Patent Description:
Carbon nanotube (CNT) materials have been proposed as an alternative to metal wire or foil heating elements in ice protection systems. CNTs are carbon allotropes having a generally cylindrical nanostructure. They have unusual properties that make them valuable for many different technologies. For instance, some CNTs heating elements can have high thermal and electrical conductivity, making them suitable for replacing metal heating elements. Due to their much lighter mass, substituting CNTs for metal heating components can reduce the overall weight of a heating component significantly. Furthermore CNT heaters have low thermal mass, therefore it has a potential to heat up and cool fast and save peak power. These make the use of CNTs of particular interest for aerospace electrothermal de-icing applications.

However, carbon-based fabric heating elements for ice protection are subject to carbon debris migration across the heating element during heater fabrication. The resulting heater may have electric short or liable to dielectric breakdown due to the CNT particles migration. Each of <CIT>, <CIT>, <CIT>, <CIT> and <CIT> discloses a known electric heater for an aircraft.

According to a first aspect, there is provided an ice protection system according to claim <NUM>.

According to a second aspect, there is provided a method according to claim <NUM>.

Carbon-based fabric heating elements for ice protection can contain carbon nanotubes, graphite fibers, graphite nanofibers or graphene. These fabrics can be prepared as pre-impregnated fabrics with thermosetting polymers such as epoxy resins. Alternatively, these fabrics can be coupled with thermosetting film adhesives to allow multiple plies or attachments to metal skins. These CNT fabric heating elements can form de-icer or anti-icer assemblies. In a given carbon-fiber based composite layer heating element, carbon debris have a tendency to migrate between plies, for instance between layers of a heating element and airfoil skins or between heating elements, causing electric shorting. They are additionally liable to dielectric breakdown. If these carbon based fabrics are pre-cured as a pre-impregnated layer before being cured with other plies and layer, the CNT heating element usually has a lack of conformability for de-icing surfaces. Additionally, carbon-based fabrics cannot be tailored to specific resistivity once cured.

The present disclosure concerns the use of thermally stable thermoplastic sheets containing carbon allotropes for heating elements. This construction of the sheet prevents migration of carbon debris across layers within the structure of a composite ice protection system. <FIG> is a schematic view of a carbon additive loaded electrothermal ice protection heating element. Heating element <NUM> includes thermoplastic <NUM> and carbon additives <NUM>.

Thermoplastic <NUM> is a thermally stable plastic, such as a polyetherether ketone (PEEK), polyetherimide (PEI), polyethlylene (PE), polyether sulfone (PES), polylactic acid (PLA), Nylon ®, polyethylene-naphthalate (PEN), polybenzimidazole (PBI), polyimide (PI), poly methyl methacrylate (PMMA), or combinations thereof. Thermoplastic <NUM> should be thermally stable.

Carbon additives <NUM> can be carbon nanotubes, graphene, carbon nanofibers, graphite powder, graphene nanoribbons, or other appropriate electrically conductive material for carbon-based heating elements. Carbon additives <NUM> can be loose particles added to thermoplastic <NUM>, or can be a carbon fabric to which thermoplastic <NUM> is applied.

Resulting heating element <NUM> is used for ice protection. Due to its thermoplastic nature, heating element <NUM> can be applied to surfaces with varying shapes, such as airfoils, nacelle components, and other areas of an aircraft needing ice protection. Heating element <NUM>, when used as a ply in a composite heating element, or combined with multiple carbon-based heating element layers, limits carbon debris migration. The amount of carbon additives added into heating element <NUM>, and the amount of CNT heating elements in an assembly, can be readily varied to change resistivity or sheet resistivity. Thermoplastic <NUM> holds carbon additives <NUM> in place, whether carbon additives <NUM> are woven, unwoven or randomly distributed. The resulting heating element can have electrical sheet resistivity between <NUM> ohms per square (Ω/sq) and <NUM> ohms per square (Ω/sq), preferably between <NUM> ohms per square (Ω/sq) and <NUM>Ω/sq.

Finally, handling of heating element <NUM> is airborne safer for an end user applying heating element <NUM> to an ice protection purpose. A person who is applied heating element <NUM> to a surface is not working directly with carbon nanotubes, carbon fibers, or other carbon additives as he would be with carbon fabrics used in previous heating systems. Instead, a handler is working with a thermoplastic sheet or strip. Thus, handling of heating element <NUM> is safer.

<FIG> is a schematic diagram of ice protection system <NUM> with a carbon additive loaded electrothermal ice protection heating element. System <NUM> includes fiberglass layers <NUM>, film adhesive layers <NUM>, carbon additive loaded electrothermal ice protection heating element <NUM>, and skin layer <NUM>. Heating element <NUM> is similar to heating element <NUM> of <FIG> in its composition. Heating element <NUM> is supported by fiberglass layers <NUM>, and skin layer <NUM>, which are adhered to heating element <NUM> by film adhesive layer <NUM>. Skin layer <NUM> can be a metallic skin or a composite (such as a fiberglass or carbon fiber composite) suitable for ice protection.

Heating element <NUM> is a thermoplastic carbon heater and is being used in system <NUM> as the substructure of an airfoil. The coefficient of thermal expansion (CTE) of heating element <NUM> is compatible with other layers <NUM> and <NUM> to prevent delamination under thermal cycles, particularly between -<NUM> and <NUM> degrees Celsius. Additionally, system <NUM> has a shear strength of at least <NUM> PSI and sufficient bird and hail strike resistance.

The embodiment in <FIG> is representative of an ice protection assembly scheme. In other embodiment, fiberglass <NUM> can be replaced with, for example, dielectric films or other pre-impregnated materials. Additionally, the number of fiberglass layers <NUM> can be increased or decreased based on ice protection needs. In some embodiments, film adhesive layers <NUM> are not needed because pre-impregnated layers are sufficiently adhesive or if the ice protection application requires less stringent bonding requirements. Thus, ice protection system <NUM> can be altered depending on ice protection needs.

<FIG> is a flow chart depicting method <NUM> of making a carbon additive loaded electrothermal ice protection heating element. In method <NUM>, a carbon-polymer mixture is made in step <NUM>, a carbon-polymer sheet is formed in step <NUM>, and the sheet is post-treated in step <NUM>.

First, in step <NUM>, a carbon-polymer mixture is made. The mixture contains a polymer, such as a polyetherether ketone (PEEK), polyetherimide (PEI), polyethlylene (PE), polyether sulfone (PES), polylactic acid (PLA), Nylon ®, polyethylene-naphthalate (PEN), polybenzimidazole (PBI), polyimide (PI), poly methyl methacrylate (PMMA), or combinations thereof.

A carbon additive is integrated into the polymer by standard methods, such as by dissolving a base polymer resin and mixing in a carbon allotrope. Alternatively, a traditional plastic compounding process such as extrusion or internal mixing can be used. Appropriate carbon additives include, for example, carbon nanotubes, graphene, carbon nanofibers, graphite powder, graphene nanoribbons, or other appropriate electrically conductive material for carbon-based heating elements.

In step <NUM>, a sheet is formed from the carbon-polymer mixture. If a method such as dissolution of a base polymer and mixing with a carbon allotrope is used, the mixture can be formed into a sheet and remaining solvent can be removed. If traditional plastic compounding processes are used, then a sheet can be created from a cast or blown extrusion film process. Alternatively, the polymer can be applied to a woven or non-woven carbon fiber sheet. Additionally, heating element <NUM> can be created as a three dimensional shape instead of a sheet by molding, allowing tailoring to large ranges of electrical resistivity.

In step <NUM>, the sheet can be tailored in post-treatment processes as desired. The resulting carbon additive filled polymer can have a thickness of between <NUM> inches and about <NUM>. 010inches, depending on a surface to which it will be applied for ice protection.

The resulting carbon additive filled polymer sheet is lightweight, electrically conductive, and does not cause carbon fiber migration problems when used in composite layers and its resistivity can be readily tailored by carbon additive loading.

An electrothermal ice protection article includes a thermally stable thermoplastic sheet containing a carbon allotrope additive.

The article of the preceding paragraph can optionally include, additionally and/or alternatively, any one or more of the following features, configurations and/or additional components:.

The thermally stable thermoplastic sheet is made of a material selected from the group consisting of polyetherether ketones, polyetherimides, polyethlylenes, polyether sulfones, polylactic acid, nylon, polyethylene-naphthalates, polybenzimidazole, polyimides, poly methyl methacrylates and combinations thereof.

The carbon allotrope additive is selected from the group consisting of carbon nanotubes, graphene, carbon nanofibers, graphite powder, and graphene nanoribbons.

The article has a uniform thickness between <NUM> inches (<NUM>) and about <NUM> inches (<NUM>).

The article has a uniform thickness between <NUM> inches (<NUM>) and <NUM> inches (<NUM>).

The article has an electrical sheet resistivity between <NUM> ohms per square and <NUM> ohms per square.

The article has a first electrical resistivity in a first portion of the article, and a second electrical resistivity in a second portion of the articles; and wherein the first resistivity and the second resistivity differ.

A method of making an electrothermal ice protection system includes creating a polymer and carbon additive mixture, forming a sheet from the polymer and carbon additive mixture, and post-treating the polymer and carbon additive mixture.

The method of the preceding paragraph can optionally include, additionally and/or alternatively, any one or more of the following features, configurations and/or additional components:
Creating a polymer and carbon additive mixture comprises dissolving a base polymer resin and mixing the carbon additive into the base polymer resin.

Creating a polymer and carbon additive mixture comprises mixing a polymer resin and the carbon additive in a plastic compounding process.

The plastic compounding process comprises heating the polymer resin to allow incorporation of the carbon additive to create a film.

Forming a thermoplastic sheet from the polymer and carbon additive mixture is done by placing the film into a cast to form a sheet.

The thermoplastic sheet is made of a material selected from the group consisting of polyetherether ketones, polyetherimides, polyethlylenes, polyether sulfones, polylactic acid, nylon, polyethylene-naphthalates, polybenzimidazole, polyimides, poly methyl methacrylates and combinations thereof.

Forming a sheet comprises molding the mixture into a complex shape.

Forming a mixture comprises injection the polymer with the carbon additive.

An ice protection system includes a carbon heating element comprising a thermally stable thermoplastic sheet containing a carbon allotrope additive, a first fiberglass layer adhered to the carbon heating element by a film adhesive, a second fiberglass layer adhered to the carbon heating element opposite the first fiberglass layer by a film adhesive, and a skin layer adhered to the second fiberglass layer opposite the carbon heating element by a film adhesive.

The system of the preceding paragraph can optionally include, additionally and/or alternatively, any one or more of the following features, configurations and/or additional components:.

The skin layer comprises a metallic layer or a composite layer.

Claim 1:
An ice protection system (<NUM>) comprises:
a first layer (<NUM>) comprising fiberglass, a dielectric film or a pre-impregnated material;
an electrothermal ice protection heating element (<NUM>) comprising a thermoplastic sheet (<NUM>) containing a carbon allotrope additive (<NUM>);
a second layer (<NUM>) comprising fiberglass, a dielectric film or a pre-impregnated material; and
a skin layer (<NUM>) comprising a metallic material or fiberglass, wherein the ice protection system optionally comprises an adhesive layer (<NUM>) positioned between one or more of the first layer (<NUM>) and the electrothermal ice protection heating element (<NUM>), the electrothermal ice protection heating element (<NUM>) and the second layer (<NUM>) and the second layer (<NUM>) and the skin layer (<NUM>);
wherein the first layer, the second layer and the skin layer provide support to the electrothermal ice protection heating element;
wherein the electrothermal ice protection heating element is disposed between the first layer and the second layer;
wherein the ice protection system (<NUM>) has a shear strength of at least <NUM>,<NUM> kilopascals (<NUM> pounds per square inch); and
wherein the electrothermal ice protection heating element (<NUM>) has a uniform thickness between <NUM> micrometers (<NUM> inches) and <NUM> (<NUM> inches).