Abstract:
Provided are thermosetting powder coatings based on a resin based on dimethyl 2,6-decalindicarboxylate and neopentyl glycol. The resin may be modified with other diacids or diols but must have a suitable glass transition temperature (Tg) required for powder coatings. We have found that this aliphatic polyester meets the basic Tg requirement of powder coatings and found that it provides powder coating with excellent weatherability. The polyester resin has a number average molecular weight (Mn) of from about 1,500 to about 10,000, a glass transition temperature (Tg) of about 45° C. to 100° C., and a hydroxyl or acid number of from about 20 to about 100.

Description:
FIELD OF THE INVENTION 
     This invention belongs to the field of powder coatings. In particular, this invention relates to weatherable thermosetting powder coatings based preferably on resins comprised of dimethyl 2,6-decalindicarboxylate and neopentyl glycol. 
     BACKGROUND OF THE INVENTION 
     Plastic materials used in the manufacture of powder coatings are classified broadly as either thermosetting or thermoplastic. In the application of thermoplastic powder coatings, heat is applied to the coating on the substrate to melt the particles of the powder coating and thereby permit the particles to flow together and form a smooth coating. 
     Thermosetting coatings, when compared to coatings derived from thermoplastic compositions, generally are tougher, more resistant to solvents and detergents, have better adhesion to metal substrates and do not soften when exposed to elevated temperatures. 
     Resins which exhibit good weatherability and meet the basic Tg (glass transition temperature) requirements for powder coating compositions are becoming increasingly important, particularly for outdoor applications. 
     Powder coatings based on acrylic resins are known to have excellent weathering performance but are generally more expensive. 
     British Patent No. 962,913 discloses polyesters containing cyclohexanedicarboxylic acid (CHDA) and 2,6-decalindimethanol useful as film and molding plastics. 
     U.S. Pat. No. 3,313,777 discloses polyesters containing CHDA and 2,2,4,4-tetramethyl-1,3-cyclobutanediol useful as film and molding plastics. 
     U.S. Pat. No. 4,363,908 discloses copolyesters containing CHDA and 2,2,4,4-tetramethyl-1,3-cyclobutanediol useful as adhesives. 
     U.S. Pat. No. 4,525,504 discloses stabilized polyesters with improved weatherability based on CHDA and 2,2,4,4-tetramethyl-1,3-cyclobutanediol. These polyesters are high molecular weight polyesters useful in molding plastics. 
     U.S. Pat. No. 5,097,006 discloses all-aliphatic polyester resins based on 1,4-cyclohexanedicarboxylic acid and hydrogenated bisphenol A and/or cycloaliphatic diols, useful as weatherable powder coating resins. 
     Powder coatings which exhibit good weatherability and meet the basic Tg (glass transition temperature) requirements for a thermosetting powder coating composition are becoming increasingly important, particularly for outdoor applications. We have found that polyester resins based on dimethyl 2,6-decalindicarboxylate and neopentyl glycol meet the basic Tg requirement for powder coatings and can be formulated with a crosslinker into powder coatings having excellent weatherability. 
     SUMMARY OF THE INVENTION 
     This invention relates to thermosetting powder coatings based on a resin prepared from dimethyl 2,6-decalindicarboxylate and neopentyl glycol. 
     The resin may be modified with other diacids or diols but must have a Tg suitable for powder coatings. We have found that this aliphatic polyester meets the basic Tg requirement of powder coatings and found that it provides powder coating with excellent weatherability. The polyester resin has a number average molecular weight (Mn) of from about 1,500 to about 10,000, preferably from about 2,000 to 6,000 and a glass transition temperature (Tg) of about 45° C. to 100° C., preferably 50° to 70° C. and hydroxyl or acid number of from about 20 to about 100, preferably from about 30 to about 80. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention provides a curable polyester comprising 
     (a) about 40 to about 100 mole percent of residues of 2,6-decalindicarboxylic acid; 
     (b) about 40 to about 100 mole percent of residues of 2,2-dimethyl-1,3-propanediol; 
     and any remaining residues comprised of residues of polyols and polycarboxylic acids; 
     said polyester having a number average molecular weight of from about 1500 to 10,000, a glass transition temperature of about 45° C. to about 100° C., and a hydroxy or acid number of from 20 to 100. 
     In the above polyester, it should be appreciated that the total mole percent of (a) and polycarboxylic acids will always equal 100 percent; likewise, the total mole percent of (b) and polyols will always equal 100 percent. 
     The above polyesters are useful as binders in thermosetting powder coating compositions. As noted above, the polyesters contain 40 to about 100 mole percent of residues of 2,6-decalindicarboxylic acid and 40 to 100 mole percent of resides of 2,2-dimethyl-1,3-propanediol, with 70 to 100 and 70 to 100 being preferred, respectively. In a further preferred embodiment, the polyester will have from about 80 to about 100 mole percent of residues of 2,6-decalindicarboxylic acid and 80 to 100 mole percent of resides of 2,2-dimethyl-1,3-propanediol. 
     In the above polyesters, the remaining percent of dicarboxylic acid and polyol residues can be selected from other known aliphatic polyol and aliphatic diacid residues. 
     Examples of such polyols includes polyester polyol branching agents such as trimethylolpropane, trimethylolethane, glycerol, and the like. Other examples of suitable polyols include 1,4-butanediol, ethylene glycol, propylene glycol, 1,3-propanediol, 2,4-dimethyl-2-ethylhexane-1,3-diol, 2-ethyl-2-butyl-1,3-propanediol, 2-ethyl-2-isobutyl-1,3-propanediol, 1,3-butanediol, 1,5-pentanediol, 1,6-hexanediol, thiodiethanol, 1,2-, 1,3- and 1,4-cyclohexanedimethanol, 2,2,4,4-tetramethyl-1,3-cyclobutanediol, hydrogenated Bisphenol A, 1,4-xylylene-diol and the like. 
     Preferably the other dicarboxylic acid residues of the polyesters are selected from residues of oxalic; malonic, dimethylmalonic; succinic; glutaric; adipic; trimethyladipic; pimelic, 2,2-dimethylglutaric; azelaic; sebacic; fumaric; maleic; itaconic; 1,3-cyclopentanedicarboxylic; 1,2-cyclohexanedicarboxylic; 1,3-cyclohexanedicarboxylic; 1,4-cyclohexanedicarboxylic; phthalic; terephthalic; isophthalic; 2,5-norbornanedicarboxylic; 1,4-naphthalic; diphenic; 4,4&#39;-oxydibenzoic, diglycolic; thiodipropionic; 4,4&#39;-sulfonyldibenzoic; and 2,6-naphthalenedicarboxylic acids. 
     As used herein, the terms &#34;hydroxyl-functional&#34; and &#34;carboxyl-functional&#34; as used to describe the polyester is used in its ordinary art-recognized meaning. In other words, such terms denote whether the polyester (or binder) is predominantly carboxyl- or hydroxyl-functional in character, thereby dictating the choice of crosslinker. In such a case, for example, a hydroxyl functional resin will have an acid number of less than about 15 (and a hydroxyl number of 20 to 100) and a carboxyl functional resin will have a hydroxyl number of less than about 15 (and a carboxyl or acid number of 20 to 100). 
     As a further aspect of the present invention, there is provided a thermosetting powder coating composition comprising 
     (I) a curable polyester comprising 
     (a) about 40 to about 100 mole percent of residues of 2,6-decalindicarboxylic acid; 
     (b) about 40 to about 100 mole percent of residues of 2,2-dimethyl-1,3-propanediol; 
     and any remaining residues comprised of residues of polyols and polycarboxylic acids; 
     said polyester having a number average molecular weight of from about 1500 to 10,000, a glass transition temperature of about 45° C. to about 100° C., and a hydroxy or acid number of from 20 to 100; 
     (II) a crosslinking effective amount of a cross-linker reactive with said polyester. 
     In a preferred embodiment, the thermosetting powder coating composition of the present invention is comprised of 
     A. a carboxyl polyester resin, 
     B. a weatherable epoxy crosslinker such as triglycidylisocyanurate (TGIC) commercially available as Aradite PT-810 by Ciba Geigy or a Beta-hydroxylalkyl amide such as Bis(N,N-dihydroxyethyl)adipamide commercially available from Rohm and Haas as Primid XL552 or 
     A. a hydroxyl polyester resin described above 
     B. a blocked polyisocyanate crosslinker or a crosslinker from the glycoluril family of &#34;aminoplast&#34; crosslinking agent, such as tetramethoxymethyl glycouril commercially available as Powderlink 1174. 
     The blocked polyisocyanate crosslinkers are those well known in the art of powder coatings. 
     Examples include those which are based on caprolactam blocked isophoronediisocyanates such as Huls B1530, Cargill 2400, Rucote NI-2 or those which are based on caprolactam-blocked tolylene 2,4-diisocyanate such as commercial Cargill 2450 crosslinker or the self blocked uretdione of isophoronediisocyanate commercially available as Huls BF1540, and 
     C. additives such as benzoin, flow aids, pigments and catalyst. 
     Additives commonly used are benzoin for reducing entrapped air or volatiles, flow aid such as Modaflow III, catalyst such as dibutyltin dilaurate and pigments such as titanium dioxide. 
     In a further preferred embodiment, the curable polyester is comprised of 
     (a) 40 to 100 mole percent of residues of 2,6-decalindicarboxylic acid; 0 to 60 mole percent of residues of a compound selected from the group consisting of naphthalenedicarboxylic acid, isophthalic acid, terephthalic acid, and cyclohexanedicarboxylic acid; and 
     (b) 40 to 100 mole percent of residues of 2,2-dimethyl-1,3-propanediol; 0 to 50 mole percent of residues of cyclohexanedimethanol; 0 to 10 mole percent of residues of a compound selected from the group consisting of trimethylolpropane, glycerol, and trimellitic anhydride; 
     the total mole percent of (a) being 100 percent and the total mole percent of (b) being 100 percent. 
     In the compositions of the present invention, the most readily-available, and thus the preferred, blocked isocyanate cross-linking agents or compounds are those commonly referred to as ε-caprolactam-blocked isophorone diisocyanate, e.g., those described in U.S. Pat. Nos. 3,822,240, 4,150,211 and 4,212,962, incorporated herein by reference. However, the products marketed as ε-caprolactam-blocked isophorone diisocyanate may consist primarily of the blocked, difunctional, monomeric isophorone diisocyanate, i.e., a mixture of the cis and trans isomers of 3-isocyanatomethyl-3,5,5-trimethylcyclohexylisocyanate, the blocked, difunctional dimer thereof, the blocked, trifunctional trimer thereof or a mixture of the monomeric, dimeric and/or trimeric forms. For example, the blocked polyisocyanate compound used as the cross-linking agent may be a mixture consisting primarily of the ε-caprolactam-blocked, difunctional, monomeric isophorone diisocyanate and the ε-caprolactam-blocked, trifunctional trimer of isophorone diisocyanate. The description herein of the cross-linking agents as &#34;blocked isocyanates&#34; refers to compounds which contain at least two isocyanato groups which are blocked with, i.e., reacted with, another compound, e.g., ε-caprolactam. The reaction of the isocyanato groups with the blocking compound is reversible at elevated temperatures, e.g., normally about 150° C., and above, at which temperature the isocyanato groups are available to react with the hydroxyl groups present on the free hydroxy groups of the polyester to form urethane linkages. 
     Alternatively, the blocked isocyanate may be a cross-linking effective amount of an adduct of the 1,3-diazetidine-2,4-dione dimer of isophorone diisocyanate and a diol having the structure ##STR1## wherein R 1  is a divalent 1-methylene-1,3,3-trimethyl-5-cyclohexyl radical, i.e., a radical having the structure ##STR2##  R 2  is a divalent aliphatic, cycloaliphatic, araliphatic or aromatic residue of a diol; and X is a 1,3-diazetidine-2,4-dionediyl radical, i.e., a radical having the structure ##STR3##  wherein the ratio of NCO to OH groups in the formation of the adduct is about 1:0.5 to 1:0.9, the mole ratio of diazetidinedione to diol is from 2:1 to 6:5, the content of free isocyanate groups in the adduct is not greater than 8 weight percent and the adduct has a molecular weight of about 500 to 4000 and a melting point of about 70° to 130° C. 
     The adducts of the 1,3-diazetidine-2,4-dione dimer of isophorone diisocyanate and a diol are prepared according to the procedures described in U.S. Pat. No. 4,413,079, incorporated herein by reference, by reacting the diazetidine dimer of isophorone diisocyanate, preferably free of isocyanurate trimers of isophorone diisocyanate, with diols in a ratio of reactants which gives as isocyanto:hydroxyl ratio of about 1:0.5 to 1:0.9, preferably 1:0.6 to 1:0.8. The adduct preferably has a molecular weight of 1450 to 2800 and a melting point of about 85 to 120° C. The preferred diol reactant is 1,4-butanediol. Such an adduct is commercially available under the name Huls BF1540. 
     In the β-hydroxylalkyl amide systems above, it is further preferred that a catalyst comprised of a carboxylate salt of a metal such as zinc, aluminum, or titanium, or an oxide of aluminum or zinc is present. Especially preferred as a catalyst is zinc stearate. Further description of catalyst systems for an activated β-hydroxylalkyl amide system can be found in U.S. application Ser. No. 08/084,104, incorporated herein by reference. 
     The amount of the blocked isocyanate cross-linking compound (or other crosslinker) present in the compositions of this invention can be varied depending on several factors such as those mentioned above relative to the amount of component (I) which is utilized and its hydroxyl number. Typically, the amount of cross-linking compound which will effectively cross-link the polymers to produce coatings having a good combination of properties is in the range of about 5 to 30 weight percent, preferably 15 to 25 weight percent, based on the total weight of components (I) and (II). 
     The powder coating compositions of this invention may be prepared from the compositions described herein by dry-mixing and then melt-blending components (I) and (II), and optionally a cross-linking catalyst, along with other additives commonly used in powder coatings, and then grinding the solidified blend to a particle size, e.g., an average particle size in the range of about 10 to 300 microns, suitable for producing powder coatings. For example, the ingredients of the powder coating composition may be dry blended and then melt blended in a Brabender extruder at 90° to 130° C., granulated and finally ground. The melt blending should be carried out at a temperature sufficiently low to prevent the unblocking of the polyisocyanate cross-linking compound and thus avoiding premature cross-linking. 
     The powder coating compositions preferably contain a flow aid, also referred to as flow control or leveling agents, to enhance the surface appearance of cured coatings of the powder coating compositions. Such flow aids typically comprise acrylic polymers and are available from several suppliers, e.g., MODAFLOW from Monsanto Company and ACRONAL from BASF. Other flow control agents which may be used include Modarez MFP available from SYNTHRON, EX 486 available from Troy Chemical, BYK 360P available from BYK Mallinkrodt and PERENOL F-30-P available from Henkel. An example of one specific flow aid is an acrylic polymer having a molecular weight of about 17,000 and containing 60 mole percent 2-ethylhexyl methacrylate residues and about 40 mole percent ethyl acrylate residues. The amount of flow aid present may preferably be in the range of about 0.5 to 4.0 weight percent, based on the total weight of the resin component, and the cross-linking agent. 
     The powder coating compositions may be deposited on various metallic and non-metallic (e.g., thermoplastic or thermoset composite) substrates by known techniques for powder deposition such as by means of a powder gun, by electrostatic deposition or by deposition from a fluidized bed. In fluidized bed sintering, a preheated article is immersed into a suspension of the powder coating in air. The particle size of the powder coating composition normally is in the range of 60 to 300 microns. The powder is maintained in suspension by passing air through a porous bottom of the fluidized bed chamber. The articles to be coated are preheated to about 250° to 400° F. (about 121° to 205° C.) and then brought into contact with the fluidized bed of the powder coating composition. The contact time depends on the thickness of the coating that is to be produced and typically is from 1 to 12 seconds. The temperature of the substrate being coated causes the powder to flow and thus fuse together to form a smooth, uniform, continuous, uncratered coating. The temperature of the preheated article also effects cross-linking of the coating composition and results in the formation of a tough coating having a good combination of properties. Coatings having a thickness between 200 and 500 microns may be produced by this method. 
     The compositions also may be applied using an electrostatic process wherein a powder coating composition having a particle size of less than 100 microns, preferably about 15 to 50 microns, is blown by means of compressed air into an applicator in which it is charged with a voltage of 30 to 100 kV by high-voltage direct current. The charged particles then are sprayed onto the grounded article to be coated to which the particles adhere due to the electrical charge thereof. The coated article is heated to melt and cure the powder particles. Coatings of 40 to 120 microns thickness may be obtained. 
     Another method of applying the powder coating compositions is the electrostatic fluidized bed process which is a combination of the two methods described above. For example, annular or partially annular electrodes are mounted in the air feed to a fluidized bed so as to produce an electrostatic charge such as 50  to 100 kV. The article to be coated, either heated, e.g., 250° to 400° F., or cold, is exposed briefly to the fluidized powder. The coated article then can be heated to effect cross-linking if the article was not preheated to a temperature sufficiently high to cure the coating upon contact of the coating particles with the article. 
     The powder coating compositions of this invention may be used to coat articles of various shapes and sizes constructed of heat-resistance materials such as glass, ceramic and various metal materials. The compositions are especially useful for producing coatings on articles constructed of metals and metal alloys, particularly steel articles. 
     Further examples of formulation methods, additives, and methods of powder coating application may be found in User&#39;s Guide to Powder Coating, 2nd Ed., Emery Miller, editor, Society of Manufacturing Engineers, Dearborn, (1987). 
     The inherent viscosity (I.V.), in dL/g are determined in phenol/tetrachloroethane (60/40 w/w) at a concentration of 0.5 g/100 ml. 
     The resin melt viscosity, in poise, are determined using an ICI melt viscometer at 200° C. 
     The acid number and hydroxyl number are determined by titration and reported as mg of KOH consumed for each gram of resin. 
     The glass transition temperature (Tg), is determined by differential scanning calorimetry (DSC) on the second heating cycle scanning at 20° C./minute after the sample has been heated to melt and quenched to below the resin Tg. Tg values are reported as midpoint. 
     The weight average molecular weight (Mw) and number average molecular weight (Mn) are determined by gel permeation chromatography in tetrahydrofuran (THF) using polystyrene standard and a UV detector. 
     Impact strengths are determined using a Gardner Laboratory, Inc., impact tester per ASTM D 2794-84. 
     Pencil hardness is determined using ASTM D 3363-74. The hardness is reported as the hardest pencil which will not cut into the coating. The results are expressed according to the following scale: (softest) 6B, 5B, 4B, 3B, 2B, B, HB, F, H, 2H, 3H, 4H, 5H, 6H (hardest). 
     The 20 and 60 degree gloss are measured using a gloss meter (Gardener Laboratory, Inc. Model GC-9095) according to ASTM D-523. 
     The QUV resistance is measured by the loss of gloss. QUV is run by alternately exposing the coated panel at 45° C. to a 313 nm fluorescent tube for 8 hours followed by a condensation at 45° C. for 4 hours. Gloss is monitored every 100 hours of exposure. The number of hours needed to reduce the gloss to 50% of the original is reported. 
     The coatings provided this invention are further illustrated by the following examples. 
    
    
     EXAMPLE 1 
     To a 500 ml, 3-neck round bottom flask were added dimethyl 2,6-decalindicarboxylate (191.1 g, 0.752 moles), neopentyl glycol (69.8 g, 0.67 moles), hydrogenated bisphenol A (30.2 g, 0.126 moles), trimethylolpropane (5.6 g, 0.042 moles) and 100 ppm Titanium and IRGANOX 1010 antioxidant (Ciba Geigy) (0.25 g). The contents were heated to melt at 180° C. The flask was swept with 1.0 standard cubic feet per hour (scfh) nitrogen while the temperatures was raised from 180° C. to 230° C. over a 6-hour period. The batch temperature was maintained at 230° C. for 8 hours. The molten resin was poured to a syrup can where it cooled to a solid with the following properties: 
     
         ______________________________________I.V., (dL/g)       0.202ICI Melt Viscosity at              7200° C. (poise)Acid Number        0.0Hydroxyl number    39DSC (2nd cycle)Tg, (°C.)   61______________________________________ 
    
     EXAMPLE 2 
     To a 500 ml, 3-neck round bottom flask were added dimethyl 2,6-decalindicarboxylate (198.2 g, 0.780 moles), neopentyl glycol (63.2 g, 0.61 moles), 1,4-cyclohexanedimethanol (31.2 g, 0.217 moles), trimethylolpropane (5.8 g, 0.043 moles) and 100 ppm Titanium and IRGANOX 1010 antioxidant (0.25 g). The contents were heated to melt at 180° C. The flask was swept with 1.0 scfh nitrogen while the temperatures was raised from 180° C. to 230° C. over a 6-hour period. The batch temperature was maintained at 230° C. for 8 hours. The molten resin was poured to a syrup can where it cooled to a solid with the following properties: 
     
         ______________________________________I.V., (dL/g)       0.212ICI Melt Viscosity at              7200° C. (poise)Acid Number        0.0Hydroxyl number    33DSC (2nd cycle)Tg, (°C.)   48______________________________________ 
    
     EXAMPLE 3 
     To a 500 ml, 3-neck round bottom flask were added 2,6-naphthalenedicarboxylic acid (175.23 g, 0.78 moles), neopentyl glycol (76.2 g, 0.73 moles), hydrogenated bisphenol A (20.7 g, 0.086 moles), trimethylolpropane (5.8 g, 0.043 moles) and FASCAT 4100 catalyst (0.25 g) and IRGANOX 1010 (0.25 g). The contents were heated to melt at 180° C. The flask was swept with 1.0 scfh nitrogen while the temperatures was raised from 180° C. to 230° C. over a 6-hour period. The batch temperature was maintained at 230° C. for 8 hours. The molten resin is poured to a syrup can where it cooled to a solid with the following properties: 
     
         ______________________________________I.V., (dL/g)       0.205ICI Melt Viscosity at              16200° C. (poise)Acid Number        1.0Hydroxyl number    37DSC (2nd cycle)Tg, (°C.)   57______________________________________ 
    
     EXAMPLE 4 
     A powder coating composition was prepared from the following materials: 
     
         ______________________________________164.0 g         Polyester of Example 136.0 g          HULS 1530 crosslinker100.0 g         Titanium dioxide2.0             Dibutyltin dilaurate3.0 g           MODAFLOW 20002.0 g           Benzoin2.0 g           TINUVIN 1444.0 g           TINUVIN 234______________________________________ 
    
     The above material was mixed in a Vitamix mixer and compounded in an APV extruder at 110° C. The extrudate was cooled, granulated, and pulverized in a Bantam mill with liquid nitrogen bled into the grinding chamber. The powder was classified through a 200 mesh screen. 
     The powder was electrostatically applied to a 3 in. X 9 in. metal panel and cured in a 350° F. oven for 20 minutes. The film properties are as follows: 
     
         ______________________________________Film thickness, Mil       2.0Impact strength, (in.-lb)Front                    20Reverse                 &lt;20Pencil Hardness           2HGloss20 deg                   2860 deg                   80MEK double rubs          200+______________________________________ 
    
     EXAMPLE 5 
     A powder coating composition was prepared from the following materials: 
     
         ______________________________________168.0 g         Polyester of Example 232.0 g          HULS 1530 crosslinker100.0 g         Titanium dioxide2.0             Dibutyltin dilaurate3.0 g           MODAFLOW 20002.0 g           Benzoin2.0 g           TINUVIN 1444.0 g           TINUVIN 234______________________________________ 
    
     The above material was mixed in a Vitamix mixer and compounded in an APV extruder at 110° C. The extrudate was cooled, granulated, and pulverized in a Bantam mill with liquid nitrogen bled into the grinding chamber. The powder was classified through a 200 mesh screen. 
     The powder was electrostatically applied to a 3 in. X 9 in. metal panel and cured in a 350° F. oven for 20 minutes. The film properties are as follows: 
     
         ______________________________________Film thickness, Mil       2.1Impact strength, (in.-lb)Front                    20Reverse                 &lt;20Pencil Hardness         FGloss20 deg                   6260 deg                   94*MEK double rubs         200+QUV, hours to 50% loss  1500______________________________________ *(Methyl Ethyl Ketone) 
    
     EXAMPLE 6 
     A powder coating composition was prepared from the following materials: 
     
         ______________________________________164.0 g         Polyester of Example 336.0 g          HULS 1530 crosslinker100.0 g         Titanium dioxide2.0             Dibutyltin dilaurate3.0 g           MODAFLOW 20002.0 g           Benzoin2.0 g           TINUVIN 1444.0 g           TINUVIN 234______________________________________ 
    
     The above material was mixed in a Vitamix mixer and compounded in an APV extruder at 110° C. The extrudate was cooled, granulated, and pulverized in a Bantam mill with liquid nitrogen bled into the grinding chamber. The powder was classified through a 200 mesh screen. 
     The powder was electrostatically applied to a 3 in. X 9 in. metal panel and cured in a 350° F. oven for 20 minutes. The film properties are as follows: 
     
         ______________________________________Film thickness, Mil       2.2Impact strength, (in.-lb)Front                    20Reverse                 &lt;20Pencil Hardness         HGloss20 deg                   5260 deg                   90MEK double rubs          200+QUV, hours to 50% loss   960______________________________________