Evolution in the Skies

How aircraft painting is changing for the better
#masking #curing #pollutioncontrol


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Air travel has become a relatively common experience for millions of people around the world. We’ve changed from a time where a trip to Europe would be considered a once-in-a-lifetime experience to the present, where flying halfway across the country for a weekend is not unusual for many Americans. Air travel in India is expanding so rapidly that 40% of all passengers on a given flight are first-time fliers! A Boeing airplane lands or takes off every three seconds of the day on average, and there are approximately 12,000 commercial airplanes flying today.

Despite some well-publicized issues associated with air travel, the perception of air travel as safe and effective is well deserved due to the balance of regulatory oversight and market competition at both the OEM and airline level. Part of this perception is the physical appearance of the airplane itself.

Most passengers would find it difficult to guess whether the airplane they’re boarding is a year old, 15 years old or older. And, the exterior appearance of commercial airplanes has become an increasingly major marketing and branding tool for the airlines. A dull, dirty paint job is not the image the airlines want to present to customers.

Like all areas of aerospace technology, coatings and their associated processes have evolved over the years due to changes in environmental regulations and market factors. These drivers have been instrumental in changing the coatings used today and in the foreseeable future.

Each driver has contributed to exterior decorative coatings development, but the most important factors in the past 15 years have been increasing environmental regulations rivers and changes in the in-service atmospheric environment.

In the early 1990s, the U.S. Environmental Protection Agency (EPA) proposed aerospace-specific regulations known as Aerospace National Emissions Standards for Hazardous Air Pollutants (ANESHAP). These regulations were promulgated in September 1995 for compliance in September 1998. The specific parts of the ANESHAP regs that impacted aircraft exterior coatings are:

  • Limited volatile organic compound (VOC) content of primers no greater than 350 g/L
  • Limited VOC content of topcoats no greater than 420 g/L
  • Mandated use of electrostatic or HVLP spray equipment
  • Mandated capture of solvent used for spray gun cleaning
  • Limited solvents for cleaning aircraft production parts to vapor pressure less than 45 mm Hg at 20°C
  • Mandated use of HAP-free chemical paint strippers.

These regulations resulted in the largest coatings reformulation effort in aerospace history, requiring improved communication and integration of research and development with the manufacturing organizations in order to meet the compliance deadlines. Primers and topcoats that had not changed for up to 20 years had to be replaced.

Difficulties with the first generation of reformulated coatings included increased color-to-color variation; reduced opacity in reds, yellows and oranges that used new organic pigments; and a host of quality issues related to new, high-solids chemistries. These difficulties and others were addressed, and we’ve moved from the first materials to the topcoats and primers used today.

The second major driver of changes in exterior coating formulations was changes in the in-service environment. In 1991, the Mt. Pinatubo volcano erupted in the Philippines, emitting 15–30 million tons of sulfur dioxide gas into the atmosphere. This gas combined with atmospheric moisture to form sulfuric acid, and acid in the upper atmosphere resulted in a significant increase in complaints from airlines about loss of gloss, color shift, and loss of paint adhesion from rivets (“rivet rash”).

The eruption and the resulting customer complaints led Boeing to develop a topcoat test incorporating alternating acid and UV exposure. This test confirmed the poor gloss performance of the high-solvent topcoats being used at the time and the potential for improved performance with new high-solids topcoats. The poor color retention of some colors, such as reds, was related to the cadmium-based pigments that were already phased out in the late 1980s but were still in service at the time of the eruption. Loss of coatings from rivets, a condition experienced at various levels for decades, was initially associated with the Mt. Pinatubo eruption but would take an additional ten years and significant effort at Boeing to fully resolve. This problem was eventually solved by changing primers, consolidating rivet suppliers to a single source, and changing the rivet conversion coating.

Customer Expectations

No one captures the sense of importance of the exterior appearance of airplanes better than former Continental Airlines CEO Gordon Bethune in his book From Worst to First: Behind the Scenes of Continental’s Remarkable Comeback. “When I took over, the only thing consistent in our paint schemes was that they were all peeling,” Bethune wrote. He ordered all of Continental’s planes repainted within six months.

Over the years, airlines have used exterior livery to brand their company and differentiate it from the competition. Before the 1990s, most paint liveries consisted of no more than four colors, and metallic colors were limited to small areas. Full-fuselage metallic designs were not available to customers. But increasing requests for more complex designs led aerospace coatings suppliers, working with airlines and OEMs such as Boeing and Airbus Industrie, to develop more stable metallic/mica colors and processes.

In 2001, Virgin Atlantic received the first full-body mica livery on its 747s, with Northwest Airlines following in 2003 on its 757s from Boeing and all its fleet repaints. Several other airlines have since followed this trend, but at the same time others have retreated from the metallic finish due to the difficulty in matching touch-up during routine maintenance. Spirit Airlines, which had over a dozen shades of silver mica in its airline launch livery, recently rebranded to eliminate micas and use only six solid colors.

Productivity Initiatives

The aircraft OEMs’ drive to meet customer expectations is balanced by internal continuous efforts to improve efficiency in the manufacturing process, reduce costs, and maintain a competitive advantage without compromising quality and safety.

Using the 777 as an example, Boeing integrates 3 million parts produced by more than 900 suppliers from 17 countries around the world. These suppliers, and Boeing’s Everett, WA, assembly facility, are continually tasked with improving their operations.

The decorative paint hangars at Everett are no exception. There, the following processes occur in a very compressed time frame:

  • Positioning of the airplane and masking of critical parts such as landing gear.
  • Cleaning and sanding of pre-primed composite surfaces.
  • Removal of the acrylic-based temporary protective coating from aluminum surfaces using an alkaline remover
  • Cleaning of metallic surfaces of tape, sealant splatter, drilling lubricants, and other contaminants from the assembly process
  • Abrasion or acid cleaning of the metallic surface in preparation for conversion coating
  • Conversion coating for improved primer adhesion
  • Application of solvent-based, high-solids epoxy primer with hexavalent chrome for corrosion protection
  • Application of solvent-based, high-solids polyurethane topcoat on the full fuselage or half, depending on the customer design
  • Masking and application of topcoat in the customer livery in additional paint cycles as needed
  • Application of stencils and decals for regulatory, maintenance and customer logo markings
  • Unmasking, touch-up and roll out from the paint hangar.

All of these tasks are performed manually by experienced painters, aided by either ceiling-mounted tele-platforms or adjustable under-wing stands that allow full access to the entire exterior. Paint hangars are staffed 24 hr/day to fully utilize the paint facilities. Hangars can supply up to 400,000 cfm of filtered, conditioned air flow, remove all particulate and aerosols from exhausted air, and cure at temperatures to 120°F.

While the paint hangars are the most visible indication of commercial coatings finishing at Boeing, the technical community that supports the research, qualification and implementation of the materials and processes is critical to paint operations. This task is the responsibility of the Material and Process Technology (M&PT) organization, itself a product of the merger of two engineering groups, Manufacturing Research and Development (MR&D) and Boeing Material Technology (BMT).

Previously, both groups performed coatings R&D, but MR&D focused on the manufacturing and quality aspect while BMT focused on engineering design. The merger of the two groups resulted in improved communication and better utilization of research resources. Research that was performed sequentially in the past is now done concurrently, reducing the time from laboratory testing to production implementation.

Challenge as Opportunity

Mandatory government regulations, extreme atmospheric conditions, demanding customers with complex designs, a complex global parts supply chain and internal demands to reduce flow time and improve quality—where is the opportunity in all this? The surprising answer: almost everywhere. The need for change opened the door to innovative approaches that would not have been attempted without these driving forces. In meeting the changes required by regulations and customers, Boeing has learned more and provided a higher-quality product to its customers on both new airplanes and in their repaint facilities. Following are a few examples of the improvements that have been realized.

Boeing expanded the types of tools used to analyze coatings and in-service coating problems with improved analytical capabilities for coatings analysis. Fourier transform infrared (FT/IR) spectroscopy, for example, became a key tool in coatings analysis when FT/IR spectrometers, which used to be delicate lab instruments weighing more than 100 lb, became available as portable units that could be used on the side of an airplane.

Using spectroscopy, we built databases of chemical spectra to allow rapid identification of contamination sources as well as basic material properties. Other analytical methods, such as high-pressure liquid chromatography (HPLC), electron spectroscopy for chemical analysis (ESCA), Auger electron spectroscopy, and secondary ion mass spectrometry (SIMS) have also become common tools for understanding coating properties.

Another key initiative was Boeing’s work with coatings suppliers to improve coating application consistency using rheology. During implementation of the first generation of high-solids topcoats, it became apparent that standard viscosity measurement using Zahn cups did not ensure the coatings consistency needed for the paint hangar environment. Different batches of a topcoat with the same Zahn cup values could have dramatically different application properties.

Use of computerized rheometers helped Boeing work with our coatings vendors to balance application properties of their coatings to minimize orange peel while resisting runs and sags. The rheometer also allowed us to quantify changes in rheology over time to better optimize properties at the estimated application time—typically, two months after blending of the topcoat color.

While many of the improvements made in high-solids coatings are due to the skilled chemists at our suppliers, some were also made at Boeing’s initiative. Before high-solids, one of our major coatings formulators supplied a concentrated, tin-based catalyst for adjusting the cure time of its polyurethane topcoat, often referred to as “kicker.” The ability to adjust cure time is critical for outdoor touch-up without having to return the airplane to the paint hangar.

A quart of kicker was intended to adjust at least 2,000 gal of mixed topcoat, but the quantity of catalyst ordered by production shops far exceeded projected usage. M&PT directed that no high-solids topcoats would have concentrated catalyst but would instead use “alternative thinners” with predetermined catalyst levels. All of our suppliers now use this approach, giving the using shop a choice of standard, medium, and fast-cure options that eliminate the risk of over-catalyzing the topcoat.

A second example was an effort that reduced the weight of the gray protective topcoat used on non-decorative exterior surfaces such as wings and landing gear. While qualifying a second generation high-solids topcoat from one of our suppliers, Boeing noted a significant reduction—in some cases, more than 2 lb/gal—in the base weight of certain colors such as blacks and reds.

This was due to a change in inert pigment used in the formulation originally developed to pass Boeing’s acid/UV exposure test. The topcoat supplier had changed from the standard inert pigment used in aerospace topcoats since 1971, barium sulfate, to a new, proprietary inert with a specific gravity (2.4) roughly half that of barium sulfate (4.5).

Boeing suggested this inert be incorporated into the formulation of gray colors. The white base used to make grays is highly pigmented due to the poor opacity of titanium dioxide (specific gravity 3.8–4.3), which requires almost 5 mils dry film thickness (DFT) to reach full hide. Yet grays easily reach hide at DFT of <1 mil. By incorporating the new inert, the supplier was able to reduce the weight of gray coatings by approximately 10%.

An additional benefit was improved color stability of grays, because reduced density variation improved the topcoat’s colloidal stability. This reduced pigment flood and float because the new inert is closer to the specific gravity of the carbon black (1.6) and other pigments used to make gray. Incorporation of this change, initiated by Boeing, has resulted in reduced coating weights that total thousands of pounds per year.

The Future
Advances in commercial aerospace coatings will continue as the various environmental, customer and supplier drivers become more integrated than they were in the past. Elimination of hexavalent chrome will receive the same attention as solvent reduction did a decade ago in North America, while the European and Asian regions are now requiring the high-solids coatings pioneered in the United States.

Eliminating rivet rash, once the major reason airlines repainted after four years, will lead to more emphasis on improved overall gloss and durability of topcoats. Increasing use of composites, such as the all-composite Boeing 787 fuselage, will increase the need for alternatives to remove coatings by manual abrasion methods—currently the standard at OEM and repaint facilities.

Manufacturing process optimization will lead to coatings that offer improved application properties and reduced hangar flow time requirements while maintaining or improving environmental, health and safety aspects of the operations environment. Ergonomic issues will drive changes in surface cleaning and reactivation technology. Manufacturers such as Boeing will continue to integrate new process technology, training and data management into their daily operations to create the best product and experience available to the flying public.