Low-VOC ceramic epoxy coating for high abrasion use on aircraft

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Low-VOC Ceramic Epoxy Coating for High Abrasion Use on Aircraft by Kate Kubernach and Barry Miller, KMR Consulting Inc., Seaford, Va. 'ilitary aviation has continually searched for abrasion. resistant Coatings for leading edges of aircraft. Over the years, a great deal of personnel has been exhausted keeping leading edges of wings, horizontal and vertical stabilizers, helicopter rotor blades, jet engine intake lips, etc. corrosion protected and aesthetically serviceable. Today's military budget cuts demand greater conservation of labor expended accomplishing repetitive tasks that advanced technology can solve.

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COATING

PROPERTIES

One such discovery involves a ceramic coating originally designed for use in the oil and gas industry, where corrosion and abrasion resistance is crucial. The coating is reducing laborintensive, aircraft leading-edge coating touch-up work. It is a modified epoxy resin system that is highly loaded with ceramic powders and is designed to be a self-priming topcoat. The coating's unique, patented epoxy blend of resins and ceramic powders provides outstanding adhesion, flexibility, corrosion, abrasion, and erosion resistance not found in other epoxy, polyurethane, or waterborne-type coatings. The coating is an organic compound and volatile organic compound (VOC) compliant, 1.63 lb/gal (196 g/L), and it is well below the projected maximum VOC content for industrial maintenance coatings expected from the U.S.

EPA. It is manufactured with foodgrade epoxies and inert ceramic powders and is considered a "Green" product. It is also nontoxic and contains no carcinogens or isocyanates. Adhesion strength comes from the high solids content of the coating, which produces low shrinkage during cure and results in lower stress levels in the cured film. Another factor contributing to the bonding strength is that during cure, no by-products are formed; thus, volatile agents are not generated to act as plasticizers. Several epoxies are blended to achieve the best possible combination in the coating. Increased cross-linking of the epoxies provides a higher heat distortion temperature, which produces long-term service at temperatures to 300°F (149°C) and short-term service (15 min or less) at temperatures to 700°F (371°C). The coating has the ability to maintain corrosion resistance in a wide variety of situations. It resists caustics as well as most acids, petroleum distillates, and solvents and provides excellent electrical insulating characteristics (dielectric and resistive properties) under severe operating conditions (see Table I). NONDESTRUCTIVE INSPECTION CAPABILITY

Testing has revealed that the coating does not interfere with nondestructive inspection of the substrate when applied. X-ray, ultrasound, and eddy cur-

rent inspection methods were successful when the coating was applied to various test panels. MIXING

The system is a two-component material prepackaged in two cans, Part A (base) and Part B (catalyst). The mix ratio by volume is 13 parts A to 1 part B. Part A is shaken by a mechanical shaker or mixer until all the ceramic powders are suspended in the resin. The time required to achieve full suspension depends on the length of time the material sits on the shelf and the temperature of the storage facility. Full suspension of ceramic powders must be accomplished prior to proceeding. Once proper suspension is achieved, the contents of Part B are added to Part A, and the coating is shaken again to achieve proper mixing. The amount of time required depends on the temperature of parts A and B and the working condition of the shaker or mixer. At room temperature (72°F, 22°C), with a good mechanical shaker, a 4 to 6 rain shake is all that is required. Caution must be taken to prevent excessive shaking after adding Part B, as this creates heat and shortens pot life. Normal pot life for the mixed material at 72°F (22°C) is approximately 4 to 6 hr. Colder temperatures increase pot life, and warmer temperatures decrease pot life. Proper viscosity is 34 to 38 sec utilizing a #4 Ford viscosity cup. Adjustments to viscosity can be made

Table I. Ceramic Epoxy Coating Properties

Adhesion Abrasion resistance (ASTM D 4060, Tabor Test 1,000 cycles, CS 17 wheel, 1 kg) Flexibility (ASTM D 522, Conical Mandrel Bend at 75°F) impact resistance (ASTM G 14) Static coefficientof friction (ASTM D 4518-90) Dielectric strength (ASTM D 149) Spray salt (fog), 5% solution, >1,500 hr (ASTM B 117) Continuous-usetemperature Fire rating over steel (ASTM E84-91a) Volatile organic compounds

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Physical Properties

Testing Agency

>4,000 psi (27.98 MPa) 10.9 milligrams loss >30% elongation 30 inch-poundimpact strength 0.152 1,435 volts/rail No distress observed 300°F (149°C) Smoke density-class 1; Flame spread-class 1 1.63 Ib/gal (196 g/L)

Hydro Research Centre Technical Inspection Services Inc. Technical Inspection Services Inc. Technical Inspection Services Inc. Technical Inspection Services Inc. Owens-Coming Fiberglas Corp. Southwestern Laboratories Owens-Coming Fiberglas Corp. Southwest Research Institute NphaJOwens-Corning

© Copyright Elsevier Science Inc.

M E T A L FINISHING

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under field unit conditions, in available paint facilities with no temperature or humidity controls. Temperature at time of application was 78°F (25°C), humid1oo 9s . . . . . . . . . ity was 60%, and the test coating was 9o IB applied using conventional spray equip80 [] ment. Two coats of test material approx70 [] . . . . imately 4 to 5 mil were applied over dry HOURS epoxy polyamide primer, MIL-P-23377, approximately 45 min apart. The left4o 341 [] .QEL" wing application used the leading-edge 3. "= ;11 .. ,s R, cE break point 6 in. from the leading-edge Ill IIICURED 2O ~ ll~ ~ 1= ;z .... top and bottom as the guideline for the 8~ 6~lB ,,-7.. coating. The left horizontal and vertical lo ~ ~ 711 :11 ~=m ~-''_ o l , 0 ~, i I~ ~!,' ,,I " • , III II stabilizers were coated 4 in. back on the 40 60 72 100 120 140 160 top, bottom, and sides, respectively. The TEMPERATURE, °F masking tape was removed within 4 hr of application, but the aircraft remained Figure 1, Cure schedule. A two-pass film of 6 to 8 mil air dries to a touch-dry (gel) finish within in the paint facility for an additional 18 3 hr at 720F (22.2°C) and dries to an80% cure in 12 hr, Cure times lengthen at lower temperatures hr for stenciling before being removed to and shorten at higher temperatures, another facility for further unrelated work. The aircraft started normal flight with small amounts of isopropanol alco(PMB), chemical stripping, or manual schedules within 5 days after coating. hol (99% pure) or methyl ethyl ketone sanding. Strip rates for the coating are The second aircraft was coated 1 month (MEK). Caution should be used during 0.15 to 1.2 sfm compared with 2.6 sfm later in August 1995. This aircraft had viscosity adjustments, as a small amount for standard polyurethane utilized by the right wing, horizontal and vertical of either material goes a long way. the Air Force. Tests are currently under leading edges coated with the test mateway for medium- and high-pressure rial in the same fashion as the first test water-blasting removal. It should be aircraft. Temperature at the time of coatAPPLICATION noted that removal rates b y PMB are ing was 85°F (29°C), and humidity Best results are achieved when the increased when the coating is applied ranged from 65% at the start of the procoating is spray applied. The use of conover a primer versus applications over cess to 90% by the completion. The ventional (suction or pressure feed) bare metal surfaces. same spray equipment used for the first spray equipment such as a high-volume, aircraft was utilized for the second. The low-pressure (HVLP) system utilizing a second aircraft returned to normal flying FIELD TEST APPLICATIONS 63 c.v.t, needle, a 563 c.v.t, fluid nozzle, operations within 5 days. Both aircraft and a 63 PB air nozzle is recommended. Introduction into the aviation field were tracked for 10 months. In May In all spray equipment, the use of tunghas been accomplished utilizing vari1996, one test aircraft was sent to Warner sten carbide needles and fluid nozzles is ous types of Air Force aircraft, includRobins Air Force Base for normal schedrecommended for maximum life. The air ing F-15s, F-16s, B-ls, C-17s, and KCuled depot maintenance. At that time, the source must be free of contamination 135s, assigned to various geographic two test aircraft had flown 480 total and moisture. Moisture inhibits the catareas around the world. The coating hours with no failures of the test coating; alytic reaction and can cause an incomhas been applied to leading-edge surhowever, significant failures occurred to plete cure. Airless systems can also be faces, including wings, intakes, and the standard coating. The test was comused to apply the coating to large areas. horizontal and vertical edges, and it pleted in August 1996, and due to the The material is applied in two passes, has withstood the expected abrasion/ positive results, the coating is now apeach 4 to 5 mil wet. The first coat should erosion effects for extended periods. In proved as an alternative leading-edge be allowed to sit until tacky. Normally, some cases, the coating was applied coating for the F-15 fleet. 30 to 40 min is sufficient; more time will over MIL-C-23377, epoxy polyamide In October 1995, the coating was be needed if the temperature is below primer, type 1, class 1. Coating on applied to a B-1 aircraft utilizing con72°F (22°C). The second coat is also other test aircraft was applied directly' ventional spray methods in field-unit applied at 4 to 5 mil, for a total dry fdrn over metal surfaces treated with a facilities by Air Force personnel. Inforthickness of 6 to 8 rail. The coating will chromate conversion coating. mation on temperature and humidity at achieve an 80% cure within 12 hr, and a The first test program involved two the time of spraying has not been made full cure is reached at 48 hr at 72°F F-15 aircraft. Each aircraft had half of available, but the aircraft was sprayed (22°C). Higher temperatures produce a the leading edges coated with 6 to 8 mil under normal field conditions. The airflail cure in a shorter time (see Fig. 1). of test coating. The other half of the craft returned to flying status within 5 leading edges were coated with standard days. A second B-1 aircraft was coated MIL-C-83286 polyurethane coating apin March 1996 utilizing the same criCOATING REMOVAL plied to a dry film thickness of 8 to 12 teria and facilities. Monthly inspecThe coating can be removed, if demil. The first aircraft was coated in July tions of the coating system will consired, utilizing plastic media blasting 1995. Test application was completed tinue for a 2- to 3-year period. The two 22

METAL FINISHING • MARCH 1997

test aircraft have accumulated in excess of 400 flight hours with no degradation of the coating. Due to the positive results of the initial test phases, B-1 testing has been expanded to include additional aircraft at other locations. A C-17 winglet application was accomplished in October 1995. The winglets, constructed of carbon fiber composite material, were prepared by lightly sanding with 320 grit paper and wiping clean with 99% pure isopropyl alcohol. A "tracer" coat of MIL-P23377, epoxy polyamide primer, class I, type I was applied and allowed to dry for 1 hr. (A "tracer" coat is defined as a full wet coat applied prior to the topcoat, which will be evident during removal procedures such as sanding. It provides a visual color difference to use as a stopping reference to prevent damage to the substrata.) Coating was applied to the winglet leading edges in two wet passes, each approximately 4-5 rail. Temperature at time of application ranged from 82 ° to 950F (28°-35°C), and humidity was 87%. The coating was touch dry in 4 hr, and the aircraft was taken out of the facility and placed back into service within 24 hr of coating. The aircraft has accumulated in excess of 1,000 flight hours since application in a wide variety of environmental conditions. To date, the aircraft has experienced no coating failures of the winglet trealment. A KC-135 leading-edge test, initiated in February 1996, is under way and will not have flight test results available until the test is completed. A total of eight aircraft (two aircraft per location) in four separate geographic areas is participating in the test program. Air Force personnel were trained on the application of the coating at the first application site under field conditions utilizing conventional spray equipment. Monthly reporting of coating condition is required, and upon completion of the test program, results will be published. An Air National Guard unit in Texas coated all leading edges of assigned F-16 aircraft between 1991 and 1992. Approximately 19 aircraft were coated over the 12-too period. After 4 to 5 years with coating, only minor touch-up maintenance was needed. The unit estimated more than 13,000 flying hours without requiring touchup. In addition, unit documentation validated a decrease of 11 hours of METAL FINISHING

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Circle 088 on reader information card

labor per phase inspection per aircraft with the deletion of leading-edge touch-up/repair work.

CONCLUSION Based on the data presented, it is evident that the coating has significant potential in the tough environment of aircraft leading-edge areas and can significantly reduce labor costs for routine upkeep. The coating may be adaptable to other applications requiring high abrasion and corrosion resistance in aviation; as well as other fields.

Biographies Kate Kubernach and Barry Miller are co-owners of KMR Consulting Inc. located in Seaford, Va., and will soon open an additional office in Las Vegas, Nev. They specialize in assisting companies through the complex maze of military requirements. Both are retired from the U.S. Air Force and were last assigned to the Air Combat Command Directorate of Logistics Staff. Kubemach was assigned as the Air Combat Command Corrosion Manager, and Miller was the Logistics Manager for the F-111 weapons system. MF 23