Abstract:
Novel, low viscosity, high solids paint compositions have been created which provide durable, high gloss surface finishes when they are applied to and cured over suitable substrates. The paints are based on urethane binder resins formed by initially reacting a hindered piperidinol ultraviolet stabilizer with a stoichiometric excess of aliphatic polyisocyanate. This reaction product is in turn reacted with a stoichiometric excess of polyether polyols. The paint compositions are cured by crosslinking the urethane binder resins at the unreacted functional hydroxyl groups of the polyol constituents by means of acid catalyzed, melamine formaldehyde crosslinking agents. Cure can generally be fully accomplished in a short time at temperatures below about 250° F. The cured paint has piperidine groups chemically incorporated within its molecular structure to effectively inhibit the degradation of the urethane linkages and the loss of gloss by ultraviolet radiation.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to an ultraviolet (U.V.) stable, sprayable, high solids urethane paint composition characterized by a durable, high gloss surface when applied to and cured over a suitable substrate. The invention also relates to a method of making such paint by reacting hindered piperidinol ultraviolet stabilizing agents into a paint resin which is crosslinked by an acid catalyzed, melamine formaldehyde, crosslinking agent at relatively low temperatures. 
     Urethane coating compositions are well known for their toughness, flexibility, impact resistance, and first class glossy surface finish. However, urethane paints are susceptible to loss of gloss when exposed to ultraviolet radiation (sunlight, e.g.,) and moisture. Thus, they have not generally been considered suitable for use as automotive topcoats. 
     William T. Short, an inventor of the subject invention, earlier described a U.V. and moisture resistant urethane paint system in U.S. Pat. No. 4,243,792 assigned to the assignee hereof. The patent relates to the incorporation of hindered piperidinol esters of aliphatic carboxylic acids in urethane polymers for U.V. stabilization. The paints were formed by reacting a stoichiometric excess of suitable aliphatic polyisocyanate with a mixture of organic triols, diols and the hindered piperidine stabilizer. The reactions were catalyzed by an organometallic catalyst such as dibutyl tin dilaurate. The paint itself was moisture cured at the unreacted isocyanate groups under conditions of high relative humidity and low temperatures. A disadvantage of this earlier paint system was the presence of the free isocyanate end groups on the uncured paint. These groups are highly reactive necessitating careful handling of the paint to prevent premature reaction, and some people may develop allergic sensitivity upon extended exposure. Thus, it would be preferable to have a paint composition with no appreciable amount of reactive isocyanate groups. 
     Due to the high volume nature of most automotive paint operations, it is also extremely desirable to increase the resin to solvent ratio. Currently most automotive paint systems are based on sprayed acrylic enamels. In order to obtain the physical and appearance properties required for automotive topcoats, a minimum resin molecular weight of about 15,000 is necessary. However, in this molecular weight range, acrylic resins can generally comprise no greater than about 50 (and usually less than 30) weight percent of the paint. Considerable energy is expended in evaporating solvent from low solids paints in the paint ovens. Moreover, means must be provided for handling fugitive solvent. 
     Accordingly, it is an object of this invention to provide a sprayable, high solids urethane paint composition that is characterized by a durable high gloss surface finish resistant to ultraviolet degradation. It is a further object of our invention to provide polyurethane paint resin without free isocyanate groups in which a piperidine U.V. stabilizer is chemically incorporated. A more particular object is to incorporate such piperidine U.V. stabilizer in a paint resin which is cured by crosslinking functional hydroxyl groups with an acid catalyzed, melamine formaldehyde crosslinking agent. 
     Another object of the invention is to provide a method of making such U.V. stable, low energy curing, urethane paint resins by reacting a hindered piperidinol ultraviolet stabilizing agent with excess polyisocyanate and thereafter reacting the product of this reaction with a stoichiometric excess of polyether polyol to create a paint resin that can be crosslinked by means of an acid catalyzed melamine formaldehyde crosslinking agent. 
     A specific object of the invention is to provide a production sprayable, low viscosity automotive quality topcoat paint formulation containing a minimal amount of volatile solvent. Another specific object is to create such a high solids paint which will cure in a relatively short period of time at relatively low temperatures. 
     BRIEF SUMMARY OF THE INVENTION 
     In accordance with the preferred practice of our invention, these and other objects may be accomplished as follows. An acid catalyzed, melamine formaldehyde crosslinkable polymer binder resin is prepared by mixing a hindered piperidine having a functional hydroxy group (herein piperidinol) ultraviolet stabilizing agent with a substantial stoichiometric excess of a polyfunctional isocyanate. The reaction yields a piperidine group-terminated isocyanate prepolymer. A preferred piperidinol U.V. stabilizer is 2,2,6,6-tetramethyl-4-piperidinol (TMP-4-OH), and a preferred aliphatic isocyanate is methylene bis(4-cyclohexyl isocyanate) (H 12  MDI). The isocyanate terminated prepolymer is then reacted with a stoichiometric excess of polyether polyol, preferably a mixture of polyoxypropylene diols and triols. The ratio of diols to higher functionality polyols may be adjusted to provide desired hardness, toughness and flexibility in the cured paint. In a preferred formulation, approximately equal chemically equivalent amounts of diol, triol and diisocyanate (i.e., about one third of the total equivalents each) are present in the paint resin compositions along with a few percent piperidinol U.V. stabilizer on an equivalent basis. 
     The initial reaction of the piperidinol and excess isocyanate creates an isocyanate terminated constituent having piperidine U.V. stabilizer attached to an end of a portion of the diisocyanate molecules. The piperidine attachment is a urethane linkage formed at the functional hydroxyl group of the piperidinol starting material. This urethane bond is stable during the subsequent reaction of the diisocyanate prepolymer with the polyols and during the acid crosslinking of the resin. That is, the crosslinking brought about by the reaction of a melamine formaldehyde in the presence of a weak acid catalyst does not displace the piperidine in the paint resin. It is therefore reacted into the cured paint to stabilize it against U.V. degradation. A competing reaction between the isocyanate and the piperidinol is the formation of weak urea linkage between the nitrogen of the piperidine ring and the isocyanate end groups. However, this linkage did not appear to be stable with respect to subsequent reaction with either the polyol or the acid catalyst used for crosslinking. 
     The preferred crosslinking agents for the subject paint resins are partially methylated melamine resins. These are made by prereacting less than 6 moles of formaldehyde (preferably about 3 moles) with each mole of melamine. The reaction causes the addition of hydroxymethyl groups to the amine groups of the melamine resin. In acid environments (pH preferably less than 5) at elevated temperatures (preferably about 250° F.), these melamine formaldehydes react with the hydroxy groups of the paint resin to form complex crosslinked polymer structures. One preferred acid catalyst is dimethylpyrophosphoric acid. Unique to this invention is the incorporation of a piperidine U.V. stabilizer in the paint that is not adversely affected by the acid catalyzed melamine cure. 
     Thus the subject invention provides durable paint finishes which are extremely stable to moisture and ultraviolet radiation exposure. We have found that merely combining conventional piperidinol U.V. stabilizers, isocyanates and excess polyols yields severely flocculated paints that are not U.V. stable. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     These and other objects and advantages of our invention will be better understood in view of the FIGURE which is a plot of measured gloss versus accelerated ultraviolet and moisture exposure for a U.V. stabilized paint resin of the subject invention, a conventional acrylic laquer topcoat and a polyurethane resin system without U.V. stabilizer. The invention may be further understood in view of the following description and specific examples. 
    
    
     It is well-known that saturated cyclic and aliphatic polyisocyanates are more stable to ultraviolet radiation than unsaturated cyclic, straight chain or branched isocyanates. Therefore, the preferred polyisocyanates for the subject invention include such compositions as methylene bis(4-cyclohexyl isocyanate), isophorone diisocyanate, hexamethylene diisocyanate, and 1,4-cyclohexane bis(methyl isocyanate). The first mentioned compound is a preferred diisocyanate for the subject invention. It is a liquid sterioisometric mixture originally sold under the trade name of &#34;Hylene W&#34; by Dupont that is available as of the filing date thereof as Desmadur-W®, sold by Mobay. The material is sometimes generically referred to as H 12  MDI. Analysis showed it to be greater than about 99.2% purity. The isocyanate constituent should have a functionality of at least about 2 to assure a chemically crosslinked paint product. In the subject compositions, the paint is cured by crosslinking hydroxyl groups carried on a polyether constituent. The isocyanate groups are fully reacted with hydroxyl groups during resin synthesis. 
     The flexibilizing constituents of the subject urethane paints are preferably polyether polyols. These polyols have been found to provide conventional moisture cured urethane paints with such properties as high gloss, high distinctness of image, impact resistance and chip resistance. The preferred polyethers are blends of polyoxypropylene diols and triols. Other polyether polyols, based on polyethylene oxide, e.g., may also be used if they have suitable U.V. stability and workable viscosities. Table I lists a number of polyethers that were used. 
     
                       TABLE I______________________________________SELECTED POLYPROPYLENE OXIDE POLYETHERS  Func-           Viscosity @  tion-   %       25° C.                           Hydroxyl                                  MolecularMaterial.sup.a  ality   Solids  (Centipoise)                           Number Weight______________________________________P-410  2       100      75      265     424P-1010 2       100     150      105    1050TP-440 3       100     600      398     423TP-1540  3       100     350      110    1530______________________________________ .sup.a All polypropylene oxide polyethers were obtained from BASF Wyandotte under the trade name Pluracol® resins. 
    
     Alkylated amino resins are known crosslinking agents for industrial coatings. Outstanding properties such as hardness, high gloss, clarity and durability are provided by melamine curatives. Thus, melamine crosslinking agents are preferred for our novel U.V. resistant urethane coatings. We have found partially methylolated melamine resins to be suitable. These are commercially available and may be formed by reacting less than six moles of formaldehyde with one mole of melamine in a methanol solution. The reaction yields a methylolated melamine. A preferred curing agent for this invention is trimethylol melamine dissolved in butanol that is sold by American Cyanamid under the trade designation Cymel®325. Generally, the melamine curing agent should be present in amounts adequate to react with substantially all of the free hydroxyl groups of a urethane binder resin to be crosslinked. About 40 grams of trimethylol melamine was used per equivalent of free hydroxy to cure the subject urethane binder resins. Fully methylolated (hexamethylated) melamine does not work. 
     In order to cure the subject paint resins in reasonable times at relatively low temperatures, it is necessary to catalyze, i.e., accelerate, the melamine crosslinking reaction. The preferred catalysts are weak acids soluble in organic solvents. A preferred catalyst is dimethyl pyrophosphoric acid (DMPA) sold by American Cyanamid as Cycat®296.9. ##STR1## 
     About 2.5 weight percent DMPA (excluding solvent weight) was used based on the weight of trimethylol melamine, about 3 percent being preferred. 
     Compatible solvents such as cellosolve acetate, methyl ethyl ketone (MEK), butyl alcohol, etc. are incorporated consistent with normal paint practices. Likewise, pigments, fillers and other paint additives may be incorporated so long as they do not interfere with the invention. 
     EXPERIMENTAL BACKGROUND 
     Of the isocyanates examined, methylene bis(4-cyclohexyl isocyanate) (H 12  MDI) produced the most satisfactory paints. When crosslinked with melamine formaldehyde resin, high gloss tough films were produced. Table II lists four polyurethane-polyester resin compositions characteristic of our investigation. The polyether polyols we used are described in Table I. In the practice of our invention the preferred molecular weight range for the diol constituent is about 300 to 1500 grams per mole and for the triol, about 300 to 2500 grams per mole. 
     
                       TABLE II______________________________________POLYURETHANE-POLYETHER RESIN COMPOSITIONS______________________________________1.   Urethane Resin IADiol P1010.sup.a 87.0g  (0.166 equivalents)Triol TP440.sup.a                 71.7g  (0.509 equivalents)Triol TP1540.sup.a                 44.2g  (0.087 equivalents)Diisocyanate H.sub.12 MDI.sup.b                 17.2g  (0.133 equivalents)R = 5.73, b = 0.782.   Urethane Resin IIBDiol P1010.sup.a 87.0g  (0.164 equivalents)Triol TP440.sup.a                 71.7g  (0.505 equivalents)Triol TP1540.sup.a                 44.2g  (0.084 equivalents)Diisocyanate H.sub.12 MDI.sup.b                 30.3g  (0.231 equivalents)R = 3.26, b = 0.783.   Urethane Resin IIICDiol P1010.sup.a 87.0g  (0.164 equivalents)Triol TP440.sup.a                 71.7g  (0.505 equivalents)Triol TP1540.sup.a                 44.2g  (0.084 equivalents)Diisocyanate H.sub.12 MDI.sup.b                 43.4g  (0.331 equivalents)R = 2.27, b = 0.784.   Urethane Resin IVDDiol P1010.sup.a 87.0g  (0.164 equivalents)Triol TP440.sup.a                 71.7g  (0.505 equivalents)Triol TP1540.sup.a                 44.2g  (0.084 equivalents)Diisocyanate H.sub.12 MDI.sup.b                 56.5g  (0.431 equivalents)R = 1.75, b = 0.78______________________________________ .sup.a BASF Wyandotte  Pluracol® resins .sup.b E. I. DuPont de Nemours and Company  Hylene® W R = equivalents of H.sub.12 MDI per total equivalents polyol  b = branching coefficient 
    
     Other polyether polyols with functionalities of at least two may be used. The branching coefficient (b) which is defined herein as the equivalents of triol per total equivalents of hydroxyl was held constant at about 0.78. R represents the chemical equivalents of H 12  MDI per total chemical equivalents of hydroxyl groups in the polyether polyol. In all cases, a substantial equivalent excess of polyol to isocyanate was used, the preferred value for R being in the range of about 1.4 to 5.0. The excess hydroxyl is necessary to provide sights on the urethane resin for crosslinking with melamine agents during paint cure. 
     Table III shows the molecular weight characteristics of these urethane resin compositions determined by gel permeation chromatography. Some had viscosities at 100 weight percent resin (i.e., without solvent) of less than 10,000 centipoise. This means that a substantially solvent free paint is possible that still has a viscosity low enough to apply it by spraying, dipping or other such means. 
     
                       TABLE III______________________________________MOLECULAR WEIGHT CHARACTERISTICS ANDCOMPOSITION OF POLYURETHANE/POLYETHERRESIN SYSTEMS                  Index of                          Weight  Weight                  Hetero- Fraction                                  FractionMaterial               geniety Diol    TriolDesignation   M-n    M.sup.--w                  (M.sup.--w/M-n)                          Polyether                                  Polyether______________________________________LA      1500   2900    1.93    0.270   0.297IIB     2800   4900    1.75    0.179   0.165IIIC    4200   9600    2.28    0.111   0.083IVD     7200   34100   4.74    0.061   0.035______________________________________ 
    
     Table IV gives the formulation and properties of a white high gloss topcoat formulation based on urethane resin IIB above. This film was cured for 30 minutes at 107° C. (225° F.) in the presence of acid catalyzed trimethylol melamine crosslinking agent. The subject coatings can generally be cured at temperatures of about 225° F. or higher in 30 minutes. 
     
                       TABLE IV______________________________________FORMULATION AND PROPERTIES OF A WHITETOPCOAT BASED ON POLYURETHANE RESIN IIBMaterial          Weight (g)______________________________________A.   FORMULATIONResin IIB        160Titanium Dioxide.sup.a                 128Cymel® 325.sup.b                 200Cycat 296.sup.c  10Cellosolve Acetate                 40Methyl Ethyl Ketone                 70Butyl Alcohol    40B.   PROPERTIESPigment/Binder Ratio                 0.4Polyol/Melamine Ratio                 1.0Total Weight Solids                 75%Gloss (20° meter)                 82-83Knoop Hardness   6.5-7.0 KHNForward Dart Impact                 15.8 joules (140 inch pounds)Reverse Dart Impact                 10.1 joules (90 inch pounds)______________________________________ .sup.a GliddenDurke RCl6 Pigment .sup.b American Cyanamid; 4 weight parts trimethylol per 1 weight parts butanol .sup.c American Cyanamid; equal weight parts dimethyl pyrophosphoric acid and butanol 
    
     The gloss, indicated as &#34;Gloss (20° meter)&#34; is a number that is the percent of a beam of incident light reflected at 20° to a plane perpendicular to the painted panel measured using a Hunter Lab glossmeter. Readings were taken at five different locations on each panel and averaged. Initial gloss for this white paint was 82-83 percent. The Knoop hardness was 6.5 to 7.0, and the paint had forward and reversed impacts of 15.8 and 10.1 joules, respectively. Thus, this white, glossy, urethane paint exhibited physical properties which would make it an excellent candidate for automotive topcoat applications. However, the major disadvantage of this system compared to conventional acrylic lacquer or enamel systems is its poor U.V. stability. Without stabilizing the paint against U.V. and moisture degradation, it would not be useful. 
     Hindered piperidine compounds are known to provide varying degrees of U.V. stabilization to urethane paints. One such composition is bis-(2,2,6,6-tetramethyl piperidinyl-4)sebacate. ##STR2## 
     This hindered amine U.V. stabilizing agent is sold under the trade name Tinuvin 770® by Ciba-Geigy. It has been incorporated in the polymer backbone of moisture cured urethane paint as disclosed in U.S. Pat. No. 4,243,792. 
     In the subject melamine cured system, however, Tinuvin 770® was not found to be a satisfactory U.V. stabilizer. The piperdine molecule contains an amine functionality. This amine is a strong base with a pKa value of approximately 11.0. Thus, when Tinuvin 770® was added to any of the above paint formulations, it reacted very rapidly with the acid catalyst for the melamine cure to form an insoluble amine salt. The salt precipitated out of the paint forming highly flocculated coatings. No U.V. stability was provided by the precipitated Tinuvin 770®. 
     Attempts were then made to incorporate Tinuvin 770® into the polyurethane backbone to prevent its precipitation. The approach taken was to react Tinuvin 770® with an excess of diisocyanate prior to the addition of polyether polyols. This should have resulted in end-capping of the piperidinyl sebacate with an isocyanate group. Further reaction of the polyether polyol should have then resulted in incorporation of the U.V. stabilizing agent into a polyurethane structure. However, all attempts to achieve this end result failed. We have developed the following theory to explain these results. 
     Infrared spectroscopic analysis of the reaction mixture revealed that reacting the Tinuvin 770® with excess diisocyanate formed urea linkages between them. This was clearly indicated by the appearance of a peak at 1670 CM -1  representing the carbonyl stretching vibration. However, we believe that only about 70% of the amine groups were converted to the urea and 30% of the U.V. stabilizer never reacted in at all. This is probably due in part to the extreme steric hindrance of the four methyl groups around the amine functionality of the piperidine. The results of adding the polyether polyol indicated that the reaction between the polyol and isocyanate was preferred. The piperidine stabilizer was apparently driven out of the polymerized constituent by breaking the urea bonds. The Tinuvin 770® then precipitated with the acid catalyst. 
     Attempts were made to stabilize the system with other commercially available U.V. stabilizers. Two types of stabilizers were selected, the first being an ultraviolet absorber, Tinuvin 328® (Ciba-Geigy) and the second a tertiary hindered amine, Tinuvin 144® (Ciba-Geigy). The latter material was chosen because it was advertised as an alternative to Tinuvin 770® for acid containing systems. Though these U.V. stabilizers were reported to be effective in other polyurethane systems, they were not compatible with the melamine cured polyurethane polyether systems we had developed. 
     Thus, conventional U.V. stabilizers for polyurethane paints were not found to be useful or effective in our compositions. Therefore, a novel approach was taken to stabilizing the system with hindered amine U.V. stabilizers. This approach will be better understood in view of the following specific examples. 
     EXAMPLE I 
     In accordance with this invention we have discovered a method and means for stabilizing melamine cured, acid catalyzed, urethane paints of the type discussed above against degradation due to exposure to ultraviolet radiation and moisture. Referring to Table V, a resin composition and paint were formulated as follows. 
     
                       TABLE V______________________________________RESIN COMPOSITION AND COATING FORMULATIONOF A U.V. STABLE POLYURETHANE/POLYETHERCOATING SYSTEM______________________________________1.   Resin VEDiol P1010.sup.a 174.0g (0.329 equivalents)Triol TP 440.sup.a                 143.4g (1.01 equivalents)Triol TP 1540.sup.a                  88.4g (0.168 equivalents)Diisocyanate H.sub.12 MDI.sup.b                  70.6g (0.539 equivalents)TMP-40H           6.1g (0.039 equivalents)Methyl Ethyl Ketone                 120.5gR = 3.01, b = 0.78______________________________________2.   Formulation                           PercentMaterial         Weight (g)                           Non-Volatiles______________________________________Resin VE         50        80Titanium         32        100Dioxide.sup.cCymel® 325.sup.d                 50        80Cycat® 296-9.sup.e                 2.4       50Methyl Ethyl Ketone                 37.5      0Iso-Butyl Alcohol                 18.5      0Butyl Carbitol   4.5       0Total Constituents                 194.9     58______________________________________ .sup.a BASF Wyandotte  Pluracol® resins .sup.b E. I. DuPont de Nemours and Company  Hylene® W .sup.c GliddenDurkee; RCl6 .sup.d American Cyanamid, 4 weight parts trimethlol melamine per weight part butanol .sup.e American Cyanamid, equal weight parts dimethyl pyrophosphoric acid and butanol 
    
     6.1 Grams of 2,2,6,6-tetramethyl-4-piperidinol (TMP-40H) a monohydroxy hindered piperidine, was dissolved in 120.5 g of warm dry methyl ethyl ketone. To this solution, 70.6 g (0.539 equivalents) of H 12  MDI was added and the mixture was refluxed for 3 to 4 hours at about 82° C. We believe that this causes substantially all of the piperidine molecules to react with isocyanate at the piperidine&#39;s hydroxyl functionality. Thus, the piperidinol is chemically bonded to the isocyanate constituent by means of stable urethane linkages. 
     Thereafter, all of the polyether polyol was added. A substantial chemically equivalent excess (1.507 equivalents; R=3.01) of polyol is required so that there is substantially no free isocyanate in the reaction product but there are functional hydroxyl groups available for crosslinking. The polyol is generally refluxed with the other constituents long enough to assure that substantially all of the isocyanate reacts with it. The reaction product is a urethane paint resin with a reacted-in piperidinol constituent. 
     To make a heat curable white paint, 32 g titanium dioxide and 40 g of trimethylolmelamine in 10 g of butanol (Cymel®325) were passed through a sand mill together. The resultant paste was diluted with 37.5 g MEK, 18.5 g isobutyl alcohol and 4.5 g butyl carbitol. The last added constituent was 1.2 g dimethyl pyrophosphoric acid in 1.2 g butanol (2.4 g Cycat®296-9). 
     The &#34;Percent Non-Volatiles&#34; refers to that portion of a constituent that is not volatile solvent. The composition of Table VI calculated on that basis had a solids content of 58 percent. 
     The paint was sprayed as a topcoat over Bonderite 40 phosphated steel test coupons which had been primed with Inmont Corporation&#39;s lacquer primer/surfacer (No. U28AD025). The topcoating was cured for 30 minutes at 107° C. The resultant finish was mirror smooth and had a very high initial measured 20° gloss of 83 percent. 
     These painted coupons, and like coupons coated with a conventional production acrylic lacquer topcoat, and like coupons coated with a polyurethane-polyester paint without a piperidine stabilizer were all subjected to accelerated ultraviolet radiation aging tests in a commercially available QUV Accelerated Weathering Cabinet that is widely used in the coating industry to evaluate coating systems. 
     The utility of the QUV cabinet is in obtaining accelerated testing with respect to the effects of ultraviolet radiation (simulating sunlight) and humidity on paints, plastics and other organic material. The correlation between hours of exposure to UV light in the QUV cabinet and months of exposure to sunlight varies with the material being tested. However, in general, those materials showing good resistance to accelerated weathering also are resistant to degradation by sunlight and weather. 
     Referring to the FIGURE, it is evident that the piperidinol stabilized white paint of this Example retained its gloss far better than either the conventional acrylic lacquer or the stabilized urethane topcoats. Reacting the piperidinol stabilizer into the paint by means of urethane bonds thus was found to provide the desired UV stability, as well as a high solids content and a low energy cure. 
     EXAMPLE II 
     Two more white paint formulations were made by the methods described in Example I. Their compositions are set out in Tables VI and VII. 
     
                       TABLE VI______________________________________RESIN COMPOSITIONS AND PAINT FORMULATIONOF SYSTEM SUBJECTED TO FLORIDA EXPOSURE______________________________________Resin VIFDiol P410.sup.a   304.19g (1.435 equivalents)Triol TP440.sup.a 203.77g (1.435 equivalents)Diisocyanate H.sub.12 MDI.sup.b             221.32g (1.689 equivalents)TMP-40H            40.0g (0.254 equivalents)Methyl Ethyl Ketone             135.75gR = 2.0, b = 0.5______________________________________       Index of       Hetero-       geniety    Weight Fraction                             Weight FractionM-n  M.sup.--w       (M.sup.--w/M-n)                  Diol Polyether                             Triol Polyether______________________________________1700 3400   2.00       1.109      0.036______________________________________Formulation A                      PercentMaterial         Weight (g)                      Non-Volatiles______________________________________Resin VIF        460.6     85Cymel® 325.sup.c            500.0     80RCl-6 Titanium   320.0     100Dioxide Pigment.sup.dCycat® 296-9.sup.c            24.0      50Methyl Ethyl Ketone            70.6      0(from Resin VIF)Methyl Propyl Ketone            300.0     0Butyl Carbitol   20.0      0Butyl Cellosolve 70.0      0Iso-Butyl Alcohol            173.0     0(from Resin + 61g addition)Total Constituents            1948.2    58______________________________________ .sup.a BASF Wyandotte Pluracol® resins .sup.b E. I. DuPont de Nemours and Company  Hylene® W .sup.c GliddenDurkee .sup.d American Cyanamid 
    
     
                       TABLE VII______________________________________RESIN COMPOSITIONS AND PAINT FORMULATIONOF SYSTEM SUBJECTED TO FLORIDA EXPOSURE______________________________________Resin VIIG______________________________________Diol P410.sup.a   331.24g (1.562 equivalents)Triol TP440.sup.a 221.87g (1.562 equivalents)Diisocyanate H.sub.12 MDI.sup.b             205.80g (1.571 equivalents)TMP-40H            41.60g (0.265 equivalents)Methyl Ethyl Ketone             141.20gT = 2.4, b = 0.5______________________________________                 Index of         Weight                 Hetero- Weight   FractionMaterial              geniety Fraction Diol                                  TriolDesignation   M-n    M.sup.--w                 (M.sup.--w/M-n)                         Polyether                                  Polyether______________________________________VIIG    1100   2400   2.18    0.156    0.061______________________________________Formulation B                      PercentMaterial         Weight (g)                      Non-Volatiles______________________________________Resin VIIG       470.6     85Cymel® 325.sup.c            500.0     80RCl-6 Titanium   320.0     100Dioxide Pigment.sup.dCycat® 296-9.sup.c            26.0      50Methyl Ethyl Ketone            70.6      0(from Resin)Methyl Propyl Ketone            450.0     0Iso-Butyl Alcohol            188.0     0Butyl Carbitol   45.3      0Total Constituents            2070.5    55______________________________________ .sup.a BASF Wyandotte Pluracol® resins .sup.b E. I. DuPont de Nemours and Company  Hylene® W .sup.c American Cyanamid Company .sup.d GliddenDurkee 
    
     These paints were applied as topcoats to phosphated steel sample coupons as in the example above for one year of Florida exposure testing. 
     Clear topcoat formulations were made from piperidinol containing resins VIF and VIIG by eliminating the titanium dioxide pigment and the sand milling step of the mixing. These clear coats were applied to test coupons over the white U.V. stabilized formulations A and B as well as over conventional light camel metallic and light blue metallic production acrylic paints. 
     Test coupons were coated with a white acrylic lacquer and a white acrylic enamel currently used in production for purposes of comparison. 
     All these coupons were subjected to one year of Florida exposure testing. In the test, each coupon is mounted in the bottom of a shallow, open, black box which is oriented at a 5° pitch from the horizontal facing south. These boxes are mounted on the top of a building where the coupons are exposed to the bright sunlight and damp weather of the Florida climate. This test is one of the toughest on the gloss retention characteristics of a paint. 
     The results of the Florida exposure test are set out in Table VIII. 
     
                       TABLE VIII______________________________________12 MONTH FLORIDA EXPOSURE (20° BLACK BOX)                     Gloss After                               Gloss          Initial Gloss                     One Year  RetentionSample Description          (20° Meter)                     Florida   (%)______________________________________White Formulation B          86.0       69.5      80.8White Formulation B          83.8       60.8      72.6White Formulation A          88.4       82.0      92.7White Formulation A          88.0       79.0      89.8White Formulation B          92.1       89.5      97.2Base Coat with ResinComposition VIF ClearCoatWhite Formulation B          91.4       89.5      97.9Base Coat with ResinComposition VIIG ClearCoatLight Camel Metallic Base          78.0       73.2      93.8Coat with Resin Compo-sition VIF Clear CoatLight Camel Metallic Base          77.7       65.8      84.6Coat with Resin Compo-sition VIIG Clear CoatLight Blue Metallic Base          88.2       82.3      93.3Coat with Resin Compo-sition VIF Clear CoatLight Blue Metallic Base          88.3       77.4      87.6Coat with Resin Compo-    Average   89.0sition VIIG Clear CoatWhite Lacquer - Current          77.5       50.2      64.8Production MaterialWhite Enamel - Current          83.3       59.6      71.6Production Material______________________________________ 
    
     The production enamel and lacquer coupons, respectively, retained only about 71.6 and 64.8 percent of their initial measured gloss. The average gloss retention of the subject topcoats with piperidinol stabilizer reacted into the urethane backbone on the average retained about 89.0 percent of their original gloss. 
     In summary, we have developed a novel coating composition and a method of making it. We have reacted a hindered piperidine U.V. stabilizer into the resin with stable urethane bonds. Simply mixing the constituents together does not yield an acceptable paint. Rather, our method of first reacting the piperidinol stabilizer with a suitable isocyanate and thereafter reacting in a chemical excess of polyol should be followed. 
     As may be deduced from the Examples, we prefer to incorporate (on a chemical equivalents basis) about 0.1 to 0.5 equivalents of monohydroxy hindered piperidine per equivalent polyisocyanate and about 1.4 to 5.0 times the total isocyanate equivalents polyether polyol. 
     To the best of our knowledge, ours are the only known U.V. stabilized urethane paints that can be cured at low temperatures within a reasonable time by an acid catalyzed melamine crosslinking reaction. The relatively low viscosities of the binder resins allows high solids contents in the coatings. 
     While our invention has been described in terms of specific embodiments thereof, clearly, other forms may be readily adapted by one skilled in the art. Accordingly, our invention is to be limited only by the following claims.