Glycoluril powder coating compositions
BACKGROUND OF THE INVENTION 
     A desirable property of powder coating compositions is that they maintain suitable flow out and cure characteristics during application to a substrate. Typically, a powder coating composition which is applied to a surface heated, and melts, experiences a reduction in viscosity, and flows out onto its substrate. If the viscosity of the powder coating melt remains sufficiently low for a sufficient time, then volatile byproducts in the coating can escape. Continued heating of the powder coating begins crosslinking with the consequence that the viscosity increases and any remaining defects are trapped in the coating. 
     The use of solid glycoluril type amino resin crosslinkers are generally advantageous in powder coating compositions. However, volatile byproducts, such as methanol are evolved during cure. It is important that improved formulations be developed which provide opportunity for such glycoluril amino resin byproducts to escape during the coating process to give coatings of improved appearance and thickness. 
     Glycoluril resin type amino crosslinking systems such as those containing tetramethoxymethyl glycoluril may produce pinholes in thicker films (over about 89 microns due to methanol outgassing. It has now been discovered that addition of suitable rheology control additives allows for the production of defect free films thicknesses in excess of 115 microns. 
     FIELD OF THE INVENTION 
     This invention is an improvement in amino resin based powder coating compositions. 
     SUMMARY OF THE INVENTION 
     This invention is an improved powder coating composition comprising; (i) a solid glycoluril amino resin crosslinker, (ii) a solid hydroxyl functional resin, (iii) a strong acid catalyst, and (iv) a rheology improving additive, wherein the improvement comprises using a rheology improving additive selected from the group consisting of alkali metal oxides, alkali metal hydroxides, alkali metal carbonates, alkaline earth metal carbonates, alkaline earth metal oxides, alkaline earth hydroxides, aluminum oxides, aluminum hydroxides, aluminum carbonates, zinc salts of long chain fatty acids, aluminum salts of long chain fatty acids, monoamines, and diamines. 
     This invention is also an improved method of coating substrates by applying the novel powder coating composition of this invention and heating the coating to effect flow out and cure. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a rheology curve of glycoluril type amino resin crosslinked powder coating compositions with and without the rheology improving additive, triethylenediamine. 
     FIG. 2 is a rheology curve of glycoluril type amino resin crosslinked powder coating compositions with and without the rheology improving additive, calcium carbonate. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The essential ingredients of the coating composition of this invention are as follows: 
     1. a solid glycoluril amino resin 
     2. a solid hydroxyl functional resin 
     3. a strong acid catalyst 
     4. a rheology improving additive 
     The Solid Glycoluril Amino Resin Ingredient: 
     The essential amino resin crosslinking agent of the formulation of the invention is predominantly monomeric tetramethoxymethyl glycoluril represented by the formula: ##STR1## This glycoluril crosslinker is marketed as POWDERLINK® 1174 Crosslinking Agent by American Cyanamid company (Wayne, N.J. U.S.A.) and has the following characteristics: 
     Physical form: White to pale yellow flakes 
     Non-volatiles: 98% by weight 
     Melting range: 90°-1100° C. 
     Ave. Mol. Wt.: 350 
     Equivalent Wt.: 90-125 
     The Solid Hydroxyl functional Resin Ingredient: 
     The hydroxyl functional resin which is reactive with the glycoluril amino resin crosslinker may be any solid resin which is conventionally known to have utility in amino resin systems. Such resins include hydroxyl functional acrylic resins, hydroxyl functional polyurethane resins, and hydroxyl functional polyester resins. 
     Hydroxyl functional resins based on either acrylics or polyesters may have pendent or terminal hydroxyl functionality and preferably have glass transition temperatures between 35° C. and 100° C. 
     A subset of resins having particular utility for powder coating compositions are crystalline polyesters having low melt viscosities. These resins are derived from 1,4-butanediol and trans-1,4-cyclohexanedicarboxylic acids, and are more fully described in U.S. Pat. No. 4,352,924. The polymers of the &#39;1924 patent tend to have lower melt viscosities and generally give better melt flow in powder coating compositions. 
     Other specific hydroxyl functional resins all having weight average molecular weights from 2000 to 5000 and hydroxyl numbers from 25 to 40 which are useful in the invention are as follows: 
     1. Crylcoat™ 3109 polyester resin, a product of UCB Chemicals, Belgium. 
     2. Rucote™ 101 polyester resin, a product of Ruco Polymer Corporation, 
     3. Rucote™ XP-402, an internally catalyzed sulfonate bearing polyester resin, a product of Ruco Polymer Corporation, 
     4. Alftalat™ AN 745 polyester resin, a product of Hoechst Celanese Corporation, 
     5. Joncryl™ SCX-800 acrylic resin, a product of S.C. Johnson Corporation. 
     6. Crylcoat™ 3493 polyester resin, a product of UCB Chemicals, Belgium. 
     The Strong Acid Catalyst Ingredient: 
     Strong acid catalysts, defined herein as catalysts having a pKa of less than 4, are generally recommended for use with glycoluril crosslinkers. Examples are p-toluene sulfonic acid, N-methanesulfonyl p-toluenesulfonamide (MTSI), dinonylnaphthalene disulfonic acid, dodecylbenzene sulfonic acid, and phosphoric acids. 
     Another desirable class of strong acids are the internally catalyzed sulfonate bearing hydroxyl terminated powder coating polyesters as more fully described in International Publication Number WO 91/10694. These sulfonate salt group bearing polyesters are synthesized by esterification or transesterification of glycols with polycarboxylic acids and also with a sulfonate salt bearing reactant such as 5-sodium sulfo isophthalic acid. 
     The Rheology Improving Additive: 
     The rheology improving additive of the invention is selected from the group consisting of alkali metal oxides, alkali metal hydroxides, alkali metal carbonates, alkaline earth metal carbonates, alkaline earth metal oxides, alkaline earth metal hydroxides, aluminum oxides, aluminum hydroxides, aluminum carbonates, zinc salts of long chain fatty acids (8 to 24 carbon atoms), aluminum salts of long chain fatty acids (8 to 24 carbon atoms), monoamines (e.g., 2,2,6,6,-tetramethyl-4-piperidinol), and diamines. The term, &#34;alkali metal&#34;, as used herein refers to Group Ia (Li, Na, . . . , etc.) and the term, &#34;alkaline-earth&#34;, as used herein refers to Group IIa (Mg, Ca, . . . , etc.) of the Periodic Table of Elements. 
     Surprisingly, it has been found that the addition of the above additive to the glycoluril based powder coating formulation results in a change of rheology during the critical f low out period before crosslinking begins, which has the practical and beneficial effect of increasing coating thickness and reducing defects. 
     Proportions of the Ingredients: 
     The relative proportions of the essential ingredients are not critical, but are generally found within the following limits: 
     
         ______________________________________
                  Wt. % range
Essential         based on Binder.sup.1 in
Ingredient        Powder Composition
______________________________________
Crosslinker        3 to 20
Hydroxyal functional resin
                  80 to 97
Strong acid catalyst
                  0.1 to 10
Rheology improver additive
                  0.05 to 50
______________________________________
 .sup.1 Binder is the combined weight crosslinker and hydroxyl functional
 resin.
 
    
     To arrive at the optimal amount of rheology additive it is desirable f or the user to prepare a series of rheology curves such as shown in FIG. 1. Thus, FIG. 1 shows the increasing benefit (lowering of viscosity before cure) of increasing triethylenediamine levels in curves 2 and 3. Curve 4, however, shows that too much triethylenediamine has been added and the method of judging suitable levels of rheology improving additive to be added to the powder coating composition. However, it is also sufficient to observe the appearance and physical properties of a coating made according to this invention to determine suitable rheology improver addition levels. 
     The components of the compositions according to this invention may be mixed by dry blending in a mixer or blender followed by compounding in an extruder and granulating, grinding and then screening to obtain a powder of suitable mesh size for coating. Alternatively some or all of the components may be dissolved in a solvent such as methylene chloride and spray dried at temperatures of about 50° C. by well-known techniques. 
     The Coating Method of the Invention: 
     The powder coating composition may be deposited on a substrate by use of a powder gun, by electrostatic deposition or by deposition from a fluidized bed or by other well-known methods of powder deposition. After deposition the powder is heated to a temperature sufficient to cause its particles to flow and thereafter crosslinks to form a coating on the substrate surface. 
     The following examples are submitted for a better understanding of the invention. 
     EXAMPLE 1 
     This Example describes the general procedure of testing and formulating glycoluril based amino resin powder coating compositions of the invention. 
     The following ingredients were melt mixed in a two roll mill at 125°-130° C. for 3 minutes. The resultant mixture was ground and classified to a powder paint of particle size 35 microns (plus or minus 10 microns): paint of particle size 35 microns (plus or minus 10 microns): 
     
                       TABLE 1
______________________________________
STANDARD POWDER PAINT FORMULATION
______________________________________
Crylcoat 3109 polyester
                     94.00
Benzoin              1.40
MTSI catalyst        0.50
T--O.sub.2 Pigment   40.00
Resiflow TMP-67 flow aid.sup.1
                     1.30
POWDERLINK ® 1174 resin
                     6.00
______________________________________
 .sup.1 acrylic polymer absorbed on silica, product of Estron Corp.
 
    
     Rheological cure profiles (FIG. 1 and FIG. 2) were measured for powder coating formulation using a Rheometrics RMS-605 mechanical spectrometer configured as a parallel plate rheometer. In these experiments complex viscosity was measured at a 2° C./min. heating rate, 10 rad/s oscillation frequency and varying strains. From these data were obtained flow index, average fluidity, fast cure onset temperature, minimum viscosity, and the graphs of the FIGURES. 
     The above composition was electrostatically sprayed onto iron phosphate treated cold rolled steel panels to give the film properties shown below: 
     
                       TABLE 2
______________________________________
FILM PROPERTIES FOR EXAMPLE 1 FORMULATION
______________________________________
Substrate (iron   BO 1000   BO 1000
phosphate treated CRS).sup.1
Bake Temperature (°C.)
                  175       190
Bake Time (min.)   20        20
MEK Rub            200+      200+
Film Appearance   TOP.sup.2 TOP
Film Thickness (microns)
                  50.8/55.9 45.7/71.1
KNOOP Hardness    11.8      11.9
Impact F/R (Kg.-Meters)
                  1.84/1.84 1.84/1.84
Tristimulus Color -0.05     -0.05
Gloss 60°  78.5      94.1
Gloss 20°  45.6      77.2
Salt Spray, Time (hrs)
                  1008      1008
Creepage           0         0
Appearance         9         9
Humidity, 60° C. (hrs)
                  504       504
                  No Change No Change
______________________________________
 .sup.1 cold rolled steel, product of Parker lndustries
 .sup.2 TOP = tight orangepeel, undesirable
 
    
     EXAMPLE 2 
     The procedure and formulation of Example 1 were followed except that varying levels of triethylenediamine were used as a rheology improving additive as shown in Table 3 below: 
     
                       TABLE 3
______________________________________
Effect of Triethylenediamine Rheology Improving
Additive Addition
              Triethylenediamine wt. %
              on binder
              0.0    0.33     0.41
______________________________________
Flow Index (1/Pa)
                27       72       112
Ave. Fluidity (1/Pa · s)
                0.013    0.035    0.053
Min. Viscosity (Pa · s)
                39       14       11
Fast Cure Onset (°C.)
                172      200      214
Gel Time.sup.2 (seconds)
                358      515      775
Vertical Plate Flow.sup.3 (cm.)
                7.7      9.7      18.7
Pinhole Free Film Thickness
                86.4     &gt;96.5    &gt;119.4
(microns)
Film Appearance TOP      SOP      SOP.sup.1
______________________________________
 .sup.1 SOP = smooth orangepeel, desirable
 .sup.2 at 200° C., using Powder Coating Institute Procedure No. 6,
 &#34;Gel Time Reactivity&#34;, 1800 Diagonal Rd., Alexandria, VA 22314
 .sup.3 at 190° C., similar to Powder Coating Institute Procedure
 No. 7 for Inclined Plate Flow
 
    
     An additional composition was prepared using a higher triethyleneamine content of 0.50 weight percent on binder. This composition failed to cure. 
     The results of the Example 2 triethylenediamine rheology improving additive addition to the coating composition are graphically shown in FIG. 1. Curve &#34;1&#34; of FIG. 1 corresponds to the control composition of Table 3 containing no triethylenediamine. Curve &#34;2&#34; corresponds to the 0.33% addition of triethylenediamine as shown in Table 3. Curve &#34;3&#34; corresponds to the 0.41% addition of triethylenediamine as shown in Table 3. Curve &#34;4&#34; corresponds to the 0.50% addition of triethylenediamine described in the text after Table 3. 
     FIG. 1 shows that the addition of triethylenediamine creates a larger window of reduced viscosity. During this time the volatile byproducts incident to the curing/crosslinking process have a much enhanced opportunity to escape. The escape of volatile materials which do not form part of the coating results in a coating of better appearance, greater thickness and other desirable properties. 
     EXAMPLE 3 
     The procedure and formulation of Example 1 were followed except that varying levels of calcium carbonate were used as a rheology improving additive as shown in Table 4 below: 
     
                       TABLE 4
______________________________________
Effect of Calcium Carbonate Rheology Improving Additive
              Calcium Carbonate wt. %
              on binder.sup.1
              0     10       15      20
______________________________________
Flow Index (1/Pa)
                21      42       68    90
Ave. Fluidity (1/Pa · s)
                0.011   0.019    0.026 0.032
Min. Viscosity (Pa · s)
                49      16       19    15
Fast Cure Onset (°C.)
                185     192      210   219
Gel Time (seconds)
                358     457      774   755
Vertical Plate Flow (cm.)
                7.7     8.8      7.4   8.2
Pinhole Free Film Thickness
                86.4    101.6    101.6 99.1
(microns)
Film Appearance TOP     SOP      SOP   SOP.sup.2
______________________________________
 .sup.1 calcium carbonate substituted for an equal weight of TiO.sub.2
 pigment
 .sup.2 SOP = smooth orangepeel, desirable
 
    
     This Example shows that when calcium carbonate is substituted for part of the composition TiO 2  pigment, an improvement in film thickness and appearance results. 
     The results of the calcium carbonate rheology improver addition to the coating composition are graphically shown in FIG. 2. Curve &#34;1&#34; is a control composition with no calcium carbonate as shown in Table 4. Curve &#34;2&#34; corresponds to the 10% addition of calcium carbonate as shown in Table 4. Curve &#34;4&#34; corresponds to the 15% addition of calcium carbonate as shown in Table 4. Curve &#34;4&#34; corresponds to the 20% addition of calcium carbonate as shown in Table 4. 
     FIG. 2 shows that the addition of calcium carbonate creates a larger window of reduced viscosity. During this time the volatile byproducts incident to the curing/crosslinking process have a much enhanced opportunity to escape. The escape of volatile materials which do not form part of the coating results in a coating of better appearance, greater thickness and other desirable properties. 
     The invention has been described in detail with particular reference to preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.