ALUMINUM WHEEL CLEARCOAT WITH UV ADDITIVE TO RETARD FILIFORM CORROSION

Abstract: Prolonged inhibition of filiform corrosion of an aluminum automotive road wheel is achieved by applying to a bare aluminum surface of the road wheel a powder clearcoat coating composition comprising a major amount of a thermoset resin component and a minor amount of a UV additive comprising a UV absorber and a light stabilizer compatible with the thermoset resin component, and then thermally curing the clearcoat coating composition to form a clear, adherent, protective coating on the aluminum surface.

Description: ALUMINUM WHEEL CLEARCOAT WITH UV ADDITIVE TO RETARD FILIFORM CORROSION FIELD OF THE INVENTION This invention relates to a method of inhibiting filiform corrosion of aluminum road wheels. More particularly, it relates to application of certain powder clearcoat compositions to a bare aluminum surface of a road wheel, and to the coated wheels. BACKGROUND OF THE INVENTION Aluminum is a preferred material for automotive road wheels. It is easily workable, durable, light weight and relatively cost competitive. The extraordinarily harsh automotive environment, however, has been found to lead to filiform corrosion and resultant loss of the bright, attractive finish of the wheel. The automotive environment can include temperature extremes, rapid thermal cycling through large temperature changes, exposure to humidity and moisture, corrosive road salt and a fury of grit abrasion. Also, road wheels and their coatings are subjected to stress due to vibration and shock. Consequently, a hard and tough protective coating is sought for aluminum road wheels which will provide good long term inhibition of filiform corrosion. Opaque, for example pigmented, organic film forming coatings have been used for aluminum vehicle parts in the past with generally good results. The true aluminum finish of the part is hidden by such opaque coatings, however, and designers have expressed the desire for clear protective coatings for bright- finished (e.g., machined) aluminum road wheel surfaces. A powder coating is especially desirable, since it would be usable with powder coating spray equipment presently in use in aluminum road wheel production lines. There is a significant investment made in such existing powder coating equipment, related personnel training, and associated production facilities integrated with the coating process. Early efforts to develop a suitable clearcoat for aluminum road wheels, specifically a powder clear coat, met with mixed results. A corrosion problem was found in some clear coated wheels after extended exposure to the automotive environment. In particular, filiform corrosion was observed, a thread-like corrosion of the metal at the interface of the metal surface and the protective coating. Such filiform corrosion marred the appearance of the wheel over time even without visually unacceptable degradation of the coating itself. Filiform corrosion has long been observed in various contexts and has been studied extensively. A paper prepared by Young J. Kim, Physical Metallurgy Laboratory, Materials Research Center, GE Corporate Research & Development, Schenectady, New York, entitled Filiform Corrosion And Electrochemical Impedance Technique for Evaluation of Coating System - A Review (August, 1991) reviews the literature on filiform corrosion on various metal substrates under organic coatings. The Kim paper summarizes the literature as revealing that filiform corrosion and the rate of its propagation at the metal/coating interface is dependent on any one or more of several identified factors. These factors include the humidity of the atmosphere to which the coated surface is exposed. Humidity above a certain level is said to be necessary to dissolve salts at the head of a filiform corrosion line ("a filiform"). Increasing coating thickness is said to increase the width of filiforms and finally to prevent corrosion by decreasing oxidation current density at the filiform head. A coating defect is said to be required, as is oxygen. Metal surface pretreatment procedures are identified also in the Kim paper as an important factor. Improvements in coating formulation, according to the literature reviewed by Kim, will have minimal impact on filiform corrosion if improper substrate pretreatment procedures are used before application of a coating, leading to disbonding. Study of inhibitor treatment of the substrate is recommended, for use in addition to pretreatment, to increase adhesion at the metal/coating interface. In the case of steel, both the concentration of C0 2 in the exposure environment and the stability of the interface between the metal and a phosphate coating are identified as factors controlling filiform corrosion. Even with the extensive investigations undertaken to date, as reviewed in the Kim paper, the formation and growth of filiform corrosion are still somewhat unclear. In addition to uncertainty as to all the factors necessary or important to filiform initiation and growth rate in any of the innumerable different environments and substrate/coating combinations, the precise mechanism of filiform corrosion is not fully understood. Thus, those skilled in the art will recognize that extended inhibition of filiform corrosion of aluminum automotive road wheels to which a powder clearcoat has been applied is a complex problem. Any of the factors mentioned in the Kim paper and any combination of them, could be significant or even controlling factors in the development of the particular filiform corrosion to which the present invention is directed. The present invention is significant, however, not only because of the technological difficulty of the problem solved, but also, as discussed above, because of the commercial significance of the existing needs it meets. SUMMARY OF THE INVENTION According to the present invention, filiform corrosion of exposed surfaces of aluminum automotive road wheels is inhibited over extended periods of use in typical automotive environments by applying to the bare aluminum surface of the road wheel a powder clearcoat composition comprising a major amount of a thermoset base resin component and a UV additive selected from UV absorbers and light stabilizers which are effective and compatible with the thermoset base resin component. The powder clearcoat is applied to an exposed surface of the aluminum part by electrostatic spraying and then thermally cured to form a clear, adherent protective coating on the aluminum surface. The term bare aluminum surface is used to mean a road wheel surface devoid of organic tegumentary coating. An aluminum road wheel surface which has been pretreated, such as by chromate pretreatment to form a chromate conversion coating in accordance with well known techniques, is included in the meaning of the term bare aluminum surface as that term is used herein. The precise mechanism by which the present invention prevents filiform corrosion at the interface of an aluminum road wheel surface and its protective clearcoat coating is not fully understood. Protective clearcoat coatings were used before the present invention in comparable film thicknesses over aluminum road wheels. Over time those prior coatings were observed to fail in preventing filiform corrosion of the aluminum road wheel surface. This occurred in cases in which the clearcoat itself has not suffered visually unacceptable degradation. The new method of the present invention, employing a powder clearcoat composition having the aforesaid UV additive in the base thermoset resin, provides extended inhibition of filiform corrosion even though the UV additive is not presently known to have a major, direct effect on the primary filiform corrosion factors identified in the above discussed Kim paper. Specifically, for example, it could not effect the humidity, oxygen or C0 2 content of the exposure atmosphere. In addition, it is not known to change the chemical nature of the wheel's aluminum surface. It is not known to control the degree of adhesion achieved between the coating and the aluminum substrate. Nevertheless, preferred embodiments of the invention are found to provide corrosion protection equivalent to the opaque paints still used for some aluminum road wheels. It is also advantageous that the spray application step of the method of the invention and the thermal curing step can be carried out in substantially the same manner and with the same equipment presently employed for clear coating aluminum automotive road wheels. Additional features and advantages of the invention will be understood by those skilled in the art of powder clearcoating automotive aluminum parts in light of the following more detailed description of certain preferred embodiments. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS All parts by weight given in the following disclosure are based on the total weight of the powder clearcoat composition. The term aluminum is used herein to mean aluminum alloys suitable for use as automotive road wheels, including, for example, any suitable aluminum silicon alloys, aluminum lithium alloys, aluminum copper base alloys such as aluminum bronze, aluminum magnesium, aluminum zinc, aluminum manganese and the like. The alloys can be single alloy, binary or have more than two metals other than aluminum. Preferably, aluminum road wheels of the invention are formed of standard aluminum alloy 356A or the like. The thermoset resin component which forms a major amount, preferably at least 70% by weight, of the powder clearcoat coating composition can be selected in most cases from any of the numerous suitable materials well known to those skilled in formulating powder clearcoat compositions. It will be readily apparent that the resin and its cross-linking agent should provide a solid powder coating which remains finely divided at storage and application temperatures for use in the powder coating compositions of the invention. Preferably, the thermoset resin component comprises a major amount of a thermoset resin selected from the group consisting of polyester resins and acrylic resins, the latter including, for example, polymethacrylate copolymers, together with a suitable cross-linking agent. Hydroxy functional polyester resin may be used, for example, in combination with a blocked isocyanate cross-linking agent. For example, hydroxy-functional polyesters are known to form suitable polyester urethane coatings when cured with caprolactam-blocked isophorone diisocyanate or the like. Numerous suitable hydroxyl functional polyester resins are known to those skilled in the art, including commercially available resins such as Rucote 107 available from Ruco Polymer Corp., Hicksville, New York, Cargill 3000 and Cargill 3016, both available from Cargill Company, Minneapolis, Minnesota, and Crylcoat 3109 available from UCB Chemicals Corp. , Norfolk, Virginia and the like. As noted above, blocked isocyanate curing agents (or cross-linking agents) are preferred for use with hydroxy functional polyester resins in powder clearcoat coating compositions of the invention. Numerous suitable blocked isocyanate curing agents will be apparent to those skilled in the art in view of the present disclosure, including the aforementioned caprolactam- blocked, preferably e-caprolactam-blocked, isophorone diisocyanate. Commercially available curing agents suitable for use with hydroxy functional polyester resin in powder clearcoat coating compositions of the inventions include, for example, HQls B-1530 available from Hϋls America, Piscataway, New Jersey, Cargill 2400, Cargill 2430 and Cargill 2450, available from Cargill Company, Minneapolis, Minnesota. Hydroxy functional polyester resin is used preferably in the range of about 50 to 100 parts by weight, more preferably .about 80 parts by weight. Blocked isocyanate curing agent is used together with the hydroxy functional polyester resin preferably in the range of about 1 to 50 parts by weight, more preferably about 20 parts by weight. Polyester resins having carboxyl functionality also are suitable for use in powder clearcoat coating compositions of the invention. A polyester resin having carboxyl functionality may be cured with " an epoxy functional cross-linking agent. Numerous suitable carboxyl functional polyester resins will be apparent to those skilled in the art in view of the present disclosure, including numerous commercially available resins such as, for example, Grilesta V7372 available from EMS-American Grilon, Inc. , Sumpter, South Carolina, Crylcoat 430 and Crylcoat 3010, both available from UCB Chemicals Corp. , Norfolk, Virginia, and Uralac 3400 and Uralac 3900, both available from DMS Resins USA Inc., Elmwood Park, New Jersey, and the like. Numerous suitable epoxy functional cross-linking agents will be apparent to those skilled in the art in view this disclosure, including triglycidyl isocyanurate (TGIC) , which is preferred. Suitable epoxy functional cross-linking agents include commercially available materials such as, for example, Tepic G available from Summit Specialty Chemical Co. , Fort Lee, New Jersey and PT-810 available from Ciba- Geigy Corp., Hawthorne, New York. Carboxyl functional polyester resin preferably is used in the range of about 80 to 100 parts by weight, more preferably about 93 parts by weight, together with triglycidyl isocyanurate cross-linking agent in the range of about 1 to 20 parts by weight, more preferably about 7 parts by weight. In accordance with other embodiments, an acidic polyester resin is employed together with a polyepoxide curing agent, such as triglycidyl isocyanurate to yield a TGIC polyester powder coating. As mentioned above, acrylic resins also are suitable for use with a curing agent as the base thermoset resin component of the powder clearcoat coating composition of the invention. In general, suitable acrylic resins include both polyacrylate resins and polymethacrylate copolymer resins. Such acrylic resins and their corresponding cross-linking agents, as well as suitable relative proportions of each for use in the powder clearcoat coating compositions of the invention, will be readily apparent to those skilled in the art in view of the present disclosure. Such resins and curing agents include commercially available materials well known in the coating industry. Suitable carboxyl functional acrylic resin may be used with an epoxy functional cross- linking agent. Hydroxy functional acrylic resin may be used with a blocked isocyanate cross-linking agent, preferably caprolactam-blocked isophorone diisocyanate. Suitable glycidyl functional acrylic resin may also be used, for example with dicarboxylic acid cross-linking agent or the like. Suitable glycidyl functional acrylic resins will be readily apparent to those skilled in the art in view of the present disclosure, including numerous commercially available resins such as, for example, Almatex P-7610, Almatex PD-7690 and Almatex PD-6100, all available from Anderson Development Corp., Adrian, Michigan, and Finedic 229-30 available from Dai Nippon Inc. and Chemicals, Osaka, Japan and the like. Suitable dicarboxylic acid cross-linking agents include, for example, dodecanedioic acid, which is preferred, decanoic acid and adipic acid. Additional suitable dicarboxylic acids will be readily apparent to those skilled in the art in view of this disclosure. Preferably glycidyl functional acrylic resins are used in the range of about 50 to 100 parts by weight, more preferably about 83 parts by weight, together with dicarboxylic acid cross-linking agent in the range of about 1 to 50 parts by weight, more preferably about 17 parts be weight in the case of dodecanedioic acid. Further regarding acrylic resins, a suitable powder coating resin which is found in various embodiments to have superior gloss properties employs a glycidyl ethacrylate copolymer cross-linked with a solid dicarboxylic acid, such as dodecanedioic acid. U.S. patent 3,781,380, which is incorporated herein by reference, teaches the use of a carboxy-terminated polymer as a cross-linking agent for acrylic resins, suitable for use as the thermoset resin component of the present invention. Various suitable acrylic resins are taught also in U.S. patent 3,998,768 which is incorporated herein by reference. Additional suitable thermoset resin components, including cross-linkable resins and suitable corresponding cross-linking agents, will be readily apparent to those skilled in the art in view of the present disclosure. Similarly, it is well within the ability of those skilled in the art to select thermoset resin components having desired special properties, such as high T , good storage stability, high gloss, good weatherability and the like, depending in part on the particular properties desired for the road wheel. As indicated above, the integrity and appearance of the cured powder clearcoat, most particularly the ability of the cured clearcoat to inhibit development and growth of filiform corrosion at the interface of the coating and the underlying aluminum wheel surface, is surprisingly imparted to the coating to a great degree by the addition of a UV additive, even though filiform corrosion was observed in the past to develop on aluminum road wheels whose clearcoat coating did not appear significantly adversely degraded by UV exposure. Suitable UV additives for use in the powder clearcoat composition of the invention are readily commercially available and will be apparent to those skilled in the art in view of the present disclosure. Certain preferred UV absorbers are selected from the group consisting of benzotriazole UV absorbers, 2- hyroxybenzophenone, oxanilide, diphenyl cyanoacrylate and a mixture of any of them. Especially preferred are o-hyroxyphenyl-2H-benzotriazole UV absorbers such as 2-[2'-hydroxy-3• ,5 ! -di-(α,α-dimethylbenzyl)phenyl]- 2H-benzotriazole in view of its superior performance in filiform corrosion inhibition and in view of its ready commercial availability. The use of o- hyroxyphenyl-2H-benzotriazoles as UV absorbers, including 2 - [2 *-hydroxy-3 • ,5 '-di-(α,α- dimethylbenzyl)phenyl]-2H-benzotriazole, to protect organic coatings from UV degradation is discussed in U.S. patent 4,226,763 to Dexter et al, the disclosure of which is incorporated herein by reference. Suitable commercially available UV absorbers include, for example, Tinuvin™ 900 UV absorber Tinuvin™ 328 and Tinuvin™ 1130, all available also from Ciba-Geigy, and UV-1164 available from American Cyanamid Co. , Charlotte, North Carolina. Most preferred for use as a UV additive in conjunction with a polyester urethane powder coating composition is caprolactam-2- oxohexamenthyleneimine. The UV absorber is used " in minor amount in the clearcoat coating composition, preferably in the range of .1 to 10 parts by weight, most preferably about 2 parts by weight. Suitable light stabilizers for the UV additive include, for example, hindered amine light stabilizers. Numerous suitable hindered amine light stabilizers are commercially available and will be apparent to those skilled in the art in view of the present disclosure. Preferred hindered amine light stabilizers include polymeric polyalkylpiperidine and the like. Hindered amine light stabilizers are discussed in U.S. patent 4,299,926 to Rody et al and U.S. patent 5,004,770 to Cortolano et al, the disclosures of both being incorporated herein by reference. Suitable, commercially available hindered amine light stabilizers include, for example, Tinuvin™ 622 LD, which is preferred, Tinuvin™ 292, Tinuvin™ 144, Tinuvin™ 944, all of which are available from Ciba-Geigy, Uvinol M- 40 available from BASF Corp., Parsippany, New Jersey, Sandovor 3056 available from Sandoz Color and Chemicals, Charlotte, North Carolina, and the like. Such hindered amine light stabilizers are used preferably in an amount within the range of about 0.1 to 5 parts by weight most preferably about 1 part by weight. Additional suitable light stabilizers will be readily apparent to those skilled in the art in view of the present invention. As indicated above, the UV additive of the powder clearcoat coating composition preferably comprises both UV absorber and hindered amine light stabilizer. The use of a combination of UV absorbers and hindered amine light stabilizers in powder clearcoat coating compositions is taught in U.S. patent no. 4,402,983 to Craven, that disclosure also being incorporated herein by reference. Those skilled in the art will recognize that it is a significant advance in the art that the present disclosure teaches the use of such materials in a method of preventing filiform corrosion of aluminum automotive road wheels. The superiority of powder clearcoat compositions containing UV additive comprising both UV absorber and hindered amine light stabilizer components in inhibiting filiform corrosion of aluminum automotive road wheels will be seen from the test results detailed below. Various other additives suitable for use in the present invention are well known to those skilled in the art of powder clearcoat compositions. A flow control agent, for example, may be incorporated, generally, in the range of about 0.1 to 5 parts by weight, preferably about 2 parts by weight. Exemplary suitable flow control agents include Troy EX-486 available from Troy Chemical Co., Newark, New Jersey, Resiflow P-67 available from Estron Chemical ' Inc. , Calvert City, Kentucky, Modaflow II available from Monsanto Co., St. Louis, Missouri and the like. Silica dry flow additive also may optionally be included in the powder clearcoat composition, such as Aerosil R-972 and aluminum oxide "C," both available from Charles Wagner Co., Philadelphia, Pennsylvania, Cab-O-Sil M5 and Tullanox 500, both available from Eastech Chemical Inc., Philadelphia, Pennsylvania and the like. Silica dry flow additive is used in the composition in a range generally of about 0.05 to 0.6 parts by weight, preferably about 0.2 parts by weight. Anti-oxidant may also be incorporated in the powder clearcoat composition, such as "hindered phenol" anti- oxidant available commercially as Irganox 1010 and Irganox 1076, both available from Ciba-Geigy. Such hindered phenol anti-oxidant can aid in inhibiting yellowing of the cured coating in the event of overbaking during cure. It is used generally in the range of about 0.01 to 2.0 parts by weight, preferably about 0.1 part by weight. Also phosphite type anti- oxidant may be employed, for example triphenyl phosphite. Commercially available phosphite type anti- oxidants include, for example, Irafos 168 available from Ciba-Geigy, and the like. Phosphite type anti- oxidants are used generally in a range of about 0.01 to 2.0 parts by weight, preferably about 0.1 part by weight. Another flow control agent suitable for use in the powder clearcoat coating composition to reduce pinholing in the cured coating is benzoin, available commercially as Uraflow B available from GCA Chemical Corp., Stamford, Connecticut, preferably used in the range of about 0.1 to 2.0 parts by weight, more preferably 0.25 part by weight. In addition, a solid epoxy resin having a free epoxy group may be used as an adhesion promoting epoxy resin additive. Such optional epoxy resin additive provides a significant improvement and is the subject of a patent application filed concurrently herewith by an inventive entity which includes inventors of the present invention. The epoxy resin is used in minor amount relative the above described thermoset resin component. Epoxy resins having an aromatic backbone, such as those derived from bisphenol A or bisphenol F and having an epoxide equivalent weight of from about 400 to about 1500 are preferred. Commercially available resins of this type are known to those skilled in the art and include, for example, Dow Chemical's XU 71944.00L resin. Epoxy resins generated from the glycidyl ethers of glycerol and those from the reaction of an epoxylated novolac resin with a di- or polyhydric phenol are also suitable. Although epoxy resins are well known to be useful as the main resin component of a powder coating composition, they are used herein as an optional component in minor amount along with the aforesaid thermoset resin component to increase adhesion of the cured coating to the aluminum substrate surface. The amount of such adhesion promoting epoxy additive is preferably from about 1% to about 20% by weight of the coating composition, more preferably from about 3% to about 5%. Additional suitable additives for use in the powder clearcoat coating composition of the invention will be apparent to those skilled in the art in view of the present disclosure. In the following examples, all parts are expressed as parts by weight unless otherwise stated. References to an "HPMF" value for a coating composition are determined in accordance with the following test procedure. Hot Plate Melt Flow Test Procedure Scope: This test procedure is used to determine the length of flow of ther osetting powder coating compositions under specified conditions. Apparatus: Balance — Mettler PE300 digital readout, or equivalent, capable of weighing accurately to 0.01 grams. Pellett Press — "Parr" hand operated press, or equivalent, available from Arthur H. Thomas Company, Philadelphia, PA. This includes a modified steel mold, 12.7mm diameter cylindrical cavity with face plugs to form a pellet 6mm thick. Cure Plate — Model S-200 Thermo Electric Cure Plate available from Thermo Electric Co. , Cleveland, OH, accurate to ±4*F (2'C). The cure plate is affixed at a 35° angle and set at a temperature of 375°F (191'C). Starrett C636 Steel Rule, or equivalent, capable of measuring 0.5mm. Starrett PR12245 Protractor Head, or equivalent. Surface pyrometer suitable for use at the test temperature, having calibration capability. Stop watch or electric timer which measures to the second. Sample preparation: A pellet, 12.7mm in diameter by 6mm in thickness, is pressed from the material to be tested. The pellet is re-weighed after compacting and must be within ±0.01 grams of the specified weight. Procedure: Use surface pyrometer to determine the temperature of the hot plate. Adjust hot plate to appropriate temperature, if necessary. Press the pellet onto the plate using as little pressure as possible and releasing immediately. Then start the timing device. The pellet is allowed to melt and flow for at least 5 minutes. Measurement is taken after the 5 minute test duration of the distance from the uppermost point of the original position of the pellet to the extreme lower point of flow. The measurements are taken with the specimen on the hot plate. The total length of flow is measured with a steel rule to the nearest 0.5mm. Note: If the pill leaves a void at any point on the plate, a new pill should be made. Example 1 Twenty-four hundred parts of a hydroxy-functional terephthalate reaction product of a mixture of neopentyl glycol and trimethylolpropane having, an OH value of 50, 600 parts of caprolactam blocked isophorone diisocyanate, 60 parts of Troy EX-486 (Troy Chemical Co.) resinous flow control agent, 60 parts of Tinuvin™ 900 (Ciba-Geigy) UV absorber, and 30 parts of Tinuvin™ 622 (Ciba-Geigy) light stabilizer were blended together and the blend was then compounded by passing it twice through a single screw Buss extruder which allowed a maximum temperature of 180*F. The extrudate was then cooled rapidly and broken up into chips. The chips were mixed with 0.1% by weight of Aerosil R-972 (Charles Wagner Co.) silica dry flow additive before being passed through a Brinkman grinder having a 12 pin rotor and a 1.0 mm screen to obtain a powder which passes through a 200 mesh sieve. The clearcoat coating composition had a gel time at 400"F of 89 seconds and an HPMF at 375"F/.25 grams of 72mm. It provides good inhibition of filiform corrosion of an aluminum automotive road wheel when applied to a thickness of 2 mils by electrostatic spray method and cured. Example 2 A blend was made of 2320 parts of the hydroxy- functional terephthalate of Example 1; 580 parts of caprolactam blocked isophorone diisocyanate; 58 parts of the resinous flow control agent; 58 parts of the UV absorber; 29 parts of the light stabilizer; and 116 parts of a solid bisphenol A epoxy resin sold by Dow Chemical under the designation XU 71944.00L and having an epoxy equivalent weight of 877 and a kinematic viscosity of 3826 centistokes. Only one pass through the extruder was needed for compounding, with a maximum temperature of 180"F. The chipping and the grinding were generally the same as in Example 1. The -200 mesh powder had a gel time at 400*F of 79 seconds and its HPMF at 375°F/.25 gram was 77mm. It provides good inhibition of filiform corrosion of an aluminum automotive road wheel when applied to a thickness of 2 mils by electrostatic spray method and cured. Example 3 A blend was made of 2407 parts of poly(glycidyl methacrylate) resin having an epoxy equivalent of 510- 560* and a melt index of 50-58**, sold by Mitsui Toatsu Chemicals, Inc. under the trademark Almatex PD- 7610; 493 parts of dodecanedioic acid; 116 parts of the Dow Chemical epoxy resin XU 71944.OOL used in Example 2; 58 parts of the resinous flow control agent Troy EX-486; 58 parts of the Tinuvin 900 UV absorber; and 29 parts of the Tinuvin™ 622 LD light stabilizer. The blend was compounded by passing it through the Buss extruder at a maximum temperature of 260"F. The chipping and grinding steps were generally the same as in Example 1 to obtain a powder passing a 200 mesh sieve. The powder had a gel time at 400 β F of 27 seconds and the HPMF at 375°F/0.25 gram was 112 mm. It provides good inhibition of filiform corrosion of an aluminum automotive road wheel when applied to a thickness of 2 mils by electrostatic spray method and cured. Notes: * HCl-pyrimidine method. ** 125"C, 2160g (ASTM D-238-57T) Example 4 The general procedure of Example 1 was followed except that 2790 parts of a carboxylated polyester resin having an acid value of 32-35* and a T g of about 60*C, sold under the trademark Grilesta V 73-72 by Ems-Chemie AG of Zurich, and 210 parts of triglycidyl isocyanurate (TGIC) were substituted for the terephthalate resin and blocked isophorone diisocyanate curing agent. The 200 mesh powder had a gel time at 400 β F of 119 seconds and the HPMF at 375'F/.25 gram was 58.5 mm. It provides good inhibition of filiform corrosion of an aluminum automotive road wheel when applied to a thickness of 2 mils by electrostatic spray method and cured. Note: * DIN 53402 method. Example 5 A hydroxy-functional polyester resin (2400 parts) having an OH value of 50 and sold under the trademark Rucote 107 by the Ruco Division of Hooker Chemical and Plastics Corp. is blended with 600 parts caprolactam blocked isophorone diisocyanate, .50 parts of Troy EX- 486 flow control agent, 60 parts of Tinuvin™ 900 UV absorber, and 30 parts of Tinuvin™ 622 light stabilizer. The blend is then compounded by passing it through the Buss extruder at a maximum temperature of 180°F. The chipping and grinding steps are generally the same as in Example 1 to obtain a powder passing a 200 mesh sieve. The powder has a gel time at 400°F of 131 seconds and the HPMF 375 β F/.25 gram was 150 mm. It provides good inhibition of filiform corrosion of an aluminum automotive road wheel when applied to a thickness of 2 mils by electrostatic spray method and cured. Examples 6 - 10 Each of the following powder clearcoat coating compositions set forth in Table A, below, provides good inhibition of filiform corrosion of an aluminum automotive road wheel when applied to a thickness of 2 mils by electrostatic spray method and cured. The coating compositions are prepared in accordance with the method described in Example 1, above. TABLE A Example Component 8 10 (parts by weight) Hydroxyl functional polyester resin (Rucote 107, Ruco Polymers Corp.) 80 80 Caprolactam blocked isophorone diisocyanate (Huls B-1530, Huls America Corp.) 20 20 Glycidyl functional acrylic resin (Almatex P-7610, Anderson Development Corp. ) 83 83 Dodecanedioic acid 17 17 TABT.T- ("continued . Example Component 7 8 10 (parts by weight) Carboxyl functional polyester resin (Grilesta V-7372, Ems-Chemie AG, Switzerland) 93 Triglycidyl isocyanate (Tepic G, Summit Specialty Chemical Corp. ) Epoxy resin (XU 71944.00L, Dow Chemical Co. ) Flow control agent (Troy EX-486, Troy Chemical Co. ) 2 Benzotriazole UV absorber (Tinuvin™ 900, Ciba-Geigy) 2 Hindered amine light stabilizer (Tinuvin™ 622LD, Ciba-Geigy) ι Silica dry flow additive (Aerosil R-972, Charles Wagner) .2 .2 .2 "Hindered Phenol" Antioxidant (Irganox 1010, Ciba-Geigy) .1 Phosphite Type Antioxidant (Irafos 168, Ciba-Geigy) .1 Benzoin (Uraflow B, (GCA Chemical Corp.) .25 The superiority of the powder clearcoat coating compositions of Examples 1-4 is shown by the results of a filiform corrosion test conducted in accordance with the following test method. FILIFORM CORROSION TEST PROCEDURE The back rim is cut from a standard aluminum automotive road wheel formed of aluminum alloy 356A. The wheel face then is sectioned into six equal pie- shaped pieces, care being taken to prevent scratches and digs on the test surface. The thickness of the clearcoat coating on the test surface is measured in a minimum of four locations for each pie section with a film thickness gauge such as the Minitector available from Elcometer, Inc., Birmingham, Michigan or the equivalent. Each section is exposed to accelerated weathering in a weatherometer, such as Model XW Weather-O-Meter available from Atlas Electrical Devices Company, Chicago, Illinois or the equivalent, for 400 hours. A line is scribed through the clearcoat coating into the aluminum substrate surface using a scriber, such as carbide tip scriber no. 90C available from Aircraft Specialties, Inc., Lapeer, Michigan, or carbide tip scriber no. CM88 available from GTS, Detroit, Michigan or the like. Distilled or deionized water (195 ml) (20 microohms/cm at 25 β C maximum conductivity per ASTM 1125) is added directly to the bottom of a controlled atmosphere chamber, such as the controlled atmosphere chamber available from Ford Motor Company, Central Laboratory - Electronics Section, Dearborn, Michigan (constructed per FLTM BJ 107-6 except that sample slot holders are omitted and a 100 mm watch glass is substituted for the 80 x 40 mm crystallization dish) . Hydrochloric acid (reagent grade) is added to the water and the watch glass then is placed centrally on the bottom of the controlled atmosphere chamber. Each wheel section is placed in the controlled atmosphere chamber, the lid is replaced and the motor is turned on for one hour at room temperature (22*C±2*C). The wheel sections then are removed from the controlled atmosphere chamber and, without rinsing, placed directly in an environmental cabinet, that is, a controlled temperature and humidity chamber with 24 hour recorder or equivalent, such as the environmental cabinet available from Ther otron Industries, Holland, Michigan. Each test piece is visually inspected every 24 hours for filiform corrosion. Filiform corrosion growth is measured and recorded from the scribe, as well as any sharp edge of the sample, such as openings, ribs or cut edges. The test samples then are removed from the environmental cabinet, recording the total hours each sample remained therein. The tested aluminum wheels had been given a chromate pretreatment as follows: Chromate Pretreatment The bare aluminum surface of the road wheel is cleaned by exposure to a mild alkaline detergent solution, for example. A-126 solution available from Industrial Chemical Products, City of Industry, California. The surface is treated with such solution preferably at room temperature for about 40 to 60 seconds. The surface then is subjected to a deoxidation step by exposure to a mild sulfuric acid solution, preferably at room temperature for about 2 minutes. Suitable sulfuric acid solutions are available commercially, for example, solution NC available from Industrial Chemical Products. A chromate conversion coating then is formed on the aluminum surface by exposure to a chromate salt solution. Quite surprisingly, it has been discovered that substantial enhancement of the inhibition of filiform corrosion provided by the above discussed preferred embodiment is achieved by forming the chromate conversion coating to a thickness not less than 31 nanometers. This discovery represents a highly significant technological advance. The 31 nanometer (or greater) thickness is accomplished preferably by exposing the cleaned and deoxidized surface to the aforesaid chromate salt solution for a minimum of about 90 seconds, preferably about 90 to 120 seconds at controlled temperature, preferably at least about 84"F, more preferably about 87"F. Suitable chromate salt solutions are commercially available, for example, solution 1155-P available from Industrial Chemical Products. The aluminum surface (still a "bare aluminum surface" as that term is used herein) is then dried, for example, in a convection oven for about 10 to 20 minutes at a maximum metal temperature of 140*F. Coating Procedure The aluminum wheel sections were coated in a Ransberg Electrostatic Spray System at 60KV with the powder clearcoat coating compositions of Examples 1, 2, 3 and 4 and then cured at a peak metal temperature of about 275°F to 425*F, preferably 320'F to 400 β F, more preferably about 325*F, although this will depend on the particular resin materials employed. The composition of Example 3 was cured for 15 minutes at a peak metal temperature of 350*F. The other three were cured for 15 minutes at a peak metal temperature of 400"F. Each wheel test section had a minimum coating thickness of 2 mils. The wheel test sections then were exposed to accelerated weathering in a Model XW Weather-O-Meter for 400 hours. The test panels were then subjected to the above described filiform corrosion test procedure. No loss of adhesion was observed on any wheel test section. The growth of filiform corrosion for various periods of time is given in mm in the table below. The precision of measurement was ±0.2 mm. The results are shown also for a control powder clearcoat coating. The control coating was a commercially available TGIC coating for aluminum automotive road wheels, not having the UV additive of the present invention. It was applied and cured to a thickness of 2 mils using the same coating procedure described above, on the same substrate, i.e., a 356A aluminum road wheel section prepared by the chromate pretreatment described above. FILIFORM CORROSION GROWTH Exposure Duration Example No. Control 1 2 3 4 300 hrs. 0.0 0.0 0.0 0.0 > 1.6 408 hrs. 0.0 0.0 0.0 0.0 816 hrs. 0.4 0.0 0.4 0.4 1008 hrs. 0.8 0.0 1.2 0.8 As can be seen, all four cured powder clearcoat coating compositions provided good inhibition of filiform corrosion on a bare aluminum surface of an aluminum automotive road wheel. In contrast, the control powder clearcoat coating composition was observed to have filiform corrosion exceeding the test limit 1.6mm after only 300 hours exposure duration. Examples 11 and 12 Comparative examples were run to demonstrate the improved inhibition of filiform corrosion obtained by powder clearcoat coating compositions of the present invention. As seen in the Formulation Table below, the powder clearcoat coating composition of Example 11 is the same as that of Example 12 except that Example 11 does not have UV additive. The coating compositions were prepared as described in Example l. They were applied to aluminum 356A automotive road wheel sections and cured as described in the Coating Procedure section above. The wheel sections had been prepared as described in the Chromate Pretreatment section above. The degree of filiform corrosion was determined in accordance with the Filiform Corrosion Test Procedure section above, and the results are presented in the Filiform Corrosion Results Table below. FORMULATIONS TABLE Example Component 11 12 (parts by weight) Polyester Resin of Example 1 80 80 Huls B 1530 20 20 Troy EX 486 2 2 Tinuvin 900 0 2 Tinuvin 622 LD 0 1 XU 71944.00L 4 4 Aerosil R 972 0.1 0.1 As shown above, the powder clearcoat coating composition of Example 12, which is an embodiment of the present invention, provided much better filiform corrosion inhibition of the aluminum road wheel than did the same composition without UV additive, that is, the composition of Example 11. Various specific embodiments have been described above in detail for the purpose of illustration. Such detail is solely for the purpose of illustration and those skilled in the art of powder clearcoat coating compositions, with the aid of the present disclosure, will recognize that variations can be made without departing from the true spirit and full scope of the invention as defined by the following claims. All parts by weight recited in the claims below are based on total coating composition weight.

Claims: We claim: 1. A method of inhibiting filiform corrosion of an aluminum automotive road wheel comprising the steps of A) spray applying to a bare aluminum surface of the wheel a powder clearcoat coating composition comprising a major amount of a thermoset resin component and a minor amount of a UV additive comprising a UV absorber and a light stabilizer compatible with the thermoset resin component, and B) thermally curing the clearcoat coating composition to a clear, adherent, protective coating on the aluminum surface. 2. The method of inhibiting filiform corrosion of an aluminum automotive road wheel in accordance with claim l wherein the thermoset resin component consists essentially of a) at least one resin selected from polyester resins and acrylic resins, and b) a cross-linking agent reactive therewith. 3. The method of inhibiting filiform corrosion of an aluminum automotive road wheel in accordance with claim 1 wherein the thermoset resin component consists essentially of a carboxylated polyester resin and a triglycidylisocyanurate. 4. The method of inhibiting filiform corrosion of an aluminum automotive road wheel in accordance with claim 1 wherein the thermoset resin component consists essentially of a hydroxy-functional polyester resin and a cycloaliphatic blocked diisocyanate. 5. The method of inhibiting filiform corrosion of an aluminum automotive road wheel in accordance with claim 1 wherein the thermoset resin component consists essentially of acrylic resin and a cross- linking agent reactive therewith. 6. The method of inhibiting filiform corrosion of an aluminum automotive road wheel in accordance with claim 5 wherein the acrylic resin consists essentially of glycidyl functional methacrylate copolymer and the cross-linking agent consists essentially of dicarboxylic acid. 7. The method of inhibiting filiform corrosion of an aluminum automotive road wheel in accordance with claim 1 wherein the UV absorber is selected from the group consisting of benzotriazole, 2-hydroxy benzophenone, oxanilide, diphenyl cyanoacrylate and a mixture of any of them. 8. The method of inhibiting filiform corrosion of an aluminum automotive road wheel in accordance with claim 1 wherein the light stabilizer is selected from the group consisting of hindered amine light stabilizers and a mixture of any of them. 9. A method of inhibiting filiform corrosion of an aluminum surface of a 356A aluminum alloy automotive road wheel comprising the steps of: A) chromate conversion coating the aluminum surface by exposure to a chromate salt solution for at least 90 seconds at a temperature of at least 84 β F, followed by drying at a maximum metal temperature of 140 * F; B) then electrostatically spray coating the aluminum surface with a 1.8 to 3.0 mils thick powder clearcoat coating composition comprising 50 to 150 parts by weight of a thermoset resin component comprising a major amount of a glycidyl functional methacrylate copolymer resin and dodecanedioic acid, 0.2 to 15.0 parts by weight of a UV additive consisting essentially of .1 to 10 parts by weight of a benzotriazole UV absorber and .1 to 5 parts by weight of a hindered amine light stabilizer, and minor amounts of flow control additive and antioxidant additive; and C) then thermally curing the powder clearcoat coating composition at 275*F to 425 β F to a clear protective coating adherent to the aluminum surface. 10. The method of inhibiting filiform corrosion of an aluminum automotive road wheel in accordance with claim 9 wherein the UV absorber is 2-[2'-hydroxy- 3 ' ,5'-di-(c_,α-dimethylbenzyl)phenyl]-2H-benzotriazole and the hindered amine light stabilizer is at least one polymeric polyalkylpiperidine. 11. A method of inhibiting filiform corrosion of an aluminum surface of a 356A aluminum alloy automotive road wheel comprising the steps of: A) chromate conversion coating the aluminum surface by exposure to a chromate salt solution for at least 90 seconds at a temperature of at least 8 "F, followed by drying at a maximum metal temperature of 140 β F; B) then electrostatically spray coating the aluminum surface with a 1.8 to 3.0 mils thick powder clearcoat coating composition comprising 50 to 150 parts by weight of a thermoset resin component comprising a major amount of an hydroxy functional polyester resin and caprolactam blocked isophorone diisocyanate cross-linking agent, 0.2 to 15.0 parts by weight of a UV additive consisting essentially of .1 to 10 parts by weight of a benzotriazole UV absorber and .1 to 5 parts by weight of a hindered amine light stabilizer, and minor amounts of flow control additive and antioxidant additive; and C) then thermally curing the powder clearcoat coating composition at 275*F to 425"F to a clear protective coating adherent to the aluminum surface. 12. The method of inhibiting filiform corrosion of an aluminum automotive road wheel in accordance with claim 11 wherein the UV absorber is 2-[2'- hydroxy-3 • , 5 • -di-(α,α-dimethylbenzyl)phenyl]-2H- benzotriazole and the hindered amine light stabilizer is at least one polymeric polyalkylpiperidine. 13. A filiform corrosion resistant aluminum automotive road wheel having a filiform corrosion inhibiting clearcoat coating on a bare aluminum surface thereof, the clearcoat coating being the cured product of a thermally curable powder clearcoat composition comprising a major amount of a thermoset resin component and a minor amount of a UV additive comprising a UV absorber and a light stabilizer compatible with the thermoset resin component. 14. A filiform corrosion resistant aluminum automotive road wheel comprising a 356A aluminum alloy wheel member having a chromate conversion coated bare aluminum surface and a 1.8 to 3.0 mils thick thermoset clearcoat coating on said bare aluminum surface, the clearcoat coating comprising the thermally cured product of an electrostatically spray applied powder clearcoat coating composition comprising 50 to 150 parts by weight of a thermoset resin component comprising a major amount of a glycidyl functional methacrylate copolymer resin and dodecanedioic acid, 0.2 to 15.0 parts by weight of a UV additive consisting essentially of .1 to 10 parts by weight of a benzotriazole UV absorber, .1 to 5 parts by weight of a hindered amine light stabilizer, and minor amounts of flow control additive and antioxidant additive. 15. A filiform corrosion resistant aluminum automotive road wheel comprising a 356A aluminum alloy wheel member having a chromate conversion coated bare aluminum surface and a 1.8 to 3.0 mils thick thermoset clearcoat coating on said bare aluminum surface, the clearcoat coating comprising the thermally cured product of an electrostatically spray applied powder clearcoat coating composition comprising 50 to 150 parts by weight of a thermoset resin component comprising a major amount of an hydroxy functional polyester resin and caprolactam blocked isophorone diisocyanate cross-linking agent, 0.2 to 15.0 parts by weight of a UV additive consisting essentially of .1 to 10 parts by weight of a benzotriazole UV absorber, .1 to 5 parts by weight of a hindered amine light stabilizer, and minor amounts of flow control additive and an antioxidant additive.

Inventors: ENDO MACKENZIE KENJI, OCOBOCK ROBERT WILLIAM, SAMMEL RICHARD KARL, SIMMONS ROBERT NEIL, ENDO, MACKENZIE, KENJI, OCOBOCK, ROBERT, WILLIAM, SAMMEL, RICHARD, KARL, SIMMONS, ROBERT, NEIL
Applicants: MORTON INT INC, MORTON INTERNATIONAL, INC.
Application year: 1993
Main classification: B32B15
IPC codes: B05D3, B05D5, B05D7, B32B15, B60B3, C09D4, C09D5, C09D123, C09D133, C09D167, C09D175