By Cynthia Challener, CoatingsTech Contributing Writer
Healthy growth in the global aerospace industry is driving strong demand for coatings, adhesives, sealants, and elastomers (CASE) used for interior, exterior, and structural aircraft, spacecraft, and unmanned aerial vehicle applications.
Market research firms Markets and Markets and Technavio predict the aerospace coatings market will grow at an annual rate of 6.9% and 7%, respectively. Markets and Markets estimates the value of the market will increase from $1.42 billion in 2017 to $1.98 billion by 2022. Global Market Insights’ numbers are much more conservative, coming in at just $1 billion by 2024 (16 kilotons in volume). This firm estimates that 89% of aerospace coatings are used in exterior applications, while OEM consumption accounts for 58% of all coating sales, and 35% of coatings are used on commercial aircraft. Just over a third of aerospace coatings are sold in North America, but the Asia-Pacific region is experiencing the fastest growth rate.
The global aerospace adhesives and sealants market is estimated to be growing at a slower annual rate of 4.69% to 5.46% according to market research firms Mordor Intelligence and Markets and Markets, respectively. The former pegs the value of the market at $632.87 million in 2017, with North America grabbing a 40% share. The latter once again has higher numbers, with the market expanding from $692.4 million in 2016 to $954.7 million by 2022.
Rising passenger air traffic, particularly in the Middle East and Asia-Pacific regions and ASEAN countries, increasing international trade, and expanding investment in the aviation industry in emerging markets are all important drivers of growth for the aerospace industry. Advances in CASE technologies are helping aircraft manufacturers not only extend aircraft life, but reduce weight and thereby fuel consumption. “CASE applications are designed in throughout both legacy and next-generation airframes and major components,” observes Marc Gomez, global business development manager for Henkel Corp. “Generally speaking,” he continues, “CASE technologies solve common problems, like design gaps, contact corrosion from dissimilar metals, fluid leaks from thermal expansion and contraction variances, and structurally adhering rigid surfaces to more flexible surfaces. CASE products address all these design variances in a proven and known manner, so aircraft manufacturers and parts designers can depend on consistent product performance with fully transparent failure modes.”
Common Technology Drivers
Urbanization is driving the need for more aircraft as more people fly for both business and pleasure. Build rates today are at levels never before seen in the industry and all indications are that rates will increase even more for the known future, according to Gomez. “The most significant development in the last few years is the staggering rate requirements for commercial, defense, and space platforms. These are good problems for a supplier, as innovation is needed throughout the global supply chain in order meet the demands of today and tomorrow,” he asserts.
The rapid growth is driving interest in automation technologies to speed aircraft assembly, including the application of CASE systems. “As evidenced by the automotive industry, many adhesive applications can be mixed in-line and dispensed robotically, and it is only a matter of time before the business case justification for such practices are realized in the aerospace industry,” Gomez notes. The same is true for coatings.
“Easy, fast, economical, long-lasting, reliable, and consistent manufacturing techniques are always of prime importance,” adds Rohit Ramnath, a senior product engineer with Master Bond. “Customization of solutions and the ability to respond quickly is vital as manufacturers make frequent improvements in better use of processing equipment, enhancing quality control, meeting difficult regulations, and offering customers better value while meeting competitive pressures,” he observes. As a result, coatings and adhesives that help improve the productivity of the manufacturing process (e.g., ease of application) and offer design improvements and savings are increasingly important in the aerospace industry, according to Jack Forsythe, industrial coatings segment lead market manager at Covestro LLC.
Another key trend occurring in both the aerospace and automotive industries is lightweighting to reduce fuel consumption for reduced costs and lower emissions. “Lightweight aluminum alloys, like aluminum lithium, require a new family of surface treatment products both for metal pretreatment of the base substrate and aftermarket cleaning and de-painting of finished parts,” Gomez explains. This need for more robust surface solutions also extends to other lightweight metals such as magnesium and titanium. Beyond metal alloys, he notes that carbon fiber prepreg parts also require new adhesive and surface treatment products during OEM assembly and maintenance, repair, and overhaul (MRO) of aircraft. The use of many dissimilar substrates is another consequence of the lightweighting trend, and there is a growing need for adhesives that can bond these materials to one another and coatings that can adhere to many different surfaces, according to Ramnath. Coatings that minimize drag in the air and reduce debris build-up and thus reduce airplane fuel consumption while maintaining necessary properties are also sought, according to Forsythe.
Reducing the total cost of ownership is a key driver that influences technology developments in aerospace coatings at AkzoNobel. “Total cost of ownership includes coating durability, ease of application, process time, and the cost per liter/surface area,” comments John Griffin, segment director for Aerospace & Film with AkzoNobel Automotive & Specialty Coatings. “Durability is by far the biggest contributor to total cost of ownership. Delaying the need to repaint aircraft or components saves significant costs in terms of material and application. Additionally, if aircraft do not need to be grounded for repainting, they can continue to generate revenue or be mission-ready,” he says.
Similar Regulatory Challenges
By far the biggest challenge for CASE technology developers, according to Griffin, is continuing to meet the requirements of aircraft manufacturers and operators while still complying with the increasing number of environmental regulations. “The development of aerospace coatings takes longer compared to other industries, and it can take three or more years to develop and qualify a product. Regulations that change from year to year present an added challenge, and the complexity continues to grow as countries set their own regulations and requirements,” he remarks.
One of the biggest technology hurdles for the aerospace industry is finding alternatives to chrome-based corrosion-prevention systems. “For a long time, this industry has been relying on chrome-based coatings to protect planes against intense UV exposure at higher altitudes, as well as for corrosion and temperature fluctuations,” Forsythe explains. Hexavalent chromium chemistries (i.e., strontium chromate and barium chromate) are still quite prevalent in treating aluminum, the most common metal substrate used in aircraft structures and parts, against corrosion; however, there is a major industry push towards more sustainable trivalent chromium technologies. There is also a desire to develop chrome-free coatings and primers for adhesive bonding, according to Forsythe. “Improving and replacing systems containing chromate anticorrosive pigments stands out as the most significant challenge. Strontium chromate is the leading anticorrosive pigment, and even after years of research into alternatives, it is still the leading ingredient,” he notes.
Sustainability in general is having a broad impact on the technologies used in the aerospace industry, according to Gomez. In addition to driving new regulations, such as REACH in Europe, that are impacting the use of traditional chrome systems, he notes that sustainability is also driving the design discussion and investment towards additive manufacturing practices like 3D printing. “More environmentally impactful subtractive manufacturing processes like chemical milling of aluminum substrates is becoming more unique, which impacts the market need for traditional solvent-based maskants. Maskants for chemical milling are also being challenged for solvent reduction with two-part and light-cure chemistries,” he explains. On a positive note, Gomez sees the move towards additive manufacturing creating new opportunities for surface finishing of these parts. “More and more original equipment manufacturers (OEMs) are working with leading chemical manufacturers to ensure sustainable solutions for current and future aircraft fleets. Most leading aircraft OEMs and suppliers are working in earnest to identify suitable solutions, while ensuring the dependable quality and safety that is expected throughout the industry,” he asserts.
Spotlight on Aerospace Coatings
Aerospace coating systems largely consist of epoxy primers and polyurethane topcoats. Epoxies are preferred as primers due to their excellent adhesive and chemical resistance properties, according to Griffin. They can be formulated as traditional solvent-based systems with conventional solids contents, high solids solvent-based systems, or water-based systems, but high-solids formulations dominate. Polyurethanes are preferred for topcoats because they provide durability, chemical and fluid resistance, flexibility, UV resistance in acidic atmospheric conditions, color and gloss retention, cleanability, and stain resistance, according to Forsythe. Polyurethane topcoats are formulated almost exclusively as solvent-based high solids, according to Griffin, although he notes that some water-based polyurethane dispersions are finding applications as aircraft cabin coatings. While typically applied at a dry film thickness of 2–3 mils, some OEMs apply polyurethane topcoats at even higher coating thicknesses, according to Forsythe.
Over time, aerospace coating technologies have evolved to improve durability and reduce application time, notes Griffin. As one example, AkzoNobel has developed basecoat/clearcoat technologies to address these market needs. The company has also found ways to optimize color formulation to achieve optimum brightness and opacity, leading to the need for fewer layers and lower film builds, potentially reducing weight. A new interior cabin coating system tailored for ease of application by MRO service providers and OEMs was also recently launched by AkzoNobel. “These one-component polyurethane dispersions dry quickly and meet all FAA and OEM performance requirements for aircraft cabin interiors. They are highly fire- and stain-resistant and designed to be used in molding and thermoforming applications over interior cabin parts, but can also be used as part of laminate constructions,” Griffin says. He adds that offering both full liquid and film systems allows the company to provide versatile options for aircraft cabins.
Eliminating hexavalent chromium continues to be a main focus area for the industry, and significant progress is being made in the development of safe and effective corrosion-inhibiting coatings for both exterior and structural applications on aircraft. Chromate-free primers for aluminum are currently used on aircraft sections that are easily accessible and can be inspected for any deficiencies, and roughly 15% of all primers in aerospace are chromate free, according to Forsythe, although interior primers still nearly always contain chromate primers. AkzoNobel is in the late stages of qualifying its Aerodur 2111 chromate-free primer to Boeing specification BMS 10-72. Its Aerodur 2118 product has already been approved to meet the AMS 3095A specification and is currently used by several major airlines, according to Griffin. The company is also working with universities and leading aerospace companies to evaluate and refine its patented magnesium oxide/lithium salt technology and its licensed magnesium particle inhibitor technology. Much of this effort is focused on developing test methods that more accurately assess a paint system’s ability to protect aluminum and steel structures. “The biggest challenge to developing and approving chromate-free corrosion inhibiting coatings is to extrapolate test data into service life,” says Griffin. He adds that the standard salt fog tests do not accurately predict long-term corrosion-inhibiting performance.
Primerless polyurethane topcoats are also being developed and used by the military, according to Forsythe. “These coatings eliminate the necessity for primer and as ‘monocoats’ offer lower film weights, reduced labor, and reduced VOC emissions—all important attributes for the aerospace industry,” he observes. He adds that return-to-service time is also critical for the military, which has need for a quick “dry to fly” time in areas of conflicts and action. Some radiation-cured UV topcoats have been evaluated to address this issue, but there are no reports of large scale use yet. AkzoNobel is one company developing new UV-curable coatings that can speed up production and maintenance processes. It is particularly valuable for clearcoats, according to Griffin. The drying time of the clearcoat in basecoat/clearcoat systems is the single largest contributor to process time because to achieve proper flow and leveling, at least 8–10 hours are needed. Reducing this time can help speed up the painting process by providing cycle time savings.
Application efficiency is also a key concern, according to Griffin. “The amount of liquid paint that coats a surface is < 50% of the amount that is mixed and sprayed, leading to significant waste in the spray process. As more aircraft and aircraft parts are being constructed from composites, we have the opportunity to apply finishes in the mold rather than spraying on the surface afterwards,” he notes. AkzoNobel is developing films that can be applied in the mold and cured using a patented latent cure mechanism that allows the dry film to further crosslink with the substrate under heat and pressure. “We expect this process will yield a protective coating for structural and non-structural composites that has better adhesion, flexibility, and durability compared with traditional spray-applied primers and coatings,” Griffin says.
Other interesting technologies are finding use as the range of substrates used in aircraft construction expands. Forsythe points to waterborne anionic epoxies for use on OEM parts, conductive coatings for lightning strike protection on composite aircraft parts before application of a chromate-free primer, and solventborne one- and two-part elastomeric polyurethane primers used by the military that provide elongation of approximately 200 (compared to just 10% for epoxies). “This flexibility allows polyurethane primers to withstand the stress applied to fasteners without cracking, which prevents water and chemical penetration,” he explains.
Focus on Aerospace Adhesives and Sealants
Bonding applications throughout an aircraft have varying requirements depending on the position and criticality of the application, according to Gomez. Parts in the interior of the aircraft require materials that have flame retardant capabilities, while parts around the aircraft engine require materials that provide added toughness at elevated temperatures. “Adhesive manufacturers provide a profound array of design solutions like epoxy adhesives for load-bearing applications, anaerobic adhesives for thread sealing, and silicones for high-temperature sealing,” he says.
The push into more lightweight airframes is driving the expanded use of carbon fiber prepregs for large structures, which in turn is driving the need for adhesive solutions, because composite parts cannot be riveted like their metal predecessors, according to Gomez. He also notes that thermoplastic applications are increasing as well, which creates opportunities for instant and light-cured adhesives.
Liquid, paste, and film adhesives and sealants exhibit a wide range of properties, can be conveniently applied, and have versatile cure schedules ranging from room-temperature cures to heat activated systems, according to Ramnath. They are used for the assembly of aircraft structures, components, interiors, and MRO applications; must meet stringent specifications; and feature high strength, humidity/chemical resistance, thermal stability, and protection against abrasion, corrosion, fungus, fatigue, creep, vibration, impact, and shock. Various standards must be met depending on the application, including NASA certification for low-outgassing, Federal Aviation Regulation 25.853(a) for flame retardancy, Boeing/Airbus standards for low smoke and toxicity, and MIL STD 810G for fungus resistance.
Adhesives can be conductive or nonconductive. Unique grades are formulated for special applications and provide properties such as exceptional compressive strength and high glass transition temperatures, according to Ramnath. “Special packaging solutions for aerospace applications are also engineered to optimize productivity, reduce waste, and improve reliability,” he adds. Examples include premixed and frozen syringes, Semkit packaging, premeasured standard packaging kits, and dual cartridges in gun applicators.
Epoxies are widely used for bonding composites and other substrates because they have a very low density and help in aircraft lightweighting, adds Forsythe. Structural acrylics are used throughout the aircraft for diverse applications, while cyanoacrylates are mostly used for quick fixes on damaged interior trim parts. Adhesive tapes and films are also used for a variety of needs. Silicone compounds, according to Ramnath, have met challenging aerospace needs including low outgassing, vibration damping, thermal management, light lens sealing, cockpit instrument sealing, and stress-relief protection of circuits. “One- and two-component silicones are noted for their exceptional temperature stability, low temperature flexibility, excellent dielectric properties, excellent thermal cycling capability, and low shrinkage upon cure. Low modulus gels, conformal coatings, and thermal interface materials have played a key role in enhancing the performance of power modules, electro-optical components, and precision instruments used in space,” he asserts.
Developing new adhesives and sealants for aerospace applications can be quite challenging. “Formulations developed using new ingredients that offer improved capabilities must still satisfy the multiple processing requirements for aerospace applications. Unfortunately, when developing these systems there are many tradeoffs that must be taken into consideration. Environmental exposures may vary between the numerous applications. A combination of conditions could have a deleterious effect on a joint in comparison to a test for a single type of exposure. Accelerated testing methods for all types of conditions have to be done fastidiously to take into account all possibilities and ensure that the adhesive will exhibit long-term durability,” says Ramnath. Simulation of the exact conditions the adhesive will see in service is the best technique to guarantee the desired results.
These requirements result in long development cycles for new aircraft and engine platforms, according to Gomez, and few players can maintain the investment requirements for developing new chemistry to meet tomorrow’s challenges. “Initial volume requirements are typically very small and production ramp up can take a few more years. For the most critical applications raw materials for new formulations need to be locked in so the OEM and MRO can depend on the chemical solution 30 years from now as the age of the aircraft dictates,” he observes. Furthermore, OEMs have large budgets at the initiation of a program that dwindle as the aircraft goes into service, so application and development windows close relatively early in the life of a program. “Raw material suppliers to chemical formulators find it difficult to supply initial requirements, let alone justify a business case to continue supply of same raw material 30 years from now,” he adds.
Complicating the situation is the importance of application technique, ease of handling, cure speed, safety, and sustainability—all of which are also vital to selecting the proper system, according to Ramnath. In addition, proper surface preparation and cleaning techniques must be employed. “An adhesive will only perform well if the surface is properly prepared,” he states. Dispensing the adhesive must be considered as well, and can be done manually or robotically, the choice of which will depend on the compound used, parts configuration, adhesive viscosity and cure speed, and the volume of parts being mated.
In recent years, both the functionality and application methods for aerospace adhesives and sealants have been greatly improved with respect to efficiency, capability, long-term durability, miniaturization, speed, weight reduction, versatility, energy savings, safety, aesthetics, strength, and the comfort of the parts/products being produced, according to Ramnath. “Systems have been developed to enhance thermal management as the need for heat dissipation has become increasingly critical for the reliability of electronic devices. Low thermal resistance compounds can be applied in bond lines as thin as 10–15 microns and maintain strength properties even in hostile environments. Both electrically insulative systems and electrically conductive products have been compounded for unmatched heat transfer properties. They also offer dimensional stability, a low CTE, and a low shrinkage upon curing,” he explains. NASA low-outgassing approved compositions have a paste consistency and can also be used in high-temperature applications. “As electronic devices grow ever smaller and higher density devices gain popularity, these thermal conductive systems will continue to play a key role in extending operating life,” Ramnath comments.
Silicone and epoxy adhesives have also been developed to meet met UL94V-0 and UL94V-1 testing requirements for flame retardancy. They cure in thicknesses of up to and beyond 1–2 inches, have a long working life, low exotherm, low shrinkage, and withstand vibration, impact, and shock, according to Ramnath. They also have good flow properties, superior electrical insulation properties, and produce low smoke levels for potting/encapsulation. Second-generation, non-halogenated epoxy systems that meet rigorous vertical and horizontal burn test requirements under the FAR standard CFR 25.853(a) are used in baggage equipment areas, aircraft windows, and lighting assemblies. Systems recommended for interior panels, door frame lining, and floor and door assemblies have passed Boeing specification BSS 7283 Revision C for low smoke emission and 7237 Revision A for toxicity. Master Bond has also created a specialty epoxy that passes Airbus flame retardancy, smoke emission, and toxicity level testing specifications (ABD 0031, Issue F, 6/8/05) for use in interior panels, floor/door assemblies, and frame linings.
Nanosilica-filled epoxy systems have gained traction due to their combination of abrasion resistance, superb physical strength characteristics, and enhanced temperature and chemical properties, according to Ramnath. “These products, which contain nano-sized fillers of < 50 nm in diameter and can be UV-cured or cured in ‘shadowed out’ areas with the use of moderate heat, are very popular because they offer excellent solutions, do not require mixing, and provide quick cures at ambient temperature,” he says.
Despite these advances, Ramnath notes that marketplace demands continue to grow exponentially. “Expectations are high for systems with increased tensile/peel strength, impact resistance, and thermal shock resistance, as well as adhesives that are non-drip even when cured at high temperatures, have low thermal expansion coefficients and low densities, and offer better surface wetting,” he observes. “In addition, higher electrical/thermal conductivities and improved glass transition temperatures are also required, and much emphasis has been placed on environmental concerns, shortening lead times, eliminating waste, reducing processing times, and lowering energy usage.”