P2 Option: Paint Gun Washer

P2 Option: Water Reducible Epoxy Primer

P2 Option: Nonchromated Primer

The Koroflex primer is a polyurethane coating that is applied to larger aircraft (e.g. transport aircraft) to improve the flexibility of the coating system, thereby increasing its resistance to cracking and adhesion loss due to flexing of the structure during flight. The Koroflex primer also provides a barrier, thus lending an increased level of protection to the substrate. In many cases, the Koroflex primer is applied directly to the metal and overcoated with MIL-C-85285 or MIL-C-83286 topcoat.

To date, a nonchromated version of the Koroflex primer has not been developed and qualified.

The Koroflex primer would be considered a low usage item compared to the epoxy primers. Once the nonchromated coating technology matures, the nonchromated corrosion inhibiting pigments can be formulated into a polyurethane resins system. Currently, there are two qualified suppliers of Koroflex: Deft and Courtalds. These companies would be in the best position to reformulate the Koroflex primer using nonchromated corrosion inhibitors. However, since neither of these companies has successfully formulated a nonchromated primer capable of meeting 2000 hour salt spray resistance, it will be some time before a nonchromated Koroflex primer will be available. Presently, Spraylat is in a good position to accomplish this, however, Koroflex is not included in their present product line.

Non-chromate substitutes equivalent to the chromate corrosion inhibitors formulated in primers and/or paints for the protection of high-strength aluminum alloys were studied. Quaternary ammonium dimolybdate salts (Q-Mo) were found to prevent ocean-water salt-spray corrosion for 2000 hours, equivalent to the strontium chromate-filled epoxy-polyamide primer, MIL-P-23377. Quaternary ammonium nitrate, phosphate, and borates were included in the evaluations. However, the Q-Mo was superior in every regard. The dimolybdate quaternary ammonium salt is a developmental inhibitor of high purity containing less than 1,000 ppm sulfate and 100 ppm chloride. Optimization of the molybdate in the primer was successful in achieving the physical, corrosion, and fluid resistance, and working properties required for demonstration testing and evaluation.

Each Chemical Agent Resistant Coating (CARC) camouflage topcoat color and primer used has a different VOC concentration. CARC paint is extensively used on many different types of equipment including all tactical helicopters, ground equipment, tanks, trucks, generators, and radar sets. CARC paints typically contain high VOCs, isocyanates, and chromium. Paint guns applying CARC typically require methyl ethyl ketone for clean-up.

P2 Option: Waterborne CARC

P2 Option: Waterborne Camouflage Coating

Despite the fact that waterborne coatings are relatively common, research continues to address limitations in application and improve performance and durability as well as further reducing VOC and HAP content. The following are examples of some of this research.

Wright Laboratory researches polyurethane paints under the High Performance Aerospace Coating System Program. Waterborne polyurethane systems are being examined under a test program. It is reported that the isocyanates in waterborne (as opposed to the usual solvent-borne) systems are blocked from reacting, which may reduce the exposure potential for workers spraying the paint.

Waterborne paints differ from conventional coatings in that water has been substituted for certain organic solvents in the coating that are used as the dispersal medium for synthetic resins and pigments in the coatings. These coatings may still contain between 5 and 20 percent organic solvents for wetting, viscosity control, and pigment dispersion. Coalescing solvents allow the particles of resin to fuse together as the water evaporates to form a continuous coating. A wide variety of waterborne coatings are commercially available such as emulsions (latex), colloidal dispersions, or solutions. Many of these coatings have performance characteristics similar to the more traditional organic solvent-based coatings and can be applied using a variety of proven methods.

VOC Content

Waterborne coatings may still contain organic solvents, with typical ranges between 5 percent and 20 percent.

HAP Content

The HAPs content tends to be lower due to reduced concentrations of solvents. Concentrations of metals and other HAPs are specific to the coating.

Benefits

Waterborne coatings tend to have these benefits when compared to conventional coatings:

• VOC and HAPs emissions are significantly reduced.
• A wide variety of coatings are available.
• Waterborne paints are generally easier to apply and clean up.
• Waterborne coatings tend to display good to excellent surface properties including gloss, rub resistance, anti-sealing effect, and non-yellowing film.
• The overspray from some waterborne paints can be recovered and recycled, effectively increasing transfer.
• Waste disposal requirements for waterborne paint waste are less restrictive. Depending on local requirements and/or waste quantities, dried waterborne paint waste may be sent to landfills as non-hazardous waste.
• Health and safety requirements for workers are significantly reduced or eliminated due to the reduced presence of solvents in the workplace.

Limitations

There are certain limitations that are common to waterborne coatings:

• Some waterborne coatings have a shorter shelf life, depending on the coating’s composition.
• Waterborne coatings can react with other materials (e.g. cause steel to rust).
• Waterborne coatings can have a lower chemical and solvent resistance than solvent borne coatings.
• These coatings tend to be more sensitive to temperature.
• Humidity controls may be needed because waterborne coatings are sensitive to humidity; low humidity can cause coatings to dry extremely fast, creating craters in the final film, while high humidity can delay dryings, resulting in sagging.
• The quality of the application is dependent on surface cleanliness; the water's high surface tension prevents the wetting of some surfaces and causes poor coating flow characteristics. Surfaces with grease and other contaminants are especially a problem.
• Emulsion coatings have poor penetration and adhesion properties on porous surfaces, e.g. wood. This prevents good adhesions to old, chalky surfaces.
• Lower abrasive resistance may translate into more frequent painting cycles.

Applications

Existing coating delivery systems can usually be adapted to handle waterborne coatings. Waterborne coatings can be applied using a variety of methods, including air or airless spraying, auto deposition, curtain coating, electrostatic spraying, flow coating, or fluidized bed. When using waterborne paints, the application system must be electrically isolated from the factory structure, not just grounded.

Economics

In general, the price of waterborne paints is comparable to the price of solvent-based paints. Solvent-based paint systems can usually be converted to waterborne paint systems with a limited capital investment. However, if it is found that painting is required more frequently the life cycle costs may be greater.

Examples

• MIL-P-85582 is a Waterborne Epoxy Primer Coating that is corrosion inhibitive, and chemical and solvent resistant. Formulated primarily for spray application, these primers are compatible with polyurethane and epoxy topcoats. The maximum VOC content of the admixed primer coating is 340 g/L. The product is available in two types and classes:

• Type 1 = Standard pigments
• Type 2 = Low infrared reflective pigments
• Class C1 = Barium chromate based corrosion inhibitors
• Class C2 = Strontium chromate based corrosion inhibitors

• Fairchild AFB Washington, currently uses a latex paint (DTYM coating by Sherwin Williams) to paint the interiors of its KC-135. Only the floors and removable parts are painted with other materials. Floors are painted with epoxy paint and removable parts are still painted with a solvent based polyurethane paint. Diluted liquid paint waste is treated in the wastewater treatment plant and solid paint waste is fed to the base’s waste-to-energy plant.

P2 Option: Disposable Paint Applicator Pen SEMPEN PEN^TM

P2 Option: Reusable Spray Cans

P2 Option: Plural Component Dispensing of Aircraft Primer

P2 Option: Latex Painting of Vehicles

P2 Option: Latex paints.

P2 Option: Ion Vapor Deposition

P2 Option: Other Options

Lockheed Martin, Fort Worth has identified and partially qualified an alternate anodizing process that does not use EPA 17 chemicals. The process is Thin Film Sulfuric Acid Anodizing (TFSAA). The F-16 Program Office is funding the construction of a pilot scale TFSAA line.

Thiokol developed a nonchromated surface treatment process for aluminum that greatly enhances the adhesion of applied paints and coatings. The process consists of a metasilicate and organofunctional silane that provides a covalent chemical bond with the paint or coating. An evaluation for NASA (Space Shuttle) determined this process to be equivalent to unpainted chromate chemical conversion coating (CCC) in corrosion resistance. This process still needs military qualification. The Thiokol pretreatment process is a two-step process that is applicable to immersion applications and, potentially, spray applications.

The Sacramento Air Logistics Center (ALC) Science and Engineering Laboratory (TIELE) directed The MITRE Corporation to research viable alternatives to two surface-finishing processes that use hexavalent chromium. The resulting research identified alternative processes or material substitutions involving less use and fewer emissions of chromium. The two processes were hard chrome plating of steel and chromate conversion coating of aluminum. Each process was addressed in a separate report. The hard chromium plating report presents the results of the engineering study conducted by The MITRE Corporation for the Sacramento ALC TIELE under Delivery Order 5009 of Contract F04699-91-D0065. To obtain an electronic copy of the report, contact the POCs listed in the Forward of this document.

Powder Coating is the application of finely ground plastic powders by electrostatic means followed by curing to melt and fuse the powder into a continuous coating. It provides a durable, corrosion resistant surface and emits no VOCs and HAPs. Powder coats can be either thermosetting powders or thermoplastic powders. Thermosetting powders melt and flow to coat the substrate and then crosslink with other particles in the powder formulation to form high molecular weight polymers. Examples include acrylic, epoxy/polyester hybrid, functional epoxy, thin film epoxy, and urethane polyester. Because thermoplastic powders do not undergo chemical change, cured thermoplastic coatings will melt when heated. Examples include cellulose ester, polyamides (such as Nylon-11) polyester, and polyvinyl chloride (PVC).

P2 Option: Powder Coatings

VOC Content

The VOC content from powder coatings is always less than 10 percent.

HAP Content

The HAP content is considerably reduced due to the lack of solvent in the coatings. The presence of any HAPs in the coating is dependent upon the specific formulation of the coating.

Benefits

There are a number of advantages to using powder coats in lieu of conventional coatings:

• Powder coatings are very durable and corrosion resistant.
• There are no problems with running, sagging, and bubbles.
• An air permit is not required; powder coating is a clean technology, virtually without air emissions.
• Transfer efficiencies are nearly 100 percent and any overspray can be recycled for reuse.
• Worker health and safety conditions are improved, leading to increased productivity.
• There is no liquid mixing or pumping, or need for liquid viscosity monitoring.
• Reduced use of hazardous materials in paint; however, there may be environmental issues associated with pretreatment (surface preparation).
• May be used to coat a variety of substrates, including aluminum and steel.

Limitations

Discussed below are some of the limitations to using powder coatings instead of conventional solvent-based coatings:

• Coatings cannot be <1 mil thickness.
• A uniform film thickness is difficult to maintain.
• Coatings are subject to powder handing (e.g., changing colors during the process).
• Surfaces must be totally clean and dry before powder is applied. No cleansing action is available as the coating is applied to the part because powder contains no organic solvents.
• Powder coating may not be practical for repainting vehicles, aircraft, and ships because curing requires elevated temperatures.
• Cured thermoplastic coatings may soften when heated but will not melt.
• The system must be reconfigured for each application.

Applications

At this time, powder coating is generally limited to small parts. The coating thickness can range from 1.5 to 60 mils.

Economics

Key economic considerations when evaluating powder coating as an alternative to conventional coating systems include:

• Capital cost for equipment and the cost of some powders are comparatively expensive.
• Costs for labor, energy, and environmental and safety compliance are lower.

Research

The Air Force Advanced Aircraft Coating Program is investigating powder coating for large parts. Tasks are being undertaken to develop, optimize and produce powders that will provide the desired improvements in coating systems, improve aircraft coating performance and increase environmental acceptability. Wright Laboratory has programs designed to develop environmentally compliant paint systems, such as: high performance aerospace coatings; advanced aircraft paint systems; low temperature plasma coatings and surface treatment; zero VOC coatings; large area powder coatings; durable, cleanable coatings; and high velocity thermal spray coatings.

Kelly AFB in San Antonio Texas recently installed a prototype epoxy powder coating system. They will begin using this process in the near future.

The U.S. Army Armament Research Development and Engineering Center (ARDEC) has been developing a Powder Coating Technology for Small Arms Bullet Tip Identification. The dry paint is applied using a spray or fluidized bed after the projectile surface has been heated to between 250° and 350°F. In addition to coating projectiles, this technology may be applicable to commercial small caliber, cannon caliber, and large caliber ammunition.

Courtlands Aerospace has reportedly developed a powder coat "torpedo paint" that may meet all required characteristics. Questions remain regarding depainting.

The Unitized Coating Application Facility, located at the National Defense Center for Environmental Excellence (NDCEE) is a DoD demonstration/training center for E-coat and powder coat technologies. Research efforts include feasibility testing, process optimization, and process validation. NDCEE will provide system design, installation and start-up support, training, and follow-up support as needed. A second NDCEE task focuses on demonstrating the applicability and technical and economic feasibility of powder coating for small businesses.

Other Points of Contact:

Powder Coating Institute
1800 Diagonal Road, Suite 600
Alexandria, VA 22314
Phone: (703) 684-1770
Fax: (703) 684-1771

Scott Mauro
Naval Facilities Engineering Service Center
1100 23^rd Ave.
Port Hueneme, CA 93043-4370
Phone: (805) 982-4889 DSN 551
Fax: (805) 982-4832

Background: Low VOC, High-Solids Coatings

High-solids coatings and primers contain higher concentrations (40 to nearly 100 percent) of non-volatiles than conventional paints (which typically contain 8-30 percent solids). Because these formulations use low-molecular weight resins, they require less solvent to attain the viscosity needed for the application. The resins have highly reactive sites to promote polymerization. High solids coatings may be one or two component systems based on acrylic, alkyd, epoxy, polyester, or urethane resins and can be cured at ambient air or in high temperature bake ovens.

VOC Content

The VOC content of high-solids coatings are lower than conventional coatings. VOC concentrations typically range between 0 percent and 60 percent, depending on the coating formulation.

HAP Content

The emission of HAPs from the application of high solids coatings tends to be less than with conventional coatings due to reduced concentrations of solvent. Emissions of other HAPs such as isocyanates and chromates are dependent on the specific coating formulation.

Benefits

In general, high-solids coatings have certain advantages over conventional coatings:

• VOC emissions are significantly reduced.
• Coatings can often be applied using existing equipment.
• Less paint is needed to achieve desired film thickness.
• Curing can take place at both ambient temperatures or in high energy bake ovens.

Limitations

• High-solids coatings tend to have the following limitations:
• To ensure paint adhesion the surface must be especially clean of oils, greases, and other surface contaminants that would otherwise be dissolved in solvents.
• These coatings may be difficult to atomize; special equipment may be required for some coatings, particularly those approaching 100 percent solids.
• May be necessary to heat paint to maintain a workable paint consistency.
• Color matching may be difficult because of difficulty to blend.
• Cleanup may be more difficult.
• Pressure may have to be increased in spray applications or 2-component formulations and plural component equipment that meters and mixes the materials at the spray gun.

Examples

• Some of the high-solids coatings that are listed on the Qualified Products List include:
• MIL-C-85285 High Solids Content Polyurethane Coatings (Type I for aircraft and Type II for ground equipment) are an alternative to MIL-C-22750 and MIL-C-83286. QPL-85285-6 provides more than 28 products that meet the performance specifications.
• MIL-C-4168 Type IV: This Chemical Agent Resistant Polyurethane Coating (CARC) is an alternative to MIL-C-4618 Type II coatings. Type IV coatings are formulated to meet the South Coast Air Quality Management District VOC requirements. QPL-46168-6 lists at least 55 Type IV coatings with 13 colors listed.
• MIL-P-553022B Type II. This Lead- and Chromate-Free Corrosion Inhibiting Primer is an alternative to MIL-P-23377. QPL-53022-11 lists at least 12 primers that meet the performance specifications.

Background: Self-Priming Topcoat

UNICOAT is a one coat painting system for aircraft that replaces the traditional two coat primer and topcoat systems. It has been successfully demonstrated on both Air Force and Navy Aircraft and has been issued a Federal specification, TT-P-2756. Originally developed at the Naval Air Warfare Center (NAWC) Warminster in 1990, UNICOAT provides the adhesion and corrosion resistance of a primer and the chemical resistance, durability, and flexibility of the original topcoat. UNICOAT is lead-free and chromate-free and is a blend of non-toxic organic and inorganic zinc compounds. UNICOAT is a polyurethane with corrosion inhibitors and adhesion promoters added to the formula.

VOC Content

UNICOAT is VOC compliant with VOC levels of <420 g/L. By eliminating the need to spray a separate coat of primer, the coating system reduces emissions of VOCs.

HAP Content

Most UNICOAT contains no toxic pigments (i.e. chromate, lead, etc.); however some free isocyanates may be released during mixing and spraying.

Benefits

UNICOAT offers certain advantages over the conventional 2-step primer/coating systems:

• VOC emissions and hazardous waste generation are reduced by 50 - 70%.
• The paint weight is reduced, increasing aircraft fuel efficiency.
• Because UNICOAT is a one coat painting system, it requires less operator-time to apply.
• The Navy reports that UNICOAT:

- Exhibits good durability and cleanability.

- Reduces the amount of corrosion control maintenance required.

- Cleans more easily than MIL-C-85285.

Limitations

There are some cases where UNICOAT is not as good as traditional primer coating systems:

• Wright Labs reports that corrosion protection may not be comparable to two coat coatings.
• Pensacola NAS reports that UNICOAT may fail the pot test.
• Tinker AFB reports substandard corrosion protection on aluminum and magnesium substrates, inadequate adhesion when applied directly to 3-M leading edge tape, and insufficient Skydol resistance on K-10 aircraft.
• There have been complaints by rework facilities that adhesion to the metal surface is poor. To alleviate this problem, the rework facilities use an epoxy primer (chromated) under the UNICOAT. Obviously, this defeats the purpose of combining the primer and topcoat into a single system.
• Frequent color changes are not practical.

Applications

UNICOAT is best used as an overcoat on flat coatings and may not be suitable for all coating operations because of high viscosity of formulations. It can be applied directly to metal including aluminum substrates. UNICOAT is already being used at several military installations including Tinker AFB (OC-ALC).

Economics

Costs will vary depending on the specific application. In general, however, savings will be realized due to:

• Fewer gallons of coating purchased.
• Less painting time because only one coat is applied.
• Reduced costs associated with stripping.

Examples

A Federal specification, TT-P-2756: Low VOC Self-Priming Polyurethane Topcoat, has been developed for this technology. This material is intended for use on aircraft, weapons systems, and other applications that require protection for aluminum, steel, magnesium, or polymeric substrates. The GSA offers UNICOAT in full gloss, semigloss and flat finishes in more than 30 colors. All coatings are chromate- and lead-free and are low-VOC.

Table 3.11.1 UNICOAT SUBSTITUTES FOR MIL-C-83286

The preceding table identifies UNICOAT substitutes for MIL-C- 83286 that are VOC compliant and do not contain chromium.

The following tables (Tables 3.11.2 through 3.11.4) are from the Qualified Products List as of 2 September 1997 of self-priming polyurethane coatings that are VOC compliant and do not contain chromium or lead.

Table 3.11.2: Coatings that Dry to a Full Gloss Finish

Table 3.11.3: Coatings that Dry to a Semigloss Finish

Table 3.11.4: Coatings that Dry to a Flat Finish

Radiation-Curable Coatings are formulated to cure quickly by exposure to ultraviolet (UV), electron beam (EB), infrared (IR), or microwave radiation. Radiation-Curable Coatings have a higher solids content and consist of a low-molecular weight olefin resin (with carbon-carbon double bonds), a reactive solvent containing unsaturated groups, and a photointiator. Radiation-Curable Coatings are usually clear, but can be pigmented and tend to exhibit good resistance to abrasion, heat staining, and weathering.

VOC Content

Recently VOC compliant radiation curable coatings have been developed with VOC contents as low as 360 g/L.

HAP Content

The HAP content of these coatings is dependent on the composition of the specific coating. Some monomer emissions may be in the exhaust. In most cases, however, the HAP content should be less than that found in conventional coatings.

Benefits

• Production rates tend to be higher because the coated items cure faster than other methods.
• Radiation Ovens require 50-75 percent less floor space than conventional curing ovens.

Limitations

• Certain parts cannot be coated because of their shape and geometry.
• Coatings must be blanketed by an inert gas to prevent tacky surface (air retards polymerization).
• Vapors from the coatings are an eye and skin irritant.
• Some radiation curable coatings are oncogenic or carcinogenic.

Applications

• These coatings can be applied using air or airless spraying, autodeposition, curtain coating, electrostatic spraying, flow coating, or roller/coil coating.
• Radiation curable coatings can be used on weather-sensitive substrates.

Economics

• These coatings tend to have high capital costs, but low energy costs when compared to thermally cured coatings. Production rates tend to be higher because of the decreased time needed for curing.

Supercritical CO₂ Spraying is a paint spraying process that substitutes supercritical (above its critical temperature and pressure) carbon dioxide (CO₂) for as much as 80 percent of the solvents that are used in other coating formulations. Supercritical CO₂ reduces paint viscosity and produces a vigorous atomization and a quality finish. The technology is commercially available.

VOC Content

VOC content and emissions may be reduced up to 80 percent.

HAP Content

Emissions of HAPs may be reduced up to 90 percent.

Benefits

• Existing spray equipment may be retrofitted in some cases.
• Transfer efficiencies approach 60 percent, reducing the amount of coating used and emission rates.
• This technology can produce a higher quality finish than other applications.
• Because less solvent is present, fire/explosion hazards are reduced.
• Because most of the remaining solvents in the coatings are slower evaporating, solvent that evaporates in a bake line or curing oven can be recovered or incinerated.

Limitations

• The availability of coatings are limited: conventional coatings need to be reformulated.
• Frequent color changes are not practical.

Applications

This technology is especially applicable where a high quality finish is needed.

Economics

• Capital costs are higher.
• Expenditures on coatings may be reduced because the unit cost of coatings may be lower, and the transfer efficiencies tend to be slightly higher.

Many companies have committed substantial resources to the development of nonchromated primers. Deft, Courtalds, Lord, Spraylat, and Pratt & Lambert have formulated nonchromated primers that are currently under evaluation at NAWCAD, Pax River. At this time, however, Spraylat is the only company that has qualified a mil-spec non-chrome primer. Northrop Grumman has developed a nonchromated primer that was originally intended for composite surfaces contacting with metal. This primer (DLP-131) is a solvent based, high solids primer and was tested in accordance with MIL-P-23377G. Scale-up efforts will be complete during the 2nd Quarter 1996. Because of the excellent corrosion resistance achieved with this primer (3000 hour salt spray resistance), it is being considered as a candidate for qualification under MIL-P-23377.

• MIL-P-23377 Class N: This primer is corrosion inhibitive, chemical resistive, and able to be stripped. Class 2 versions of both Type I (standard pigment) and Type II (low infrared reflective pigment) primers do not contain chromium. This type of primer is produced by Spraylat and Deft.

• MIL-P-53030: This water reducible epoxy primer is intended for use on ferrous and non-ferrous substrates. It is a lead- and chromate-free primer that may be used to replace MIL-P-52192 and MIL-P-23377 in some cases. The Army has replaced use of MIL-P-23377 with MIL-P-53030 in many applications. The primer is compatible with chemical agent resistant and other aliphatic polyurethane topcoats. The primer contains no more than 340 grams per liter (2.8 pounds per gallon) of volatile organic compounds (VOC). The primer described in this specification is intended for use on clean, chemically pretreated metal surfaces. It is compatible with MIL-C-46168, MIL-C-83286, MIL-C-85285 and MIL-C22750 topcoats.

The Air Force Advanced Aircraft Coating Program’s work on advanced corrosion resistant aircraft coatings includes qualifying a near-term coating system that will meet Aerospace NESHAP requirements and a long-term, totally "green" system. The near-term system includes non-chromated conversion coatings, non-chromated and low VOC primers and topcoats. The long-term solution will utilize sol-gel, a ceramic technology, to replace conversion coatings and interface coatings, and low VOC, non-isocyanate topcoats. This project is targeted toward tanker/transport coatings systems but the technologies will be applicable to all conventional (non-Low Observable) aircraft. The C/KC-135 aircraft has been selected as a test-bed.

The Coating Technology Integration Office (CTIO) at Wright laboratory is working on qualifying non-chromated primers for use during scuff sanding and over-coating of aircraft (i.e., the aircraft is repainted without stripping to bare metal).

CERAM-KOTE 54 is a multi-use, single coat, spray applied, air-dried, flexible, self-priming, ceramic-based coating that was applied on several Texas Air National Guard aircraft with favorable results. It was also applied on the following:

• F-4 Phantom nose cone
• F-15 Eagle leading wing edges, vertical and horizontal stabilizers, and jet intakes
• B-1 Bomber leading edges including slats
• F-16 leading edges, horizontal and vertical stabilizers
• Jet engine lips

Benefits

Wright Laboratory engineers tested the product on a Whirley Arm Tester at Wright-Patterson AFB. The standard polyurethane coating lasted only eight minutes while CERAM-KOTE 54 lasted 15 minutes.

Limitations

CERAM-KOTE has two noted drawbacks. It is not covered by a military specification, and it is reported to be difficult to remove. While the proponents of CERAM-KOTE question the need to remove the material, some Air Force units want to be able to strip an aircraft's leading edge down to bare metal. Concerns also exist about colorfastness and infrared invisibility of the material.

The Navy has conducted some IR tests on the material.

Randolph AFB recently tested non-chromate surface conversion coatings as replacements for alodine, which contains large amounts of chromate. As chromate is being phased out of the aircraft painting process, X-IT Prekote, and SOL GEL Primer were tested as potential replacements for alodine. These new surface conversion coatings are chromate free and non-hazardous. They have the potential to change a large and costly waste stream from hazardous to non-hazardous.

The new surface conversion coatings were applied to two T-38A and two T-37B aircraft. All tests were conducted by 12 Flying Training Wing (FTW) and 14 FTW personnel under the supervision of the manufacturers. The X-IT worked very well and is going to be advanced for testing on three more aircraft (T-38s) at Columbus AFB. If those tests are successful it may be approved. SOL GEL Primer did not work as it caused problems with adhesion and magnesium coating. It was dropped from further testing.

A non-chromated formulation of MIL-P-85582 by Spraylat has been approved. Field testing is in progress. Other formulations are under evaluation by the Navy.

Thiokol developed a nonchromated surface treatment process for aluminum that greatly enhances the adhesion of applied paints and coatings. The process consists of a metasilicate and organofunctional silane that provides a covalent chemical bond with the paint or coating. An evaluation for NASA (Space Shuttle) determined this process to be equivalent to unpainted chromate chemical conversion coating (CCC) in corrosion resistance. This process still needs military qualification. The Thiokol pretreatment process is a two-step process that is applicable to immersion applications and, potentially, spray applications.

Vapor Deposition is a group of technologies that is used in decorative coating, tool coating and other equipment coating applications. It is fundamentally an evaporative process where there is an atom-by-atom transfer from the solid phase to the vapor phase and back to the solid phase with a gradual build-up of a film on the surface to be coated. It is mainly considered an alternative to electroplating, but may have some painting applications as well. The U.S. Army Construction Engineering Laboratory (CERL) is investigating ion vapor deposition (IVD), also known as ion vapor plating.

The CERL is continuing research by using ion vapor deposition to coat anodes in cathodic protection systems and gaskets in communication shelters. CERL holds the patent for "Mixed Metal Oxide Coated Substrates".

VOC Content

There are essentially no VOC emissions associated with this process because the process is conducted in a vacuum.

HAP Content

No HAPs are emitted by IVD.

Benefits of IVD

• Can be used with virtually any type of coating
• Can be applied to most metals and some plastics
• Coatings are typically soft, ductile, adherent, and corrosion resistant
• Generates limited quantities of non-hazardous waste

Limitations of IVD

• Limited to small parts applications
• Cannot be used where fine tolerances are required or there is a small diameter opening
• Requires a cooling water system
• Typically does not work well where lubrication occurs (e.g., on fasteners)

Applications

Vapor deposition technologies can be used on most metals and some plastics.

Economics

The capital costs of this option are high (as much as $500,000 or more).

Through the Joint Strike Fighter (JSF) Paintless Airplane Program (JPAP), the Boeing Company and 3M have been demonstrating feasibility of replacing paint with appliqués, an adhesive sheet.

Extensive material testing and a series of flight test were performed prior to full-coverage application of an F/A-18B supersonic aircraft at the Naval Air Warfare Center, Pautuxent River. The JPAP's will be quantifying the reduction in aircraft support costs associated with appliqués by estimating the total life-cycle costs for fleet use of appliqués and comparing them to paint. Other JPAP objectives include demonstrating the suitability of appliqués for maritime and carrier environments, and for supersonic aircraft.

To accomplish these objectives, Boeing and 3M have conducted materials, environmental, wind tunnel and flight tests. The flight tests were broken into a series of progressively more difficult demonstrations beginning with a small patch of appliqués placed on the surfaces of a Boeing T-33 chase plane. In September 1996, most surfaces of an F-18-B were covered with an appliqué, and in October a one-year flight test began. Material flight-qualification testing and T-33 flight tests were conducted at the Boeing Developmental Center in Seattle, along with development of application, cutting and removal techniques. All F-18 flight-testing is being performed at the Naval Air Station Patuxent River, MD, in combination with JPAP partners, Naval Air Systems Command and Naval Air Warfare Center Aircraft Division.

VOC Content

There are no volatile emissions.

HAP Content

There are no HAP emissions.

Benefits

Appliqués have the following benefits in addition to zero emissions:

• There would be no need for stripping and repainting of aircraft.
• Hazardous waste generation would decrease significantly.
• Because old appliqués are peeled off, there will be no "weight growth".
• Appliqués can be applied concurrently with other maintenance operations.

Limitations

Listed below are some of the current limitations with appliqués that need to be addressed:

• A chromate primer is still needed to prevent corrosion.
• Appliqués are difficult to apply to panel edges and fasteners.
• Improvements are needed in linking the adhesive to the film.
• It is suspected that chromates in the primer may leach into the appliqué.

Applications

While this technology is promising, it is still in the research phase, and so not yet ready for the field.

Economics

This technology has the potential to cut maintenance and environmental costs.

Isocyanates are used as hardeners in polyurethane paint systems. The Air Force, other military services, and commercial aircraft manufacturers use polyurethane paint systems almost exclusively on their aircraft exterior surfaces. These paints are two-part paint systems, in which the isocyanate groups in Part 1 react with the hydroxyl groups in Part 2 to generate a urethane cross-link, which hardens or "cures" the paint.

The advantages of isocyanate based paint systems include rapid curing time, low curing temperature, and excellent abrasion and impact resistance. Unfortunately, isocyanates have both health and environmental hazards, prompting the Air Force Air Logistic Centers (ALCs) to request research resources for the investigation of a replacement for isocyanates in polyurethane paint systems.

Vapors or mists of isocyanates can be generated during paint spraying. Even brief exposure can be irritating to the nose, throat, and lungs. Sensitization may result from excessive exposure. Subsequent exposure to low concentrations has been known to provoke allergic reactions with asthma-type symptoms. Repeated or prolonged skin contact may cause irritation, blistering, dermatitis, or skin sensitization. The Air Force protects Corrosion Control personnel by requiring the use of coveralls, supplied air respirators, and in many cases, mechanical ventilation.

Some isocyanates are also considered environmental hazards. A few have been added to EPA's Aerospace Industry NESHAP list of HAPs. The list now includes hexamethylene 1,6-diisocyanate, methyl isocyanate, methylene diphenyl diisocyanate, and 2,4 toluene diisocyanate.

The potential for exposure to isocyanates is increased when the personal protective equipment (respirators, etc.), engineering controls (paint booths), and administrative controls (prohibiting occupancy of painting areas for hours after painting) are bypassed. These protective controls are not always followed by the painters, or those who work near painting areas.

In recent years, chemical substitution within polyurethane paint systems has been the topic of industrial research. But these efforts have focused on the elimination of chromium, and the reduction of volatile organic compounds (VOCs) in the paint, and not on the elimination of isocyanates. To date, no replacement for isocyanate-containing polyurethane paints has been developed for use in the aerospace industry.

• Spraylat Corporation is a paint manufacturing firm (Chicago, IL) that supplies polyurethane paints, and performs research into future coatings. The firm has developed substitutes for polyurethane paints for use in general industry (cycloaliphatic epoxy systems). However, these paints reportedly will not meet the specifications required by the aerospace industry (flexibility tests, etc.). Since present research is focused in other areas Spraylat predicts that the coatings industry will not develop a suitable substitute for isocyanate-containing polyurethane paint for the aerospace industry for at least 10 to 20 years.

• Courtalds Aerospace’s paint research has focused on ridding the paint of metals and VOCs. Isocyanate-substitution is a long-range research issue, with some testing going on at the Courtalds facilities in England. Courtalds is experimenting with one-component polyurethane systems. These systems contain isocyanates that are pre-reacted, which is believed to be less hazardous to workers applying the paints. But Courtalds reports that these systems do not offer the characteristics required by the aerospace industry. They are currently used inside some aircraft, but not on the exterior skin of the aircraft.

• Air Products, Inc. specializes in Navy aircraft coatings and materials development. They have an active R&D department that has proprietary coating systems in development that may be applicable to aircraft in five to ten years. The competitive nature of the commercial coatings industry prevents this and most companies from sharing details of the research.

• Wright Laboratory is conducting research on waterborne polyurethane paints under its High Performance Aerospace Coating System Program. The isocyanates in waterborne (as opposed to the usual solvent-borne) systems are blocked from reacting, which may reduce the exposure potential for workers spraying the paint. This may be a promising alternative to the system currently in use.
• Wright Laboratory’s Air Force Advanced Aircraft Coating Program and Boeing Defense and Space are working to develop advanced corrosion resistant aircraft coatings for aluminum alloys which are environmentally benign and meet DoD performance demands. The program employs a two-part parallel effort to develop a near-term coating system that will meet Aerospace NESHAP requirements and a long-term, totally "green" system that will include low VOC, non-isocyanate topcoats. It is targeted toward tanker/transport coatings systems but the technologies will be applicable to all conventional (non-Low Observable) aircraft. The C/KC-135 aircraft has been selected as a test-bed.

• Aircraft Film Technology has been studied by the Air Force. Although the film is an isocyanate-containing polyurethane, it is a hand-applied film material, and not sprayed onto the surface of the aircraft. Therefore, the film would have a lower potential for isocyanate exposure to workers. Various types of film are currently used on portions of military aircraft. Examples include polyurethane radome boots on the F-14, F-15, and F-16; polyurethane rotor boots on Army helicopters; and polyurethane leading edge tape. Concerns exist for both the Air Force and the commercial aircraft industry regarding corrosion, adhesion, and resistance to service fluids.

• The Naval Air Warfare Center, Aircraft Division (NAWCAD), that investigates the surface coatings used on Navy aircraft, reported that the Navy uses MIL-C 85285 and MIL-C 83286 polyurethane topcoats. The office suggests that a one-component polyurethane system may be used for touch up painting of aircraft.

Industry continues to develop lower VOC coatings. Deft, Inc. is currently in the process of field testing a zero VOC polyurethane topcoat. The coating is a two-component polyurethane that utilizes water as a carrier. Typical properties of a zero VOC gloss polyurethane coating include the following:

Table 3.11.5: Typical Properties of Zero VOC Gloss Polyurethane

Based on the performance properties outlined above, it appears that zero VOC topcoats will offer comparable performance to the polyurethane topcoats currently used for military applications (MIL-C-85285 and MIL-C-83286). However, the application process for this technology may be more sensitive than polyurethane topcoats to environmental conditions. Because the carrier used in these coatings is water, temperature and humidity conditions will have a greater effect on the cure. Temperature or humidity controls for paint booths or hangars may be necessary when using this product in some climates.

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