Carbon-based nanomaterial-enhanced elastomer coating for passive ice accretion prevention

Abstract

An elastomer coating for application to a surface to passively prevent or reduce ice accretion in applications such as aircraft control surfaces and propellers or large wind turbines or ship structures or oil rigs. The elastomers are preferably silicone. Low loadings of carbon-based nanomaterials, such as functionalized graphene oxide, reduced graphene oxide, carbon nanotubes and graphene nanoplatelets, are dispersed evenly into the elastomer to enhance both durability and icephobicity.

Description

[0001] CARBON-BASED NANOMATERIAL-ENHANCED ELASTOMER COATING FOR PASSIVE ICE ACCRETION PREVENTION

[0002] TECHNICAL FIELD OF THE INVENTION

[0003] The present invention relates to methods and systems for reducing ice accretion on surfaces, and more particularly to passive ice accretion prevention methods and systems.

[0004] BACKGROUND OF THE INVENTION

[0005] It is known in the art of ice accretion prevention that surfaces may be susceptible to icing under certain environmental conditions, such as for example icing of outer aircraft surfaces. FIG. 1 is a photograph showing such ice accretion on a rubber deicing boot along the leading edge of a wing. Where aircraft encounter such icing conditions, it can pose a threat to flight safety, with the resulting build-up of ice on exposed surfaces leading to higher drag, degradation of control authority, and stall occurring at higher speeds and lower angles of attack. Ice buildup also occurs in other settings, such as on wind turbines and buildings, where such buildup creates public hazards.

[0006] To counter this threat, many aircraft employ ice protection systems (IPS) that use thermal energy to prevent the build-up of ice on critical surfaces such as leading edges of wings, engine nacelles, and propellers. One thermal energy technique is bleed air systems, which are used by most large aircraft with jet engines or turboprops. Hot air is "bled" off one or more engines' compressor sections into tubes routed through wings, tail surfaces, and engine inlets. The cooled, spent air is exhausted through holes in the underside of the wings. However, with advances towards more efficient aircraft design and more electric aircraft (such as unmanned aerial vehicles or UAVs), the sources of energy that traditional IPS use will become limited. Consequently, it will be necessary to provide alternative anti-icing systems that require lower or even no power input.

[0007] One older prior art system employs a rubber deicing boot that is a thick piece of inflatable rubber attached to the leading edges of the aircraft’s control surfaces (see FIG. 1). Typically, such inflatable boots are located on the wings, as well as the horizontal and vertical stabilizers. As ice builds up on the leading edges, a pneumatic system pumps air into the boots and inflates them. As the boots inflate, the ice cracks and flies off due to aerodynamic forces. Deicing boots typically need to be replaced every few years, and if they have leaks, then their effectiveness is significantly reduced. They can also have difficulty removing ice in severe icing conditions.

[0008] First developed in World War II, TKS™ systems were created for Royal Air Force bombers as an alternative to pneumatic boot deicing. Today, the technology continues to buy crucial time for light aircraft pilots as they work to implement exit strategies when faced with dangerous icing conditions. TKS™ systems dispense an ethylene glycol-based fluid with a freezing point below -57°C through porous titanium panels attached to the leading edge of the wing and tail assembly. The fluid is released through thousands of the panels’ laser-drilled holes, which are not much larger than the size of a human hair. As air flows over the wing and empennage, it disperses the fluid, coating the surfaces and preventing the formation and adherence of ice. TKS™ systems also employ slinger rings to prevent ice accumulation on the propeller blades. As these metal rings spin right alongside the propeller, they fling TKS™ fluid onto the propeller blades and consequently reduce the freezing point of the moisture in the area. In certain aircraft, nozzles also spray TKS™ fluid onto the windshield. Depending on the flow rate, TKS™ systems can provide anywhere between one to three hours of protection to allow for a safe exit from icing conditions.

[0009] Another potential solution that has been considered is the use of a passive ice accretion prevention method in the form of an icephobic coating. These coatings lower the adhesive bond at the surface to promote ice removal by aerodynamic, vibration, and other forces. For example, coatings comprising silicone elastomers or polysiloxane (e.g., NuSil Corporation silicone coatings; Sivas et al. “A Silicone-Based Ice-Phobic Coating for Aircraft” (2007); EP 1 849 843 to Watson et al.; Zhuo et al. “Polysiloxane as icephobic materials - The past, present and the future” (2021)) and fluorocarbon elastomers (e.g., US Patent Application 2006 / 0281861 to Putnam) have been proposed for the passive reduction of accreted ice.

[0010] SUMMARY OF THE INVENTION

[0011] The present invention is directed to carbon-based nanomaterial-enhanced elastomer coatings that can be applied to surfaces to reduce ice accretion. The invention involves the dispersal of low loadings of nanomaterials evenly through an elastomer base to enhance both the durability and icephobicity of the elastomer for use in applications where it is desirable to control ice accretion, such as the reduction of significant ice accretion on the leading edges of aircraft control surfaces and propellers. Addition of carbonaceous nanofillers may also increase the mechanical and thermal stability properties of the coatings. The present invention could also find application in other settings such as large wind turbines, ship structures (railings, etc.) and oil rigs, especially in Arctic operations along with building structures where ice build-up could pose a public hazard.

[0012] Enhancing an elastomer with carbon-based nanomaterials was tested to improve the mechanical strength and durability of elastomer coatings, but surprisingly the enhanced coating also manifested relatively strong icephobicity. Preliminary testing has shown that an elastomer coating enhanced with carbon-based nanomaterials such as functionalized graphene oxide, reduced graphene oxide, carbon nanotubes and graphene nanoplatelets results in a durable icephobic coating.

[0013] According to a first broad aspect of the present invention, there is provided a coating for reducing ice accretion on a surface, the coating comprising: an elastomer; and a carbon-based nanomaterial dispersed in the elastomer to form the coating, the carbon-based nanomaterial comprising 1% or less by weight of the coating.

[0014] In some exemplary embodiments of the first broad aspect of the present invention, the elastomer is a two-part room-temperature-vulcanizing (RTV) silicone. The carbon-based nanomaterial is preferably selected from the group consisting of functionalized graphene oxide, reduced graphene oxide, carbon nanotubes, and graphene nanosheets and nanoplatelets, although other carbon-based nanomaterials may be useful with embodiments of the present invention. The graphene nanoplatelets preferably comprise thermomechanically exfoliated Albany graphite. The carbon-based nanomaterial may be graphene nanoplatelets where the graphene nanoplatelets comprise 0.5% to 2.0% by weight of the coating, or it may be functionalized graphene oxide where the functionalized graphene oxide comprises 0.05% to 1% by weight of the coating. The carbon-based nanomaterial may be reduced graphene oxide where the reduced graphene oxide comprises 0.5% to 1% of the coating, or it may be carbon nanotubes where the carbon nanotubes comprise 0.01% to 0.05% of the coating, or sulfur- doped reduced graphene oxide (S-doped rGO). The carbon-based nanomaterial is preferably dispersed in the elastomer by shear mixing. In some exemplary embodiments ice adhesion strength values for rime ice are reduced to 20.04 to 29.34 kPa, and ice adhesion strength values for glaze ice are reduced to 33.21 to 54.39 kPa.

[0015] According to a second broad aspect of the present invention, there is provided a method for reducing ice accretion on a surface comprising the steps of: a. providing an elastomer; b. providing a carbon-based nanomaterial; c. dispersing the carbon-based nanomaterial in the elastomer to form a coating, the carbon-based nanomaterial comprising 1% or less by weight of the coating; and d. applying the coating to the surface.

[0016] In some exemplary embodiments of the second broad aspect the elastomer is silicone. The elastomer may be a two-part room-temperature-vulcanizing silicone elastomer.

[0017] The carbon-based nanomaterial is preferably selected from the group consisting of functionalized graphene oxide, reduced graphene oxide, reduced graphene oxide doped with heteroatoms such as sulfur, carbon nanotubes, and graphene nanoplatelets. The graphene nanoplatelets may comprise thermomechanically exfoliated Albany graphite.

[0018] In some exemplary embodiments, the step of dispersing the carbon-based nanomaterial in the elastomer is conducted by shear mixing. The surface may be pre-treated before the applying of the coating. Such pre-treating may be by polishing, cleaning and applying a chemical primer as a binder between the surface and the coating.

[0019] Previous work has shown that certain coatings demonstrate desirable levels of icephobicity; however, these studies have largely been limited to a laboratory environment. Questions remain about the durability necessary to consider them for a practical application such as an aircraft IPS. The addition of carbon-based nanomaterials dispersed throughout an elastomer coating addresses this durability issue through the enhancement of the coating’s bulk mechanical properties. Additionally, testing demonstrated that the carbon nanomaterials enhanced the icephobicity of the elastomer so that a lower shear force was required to shed any accreted ice (a low as 12.4 kPa). A detailed description of exemplary embodiments of the present invention is given in the following. It is to be understood, however, that the invention is not to be construed as being limited to these embodiments. The exemplary embodiments are directed to particular applications of the present invention, while it will be clear to those skilled in the art that the present invention has applicability beyond the exemplary embodiments set forth herein.

[0020] BRIEF DESCRIPTION OF THE DRAWINGS

[0021] In the accompanying drawings, which illustrate exemplary embodiments of the present invention:

[0022] FIG. 1 is a photograph of a King Air™ aircraft wing with rime ice accretion along its leading edge (source: https: / / airfactsjournal.com / 2016 / 03 / icing-cold-hard-air-facts / ).

[0023] FIG. 2 is photographs of accreted rime ice and glaze (clear ice) along the leading edge of a wing.

[0024] FIG. 3 is a plot of average shear stress (kPa) for rime ice for each carbon-based nanomaterial- enhanced elastomer at different loadings.

[0025] FIG. 4 is a plot of percent shear stress (kPa) reduction compared to the control for rime ice for each carbon-based nanomaterial-enhanced elastomer at different loadings.

[0026] FIG. 5 is a plot of average shear stress (kPa) for glaze ice for each carbon-based nanomaterial-enhanced elastomer at different loadings.

[0027] Exemplary embodiments will now be described with reference to the accompanying drawings.

[0028] DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

[0029] Throughout the following description, specific details are set forth in order to provide a more thorough understanding to persons skilled in the art. However, well-known elements may not have been shown or described in detail to avoid unnecessarily obscuring the disclosure. The following description of examples of the invention is not intended to be exhaustive or to limit the invention to the precise form of any exemplary embodiment. Accordingly, the description and drawings are to be regarded in an illustrative, rather than a restrictive sense.

[0030] The present invention is directed to enhancing an elastomer using dispersed carbon nanomaterials to improve not only the mechanical strength and durability of the elastomer as a coating but also the icephobicity of the elastomer coating. The term icephobicity is used to represent the amount of shear stress required to remove ice from a surface (shear strength of surfaces). In the test results below, two methods were used to test the shear strength of ice adhesion, namely (1) a static ice adhesion test rig that can be used in a cold room and applies a side force to a frozen ice block, and (2) a spin rig that is installed in an Altitude Icing Wind Tunnel (AIWT) and uses centrifugal force to shed the ice from the sample (a simulated in-flight icing environment).

[0031] Initially, the carbon-based nanomaterials were added to the elastomer coating to enhance its mechanical properties and durability. One unanticipated and surprising additional benefit was that the icephobicity of the coating was increased when low loadings of carbon-based nanomaterials were added.

[0032] TEST RESULTS

[0033] Turning now to FIG. 3 to 5 and Table 1 and Table 2 below, the icephobic effect of exemplary enhanced coatings is illustrated.

[0034] A control material was used in the testing, specifically Cenusil™ M825 A / B RTV-2 silicone produced by Wacker Chemie AG, with no carbon nanomaterial additives.

[0035] The elastomer base selected for the testing was an RTV-2 silicone elastomer (a two-part room- temperature-vulcanizing silicone elastomer). Various carbon-based nanomaterial additives were tested, mixed in different weight percentages with the elastomer base under two different mixing techniques. The tested carbon-based nanomaterials were functionalized graphene oxide (GO), reduced graphene oxide (rGO), carbon nanotubes (CNT) and graphene nanoplatelets. The graphene nanoplatelets were in the form of thermomechanically exfoliated Albany graphite (TMZ) produced according to the teaching of PCT Application WO 2019 / 148295 to Osazuwa et al.

[0036] Two shear mixing techniques were used to create the enhanced coatings for testing purposes. The first technique used in the testing (results in Table 1) was taught in PCT Application W02020 / 124263 to Angammana et al., and the second was a high shear mixing technique (results in Table 2). The test results are shown below in Tables 1 and 2 and graphically in FIG. 3 to 5, inclusive.

[0037] Table 1 - Results from First Mixing Technique Table 2 - Results from Second Mixing Technique

[0038] In round #1 of the AIWT Test Rig testing (Table 1), it is noted that all samples consistently yielded ice adhesion strength values substantially less than the control elastomer, in the range of 20.04 to 29.34 kPa for the rime ice testing kPa compared to 43.93 kPa for the control elastomer. In round #2, which was directed instead to glaze ice and did not include the control elastomer, the exemplary coatings achieved ice adhesion strength values of 33.21 to 54.39 kPa. Two different sets of tests were conducted under different conditions that resulted in different accreted ice (rime ice vs. glaze ice, FIG. 2). Rime ice is formed when supercooled water droplets instantly freeze onto cold (subzero Celsius) surfaces while glaze or clear ice occurs when the droplets do not freeze on impact but flow back along the wing surface before freezing. Small air bubbles are trapped in rime ice that imparts a milky, crystalline appearance. Glaze ice on the other hand can be nearly transparent and has a smoother surface, sometimes with a waxy appearance. The glaze ice has a higher density and adheres more strongly to the surface than the rime ice and consequently it has higher adhesion strength. For comparison purposes, the ice adhesion strength of a bare aluminium alloy is -500 kPa, and Teflon™ is -238 kPa, whereas the threshold of icephobicity is considered to be 100 kPa.

[0039] Results for the Static Ice Adhesion Test Rig testing are presented in both Table 1 and Table 2. Most of the tested coatings performed better than the control elastomer. The 0.5% TMZ and 0.05% GO loadings produced the best results using the second mixing method and the Static Ice Adhesion Test Rig, as shown in Table 2, generating results as low as 12.4 kPa.

[0040] The foregoing is considered as illustrative only of the principles of the present invention. The scope of the claims should not be limited by the exemplary embodiments set forth in the foregoing, but should be given the broadest interpretation consistent with the specification as a whole.

Claims

CLAIMS1. A coating for reducing ice accretion on a surface, the coating comprising: an elastomer; and a carbon-based nanomaterial dispersed in the elastomer to form the coating, the carbon-based nanomaterial comprising 1% or less by weight of the coating.

2. The coating of claim 1 wherein the elastomer is silicone.

3. The coating of claim 2 wherein the elastomer is a two-part room-temperature- vulcanizing silicone elastomer.

4. The coating of claim 1 wherein the carbon-based nanomaterial is selected from the group consisting of functionalized graphene oxide, reduced graphene oxide, reduced graphene oxide doped with heteroatoms, carbon nanotubes, and graphene nanosheets and nanoplatelets.

5. The coating of claim 4 wherein the graphene nanoplatelets comprise thermomechanically exfoliated Albany graphite.

6. The coating of claim 1 wherein the carbon-based nanomaterial is dispersed in the elastomer by shear mixing and a homogenizer.

7. The coating of claim 2 wherein the carbon-based nanomaterial is graphene nanoplatelets where the graphene nanoplatelets comprise 0.5% by weight of the coating.

8. The coating of claim 2 wherein the carbon-based nanomaterial is functionalized graphene oxide and the functionalized graphene oxide comprises 0.05% to 1% by weight of the coating.

9. The coating of claim 2 wherein the carbon-based nanomaterial is reduced graphene oxide and the reduced graphene oxide comprises 0.5% to 1% of the coating.

10. The coating of claim 2 wherein the carbon-based nanomaterial is carbon nanotubes and the carbon nanotubes comprise 0.01% to 0.05% of the coating.

11. The coating of claim 1 wherein ice adhesion strength values for rime ice are reduced to 20.04 to 29.34 kPa.

912. The coating of claim 1 wherein ice adhesion strength values for glaze ice are reduced to 33.21 to 54.39 kPa.

13. A method for reducing ice accretion on a surface comprising the steps of: a. providing an elastomer; b. providing a carbon-based nanomaterial; c. dispersing the carbon-based nanomaterial in the elastomer to form a coating, the carbon-based nanomaterial comprising 1% or less by weight of the coating; and d. applying the coating to the surface.

14. The method of claim 13 wherein the elastomer is silicone.

15. The method of claim 14 wherein the elastomer is a two-part room-temperature- vulcanizing silicone elastomer.

16. The method of claim 13 wherein the carbon-based nanomaterial is selected from the group consisting of functionalized graphene oxide, reduced graphene oxide, reduced graphene oxide doped with heteroatoms, carbon nanotubes, and graphene nanosheets and nanoplatelets.

17. The method of claim 16 wherein the graphene nanoplatelets comprise thermomechanically exfoliated Albany graphite.

18. The method of claim 13 wherein the step of dispersing the carbon-based nanomaterial in the elastomer is conducted by shear mixing and a homogenizer.

19. The method of claim 13 wherein the surface is pre-treated before the applying of the coating.

20. The method of claim 19 wherein the surface is pre-treated by polishing, cleaning and applying a chemical primer as a binder between the surface and the coating.

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