. This refusal to react causes stratification in the final coating. 
     The language &#34;essentially nonfunctional&#34; is thus intended to include small amounts of reactive groups in a proportion which is too small to prevent stratification on subsequent cure. In preferred practice, the equivalents of reactive functionality per 100 grams of resin solids should be less than 0.1. 
     This refusal of one of the three resin components to chemically combine with either of the other two can be seen by applying a drop of methyl ethyl ketone onto the cured film in a horizontal position. After the ketone evaporates, an oily spot is visible at the periphery of the area where the drop had been. This establishes that the nonfunctional reaction product is free to be concentrated by the drop of solvent. If the film were cured at the surface, the drop of solvent would dry, and after evaporation no detectable blemish would be observable. 
     The polyester and the polysiloxane should be coreactive and their functionalities should be in substantial balance as previously indicated. Also, the molecular weight of the polyester and the polysiloxane prior to reaction should be limited to avoid gelation. This is broadly specified herein by requiring the essentially nonfunctional reaction product to be organic solvent-soluble. 
     The polyester may be oil-free, or it may contain a fatty oil or acid derived therefrom, such as linseed fatty acid, in an amount of up to about 40% of the weight of the polyester. The fatty groups may be saturated or unsaturated, and, when used, preferably contain 16-22 carbon atoms. 
     The hydroxy or alkoxy-functional organic polysiloxanes which are used herein are subject to wide variation and are conventional and well known materials. 
     The preferred polysiloxanes are methyl and/or phenyl-substituted polysiloxanes having an average molecular weight in the range of about 400 to about 6000, preferably 500 to 3000. These desirably contain from 1 to 10, preferably from 2 to 6 reactive groups per molecule. These polysiloxanes are illustrated by a methyl and phenyl substituted polysiloxane having a molecular weight of about 800 and a methoxy content of 15% by weight. Methoxy functionality is preferred and the molecular weight is by number average. Commercially satisfactory polysiloxanes are shown in the examples. 
     The heterocyclic aminoplast resins are typified by hexamethoxymethyl melamine, though the ethyl propyl and butyl ether derivatives of hexamethylol melamine are also useful. Benzoguanamine may replace the melamine. 
     The invention is illustrated as follows: 
    
    
     EXAMPLE 1 
     (Preparation of reactive high molecular weight polyester) 
     1040 grams of phthalic anhydride, 280 grams of isophthalic acid and 250 grams of adipic acid were esterified at 210° C. with the following polyols: 
     neopentyl glycol: 822 grams 
     hexanediol: 310 grams 
     trimethylol propane: 45 grams 
     The polyesterification was continued to an acid value of 15.2. During the reaction 236 grams of water distilled off. 1160 grams of aromatic solvent (Ashland Hi-Sol 4-1) and 130 grams of butanol were added. The Polyester so-produced has an hydroxy number of 90 and the solution has the following characteristics: 
     Solids content: 66.7% 
     Viscosity (Gardner-Holdt): Z 1   
     Acid Value: 15.2 
     Color (Gardner): 2-3 
     EXAMPLE 2 
     (Preparation of reactive, low molecular weight branched polyester) 
     The following components are mixed together and refluxed at 230° C. until 148 cc of water are removed. 
     Trimethylol propane: 446 grams 
     Neopentyl glycol: 124 grams 
     Isophthalic acid: 250 grams 
     Adipic acid: 292 grams 
     2-ethoxy ethanol acetate: 15 grams 
     Xylol: 15 grams 
     Upon removal of the desired amount of water, the acid value is 4.6 and the product is cooled to 140° C. 
     EXAMPLE 2A 
     (Preparation of nonfunctional reaction product) 
     810 grams of 2-ethoxy ethanol acetate, 950 grams of General Electric siloxane SR-191 (methyl and phenyl substituted polysiloxane containing 15% methoxy and a molecular weight of about 800) and 1.0 gram of tetraoctyl titanate catalyst are premixed and added to the polyester product previously produced. The mixture is heated to 150° C. and 80 grams of methanol are removed. The product is held for a Gardner-Holdt viscosity of U-W. 76 grams of butanol are than added and the product is cooled to 110° C. 
     The polyester component described previously was produced using 12.2 mols of hydroxy functionality and 7.0 mols of carboxyl functionality which supplies an excess of 5.2 mols of hydroxy. The siloxane provides 4.8 mols of methoxy functionality, so the final product has very little residual functionality after the methoxy functionality has consumed most of the hydroxy functionality in the polyester. 
     EXAMPLE 3 
     71 parts by weight of the solution of Example 1 are blended with 17.8 parts of the solution of Example 2A and diluted with 11.2 parts of aromatic hydrocarbon solvents (Solvesso 150). This provides a final solution coating composition containing about 60% by weight of nonvolatile resin and having a silicone resin content of about 10% by weight. 
     EXAMPLE 4 
     A pigmented coating composition is prepared by mixing the following to a smooth paste and dispersing to a 71/2 fineness (North Standard Gauge) in a sand mill: 
     
         ______________________________________                                    
Parts   Component                                                         
______________________________________                                    
0.94    50% solution of Flexowax &#34;C&#34; (Glyco Chemicals,                    
        Inc.) in Solvesso 150                                             
1.60    Iron oxide pigment                                                
0.66    Light chrome yellow pigment                                       
11.86   Titanium dioxide, rutile pigment                                  
1.7     Butanol                                                           
1.41    Solvesso 150 solvent                                              
7.16    Solution product of Example 3                                     
______________________________________                                    
 
    
     The following is dispersed by intensive mixing under a Cowles disperser to a fineness of 51/2 (North Standard Gauge) and blended with the sand mill dispersed paste described above. 
     
         ______________________________________                                    
Parts              Component                                              
______________________________________                                    
4.71               Amorphous Silica (Syloid 74 from                       
                   W. R. Grace Co.).                                      
8.00               Solvesso 150 (aromatic hydrocarbon                     
                   solvent)                                               
16.71              Resin Blend of Example 3.                              
       The following ingredients were added in the                        
       order shown, while mixing:                                         
39.55              Resin Blend of Example 3                               
4.24               Melamine Aminoplast resin                              
                   (Monsanto Product Resimene X-740)                      
0.33               25% by weight, p-toluene sulfonic                      
                   acid in butyl alcohol.                                 
         Shade in:                                                        
1.13               20% by weight Bone Black Pigment                       
                   (Hoover Color Corp. 3495), ground                      
                   in Resimene X-740, 80% by weight.                      
100.00%  total                                                            
______________________________________                                    
 
    
     EXAMPLE 5 
     An oil free siliconized polyester solution containing 15% silicone by chemical reaction in the conventional manner, was adjusted to 60% total nonvolatile resin by weight using Solvesso 150. This was used to prepare a pigmented coating composition in exactly the same manner used in Example 4. 
     EXAMPLE 6 
     A pigmented coating composited was prepared as in Example 4 using Aroplaz 1710-R-60. This is a 60% solids (by weight) polyester resin solution containing 50% silicone modification, based on the total nonvolatile resin. 
     EXAMPLE 7 
     A pigmented coating composition was prepared as in Example 4 using Aroplaz 1711-A9-60. This is a 60% solids (by weight) polyester resin solution containing 30% silicone modification based on the total nonvolatile resin. 
     EXAMPLE 8 
     The pigmented coating compositions of Examples 4, 5, 6 and 7, were separately applied onto 0.023 inch thick Parker 721 S treated aluminum panels using a #26 wire wound rod. Thermal conversion (or curing) to solid, solvent insoluble and thermally infusable films was done in a gas fired chamber oven for 30 seconds to a peak metal temperature of 450° F. 
     EXAMPLE 9 
     The panels of Example 8 were exposed in an Atlas XW-R unshielded Dew Cycle Weatherometer (Atlas Electric Devices, Chicago, Ill.) which was operated 60 minutes arc light on and 60 minute light off, with cooling deionized water spray on backs of panels during light off cycle. The panels were periodically removed to measure color change and free chalk formation. The color changes were determined in conventional manner by comparing to unexposed control panels using a Hunter Color/difference meter D25-2 instrument (Hunter Associates Laboratory, Inc., Fairfax, Va.) and are expressed in ΔE units as calculated using the Hunter Lab ΔE computor; one ΔE (National Bureau of Standards) unit being that which is just barely discernible as a difference in color to the average human eye. The greater the color change, the larger the ΔE value will be. Chalk values were determined using ASTM standard test designation D569-44 procedure employing a 7/8 inch Round Model Jackobsen Chalk Tester (Gardner Laboratories, Inc.). Chalk values are expressed numerically from 10 to 1, with 10 being no chalk, and 1 being most chalky. 
     The results of this exposure after a total of 500 hours in the Weatherometer were: 
     
         ______________________________________                                    
Example                  Chalk Value                                      
                                    ΔE                              
______________________________________                                    
4        10% Silicone Blend                                               
                         10         2                                     
5        15% Silicone Reacted                                             
                         6.5        8                                     
6        50% Silicone Reacted                                             
                         10         3                                     
7        30% Silicone Reacted                                             
                         10         6                                     
______________________________________                                    
 
    
     This testing demonstrates the superior durability of the silicone blend of this invention.