Chemical reactivation method for direct adhesion of coatings to metal surfaces

Abstract

The present invention relates to coatings for metal surfaces, and more particularly to an improved method for adhering coatings to metal surfaces in aerospace applications. A method (400) for adhering a coating to a metal substrate includes exposing a surface of the metal substrate to a solvent (402), applying a sol-gel treatment to the metal substrate (404), drying the treated metal substrate (406), applying a chemical reactivator to the treated metal substrate (408), and applying a polymer coating to the metal substrate (410). The method can further include cleaning the metal substrate with an alkaline cleaner before exposing the surface of the metal substrate to the solvent. The polymer coating can include polyurethane or a clear polymer coating.

Description

[Technical Field]

[0001] The present disclosure relates generally to coatings for metal surfaces, and more particularly to improvements in methods for adhering coatings to metal surfaces in aerospace applications. [Background technology]

[0002] Aircraft and aerospace vehicle vertical stabilizers and cockpit window frames are high-risk areas that may be coated with polyurethane clear coats or other protective coatings, for example, as paint or coating stencils or paint edge seals, or on other parts of an aircraft, where such coatings may be applied directly to metal surfaces. On aerospace vehicles such as the Boeing 737, clear coat adhesion failure can frequently occur during demasking procedures or test flights. In such cases, the protective coating will not meet the appearance or performance standards required for effectiveness in high-risk areas.

[0003] Current coating methods involve using solvents to clean metal aircraft surfaces before applying a clear coat or other surface coating to the surface, which can be tested and demonstrated using a raindrop erosion test to predict erosion defects. Neither traditional sol-gel coatings used alone nor adhesion promoters used separately as coatings have been proven effective in mitigating erosion defects during aircraft operation.

[0004] Therefore, it is desirable to develop a durable and effective erosion resistant coating system for aerospace applications. Summary of the Invention

[0005] The following is a summary intended to provide a basic understanding of some aspects of one or more embodiments of the present teachings. This summary is not intended to be an exhaustive overview of the present teachings, to identify key or essential elements, or to delineate the scope of the present application. Rather, this summary's primary purpose is to present one or more concepts in a simplified form as a prelude to the more detailed description that is presented later.

[0006] A method for adhering a coating to a metal substrate is disclosed. The method for adhering a coating to a metal substrate includes exposing a surface of the metal substrate to a solvent. The method for adhering a coating to a metal substrate further includes applying a sol-gel treatment to the metal substrate. The method for adhering a coating to a metal substrate further includes drying the treated metal substrate. The method further includes applying a chemical reactivator to the treated metal substrate. The method further includes applying a polymer coating to the metal substrate. Implementation of the method for adhering a coating to a metal substrate may further include cleaning the metal substrate with an alkaline cleaner before exposing the surface of the metal substrate to a solvent. The sol-gel treatment is applied to a thickness ranging from about 0.5 microns to about 2 microns. The chemical reactivator is applied to a top surface of the treated metal substrate after drying the treated metal substrate to a thickness of less than about 100 nanometers. The method for adhering a coating to a metal substrate may further include applying an additional coating or treatment over the polymer coating. The solvent may include water. The solvent may have a pH level between 8 and 9. The metal substrate may include aluminum or an alloy thereof. Drying the treated metal substrate may include exposing the metal substrate to ambient conditions in a controlled environment for at least 1 hour. The solvent may further include a mild detergent. The polymer coating may include polyurethane. The polymer coating is a clear coat or is transparent in the visible range. The polymer coating may further include a pigment or additive.

[0007] Also disclosed is a method for preventing leading edge erosion of a metal surface. The method for preventing leading edge erosion of a metal surface includes treating the metal surface with a solvent. The method further includes applying a sol-gel treatment to the metal surface. The method further includes applying a chemical reactivator to the treated metal surface. The method further includes applying a polyurethane coating to the metal surface. Implementing the method for preventing leading edge erosion of a metal surface can further include drying the metal surface after applying the sol-gel treatment to the metal surface. The sol-gel treatment is applied to a thickness ranging from about 0.5 microns to about 2 microns, and the polyurethane coating has a thickness ranging from about 50 microns to about 150 microns. The metal surface can include aluminum or an alloy thereof.

[0008] An article is disclosed that includes a metal substrate. The article further includes a sol-gel coating layer disposed on a surface of the metal substrate. The article further includes a chemical reactivator layer disposed on the sol-gel coating layer. The article further includes a polymer coating disposed on the chemical reactivator layer. Implementations of the article include where the polymer is a transparent polyurethane. The metal substrate may include aluminum or an alloy thereof.

[0009] The above-described features, functions, and advantages may be achieved individually in each embodiment or may be combined in yet other embodiments, details of which will become apparent with reference to the following description and drawings. [Brief explanation of the drawings]

[0010] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate several embodiments of the present teachings and, together with the description herein, serve to explain the principles of the present disclosure, although some details have been simplified in favor of clarity of the present teachings rather than maintaining strict structural accuracy, detail, and scale.

[0011] [Figure 1A]Examples of application of one or more surfaces and methods comprising the adhesive composition to an aerospace vehicle are provided. [Figure 1B] FIG. 1B is an exploded view of a portion of the aerospace vehicle of the present disclosure shown in FIG. 1A. [Figure 2] 1 is a cross-sectional view of a coating structure applied to a metal surface according to the prior art to the present disclosure; [Figure 3] 1 is a cross-sectional view of a coating structure applied to a metal surface using a chemical reactivation process according to the present disclosure. [Figure 4] 1 is a flow chart illustrating a chemical reactivation process for adhering a coating to a metal surface according to the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0012] Exemplary embodiments of the present teachings will now be described in detail with reference to examples of the present teachings illustrated in the accompanying drawings, wherein, wherever possible, the same reference numerals are used throughout to refer to the same or like elements.

[0013] For purposes of this disclosure, leading edge refers to the forward-facing or leading-most exposed portion of a vehicle or aircraft airfoil, such as a fin, wing, stabilizer, or propeller blade, etc. Such structural portions are subject to erosion during operation of the vehicle or aircraft.

[0014] In this disclosure, ambient conditions refer to the environmental conditions prevailing in a given area. For example, if the ambient conditions of a test environment are described, the ambient conditions refer to the environment prevailing during the test procedure being performed. In the example related to the use environment of a vehicle part, the ambient conditions refer to the environmental conditions to which the part or leading edge of the part is exposed during use. Ambient conditions include temperature, light exposure, chemical exposure, humidity exposure, salt exposure, etc.

[0015] In this disclosure, a controlled environment refers to an enclosed area in which certain parameters, such as air temperature, pressure, light, airflow, particle levels, and humidity, are regulated. An example of a controlled environment is a laboratory or space in which the aforementioned parameters are controlled and which is isolated from other operations, spaces, environmental conditions, or parameters. A controlled environment can include an enclosed space in which certain environmental conditions are actively managed. A controlled environment can include a room temperature in the range of about 70°F to about 80°F and a relative humidity (RH) in the range of about 75% to about 90%, or a temperature of about 75°F and a relative humidity of 90%.

[0016] The present disclosure provides a method or process for directly adhering a polymer coating, such as a polyurethane coating, to a metal surface without the need for an epoxy or other primer. Currently, there are products that use an epoxy primer to directly adhere similar coatings to metal surfaces. This product, a sol-gel treatment coating, is widely used in production systems and on aircraft and airplanes. Adhesion promoters, such as Paintbond SM-1, are also used to promote adhesion between two polyurethane coating layers. The disclosed method combines a sol-gel treatment coating and an adhesion promoter, also referred to herein as a chemical reactivator, by first applying the sol-gel treatment to the metal surface. After drying, a primer or adhesion promoter is applied directly onto the sol-gel treatment. When a polyurethane or other polymer coating is applied over this combined layer, the resulting coating has been demonstrated to exhibit acceptable adhesion in both raindrop erosion tests and operational performance. This combination has no anticipated drawbacks other than the additional manufacturing steps of alkaline cleaning and applying the sol-gel material directly to the metal areas of the vehicle or aerospace part, thus allowing the coating to be applied directly to the metal surface without the need for a primer.

[0017] The sol-gel treatment can be used in one or more aircraft or aerospace vehicle manufacturing systems. In the disclosed method, the sol-gel treatment is applied to a metal surface as a two-component formulation, allowed to dry, and then a chemical reactivator is applied directly over the sol-gel treatment. Applying a polyurethane or other polymer coating over this combined layer according to the disclosed method has been demonstrated to provide acceptable adhesion in both raindrop erosion tests and operational performance. Examples of sol-gel treatments are described, for example, in U.S. Patent No. 5,789,085, issued August 4, 1998. <Sol coating>

[0018] Sol coatings of metals bond the resin to the substrate through chemical bonds (covalent bonds, hydrogen bonds, or van der Waals forces) while minimizing the environmental impact of the previously harmful use of highly diluted metals. As described herein, a preferred sol for forming a sol coating (also called a sol-gel film) on a metal contains an organozirconium compound (e.g., tetra-n-propoxyzirconium) covalently bonded to the metal via Zr, and an organosilane (e.g., 3-glycidoxypropyltrimethoxysilane) covalently bonded to an organic primer, adhesive, or resin (with an aqueous formulation containing acetic acid catalyst as a catalyst and Zr hydrolysis rate stabilizer).

[0019] With proper surface treatment, the typical failure mode of adhesively bonded specimens in high-temperature, high-humidity environments is cohesive failure within the organic adhesive layer. In this case, the sol-gel film is stronger than the adhesive bulk, resulting in the highest possible bond strength. However, it has been shown that adhesion with sol-gel treatments may not always be sufficient for some combinations of certain metal substrates and paint films or polymer coatings.

[0020] In some embodiments, the sol-gel process can be used to create binder coatings approximately 20 nm to 500 nm thick that transition from the metal surface through a hybrid organometallic sol-gel film to the adhesive. Preferred sol coatings herein contain organosilanes and organozirconium compounds to improve bond strength and durability. The organosilanes bond covalently or otherwise to the organic adhesive resin or primer. Ideally, covalent bonds are also formed at the interface between the sol-gel and the metal surface. Depending on the design of the sol coating (e.g., porosity, microstructure), mechanical interactions may play a role. The durability of sol-gel films in wet conditions depends on whether the film rehydrates. If the film is too thick, it becomes glassy.

[0021] The term "sol-gel" is a contraction of solution-gelation and refers to a series of reactions in which soluble metal species (typically metal alkoxides or metal salts) hydrolyze to form metal hydroxides. The soluble metal species usually contain organic ligands tailored to fit the resin within the bond structure. The metal hydroxides condense (peptize) in solution to form organic-inorganic hybrid polymers. Depending on the reaction conditions, the metal polymers either condense into colloidal particles or grow to form network gels. The ratio of organic to inorganic materials in the polymer matrix is ​​adjusted to optimize performance for specific applications.

[0022] Many metals are known to undergo sol-gel reactions, and the sol-gel systems of silicon and aluminum have been widely studied. Equations (1) and (2) show typical sol-gel hydrolysis and condensation reactions for silicon. Si(OEt)4+2H2OSi(OH)4+4EtOH Hydrolysis (1) Si(OH)4SiO2+2H2O condensation (2) In the formula, Et represents CH3-CH2-. When the hydrolysis and condensation reactions are complete, the conversion to the metal oxide or hydrated metal hydroxide is complete. In some cases, the reaction is partial, leaving alkoxide functional groups in the gel after the reaction. Depending on the reaction conditions, the reactions of equations (1) and (2) can produce discrete oxide particles, as demonstrated in the synthesis of nanoscale particles, or they can form crosslinked gels that can be used for film formation. The solubility of the resulting gel in solvents depends on the particle size and the degree of crosslinking.

[0023] A clean and active metal surface is important for the adhesion of sol-gel films formed from sprayed, dipped, or drenched sols. Cleaning is a key factor for good adhesion. A dirty surface can inhibit adhesion or cause adhesion between the sol and the dirt instead of between the sol and the surface. A chemically active surface is also important. Titanium forms a passive oxide surface. Bare, pure titanium surfaces oxidize rapidly in air or dry oxygen, resulting in the formation of a titanium oxide film approximately 2 nm to 4 nm (20 Å to 40 Å) thick. Titanium surface oxides do not hydrolyze as readily as aluminum surface oxides to form active metal hydroxides. However, water chemisorbs to the titanium oxide surface. Aluminum oxidizes in air at a similar rate or even faster.

[0024] Etching titanium alloys with HNO3-HF removes the TiO2 alpha-case film but results in a smooth surface that is unsuitable for bonding. Conventional alkaline etchants, such as TURCO 5578 and OAKITE 160, produce a roughened surface more suitable for adhesive bonding, but also form a sticky smut layer. Smut significantly reduces adhesion. Even extended immersion in hot HNO3 after alkaline etching can still leave some darkening. In the preferred process, the surface is cleaned and rinsed, etched with HNO3-HF, rinsed again, and alkaline etched. After a further rinse, the surface is desmutted with BOECLENE in one or more steps. This results in a clean, active surface ideal for adhesive bonding with the sol-coated coatings disclosed herein.

[0025] A common model for sol-gel film formation on titanium involves a Lewis acid / base interaction between the hydrolyzed zirconium alkoxide, organosilane, or both in the sol and the titanium oxide surface. This interaction may be facilitated by chemisorbed water, leading to the formation of Zr-O-Ti or Si-O-Ti bonds or new Ti-OH bonds on the surface. A similar reaction occurs on aluminum surfaces. The ability of metal alkoxides to bond covalently to the metal surface likely requires more energy to bond with titanium than with aluminum. Complete and covalent bonding with titanium alloys may not occur until the part reaches high temperatures, such as those encountered during adhesive curing.

[0026] The sol-gel process has a wide range of applications. Reaction conditions (e.g., concentration of reagents and type of catalyst) control the relative rates of hydrolysis and condensation reactions. Sol-gel solutions can be tailored to facilitate the formation of thin films or condensation into fine colloidal particles. Depending on the starting materials and reaction conditions, films can be formed with morphologies similar to surface coatings formed by anodization and etching. Controlling the chemical composition of the sol allows for tuning of density, porosity, and microstructure.

[0027] The sol-gel condensation reaction is affected by the acid-base properties of the metal / oxide surface. Titanium has a higher isoelectric point (IEP = 6.0, an indicator of surface pH) than aluminum (IEP = 9.2), which changes the interfacial chemistry between the metal and the sol.

[0028] An exemplary zirconium compound for forming the sol is represented by the general formula (RO)Zr. In this formula, R is a lower aliphatic compound having 2 to 5 carbon atoms, particularly a linear aliphatic (alkyl) group. Tetra-n-propoxyzirconium is preferred because of its readily available commercial availability. Branched aliphatic, alicyclic, or aryl groups are also believed to work well. For applications involving prolonged exposure to high-temperature / humid environments, adjusting the organic group on the zirconium can improve thermal oxidative stability.

[0029] Exemplary organosilane compounds (available from Petrarch or Read) for making sols include: 3-aminopropyltriethoxysilane, 3-glycidoxypropyltrimethoxysilane, p-aminophenylsilane, allyltrimethoxysilane, n-(2-aminoethyl)-3-aminopropyltrimethoxysilane, 3-aminopropyltriethoxysilane, 3-aminopropyltrimethoxysilane, 3-glycidoxypropyldiisopropylethoxysilane, (3-glycidoxypropyl)methyldiethoxysilane, 3-glycidoxypropyltrimethoxysilane, 3-mercaptopropyltrimethoxysilane, 3-mercaptopropyltriethoxysilane, 3-methacryloxypropylmethyldiethoxysilane, 3-methacryloxypropylmethyldimethoxysilane, 3-methacryloxypropyltrimethoxysilane, n-phenylaminopropyltrimethoxysilane, vinylmethyldiethoxysilane, vinyltriethoxysilane, or Vinyltrimethoxysilane.

[0030] In these organometallic compounds, the organic group is preferably aliphatic or alicyclic, typically a lower n-alkoxy group having 2 to 5 carbon atoms. The organosilanes typically contain epoxy groups (for bonding with epoxy or urethane resins or adhesives) or primary amines (for bonding with polyimide resins or adhesives).

[0031] If the sol is alcohol-based, preferred alcohols include ethanol, isopropanol, or other lower aliphatic alcohols.

[0032] Such sols can be used to form sol-gel films on aluminum and titanium alloys, such as Al 2024, Al 7075, Ti-6-4, Ti-15-3-3-3, Ti-6-2-2-2-2, and Ti-3-2.5. The disclosed methods can also be used on substrates, including copper and iron, examples of which include stainless steel or Inconel alloys, aluminum alloys, titanium alloys, anodized aluminum, or combinations thereof, or chromate anodized aluminum, boric acid sulfuric acid anodized aluminum, or metals with chemical conversion coatings, or any combination thereof.

[0033] Chemical reactivators, such as those designated Paintbond SM-1, promote adhesion in the disclosed methods and coating systems. Chemical reactivators can include, for example, zirconium or other metal alkoxides in a solvent formulation.

[0034] FIG. 1A illustrates an example in which one or more surfaces of a structural member containing an adhesive composition are applied to an aerospace vehicle. FIG. 1B illustrates an exploded view of a portion of the aerospace vehicle of the present disclosure shown in FIG. 1A . The coating compositions and methods of the present disclosure are applied to an aerospace vehicle 100, where a vehicle component substrate 130 is prepared by applying a polymer coating to the substrate 130 using a method or process of the present disclosure. The vehicle component substrate 130 shown in the exploded view of FIG. 1B includes a metal substrate surface layer 132, a sol-gel composition layer 136, and a chemical reactivator layer 138 on its surface, thereby adhering a polymer coating 134 to the substrate 130 surface and / or to a structural member of the vehicle 100, particularly the leading edge of the vehicle 100. In some embodiments, the polymer coating application method of the present disclosure is applied to the exterior surface of the aerospace vehicle 100. In some embodiments, an additional coating layer, such as a paint, coating, or other protective coating, can be applied over the polymer coating 134. 1B illustrates a region near the leading edge of a typical aircraft or aerospace vehicle 100 that could benefit from the use of a polymer coating 134 with improved adhesion based on the composition and application method of the sol-gel composition layer 136 and chemical reactivator layer 138. While leading edge portions or regions, such as a cockpit window frame, tail, or wing, are illustrated, other portions or regions of the aerospace vehicle 100 or other structures may also be coated using the disclosed methods. For example, the polymer coating 134 application method may be applied to aircraft fuselages, door edges, vertical stabilizers, wingtips, engine intakes, stencil coatings, or decals applied to metal surfaces. In some embodiments, the disclosed methods may also be applied to metal substrates or surfaces containing microstructures comprised of high-strength alloys, such as 70T6, or lower-strength alloys, such as 60T6.

[0035] According to the present disclosure, a composite structure can include a metal substrate at a surface, with one or more additional composite or metallic layers adhered to at least a portion of the substrate, and a polymer coating adhered thereto. In some embodiments, the top layer of the substrate includes a metal or metal alloy, such as aluminum, stainless steel, titanium, low-carbon steel, aerospace metals, or alloys thereof. Suitable substrates can also include organic polymers, such as PEEK, PEKK, or bismaleimide (BMI) resins, or inorganic materials, including ceramics, such as silicon dioxide (SiO2) or aluminum oxide (Al2O3), in composite multi-layer stacks. Substrates can include metallic, organic, or inorganic materials and can be flat and / or have small or large radii of curvature, depending on the nature of the aerospace component or application for which the substrate is used.

[0036] In other embodiments, the composite layers of the multilayer laminate can include glass fiber, carbon fiber, aramid fiber, alumina fiber, ceramic fiber, or a combination thereof. Examples of polymeric materials (e.g., as substrates) that can be used for metal coatings and surface coatings with polymer layers according to the present disclosure include polymeric materials that function as a matrix in combination with one or more types of fibers or reinforcing materials or functional additives. In one embodiment, materials useful for implementing the present disclosure include fiber-reinforced plastics (FRPs) that combine polymeric materials with inorganic fibers, such as carbon fiber, carbon nanotubes, graphite, fiberglass, glass, metals, alloys, metallized fibers, metal-coated glass fibers, alumina fibers, and boron fibers. In one embodiment, the fiber-reinforced plastics include organic fibers, such as nylon fibers or aramid fibers. In one embodiment, the fiber-reinforced plastics can be composed of organic and / or inorganic fibers blended with a thermosetting polymer, such as an epoxy resin material. In one embodiment, the composite layers of the multilayer laminate can include multiple layers of thermosetting materials, fiber layers, or a combination thereof.

[0037] In one embodiment, polymer articles constructed from carbon fiber reinforced plastic (CFRP) or glass fiber reinforced plastic (GFRP) are suitable for use in aircraft structures and other applications. However, the present disclosure is not limited to these types of materials or specific configurations, and articles constructed from other polymers can be used with the processes of the present disclosure. In one embodiment, the polymer substrate includes a crystalline polymer. Crystalline polymers provide heat and chemical resistance to the FRP. For example, epoxy-based polymers, when crystalline, can exhibit thermal stability at temperatures up to 260°C and, depending on the formulation and additives used, even higher temperatures. In another example, the polymer substrate includes a semi-crystalline polymer. Semi-crystalline polymers provide ductility and processing advantages to the FRP in addition to the advantages of crystalline polymers. In yet another example, the polymer substrate includes an amorphous polymer. Amorphous polymers provide elasticity, ductility, and processing advantages to the FRP. In addition, certain exterior surfaces may include the composite polymer described above in combination with one or more metal components, such as metal fasteners, and the metal components may be intermittently positioned while providing a primer-free topcoat on the composite. The disclosed method allows for enhanced adhesion to the metal portions of the assembly.

[0038] FIG. 2 is a cross-sectional view of a coating structure applied to a metal surface according to prior art to the present disclosure. In existing prior art examples, surface coatings and primer systems are available for a variety of materials, and an epoxy primer can be used to directly contact a polymer coating to the metal surface. In this example, a sol-gel treatment coating 204 is applied to a metal or metal substrate surface 202. An epoxy primer 206 is then applied to the sol-gel treatment coating 204. A clear coat or pigmented polymer coating 208 is then applied. Exemplary polymer coatings include polyurethane polymers.

[0039] 3 is a cross-sectional view of a coating structure applied to a metal surface using a chemical reactivation process according to the present disclosure. The coating structure can be described as including a metal substrate 302, a sol-gel coating layer 304 disposed on the surface of the metal substrate 302, a chemical reactivator layer 306 disposed on the sol-gel coating, and a polymer coating 308 disposed on the chemical reactivator layer. In some embodiments, the polymer includes a transparent polymer, a translucent polymer, or a pigmented polymer, such as a translucent polyurethane or a transparent polyurethane, and in exemplary embodiments, a transparent polyurethane or a pigmented polyurethane. An example of a metal substrate includes aluminum or an alloy thereof. Other exemplary embodiments include substrates having a metallic surface, as described herein.

[0040] FIG. 4 is a flowchart illustrating a chemical reactivation process for adhering a coating to a metal surface according to the present disclosure. The method 400 for adhering a coating to a metal substrate includes exposing the surface of the metal substrate to a solvent 402, applying a sol-gel treatment to the metal substrate 404, drying the treated metal substrate 406, applying a chemical reactivation agent to the treated metal substrate 408, and applying a polymer coating to the metal substrate 410. In some embodiments, the method 400 for adhering a coating to a metal substrate includes cleaning the metal substrate with an alkaline cleaner before exposing the surface of the metal substrate to the solvent. Specific cleaning procedures can include surface polishing and deoxidation using a chemical acid etchant. In some embodiments, the method 400 for adhering a coating to a metal substrate includes applying the sol-gel treatment to a thickness ranging from about 0.5 microns to about 2 microns. In some embodiments, the method 400 for adhering a coating to a metal substrate includes applying a chemical reactivator to the top surface of the metal substrate after drying the treated metal substrate to a thickness of less than about 100 nanometers. In some configurations, the method 400 for adhering a coating to a metal substrate further includes applying an additional coating or treatment, such as those described herein, over the polymer coating. The solvent of the present disclosure can include water, methyl ethyl ketone (MEK), or methyl propyl ketone (MPK), e.g., MEK and MPK in a 30:70 ratio. The purpose is to completely remove oils from the metal surface, thereby rendering the cleaned or treated metal hydrophilic and facilitating coating with an aqueous sol-gel formulation. In other embodiments, the solvent used in the method 400 includes a mild detergent. In some embodiments, the pH level of the solvent is between about 8 and about 9. Conditions for drying the treated metal substrate or metal surface can include exposing the metal substrate to ambient conditions in a controlled environment for at least 1 hour, in some embodiments, at 70°F for between about 15 minutes and about 30 minutes, or until dry, and in some embodiments, up to 1 hour.In yet other embodiments, the polymer coating has optical properties, such as gloss, that are not affected by the combination of the sol-gel composition and the chemical reactivator composition. Examples of suitable polymer coatings include polyurethanes, but regardless of the polymer composition of the polymer coating, the polymer coating is a clearcoat. A clearcoat can be defined as a polymer coating that is transparent in the visible range. In some embodiments, the polyurethane coating is applied in a single layer, without any additional coatings or surface treatments designed to affect its optical properties. In alternative embodiments, the polymer coating can further include pigments or additives. In some examples, these pigments or additives may or may not affect the transparency or translucency of the polymer coating. In some embodiments, the method can include drying the metal surface after applying the sol-gel treatment to the metal surface. In some embodiments, other drying methods, such as a heat gun, oven, or compressed air, can be used as an auxiliary drying method, in which case the exposure time ranges from about 1 hour to about 4 hours at about 120°F. In some embodiments, the sol-gel treatment is applied to a thickness ranging from about 0.5 microns to about 2 microns, alternatively from about 0.1 microns to about 10 microns, or alternatively from about 0.5 microns to about 5 microns. In some embodiments, the polyurethane coating has a thickness ranging from about 25 microns to about 250 microns, alternatively from about 50 microns to about 200 microns, or alternatively from about 50 microns to about 150 microns. Metal surfaces coated by the methods described herein can include aluminum, titanium, steel, or alloys thereof, or other metals or surfaces described herein. These can include stainless steel or chrome electroplated steel (CRES). Solvents and other materials used in the erosion prevention and surface coating methods described herein can include mild detergents that do not contain aggressive chemicals. In some embodiments, environmental conditions during manufacturing are controlled, for example, at temperatures ranging from about 20°C to 25°C and humidity levels below 60%.

[0041] For aerospace applications involving high-strength alloys, mild detergent-based solvents with a pH of 8-9 can be used to clean aluminum, aluminum alloys, and other metal substrates without damaging surface integrity or adversely affecting subsequent coating steps. In some examples, for low-strength alloys such as 60T6, alkaline cleaners at room temperature (20-25°C) and humidity levels below 30% can thoroughly remove impurities while maintaining hydrophilicity.

[0042] In another embodiment, cleaning the metal substrate with a solvent includes using an aqueous solvent containing a surfactant and a weak acid, such as citric acid, under gentle agitation at ambient temperature. For example, a solution containing deionized water (DI) and 0.1% sodium lauryl sulfate can be used, which effectively removes surface contaminants without damaging the native oxide layer on the metal surface.

[0043] In some embodiments, ultrasonic cleaning techniques can be used in conjunction with a mild detergent or surfactant in deionized (DI) water at temperatures ranging from 20°C to 40°C (68°F to 104°F) and humidity levels below 60%. This method can remove grease, residual oil, and particulate matter from even complex metal surfaces without damaging the surface. After thorough rinsing with DI water, the surface can be dried at ambient temperature using compressed air or nitrogen gas. In some embodiments, a final wipe with a lint-free cloth can remove any residue and prevent redeposition in subsequent coating steps.

[0044] In some embodiments, applying a sol-gel treatment to a metal substrate involves several steps designed to enhance wetting and reactivity in subsequent coatings or other processes. The process involves first cleaning the metal surface with a mild detergent or aqueous solution, followed by thorough rinsing and drying to remove any residue. The sol-gel composition is then applied uniformly over a large area of ​​the surface of the metal substrate using conventional coating techniques, such as spraying, brushing, or rolling. The sol-gel treatment enhances the hydrophilicity of the metal surface by forming a thin film with unique chemical properties that enhance wetting and adhesion.

[0045] In some embodiments, the sol-gel composition can be applied in multiple layers to achieve optimal coating properties and uniformity over large areas, such as aircraft wings or fuselage sections. In other embodiments, application to specific areas may require additional testing to ensure consistency throughout the surface treatment. The sol-gel layer can be tailored to each metal substrate by adjusting its composition and thickness to suit the unique properties of each substrate. For example, titanium surfaces have an acid underlayer that can inhibit adhesion, so an etching step is recommended before applying the sol-gel treatment. In contrast, CRES (chromium electroplated steel) may require a special process involving a chemical reaction between the sol-gel and the metal substrate.

[0046] The thickness of each layer is typically in the range of about 0.5 microns to about 2 microns for the sol-gel composition, thinner, e.g., less than 100 nanometers, and about 50 microns to about 150 microns for the polyurethane or other polymer coating applied over the treated surface. These layers combine to create a surface energy profile that improves adhesion between the metal substrate and subsequent coatings.

[0047] In other embodiments, the process can be further accelerated by using a controlled drying environment, such as an oven or dryer. For example, an oven controlled at a set temperature of approximately 40°C can reduce overall drying time and provide more consistent results than drying under ambient conditions. In yet other embodiments, ultraviolet (UV) radiation combined with ambient air circulation can increase cure speed and promote uniformity.

[0048] Applying the sol-gel composition or chemical reactivator to specific regions or areas of a metal substrate may require additional or modified conditions or steps. This is particularly relevant when dealing with complex surface features, such as rivets, fasteners, or other irregularities, which may affect the adhesion strength of the coating to the substrate. In some embodiments, equipment such as a spin coater or dip tank can be used for mass production. Alternatively, specialized applicators can be used to apply the chemical reactivator uniformly to the dried sol-gel composition layer and minimize waste. The applicators used have adjustable nozzles or precision spray tips specifically designed for the purpose.

[0049] In yet another example, a water-based polyurethane or other polymer clear coat can be applied over the combined sol-gel and chemical reactivator layer using an electrostatic spray gun under ambient conditions with minimal air flow. In another example, the methods and structures of the present disclosure can be used in combination with UV-cured pigmented coatings to improve color retention and gloss levels.

[0050] The disclosed methods provide erosion protection without sacrificing adhesion, making them particularly useful in specialized applications such as stencil marking and edge sealing of aircraft tail and cockpit window frames. In these examples, the sol-gel treatment can be combined with a chemical reactivator and applied directly to the metal surface using a desired application technique, such as brushing or rolling. Additionally, the methods and compositions described herein are not limited to aluminum substrates, but are also applicable to other metals, such as titanium, CRES-anodized surfaces, and, in some cases, certain polymers with appropriate surface treatments. <Raindrop erosion resistance test>

[0051] Several tests can be used to predict a coating's resistance to raindrop erosion. The test described herein uses a rotating arm. For interlayer evaluation, a first topcoat is applied to the entire top surface of the test foil, and a second topcoat is applied only to a portion of the test foil, leaving only the first topcoat on the bullnose. The coated foil is then attached to a zero-lift rotating blade mounted inside a wooden drum. The entire device is housed in a blockhouse. As the blade rotates, water droplets are ejected from the periphery of the drum. The blade's rotational speed causes the foil to strike the water droplets at approximately half the speed of sound, slightly above the speed of an aircraft flying under most rainfall conditions. Raindrops striking the second topcoat near its edge create a water hammer effect, generating Rayleigh surface shock waves that cause adhesion failure between the layers. The local stresses generated in this test are an order of magnitude higher than those generated in the tape peel test according to ASTM D3359. Raindrop erosion resistance is then assessed by measuring the distance the second topcoat peels from the original painted edge.

[0052] While the present teachings have described one or more exemplary embodiments, modifications and / or variations can be made to the described examples without departing from the scope or spirit of the present disclosure. For example, even if a process is described as a series of acts or events, the present teachings are not limited to the ordering of such acts or events. Some acts may occur in a different order than described herein or may occur concurrently with other acts or events. Furthermore, not all process steps are required to implement a method in accordance with one or more aspects or embodiments of the present teachings. Of course, additional structural objects and / or process steps can be added, and existing structural objects and / or process steps can be deleted or modified. Furthermore, one or more acts described herein can be implemented as one or more separate acts and / or steps. Furthermore, when the terms "comprise," "include," "includes," "have," "having," "involving," or similar terms appear in the detailed description and claims, these terms, as well as "comprising," are intended to be inclusive. As used herein, the term "at least one" means that one or more of the listed items may be selected. Furthermore, in the description and claims, the term "on" means that two materials are in some manner in contact with one another. In contrast, the term "over" means that two materials are in close proximity to one another, including the presence of one or more intervening materials, and that contact is possible but not required. However, as used herein, neither "on" nor "over" implies a specific direction. When a coating material is described as "conformal," it means that the angular shape of the material covered by the coating material is maintained in the conformal material. Furthermore, when the term "about" is used, it means that the numerical value can vary to some extent without causing incompatibility with the process or structure of the illustrated embodiment.Terms such as "couple," "coupled," "connect," "connection," "connected," "coupled to," and "coupled" may refer to a direct connection or a connection where other elements or members are interposed. Finally, terms such as "exemplary" or "illustrative" are used to indicate that the description is merely an example, rather than an ideal case. Other embodiments of the present disclosure will be apparent to those skilled in the art upon consideration of the specification and embodiments of the present disclosure. The description of the specification and examples is given by way of example only, with the true scope and principles of the present disclosure being indicated by the following claims.

Claims

1. exposing a surface of a metal substrate to a solvent; applying a sol-gel treatment agent to the metal substrate; drying the treated metal substrate; applying a chemical reactivator to the treated metal substrate; applying a polymer coating to the metal substrate; 1. A method for adhering a coating to a metal substrate, comprising:

2. The method of claim 1 further comprising cleaning the metal substrate with an alkaline cleaner before exposing the surface of the metal substrate to the solvent.

3. The method of claim 1 , wherein the sol-gel treatment is applied to a thickness ranging from 0.5 microns to 2 microns.

4. 10. The method of claim 1, wherein the chemical reactivator is applied to a top surface of the treated metal substrate after drying the treated metal substrate to a thickness of less than 100 nanometers.

5. The method of claim 1 further comprising applying an additional coating or treatment over the polymeric coating.

6. The method of claim 1 , wherein the solvent comprises water.

7. 10. The method of claim 1, wherein the solvent has a pH level between 8 and 9.

8. The method of claim 1 , wherein the metal substrate comprises aluminum or an alloy thereof.

9. 10. The method of claim 1, wherein drying the treated metal substrate comprises exposing the metal substrate to ambient conditions in a controlled environment for at least one hour.

10. The method of claim 1 , wherein the solvent further comprises a mild detergent.

11. The method of claim 1 , wherein the polymer coating comprises polyurethane.

12. The method of claim 1 , wherein the polymer coating is a clear coat or is transparent in the visible range.

13. The method of claim 1 , wherein the polymer coating further comprises a pigment or additive.

14. treating the metal surface with a solvent; applying a sol-gel treatment agent to the metal surface; applying a chemical reactivator to the treated metal surface; applying a polyurethane coating to the metal surface; 1. A method for preventing erosion of a leading edge of a metal surface, comprising:

15. 15. The method of preventing erosion of a leading edge of a metal surface of claim 14, further comprising drying the metal surface after applying the sol-gel treatment to the metal surface.

16. The sol-gel treatment agent is applied to a thickness ranging from 0.5 microns to 2 microns, 15. The method of claim 14, wherein the polyurethane coating has a thickness ranging from 50 microns to 150 microns.

17. 15. The method of preventing erosion of a leading edge of a metal surface of claim 14, wherein the metal surface comprises aluminum or an alloy thereof.

18. A metal substrate; a sol-gel coating layer disposed on the surface of the metal substrate; a chemical reactivator layer disposed on the sol-gel coating layer; a polymeric coating disposed on the chemical reactivator layer; Items including.

19. 20. The article of claim 18, wherein the polymeric coating comprises a clear polyurethane.

20. 20. The article of claim 18, wherein the metal substrate comprises aluminum or an alloy thereof.

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